RATTAN SPINY MORPHOLOGY AND LITTER COLLECTING STRUCTURES IN ASSOCIATION WITH ANT COLONIES
LIU KUNPENG
UNIVERSITI SAINS MALAYSIA
2019
RATTAN SPINY MORPHOLOGY AND LITTER COLLECTING STRUCTURES IN ASSOCIATION WITH ANT COLONIES
by
LIU KUNPENG
Thesis submitted in fulfillment of the requirements for the degree of Master of Science
June 2019
ACKNOWLEDGEMENT
First and foremost, I would like to express my sincere gratitude to my supervisors,
Dr. Nik Fadzly N Rosely, Dr. Asyraf Mansor, Dr. Nadine Ruppert and Prof. Lee Chow
Yang. Without their assistance and dedicated involvement in every step throughout the process, this thesis would have never been accomplished. I really need to thank my supervisors’ dedicated guidance not only in my research project, but also in giving me the knowledge that will benefit my entire life.
Besides my supervisors, I would like to thank Dr. Foong Swee Yeok for always discussing with me and giving me advice in my research and daily life. In addition, I need thank Dr. Hasnuri Mat Hassan, Prof. Aileen Tan Shau Hwai and Dr. Faradina Merican
Mohd Sidik Merican for teaching and offering me favours in different fields. I also need thank all the facilities and assistance from all stuff of School of Biological Sciences,
University Science Malaysia.
My sincere thanks also go to my friends, labmates and former coursemates who gave me plenty of unforgettable memories. I really appreciate Kathrine Tan for helping me translate my abstract. Not to forget my friend Sangsang for numerous conversation that touched the deepest of my heart.
I am heartily thankful to my family, especially my parents, for supporting me not only financially but also mentally even they are thousands of miles away. Thank you for your tremendous love and understanding to let your only son study abroad without worries.
Lastly, I offer my regards and blessing to everyone who I met in any respect during the completion of this study!
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TABLE OF CONTENTS
Acknowledgement ii
Table of contents iii
List of Tables v
List of Figures vi
Abstrak ix
Abstract xi
CHAPTER 1 GENERAL INTRODUCTION
1.1 Background study 1
1.2 General Objectives 3
1.3 Thesis Structure 4
CHAPTER 2 LITERALTURE REVIEW
2.1 Plant Defensive Mechanisms 6
2.2 Ant-Plant Interactions 20
2.3 Rattan 28
CHAPTER 3 GENERAL METHODOLOGY 34
CHAPTER 4 WHO IS BETTER DEFENDED? SPINESCENCE MEASUREMNT ON 5 RATTAN SPECIES
4.1 Introduction 38
4.2 Method and Materials 40
4.3 Results 44
4.4 Discussion 54
iii CHAPTER 5 ANT-RATTAN ASSOCIATIONS
5.1 Introduction 63
5.2 Method and Materials 64
5.3 Results 64
5.4 Discussion 72
CHAPTER 6 RECYCLING IN THE FOREST: RATTAN LITTER COLLECTION AND RELATIONSHIPS WITH ANTS
6.1 Introduction 76
6.2 Method and Materials 77
6.3 Results 78
6.4 Discussion 84
CHAPTER 7 GENERAL DISCUSSION 88
CHAPTER 8 GENERAL CONCLUSION AND FURTHER STUDIES 94
REFERENCE 97
APEENDICES
LIST OF PUBLICATIONS
vi LIST OF TABLES
Page Table 4.1 Table 4.1. Spine length mean value of rattan species 44
Table 4.2 Spine width mean value of rattan species 45
Table 4.3 Spine angle mean value of rattan species 48
Table 4.4 Spine angle mean value of rattan species 49
Table 4.5 Leaf hairs measurement of D. lewisiana 54
Table 4.6 Leaf hairs measurement of C. castaneus 54
Table 4.7 Leaf hairs measurement of D. geniculate 54
Table 5.1 Ant genera found on various rattan species 65
Table 6.1 Mean value inclination of rattan spines 79
Table 6.2 Angles of different paired leaflets in rattan species 82
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LIST OF FIGURES
Page Figure 3.1 Study site locations at (a) Bukit Genting Hill, 34 (b) Penang National Park, (c) Taman rimba, Teluk Bahang and( d) Cherok Tokun, Penang
Figure 3.2 Sampling track in Bukit Genting Hill 35
Figure 3.3 Sampling track in Penang National Park 36
Figure 3.4 Sampling track in Taman Rimba, Teluk Bahang 36
Figure 3.5 Sampling track in Cherok Tokun, Penang mainland 37
Figure 4.1 The vernier scale 41
Figure 4.2 The protractor 41
Figure 4.3 The digital weight scale for tensile tests 42
Figure 4.4 The Ocean Optics Jazz spectrophotometer 43
Figure 4.5 Leaf hairs of D. lewisiana 43
Figure 4.6 Spines of D. lewisiana 45
Figure 4.7 Spines of D. geniculata 46
Figure 4.8 Spines of C. castaneus 46
Figure 4.9 Spines of P. griffithii 47
Figure 4.10 Spines of K. scortechinii 47
Figure 4.11 The spectral reflectance results from 400nm to 700nm wavelength 48
Figure 4.12 Cut image of D. lewisiana 50
Figure 4.13 Cut image of D. geniculata 50
Figure 4.14 Cut image of C. castaneus 51
Figure 4.15 Cut image of P. griffithii 51
vii Figure 4.16 Cut image of K. scortechinii 52
Figure 4.17 Leaflets of K. scortechinii 59
Figure 5.1 Philidris sp. 66
Figure 5.2 Dolichoderus thoracicus 66
Figure 5.3 Crematogaster sp1. 67
Figure 5.4 Crematogaster sp2. 67
Figure 5.5 Tapinoma melanocephalum 67
Figure 5.6 Technomyrmex sp1. 67
Figure 5.7 Camponotus sp. 67
Figure 5.8 Crematogaster sp5. 67
Figure 5.9 Crematogaster sp6. 68
Figure 5.10 Crematogaster sp3. 68
Figure 5.11 Pheidole sp. 68
Figure 5.12 Technomyrmex sp2. 68
Figure 5.13 Crematogaster sp4. 68
Figure 5.14 Camponotus beccarii 68
Figure 5.15 Technomyrmex sp3. 69
Figure 5.16 Cerataphis orchidearum 69
Figure 5.17 Live Dolichoderus thoracicus 69
Figure 5.18 Live Tapinoma melanocephalum 69
Figure 5.19 Live Camponotus beccarii 69
Figure 5.20 Live Crematogaster sp. 69
Figure 5.21 Ant genus that associated with rattan species 70
vii Figure 5.22 (a) Yellow crazy ants (Anoplolepis gracilipes) were tending 71 aphids (Cerataphis sp.) on rattan D. lewisiana; (b) Dolichoderus thoracius gathered in the knee area of Calamus diepenhorstii.
Figure 5.23 Dendrogram using Jaccard’s Coefficient similarities based on 71 the ant genus in four rattan species
Figure 5.24 (a) Ants (Dolichoderus) inside a female flower of 75 Daemonorops lewisiana. (b) Ants (Philidris) inside a male flower of Daemonorops lewisiana.
Figure 6.1 Inclination of rattan spines: (a) P. griffithii (mean 65.5°); 80 (b) Calamus castaneus (mean 103°); (c) Daemonorops lewisiana (mean 138°)
Figure 6.2 (a) Little amount of leaf litter trapped by P.graffithii; 81 (b) Extensive amount of leaf litter collected and trapped by C. castaneus
Figure 6.3 Leaflet angle of Calamus castaneus from the upper part 82 to bottom part (from 3 to 1) is becoming smaller
Figure 6.4 (a) A protruding leaflet growing on the middle of the rachis 83 that may hinder leaf debris from falling. (b) The leaf of Plectocomia griffithii does not collect leaf debris or harbour ant colonies.
Figure 6.5 (a) Leaf litter trapped on the bottom part of 83 Daemonorops lewisiana leaves. (b) Ant colony on the debris of leaves trapped by Calamus castaneus.
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MORFOLOGI DURI ROTAN DAN STRUKTUR PENGUMPUL SARAP YANG
BERKAITAN DENGAN KOLONI SEMUT
ABSTRAK
Rotan merupakan pokok palma yang biasa dijumpai di hutan Malaysia tetapi tumbuhan ini jarang diketahui ramai selain dari nilai ekonomiknya dalam pembuatan perabot dan tikar. Banyak spesies rotan mempunyai duri yang banyak dan aturan yang berlainan. Tetapi, kurang ada kajian dilakukan mengenai aspek struktur dan keunikan fungsi duri. Kajian ini memfokuskan stuktur duri rotan yang terdapat pada lima spesies rotan yang biasa dijumpai di bahagian utara Semenanjung Malaysia, iaitu Daemonorops lewisiana, Daemonorops geniculata, Calamus castaneus, Plectomia griffithii dan
Korthalsia scortechinii. Panjang, lebar, sudut condong, kepadatan dan kekuatan duri rotan telah diukur serta dibanding sesama lain untuk mengetahui spesies yang mana mempunyai keupayaan yang paling bagus dalam melindungi rotan tersebut. Ciri-ciri rambut daun pada
D. geniculata, D. lewisiana, dan C. castaneus telah direkod. Tidak ada spesies rotan mempunyai struktur pertahanan yang jauh lebih baik berbanding spesies lain dan hal ini disebabkan setiap spesies mempunyai kelebihan tersendiri. D. geniculata mempunyai duri yang paling panjang; D. lewisiana mempunyai duri yang paling kuat; duri pada C. castaneus mempunyai kepadatan yang paling tinggi dan duri yang berarah ke bawah pada
P. griffithii mungkin berkesan dalam menghalang binatang mamalia pemanjat kecil. Tiada ciri yang unik terdapat pada struktur duri K. scortechinii tetapi duri spesies ini masih dapat mempertahankan dirinya dengan koloni semut yang mendiami pada stuktur okrea duri spesies ini. Rotan bergantung pada pelbagai jenis strategi pertahanan dan struktur duri
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merupakan salah satu strategi pertahanan. Dalam kajian ini, banyak koloni semut telah dijumpai pada spesies rotan tertentu. Kajian seterusnya memfokuskan hubungan antara rotan dengan semut, spesies semut yang terdapat pada empat spesies (D. geniculata, D. lewisiana, C. castaneus, and K. scortechinii). Tiada bukti yang menunjukkan bahawa terdapat hubungan obligat antara rotan dan spesies semut. Spesies semut yang berlainan telah dijumpai pada rotan dan semut-semut tersebut mempunyai fungsi yang berbeza.
Tetapi, tiada koloni semut didapati pada spesies P. griffithii. Kecuali K. scortechinii yang menyediakan domatia (okrea yang berkembang) kepada koloni semut, tiga spesies yang lain tidak menyediakan stuktur yang sama kepada koloni semut tetapi koloni semut dapat dijumpai dalam sampah dedaun yang jatuh pada stuktur duri dan daun. Terdapat lebih banyak koloni semut pada rotan yang menyimpan sampah dedaun (D. lewisiana dan C. castaneus) berbanding dengan jenis rotan yang tidak menyimpan sampah dedaun (P. griffithii). Bab yang selanjutnya memberi fokus pada perbezaan struktur duri and anak daun yang terpadat pada dua jenis rotan. Keputusan menunjukkan bahawa P. griffithii tidak memiliki struktur yang dapat menyimpan sampah dedaun (duri yang tunjuk ke atas dan daun yang berupa corong), jadi spesies ini tidak dapat menyimpan sampah daun dari kanopi. Duri pada D. lewisiana dan C. castaneus mempunyai fungsi alternatif dalam pengumpulan sampah dedaun dan struktur tersebut dapat mengalakkan kolonisasi semut.
Kesimpulannya, suatu adaptasi yang kompleks dan baru telah (mengumpul sampah dedaun dan provisi bahan membuat sarang) didapati pada rotan dan adaptasi tersebut mengalakkan interaksi antara rotan dan semut melalui susunan daun, anak daun dan duri.
Di sebaliknya, rotan memperoleh faedah seperti pertahanan, peningkatan nutrien, serta pendebungaan daripada semut.
x RATTAN SPINY MORPHOLOGY AND LITTER COLLECTING
STRUCTURES IN ASSOCIATION WITH ANT COLONIES
ABSTRACT
Rattan is a common palm in Malaysian forests but rarely known except for their economic values in furniture or matting products. Many rattan species possess a great number of spines arrangement in various patterns. However, few studies have looked into the different aspects of those spiny structures and their unique functions.
This study focused on rattan spine structures in five different species which are common in the northern part of Peninsular Malaysia; they are Daemonorops lewisiana,
Daemonorops geniculata, Calamus castaneus, Plectomia griffithii and Korthalsia scortechinii. Spine length, width, inclination, density, and strength were measured, and comparison from every aspect was taken to find out which rattan species possess the greatest defensive abilities to protect themselves. The leaf hairs characteristics on leaflets of D. geniculata, D. lewisiana, and C. castaneus were also measured. The results showed that none of the species has an outstanding defensive weapon since every species have their advantages. D. geniculata has the longest spines; D. lewisiana has the strongest spines; C. castaneus has the greatest the number in density and P. griffithii’s down-pointing spines may effectively deter small climbing mammals. K. scortechinii has nothing special in its spiny structures but was still well defended by ant partners colonizing their ocrea structures. Therefore, a rattan plant may rely on multiple defensive strategies and spiny structures only contribute part of its defensive role. During the study, many ant colonies were found on certain species of rattan plants.
Ants were founded on four rattan species (D. geniculata, D. lewisiana, C. castaneus,
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and K. scortechinii) and the relationship between ant and rattan were studied. There was no evidence of an obligate relationship existed among the rattan and ant species.
Different ant species colonized on those rattan plants and they may serve different services to the plants. However, no ant colonies were found on rattan P. griffithii.
Except for K. scortechinii that directly provide domatia (swollen ocreas) to ant colonies, the other three rattan species have no prepared structure for ant colonies.
Instead, ant colonies were found inside the leaf litter trapped inside their spiny and leaflet structures. The presence of ant colonies in litter-collecting rattan (D. lewisiana and C. castaneus) was significantly higher compared to non-litter-collecting rattans (P. griffithii). The differences in spine and leaflet structures between the two types of rattan were studied. Results showed that P. griffithii do not possess litter collecting structures (upward-pointing spines and funnel-shaped leaves) so the plant could not collect much leaf litter from the canopy. Hence, the spines of D. lewisiana and C. castaneus have an alternative function in collecting leaf litter and encourage ants to build colonies on them. It can be concluded that a complex and novel type of adaptation (litter-collection and provision of nesting materials) for rattan which promotes interactions between the rattan and ants through the arrangements of leaves, leaflets, and spines. In return, the rattan may benefit from ants’ services, such as protection, nutrient enhancement, and pollination.
xi CHAPTER 1 GENERAL INTRODUCTION
1.1 Background study
Plants face numerous threats such as herbivores, pathogens, parasitic plants or competitors after germination. Among all the threats, herbivore may be the most hazardous to plants. Herbivores vary in size, from tiny invertebrates like homoptera which suck sap from plant vascular tissue to megaherbivores which can consume a large amount of plant tissue or even uproot the whole plant from soil (Herrera &
Pellmyr, 2009). Herbivores can learn and choose different types of plant or different parts of plants as their optimal diets so that they can balance their nutrients and maximize their fitness while avoiding plant’s defensive mechanism (Waldbauer &
Friedman, 1991; Karban & Agrawal, 2002).
Herbivores also possess countermeasures against plants’ defensive weapons.
For example, mixed function oxidases are a group of enzymes which can detoxify foreign chemicals from plant materials (Feyereisen, 1999). Some herbivores can even sequester the chemical weapons from the plants and use it in their own tissues or organs to deter predators (Duffey, 1980). If herbivores cannot consume a plant or cannot digest the plant’s tissue, the role could be carried out by their symbiotic partners. For example, leaf-cutting ants can cultivate fungus to breakdown cellulose, starch and xylan from plant tissues (Herrera & Pellmyr, 2009).
For plants, herbivory attacks are an inevitable hindrance in the growth process.
To protect from tricky and crafty herbivores, plants have two choices to deter herbivores. One is using avoidance strategies and the other one is using tolerance strategies. Avoidance strategies can be further divided into escaping strategies and defensive strategies (Rosenthal& Kotanen, 1994). Plants can escape herbivores by
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camouflaging themselves into background or mimicry of an unpalatable items from the surrounding environment. Plants can also alter their growth periods or fruiting time to avoid herbivores during peak periods.
For those plants which cannot hide from herbivores, chemical and/or physical defensive weapons are always armed and may accompanied with warning signals.
Chemical mechanisms refer to plants’ secondary compounds which may decrease the palatability of plant tissues by increase bitterness or nasty smells, lower the digestible nutrients, increase indigestible substances and also produce toxic compounds which harmful to herbivores. Physical defensive structures deter herbivores by direct contact with herbivores. Spinescence (spines, thorns and prickles), pubescence (trichomes and leaf hairs), sclerophylly (hardened leaves) and minerals (silica) are common physical weapons against a variety of herbivores (Hanley et al., 2007). Other plants which can neither decrease their attraction to herbivore nor find powerful defensive weapons to deter herbivore can only tolerate herbivore’s attack. Increasing growth rate, photosynthesis efficiency or increasing tillering etc. are common strategies in tolerance (Strauss & Agrawal, 1999). Plant can also seek for assistance from their mutualistic animal partners to chase herbivores away (Herrera & Pellmyr, 2009). For example, Macaranga plants provide food bodies and nest sites for ants and in return ants protect the plant against herbivores (Itino et al., 2001; Itioka, 2005).
The armament race between plants and herbivores has been continuing for millions of years and herbivores are the main evolutionary pressure in shaping plants’ defensive traits. Herbivores are also an important selective force to determine which plants’ traits can exhibit greater fitness. However, the defensive strategies of an important plant in Malaysia tropical rainforests are rarely studied. Rattan (Subfamily:
Calamoideae) is a well-known for the commercial value in matting, furniture mating
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and medicines (Dransfield, 1979). Previous studies were mainly focused on how to cultivate rattan plants with more economic value and how to increase their yields
(Dransfield, 1992; Xu et al., 2000). Although many rattan species have conspicuous spines, few studies explored on their interaction with other animals. What are the features that spine possesses, and what are the potential alternative functions of those spines? What kind of spine-animal interaction that occurs? Such questions remain unknown.
Several aspects in rattan spinescence were studied. This study investigated methods in measuring the characteristics of spiny structures, looking into the alternative function of their unique spine and leaves arrangements and also the plant- animal interaction. This study would potentially help us to gain more knowledge in exploiting the rattans’ potential usages and promote better rattan harvest by reducing herbivores’ damages. It is also crucial for us to save several endangered rattan species from extinction. The defensive traits on the rattan could be an insight into mutualistic or antagonistic relationship with animals.
1.2 General Objectives
To describe the spinescence of five different rattan species and to estimate their physical defensive abilities.
To study the ant-rattan relationships
To study the alternative function of spiny structures and to examine the leaf litter collecting structure in some rattan species.
1.3 Thesis structure
This thesis is the first investigation of rattan spiny structures based on protocols of plant traits measurements and close observation of rattan-ant relationships. My
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research questions are: what is the general pattern of rattan spiny structureand how to measure it? Are there differences among different species which rattan has the most effective structural defensive ability based on their spiny structures?
Several aspects of spinescence were measured and compared among different species. It could be used as a standardised method to study other plants’ spiny structures. After I observed several different rattan spiny structures, my research questions are: why they are arranged differently and are there other functions rather than defence? Spiny structures can be multi-functional and rattan plants may not only rely on spines for defence. After I observed many cases that rattan plants were bearing ant colonies. the relationship between rattan and ants were studied. What are those ants that colonized on rattan plants? Are there any mutualistic relationships between ant and rattan and what benefits they can get from each other? I noticed that some rattan plants always accommodate ant colonies while some do not. My research question is: why ant species prefer certain rattan plants only? Is it likely that ants preferred to build their colonies in rattan plants which collect leaf litter? Why do certain rattans can collect leaf litter but others cannot? I studied the unique characteristics of litter- collecting leaf and spiny structures of certain rattan species and proposed a new adaptation between ant species and litter-collecting rattan plants.
The second chapter is the literature review in theories and previous studies about plant defensive mechanism, ant-plant interactions and researches on rattan plants.
The third chapter is the general methodology about my study locations, periods and the statistics I used to analyse field data. The fourth chapter is the first working chapter talking about the spiny structures of five different rattan species. The fifth chapter is the ant species I found on different rattan plants and possible services that ant could provide for rattan plants. The sixth chapter is to study the litter collecting structures of
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certain rattan species and figure out whether ants are adapted to build their nests in litter-collecting rattans. The seventh chapter is a general discussion and the last chapter is a conclusion. Each working chapter was written as an independent manuscript and repetitive information may appear in certain parts of each chapter.
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CHAPTER 2 LITERATURE REVIEW
2.1 Plant defensive mechanisms
Plant plays the most important role in our planet as producers that absorb energy from the sun and consumed by herbivores. The earliest evidence of terrestrial herbivorous activities was recorded in Early Devonian, when plant tissues were consumed by arthropod herbivores (Labandeira, 2007). Due to the lack of escaping abilities, stationary plants have evolved a range of defence mechanisms against herbivores to increase their survival and reproductive rates. On the other hand, herbivores also evolved numerous countermeasures and a great variety of interaction between animals and plants has contributed to the macroevolution of adaptive traits in our ecosystem.
However, not all defensive strategies are always ready to deter herbivores.
Certain types of defence strategy only emerged after plants encountered an attack, which is classified as inducible defence. Some other types of defence are constantly present in plant, which I classified as constitutive defence. Spines, thorns and certain secondary products are induced defence (Herrera & Pellmyr, 2009). Inducible defence has an advantage that is they can save energy and resources when the plant face little pressure from herbivores. Therefore, the defensive traits exhibit in plants should match with the distribution of herbivores or ancient herbivores that exist once upon a time
(Burns, 2013).
Plants’ defence can also be categorized into physical defence (mechanical defence) and chemical defence. Chemical defence are well studied as researchers see a great variety of compounds in plant which has no specific role in plants’ daily routine, i.e. growth, development and reproduction (Fraenkel, 1959). Secondary compounds
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are by-products which are derived from primary metabolic pathways (Whittaker,
1970). They can be grouped as toxins if they are interfering with herbivores’ important metabolic process or they can be grouped as deterrence if they can stop herbivores from eating the plant (Herrera & Pellmyr, 2009). The deterrence compounds are always associated with features such as offensive-smelling, bitter-tasting or hard-to- digest. Some compound can possess both features, for example, cucurbitin are both toxic and awful-tasting to a large group of beetles (Metcalf et al., 1980).
There are three common groups of plant secondary substances in defence, nitrogen compounds (Alkaloids), terpenoids (monoterpenes and saponins) and phenolics (tannins and flavonoids) (Harborne, 1991). Certain compounds may serve multiple functions in plant, for example, silica may have structural functions in grasses and also act as a defensive weapon that abrades herbivores diet and lowers the digestibility (Van Soest & Jones, 1968; Herrera, 1982). Plants are not guarded by chemical compounds may also be toxic with the help of other organisms. Grasses can produce toxic compounds (alkaloids) after they harbour fungal endophytes (Clay,
1989).
Compared to the mainstream theories of plant defence strategies in chemical defence, plant structural defence are lacking in research. The structural trait that plant display can not only be obvious protuberances but also small or microscopic modification in cell wall structures (Hanley et al., 2007). Therefore, any mechanical and anatomical traits that deter herbivores directly can be considered as structural defence and plant can gain evolutional advantages by possessing structural traits.
Karban, & Baldwin (1997) emphasized that the defensive trait that confers a fitness to plant is beneficial under the presence of herbivores. Traits with no physical contact with herbivores cannot be considered as structural traits. Several types of traits that
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belong to structural traits can be categorized as spinescence (spines, thorns and prickles), pubescence (hairs and trichomes), sclerophylly (hardened leaves) and granular minerals inside plant tissues (Hanley et al., 2007).
However, the morphological structures are not necessarily for the herbivory pressure adaptation. For example, evidence showed that three Berberis displayed longer spines after fire (Gowda & Raffaele, 2004). Certain morphological changes maybe due to the responses to environmental stimuli. Nevertheless, Strauss & Agrawal
(1999) argue about defensive trait as “a trait can be view as defensive even though defence is not its primary function.” Many structural characteristics can also provide multiple functions. Leaves are densely covered by a layer of fine hairs (trichomes) which are vital in protecting leaves from herbivores (Werker, 2000). Grey willow
(Salix cinerea) increase the trichomes density on their new leaves to stop leaf beetle
(Phratora vulgatissima) from browsing (Dalin & Björkman, 2003). Pubescence may also prevent herbivore oviposition as female Papilio troilus prefer to lay eggs on the leaves without pubescence (Haddad & Hicks, 2000). Trichomes on some plants of
Datura wrightii are glandular which deter herbivores by excreting a sticky exudate
(Van Dam & Hare, 1998). Although researches also demonstrated that leaves with abundant trichomes will also deter predators’ movement and decrease their searching efficiency (Krips et al., 1999), trichomes in many plants are also important in other functions to increase their physiological benefits. For example, trichomes play a role in water balance (Levin, 1973), gas exchange and temperature maintaining (Gutschick,
1999). Hairs also help leaves reduce UV radiation impact (Manetas, 2003). In arid environment, pubescence helps lower temperatures and water loss in daytime during the hot period while maintaining a relatively high leaf temperature during cold weather at night (Press, 1999). Sclerophylly also helps leaves resist wilt and increase leaf life
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span (Chabot & Hicks, 1982), maintain water (Lamont et al., 2002) and nutrients level
(Chapin et al., 1993), this is in addition to their protective roles in reducing the palatability and digestibility of leaves or shoots tissues (Grubb, 1986) as well as decreasing the chewing effectiveness of herbivores (Perez‐Barberia & Gordon, 1998).
A trait is an adaptation evolved under certain pressure or induced by certain stimuli in an environment which may come from biotic effects or abiotic effects. The defensive trait may not be the direct response from herbivores but a ‘natural resistance’
(Edwards, 1989) and be cautious about the alternative function of a defensive trait
(Hanley et al., 2007). Spinescence is a term that describes the characteristic of plant structural trait, spines, thorns and prickles. Spines, specifically, are modified leaves, thorns originated from modified branches or twigs, while prickles come from cortical or epidermal tissue (Cornelissen et al., 2003). Grubb (1992) used ‘spine’ to describe any projection with a stiff sharply point, which is a big set of term including thorns and prickles.
Although there are various spiny structures in different plant species and they originated from different plant tissue, Cornelissen et al., (2003) argued that spinescence have an obvious function in plant’s defence and Hanley et al., (2007) claimed that spinescence play an evolutionary role in deterring herbivores. If spinescence is a weapon against herbivores, there should be an increase in number of plants with spinescence in areas with relatively higher herbivore pressures. In Africa, spiny structures are a common feature in areas where browsing pressure is high due to the megaherbivores (Brown, 1960). In arid places of southern Africa, plants in moist environments tend to be more spinescent since herbivores also prefer to assemble in sites of similar condition (Milton, 1991). When megaherbivores face pressure from
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large carnivores, their pressure exert on plant are weaker and plant species are less thorny in areas with the presence of herbivores’ predators (Ford et al., 2014).
Even though megaherbivores have gone extinct for many years, pressures from human-introduced livestock still exert onto the spinescence as palm species possess longer and denser leaf spines in areas with cattle compare to areas without cattle
(Goldel et al., 2016). In Australia and New Zealand, structural defence disappeared on plants in offshore islands where herbivores have never set foot on (Burns, 2013; Burns,
2016). Damnacanthus indicus, has smaller leaves in regions with deer compared to regions free of deer (Takada et al., 2001). Presence of spinescence does not only match with regional distribution of herbivores, it also matches the ontogeny of plant species.
For example, juvenile trees of Acacia tortilis and A. nilotica are physically well- defended compared to the adult trees because the adult trees are too high for the consumption by mammalian herbivores (Brooks & Owen-Smith, 1994). In Western
Australia, shrubs vertically increase their structural defence since megaherbivores are capable to reach and feed on adult plant while shrubs in Eastern Australia, shrubs vertically decrease their structural defence since adult plant are unreachable for herbivores (Burns, 2013). Similar patterns show in New Zealand plant Pseudopanax crassifolius where leaves are no longer spiny after they reach adult stage above 3 meters, which is the highest point an avian herbivore can reach (Fadzly et al., 2009;
Burns, 2016). The behaviour of spinescence due to the presence of herbivores indicates that plant structural traits are induced by herbivores’ pressure and several studies found a significant increase in plant spinescence after the plants was consumed by herbivores.
Leaf spines of European holly, Ilex aquifolium, exhibited more branching structures and smaller leaves with higher spinescence after herbivory attack, but undisturbed plants showed a decrease in leaf spinescence (Obeso, 1997). Spines on the leaves of
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American holly, Ilex opaca, were initially thought to help cool the plants, but they were found in more abundance on shadowed leaves rather than the leaves under higher sun exposure. American holly deployed more spines in southern areas where the number of herbivory was higher than the northern areas and herbivores preferred leaves with fewer spines (Supnick, 1983). Acacia depranolobium have longer thorns after browsing by domestic goats compared to trees that were never browsed by domestic goats (Young, 1987). Solanum lycocarpum trees showed a significant increase in spine abundance and spine length after they were attack by moths and moths preferred leaves with shorter and fewer spines (Alves-Silva & Del-Claro, 2016). The abundance of palm stem spine remained the same, but leaf spines increased significantly with the presence of livestock (Goldel et al., 2016)
Even though evidence showed that spinescence could be induced by abiotic factors (Gowda & Raffaele, 2004), majority of the spines are inducible weapon to deter herbivores and several studies proved the effectiveness of spinescence in deterring different herbivores. Cooper & Owen-Smith (1986) studied the impact of plant spinescence on megaherbivores and found that spiny structures can reduce the effectiveness of browsing by restricting the bite size, retarding biting rate, eventually reduce the tissue and foliage loss every time when a plant is fed by a megaherbivores.
Belovsky et al., (1991) found that spinescence have no effect on feeding rates, but a reduction in biomass ingested among five herbivore species existed, ranging from small mammal rabbits to large herbivores kangaroos. An increase in spine density of
Acacia tortilis can reduce the pruning rate of goats and spines can also protect twigs
(axillary meristems) which grow new leaves (Gowda, 1996). Midgley et al., (2001) suggested that thorns in African Acacia are mainly functions as protective weapons for stems but plays minor role in protecting leaves. Researches also showed that plants
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suffer more damage after the removal of spinescence, which further proved the importance of plant defensive function. Wilson & Kerley (2003) found that plant spinescence can limit the intake rate of herbivores and removal of spinescence can remove the restrictions of feeding style and herbivores can enlarge their bite and eventually achieve higher intake rate. A unique experiment designed by Cooper &
Ginnett (1998) indicated that spines on stems can decrease the foraging efficiency of small climbing mammals and woodrats, where these animals can get access to more branches of shrub after the thorns were removed from the plant. Removal of thorns on branches of Acacia seyal plant suffer significantly greater damage from browsers
(Milewski et al., 1991). Deer were feeding more on seedlings of two Acacia species after their thorns were removed (Cash & Fulbright, 2005).
Many studies above have proven that spines are defensive weapons. The effectiveness of spinescence in deterring herbivores, however, has been questioned in several cases. The marginal leaf spines of American holly, Ilex opaca, are not the key factor to deter caterpillars but the glabrous cuticle and tough margins of leaf take a greater part in deterring invertebrate herbivores. Rabbits and deer showed little discrimination between foliage with or without spinescence (Potter and Kimmerer,
1988). Considering the relative size of spines and distance between each spine, it can be assumed that spines are evolved under vertebrate herbivores’ pressure and they have little effects in deterring invertebrate herbivores.
However, consideration should be given that plants may not solely rely on spine for defence and plants may have multiple defensive traits, which were described as ‘plant defence syndrome’ by Agrawal & Fishbein (2006). Theoretically, spines may have other functions but they play an ambiguous role in defending herbivores. Other than a protective role, spines on cactus may benefit the plant in other ways, such as
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helping zoochorous dispersal (Bobich & Nobel, 2001), conducting water toward roots
(Benson et al., 1982) protecting stem from freezing temperature (Loik & Nobel, 1993), extreme hot temperature (Nobel et al., 1986) and maintaining rate of photosynthesis in extreme circumstance (Loik, 2008). Recurved spines help rattan plants or climbing palms to hold onto other plants and scramble up to higher level of canopy level of the forests (Dransfield, 1979; Putz, 1990). Spines of acacia trees are swollen, and they provide nest sites to their mutualistic ants (Young et al., 1996). The presence of multi- functioning spines could explain that spines may not be induced by one stimulus, such as pressure from herbivores, since protective role may not be the primary role of spinescence. An additional physical barrier other than spinescence is ‘divaricate branching’. Divaricate plants’ branches deploy wide angles and they interweave each other with very small leave (Burns, 2016). Divaricate branching is a common feature in plants found in New Zealand and it is considered as a defensive trait against avian herbivores, extinct moa. (Greenwood & Atkinson, 1977; Lee et al., 2010). Bond et al.,
(2014) proved that avian herbivores (emus and ostriches) faced difficulty when feeding on divaricate plants and plants with divaricate branching experienced less damage compared to non-divaricate plants. Researches also showed that the divaricate branching can tolerate wind (Darrow et al., 2001), avoid photoinhibition (Howell et al., 2002) and build microclimate to prevent water-loss and damage from frost
(McGlone & Webb, 1981). Therefore, structural traits are not always induced by herbivores and plant may keep such structural features regardless the presence of herbivores since they have alternative roles.
To avoid herbivores’ attack, the structural traits and chemical traits described above are all belong to defensive traits. However, stationary plants can also rely on other traits, such as escaping strategies, to avoid herbivores. Changing of the
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phenology of leaf production is an escaping strategy to avoid the consumption by herbivores on young leaves (Aide, 1988). Plant produce young leaves during the time when herbivores are rare (Aide, 1992; Coley & Barone, 1996). Young leaves can also flush synchronously in order to saturate the need of herbivores and some leaves may escape from damage (Aide, 1993). Yong leaves which delay greening can also avoid herbivores attention (Coley & Barone, 1996).
Another escaping strategy is certain plant can mimic other coloration to be invisible. In other words, plants try to avoid herbivores’ attention. Many herbivores rely on their vision to seek for food. Hence, plants which are visually obvious are found to be heavily defended by chemical weapons (Yamamura & Tsuji, 1995). Conversely, other plants choose to be inconspicuous in an environment so that visually dependent herbivores may only notice conspicuous plants. Camouflage strategy is the trait that helps an organism blend in with its surrounding environment. This strategy is common in the animal kingdom but plants’ camouflage strategies can be divided into two categories, which are mimicry and crypsis (Wainwright, 2017). Crypsis is described as the situation where the appearance of plant resembles its background image in order to reduce its herbivores’ detection. This strategy is normally the first strategy of plants’ ontogeny as seedling plants out of the ground can perfectly blend in background coloration. Burns (2010) concluded 4 hypothesis for the crypsis strategy to be viable,
(1)plant’s predators must be visually orientated and foraging in daytime; (2)plants need grow in a special habitat but not widely spread in every common habitat which background colour may be different; (3) height of the plant must be lower, the closer to the ground the harder to be found; (4) the background appearance must be unpalatable so that the background itself will not attract any attention from herbivores.
In New Zealand, the juvenile plants of Elaeocarpus hookerianus possess various
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shaped leaves with mottled brown in colour, which is similar to the colour of background leaf litter (Fadzly & Burns, 2010). Another plant in New Zealand is
Pseudopanax crassifolius, which seedling stage appearance also similar to the background litter colours (Fadzly et al., 2009, Burns, 2010). In tropical forests, cryptic coloration of seedling plants which resemble the background colouration can be found in Macaranga bancana and (Fadzly et al., 2016) and Amorphophallus bufo (Liu et al.,
2017). The plants changed their colour and appearance after they grow higher since the strategy is no longer effective, but plants can deploy alternative defence strategies throughout their ontogeny. The dry vegetative bracts covering Monotropsis odorata have a similar coloration to ambient litter. Klooster et al., (2009) removed the bracts of Monotropsis odorata and found the plant suffered more herbivores’ attack.
Therefore, the cryptic coverage helps the plant to avoid herbivores. Leaf colour of
Corydalis benecincta are dimorphic, which presents two different colours (grey and green). Niu et al., (2014) found that herbivores can hardly distinguish the grey leaves from background grey rocks colour so grey morphs suffer less herbivory and have higher survivor rate.
Another type of camouflage strategy is that plant mimic an unpalatable object from surrounding environment, which are called masquerade (Wainwright, 2017).
Herbivores are able to spot the presence of the plant, however, will not see it as an edible object (non-plant-mimicking defensive masquerade) or the animals deem it as an edible plant parts, but the parts are not appealing at all (plant-mimicking defensive masquerade) (Lev‐Yadun, 2014). Australian mistletoes resemble their non-palatable hosts are considered as a protective strategy against herbivores (Barlow & Wiens,
1977). Although studies showed that herbivores do not distinguish between host- mimicry plant and non-host-mimicry plant, it is found that the plant with no visually
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mimic strategy were lower in nutrient contents (Canyon & Hill, 1997). Another mimic strategy used by epiphytic woody vein, Boquila trifoliolata, mimics the leaves of their host plants’ foliage and suffer less damage compared to unsupported veins and veins climbing on leafless trunks (Gianoli & Carrasco-Urra, 2014).
Defensive masquerade strategy is not necessary to be less conspicuous. In contrast, plants may use bright colour to mimic unappealing patterns (i.e. insects damaged tunnels) to lower the attraction of herbivores or even warn herbivores with aposematic coloration (Lev‐Yadun, 2014; Lev-Yadun & Niemela, 2017). Dark spots in plant Xanthium trumarium mimics numerous ants crawling on the plants; conspicuous reddish spots on the pods of three annual legumes which resemble caterpillars and the aphid-like dark anthers on Paspalum paspaloides may have potential visual deterrence against herbivores (Lev-Yadun & Inbar, 2002). Plants can also pretend to be attacked by fungal or herbivores to reduce herbivore’s tendency to feed on the plants. The white coloration on leaves of some coastal and sand-dune plants may be the mimicry of fungal attack to deter herbivores and insects form ovipositing on the leaves (Lev-Yadun, 2006). Leaves deploy fake appearance that they were attacked or damaged by insects, especially tunnelling damaged, signalling defensive information to later attackers (Yamazaki, 2010). If herbivores consume plants tissues which are formerly infested by other insects or pathogens, they may face certain risks, such as higher competition, cannibalism or predation, induced chemical or physical defences by damaged plants (Yamazaki, 2010; Lev-Yadun & Niemela, 2017). Hence, herbivores may avoid eating plants that are occupied, damaged or infested and plants which fake those signals (pseudo-variegation) may gain an advantage in evolutionary selection.
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The visual signals of plant targeted for herbivores are not necessary to mimic images or patterns but they can also be conspicuous warning colours. The physical defensive weapons (spines, thorns and prickles) are always associated with aposematic coloration (Lev-Yadun, 2001; Lev-Yadun, 2009a). The spiny plants in genus Launaea are white variegated, which are conspicuous structural signals providing indirect protection to another four non-thorny plants with white variegated leaves (Lev-Yadun
2009b). Leaves of Silybum marianum in Israel resemble green zebras and the special pattern serves as an aposematic coloration against herbivores (Lev-Yadun, 2003). In the animal kingdom, predators will avoid colourful preys as they associate colourful signals with unpalatabilities and non-toxic animals mimic those warning signal to avoid predators (Cott, 1940; Harvey et al., 1982). Hence, herbivores may also acquire the sense to avoid plants with aposematic colorations.
The intention of acquiring these escaping and defensive traits is to avoid herbivores’ attack. Some other plants do not develop such strategies to avoid herbivory, but they choose to tolerate attack from herbivores or environments. Tolerance is defined by Strauss & Agrawal (1999) as “the degree to which plant fitness is affected by herbivore damage relative to fitness in the undamaged state.” To say that a plant can tolerate herbivory means that it can regrowth and reproduce after certain parts are consumed by herbivores. Tolerance is always interchanged with the term
‘compensation’ and compensation can be used to indicate the degree of tolerance which a plant possesses. The mechanisms behind plants tolerance against herbivores may involve several following compensatory responses, such as increasing their leaf photosynthetic rate (Houle & Simard, 1996), increasing the shoot regrowth rate
(Danckwerts, 1993), increasing number of tillers/branches and leaves (Rosenthal &
Welter, 19995) or higher reproductive efficiency and percent fruit set, relocation of
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biomass from roots to shoot, decrease leaf longevity etc. (Mabry & Wayne, 1997).
Considering the cost that plant needed to regrowth after herbivory, the allocation of resource to one strategy (tolerance) may suppress the resource to another strategy
(defence), which is evident by the existence of trade-off between tolerance and defence
(Fineblum & Rausher, 1995). Nevertheless, positive relation between tolerance and defence also proved in plants Arabidopsis thaliana (Mauricio et al., 1997) and Salix planifolia (Rosenthal & Welter, 19995). The mechanism of the trade-off between tolerance and resistance are still unknown but tolerance may influence the evolutionary path of plants’ resistant traits and the diverse composition of plant communities
(Rosenthal & Kotanen, 1994; Strauss &Agrawal 1999).
Since defensive and escaping traits are costly, plants’ avoidance strategies directing resources to secondary compounds or structural weapons may eventually reduce those resources for growth or reproduce. Therefore, several hypotheses predict that there is a negative association between plant growth and defence (Stamp, 2003).
Optimal Defence (OD) hypothesis predicts that there is a trade-off between a high- level of defence and a great impact on plant fitness from herbivores’ damage (McKey,
1974; Rhoades, 1979). The Growth-Differentiation balance (GDB) hypothesis (Lorio,
1986; Herms & Mattson; 1992) and The Carbon-Nutrient balance (CNB) hypothesis
(Bryant et al., 1983) have a similar prediction that plant will balance the allocation between growth and differentiation-related process (defence) and the resource directed to defence will result in resource diversion from other needs. The optimal defence theory will consider the value of plant tissues or organs and the probability of being attack, then allocate more resource to defend the most precious plant tissues or organs and the parts which has the highest chance to be consumed (McKey, 1974; Rhoades,
1979; Stamp, 2003). This hypothesis assumes that herbivores are the primary selective
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force in shaping plants’ defensive traits and when herbivores are absent, the less- defended plants have higher fitness than well-defended plants since they have more resource to be allocated in growth and reproduction (Stamp, 2003).Similarly, Feeny
(1976) argued the relationship between ‘apparent’ plant and degree of defensive traits by predicting that the ‘apparent’ plant occupying enormous landscape or persisting for a long time should invest more in defence, while the ‘unapparent’ plant live in specific spots transiently with low possibility to be found by herbivores should invest relatively less in defence. Coley et al., (1985) considered the impact of nutrient availability on plant defence, which predicts that the plants in nutrient-poor sites are expected to invest more resource in defence as they have no fast-recovering ability while plants in nutrients rich areas are expected to invest lesser nutrients in defence since they have stronger ability to tolerate attack from herbivores. Nevertheless, Edwards (1989) pointed out that the primary selective force may not come from herbivores but ‘neutral resistance’. Apart from defending herbivores, a defensive trait may also help resist harsh environment and the trait may be evolved before a plant experience the herbivory pressure. Therefore, a defensive trait may display alternative functions and the growth/reproductive abilities are not mutually exclusive with defence.
The resistant behaviours that plant deploy are a direct defence against herbivores, whereas plant can interact with animals in more diverse ways. Certain plants can protect themselves indirectly by producing a mixture of volatiles to attract herbivores’ predators, parasites or other natural enemies (Aljbory & Chen, 2018).
Plants can also build mutualistic allies with certain animal species and their animal partners may protect or help them in various ways. (Herrera & Pellmyr, 2009).
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2.2 Ant-Plant interactions
The consumption from herbivores and the deterrence from plants are just one way in animal-plant interaction. In many ecosystems, no other animals can interact with plants as diverse as ants
Ants are distributed in extraordinary geographic range of ecosystem and they make up more than 10% of biomass in various ecosystems. They can also alter the local habitat dramatically as leading predators in many habitats and help direct energy/nutrients cycles (Herrera & Pellmyr, 2009). Ants belong to Family Formicidae under the order Hymenoptera. There are more than 10 000 species under 296 genera has been described around the world (Bolton, 1994). Ants existed approximately 100 million years ago and the oldest ant fossil (Sphecomyrma freyi) was found in the amber of Upper Cretaceous age (Wilson et al., 1967). Ants are highly socialized animals that live in organized colonies. Ants in their colonies can be divided as sterile workers and soldiers and fertile drones and queens based on their reproductive ability. Each individual of ants can work independently and cooperate other ants to show group behaviours so that an increase in overall success in duties can be achieved. The great teamwork as well as the diverse ways interacting with plants have contributed to ants’ success in today’s ecosystems (Herrera & Pellmyr, 2009).
Certain ant species are antagonistic to plants. The leaf-cutter ants are the herbivores of many plant species in tropical forests and savannahs of South America.
The leaf-cutting ants (Attini) mainly cut grass or some dicots, some even collect flowers and fruits (Herrera & Pellmyr, 2009). Leaf-cutting ants cannot digest those leaves directly but rely on their mutualistic partner, a fungus (leucoagaricus gongylophorus) to produce enzymes to breakdown the plant tissues (cellulose, starch and xylan). They also detoxify insecticides and ant repellent produced by plants (North
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et al., 1997). In return, ants remove other fungus competitors such as yeast, bacteria or alien fungus spores by antibiotic compounds produced by ants and the constant licking behaviours (North et al., 1997, Herrera & Pellmyr, 2009). Ants will also reject harvesting plant materials with fungicide which can damage their mutualistic fungus
(Ridley et al., 1996). Currie et al., (1999) found that bacteria (Streptomyces sp.) on the cuticle of ants can produce specific antibiotics to supress the growth of parasite fungus (Escovopsis) and promote the development of mutualistic fungi as well. By the help of cultivated fungi, leaf-cutting ants are the leading herbivores in many habitats as garden fungi help ants digest nutrient from the plants and ants help maintain the fungi garden from infection and invasion from other organisms.
Nevertheless, the relationship between ant and plants are not always antagonistic. In contrast, myriads of ant species build mutualistic relationships with plants in various ways. Ants are dominant predators which forage a range of prey. The ancient Chinese take a good advantage of this feature of weaver ants. Colonies of weaver ants were placed on branches of orchard trees and prevented from leaving the tree. After weaver ants build colonies on the tree, a considerable number of invertebrate herbivores were removed and trees with ants’ protection produce more fruits than trees without protection (Beattie, 1985). Experiments demonstrated that predation of wood-ants on trees significantly suporessed the population of non-tended aphid species and the trees which were protected by ants had significant lower number of Lepidoptera larvae and significant lower rate of defoliation than trees without ants’ protection (Skinner & Whittaker, 1981).
As plant species enjoy the protection from ants, certain plant species offer rewards to their symbiotic ant species. Certain plants provide nest sites and domatia so that ants can live inside the plant and defend their colonies along with the plants.
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Nest sites can be cavities, cracks or excavated tunnels by other animals and ants can modify those natural cavities into other shape or size by usage of soil particles, debris or carton (Beattie, 1985). The domatia of plants is a specific adaptation for ant colonies and ants were encouraged to occupy the interval cavities of the domatia on the particular part of plants (Herrera & Pellmyr, 2009). Plant species bearing domatia are known as myrmecophytes. The domatia structure evolved by plants can always be found on stems, leaves or spines. A famous example is the domatia of some Acacia species in America with expanded and hollow thorns (Janzen, 1966). Ants
(Psedomyrmex) gnaw a small entrance out at the tip of the hollow thorns and moved in. Acacia plants also secrete nectar and Beltian bodies (food bodies containing proteins, lipids and carbohydrates) to ants and in return ants deter not only invertebrate herbivores but also mammalian herbivores, gnaw off invading veins or other plant competitors (Janzen, 1966; Janzen, 1967).
The mutualistic relationship between ants (Pheidole) and their host plant, piper
(Piperaceae), showed positive results in removing stem borers, fungal spores and suppressed fungal activities after ants built colonies from the hollow cavity of sheathing leaf base to the stem pith tissue of the entire plant (Letourneau, 1998). myrmecophytes thrived in many tropical habitats, which takes up 380 individuals (16 plant species) per hectare associating with 25 species of ants (Fonseca & Ganade,
1996). Certain ant species even provide extra nutrients to the plants that offer them domatia. Stem cavities of Hydnophytum formicarum receive animal wastes from cohabiting ants and broke down those wastes for nutrient (Rickson, 1979). Philidris ants can also ‘feed’ on their mutualistic epiphyte Dischidia major by placing organic debris into their modified leaves (pitchers) where tissue of leaf wall could use those nutrients (Peeters & Wiwatwitaya, 2014).
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Food bodies are a common reward from plants that provide lodging with ants.
Food bodies are a great variety of small epidermal structures containing various nutrient such as protein and lipids to attract ant foragers (Beattie, 1985). Food bodies in which predominant metabolite substance is lipid are known as Pearl bodies while the vascularized food bodies with protein and lipid are known as Beltian bodies
(Herrera & Pellmyr, 2009). Rickson (1969) observed the food bodies from the tip of each rachis and pinnule of Acacia cornigera as food source to their symbiotic ant
Pseudomyrmex ferruginea. Tropical plant species from genus Cecropia even produce food bodies containing a great amount of animal glycogen, which is extremely rare in plants and ants were found collecting those Mullerian bodies avidly in order to build a mutualistic relationship with the plant and serve as protectors (Rickson, 1971; Rickson,
1973). Plant Piper cenocladum will not produce any food bodies if their specialist ant
Pheidole bicornis are not present and the production of food bodies will be rebooted after their symbiotic ant species reinvade the plants (Risch & Rickson, 1981). Another famous example of plants defended by mutualistic ant partner is the Macaranga plants in Southeast Asian tropical forests. Heil et al., (1997) found that Macaranga plants invested up to 9% of above-ground biomass costs into their Beccarian bodies (type of
Pearl bodies) and ant-inhabited plants produced much more food bodies than ant-free plants. Although the investment on food bodies is expensive to the plants, Macaranga can still benefit from the effective protection from ants’ anti-herbivores strategies. The plant has a regulatory system to ensure that food bodies’ investments were kept in high rates when ant partners exited (Heil et al., 1997).
Extra-floral nectaries (EFNs) are another common reward provided by plants.
EFNs are secreted from leaves, twigs or flowers’ external surface so they are not involved in the plants’ pollination systems. They are not the rewards to pollinators
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such as bees or wasps but as an attractant to ants for seeking their protection as a number of studies demonstrated that the exclusion of ant partner on the plants bearing
EFNs resulted in an increased damage from herbivores and seed predators (Beattie,
1985). However, the effect of protection may not be consistence as Barton (1986) revealed that the different level of ant protection due to the densities of the ants and the type of herbivores among three separate population of Cassia fascicul. EFNs are a mixture of nutrients of sugars, amino acids and lipids that not only attract ants but also various other predatory animals such as wasps and spiders. Studies have shown that they can also benefit the plant offering EFNs by attacking herbivores (Beattie,
1985; Herrera & Pellmyr, 2009).
Plants that benefit from ants’ protection may not only from direct protection, but also from indirect protection. The indirect ant-guard system is always complicated by involving herbivores in a three-way relationship (ants, plants and herbivores). The herbivores are Homoptera (Beattie, 1985) or lepidopteran larvae (Herrera & Pellmyr,
2009). Homoptera are sap-sucking herbivores such as aphids, leafhoppers, scale insects, mealybugs and coccids. Homoptera possess slender mouthparts (proboscis) penetrating vascular tissue and sucking sap from phloem directly. After passing through the gut of homoptera, sap will come out from the anus and become honeydew droplets for ants (Beattie, 1985). This honeydew contains a variety of nutrients such as different kinds of sugar, amino acids, alcohols, plant hormones, salts, amides and vitamins (Brian, 1977). Ants harvest honeydew from homopterans and in return they will tend and protect the insects from predators and parasites. Even though lepidopteran larvae are not plant-suckers, they will also produce honeydew for ants after chewing plant tissues (Herrera & Pellmyr, 2009). In addition to protection, ants also keep colonies of homoptera in good hygiene, which is crucial for their survival
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(Majer, 1982). The homoptera and lepidopteran larvae are considered pests of plant species since they consume plant materials and cause certain damage to their host plants. Furthermore, tending and guarding service by ants may significantly maximized the population of those pests as well as the damage they may cause.
Nevertheless, the net effects of this complex three-way interactions are not always negative. For example, although plant species Solidago altissima bearing treehoppers (Publilia concave) tended by Formica ants which also attack two primary herbivores beetles (Trirhabda virgata and T. borealis), and plants with ants possessed higher seed yields and grower taller than plants without ants (Messina, 1981). Buckley
(1983) found overall negative effects in the ants (Iridomyrmex sp.), host plants (Acacia decurrens) and sap-feeding treehoppers (Sextius virescens) three-way systems. An ants-expel study showed that although plants (Pluchea indica) without ants (Pheidole megacephala) bear significantly less scale insects (Coccus viridis) population than those with ants, host plants suffered more from infestation of sooty mold and followed by higher rates of leaf death (Bach, 1991). This study demonstrated that the protection of homopterans from ants will also remove plants’ herbivores at the same time, which benefits more than the costs from homopteran pests. Janzen (1979) argued that the cost due to the sap-taking homopteran pests is the same way as plants offer external-flora nectaries as a reward to ants and the presence of homopterans will also attract ants to the host plants. Therefore, the net-effect of the three-way system can be a triple-win system. Furthermore, studies showed that ants can regulate the population of their insect mutualists by switching from tending them to eating them in response to the densities of homopterans and quality of honeydew (Beattie, 1985; Cushman 1991).
Hence, the damage from homopterans can be limited and eventually benefit the host plants.
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Several studies reported that ants can serve as pollinators. For example, Wyatt
(1981) found plant Diamorpha smallii was pollinated by ants from genus Formica.
But compared to their hymenopteran relatives such as bees and wasps, ants are considered as a rare group of pollinators. Furthermore, studies showed that certain flowers even inhibit ants from visiting their flowers. Flowers of Polemonium viscosum display different types of floral form by changing the long and broad flared flowers to short and narrow flowers so that ants can be excluded. Meanwhile, Polemonium viscosum which restricts ant visitation reproduce more seeds than those with free access to ant visit (Galen, 1999).
By comparing with bees or wasps, entomologists proposed that several features which ants do not have may cause the rare cases of ant pollination. However, Beattie
(1985) argued that ants are capable of pollinating flowers. The integument of many ants is hairy or even hairier than bees. The non-hairy ants’ integument is rugged with many furrows and grooves, which allows pollen to adhere to their integument and transferred to another flower (Wyatt, 1981; Beattie, 1985). Ants groom themselves as frequent as bees but they are still effective pollinators since flowers’ stigma receive pollen is less accessible to be groomed on that particular part of the integument.
(Beattie, 1971). Furthermore, although ants cannot fly, their foraging ability can be far and systematic by their sophisticated orientation and sensory systems (Beattie, 1985).
Other studies suggested that the antimicrobial compounds produced by ants’ metapleural glands can kill or disable pollen grains that resemble microorganisms and the secretion of antimicrobial may be the obstruction in the coevolution between flowering plants and ants (Beattie, 1985; Beattie, 1991). Nevertheless, researches demonstrated that certain plants deploy strategies to avoid the antimicrobial problems
(Peakall, 1989; Peakall & Beattie, 1989). So far, no hypothesis are sufficient to explain
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the comlexity of ant pollination and further studies are required (Beattie, 1985; Herrera
& Pellmyr, 2009).
Ants can also help plants to disperse their seeds and this particular ant-plant interaction are known as myrmecochory. Majority of the myrmecochorous plants’ seeds bear elaiosome which is a lipid-rich attachment that attract many ant species and is a specific adaptation to promote seed dispersal since ants acquire nutrients by removing elaiosome and seeds are dispersed subsequently (Beattie, 1985; Herrera &
Pellmyr, 2009). Benefits for ants are lipid-rich food with additional amino acids, sugars and protein once ants remove and eat the elaiosome. On the other hand, plants seeds are dispersed and normally placed on nests of ants. A successful seed dispersal rely on the ant communities’ behaviour and features. For example, the seeds removed from plants in morning may avoid rodents’ predation at night (Herrera & Pellmyr,
2009) or the rapid seed-bury behaviour of ant colonies can hide seeds from predators’ notice (Bond & Breytenbach, 1985). Seeds survivorship can be improved if ant nests are nutrient-enriched and rare in competition with sibling plants (Hughes, 1991).
Protection from natural hazards such as fire from burial at certain depth by ants are also beneficial to seeds (Hughes et al., 1994).
Studies showed that secondary dispersal of fruits and seeds from animal faeces by ants can also benefit seeds by decreasing fungal infestation, avoiding seeds predators, avoiding fire and reduce intra-/interspecific competition (Herrera & Pellmyr,
2009). Certain ant species harvest seeds as food and this behaviour will eventually change the distribution of local plants (Beattie, 1985, Herrera & Pellmyr, 2009).
Studies found that seeds remaining from ants colonies were viable and seedling germinated from those seeds had higher survival rates and faster growth rate (Levey
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& Byrne, 1993). Therefore, harvesting ants can be both predators and dispersers and seeds may still benefit from complex relationships.
Ant-plant mutualism through direct interaction may not be ubiquitous form, instead, more ants interact with plants indirectly by modifying the physical, chemical and micro-organisms on the soil around plants. Ants can affect aeration and drainage like earthworms. The nests of ants can collect and gather a higher concentration of organic compounds and microbial population surrounding the plant can be enriched, which may raise the success rate of germination, seedling recruitment and growth rate
(Herrera & Pellmyr, 2009).
2.3 Rattan
Rattan (Arecaceae) is a very important palm species belong to subfamily
Calamoideae. There are approximately 600 species under 13 different genera of rattan in this world. (Dransfield, 2002). True rattan only can only be found in old world forests and they are mainly distributed in tropical and subtropical areas of Africa and
Asia (Sunderland & Dransfield, 2007). In Peninsular Malaysia, there are approximately 105 species of 9 genera of rattan species (Dransfield, 2002) and half of them are endemics (Dransfield, 1979). Rattan plants predominate in tropical rainforests and monsoon forests while certain species also are adapted in secondary forests (Dransfield, 2002). In Peninsular Malaysia, rattan may be found in a wide range of habitats from sea level up to high altitude mountains (Dransfield, 1979). In many countries, rattan play an important role in local economy and commercial important species take up 10% of total species (Dransfield et al., 2002).
The flexibility and strength of the cane id the deciding factor for the commercial value of rattan species. A good cane is stripped from a rattan stem which
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is solid, strong and very flexible. A very diverse usage of rattan canes in binding, matting, furniture making and even weapons (Wan Razali et al., 1992). Apart from rattan stems, other parts from certain rattan species were used in many traditional ways.
For example, palm heart can be used as foods or traditional medicines; leaves can be used as cigarette paper, cover of houses (thatching) and vermifuge after chewing; leaf sheath may be made into toothbrush or graters (Sunderland & Dransfield, 2007). Not all rattan species are used by human, a roughly 20% of rattan have commercial value
(Dransfield & Manokaran, 1993). Therefore, the economic value of rattan species resulted in over exploitation of forests rattan resources. Degradation of natural forests and many other artificial factors threatened the growth of rattan species (Dransfield et al., 2002). More than a hundred of species of rattan were reported as being threaten
(Walter & Gillett, 1998) and more than 20 species of rattan were listed as endangered species (Johnson, 1996). It also triggered the formulation of guidelines of rattan cultivation (Wan Razali et al., 1992).
Many rattan species are capable to climb to the canopy level of the forest by their specialized climbing organs, cirrus and flagellum. Cirrus is extended from the leaf rachis of leaflets while flagellum is extended from the top of the leaf sheath. Both of them are whip-like and armed with recurved spines for grabbing and holding other trees. Cirri and flagella are normally mutually exclusive and flagella can only be found in genus Calamus. (Dransfield, 1979). There are also many rattan species are non- climber and possess no climbing organs. Stems of most rattan species were covered by numerous spines whitch grow on the leaf sheaths. The spiny leaf sheaths protect the stem inside and younger parts of the stem are always well protected, for example, the true apex of meristem is armed by elongated spiny sheaths of the upmost leaves
(Dransfield, 1979). Rattan species can grow in clusters or solitary and certain species
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(i.e. Calamus subinermis) can be both (Sunderland & Dransfield, 2007). Rattans, similar to other palm species, are monocotyledons with only one apical meristem.
Stems are very precious since they cannot deploy secondary thickening for vascular bundles or external bark once they are produced (Tomlinson, 1990). Although without secondary thickening, the diameter of rattan stem among different species varies from less than 1 cm up to 20 cm and the basal part of the stem is wider than stems above it
(Dransfield, 1979). Rattan leaf can be divided into the sheath, the petiole and the laminar (some species may have cirrus). Leaf sheath is the key in determine the species’ taxonomic features (Dransfield, 1979). Not all rattan possesses a spiny leaf sheath and not the entire sheath is covered by spines. Spines can be found in exposed areas. The structure and arrangements of spines vary among different rattan species. The texture of spines can be soft, papery to tough but some are brittle (Dransfield, 1979). Certain species have an erect ligule-like structure at the mouth part of the leaf sheath, which is called ocrea. Ocrea in some species of Korthalsia is inflated and divergent, which represented their value in taxonomy (Dransfield, 1979).
Rattans leaflet are normally pinnate in form. The margin of leaflet can be entire margined or erose-margined, which is the main distinction of leaflet shape. The erose- magined leaflet can be described by the term Praemorse, which represented the upper magins of leaflets are erose in shape, as if parts of leaflets are bitten off. This type of leaflet is always associated with the broad rhomboid leaflets from genus Korthalsia
(Dransfield, 1979). The inflorescence of rattan can be hapaxanthic (flowering simultaneously and stems die after flowering or fruiting) or pleonanthic (flowering continuously can stem can still grow after flowering). Pollination of rattan flowers are less studied, moths (microlepidoptera), nitidulid bettles, stingless bees (Trigona) and paper wasps (Vespidae) were observed visiting flowers of rattan and may help transfer
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pollen (Dransfield, 1979; Lee et al., 1995; Kidyoo & McKey, 2012). Rattan fruits are covered by overlapping scales. Birds such as hornbills, primates like pig-tailed macaque (Macaca nemestrina) were observed to contribute to rattan seeds dispersal
(Sunderland & Dransfield, 2007; Ruppert et al., 2014). Herbivores of rattan are varied in size. Numerous insects were reported attacking rattan’s stems and leaves (Xu et al.,
2000; Dransfield, 2002). Small climbing mammals may attack the apex of meristems of rattan stems despite protection by spines and stems were frequently destroyed by megaherbivores like wild cattle, wild boar, monkeys and elephants for the crisp, sweet tissue of stem under leaf sheath (Dransfield, 1979).
Many rattan species build complex relationships with several ant species.
Dransfield (1979) concluded 6 different types of association between rattan and ants.
1) The casual relationships, which describes as ants randomly build their carton domes on the rattan plant; 2) Simple associations with leaflets, which shows ants build their nest by enclosing space between the lowermost leaflets and the leaf sheath; 3)
Adaptation involving inflorescence, ants move in bracts of inflorescences of rattan plants; 4) Adaptation in leaf sheath auricles, which describes rattan Daemonorops ursina’s auricles are used by ant species with their narrow and spiny leaf sheath; 5
Ocrea, which is found in several species of Korthalsia and Calamus. Their swollen and hollow ocrea are constantly used by ants and some rattans build obligate relationships with ants; 6) Spine whorls in several species of Daemonrops and
Calamus possess interlocked spaces by their whorls-arranged spines in different level along the stem and ants construct colonies in those spaces and build tunnels to connect each other. Homoptera such as aphids or scale insects were also found to be involved in ant-rattan associations and more complex relationships may be built among the three different species.
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Ant-rattan interactions were found in several different habitats around world.
In tropical forests of Africa, Sunderland (2004) reported 7 rattan species from three genera (Eremospatha, Laccosperma and Oncocalamus) were associated with 21 species of ants and certain level of ant-rattan specificity may exist among them. In
Peninsular Malaysia, 9 cases of ant-rattan relationships were known by 4 species of
Korthalsia, 4 species of Daemonorops and 1 species of Calamus with 16 species of ants and they were just a fraction of the obvious myrmecophytic associations between ant and rattan (Moog et al., 2003). The inflated sac-like ocrea in Calamus longipinna were reported to function as dormatia for ant species (Merklinger et al., 2014). In
Singapore, two species of Korthalsia were associated with ants from three genera
(Iridomyrmex, Dolichoderus and Philidris) and aphids were also involved in their association (Chan et al., 2012). The inflated sac-like ocrea in Calamus longipinna were also reported serving as dormatia for ant species (Merklinger et al., 2014). Ants which built colonies in overlapping spiny structures of two Daemonorpos rattan species were found enhancing the nutrient absorption of plants (Rickson & Rickson, 1986).
As rattan provides nest sites to ant colonies, there are more detailed studies concerning ants’ service to rattan Korthalsia in Borneo. In Sabah of Borneo, ants
(Camponotus sp.) built obligate relationships with rattan Korthalsia robusta (Mattes et al., 1998). Mattes et al., (1998) also found two species of aphids tended by ants and ants guarded the apex of the plants from day to night. Intruding behaviours would trigger aggressive defensive behaviours of ant colonies. Rattan Korthalsia furtadoana were reported well defended by obligate ant species and showed higher growth rates than Korthalsia furtadoana colonised by ‘parasite’ ants which rarely defend their host plants (Edwards, 2010). Miler et al, (2016) also reported that rattan Korthalsia
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furtadoana’s leaflets have less damage and less covered by epiphylls if they were guarded by patrolling ants.
In conclusion, rattan is a very important plants in tropical rainforests and many species of them possess commercial values and certain species are endangered by over exploitation. There is a lack of knowledge for rattan even for certain species with high economic value. Previous studies mainly focus on the cultivation of few valuable rattan species. Rattans with little economic values are even more disregarded in terms of research. Although rattans have very conspicuous spiny structures, their structural characteristics has yet been recorded and their defensive mechanism remain unknown.
Most studies about ant-rattan associations only reported the existence of ant colonies on rattan plants. Except for rattan Korthalsia, few studies investigated ant interaction with other rattan species.
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CHAPTER 3 GENERAL METHODOLOGY
Surveys on rattan-ant relationship were conducted in northern Peninsular
Malaysia (Figure 3.1), Bukit Genting Hill (5°18’31.9”N, 100°13’14.0”E), Penang
National Park (5°28’01.4”N, 100°11’56.0”E) and Taman Rimba, Teluk Bahang
(5°26'52.51"N, 100°13'4.89"E) in Penang island, and in Cherok Tokun, Penang mainland (05°21’54.6”N, 100°28’58.7”E; Figure 1). Rattan-ant relationships were recorded during August 2017 to July 2018. Measurements of Rattan spiny structure were conducted during February 2018 to August 2018.
Rattan species were identified in the forests and parts of plant material were collected by garden shears while ants on rattan were collect by forceps and preserved in 70% alcohol solutions.
Comparison of differences of spiny structures and leaf angles in statistics were conducted by one-way ANOVA and the Tukey post hoc test were used to locate the group that differ from others. Data was log transformed to achieve normality. A more detailed methodology is outlined in each working chapter.
Figure 3.1 Study site locations at (a) Bukit Genting Hill, (b) Penang National
Park, (c) Taman rimba, Teluk Bahang and (d) Cherok Tokun, Penang
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Specificlly, three Daemonorops lewisiana and three Daemonorops geniculata can be found along the track (Figure 3.2) of Bukit Genting Hill, ants in one D. geniculata and two D. lewisiana were sampled and identified. In Penang National Park， thirty D. lewisiana and ten Plectomia griffithii were found along the track (Figure 3.3). Thirteen individuals of D. lewisiana were found had at least one ant colonies and they were collected for identification. In the forsts of Taman Rimba, Teluk Bahang, six Calamus castaneus and nine P. griffithii were found alone the track (Figire 3.4) and ants from three C. castaneus were sampled. Nine C. castaneus, five Korthalsia scortechinii and five D. geniculata were found along the track in the forests of Cherok Tokun. Ants in six individuals of C. castaneus, three individual of K. scortechinii and three individuals of D. geniculate were collected and identified in laboratory.
Figure 3.2 Sampling track in Bukit Genting Hill.
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Figure 3.3 Sampling track in Penang National Park.
Figure 3.4 Sampling track in Taman Rimba, Teluk Bahang.
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Figure 3.5 Sampling track in Cherok Tokun, Penang mainland.
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CHAPTER 4 WHO IS BETTER DEFENDED?
SPINESCENCE MEASUREMENT ON five RATTAN SPECIES
Introduction
Plants, the most important producer in our ecosystem, are inevitably consumed by herbivores. Therefore, plants have evolved various traits to deter various herbivores.
Toxic chemicals or repellents produced by some plants are considered as chemical defence. Whereas, mechanical defence are physical structures that plants possess to deter herbivores or decrease the efficiency of herbivory activities. Among all those protuberant structures, spines are one of the most prominent physical defensive traits in many plant species and they may act as the first barrier in deterring an array of herbivores (Supnick, 1983; Cooper & Owen-Smith, 1986; Grubb 1992). However, little attention has been paid to structural defence in contrast to the large interest on studying plants’ chemical defence traits (Hanley et al., 2007).
Spinescence describes the characteristics of plant protruded structures, spines thorns and prickles. Spines are modified leaves with sharp-pointed stipule or petiole, while thorns come from modified branches or twigs and prickles are originated from cortical or epidermal tissue (Cornelissen et al., 2003; Grubb, 1992). Spinescence structures are deemed as a defensive weapon to deter herbivores since regional studies showed that well-defended plants correspond with the areas which are under heavy- browsing pressure (Milton, 1991; Takada & Miyashita, 2001; Burns, 2013; Ford et al.,
2014; Goldel et al., 2016).
Although majority of the browsing herbivores have become extinct in tropical
America, many plants still have spines and they are spinier or thornier under the
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existence of introduced livestock exist (Goldel et al., 2016). As large carnivores can suppress the number of herbivores, plants in areas with large carnivores are less defended by spines compared to areas without large carnivores (Ford et al., 2014). Not only mammalian herbivores, Australia and New Zealand plants possess similar spiny structures to deter large, avian herbivores but spiny structures disappeared in many offshore islands as no history of avian herbivores exist at those areas (Burns, 2016).
The defensive abilities of spinescence also match with the ontogeny of plant as some species deploy more spiny structures in their juvenile stage when heavy browsing happened but less defended by such structures when they reach mature stage (Brooks
& Owen-Smith, 1994; Barton & Koricheva,2010; Fadzly et al., 2009; Burns, 2016).
Researches on interaction between different kinds of herbivores and spiny plants showed that spines can decrease the biomass loss caused by herbivores (Belovsky et al., 19091) and reducing the browsing efficiency of small climbing mammals (Cooper
& Ginnett, 1998). Few studies demonstrated the relationships between spinescence and invertebrate herbivores. Therefore, spiny structures are more likely to offer deterrence against vertebrate herbivores rather than against invertebrate herbivores.
However, studies also showed that the protective ability by spines is limited (Potter &
Kimmerer, 1988).
Spinescence in Southeast Asian tropical rainforests are less studied. Rattan plants (subfamily Calamoideae) are common spiny palm species in Malaysia. There are around 600 species worldwide and 110 species can be found in Peninsular
Malaysia (Dransfield, 1979; Dransfield, 2002). Rattan stems are used widely in local commercial market as furniture and matting materials (Dransfield, 1979; Dransfield,
1992). Some of the most distinguished features in rattan are the massive range of variety and arrangement of spiny structures on stems of various rattan species
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(Dransfield, 1979). Since rattan may encounter different kinds of invertebrate herbivores (Xu et al., 2000) and vertebrate herbivores (Dransfield, 1979), numerous spines on rattan are considered as a defensive weapon to deter herbivores (Dransfield,
1979). However, characteristics of spinescence have yet been measured and studied before.
In this study, I referred the standardised measurement used by Cornelissen et al., (2003) and Peìrez-Harguindeguy et al., (2013) on plant functional traits on five different rattan species (Daemonorops lewisiana, Daemonorops geniculata, Calamus castaneus, Plectomia griffithii and Korthalsia scortechinii). I proposed a new method in measuring the strength of the spine since methods employed by Cornelissen et al.,
(2003) and Peìrez-Harguindeguy et al., (2013) are hardly quantitative. I compared the different spiny structures among the five species to learn the defensive power of each rattan species as well as the potential function of their different spiny arrangements.
Method and Materials
Five different species of rattan (Daemonorops lewisiana, Daemonorops geniculata, Calamus castaneus, Plectomia griffithii and Korthalsia scortechinii) were selected in this survey. For every species, five individuals of rattan were randomly sampled for their spines’ information. Ten spines were randomly selected on each individual of rattan and measurements on length and width were using a vernier scale
(Figure 4.1). Length of spine was measured from the base to the tip of each spine.
Width of spines was measured at base part of each spine. Length and width were measured directly on the stem.
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Figure 4.1 The vernier scale.
Inclination of spines (10 from each plant) was measured by a protractor (Figure
4.2). Density of spines was measured by estimating the number of spines on the densest part of each individual of plant, which is normally at base part of the plant. Recurved spines on cirrus or flagellum were ignored as they are climbing organs.
Figure 4.2 The protractor
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10 spines ranging from 1.5 to 4.0 cm were randomly collected for tensile test.
Both sides of the spine were fixed by clips. By pushing a digital weight scale in the middle part of each spine, the value will be recorded at the moment when the spines were broken off (Figure 4.3). Reading was recorded in the unit gram from the digital scale and it was transferred to Newton (1 gram = 0.0098 Newton on earth).
Figure 4.3 The digital weight scale for tensile tests
One stem of each species was taken out and image was taken from above to bottom to investigate their cut image. The cut image may provide a different viewing angle of herbivores’ perspective
Colour of spines were recorded by taking pictures and one spine from each species was taken out for spectral measurement. The spectral measurement was using
Ocean Optics Jazz spectrophotometer (Figure 4.4) and analysed by SpectraSuite software.
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Figure 4.4 The Ocean Optics Jazz spectrophotometer
Leaf hairs can only be found on D. lewisiana, C. castaneus and D. lewisiana.
Length, angle and number of hairs on both sides and margin of leaflets from the three species were measured (Figure 4.5).
Figure 4.5 Leaf hairs of D. lewisiana.
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Results
Length/width (mm)
The length of spines of the five different rattan species (Table 4.1) were significantly different [F (4,245) =52.287, P=0.000]. A Tukey post hoc test revealed that spine length of K. scortechinii (3.17±1.05) were significantly shorter than D. lewisiana (21.19±8.87); the spine length of D. lewisiana (21.19±8.87); P. grifffithii
(27.75±9.16) and C. castaneus (28.50±14.39) has no significant differences. But the spine length of D. geniculata (43.89±25.58) is significantly longer than C. castaneus
(28.50±14.39), which means D. geniculata has the longest spines among all of the five species.
Table 4.1. Spine length mean value of rattan species. Alphabets in superscript
denotes significant differences in Tukey Post Hoc.
Species D.geniculata D.lewisiana C.castaneus P. griffithii K. scortechinii
Length(mm) 43.89±25.58a 21.19±8.87b 28.50±14.39b 27.75±9.16b 3.17±1.05c
The width of spines of the five species of rattan (Table 4.2) were significantly different [F (4,245) =82.240, P=0.000]. A Tukey post hoc test revealed that spine width of K. scortechinii (1.32±0.30) and P. grifffithii (1.64±0.51) have no significant difference and they are the thinnest spines among the five species. Spine width of D. lewisiana (2.55±0.62) are significantly wider than P. grifffithii (1.64±0.51); spine width of D. geniculata (3.12±0.82) are significantly wider than D.lewisiana
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(2.55±0.62); spine width of C. castaneus (4.14±1.59) are significantly wider than D. geniculata (3.12±0.82) and it has the widest spines among the five rattan species.
Table 4.2. Spine width mean value of rattan species. Alphabets in superscript
denotes significant differences in Tukey Post Hoc.
Species C.castaneus D.geniculata D.lewisiana P. griffithii K. scortechinii
Width(mm) 4.14±1.59a 3.12±0.82b 2.55±0.62c 1.64±0.51d 1.32±0.30d
Colour of spines was recorded by photos. Spines in D. lewisiana are normally black colour with yellow base (Figure 4.6). Spines in D. geniculata are usually yellow to brown (Figure 4.7). Spines in C. castaneus are normally bright yellow but some spines were turned to brown and black (Figure 4.8). Spines in P. griffithii are green to yellow colour, some are brown colour (Figure 4.9). Spines in K. scortechinii are close to stem skins, which are green to yellow with a black tip (Figure 4.10). The spectral reflectance results from 400 nm to 700 nm wavelengths is Figure 4.11.
Figure 4.6 Spines of D. lewisiana
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Figure 4.7 Spines of D. geniculate
Figure 4.8 Spines of C. castaneus
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Figure 4.9 Spines of P. griffithii
Figure 4.10 Spines of K. scortechinii
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Figure 4.11 The spectral reflectance results from 400 nm to 700 nm
wavelengths.
The mean angle of spines of five different rattan species (Table 4.3) were divided into two groups. One group has an acute angle (P. griffithii and K. scortechinii), while another group has an obtuse angle (D. lewisiana, D. geniculata and C. castaneus). Statistics showed a significant difference among the five groups [F (4, 70)
=25.721, P=0.000]. The Tukey post hoc test revealed that the angle of P. griffithii
(68.33 ±17.29°) are significantly smaller than the angle of D. geniculata
(107.00±15.79°). In other words, the acute angle group are significantly smaller than obtuse group. The spines with an acute angle were pointing downwards to the ground but the spines with an obtuse angle were pointing upwards to the canopy.
Table 4.3. Spine angle mean value of rattan species. Alphabets in superscript
denotes significant differences in Tukey Post Hoc.
Species D.lewisiana C.castaneus D.geniculata P. griffithii K. scortechinii
Angle (°) 125.33±25.88a 107.00±15.79a 105.33±22.48a 68.33±17.29b 67.33±14.38b
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By estimating the numbers in every 20 cm of the stems, K. scortechinii has around 30-90 spines; D. lewisiana has 150-200 spines; D. geniculata has 200- 250 spines, P. griffithii has 200-300 spines and C. castaneus has 300 - 400 spines. The species with the lowest spine density was K. scortechinii while the hightest spine density species was C. castaneus.
The strength to break down a spine ( Table 4.4) was significantly different among the four groups [F (3, 36) =9.374, P=0.000]. The Tukey post hoc test revealed that the strength to break down the spines of D. lewisiana (4.55±1.53N) was significantly higher than the strength to break down the spines of D. geniculata
(2.95±0.46N). No significant difference was found among D. geniculata, C. castaneus and P. griffithii).
Table 4.4. Spine angle mean value of rattan species. Alphabets in superscript
denotes significant differences in Tukey Post Hoc
Species D. lewisiana D. geniculata C. castaneus P. griffithii
Strength (N) 4.55±1.53a 2.95±0.46b 2.66±1.72b 1.80±0.38b
Cut image of D. lewisiana, D. geniculata, C. castaneus, P. griffithii and K. scortechinii is shown in Figure 4.12 to Figure 4.16.
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Figure 4.12 Cut image of D. lewisiana.
Figure 4.13 Cut image of D. geniculata
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Figure 4. 14 Cut image of C. castaneus
Figure 4.15 Cut image of P. griffithii
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Figure 4.16 Cut image of K. scortechinii
Specifically, for rattan D. geniculata (Figure 13), spine length on the right side
(72.03±136mm) of the is significantly different from the spine length on the left side
(2432±588mm), t (12) =10.22, p=0.000. Angle of spines from right side (123.0±17.5°) is significantly larger than angle of spines from left side (87.40±6.02°), t (4) =4.29, p=0.013.
Leaflet Hairs:
Only three species (D. lewisiana, C. castaneus and D. geneculata) possessed small hairs on the surface, back and both margins of every leaflet. The arrangement of hairs on leaflets are different. D. lewisiana’s leaflets have three rows of hairs on the surface but only one on the back. In C. castaneus and D. geniculate, however, the leaflet hairs were arranged in a reverse way, three rows on the back and one row on
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the surface of every leaflet. Margins of both sides are full of hairs in all three species’ leaflets.
There were 34.8±3.96 hairs on the surface of D. lewisiana’s leaflets. The average interval between each hair was 16.46±6.42 mm. They were 4.01±1.11 mm in length. Average inclination of hairs was 45.00 ±9.13°, pointing to the tip of every leaflet. 78±14.27 hairs on the back side of leaflet, with an average interval of
3.68±2.26 mm. They were1.90±0.73 in length. Hairs were pointing to the tip of leaflet with 48.00±8.88°. On the margins of leaflets, there were 81.80± 11.92 hairs on one side of the leaflet (interval of each hair: 4.63±1.95mm) with the length of 1.61±0.35 mm and 2.5±6.43° of inclination towards the tip of the leaflet (Table 4.5)
On C. castaneus, 15.4±4.39 hairs were found on the surface, with an average interval of 8.24±3.05 mm between each hair. Average length of hairs was
2.55±0.72mm and average inclination of hairs was28±5.37°, pointing to the tip of every leaflet. On the back of every leaflet, 61.2±13.44 hairs with an average interval of 14.37±6.92 mm between each hair were found. 3.91±0.85 mm in length and 56±8.43° in angle were pointing to the tip of every leaflet. On the margin of leaflet, most of hairs were very short (less than 1 mm) and clung to the margin (angle smaller than 10°). The average interval of every leaflet was 4.81±2.03mm (Table 4.6)
On D. geniculata, 6.4±2.07 hairs on the surface with an average length of 2.19
±0.59mm were found. Each hair had an average interval of 25.80±26.17 mm. The average angles of hairs pointing to the tip of leaflets was 22.5±4.56°. On the back of the leaflet, 51.4±19.42 hairs have an average length of 2.60±0.75 mm and an average length 8.5±4.46 mm between every leaflet. Hairs were pointing to the same direction
(47.5±10.61°). On the margin of every leaflet, 35.0±6.86 hairs had an average interval
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of 8.2±3.69 mm. But the length of most hairs was short (less than 1 mm) and clung to the margin tightly (angle smaller than 10°) (Table 4.7)
Table 4.5 Leaf hairs measurement of D. lewisiana
D. lewisiana Length (mm) Angle (°) Numbers Interval (mm)
Surface 4.01±1.11 45.0±9.13 34.8±3.96 16.5±6.24
Back 1.90±0.73 48.0±8.88 78.0±14.27 3.68±2.26
One side of margin 1.61±0.35 25.5±6.43 81.8±11.92 4.63±1.95
Table 4.6 Leaf hairs measurement of C. castaneus
C. castaneus Length (mm) Angle (°) Numbers Interval (mm)
Surface 2.55±0.72 28.0±5.37 15.4±4.39 8.24±3.05
Back 3.91±0.85 56.0±8.43 61.2±13.44 14.37±6.92
One side of margin < 1.00 < 10.0 117.2±21.68 4.81±2.03
Table 4.7 Leaf hairs measurement of D. geniculata
D. geniculata Length (mm) Angle (°) Numbers Interval (mm)
Surface 2.19±0.59 22.5±4.56 6.40±2.07 25.8±26.17
Back 2.60±0.75 47.5±10.61 51.4±19.42 8.50±4.46
One side of margin < 1.00 < 10.0 35.0±6.86 8.20±3.69
Discussion:
From the measurements of different aspects of stem spines from 5 rattan species as well as the features of leaf hairs from 3 rattan species, spines are varies among the five species in different aspects, but leaf hairs on the three species (D.
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lewisiana, C. castaneus and D.geneculata) are very similar to each other with a slightly different arrangement on both sides of leaf surface. Measurements on spinescence are used to investigate the effectiveness of structural defenses against herbivores damages.
Peìrez-Harguindeguy et al. (2013) categorized 5 orders to indicate the defensive power of spinescence and the least defensive power is the plant with no spines while the most defensive power is the plant with the longest, the highest density and the hardest spines that could harm herbivores or human.
From my measurement of stem spines of the five rattan species, I could hardly categorize them from the least spinescent to the most spinescent. D. geniculata has the longest spines and K. scortechinii has the shortest spines, which is the only species that all spines are less than 10 mm. The length of spines from D. lewisiana, C. castaneus and P. griffithii have no significant difference. Theoretically, the longer spine should be more powerful in defending against herbivores compared to shorter spines. Hence, D. geneiculata’s spines may be the most effective in deterring herbivores while K. scortechinii’s spines may be too short to defend themselves. C. castaneus has the widest spines while P. griffithii and K. scortechinii have the thinnest spines. With similar length, wider spines may be more stable than thinner spines. The length of C. castaneus spines is similar to D. lewisiana and P. griffithii and shorter than D. geniculata. Therefore, it may be harder to remove those wider spines in C. castaneus compared to spines on other rattan species. However, since the very short spines on K. scortechinii, it is extremely hard to remove their spines from the stems.
The densest stem spines are found on C. castaneus, with more than 300 spines per 20 cm on the base part of their stem, but the sparsest stem spines are on K. scortechinii, not more than 100 spines per 20 cm on their stems. The other three rattan species have similar density of spinescence. Studies have shown that longer spines and higher
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density can help plants reduce the efficiency of herbivory (Gowda, 1996; Cooper &
Ginnett, 1998; Midgley & Balfour, 2001).
However, the defence of spines may rely on more specific aspects. The orientation of the spines is divided into two groups. K. scortechinii and P. griffithii have acute spine angles, which means that the majority of their spines were pointing downwards to the ground. The spines of the other three rattan species, D. geniculata,
C castaneus and D. lewisiana, have obtuse spine angle. Many of their spines were pointing upwards to the sky. The down-pointing spine angles may be more effective in deterring small climbing mammals which consume the apical meristem from the bottom to top. The strength of the spines is crucial in playing defensive roles as well.
The stronger the spines, the tougher they are and harder to be broken down by herbivores. D. lewisiana has the strongest spine strength while spines in C. castaneus,
P. griffithii and D. geniculate have weaker spines with little difference among each other. Hence, D. lewisiana’s solid spines may wound herbivores’ mouthpart or may be easier to penetrate into herbivores’ thick skins than other species spines. The strength of the spines is always neglected by previous studies as it is not easy to quantify the strength of spines although it is an important feature of spines that help plants in defending herbivores.
The colour of spines can also contribute to deterring herbivores, especially for herbivores that forage by vision. Plants’ spines around the world display different colourations, African Acacia species’spines are white colour (Midgley, 2004). In
Pseudopanax crassfolius, conspicuous colour flanking spines only exist in juvenile leaves but disappeared in the adult stage, which matches with the period that the plant could encounter herbivores (Fadzly, 2009). Ronel& Lev-Yadun (2012) found 167 species’ spine, thorn and pickles colour are yellow, red, orange and white in Israel.
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From my investigation, many spines of D. geniculate, D. lewisiana, and C. castaneus are always covered by black colour. The tip of spines on K. scortechinii are also in black colour. Spines on P. griffithii are green to yellow colour. The conspicuous colouration is described as aposematic (warning) colouration by Lev-Yadun, (2001) and it serves as similar effect as toxic animals’ warning decoration which tells the predators that the plants are dangerous to be eaten. Therefore, aposematic colouration in spiny structures may display the dangerous signals to herbivores that they are not palatable. (Lev-Yadun, 2009). The black colour on those spines could also be mimicry of ants. Black spots were also found on stems and branches of Xanthium trumarium
(Asteraceae) by Lev-Yadun & Inbar, (2002), flowers of many Passiflora species by
Lev-Yadun (2009b) and petioles of Amorphophallus bufo (Ridl) by Liu et al., (2017).
Numerous black spines on the stem of rattan species are also similar with a swan of ants patrolling on their stem. Hence, herbivores could avoid plants that guarded by ants. From our observation, many individuals of rattan from D. geniculate, D. lewisiana, C. castaneus and K. scortechinii were colonized with ants but no ant colonies found in P. griffithii, which matches with the presence of black spines on those four rattan species with ant colonies.
From the stem spiny structures of those five rattan species, K. scortechinii seems to have the weakest spiny structures since their spines are the shortest and the scarcest. However, they may have the strongest defensive ability against not only the vertebrate herbivores but also the invertebrate herbivores with the help of obligate ant mutualistic partners. Agrawal & Fishbein (2006) introduced a new concept as plant defensive syndromes, which describes that plant may develop a series of defensive strategies rather than a single trait to deter herbivores.
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Rattan species may also possess other defensive strategies, such as increasing the hardness of leaves or the granular minerals in plant tissues and increase the number of secondary compounds in plant tissues. Plants that cannot produce toxic chemical compounds can also deter herbivores by chemicals produced by symbiotic organisms such as bacteria or fungal endophytes (Clay, 1989). Apart from defensive strategies, plants can also deploy avoidance strategies such as lowering the nutrient content in plant tissue or delayed their growth to avoid herbivores in peak periods (Coley &
Barone, 1996). Plants with sufficient nutrients regrowth more rapidly without defensive strategies, which are considered as tolerance strategies (Rosenthal &
Kotanen, 1994).
Rattan leaves are normally enormous, extending from the top of every stem.
Spines on the stem can hardly prevent herbivores from eating their leaves. Leaf hairs on common mullein, Verbascum thapsu, are a defensive weapon against grasshoppers
(Woodman & Fernandes, 1991). Leaf hairs on rattan species, D. geniculata, D. lewisiana, and C. castaneus, are arranged in different ways. But they all have numerous hairs placed on the surface, back and both sides of margins. All the hairs in every leaflet are pointing to the tip of the leaflet and majority of the hairs accumulate at the front part of the leaflet. They may discomfort herbivores’ mouthpart if herbivores eat leaves from the tip to the end. Moreover, leaf hairs may have multiple functions (Hanley et al., 2007).
Although I do not find hairs on rattan P. griffithii and K. scortechinii, they may have alternative strategies to protect their leaves. Leaflets of K. scortechinii are praemorse, a term which describes the upper margins of the leaflet are erose or zigzag as if bitten off (Dranfield, 1979) (Figure 4.17). The shape may indicate that part of one part of the leaflet was eaten by herbivores. Lev-Yadu & Niemelä (2017) described a
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strategy as pseudo-variegation of leaves by displaying visual signals to pretend damages or occupation by herbivores may decrease the tendency of later attackers.
Furthermore, rattan Korthalsia has close relationships with ant colonies and their leaves suffer less damage if there are patrolling ants protecting the rattan plants (Miler et al., 2016). During our survey, ants in K. scortechinii were aggressive and displayed defensive behaviours so the ants may help to protect rattan leaves. The leave protection of P. griffithii is poorly studied and there is a layer of white powder at the back of every leaflet may act like trichomes for protection.
Figure 4.17 Leaflets of K. scortechinii
Spines are normally considered as induced plant response to not only herbivores (Karban & Myers, 1989) but also natural resistance, such as fire (Gowda &
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Raffaele, 2004). Hence, in many plant species, the deployment of spiny structures may only happen when plants encountered herbivory or natural resistance. However, throughout my investigation, the same rattan species from different locations deployed similar patterns of stem spines. Stems of many rattan species are constantly covered by numerous spines (Dranfield, 1979) and stem spines may not be inducible. The non- inducible feature of rattan stem may result from two factors. One may be the vital value of rattan stems in terms of surviving and the other may be the alternative function of stem spines.
Palms (Family: Arecaceae) are a group of monocotyledons that only possesses one apical meristem. Leaves and flowers all rely on the growth of meristem and no secondary thickening for vascular bundles or external bark (Dransfield, 1979;
Tomlinson, 1990). Therefore, the single apical meristem, or the true apex which is called the edible “cabbage” by Dransfield (1979), is very vulnerable to the destruction from herbivores and the death of the single meristem will cause the death of the whole stem (Tomlinson et al., 2011). According to the Optimal Defence Hypothesis that defence should be contributed to the plant organs, structures of tissues which contain high evolutionary value (McKey, 1974; Stamp, 2003). Every meristem of the rattan plants is the indispensable part that fatal to every individual. Hence, much defensive effort was contributed to stems so that a well-defended stem could maximize the fitness of every single plant. In many species, rattan spines on sheaths are always longer than the apical meristem (Dranfield, 1979). Similarly, investigation of Brazil palm species also showed no difference in stem spines with or without the presence of livestock (Goldel et al., 2016). Therefore, stem spines on rattan species may be a constant trait without much change through the time and different locations.
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Defence is defined by Strauss & Agrawal (1999) as “a trait can be viewed as defensive even though defence is not its primary function.” Researches on red pigments (anthocyanins) in leaves demonstrated that red pigments may serve as defensive signal to avoid herbivores’ attack (Archetti, 2000; Hamilton & Brown, 2000;
Fadzly et al., 2016) and they may also protect leaves from photoinhibition (Gould et al., 1995) and light damage (Lee & Gould,2002). Other than secondary compounds, plants structural defences may also play alternative roles. For example, divaricate branching in New Zealand flora is considered a defensive trait to deter browsing birds
(Greenwood & Atkinson, 1977; Bond et al., 2004; Burns, 2016). The branching structure is also responsible for resisting frost (Darrow et al., 2001), maintaining microclimate inside the shrub to protect growing points and leaves from wind and dehydration (McGlone & Webb, 1981). Therefore, a trait which served multiple functions is unlike to change through time or locations.
Rattan spines may also deploy various roles besides defence. Referring to chapter 6, rattan D. lewisiana and C. castaneus collect leaf litters from canopy level with the help of their up-pointing spines and litters on their stem may form a nutrient trap and it also serves as a suitable environment for ant colonies. Ants on those rattans may enhance the nutrient content for the rattan and protect the rattan plants from herbivores.
In our survey, D. geniculata’s spiny structures can also collect leaf litter. The longer spines on one side are always pointing up and formed a closing environment around the main stem where much litter were collectted and ant colonies may also exist inside. The spiny structures of D. geniculata are very special as two distinct groups can be seen from their stem (Figure 4.13). One group of spines are shorter with acute angles pointing to the ground while the other group are longer with obtuse angles
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pointing to the canopy level. The shorter spines may function in defence since their acute spines could deter small climbing mammals from climbing up. The longer spines serve as collecting tools for litters so that leaves from canopy level could fall and trapped by their up-pointing spines. Spines in the same plant may also serve as different functions by their growth patterns. Hence the multi-functioning spines on stems can still benefit the plant even no protective role is needed for deterring herbivores. The two reasons above may explain why stem spines from different individuals but the same species exhibited little differences.
In conclusion, the presence of rattan spines on stems is the trait that may serve multiple functions. Spines on stems are a constantly heavy defensive trait that deters herbivores from damaging the important stem. The up-pointing spines on D. lewisiana,
C. castaneus and D. geniculata provide an alternative function as little-collecting structure, which may serve as a nutrient trap for accumulate water and nutrients as well as attract ant symbiotes. The down-pointing spines on P. griffithii and K. scortechinii mainly function as deterrence against small climbing mammals to protect meristem. It is hard to make the conclusion in which rattan species possesses the strongest defensive weapons based on their spiny structures as spines may serve other functions and rattan may rely on various defensive strategies. Further study on rattan spiny structures may help us understand the interaction between herbivores and spiny palms in the tropical forest.
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CHAPTER 5 ANT-RATTAN ASSOCIATIONS
Introduction Rattans are spiny palms belonging to the subfamily Calamoideae. About 600 species of rattans from 12 genera are recorded worldwide (Dransfield, 2002). Some rattans are climbers, which possess climbing structures, such as cirrus or flagellum, but some are acaulescent or non-climbing species (Dransfield, 1979). The flexible stems of certain rattan species are used by humans for matting, binding, and furniture production (Dransfield, 1992). In Peninsular Malaysia, rattans can be found from sea level to mountains over one thousand meters high. They are an important component of the tropical primary rainforest and secondary forests but remain poorly understood
(Dransfield, 1979).
Ants are often regarded as the most successful eusocial animals. Their relationships with plants have long been established. These interactions include leaf- harvesting for fungal gardens (Wirth et al., 2013); seed-harvesting and seed-dispersal
(Berg, 1975; Beattie &Culver, 1981); Plants bearing domatia for ant colonies are called myrmecophytes, benefiting from symbiont ants’ protection against herbivores (Janzen,
1966; Buckley, 1982; Beattie, 1985; Huxley &Cutler, 1991; Murase et al., 2003;
Herrera &Pellmyr, 2009); and protection from Crematogaster, which are the obligate ant partner of Macaranga, regulated by the food supply (food bodies) of their host plants (Itino et al., 2001). Several cases showed that ants can also serve as pollinators
(Peakall et al., 1991).
Ant and rattans association have been reported on Africa (Sunderland, 2004), which seven rattan species were found ant colonies. In Singapore, two rattan species from Korthalsia were reported that ants from three different genera (Iridomyrmex,
Dolichoderus and Philidris) occupying their ocrea structures (Chan et al., 2012).
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Several studies also found interactions among rattan, ants and Homoptera in Malaysia forests (Mattes et al., 1998; Moog et al., 2003). However, to the best of my knowledge, there is no study that had been conducted about ant-plant relationships in Northern
Peninsular Malaysia. In this study, I carried out my survey on ant-rattan association in
Penang Island and Peninsular Malaysia. Ant colonies and also their behaviours were recorded and I proposed that ants may provide multiple services for rattan plants.
Materials and Methods
Rattans were identified based on keys developed by Dransfield, (1979). The associated ants were collected using forceps, kept in 70% ethanol, and brought back to the laboratory for identification using keys developed by Hashimoto (2003).
To visualize similarities in ant associations by the different rattan species, cluster analysis was applied by using Multi Variate Statistical Package (MVSP) software. We used Unweighted Pair Group Method with Arithmetic Mean (UPGMA) with Jaccard’s Coefficient for the similarity matrix. Jaccard’s index was used since the data consisted of the absence and presence data (of the ants). Value of 1 indicates perfect similarity and 0 indicates complete dissimilarity.
Results:
In total, four rattan species (Daemonorops lewisiana, Calamus castaneus,
Daemonorops.geniculata and Korthalsia scortechinii), with 31 individuals of rattan, showed signs of association with seven ant genera (Philidris, Dolichoderus,
Crematogaster, Tapinoma, Technomyrmex, Camponotus and Pheidole) (Table 5.1).
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Table 5.1. Ant genera found on various rattan species.
Individual Rattan species Location Ant species
Daemonorops
1 D. lewisiana PNP Philidris sp. (Figure 5.1)
2 D. lewisiana PNP Dolichoderus thoracicus (Figure 5.2)
3 D. lewiisana PNP Crematogaster sp1. (Figure 5.3)
4 D. lewiisana PNP Dolichoderus thoracicus
5 D. lewisiana PNP Philidris sp.
6 D. lewisiana PNP Philidris sp.
7 D. lewisiana PNP Philidris sp.
8 D. lewisiana PNP Crematogaster sp2. (Figure 5.4)
9 D.lewisiana PNP Philidris sp.
10 D. lewiisana PNP Philidris sp.
11 D. lewisiana PNP Tapinoma melanocephalum (Figure 5.5)
12 D. lewisiana PNP Philidris sp.
13 D. lewisiana PNP Philidris sp.
14 D. lewisiana BGH Technomyrmex sp1. (Figure 5.6)
15 D. lewisiana BGH Technomyrmex sp1.
16 D. geniculata BGH Camponotus sp. (Figure 5.7)
17 D. geniculata CT Crematogaster sp5. (Figure 5.8)
18 D. geniculata CT Crematogaster sp6. (Figure 5.9)
19 D. geniculata CT Dolichoderus thoracicus
Calamus
20 C. castaneus TRTB Crematogaster sp3. (Figure 5.10)
21 C. castaneus TRTB Crematogaster sp3.
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22 C. castaneus TRTB Crematogaster sp3.
23 C. castaneus CT Tapinoma melanocephalum
24 C. castaneus CT Dolichoderus thoracicus
25 C. castaneus CT Pheidole sp. (Figure 5.11)
26 C. castaneus CT Technomyrmex sp2. (Figure 5.12)
27 C. castaneus CT Crematogaster sp4. (Figure 5.13)
28 C. castaneus CT Pheidole sp.
Korthalsia
29 K. scortechinii CT Camponotus beccarii (Figure 5.14)
30 K. scortechinii CT Technomyrmex sp3. (Figure 5.15)
31 K. scortechinii CT Dolichoderus thoracicus
* PNP is Penang National Park. BGH is Bukit Genting Hill. CT is Cherok
Tokun. TRTB is Taman Rimba, Teluk Bahang.
Furthermore, one hemipteran aphid species, Cerataphis orchidearum was found on two individual of D. lewisiana. (Figure 5.16).
Figure 5.1 Philidris sp. Figure 5.2 Dolichoderus thoracicus
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Figure 5.3 Crematogaster sp1. Figure 5.4 Crematogaster sp2.
Figure 5.5 Tapinoma melanocephalum Figure 5.6 Technomyrmex sp1.
Figure 5.7 Camponotus sp. Figure 5.8 Crematogaster sp5.
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Figure 5.9 Crematogaster sp6. Figure 5.10 Crematogaster sp3
Figure 5.11 Pheidole sp. Figure 5.12 Technomyrmex sp2.
Figure 5.13 Crematogaster sp4. Figure 5.14 Camponotus beccarii
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Figure 5.16 Cerataphis orchidearum Figure 5.15 Technomyrmex sp3.
Certain images from light microscope were not clear and live photos of several ant species (Dolichoderus thoracicus, Tapinoma melanocephalum, Camponotus beccarii and a species of Creematogaster) were taken from Antwiki
(www.antwiki.org) for further identification.
Figure 5.17 Live Dolichoderus Figure 5.18 Live Tapinoma thoracicus melanocephalum
Figure 5.19 Live Camponotus beccarii Figure 5.20 Live Crematogaster sp.
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Specifically, Philidris and Crematogaster were the most common genera of ants found. All Philidris were only found on D. lewisiana, while Crematogaster were found in three rattan species. Other ants were found in relatively lower numbers, with no clear patterns of their relationship to specific rattan species (Figure 5.21). However, the presence of those ant colonies on rattan plants was unlikely to be incidental, because of the high occurrence of ants on D. lewisiana and C. castaneus, but not on P. graffithii. Yellow crazy ants (Anoplolepis gracilipes) were also found tending the aphid Cerataphis orchidearum on D. lewisiana (Figure 5.22a) and Dolichoderus thoracius gathering at the surface of Calamus diepenhorstii (Figure 5.22b). However, because the ants’ nests could not be located on the rattan, I excluded both from my results.
Figure 5.21 Ant genus that associated with rattan species.
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Figure 5.22 (a) Yellow crazy ants (Anoplolepis gracilipes) were tending aphids (Cerataphis sp.) on rattan D. lewisiana; (b) Dolichoderus thoracius gathered in the knee area of Calamus diepenhorstii.
Based on the results of cluster analysis (Figure 5.23), Rattan species that associated were categorized into 3 groups by the different ant genus colonized on them.
The first group was K. scortechinii. The second group was D. geniculata and C. castaneus and D. lewisiana were categorized into the third group.
Figure 5.23 Dendrogram using Jaccard’s Coefficient similarities based on the
ant genus in four rattan species
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Discussion:
From the cluster analysis (Figure 5.23), three different clusters are formed based on the ant genus composition. The different cluster between K. scortechinii and the other two clusters may because of the difference of ant colonization. K. scortechinii possess swollen ocrea structures and those structures offer direct shelters for ant colonies. But the other three species do not such structures for ants. D. geniculata, C. castaneus and D. lewisiana do not provide direct domatia but collect litter by their spiny structures and ant colonization is encouraged. However, the litter collecting structures is very similar between C. castaneus and D. lewisiana but a slightly differ from D. geniculata, which may explain the same cluster formed by C. castaneus and
D. lewisiana but not with D. geniculata. However, this cluster analysis can hardly explain the similarities of rattan species based on ant species since many ant samples we collected can only be identified at genus level. Therefore, the real similarity in ant association by the four rattan species may be different.
In the four species of rattan plants, Korthalsia scortechinii is the only species that possess swollen ocrea structures. The ocrea structures are considered as an adaptation with ant species, which ants chew a hole on ocrea and build nests inside
(Dransfield, 1979). The other three rattan species do not offer any hollow structures for ant residents directly. However, they do collect leaf litter among their numerous leaf and spiny structures and ants built their nest inside those leaf litter. Apart from nests sites, rattan do not provide other rewards suck as food bodies or extra-floral nectars. In return, ants may provide multiple services to rattan plants.
Ants are capable to provide both direct and indirect protection to the plant
(Beattie 1985). Direct interaction is established by plants that provide domatia
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(shelters), food bodies or extrafloral nectaries directly to attract ants. In return, the ants guard and protect their host plants against herbivores and seed predators. Macaranga provides shelter and food bodies for their symbiotic ants (Crematogaster) and ants significantly reduce herbivore damage (Itioka et al., 2000). In my study, no rattan provided any food bodies or extrafloral nectaries to their ant residents. Although I observed that many ants (Dolichoderus thoracius) gathered at the leaf sheath of the rattan Calamus diepenhorstii (Figure 19b), there could be extra attraction for that particular ant species but so far I was unable to detect the reason for their accumulation.
Previous researches showed ants help to protect rattans together with their shelters which rattan provides. Korthalsia furtadoana experience less leaf damage when their ocreas where colonized by patrolling ants (Miler et al., 2016). Unlike rattan Korthalsia possessing ocreas that could serve as domatia for ant colonies, rattans D. lewisiana and C. castaneus do not directly provide shelter for ants. However, the litter that they collect may also encourage ants to build nests and protect the plant.
In my study, most ants showed protective behaviours (accumulating and biting) for the rattan when I was collecting the rattan and ant specimens. Protective behaviours were evident in Philidris, some Crematogaster species, Tapinoma, Camponotus and
Technomyrmex ant species. Ant protection is crucial to a rattan’s development since ants may not only disturb some megaherbivores like elephants, wild cattle, pigs and small herbivores like squirrels (Dransfield 1979), but also help eliminate invertebrate herbivores like grasshoppers (Chondracris rosea, Choroedocus capensis, Pachyacris vinosa), moths (Sesamia inferens), butterflies (Gangara thysis) and more. (Xu et al.,
2000). Furthermore, ants can also help the plant to eliminate plant competitors. Ants constantly cut off foreign plant parts that affect their host plants Macaranga (Fiala,
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1996). The leaves of K. furtadoana were covered with fewer epiphylls if patrolling ants were presented (Miler et al., 2016).
Indirect interactions also existed among rattans, ants and Cerataphis
(Homoptera). Cerataphis is a rattan pest which sucks the sap from the rattan plant.
However, they are constantly tended and guarded by ants since they excrete honeydew for ants (Stern et al., 1995). In this complicated three-way system, even though plants are damaged by homopteran pests, plants may still benefit from the ants, since ants will also protect the host plant from herbivores (Beattie, 1985; Herrera & Pellmyr,
2009).
Protective effects may even come from the visual imitation of ants crawling on the rattan. Lev-Yadun and Inbar (2002) suggested that black spots on the surface of stem and branches of Xanthium trumarium (Asteraceae) looks like ants swarming on the plants. This mimetic strategy may serve as a warning signal to deter herbivores.
Similar patterns of black spots were also found on Amorphophallus bufo, an understory plant in Malaysia forests which exhibits numerous black spots that may serve as a defensive strategy by mimicking ants (Liu et al., 2017). Therefore, even if the ants on rattans did not show any protective behaviours, having them constantly moving around the plant may indicate to herbivores that the plant is being protected. Herbivores may avoid such a plant if they have previously experienced ant bites.
Ants may serve as pollinators. Although they are a less common pollinator than bees and wasps, a few cases showed that certain ants are capable to collect pollens from flowers and visit flowers systematically (Beattie, 1985; Herrera & Pellmyr, 2009).
Of the two rattan species hosting ants in this study, C. castaneus is mainly pollinated by stingless bees (Trigona) and paper wasps (Vespidae) (Kidyoo & McKey, 2012).
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However, the pollinators of D. lewisiana are unknown. In our observations, the flowers of D. lewisiana were always enclosed by spiny bracts. Of all potential insect pollinators that visited the flowers, only ants could crawl inside their female flowers
(Figure 5.24a) and male flowers (Figure 5.24b). This may suggest the role of ants in the pollination process of D. lewsisana. More research is required to further substantiate this observation.
Figure 5.24 (a) Ants (Dolichoderus) inside a female flower of Daemonorops
lewisiana. (b) Ants (Philidris) inside a male flower of Daemonorops lewisiana.
Furthermore, ants could increase the nutrient level in those litter traps and rattans may use that nutrient to gain an advantage in resources competition in tropical forests. The nutrient-enhancement services will be discussed in following chapter.
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CHAPTER 6 RECYCLING IN THE FOREST: RATTAN
LITTER-COLLECTING STRUCTURES AND RELATIONSHIPS
WITH ANTS
Introduction: Several studies have been reported that certain level of association exist among rattan and ant species. Dransfield (1979) summarised six adaptation between ant and rattan species through 1) Casual relationships, 2) leaflets, 3) inflorescences, 4) Leaf sheath auricles, 5) ocreas and 6) spine whorls. Many adaptations are established by rattan providing nest sites directly to ant species and ants, in return, ants provide several services to rattan plants.
Ants are a good mutualistic partner as they can help plants in various aspects.
Beattie (1985) concluded that ant could protect plants against a range of herbivore, from small invertebrates to small climbing mammals, even big herbivores like wild boars and elephants are not able to tolerate numerous ant bites. Furthermore, ants can feed certain plant species and those plant relying on extra nutrient from ants were called myrmecotrophic plants (Beattie, 1985, Herrera & Pellmyr, 2009). In tropical forests of Thailand, an epiphyte Dischidia major were reported absorb nutrients provided by ants Philidris (Peeters & Wiwatwitaya, 2014).
Many understory palms face competitive nutrients intake in tropical forests.
An understory palm, Asterogyne martiana, solved this problem by capturing nutrients in litter fall and precipitation (Raich, 1983). The palm species deployed a funnelled structure and collect litter and water in the centre of their crown and nutrient inside the trap could be direct to plants’ root system. If ants working together with plants’ nutrient trap strategies, they could enhance the nutrients contents by placing preys or
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produce wastes inside the nutrient traps, similar to the feeding behaviour involving in the myrmecotrophic plants.
In Malaysia tropical forests, Certain rattan species have always been found breading ant colonies while others do not. And the rattans interacting with ant species always collect plenty of leaf litter. In this study, I investigated the unique litter- collecting structures of Daemonorops lewisiana and Calamus castaneus and compared the spine and leaf structures of Plectomia griffithii which cannot collect much litter.
The ant colonies among the three rattan species were also been compared to see which structures encourage interaction with ant species and whether ants may potentially increase the nutrient contents in rattan plants.
Materials and Methods
Three rattan species that were found to be most abundant in Penang National
Park and Taman Rimba, Teluk Bahang, included Plectomia griffithii, Daemonorops lewisiana and Calamus castaneus. For each individual rattan clump (stem length >
50cm), whether an ant colony was present, absent or abandoned were recorded Any rattan with a sign of tunnels on the surface, but an absence of ants, was considered abandoned. Rattan seedlings (< 50 cm) were ignored as they were too small to collect leaf litters.
To examine the leaf-collecting structures, the inclination of spines from the stem were measured. Ten spines were randomly chosen from each species. I compared the spine angles among the three species. The paired leaflet angles were also measured in D. lewisiana and C. castaneus. Using a protractor, The angles of paired leaflet from five randomly selected leaves of D. lewisiana and C. castaneus were measured The angles among different pairs of leaflets within the same leaf (the 1st, 10th and the last
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pair of leaflets from bottom to top) were compared to determine the changes in the angle from the first to the last pair of leaflet.
Results:
Thirty individuals of D. lewisiana were found in Penang National Park, of which 13 were associated with at least one ant colony. Another four of these showed signs of abandonment by ants. Calamus castaneus were found in Taman Rimba, Teluk
Bahang and Cherok Tokun. Of the 15 C. castaneus found, nine had at least one ant colony. Plectomia griffithii were found in all the three study areas, but none had ant colonies or signs of abandonment in the 19 individuals of P. griffithii. D. lewisiana is significantly associated with ant (57%) compare to P. griffithii (2 (1, N=50) =13.44, p<0.01). There was also a significant difference between C. castaneus (60%) and P. griffithii (0%) (2 (1, N=34) =12.58, p<0.01) in terms of their association with ants.
Hence, it is significantly more likely to find an ant colony in leaf litter collecting rattans
(D. lewisiana & C. castaneus) than non-litter collecting rattans (P. griffithii). In Bukit
Genting Hill, we found that two D. lewisiana and one D. geniculata had ant colonies.
In Cherok Tokun, Penang, ants were present on three D. geniculata and three
Korthalsia sortechinii.
Although P. griffithii, D. lewisiana and C. castaneus had many spines on their leaf sheaths and stems, the patterns of spine arrangement differed. Spines of P. griffithii normally grew in clusters. Every cluster had three to ten spines. On the main stem, clusters of spines might link together to form a row of spines. Most spines were pointing downwards (angles of spines ranging from 60° to 90°; Figure 6.1a). Only small amount of leaf litters was trapped by those spines (Figure 6.2a). The arrangement of spines on C. castaneus was more irregular compared to P. griffithii. The inclination
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of spines was vertical to the stem or pointing slightly upwards (angles of spines ranging from 90° to 130°; Figure 6.1b). Spines were rarely pointing downwards, and a large amount of leaf litter was trapped by those spines (Figure 6.2b). The spines of D. lewisiana were shorter (< 4 cm) than those of C. castaneus (> 6 cm) but were also able to trap a substantial amount of leaf litter. Most spines had larger angles than C. castaneus, ranging from 120° to 170° (Figure 6.1c). The spine angles of the three species were significantly different [F (2, 27) =31.293, P=0.000). A Tukey post hoc test revealed that spine angles of P. griffithii (65.5 ± 21.4°) were significantly smaller than the spine angles of C. castaneus (103.0 ± 10.6°, p = 0.001) and D. lewisiana
(138.0 ± 26.3). Angles of C. castaneus was also significantly smaller than D. lewisiana.
(Table 6.1)
Table 6.1. Mean value inclination of rattan spines. Alphabets in superscript denotes significant differences in Tukey Post Hoc
Species Angles of spines (°)
Plectocomia griffithii 65.5 ± 21.4 a
Calamus castaneus 103.0 ± 10.6 b
Daemonorops lewisiana 138.0 ± 26.3 c
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Figure 6.1 Inclination of rattan spines: (a) P. griffithii (mean 65.5°); (b)
Calamus castaneus (mean 103°); (c) Daemonorops lewisiana (mean 138°).
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Figure 6.2 (a) Little amount of leaf litter trapped by P. graffithii. (b)
Extensive amount of leaf litter collected and trapped by C. castaneus.
Leaf shape also facilitated litter accumulation in two rattans (D. lewisiana and
C. castaneus). D. lewisiana and C. castaneus have a similar leaf shape, with long and narrow leaflets asymmetrically arranged on both sides of the rachis. Every leaflet pair on both sides formed an angle, and the angles changed from wide to narrow from the upper to the lower part of the leaf. Leaflets on the upper parts had a wider angle (180°).
On the bottom part of the leaf, the leaflets narrowed towards each other and the angle between the leaflets became smaller (ranging from 45° to 90°; Figure 6.3). For D. lewisiana, there were a significant difference among leaflet angles of different positions [F (2, 12) =122.757, p=0.000]. Tukey post hoc test showed that the first pair of leaflets (73.0 ± 12.0°) was significantly smaller than the 10th pair of leaflets (129.0
± 14.3°) and the last pair of leaflets (180 ± 0.0°). For C. castaneus, there was also a significant difference of leaflet angles of different positions [F (2, 12=161.110, p=0.000]. Tukey post hoc test showed that the first pair of leaflets (45 ± 11.2°) was significantly smaller than the 10th pair of leaflets (144 ± 18.2°) and the last pair of leaflets (180 ± 0.0°; Table 6.2). The leaflets at the bottom part formed an inverted funnel that trapped leaf litters.
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Table 6.2. Angles of different paired leaflets in rattan species. Alphabets in superscript denotes significant differences in Tukey Post Hoc.
Paired leaflet position Daemonorops lewisiana (°) Calamus castaneus (°)
1st 73.0 ± 12.0 a 45 ± 11.2 α
10th 129.0 ± 14.3 b 144 ± 18.2 β
Last 180 ± 0.0 c 180 ± 0.0 γ
Figure 6.3 Leaflet angle of Calamus castaneus from the upper part to bottom
part (from 3 to 1) is becoming smaller.
In D. lewisiana, some leaves showed one protruding leaflet growing around the middle part of the rachis (Figure 6.4a). This particular leaflet arrangement may be able to trap litter from falling to the ground. Due to this unique leaf shape of D. lewsiana and C. castaneus, leaf litter from the canopy was always trapped at the bottom part of the leaves (Figure 6.5a) Ant colonies may use the leaf litter to establish their nests (Figure 6.5b). The leaves of P. griffithii did not demonstrate any leaf- collecting capability and no leaf litter or ant colonies were found. (Figure 6.4b).
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Figure 6.4 (a)A protruding leaflet growing on the middle of the rachis that may hinder leaf debris from falling. (b) The leaf of Plectocomia griffithii does not
collect leaf debris or harbour ant colonies.
Figure 6.5 (a)Leaf litter trapped on the bottom part of Daemonorops
lewisiana leaves. (b) Ant colony on the debris of leaves trapped by Calamus
castaneus.
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Discussion:
In this study, ants were more likely to associate with litter-collecting rattans.
However, there was no clear evidence of an obligate relationship between the rattan and ant species. There are few studies on rattan-ant relationship. Mattes et al., (1998) reported that two species of Camponotus formed obligate association with Korthalsia robusta in Sabah, Malaysia. Edwards et al. (2010) stated that the only possible obligate symbiont of Korthalsia furtadoana were two species of Camponotus in lowland dipterocarp forests of Borneo. Moog et al. (2003) found that four species of Korthalsia, one species of Calamus and four species of Daemonorops had a high possibility of associations with Camponotus sp. in Peninsular Malaysia. Chan et al. (2012) reported seven individual clumps of Korthalsia echinometra were colonized by Iridomyrmex sp. and four clumps of Korthalsia rostrata were colonized by Dolichoderus sp. and
Philidris sp. in Singapore. Sunderland (2004) surveyed in Africa and found 11 species of rattans associated with 12 species of ants, but the evidence for a symbiotic relationship between rattan and ants was weak.
In this study, I propose a complex and novel type of rattan adaptation to promote ant association by leaf litter-collection and provisioning of nest materials through their arrangement of leaves, leaflets and spines which is not mentioned by
Dransfield (1979) in the six ant-rattan adaptations (1.casual adaptations; 2 leaflets; 3 inflorescences; 4 leaf sheath auricles; 5 ocreas; and 6 spine whorls). The ocrea in
Korthalsia was the most studied adaptation (Mattes et al., 1998; Edwards et al., 2010;
Chan et al., 2012; Hu, 2014 and Miler et al., 2016). The ocrea in Calamus was also colonized by ants (Merklinger et al., 2014). Rickson & Rickson (1986) reported spine whorls associated with Camponotus. Other adaptations were rarely studied and poorly understood.
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My results showed that rattans (C. castaneus and D. lewisiana) had leaf arrangements which extended in every direction and expanded horizontally in the shape of a vase or bowl able to collect leaf litter. At the bottom part of the rattan leaf, the angles of the leaflets become smaller and a funnel-like structure was formed at the end of every leaf. In addition, the gaps between two abreast leaflets were smaller than falling litter from canopy. Hence, leaves falling from the canopy were frequently trapped at the end of the rattan leaves.
Spines are normally considered to be a defensive weapon against herbivores.
Small climbing mammals (e.g. squirrels) eat the apex of rattan stems (Dranfield, 1979).
Therefore, if spines are pointing down like P. griffithii, this may potentially deter squirrels from climbing up. On the contrary, the spines on D. lewisiana and C. castaneus were always pointing upwards, and hence may not be effective to prevent small climbing mammalian. I suggested that the main function of spines on D. lewisiana and C. castaneus is to collect leaf litter. Leaves falling from the canopy can easily be pierced by these spines and trapped along the rattan stems.
The leaf litter collecting structure on an understory palm (Asterogyne martiana) was first described by Raich (1983) in the rainforests of Costa Rica. It also has a funnel shape that collects nutrients in precipitation and organic debris. This debris also traps a large amount of rainfall that is diverted down to the stem and is finally absorbed by the roots. The nutrient content from the stem flow was much higher than nutrients from rainfall (Raich, 1983). This nutrient-capturing hypothesis may also apply to rattan that have similar structures for collecting leaf litter
The nutrient-capturing hypothesis can be further understood with the help of ant colonies. Beattie (1985) described a unique ant-plant relationship known as
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myrmecotrophy, in which the plants gain benefits from being “fed” by ants. Janzen
(1974) observed this phenomenon between several epiphytes and ant species in forests of Sarawak. Ants “fed” those epiphytes by placing preyed insect in certain cavities of the plant. The plant wall of those cavities was able to absorb those prey bodies and their nutrients. Prey bodies were marked with radioactive elements and these markers were found inside the plants, showing that insect bodies were absorbed by plant tissue
(Rickson, 1979). In Thailand, the ants Philidris, Crematogasters and Echinopla collected organic debris to build nests inside the pitchers (modified leaves) of the epiphyte Dischidia major. Ants gained shelter, while their nest-building behaviour benefited plants by providing them with extra nutrients (Peeters & Wiwatwitaya, 2014).
Myrmecotrophy between rattans and ants was reported by Rickson & Rickson (1986).
Daemonorops verticillaris and D. macrophylla in Peninsular Malaysia have overlapping spines where ants build nests inside and they also collected falling litters at the top of the plant. Isotope tracers were used to demonstrate that water and nutrients from ant nests were absorbed into the plants and assisted in the growth of the plant
(Rickson & Rickson 1986).
Organic materials trapped by the plant may provide nesting materials and harbourage for ants, and in return, the plant may receive more nutrients that are brought in by the ants. Overlapping leaf litter provides myriads of cavities for ants to build nests inside and debris or detritus from dead leaves can be used as building materials for ant colonies. For example, some Crematogaster species are arboreal foragers that collected decayed woods particles or rotting logs to build nests (Eguchi et al., 2011).
Rattan spines can provide structure support (frames) for their carton nests. Moreover, litter that traps rainfall (Raich, 1983) can provide a moist environment (water storage) for ant colonies. Water is crucial for colony formation in many ant species. For
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example, Dolichoderus sulcaticeps collects nesting materials from plant surfaces.
Those materials can only be used for construction of nests on the plant when soaked with water (Rohe & Maschwitz, 2003).
In conclusion, leaf litter collection of certain rattan species may be a unique adaptation to attract symbiotic ant colonies as leaf litter is a nutrient trap that offers a suitable environment for nesting ants. For the rattan, its nutrient household could be greatly improved by ant colonies and ants could also protect the rattan from herbivores or seed predators and even pollinate the rattan flowers. The relationship between these rattans and ants may not be obligate. Future studies should further evaluate and elucidate these complex associations.
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CHAPTER 7 GENERAL DISCUSSION
Resistance strategies of plant can be divided as avoid strategies and tolerance strategies. In tropical forests, strategies that rattans deploy are closer to avoid strategies rather than tolerance strategies. Coeley et al. (1985) argued that plants grow in nutrient-rich sites are capable to allocate enough resources to regrowth back after certain level of defoliation so plants may invest little resources in their defensive strategies. One the other hand, plant grow in nutrient-poor spots may tend to invest more in defensive weapons since plants are less well able to recover back from herbivores’ feeding. The nutrients in soil of tropical forests is complex and diverse systems (Vitousek & Sanford, 1986; Brown & Lugo, 1990). Whittaker (1975) statements on nutrient level of tropical forests as “The tropical rainforest thus has a relatively rich nutrient economy perched on a nutrient poor substrate” because the high turnover of nutrients. Drenching rain in the forests are so effective that most of surface nutrients will be stripped away and recaptured by decomposer such as fungi and roots which sharing plant-fungus relationships (Whittaker, 1975). In other words, nutrients in tropical forests are not low but competition is too high to take an advantage of nutrients efficiently. A research in tropical forests of Costa Rican also reported that the nutrients level in soils were negatively correlated with species richness (Huston, 1980).
Therefore, rattan face a high level of intra/interspecific competition, which results in a shortage of nutrients.
Rattan plants may contribute to defensive strategies in priority than tolerance according to hypothesis by Coeley et al. (1985). It may also explain why certain rattan species are collecting leaf litter from forests. Litter collecting structures of D. lewisiana,
C. castaneus, and D. geniculata may help them enhance the nutrients and water allocative abilities. Ants’ foraging behaviours and wastes that they produce may
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significantly increase the nutrients level in those litter-traps on rattan plants. Rattans with litter collecting structures, hence, gain an advantage in nutrients competition in a serious nutrient-poor environment,
Furthermore, the irreplaceable meristem is the key restriction for resistance strategies to be chosen. Rattan plants must choose defensive strategies to protect their stems since their stem can hardly tolerate too much damages. The stems of rattan lack of secondary thickening is a crucial weakness in defending herbivores. Once the only meristem is eaten by herbivores like small climbing mammals or megaherbivores, such as wild boars or elephants, the whole stem can hardly grow back and it may cause the stem or even the plant to die. According to the optimal defensive hypothesis by McKey
(1974), plants will invest more resources in defending the more valuable plant tissues and organs. A well-defended rattan stem has significantly higher fitness compare to a less-defended stems. Studies conducted on Brazil palm species also found that stem spines are constantly defended with or without the presence of herbivores (Goldel et al., 2016). Although leaf spines are considered as an inducible weapon which can increase in length and density after herbivores’ attack, the stem spines in rattan stem are more likely an irreducible defensive weapon and there is no intraspecific changes in their stem spinescence trait. A continuous physical defence by spiny structures are necessary for rattan plants since they can tolerate the loss of one leaf, but they can hardly tolerate the loss of a stem.
The defensive mechanisms of rattan plants are a complex system. Rattan is face various herbivores in tropical forests. Hence, rattan can hardly use single defensive strategy to deter different kinds of herbivores. Most rattan plants bearing numerous spiny structures on their stems (Dransfield, 1979), spinescence should contribute to the defense of valuable stems, especially the irreplaceable apex meristem. However,
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we should bear in mind that rattan species may not only rely on spiny structures in defense so the degree of defense from spinescence is differ among species. For example, rattan Korthalsia rely on protection from mutualistic ant species more than spiny structures. The ocrea structures of some Korthalsia is the only obligate ant-rattan mutualism found in rattan family (Mattes et al., 1998) and their symbiotic ants aggressively attack invader not only animals, but competitive plants (Edwards, 2010;
Miler et al, 2016). Therefore, Korthalsia may not only rely on their spiny structures when herbivores attack them and the measurements of spinescence showed that spines on Korthalsia were the shortest in length and the sparsest in density among the five rattan species. It may indicate the degeneration of spiny structures of Korthalsia with the help of ants’ effective protection.
Ant-rattan relationships from another three species (D. lewisiana, C. castaneus, and D. geniculata) which we investigated showed they built relationships with multiple species of ants and evidence of obligate relationship was low. Hence, the defensive service provided by different ant species may be uneven. Certain ant species may play little in protective role or even do not protect their host plants at all. Edwards et al., (2010) found that even rattan Korthalsia furtadoana which build a compulsory relationship with Camponotus ants will also infested by other ant species with little or no help for rattans.
Therefore, the three rattan species (D. lewisiana, C. castaneus, and D. geniculata) need still invest more resource to their spiny structures in protecting their stems. The investment on spines are worthy since spines are not only protecting the stems but also collect litter for themselves. Edwards et al., (2010) describe those ants occupying ocreas but serving nothing for their host plant as ‘parasite’. At the same time the host plant, K. furtadoana were unable to punish or expel their parasitic lodgers.
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Nevertheless, ant species in my investigation may be a different story. Ants which offer little or no protection to their host rattan (D. lewisiana, C. castaneus, and
D. geniculata) may not be parasitic species. First, their group movements on rattans may be an effective warning signal to herbivores. Plants that with black spots on their stems or petioles may have potential visual deterrence against herbivores or oviposition by herbivores (Lev-Yadun, 2009c; Liu et al., 2017). Numerous ants moving around on rattan’s stems and leaves is a better advertisement comparing to static black spots. Herbivores that foraging by visual signals and experienced the pain from ant bite before may leaned how to avoid feeding on those plants. Furthermore, the three rattan species (D. lewisiana, C. castaneus, and D. geniculata) all possess spines with part of black colour on themselves may also be an ant-mimicry patter as warning visual signals to herbivores.
Second, ants can provide more nutrient to the leaf litter traps which rattan collect. Remnants of preys captured by ants and wastes left by ants on leaf litter should increase the organic nutrients level and water can redirect those nutrients to their roots and absorbed by rattans’ roots. The ant nests on Daemonorops verticillaris and D. macrophylla can even increase the nutrients intake efficiency of rattan by absorbing precipitation alone with nutrients through their nest materials and direct into skin of rattan stems (Rickson & Rickson, 1986). Ant colonies are beneficial to rattans’ nutrient traps and may help rattan absorb more resources so the nutrient-poor environments may favour the plants can grasp more resources and energy.
Third, ants in D. lewisiana’s flower were always colonized by resident ants and their pollens may be carried by those ants for pollinating purposes. Ants in the litter collecting rattans, thus, are unlikely parasite if they do not aggressively chase foreign invaders away because they may provide more services and benefits to their host plants.
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If rattan has the ability to choose, they may pick the ants which are capable in protecting, increasing nutrients level, pollinating and more services since better services may help maximize the benefits to rattan plants. Rattan won’t suffer too much from bearing an ant colony even if the ants cannot provide any benefit the plants at all.
The reason is because it costs little or nothing to attract ant partners. Many plants in ant guard systems have to offer different types of food to their symbiotic ants. For example, food bodies from Macaranga trees (Itino et al., 2001) and extra-flora nectars from ferns (Koptur, 1992). All those foods contain a mixture of nutrients suck as amino acids, lipids, sugars and so on (Beattie, 1985). Thus, costs of those attractants to ants is much higher compare to rattan which produce no food bodies or no extra-flora nectars. Korthasia and several species of Calamus only provide nests site for ant colonies so the costs are just the swollen ocrea structures. The ocrea nests are a direct connection to bridge the cooperation of rattans and ants.
One the other hands, rattan offer nesting spots by litter-collecting structures are indirect association between rattan and ants. The rattan spines do not encourage colonization of ant species since rattan Plectomia griffithii also have numerous down- pointing spines but few ant colonies lived there. D. lewisiana, C. castaneus, and D. geniculata collect enough litter and debris from other higher plants first, then encouraging other ants build colonies in those small cavities formed by overlapping of litter and debris. The costs of ant’s nests are almost zero since litter or debris are from litter abandoned by other plants. The only costs may count on those stem spines that hold and traps the leaf litter. But spines on stem are an indispensable structure for protection. Litter that cover the bottom part of leaflets may affecte their photosyhthesis efficiency, but it only covers a small portion of their leaves. Hence, cost of litter trapping structures can be neglected.
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Homoptera that were found on certain rattan species may result in damage since they suck sap from the plant and offer honeydew to ants. If the number of homoptera is controlled by their host ants, the damage of homoptera is limited. And ants can still benefit the rattan plants by protectiong both aphids and their host plants (Beattie, 1985,
Herrera & Pellmyr; 2009). Therefore, the three-way interactions may benefit to rattan plants, homoptera and ants.
The benefits from the protection of ant species may still benefits the whole plants and a winning in three-aspect situation may benefit to the plants, homoptera and ants (Beattie, 1985, Herrera & Pellmyr; 2009).
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CHAPTER 8 GENERAL CONCLUSION AND FURTHER
STUDIES
Rattans are important palms in Malaysia tropical rainforests and this is the first research to focus on their defensive mechanism of rattan and their interaction with other animals. There are few previous studies I can refer to measuring plant’s spiny structures from different aspects. The protocols of spinescence measurement are very simple and no detailed example is given. The biggest problem is the lack of reliable measurement of spines’ strength. Guidance from two previous protocols all suggested using our fingers to bend down every spine and estimate the strength of it. This method is a rough imprecise method since everyone has different strength and it can hardly be quantitative. I invented the method by pressing the spine by a digital weight scale so that I can quantify the strength I used to break down a spine.
Hence, I can use the quantified data to compare the different strength between different species. Although it is still a primitive method to analyse of spines’ strength and the results are not very precise, it is the easiest and the most cost-effective way I devised to complete this measurement. The second problem I met is the rarity of studies in ant-rattan relationships. Most rattan species have not been investigated in terms of the relationship with ants. Ants capture from those rattan were very hard to identify. The litter collecting structures of plants was also rarely studied. There is no former evidence showing spines possess alternative function in collecting litter.
However, some researchers suggested plants may capture more nutrient from the litter trap and isotope tracking technique even proved that ants could help rattan to absorb extra nutrients. However, I can hardly run similar test in my study due to budget and equipment constraint.
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Five different rattan species (Daemonorops lewisiana, Daemonorops geniculata, Calamus castaneus, Plectomia griffithii and Korthalsia scortechinii) were investigated and the details and features of their spiny structures were recorded.
Rattan’s stems were constantly covered by numerous spines on their leaf sheathes and leaves of Daemonorops lewisiana, Daemonorops geniculata and Calamus castaneus were covered by plenty of hairs which could potentially deter herbivores. Spiny structures were different among the five species and there is no evidence in which species possessed the most powerful defensive ability based on their spinescence.
Spines might provide alternative functions and rattans were not solely rely on physical defence. Ant species were always associated with Daemonorops lewisiana and
Calamus castaneus but never found in Plectomia griffithii because D. lewisiana and
C. castaneus deployed unique structures in collecting leaf litter but P. griffithii can hardly hold or trap leaf litter. Litter were collected in a great amount and formed a nutrient trap. Numerous cavities in leaf litter were suitable nest sites for ant colonies and a diverse group of ant species colonized on different rattan plants. Although little evidence showed obligate relationships exist between rattan and ants. Ants get nests sites from rattan plants and in return they may serve the rattan in multiple ways, such as protecting rattan from foreign animal and plant invaders, enhance the nutrient level in rattan’s nutrient trap and may pollinate flowers of D. lewisiana.
Further studies need focus on details of animal-rattan interactions. The effectiveness of spinescence on different rattan species and the effective protection from different ant species need tested by different types of herbivores. The enhancement in nutrient content in litter trap of rattan plant and the mechanism of how rattan absorbing nutrients from it could be tested by isotope tracking techniques. The cost-benefit studies of the rattan plants if it is bearing ants and homoptera pests. All
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those studies could increase understanding of rattan plants and leads to better protection of rattan plants in tropical forests.
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LIST OF PUBLISHCATION
Liu, K., Fadzly, N., Mansor, A., Zakaria, R., Ruppert, N., & Lee, C. Y. (2017).
The dual defensive strategy of Amorphophallus throughout its ontogeny. Plant
Signaling & Behavior, 12, e1371890.
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APPENDICES
Figure 1 Daemonorops lewisiana
Figure 2 Calamus castaneus
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Figure 3 Daemonorops geniculata
Figure 4 Plectomia griffithii
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Figure 5 Korthalsia scortechinii
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Figure 6 Ant colonies in D. lewisiana.
Figure 7 Abandoned ant colony in D. lewisiana.
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