ORIBATID MITE (ACARI: ORIBATIDA) ASSEMBLAGE RESPONSE TO CHANGES IN LITTER DEPTH AND HABITAT TYPE IN A BEECH-MAPLE FOREST IN SOUTHWESTERN QUEBEC
By ZACHARY A. SYLVAIN
Department of Natural Resource Sciences McGill University, Montreal December 2007
This thesis is submitted to McGill University in partial fulfillment of the requirements of the degree of Master of Science, Entomology
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While these forms may be included Bien que ces formulaires in the document page count, aient inclus dans la pagination, their removal does not represent il n'y aura aucun contenu manquant. any loss of content from the thesis. Canada TABLE OF CONTENTS
LIST OF TABLES 3
LIST OF FIGURES .4
LIST OF APPENDICES 5
ACKNOWLEDGEMENTS 6
PREFACE 7
CONTRIBUTION OF AUTHORS 8
ABSTRACT 9
RESUME 10
CHAPTER 1: GENERAL INTRODUCTION AND LITERATURE REVIEW 11
Biodiversity and ecosystem functioning 11
The soil/litter system in forest ecosystems 13
Arthropod study in ecology 15
Ecology of the oribatid mites 17
THESIS OBJECTIVES AND RESEARCH QUESTIONS 22
LITERATURE CITED..... 25
CONNECTING STATEMENT 31
CHAPTER 2: HABITAT TYPE AS A DETERMINANT OF ORIBATID MITE (ACARI: ORIBATID A) ABUNDANCE, SPECIES RICHNESS AND ASSEMBLAGE COMPOSITION 32
ABSTRACT 32
INTRODUCTION 33
METHODS 35
RESULTS.... 40
1 DISCUSSION 42
LITERATURE CITED 45
CONNECTING STATEMENT 57
CHAPTER 3: THE EFFECTS OF LITTER DEPTH ON ORIBATID MITE (ACARI: ORIBATIDA) ASSEMBLAGES IN A BEECH-MAPLE FOREST IN SOUTHWESTERN QUEBEC 58
ABSTRACT 58
INTRODUCTION 59
METHODS 61
RESULTS 66
DISCUSSION 68
LITERATURE CITED 72
CHAPTER 4: GENERAL CONCLUSION 84
LITERATURE CITED 86
2 LIST OF TABLES
Table 2.1: ANOVA comparing oribatid species richness and oribatid abundance in soil (four forest stand and three open field site types) and litter (four forest stand site types) samples. Data were log transformed prior to analysis, and bold type denotes significant effects 48
Table 2.2: MRPP of oribatid assemblage composition in litter from four forest stand types at the Morgan Arboretum. Differences between groups (A) were evaluated with the Sorenson (Bray-Curtis) distance measure. Data were log transformed prior to analysis, and bold type denotes significant effects 48
Table 2.3: Indicator species analysis reporting significant indicator values (a < 0.05) for oribatid mites by forest stand type. Data were pooled among dates. Monte Carlo test of 1000 runs used to test significance of maximum indicator value (IV) 49
Table 3.1: Experimental design for one set of replicates within each stand type. Four replicates were used for each treatment, with samples taken on two dates from the Morgan Arboretum 75
Table 3.2: ANOVA comparing oribatid species richness and oribatid abundance in soil and litter samples collected from beech stands. Data were log transformed prior to analysis 75
Table 3.3: ANOVA comparing oribatid species richness and oribatid abundance in soil and litter samples collected from maple stands. Data were log transformed prior to analysis 75
Table 3.4: Results of two-tailed Student's t-test comparing mean oribatid abundance and raw species richness of control treatments from beech and maple stands (a = 0.05) 76
Table 3.5: MRPP of oribatid assemblage composition in litter treatments collected from maple and beech stands at the Morgan Arboretum. Differences between groups (A) were evaluated with the Sorenson (Bray-Curtis) distance measure. Data were log transformed prior to analysis, and bold type denotes significant effects 76
Table 3.6: Indicator species analysis reporting significant indicator values (a < 0.05) for oribatid mites by litter depth. Data were pooled among sampling dates. Monte Carlo test of 1000 runs used to test significance of maximum indicator value (IV). .77
3 LIST OF FIGURES
Fig. 2.1: Site locations within the Morgan arboretum of southwestern Quebec. The solid line indicates border of the Arboretum. Agricultural samples were taken from a corn field 1.6 km southwest of the Arboretum 50
Figure 2.2: Rank abundance curve for the ten most common species (full names can be found in Appendix 2.1) collected from all samples, and by contribution by forest stand type 51
Fig. 2.3: Means for a) log abundance (± SE) and b) log species richness (± SE) of oribatid mites collected from leaf litter in four forest stands at the Morgan Arboretum; log-transformed data was used in analysis, and significant differences (p < 0.05) indicated by different letters above error bars 52
Fig. 2.4: Rarefaction curve showing species richness (+ S.D.) of oribatid mites collected from leaf litter in a) July 2005, b) September 2005, and c) June 2006, separated by habitat type .53
Fig. 2.5: Three-dimensional non-metric multidimensional scaling (NMS) ordination of log-transformed oribatid mite data pooled by date. Percent variation by axis shown in figure (final stress = 8.61, axis 1: p = 0.020, axis 2: p = 0.020, axis 3: p = 0.020) 54
Fig. 3.1: Site locations within the Morgan Arboretum of southwestern Quebec. Solid line indicates border of the Arboretum 78
Fig. 3.2: Rank abundance plot for the fourteen species most commonly collected (>100 individuals from all samples), separated by stand type (beech and maple). Species codes are found in Appendix 2.1 79
Fig. 3.3: Rarefaction curves showing species richness (±S.D.) of oribatid mites collected from leaf litter in a) maple stands and b) beech stands, separated by litter depth treatment 80
Fig. 3.4: Three-dimensional non-metric multidimensional scaling (NMS) ordinations of log-transformed oribatid mite data pooled by date. Percent variation by axis shown in figure (Final stress = 9.22, axis 1: p = 0.020, axis 2: p = 0.020, axis 3: p = 0.020) 81
4 LIST OF APPENDICES
Appendix 2.1: Oribatid mites collected from soil and litter samples within seven habitat types at the Morgan Arboretum of McGill University in Montreal, Quebec 55
Appendix 3.1: Oribatid mites collected from soil and litter samples within litter depth treatments at the Morgan Arboretum of McGill University in Montreal, Quebec 82
5 ACKNOWLEDGEMENTS
I would like to thank my supervisor, Dr, Chris Buddie, for first introducing me to the world of soil ecology and for all the help and advice he has provided over the last two years. Through his unending support, motivation and enthusiasm, Dr.
Buddie served to deepen my understanding of and appreciation for the field of ecology, and I greatly appreciate the opportunity to have worked with him. I would also like to thank my committee members, Dr. Terry Wheeler and Dr. Frederic
Beaulieu, who in addition with Dr. Buddie provided assistance and guidance in the development of my project. This research was partially funded by the National
Science and Engineering Research Council of Canada, the Le Fonds quebecois de la recherche sur la nature et les technologies, and the Department of Natural Resource
Sciences (McGill University).
I would also like to extend my thanks to Christina Idziak, the director of the
Morgan Arboretum, for her help in allowing me to conduct my research and for providing me with materials such as maps and site information regarding various aspects of the forest. Working my way through the mire of mite identifications would have been impossible without the infinitely patient help of Dr. Valerie Behan-
Pelletier of Agriculture and Agri-Food Canada, and the long hours at the microscope were made more enjoyable by the company of fellow mite-researcher, Andrea
Dechene. I would also like to thank all the members of the Insect Ecology laboratory, for all the aid, entertainment and experiences they've provided me with.
Lastly (but certainly not least), I would like to thank my family for their limitless support and understanding in my pursuit of this degree.
6 PREFACE
This thesis contains four chapters.
Chapter 1
This chapter provides a general introduction and literature review for the thesis
Chapter 2
This chapter is a manuscript in preparation for submission to Pedobiologia.
Sylvain, Z.A. and Buddie, CM. Habitat type as a determinant oforibatid mite
(Acari: Oribatida) abundance, species richness and assemblage composition.
Chapter 3
This chapter is a manuscript in preparation for submission to The Canadian
Entomologist.
Sylvain, Z.A. and Buddie, CM. The effects of litter depth on oribatid mite (Acari:
Oribatida) assemblages in a beech-maple forest in southwestern Quebec.
Chapter 4
This chapter provides a general conclusion for the thesis
7 CONTRIBUTION OF AUTHORS
Both authors designed the two studies presented in chapters 2 and 3. Z.A.
Sylvain performed all data collection, identified all specimens, conducted the statistical analyses and presented the results. CM. Buddie supervised the research and provided general advice on experiment design and analysis, and editing for all chapters of this thesis.
8 ABSTRACT
I investigated oribatid mite assemblages in a beech-maple forest in southwestern Quebec. I first examined the effects of four forest stand types
(American Beech (Fagus grandifolia) dominated, Sugar Maple (Acer saccharum) dominated, mixed deciduous and coniferous plantations) and three open site types
(agricultural field, fallow pasture and unmanaged hay field) in structuring oribatid mite assemblages. My second study focused on the effects of changes in litter depth
(a factor that varies by stand type) on the structure of oribatid assemblages.
Stand type was shown to be an important factor in determining oribatid mite abundances, species richness and assemblage composition. Results from the second study confirm this, but revealed no effect of changes in litter depth on oribatid mite assemblages. These findings serve to demonstrate that while examining specific environmental factors as determinants of oribatid mite diversity and distribution is important, more general factors such as habitat type cannot be ignored.
9 RESUME
J'ai investigue les assemblages des mites oribates dans une foret hetre-erable dans le sud-ouest du Quebec. Premierement, j'ai examine les effets de quatre peuplements domines par le hetre americain (Fagus grandifolia) et l'erables a sucre
{Acer saccharum), foret feuillue melangee et plantations des coniferes) et trois sites de type ouvert (champ agricole, paturage, et champ de foin non-entretenu) sur les assemblages des mites oribates. Ma deuxieme etude a ete devouee sur les effets des changements de la profondeur de la litiere (un facteur qui varie selon le peuplement) sur la structure des assemblages oribates.
Le peuplement a ete un facteur important dans la determination de l'abondance des mites oribates, la richesse de l'espece et de la composition des assemblages. Les resultats de la deuxieme etude confirme ce fait, mais n'a pas demontre des changements dans la profondeur de la litiere sur les assemblages des mites oribates. Ces resultats peuvent montrer que meme si des facteurs environnementaux specifiques comme determinants de la diversite des mites oribates et la distribution sont importants, il y'a d'autres facteurs generaux comme le type d'habitat qui ne peuvent pas etre exclus.
10 CHAPTER 1: GENERAL INTRODUCTION AND LITERATURE REVIEW
Biodiversity and Ecosystem Functioning
One of the primary questions within the field of ecology deals with the mechanisms that cause and maintain biodiversity (Bardgett, 2002; Lawton et al,
1998; Loreau et al, 2003; Rainio and Niemela, 2003). The role and importance of certain environmental factors such as climate (Thullier et al, 2005) and habitat size
(Didham et al, 1998; MacArthur and Wilson, 1967) in structuring the assemblages of many organisms is becoming better understood. In addition to these abiotic factors, inter- and intra-specific interactions such as predator-prey dynamics and competition (Tilman, 1994) also influence the distribution of organisms throughout ecosystems.
Studies of biodiversity in ecosystems have also begun to investigate the alternative question of what effect biodiversity has on ecosystems. Initially brought into general awareness by Tilman (e.g. Tilman, 1996; Tilman, 1999; Tilman et al,
1996) due to his studies on the effects of varying levels of species richness of grasses on the productivity and stability of grassland ecosystems, the importance of biodiversity on ecosystem functioning is beginning to gain credence (see discussion by Naeem (2002)). A respectable body of work has been accumulated supporting the importance of biodiversity in promoting and maintaining ecosystem services (Giller and O'Donovan, 2002; Hooper et al, 2005; Naeem, 2002), and several potential hypotheses for how biodiversity influences ecosystem function have been proposed.
11 Hypotheses to explain the particular role of biodiversity on ecosystem functioning fall into several categories. Some postulate an equal contribution of all species within the system (e.g. rivet, modified rivet and uniqueness hypotheses), while others highlight the importance of a single "keystone" species that drives the stability of the entire system. An additional category consists of hypotheses proposing maintenance of functioning through functional redundancy of roles within the ecosystem, with multiple species performing the same role (Giller and
O'Donovan, 2002). In general, work with animal assemblages has found positive effects of biodiversity on ecosystem functioning, including increases in productivity
(Naeem et al, 1995; Naeem et al, 2000; van der Heijden et al, 1998), increased rates of decomposition (Heneghan et al, 1999) and increased resistance to drought
(Tilman and Downing, 1994). Resilience of ecosystems (Loreau et al, 2003;
Maraun et al, 1998) may also be influenced by biodiversity.
The services provided by ecosystems have been variously estimated, with one study suggesting the monetary value to amount to over $33 trillion USD annually
(Costanza et al, 1997) and include biological control, climate regulation, nutrient cycling and pollination (Costanza et al, 1997). Loss of species therefore has direct economic impacts beyond the obvious loss of functioning. Understanding the processes that underlie the maintenance of biodiversity therefore aids us in maintaining economic as well as ecosystem services, and this knowledge is crucial to environmental planning of conservation efforts (Giller and O'Donovan, 2002).
Given the large number of species that remain unknown to science, and the
12 current loss of species due to human impacts on the ecosystem (Wheeler et al,
2004), the determination of factors supporting species richness within ecosystems is crucial.
The Soil/Litter System in Forest Ecosystems
Much work has been done on the biodiversity of terrestrial systems, and yet until recently the soil and litter systems were largely ignored (Bardgett, 2002) and represent one of the least well understood ecosystems in ecology (Behan-Pelletier and Newton, 1999). This is surprising, as the soil/litter system provides key services to terrestrial ecosystems, such as conversion of the net products of primary production into usable nutrients through the processes of decomposition and nutrient cycling (Seastedt, 1984). The importance of belowground assemblages on overall ecosystem stability and functioning is only beginning to be understood (Hooper et al, 2005; Wolters et al, 2000).
The majority of biodiversity in terrestrial systems is to be found within soil systems (Hansen, 2000; Mesibov, 1998; Wardle, 2002), and yet this diversity is largely unknown (Behan-Pelletier and Newton, 1999). Thus, the soil/litter system has been referred to by some as the "poor man's tropical rainforest" (Behan-Pelletier and Newton, 1999). The soil system encompasses several layers within the soil itself, the heterogeneous nature of which serves to increase biodiversity through microhabitat complexity. The O horizon encompasses the organic layer of fallen leaves and woody material in varying stages of decomposition, and rests above the A
13 horizon (the top layer of the soil proper). This horizon is characterized by incorporation of organic matter in with the mineral substrate, and below it is located the B horizon (or subsoil) which primarily consists of mineral soil and clay. The C horizon lies below both the A and B horizons, and is comprised of the parent material from which the A and B horizons developed.
Recent work has sought to highlight the importance of biodiversity within the soil/litter system on overall ecosystem functioning and stability (Hooper et al,
2005), with several studies documenting the exchanges between belowground and aboveground systems. The importance of soil organisms in the processes of decomposition and nutrient cycling is well documented (Brussaard et al, 1997;
Moore et al, 1988; Seastedt, 1984), with aboveground plant productivity reliant upon these processes (Mikola et al, 2002). Some studies have also attempted to demonstrate food web linkages involving soil and litter organisms with aboveground predators (Johnston, 2000).
Certain human activities have long been understood to have negative effects on the environment, such as from displacement of natural habitat (clear-cutting, heavy agriculture, urbanization) (Vitousek et al, 1997). This is especially true for impacts on the soil system. Industrial activities nearly always have negative impacts on the soil/litter system; mining activities release heavy metals and pollutants into soils (Ledin and Pedersen, 1996), agriculture severely disturbs the habitat substrate
(Brussaard et al, 1997) and forestry practices such as timber harvesting and clearing both disturb the habitat and reduce organic input into the system (Battigelli et al.,
2004). Frequent disturbance events create conditions that are generally unfavorable
14 to many organisms and those inhabiting soils and litters are no different - certain soil fauna such as some mites and collembola are extremely susceptible to changes in the local habitat (Maraun et al, 2003).
Although there is a resurgence of interest in soil and litter ecosystems, our comprehension of these systems remains limited. Knowledge of soil biodiversity is especially poor, with a majority of species being undescribed (Behan-Pelletier and
Newton, 1999). Similarly, the mechanisms underlying soil biodiversity are also poorly known (Bardgett, 2002; Osier et al, 2006). Arthropods and other invertebrate groups are among the most diverse within soils (as with all ecosystems), and are integral for proper functioning of soil and litter processes (Paoletti et al, 2007).
Arthropod study in Ecology
Arthropods fill nearly every functional niche in ecosystems, ranging from predators to saprophages, and they occupy every stratum of terrestrial ecosystems.
Additionally, arthropods have been much used to test ecological models with a notable example being MacArthur and Wilson's (1967) island theory of biogeography, tested by Simberloff and Wilson (1969). Due to their life history traits including small size and (generally) high rates of reproduction, arthropod groups such as insects are ideal for testing ecological theory as they allow for microcosm designs and can be run for several generations within a useful timeframe
(Srivastava et al, 2004).
15 Arthropods provide useful contributions to ecosystem services, including pollination (e.g. hymenopterans), pest control (e.g. spiders and parasitoid wasps) and decomposition (e.g. beetles, mites, etc.). Arthropod contributions to decomposition generally take two forms; breakdown of coarse woody debris (e.g. Hammond, 1997;
Hammond et al, 2001) and breakdown of finer organic material (Paoletti et al,
2007). The former contribution to decomposition occurs in aboveground systems, while the latter typically occurs in the soil/litter system (although may also occur in suspended soils in the canopy).
The largest representative lineage of arthropods within soil and litter is that of the order Acari (the mites) (Behan-Pelletier and Newton, 1999). Worldwide there are an estimated 45,000 described species of mites, of which approximately 30,000 are found living within soils, and this is estimated to be only 5% of total mite diversity (Walter et al, 1996). Mites can inhabit every soil system on every continent in the world, regardless of altitude, latitude or soil quality, and can be found everywhere from the canopy to 10 m deep in soils (Walter, 1995). This cosmopolitan distribution (as well as extreme diversity) arises due to the ancient nature of the lineage—fossil mites date back to the mid-Devonian (~450 mya)
(Norton et al, 1988), and over this time period several genetic systems have evolved
(Norton et al, 1993).
Along with a vast phylogenetic diversity goes diversity of functional roles within the ecosystem, occupying a wide range of feeding guilds from predators to scavengers, all manner of plant feeders and different types of decomposers (Moore et al, 1988). As a consequence of this, life-history traits are equally varied;
16 Mesostigmatids and Prostigmatids (two suborders within Acari) are very similar to other arthropod groups, with high metabolic rates, high fecundities and short life spans (Norton, 1994). In contrast to this, members of the suborder Oribatida possess life-history traits more in common with k-selected organisms, such as low metabolic rates and low fecundities and long life spans (Norton, 1994).
Ecology of the Oribatid Mites
Oribatid mites are an extremely diverse group of animals, and in soils and litter, they are responsible for important contributions to the processes of decomposition and nutrient cycling (Seastedt, 1984). Most studies have been conducted on this group in Northern Europe, especially Germany (e.g. Maraun et al,
2003; Maraun and Scheu, 2000; Migge et al, 1998 etc.), while the few studies conducted in North America have been focused in Western Canada (e.g. Battigelli et al, 2004; Lindo and Visser, 2004; McLean and Parkinson, 2000) and the Southern
United States (e.g. Hansen, 2000). In north-temperate forests of Eastern North
America, there have been few inventories of mites, and a paucity of ecological/biodiversity research.
As important members of the decomposer community, oribatid mites can comprise approximately 50% of the total microarthropod diversity in many forest, grassland and desert systems (Seastedt, 1984). Within these systems, there is a general lack of understanding on the role of oribatid mites and functional groups based on assumed linkages with data known for a relatively few species (Behan-
17 Pelletier and Newton, 1999). Some general information about the mechanisms by which oribatid mites contribute to decomposition, however, is known.
Oribatids contribute to decomposition and nutrient cycling as secondary decomposers; they influence these processes through detrital feeding and by feeding on micro-organisms, chiefly bacteria and fungi (Verhoef and Brussaard, 1990;
Walter and Proctor, 1999). They aid directly in decomposition by shredding organic material into finer particles, which increases surface area for bacteria and fungi
(Lussenhop et al, 1991). Bacterial and fungal feeding helps to stimulate carbon mineralization by increasing the turnover rate, activity and respiration of bacterial colonies and fungal hyphae being fed upon (Mikola et al, 2002), whereas increased nitrogen mineralization and availability is a direct result of excretion of waste into the environment (Mikola et al., 2002).
Oribatid fecal pellets also contribute to the microstructure of the soil/litter environment, and help to distribute bacteria and fungi through the substrate as well as provide increased surface area for bacterial and fungal colonization (Lussenhop et al, 1991). The presence of oribatids within soils has also been noted to reduce the impact of plant pathogens within the system (Lussenhop et al, 1991), and oribatid assemblages appear to aid in the recovery of fungal communities after disturbance
(Maraune^a/., 1998).
Despite their overall importance to ecosystem functioning, very little is known of the causes for oribatid diversity and distribution (Osier et al, 2006).
While the biodiversity of oribatids is a common subject of study, work to determine
18 mechanisms that may be responsible for structuring oribatid communities has been conducted much less frequently (Maraun and Scheu, 2000).
The strongest response to environmental factors in structuring oribatid assemblages seems to result from disturbance to the soil/litter system itself. In a study designed to test differing rates of disturbance on oribatid assemblages, even bi weekly perturbation of the litter layer resulted in reduced densities of oribatids
(Maraun et al., 2003). Similarly, the presence of earthworms such as Dendrobaena octaedra has been observed to result in altered oribatid assemblages (positive correlation with diversity and richness and negative correlation with dominance in the L layer, negative correlation with abundance in the FH layer) (McLean and
Parkinson, 2000). This is possibly through disturbance or alteration of humus form and consumption of microarthropods and other soil invertebrates, which can disrupt food webs. Even the disturbance caused by simply walking through an area may be sufficient to impact oribatid assemblages (Garay and Nataf, 1982).
One of the common factors thought to structure oribatid mite assemblages is that of soil pH, as many species of oribatid mites appear to prefer more acidic soils
(Van Straalen et al, 1988). However Maraun and Scheu (2000) note that diversity in more basic soils can also be high, and that a more likely candidate responsible for structuring oribatid assemblages is humus form, which often correlates with pH. The authors note, however, that as many factors covary with humus form this serves little purpose in explaining the diversity and distribution of oribatids (Maraun and Scheu,
2000); they further comment that the amount and quality of food resources may help
19 to determine assemblage structure, but note that little work has been done to support this.
Battigelli et al. (2004) found that oribatid diversity (average number of species per sample) and densities were depressed with loss or delayed development of fungi within the organic layer of soils, and note that the loss of organic material would serve to increase microclimatic fluctuations. In contrast to this, increases in litter depth (organic matter cover) were found to result in decreased oribatid densities
(Osier et al., 2006). The composition of litter cover has been demonstrated to affect the structure of oribatid communities, with more complex litters (i.e. litters with inputs from multiple plant species) supporting more diverse and abundant assemblages (Hansen, 2000). In contrast with this, however, Kaneko et al. (2005) found that within three levels of canopy species richness (high, medium and low) there was no significant difference in the structure of their associated assemblages.
In a similar line of inquiry, the assemblage composition of oribatids was compared between beech and spruce stands, and was found to support similar assemblages
(Migge etal., 1998).
Initial studies report conflicting results on the importance of plant species diversity for structuring oribatid assemblages and isolating particular aspects of the environment for study has produced mixed success in identifying potential factors structuring oribatid assemblages. Returning to the question of differences in stand type over a larger variety of natural habitats may be beneficial to facilitate better determination of factors that may influence oribatid mite distributions. While many factors covary with stand type, being able to compare stands with differing
20 assemblage structures may help to reduce the number of possible factors to investigate.
Litter depth, itself, may also be a key factor in structuring Oribatid communities. An early study by Gill (1969) investigated the effects of litter as both a resource and regulator of microclimate using both natural and artificial litter
(Dacron), and found that the effects of litter on oribatid assemblages are more likely to be due to insulation or increased habitat space than to an increase in resources. A more recent study also found that increases in litter depth resulted in a shift in oribatid mite relative abundances for litter from a single tree species (Betula pubescens, Ehrhart) (Osier et al, 2006), however aboveground plant diversity has been suggested as an important structuring factor for oribatid assemblages (Hansen,
2000; St. John et al, 2006). As litter depth is a simple factor to manipulate, studies in more complex litters may be worth conducting.
21 THESIS OBJECTIVES AND RESEARCH QUESTIONS
I conducted my research in the Morgan Arboretum of McGill University, located on the western edge of the island of Montreal in south-western Quebec. The
Morgan Arboretum is a 254 ha research forest comprising a large variety of natural forest stands and plantations, with large sections dominated by a mixture of Sugar
Maple (Acer saccharum) and American Beech (Fagus grandifolia) characteristic of the region. The aim of this study is to investigate the effects of habitat type on oribatid diversity, abundance and assemblage composition. Previous work has suggested that tree species diversity may influence the structure of oribatid assemblages, with more complex litters supporting more diverse assemblages
(Hansen, 2000), and so I have selected a range of habitat sites including four forest stands of differing composition and tree species dominance and three open field sites including an agricultural field, an abandoned pasture being colonized by the surrounding forest and an unmanaged hay field.
In this thesis, Chapter 2 deals with the role of broad-scale factors due to different habitat types on oribatid mite assemblages. Chapter 3 investigates the more specific influence of a factor that varies with stand type, that of litter depth. Litter depth presumably influences several aspects of the soil/litter habitat, including the stabilization of microclimate and the total available habitat space available to organisms.
22 Analysis in both chapters will focus on determining any changes in the abundance, species richness and overall assemblage composition of oribatid mites due to changes in habitat type or litter depth.
In Chapter 2 my objective is to determine how habitat type influences litter- and soil-dwellins oribatid mites in and around the Morgan Arboretum
In this study, I investigate the following question: What are the effects of changes in habitat type on the abundance, species richness and assemblage composition of oribatid mites in the Morgan Arboretum?
Litter samples were collected from four forest stand types (greater than 50% beech cover, greater than 50% maple cover, mixed deciduous with no single dominant species and coniferous plantations) and soil samples were collected from the four forest stands and three open habitats (agricultural field, abandoned pasture being colonized by surrounding forest and unmanaged hay field). Samples were taken on three sample dates: 27 July and 1 September, 2005 and 27 June, 2006. The null hypothesis posits that there will be no difference in the relative abundance, species richness of oribatids, and that there will be a uniform community composition. I predict that there will be an effect of habitat type, however, and expect that forested sites will support more abundant and species rich assemblages than open sites, owing to an increase in ground cover and more species rich plant assemblages (Hansen, 2000) that will provide key microhabitats and protection from
23 changes in local microclimate. I also expect that each habitat type will support a characteristic and distinct assemblage composition.
In Chapter 3 my objective is to determine the effects of changes in litter depth on oribatid mite assemblages
In this experiment, I address the following question: What effects on the relative abundance, species richness and assemblage composition of oribatid mites result from changes in litter depth within two stand forest types in the Morgan
Arboretum?
Four litter depth treatments each were established in stands of American
Beech (control, complete litter removal, removal to 3 cm and a disturbance control at natural depth) and Sugar Maple (control, complete litter removal, addition to 6 cm and a disturbance control at natural depth); samples were taken on 5 July and 8
September, 2006. The null hypothesis is that there will be no change in oribatid communities due to changes in litter depth. I predict that beech stands will support more abundant and species rich assemblages than maple stands, as beech litter is naturally thicker within the arboretum, and decomposes more slowly than maple litter (Sariyildiz and Anderson, 2003). I also predict that relative abundance and species richness will increase with an increase in litter depth and decrease with higher rates of disturbance.
24 LITERATURE CITED
Bardgett, R.D. 2002. Causes and consequences of biological diversity in soil. Zoology. 105: 367-374
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30 CONNECTING STATEMENT
Chapter 1 provided a summary of the relevant literature regarding the importance of biodiversity, soil systems and oribatid mites. The knowledge of what factors control the diversity and distribution of oribatid mites is lacking, and general indicators of potential structuring factors are needed. Chapter 2 examines the general effect of habitat type on oribatid mite assemblages to help identify potential environmental factors for study to tease apart mechanisms underlying the structure of oribatid assemblages.
31 CHAPTER 2: Habitat type as a determinant of oribatid mite (Acari:
Oribatida) abundance, species richness and assemblage composition
ABSTRACT
Oribatid mites represent one of the most diverse arthropod groups within the soil/litter system and perform important roles in the processes of decomposition and nutrient cycling. Despite this importance, little is known of the mechanisms structuring oribatid assemblages. Oribatids were sampled from four forest stand types and three open field habitats to determine the importance of habitat type on the structure of oribatid assemblages within the Morgan Arboretum of McGill University in Montreal, Quebec.
Data from litter samples indicates that all four forest stand types supported significantly different abundances of oribatids. Species richness was highest in beech stands and coniferous plantations and significantly lower in maple and mixed deciduous stands. Ordination analysis revealed that each stand type supported distinctive assemblage compositions. This data supports the importance of stand type in structuring oribatid assemblages, and may prove useful in highlighting specific aspects of the environment that may drive these changes.
32 Introduction
The soil/litter system is one of the least well understood within ecology
(Behan-Pelletier and Newton, 1999), and yet soils are among the most important aspect of terrestrial systems. The processes of decomposition and nutrient cycling are performed within this system, thus providing integral ecosystem functions upon which terrestrial ecosystems are built (Bardgett, 2002). A large proportion of the overall diversity of terrestrial systems such as temperate forests is typically found in the soil/litter system (Adams and Wall, 2000; Bardgett, 2002; Behan-Pelletier and
Newton, 1999; Tsiafouli et al., 2005), and the majority of this biodiversity is unknown. This has led to the soil ecosystem being referred to as the "poor man's tropical rainforest" (Behan-Pelletier and Newton, 1999), as it represents an easily reachable biodiversity "hotspot" open to study.
Within the soil/litter system, most animals are members of the decomposer community (Hansen, 2000; Paoletti et al, 2007). This trophic group is responsible for converting the majority of primary production input from coarse sources such as fallen leaves and woody debris into more readily utilized nutrients within the soil
(Seastedt, 1984). Within temperate forests, the factors that control the organization of decomposer communities, and what aspects of the habitat favor or exclude the presence of a given species are not well understood (Kaneko et al., 2005).
Microarthropods comprise one of the most diverse groups of soil and litter fauna, and among the most diverse of these are mites of the order Acari (Behan-
Pelletier and Newton, 1999), which number over 30,000 species globally (Walter et al, 1996). The suborder Oribatida is the most numerous representative of Acari in
33 the soil/litter system, and these mites make essential contributions to the decomposition and nutrient cycling processes (Seastedt, 1984).
Over the past decade, there has been an increase in research about oribatid mite diversity, community structure, and ecological importance (e.g., see Battigelli et al, 2004; Hansen, 2000; Maraun and Scheu, 2000; Maraun et al, 2003 etc.). In particular, understanding mechanisms behind the population dynamics of these organisms is important in order to understand how the soil habitat interacts with the remainder of terrestrial systems, how nutrient cycling can be improved, and can potentially have impacts on conservation (Migge et al, 1998). Owing to their k- selected life history traits (Norton 1994), it has been suggested that oribatid mites maybe suitable as bioindicators of ecosystem health (Paoletti et al, 2007). Further, understanding what dictates the composition of oribatid communities may help in improving industries such as agriculture and silviculture by promoting higher rates of nutrient cycling (Battigelli et al, 2004).
Previous work regarding potential organizing factors of oribatid communities have served to highlight that although populations of oribatids tend to be stable within the soil/litter system, they remain tenuous, as simple mechanical disturbance in the form of mixing litter layers has been shown to have a significant detrimental effect on many species (Maraun et al, 2003). Other research has focused more directly on the habitat itself, such as examining the effects of soil pH, humus form
(Maraun and Scheu, 2000) and litter complexity on oribatid communities. The latter has shown that litters consisting of inputs from multiple plant species support more species rich assemblages than monocultures (Hansen, 2000), but little work has been
34 done to examine what effect litter mixtures composed of differing tree species inputs
(such as changes between types of forest stands) have on oribatid assemblages, especially in a natural setting such as those found within different habitat types in forested ecosystems.
I examined whether habitat type influences the species richness, relative abundance and community composition of litter- and soil-dwelling oribatid mites in and around the forests of the Morgan Arboretum in southwestern Quebec. Litter- dwelling oribatid assemblages were compared within and between forested habitat types, and soil-dwelling oribatid assemblages were compared within and between forested and open habitats; I predict that forested habitats will support more abundant and species rich assemblages than open sites, owing to an increase in ground cover and diverse plant species (Hansen 2000), that provide microhabitats for mites and protection from local climate change (e.g. seasonal temperature fluctuations). I also expect community composition to be characteristic and distinct by stand type.
Methods
Study site - The study was carried out at the Morgan Arboretum of the Macdonald campus of McGill University, located at the western tip of the island of Montreal, in
Southwest Quebec, Canada (45 25'N, 73 56'W). The Arboretum encompasses 254 ha, and my sample sites were located throughout both forested areas and adjacent open habitats. Dominant forest cover is American Beech (Fagus grandifolia
Ehrenberg) and Sugar Maple {Acer saccharum Marshall), with interspersed stands of other tree species and isolated plantations, notably conifers such as Red Pine (Pinus
35 resinosa Solander and Aiton) and Norway Spruce (Picea abies L.). Open sites ranged from meadows in various states of succession to those actively used for agricultural production of corn (Zea mays L.) and alfalfa (Medicago sativa L.).
Design - Four forest stand types (beech dominant, sugar maple dominant, mixed deciduous and mixed coniferous) and three field types (corn field, open meadow and abandoned pasture) were selected for sampling, and within each habitat type four replicated sites were established for a total of 16 forest sites and 12 field sites (Fig.
2.1). All forest stands were located within the Morgan Arboretum, and the agricultural sites were located approximately a kilometer to the southwest.
For forested sites, stand type was determined by calculating percent cover for each tree species present within 10 m of the site marker. This was accomplished by calculating the total diameter at breast height (dbh, measured 1.4 m above the ground) of each tree within the 10 m radius, and then by calculating the percentage contribution each species made to the overall site cover. American Beech and Sugar
Maple stands comprised a minimum of 50% cover of the respective tree species, while mixed deciduous stands had no single dominant tree species present. Each site was approximately 0.01 ha in size, with a minimum distance of 40 m between sites.
Open sites within the arboretum were located within two field areas; an abandoned pasture gradually being colonized by the surrounding forest located in the northeast portion of the arboretum, and a hayfield left unmanaged located in the northern portion. Agricultural sites were located to the southwest of the arboretum in a field planted with corn in the summer of 2005 and alfalfa in 2006—sites at this field were
36 located by measuring distances from field edges, to standardize locations between sampling periods. All open sites were approximately 25 m2 in size, and separated by a minimum distance of 15 m.
Sampling protocol - Samples were taken from each site on 27 July and 1 September in 2005 and 27 June in 2006. Soil samples were taken from sites within all habitat types, and litter samples were taken only from forested sites. Litter samples
(material comprising fallen leaves and woody debris) were taken by manually removing 1 L (densely packed) of leaf litter from the LFH horizons, including loose litter fragments down to the soil surface. Soil samples were taken from the O and A horizons using a bulb-planter to obtain a soil core 6 cm deep and 6 cm in diameter
(~170 cm ). All samples were taken haphazardly within a 2 m radius from the marker denoting each site. Samples from agricultural fields were taken haphazardly within a 2 m radius from a central pre-determined point measured out from field edges prior to the first sample date.
Oribatid mites were extracted from litter samples using modified Tullgren funnels (Crossley and Blair, 1991) for 5 days at a temperature of 32° C ± 1° C. Soil samples were extracted in a modified Kempson apparatus (Kempson et ah, 1963), also kept at 32° C ± 1° C for 5 days. Mites were extracted by gradually drying out the soil and litter substrate and causing a downward migration as the organisms continually descend through the material to avoid desiccation, eventually falling into a collection vial containing 70% ethanol for preservation and storage.
37 All specimens were identified to species using a Leica DM2500 compound- light microscope and a Nikon SMZ1500 dissecting microscope, with various taxonomic keys (e.g., Balogh, 1983; Niedbala, 2002; Weigmann, 2006) as well as unpublished keys compiled by Drs. R. Norton (SUNY-ESF, Syracuse) and V.
Behan-Pelletier (Agriculture and Agri-Food Canada, Ottawa) and material assembled from Marshall et al, 1987. Identified specimens were labeled and stored in 70% ethanol, and Dr. V. Behan-Pelletier confirmed identifications. Voucher specimens were deposited at the Lyman Entomological Museum (Ste. Anne de Bellevue,
Quebec).
Statistical methods - For all analyses, soil and litter samples were treated separately.
Analyses for changes in species richness and abundance were done using Analysis of
Variance (ANOVA) using general linear model (GLM) procedures in SAS software version 8 (SAS Institute, 1999). Data were analyzed using the two-factor model y = habitat + date with the main effects being habitat type and sample date, and a post hoc Tukey's test (a = 0.05) was used to compare differences between individual means. Data were tested to meet the assumptions of parametric statistics, and transformed if required.
Raw species richness is not always the best diversity metric since it is sensitive to sample size (Buddie et al., 2005). Therefore, standardized species richness estimates were computed using Individual-based rarefaction analysis, using
Ecosim Version 7.2 (Gotelli and Entsminger, 2004). Rarefaction techniques allow comparisons between sample types at the maximum abundance of the least abundant
38 sample by repeated Monte Carlo subsampling at pre-designated abundance points.
Resulting curves allow for assessing whether sampling effort was sufficient, can provide estimates of species richness by habitat type, and associated estimates of variance allow for statistical comparisons.
Assemblage composition was compared between stand types using nonmetric multidimensional scaling (NMS) ordinations and the Bray-Sorenson distance measure. NMS is a non-parametric ordination technique that makes no a-priori assumptions of spatial relationships and utilizes rank order information to compare similarities between samples (McCune and Grace, 2002). Ordinations were conducted in PCOrd version 4.17 (McCune and Mefford, 1999) using log- transformed data, and a Detrended Correspondence Analysis (DCA) ordination was used for the starting configuration rather than random starting coordinates to reduce stress levels and avoid local minima (Work and McCullough, 2000). An initial six- dimensional ordination was run to aid in selection of the number of axes for the final ordination solution, as well as to evaluate stress reduction, and a Monte Carlo test was used to assess significance of each axis (McCune and Grace, 2002).
Multi-response permutation procedures (MRPP) analysis was performed on log-transformed data using PCOrd version 4.17 (McCune and Mefford, 1999).
MRPP is a nonparametric procedure used to test hypotheses of no difference between groups, which are required to be pre-existing (McCune and Grace, 2002). This form of analysis is more powerful than other similar tests such as MANOVA, as MRPP has no requirement of normality or homogeneity of variance, assumptions which are rarely met by ecological data (McCune and Grace, 2002). For this study, habitat
39 types were used to designate groups, and the Sorenson (Bray-Curtis) distance measure was used to analyze differences between groups.
Indicator species analysis (Dufrene and Legendre, 1997) was used to assess affinities between species and habitat types, using the software PCOrd version 4.17
(McCune and Mefford, 1999). Statistical significance of indicator values were tested using a Monte Carlo test of 1000 runs. Indicator species analysis is an ideal pair with MRPP techniques, as an aid in describing how well species separate among groups (McCune and Grace, 2002).
Results
A total of 7,112 individuals within 53 species was collected for this study
(Appendix 1). Litter samples yielded 6920 individuals representing 51 species, soil samples yielded 192 individuals and 26 species, and 24 species were shared between the two strata. More individuals (3375) were collected in June of 2006 than on the other sampling dates. The most abundant species collected was Anachipteria magnilamellata (Ewing) with 810 individuals (11.4% of total individuals sampled).
Four species were collected only once in the study (i.e. singletons) and 3 species were collected only twice (doubletons). Of the ten most abundant species collected in this study (Fig. 2.2), most were found in the coniferous stands.
Abundance and Species Richness
My first objective was to compare oribatid abundance and species richness by habitat type. Results from ANOVA tests (Table 2.1) indicated a significant effect of
40 habitat type and sample date on litter mite abundance (Fig. 2.3a) and species richness
(Fig. 2.3b). No significant effects of habitat type were observed for oribatids collected from soil samples.
Within the litter layer, post hoc Tukey's test comparisons revealed that stand types had different mite abundances (all at p < 0.05) (Fig. 2.3a). Mite abundance was greater in June 2006 (mean log abundance ± SE = 4.73 ±0.194 individuals) than
July 2005 (3.92 ± 0.194), although mite abundance in September 2005 (4.25 ± 0.201) was not different from that in July 2005 or June 2006.
Post hoc Tukey's test comparisons of litter species richness data revealed that coniferous and beech stands were significantly different from mixed deciduous and maple stands, the latter two of which were not significantly different from each other
(all at p < 0.05) (Fig. 2.3b). Species richness was greater in June 2006 (2.45 ± 0.066 species per 1000 cm 3) than July 2005 (2.45 ± 0.066 species per 1000 cm"3) and
September 2005 (2.19 ± 0.068 species per 1000 cm"3).
There were too few mites to perform rarefaction analyses on soil samples; therefore I restricted this approach to litter samples. Since ANOVA results indicate significant effects of sample date for mite abundance and species richness, data were kept separate by sample date. There were enough mites in coniferous and beech stands for all sample dates to generate rarefaction curves with an asymptote (Fig.
2.4), but it was not possible to assess rarefied species richness in maple and mixed stands. Overall, beech stands supported the greatest species richness (rarefied estimates), with significantly more species than coniferous stands; this pattern was consistent for all sample dates (Figs 2.4a, b and c). Litter in maple and mixed stands
41 seemed to contain fewer oribatid species than beech and confierous stands, but additional sampling would be required to verify this pattern (Fig. 2.4).
Community composition
Ordination analyses were used to compare the compositional differences of oribatid mite assemblages in various habitats. Mites from soil samples were too few to warrant multivariate analyses, so the NMS ordination represents mites from litter collected in forest stands (data were pooled among sampling dates within each stand). The NMS explained 87.4% of total variation within the data set and stands clearly cluster apart, with the exception of one mixed-deciduous site (Fig. 2.5).
Results of MRPP analysis indicate that stands of beech support assemblages that are significantly different from those of all other stand types, as do coniferous stands; maple and mixed deciduous stands, however, support similar assemblages (Table
2.2). Indicator species analysis revealed five species with significant affinities to beech stands and four species associated with coniferous stands (a < 0.05) (Table
2.3). Two species of oribatids, Nanhermannia elegantula (Berlese) and
Trhypochthonius americanus (Ewing) were only found in beech stands (IV = 100.0)
(Table 2.3).
Discussion
This study revealed changes in abundance, species richness and assemblage composition of litter-dwelling oribatid mites within different forest stand types.
These findings supported my prediction that the composition of oribatid assemblages would be distinctive by stand type; due to the extremely low numbers of oribatids collected from soil samples, I was unable to test my prediction of whether forested
42 sites are more abundant and species rich than open sites. These results indicate that even an environmental factor on coarse scales such as forest stand type can be important in structuring oribatid assemblages.
Previous studies have sought to explore the role of aboveground plant richness on belowground oribatid assemblages (Hansen, 2000; St. John et ah, 2006).
The authors of these studies found that more diverse aboveground assemblages support more diverse mite assemblages, however my results suggest this pattern may be more complicated than it originally appears. Coniferous stands consisting of only two tree species per site supported the most abundant oribatid assemblages of all stand types and were among the most species rich. This seems contradictory to previous findings where more complex litters support more diverse assemblages, but this contradiction may be due to the thick nature of the coniferous litter at the
Morgan Arboretum. This thickness may simulate an increase in habitat space provided by more complex litters, and habitat heterogeneity may be established by varying stages of litter decomposition rather than solely through inputs from many tree species.
The high species richness in what might otherwise be considered a simple litter may also be due to differences in humus form. Humus form may control oribatid assemblage composition, although what mechanisms might be responsible in this association are unclear (Maraun and Scheu, 2000). In this study, maple and mixed deciduous stands tended to feature less dense litters of approximately 3 cm in depth, while coniferous plantations and beech stands both featured dense litters approximately 5 cm or more in depth (personal observation). As rarefaction curves
43 display, sampling effort in maple and mixed stands was lower than in beech and coniferous stands and this may serve to explain why a "simple" litter supported a more diverse assemblage than other more "complex" litters. Rarefaction analysis also indicates that beech stands are more species rich than coniferous stands, and in general appears to suggest that coniferous stands may not be statistically different from at least maple stands, although again, the low sampling effort in maple and mixed stands makes such assessments difficult.
Indicator species analysis found nine species to be significantly related to specific forest stand types, two of which were only found within beech stands, further highlighting the influence of habitat type on oribatid mite assemblages. The strong effects revealed by multivariate analysis demonstrate that while specific aspects of the soil/litter system such as disturbance (Maraun et ah, 2003), presence of earthworms (McLean and Parkinson, 2000) and litter complexity (Hansen 2000) are important in structuring oribatid assemblages, the influence of broader-scale factors such as habitat type cannot be overlooked. Coarse factors such as forest stand type may not specify what mechanisms underlay the differences in assemblage structure and drive changes in abundance or species richness; however they may help to pinpoint habitat specific factors (such as moisture regime, soil pH, soil type and others) that can lead to a better understanding of oribatid mite distributions. When paired with an understanding of oribatid mite life-history traits, knowledge of the effects of different forest stands on oribatid mite assemblages may help to tease out particular environmental factors to be tested for influence in structuring oribatid mite communities.
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47 Table 2.1: ANOVA comparing oribatid species richness and oribatid abundance in soil (four forest stand and three open field site types) and litter (four forest stand site types) samples. Data were log transformed prior to analysis, and bold type denotes significant effects. Data set Treatment Model MS ± S.E F, D.F. F P-value Oribatid abundance Litter Habitat type 13.399 ± 0.2268 F3.41 22.21 <0.0001 Sample date 2.668 + 0.1965 ^2,41 4.42 0.0182 Oribatid richness Habitat type 0.7796 ± 0.0771 F3.41 11.20 <0.0001 Sample date 0.3184 ± 0.0667 ^2,41 4.57 0.0161 Oribatid abundance Soil Habitat type 0.5809 + 0.2161 F6.50 1.72 0.137 Sample date 0.7471 ±0.1395 F2.50 2.21 0.121 Oribatid richness Habitat type 0.1393 + 0.1260 F6.50 1.21 0.317 Sample date 0.1541 ±0.0814 F2.50 1.34 0.271
Table 2.2: MRPP of oribatid assemblage composition in litter from four forest stand types at the Morgan Arboretum. Differences between groups (A) were evaluated with the Sorenson (Bray-Curtis) distance measure. Data were log transformed prior to analysis, and bold type denotes significant effects. Grouping variable A P-value Beech vs. Maple 0.3102 0.006 Beech vs. Mixed 0.4074 0.005 Beech vs. Coniferous 0.3148 0.004 Maple vs. Mixed 0.1250 0.076 Maple vs. Coniferous 0.4167 0.006 Mixed vs. Coniferous 0.3102 0.007
48 Table 2.3: Indicator species analysis reporting significant indicator values (a < 0.05) for oribatid mites by forest stand type. Data were pooled among dates. Monte Carlo test of 1000 runs used to test significance of maximum indicator value (IV). Species Family Genus Species Author Max habitat Mean Std. Dev. IV code Atrstr Phthiracaridae Atropacarus striculus (C.L. Koch) coniferous 33.0 3.99 44.4 Chacus Chamobatidae Chamobates cuspidatus (Michael) coniferous 33.3 9.41 57.5 Eregra Eremobelbidae Eremobelba gracilior Berlese beech 30.4 13.04 88.4 Nanele Nanhermanniidae Nanhermannia elegantula Berlese beech 26.8 13.27 100 Oppnov Oppiidae Oppiella nova (Oudemans) beech 29.0 11.52 61.4 Opprig Oppiidae Oppia nr. rigida (Ewing) beech 31.4 10.53 55.0 Phtbor Phthiracaridae Phthiracarus boresetosus Jacot coniferous 30.1 2.56 38.6 Rhyard Euphthiracaridae Rhysotritia ardua (C.L. Koch) coniferous 30.5 2.66 35.4 Trhame Trhypochthoniidae Trhypochthonius americanus (Ewing) beech 28.2 13.97 100 Fig. 2.1: Site locations within the Morgan arboretum of southwestern Quebec. The solid line indicates border of the Arboretum. Agricultural samples were taken from a corn field 1.6 km southwest of the Aboretum.
50 1000 I I Total abundance i^^ Beech abundance K&%a Coniferous abundance 800 A R^^l Maple abundance tn CO Mixed deciduous abundance
> S 600 _C %, CD O C CD 400 "O c 3 .Q < 200
Figure 2.2: Rank abundance curve for the ten most common species (full names can be found in Appendix 2.1) collected from all samples, and by contribution by forest stand type.
51 a)
LU CO I
CO A t E o B o 5H o T o a> o c I D CO •co JD CO O) O
^ 1 H co
O o c,\o o\o # * ^ / ^ o° # ^ A $ 2.5 - T .1. LU B CO T -: B ?,o - T to CD 1 c o 1.5 • a) o a> a. m 1.0 - D) o 0.5 - «$T J? J? Fig. 2.3: Means for a) log abundance (± SE) and b) log species richness (± SE) of oribatid mites collected from leaf litter in four forest stands at the Morgan Arboretum; log-transformed data was used in analysis, and significant differences (p < 0.05) indicated by different letters above error bars. 52 a) 26 - 24 - 22 • Beech cover > 50% Coniferous plantations Maple cover > 50% Mixed deciduous 200 300 400 Individuals 26 - 24 - 22 - Q " 20- ^ 18 • w Ui c(U 16 - 03 '8 12- Q. CO Beech cover > 50% Coniferous plantations Maple cover > 50% Mixed deciduous 600 800 1000 1200 1400 1600 Individuals Q " 30-^ Beech cover > 50% Coniferous plantations Maple cover > 50% Mixed Deciduous Individuals Fig. 2.4: Rarefaction curve showing species richness (+ S.D.) of oribatid mites collected from leaf litter in a) July 2005, b) September 2005,- and c) June 2006, separated by habitat type. 53 • Beech cover > 50% O Conifer plantations T Maple cover > 50% A Mixed deciduous (no species > 50%) Fig. 2.5: Three-dimensional non-metric multidimensional scaling (NMS) ordination of log-transformed oribatid mite data pooled by date. Percent variation by axis shown in figure (final stress = 8.61, axis 1: p = 0.020, axis 2: p = 0.020, axis 3: p = 0.020). 54 Appendix 2.1: Oribatid mites collected from soil and litter samples within seven habitat types at the Morgan Arboretum of McGill University in Montreal, Quebec Litter Soil Code Family Genus Species Author Beech Coniferous Maple Mixed Beech Coniferous Maple Mixed pasture Hay Agr. Total Aclicol Achipteriidae Achipteria coleoptrata L. 0 183 0 0 0 0 0 0 0 0 0 183 Adraiag Gymnodamaeidae Adrodamaeus magnisetosus (Ewing) 7 0 0 0 0 0 0 0 0 0 0 7 Anamag Achipteriidae Anachipteria magnilamellata (Ewing) 29 742 3 0 0 36 0 0 0 0 0 810 Arcros Mesoplophoridae Archoplophora rostralis Willmann 48 0 0 0 0 0 0 0 0 0 0 48 Atrstr Phthiracaridae Atropacarus striculus (C.L. Koch) 58 544 18 33 0 3 1 4 0 0 0 661 Carpol Carabodidae Carabodes polyporetes Reeves 11 19 1 0 0 0 0 0 0 0 0 31 Cepcor Cepheidae Cepheus corae Jacot 6 1 0 0 0 0 0 0 0 0 0 7 Cergra Ceratozetidae Ceratozetes gracilis (Michael) 12 33 1 14 1 0 0 1 0 3 4 69 Cenned Ceratozetidae Ceratozetes mediocris Berlese 0 8 0 0 0 0 0 0 0 0 0 8 Chacus Chamobatidae Chamobates cuspidatus (Michael) 174 485 1 46 5 0 0 3 1 0 0 715 Culbic Astegistidae Cultroribula bicultrata (Berlese) 0 10 0 0 0 0 0 0 0 0 0 10 Diahum Ceratozetidae Diapterobates humeralis (Hermann) 18 0 0 0 3 0 0 0 0 0 0 21 Enimin Eniochthoniidae Eniochthonius minutissimus (Berlese) 36 6 0 0 0 0 0 0 0 0 0 42 Epican . Damaeidae Epidamaeus canadensis (Banks) 1 0 0 0 0 0 0 0 0 0 0 1 Epispl Damaeidae Epidamaeus spl 1 0 0 0 0 0 0 0 0 0 0 1 Eregra Eremobelbidae Eremobelba gracilior Berlese 160 0 3 0 0 0 1 0 0 0 0 164 Eupfla Euphthiracaridae Euphthiracaridae flavus (Ewing) 2 0 0 0 0 0 0 0 0 0 0 2 Euzspl Euzetidae Euzetes spl 34 159 86 17 2 5 8 1 0 3 0 315 Guspar Gustaviidae Gustavia parvula (Banks) 0 0 0 0 0 0 0 0 0 1 0 1 Gymom Gymnodamaeidae Gymnodamaeus ornatus Hammer 5 0 0 0 1 0 0 0 0 0 0 6 Herocc Hermanniellidae Hermanniella occidentalis Ewing 6 0 0 0 0 0 0 0 0 0 0 6 Hypruf Hypochthoniidae Hypochthonius rufulus C.L. Koch 12 58 41 2 0 0 1 0 0 1 0 115 Lepsin Tegoribatidae Lepidozetes singularis Berlese 0 0 0 0 5 0 1 0 0 0 0 6 Liaspl Liacaridae Liacarus spl 11 0 11 3 0 0 0 0 0 1 0 26 Liasp2 Liacaridae Liacarus sp2 4 0 0 0 0 0 0 1 0 0 0 5 Liasp3 Liacaridae Liacarus sp3 8 0 2 1 0 0 0 0 0 0 0 11 Myccon Mycobatidae Mycobates conitus Hammer 37 0 0 0 0 0 0 0 1 0 0 38 Nanele Nanhermanniidae Nanhermannia elegantula Berlese 27 0 0 0 1 0 0 0 0 0 0 28 Notaiia Nothridae Nothrus anauniensis Can. & 0 2 7 2 0 0 0 0 0 0 0 11 Fanzago Notbor Nothridae Nothrus borussicus Sellnick 0 2 0 0 0 0 0 0 0 0 0 2 Notmon . Nothridae Nothrus monodactylus (Berlese) 5 8 0 3 0 0 0 0 0 1 0 17 Notpal Nothridae Nothrus palustris C.L. Koch 1 11 1 0 0 0 0 0 0 0 0 13 Notpra Nothridae Nothrus pratensis Sellnick 3 51 0 0 0 0 0 0 0 0 0 54 Notsil Nothridae Nothrus silvestris Nicolet 15 0 1 0 0 0 0 0 0 0 0 16 Litter Soil Code Family Genus Species Author Beech Coniferous Maple Mixed Beech Coniferous Maple Mixed Pasture Hay Agr. Total Oppnov Oppiidae Oppiella nova (Oudemans) 178 66 0 0 4 0 2 0 0 0 0 250 Opprig Oppiidae Oppia nr. rigida (Ewing) 86 16 33 2 2 1 2 0 0 0 0 142 Oppspl Oppiidae Oppia spl 3 0 0 0 0 0 0 0 0 0 0 3 Oppsub Oppiidae Oppiella subpectinata (Oudemans) 3 15 2 2 1 0 0 0 0 0 0 23 Oricar Oribotritiidae Oribotritia carolinae Jacot 33 5 4 1 0 0 0 0 0 0 0 43 Oriden Oribatulidae Oribatula dentaticuspis Ewing 2 0 0 0 0 0 0 0 0 0 0 2 Oriqua Oribatellidae Oribatella quadricornuta (Michael) 49 0 0 0 0 0 0 0 0 0 0 49 Oritib Oribatulidae Oribatula tibialis (Nicolet) 50 62 99 0 0 0 0 0 0 0 0 211 Perema Galumnidae Pergalumna nr. emarginata (Banks) 17 51 96 27 2 0 10 1 0 3 0 207 Phtbor Phthiracaridae Phthiracarus boresetosus Jacot 68 531 50 76 1 1 3 5 0 0 0 735 Phtlon Phthiracaridae Phthiracarus longulus (C.L. Koch) 9 133 4 8 0 0 0 1 0 0 0 155 Pilbin Galumnidae Pilogalumna nr. bindalares (Jacot) 109 277 154 30 0 1 4 8 0 0 0 590 Plapel Camisiidae Platynothrus peltifer (C.L. Koch) 108 8 0 0 0 0 0 0 0 0 0 116 Rhyard Euphthiracaridae Rhysotritia ardua (C.L. Koch) 21 107 35 26 0 0 2 0 0 0 0 191 Schpal Scheloribatidae Scheloribates pallidulus (C.L. Koch) 261 340 30 7 3 3 2 0 3 1 8 658 Schspl Scheloribatidae Scheloribates spl 13 0 0 0 0 0 0 0 0 0 0 13 Sucfro Suctobelbidae Suctobelbella frothinghami Jacot 0 0 1 0 0 0 0 0 0 0 0 1 Tecvel Tectocepheidae Tectocepheus velatus (Michael) 152 24 23 36 3 1 0 3 1 2 1 246 Trhame Trhypochthoniidae Trhypochthonius americanus (Ewing) 27 0 0 0 0 0 0 0 0 0 0 27 56 CONNECTING STATEMENT Chapter 2 revealed that, at least for forested sites, habitat type does influence the structure of oribatid communities. Chapter 3 focuses on one aspect of the environment that varied by habitat type, litter depth, and examines what effect changes in litter depth might have on oribatid assemblages. 57 CHAPTER 3: The effects of Utter depth on oribatid mite (Acari: Oribatida) assemblages in a beech-maple forest in southwestern Quebec ABSTRACT Oribatid mites are important members of the decomposer community in soil and litter. Despite this importance, mechanisms underlying their assemblage structure are poorly understood. This experiment examined the effect of litter depth on oribatid assemblages; litter treatments consisting of a control, disturbance at natural depth, complete removal and either a partial removal or addition were established in the Morgan Arboretum of McGill University in Montreal, Quebec. Treatments were established in two forest stand types dominated by either American Beech (Fagus grandifolia) or Sugar Maple {Acer saccharum). Analysis of relative abundance and species richness within litter depth treatments revealed no significant effects for oribatids. Ordination analysis revealed a significant effect of stand type, but not of litter depth. These results suggest that while forest stand type is important in structuring oribatid assemblages, the litter depth within stands is not, and thus some other factor varying with stand type must be responsible. 58 Introduction The Acari (mites and ticks) represent a large proportion of the total soil animal diversity, especially in temperate forests; over 45,000 species have been described worldwide, of which 30,000 are found in the soil and litter ecosystem (Walter et al, 1996), although this is thought to represent only 5% of total mite diversity (Walter and Proctor, 1999). Mites are represented in nearly every feeding guild (Moore et al, 1988), and are represented by the suborder Oribatida within the decomposer community in soils and litter. Oribatids are ubiquitous in the soil and litter of temperate forests, and are important secondary decomposers (Evans, 1992). These contributions to ecosystem functioning have been demonstrated to greatly contribute to efficiency of nutrient cycling (Seastedt, 1984) and help to promote resilience within the soil system (Maraun et al, 1998). Relatively little is known about oribatid ecology or the factors responsible for structuring oribatid assemblages. Life history traits and memberships within feeding guilds are extrapolated from data on few species, and while functional redundancy of oribatid contributions to ecosystem function have been suggested, this is largely assumed with little data in support (Behan-Pelletier and Newton, 1999). Factors responsible for structuring oribatid communities are only beginning to be understood (Maraun and Scheu, 2000), making differences in assemblage structures between sites difficult to account for. Abiotic factors have been demonstrated to have impacts on structuring oribatid assemblages, notably the litter layer that comprises the habitat itself (Gill, 1969; Osier et al, 2006; Ponge, 2003). The importance of litter has been suggested 59 to be primarily due to the quality and resource availability within the litter (Scheu et al, 2003), however the importance of litter for providing habitat space has also been suggested (Hansen, 2000). Litter has the added benefit of being easily manipulated to examine effects on oribatid assemblages. Maraun et al. (2003) demonstrated that disturbance to the litter layer of the soil/litter system significantly reduced oribatid diversity and densities. Having input from multiple tree species to the litter layer has also been shown to result in more diverse assemblages than simpler litters (Hansen, 2000). Litter depth is also a key factor likely to influence mite assemblages. The depth of litter will have an insulating effect on the underlying layers of the habitat as well as minimizing fluctuations in microclimatic variables and deeper, more decomposed litters are likely to also provide increased habitat space and nutrients. Gill (1969) investigated the effects of litter both as a resource and as a regulator of microclimate, using treatments of Dacron at natural litter depth. The effect of litter in this study was found to be more likely due to insulating properties or habitat space availability than resource availability (Gill, 1969). Osier et al. (2006) examined direct effects of litter mass on oribatid assemblages using six litter treatments (1, 2, 4, 8 and 12 times natural litter mass, as well as litter removal) within a mature birch woodland in northeastern Scotland. They showed that distinct assemblages were found within the soil and litter layers, and that within the litter layer density of oribatids displayed a negative relationship with increasing litter mass, with an accompanying shift in relative abundance of species (Osier et al., 2006). The study used litter input from birch trees (Betula 60 pubescens, Ehrhart), and so according to Hansen (2000), may support less abundant and diverse assemblages than might be found in a more natural forest habitat containing multiple tree species. I investigated the effects of changes in litter depth on the relative abundance, species richness and assemblage composition of oribatid mites in the soil and litter of a beech-maple forest in southwestern Quebec. Assemblages within three litter depth treatments were compared both within and between beech and maple stands, while soil assemblages were compared for four treatments within and between both stand types. I predict that beech stands will support more abundant and species rich assemblages than maple stands, as the litter of the former is naturally thicker at my study sites and decomposes more slowly than litter of other species (Sariyildiz and Anderson, 2003). I also expect to find a lower abundance and lower species richness as litter treatments increase in disturbance and decrease in depth. These changes should be mirrored in soil assemblages, as disturbance and decreases in litter depth can potentially alter the microclimate of the soil habitat. Methods Study site - The study was carried out at the Morgan Arboretum of the Macdonald campus of McGill University, located at the western tip of the island of Montreal, in Southwest Quebec, Canada (45 25'N, 73 56'W). The arboretum covers 254 ha with a dominant forest cover of American Beech (Fagus grandifolia, Ehrenberg) and Sugar Maple (Acer saccharum, Marshall), and contains stands of other tree species and isolated plantations, notably coniferous species such as Red Pine [Pinus resinosa, Solander and Aiton) and Norway Spruce (Picea abies, L.). 61 Design - Litter depth manipulations were established in two types of forest stands, one dominated by American Beech and the other dominated by Sugar Maple (Fig. 3.1). Sites were considered species-dominant if the respective tree species comprised greater than 50% of the total diameter at breast height (dbh, measured 1.4 m above the ground) for all trees within the stand. Four replicated sites were established for each stand type (Fig. 3.1), and each site was approximately 0.01 ha in size with a minimum distance of 40 m between sites. Seven total litter treatments (four per stand type) were used for this study. Three treatments were unique to each stand type, consisting of a natural litter depth (6 cm for beech stands and 3 cm for maple), a depth manipulation (reduction of litter to 3 cm for beech and addition to 6 cm for maple), and a disturbance manipulation at natural depth, with one treatment (complete litter removal) held in common (Table 3.1). This difference in treatments was done to compare assemblage characteristics of natural depth between stand types. The depth manipulation and disturbance control treatments were established by first removing all litter within the enclosure, mixing the litter layer and then depositing litter to the required depth of the treatment. This was done so that no stratum of litter containing more oribatids by chance was likely to be introduced to any one depth manipulation treatment, with the disturbance control treatments serving to account for potential effects of this disruption when compared to the control treatments. Litter treatments were established in enclosures 60 cm square, delimited with pieces of aluminum flashing 20 cm square and half-buried in the soil to provide a 10 cm barrier above and a 10cm barrier below the soil surface. Breaks were left in each of the enclosure walls 10 cm 62 wide in the center and 10 cm on either corner in order to allow colonization of litter after the treatments were created. Litter manipulations were done on 21 and 26 April 2006. Sampling protocol - Samples were collected from each site on 5 July and 8 September 2006. Soil samples were taken from all sites and treatments, and litter samples were taken only from treatments with litter present. Litter samples (collected material comprising fallen leaves and woody debris) were taken by manually removing 1 L of leaf litter from the LFH horizon, including all loose litter fragments down to the soil surface. Soil samples were taken from the O and A horizons using a bulb-planter to obtain a core 6 cm deep and 6 cm in diameter (~170cm ). To remove the effects of sampling interference between dates, aluminum flashing was used to designate quadrats within each enclosure 30 cm on a side. Samples for July 5 were taken haphazardly from the center of one quadrant of each enclosure, with samples from September 8 being taken from the quadrat located diagonally from that used for the July 5 sampling date. Oribatid mites were extracted from litter samples using modified Tullgren funnels (Crossley and Blair, 1991) for 5 days at a temperature of 32° C ± 1° C. Soil samples were extracted in a modified Kempson apparatus (Kempson et al, 1963), also kept at 32° C ± 1° C for 5 days. These extractors work by gradually drying out the soil and litter substrate, causing microarthropods such as oribatid mites to continually descend through the substrate to avoid desiccation, eventually falling into a collection vial containing 70% ethanol for preservation and storage. 63 All specimens were identified to species using a Leica DM2500 compound- light microscope and a Nikon SMZ1500 dissecting microscope, with various taxonomic keys (e.g., Balogh, 1983; Niedbala, 2002 and Weigmann, 2006) as well as unpublished keys compiled by Drs. R. Norton (SUNY-ESF, Syracuse) and V. Behan-Pelletier (Agriculture and Agri-Food Canada, Ottawa) and material assembled from Marshall et al. (1987). Identified specimens were labeled and stored in 70% ethanol, and Dr. V. Behan-Pelletier confirmed identifications. Voucher specimens were deposited at the Lyman Entomological Museum (Ste Anne de Bellevue, Quebec). Statistical methods - For all analyses, soil and litter samples were treated separately. Analyses for changes in species richness and abundance were done using Analysis of Variance (ANOVA) using general linear model (GLM) procedures in SAS software version 8 (SAS Institute, 1999). Data were analyzed using the two-factor model y = treatment + date with the main effects being litter depth treatment and sample date, and a post hoc Tukey's test was used to compare differences between individual means. Data were tested to meet the assumptions of parametric statistics, and transformed if required. Abundance and raw species richness of maple and beech stand control treatments for both litter and soil were compared using a Student's t- test to assess differences between stand types. Raw species richness is not always the best diversity metric, as it is sensitive to sample size (Buddie et al, 2005). To account for this, standardized species richness estimates were computed using individual-based rarefaction analysis, using EcoSim Version 7.2 (Gotelli and Entsminger, 2004). Rarefaction techniques allow 64 comparisons between sample types at the maximum abundance of the least abundant sample by repeated Monte Carlo subsampling at pre-designated abundance points. The resulting curves allow assessment as to whether sampling effort was sufficient, can provide estimates of species richness by treatment type, and associated estimates of variance allow for statistical comparisons. Assemblage composition was compared between stand types using nonmetric multidimensional scaling (NMS) ordinations and the Bray-Sorenson distance measure. NMS is a non-parametric ordination technique that makes no a-priori assumptions of spatial relationships and utilizes rank order information to compare similarities between samples (McCune and Grace, 2002). Ordinations were conducted in PCOrd version 4.17 (McCune and Mefford, 1999) using log- transformed data, and a DCA ordination was used for the starting configuration rather than random starting coordinates to reduce stress levels and avoid local minima (Work and McCullough, 2000). An initial six-dimensional ordination was run to aid in selection of the number of axes for the final ordination solution, as well as to evaluate stress reduction, and a Monte Carlo test was used to assess significance of each axis (McCune and Grace, 2002). Multi-response permutation procedures (MRPP) analysis was performed on log-transformed data using PCOrd version 4.17 (McCune and Mefford, 1999). MRPP is a nonparametric procedure used to test hypotheses of no difference between groups, which are required to be pre-existing (McCune and Grace, 2002). This form of analysis is more powerful than other similar tests such as MANOVA, as MRPP has no requirement of normality or homogeneity of variance, assumptions which are 65 rarely met by ecological data (McCune and Grace, 2002). For this study; litter depth treatments were used to designate groups, and the Sorenson (Bray-Curtis) distance measure was used to analyze differences between groups. Indicator species analysis (Dufrene and Legendre, 1997) was used to assess affinities between species and litter depth treatments, using the software PCOrd version 4.17 (McCune and Mefford, 1999). Statistical significance of Indicator values were tested using a Monte Carlo test of 1000 runs. Indicator species analysis is an ideal pair with MRPP techniques, as an aid in describing how well species separate among groups (McCune and Grace, 2002). Results A total of 5,344 individuals within 53 species was collected during this study (Appendix 3.1). Litter samples yielded 5139 individuals representing 47 species, soil samples yielded 205 individuals and 27 species, and 22 species were shared between the two layers. About the same number of individuals were collected in July (2742) as in September (2602). The most abundant species in the experiment was Scheloribates pallidulus (Koch) with 858 individuals (16% of total individuals sampled). Five species were collected only once in the study (i.e. singletons) and 3 species were collected only twice (doubletons). Fourteen species were represented by more than 100 individuals each in this experiment (Fig. 3.2). Most of these species were more commonly collected from beech than maple litter (Fig. 3.2) Abundance and Species Richness My first objective was to compare species richness and abundance by litter depth treatment. Results from ANOVA, run separately for beech (Table 3.2) and 66 maple (Table 3.3) stands indicated no significant effects of litter depth treatment or sample date, on litter or soil mite species richness and abundance. Results of the Student's t-test revealed no significant differences of abundance or raw species richness between soil and litter samples taken from beech and maple control treatments (Table 3.4). There were too few mites to perform rarefaction analysis on soil samples therefore this analysis was restricted to litter samples. Since ANOVA results indicated no significant effect of sample date for litter abundance and species richness, data were pooled by sample date. Curves were close to, or reached an asymptote, indicating that the sampling effort was sufficient to collect the majority of species present within the treatments (Fig. 3.3). In maple stands, the control and litter addition treatments appear to support higher rarefied estimates of species richness than the 3 cm disturbance treatment (Fig. 3.3a). In beech stands, the 6 cm disturbance treatment appears to support higher rarefied estimates of species richness than the control and litter reduction treatments (Fig. 3.3b). Community composition Ordination analyses were used to compare compositional differences by litter treatment type. Mites from soil samples were too few to warrant multivariate analyses. The NMS ordination was conducted on litter data pooled by date, and explained 91.6% of the total variance within the data set. While no treatment clusters apart from any other, beech treatments can be seen to cluster to the right of the ordination and maple treatments to the left (Fig. 3.4). Results of MRPP analysis indicate significant differences between beech and maple stand treatments, with only 67 the comparison of the beech control treatment with maple addition and disturbance treatments being statistically not significant (Table 3.5). Indicator species analysis highlighted five significant affinities (a < 0.05) of species with beech treatments alone (Table 3.6). Discussion In this experiment, the composition of oribatid mite assemblages differed by litter type (beech versus maple), which confirms the observation in Chapter 2 that stand type is an important factor in structuring oribatid assemblages. However, raw species richness and abundance did not differ by stand type and estimates of rarefied species richness indicate that species richness may be higher in beech stands than in those of maple. The experimental effect of either reducing or increasing the litter depth had no clear effect on litter or soil-dwelling mites (richness, abundance or community composition). My original predictions of oribatid abundances being higher in beech than maple stands, thicker than thinner litters and control than disturbance treatments were not supported, but rarefaction analysis provides support for the prediction that species richness would be higher in beech stands than in those of maple. While specific associations of the beech reduction and disturbance control treatments were found for some species with indicator species analysis (Fig. 3.6), the lack of general effects on oribatids when manipulating litter depth runs contradictory to findings by Osier et al. (2006), who found that as litter mass increased, densities of oribatid mites decreased after one year. The different results may be due to the 68 greater range of litter treatments in their experiment; Osier et al. (2006) used litter masses ranging from natural levels to 12-times natural litter mass. In my study, doubling (for maple stands) or halving (for beech stands) the amount of natural litter may not have changed the habitat sufficiently to affect mite assemblages. Indicator species analysis revealed significant associations of species with treatments only for treatments established in beech stands, with two species (Adrodamaeus magnisetosus and Trhypochthonius americanus) for the reduction treatment and three species (Nanhermannia elegantula, Oppiella nova and Oppia nr. Rigida) for the disturbed natural depth treatment. An alternative explanation for the lack of effects when litter was manipulated may relate to the time frame of my study. Samples taken only one year after manipulations may not provide enough time for oribatids to reproduce sufficiently to colonize the new habitat space. Oribatid mites exhibit life-history traits common to k-selected organisms, such as low rates of fecundity and relatively long life spans, but also often take more than a single year to complete their life cycle (Norton, 1994). The shape of the curves in my rarefaction analyses suggest that sampling effort was sufficient and that the majority of species present were collected, and suggesting that there was no effect of litter depth changes on oribatid assemblages at this study site. The rarefactions also suggest that when comparing rarefied estimates of species richness (a more meaningful metric than raw species richness—see methods), beech stands contain more species than do maple stands. It is unclear why the disturbance treatments did not differ significantly in abundance or species richness from the controls, especially as the effects of 69 disturbance on oribatid assemblages are well documented. For example, disturbance caused by epigeic earthworms alters oribatid assemblages (McLean and Parkinson, 2000) and a simple study of mechanical disturbance of varying frequencies revealing a gradual loss of species and decrease in density with increased disturbance (Maraun et al, 2003). These authors concluded that even low rates of disturbance are detrimental to oribatids and other soil groups. This is in contrast to the much-studied intermediate disturbance hypothesis first proposed by Connell (1978). This hypothesis postulates that intermediate levels of disturbance will promote increased diversity by suppressing the intensity of competitive exclusion between species. Huston (1994) further stated that higher levels of diversity should arise in locations where the overall growth rates of populations more closely match the frequency of disturbance. The hypothesis that diversity will be highest when disturbance levels match population growth rates likely does not hold true for soil mite assemblages; oribatid growth rates are already low (Norton, 1994) as a result of their life-history traits, and rates of competitive exclusion may be low to begin with as fungivores and microbial grazers are unlikely to be resource-limited (although a paucity of information exists to back up claims). In this light, disturbance would likely do little to reduce already low rates of competition, and would only serve to further reduce growth rates and result in species loss and decreases in abundance. Therefore, while the life-history traits of oribatids make them unlikely to adhere to the intermediate disturbance hypothesis, the brief period of disturbance in my treatments may have been below the threshold required to alter the assemblages in this study. 70 Although litter depth and disturbance treatments did not cluster apart from each other, beech and maple stands are clearly segregated on opposite sides of the second ordination axis. This demonstrates that habitat type is an important structuring factor of oribatid assemblages, and confirms my findings from Chapter 2. The comparison of the beech control with the maple addition treatment lends slight weight to the argument that minor changes in litter depth may have a minor effect on oribatid assemblages, especially as the comparison of the beech control with the maple disturbance treatment was marginally non-significant (p = 0.052). Indicator species analysis also supports the general finding of the importance of stand type, as the only significant indicator species were all found in association with beech treatments. Although my original predictions about the effect of litter depth on oribatid mite assemblages were not supported, the data do support previous studies' findings about causes of oribatid diversity and factors structuring oribatid assemblages, notably the effect of habitat type on oribatid community composition. Future work using litter manipulations should use a wider range of treatment depths and an increase in recovery time between the establishment of treatments and collection of samples may more insight into the effect of litter depth has on oribatid assemblages. 71 LITERATURE CITED Balogh, J. and Mahunka, S. 1983. Primitive oribatids of the Palearctic region. New York, Elsevier Science Publishing. 327 pp. Behan-Pelletier, V. and Newton, G. 1999. Linking soil biodiversity and ecosystem function - the taxonomic dilemma. Bioscience. 49: 149-153 Buddie, CM., Beguin, J., Bolduc, E., Mercado, A., Sackett, T.E., Selby, R.D., Varady-Szabao, H. and Zeran, R.M. 2005. The importance and use of taxon sampling curves for comparative biodiversity research with forest arthropod assemblages. Canadian Entomologist. 137: 120-127 Connell, J.H. 1978. Diversity in tropical rain forests and coral reefs. Science. 199: 1302-1310 Crossley, D.A. and Blair, J.M. 1991. A high-efficiency, low-technology Tullgren- type extractor for soil microarthropods. Agriculture, Ecosystems & Environment. 34: 187-192 Dufrene, M. and Legendre, P. 1997. Species assemblages and indicator species: the need for a flexible asymmetrical approach. Ecological Monographs. 67: 345 - 366 Evans, G.O. 1992. Principles of Acarology. CAB International, Wallingford, UK. 564 pp. Gill, R.W. 1969. Soil microarthropod abundance following old-field litter manipulation. Ecology. 50: 805-816 Gotelli, N.J. and G.L. Entsminger. 2004. EcoSim: Null models software for ecology. Version 7. Acquired Intelligence Inc. & Kesey-Bear. Jericho, VT 05465. http://garyentsminger.com/ecosim.htm. Hansen, R. 2000. Effects of habitat complexity and composition on a diverse litter microarthropod assemblage. Ecology. 81: 1120-1132 Huston, M. A. 1994. Biological diversity-the coexistence of species. Cambridge University Press, Cambridge. 681 pp. Kempson, D., M. Lloyd and R. Ghelardi, 1963. A new extractor for woodland litter. Pedobiologia 3:1-21. Maraun, M., Salamon, J-A., Schneider, K., Schaefer, M. and Scheu, S. 2003. Oribatid mite and collembolan diversity, density and community structure in a moder beech forest (Fagus sylvatica): effects of mechanical perturbations. Soil Biology & Biochemistry. 35: 1387-1394 72 Maraun, M., and Scheu, S. 2000. The structure of oribatid mite communities (Acari, Oribatida): patterns, mechanisms and implications for future research. Ecography. 23: 374-383 Maraun, M., Visser, S. and Scheu, S. 1998. Oribatid mites enhance the recovery of the microbial community after a strong disturbance. Applied Soil Ecology. 9: 175 — 181 Marshall, V.G., Reeves, R.M. and Norton, R.A. 1987. Catalogue of the Oribatida (Acari) of continental United States and Canada. Memoirs of the Entomological Society of Canada. 139. 418 pp. McCune, B. and Grace, J.B. 2002. Analysis of Ecological Communities. MjM Software Design. Gleneden Beach, OR. McCune, B. and Mefford, M. J. 1999. Multivariate analysis of ecological data version 4,17. MjM Software Gleneden Beach, OR. McLean, M.A. and Parkinson, D. 2000. Introduction of the epigeic earthworm Dendrobaena octaedra changes the oribatid community and microarthropod abundances in a pine forest. Soil Biology & Biochemistry. 32: 1671 - 1681 Moore, J.C., Walter, D.E. and Hunt, H.E. 1988. Arthropod regulation of micro- and meso-biota in below-ground detrital foodwebs. Annual Review of Entomology. 33: 419-439 Niedbala, W. 2002. Ptyctimous mites (Acari, Oribatida) of the nearctic region. Monographs of the Upper Silesian Museum, Bytom, Poland, 4. 261 pp. Norton, R.A. 1994. Evolutionary aspects of oribatid mites' life histories and consequences for the origin of the Astigmata. In Mites: ecological and evolutionary analyses of life-history patterns, Houck, M.A. ed. pages 99-135 Osier, G.H.R., Korycinska, A. and Cole, L. 2006. Differences in litter mass change mite assemblage structure on a deciduous forest floor. Ecography. 29: 811-818 Ponge, J.F. 2003. Humus forms in terrestrial ecosystems: a framework to biodiversity. Soil Biology & Biochemistry. 35: 935-945 Sariyildiz, T. and Anderson, J.M. 2003. Decomposition of sun and shade leaves from three deciduous tree species, as affected by their chemical composition. Biology and Fertility of Soils. 37: 137-146 Scheu, S., Albers, D., Alphei, J., Buryn, R., Klages, U., Migge, S., Platner, C. and Salamon, J-A. 2003. The soil fauna community in pure and mixed stands of beech 73 and spruce of different age: trophic structure and structuring forces. Oikos. 101: 225-238 Seastedt, T.R. 1984. The role ofmicroarthropods in decomposition and mineralization processes. Annual Review of Entomology. 29: 25-46. Walter, D.E., Krantz, G. and Lindquist, E.E. 1996. Acari. The Mites. Version 13 December 1996. http://tolweb.org/Acar/2554/1996.12.13 in The Tree of Life Web Project, http://tolweb.org Walter, D.E. and Proctor, H.C. 1999. Mites: Ecology, evolution and behaviour. CAB International, Wallingford, UK. 322 pp. Weigmann, G. 2006. Hornmilben (Oribatida). In: Dahl, Die Tierwelt Deutschlands, Bd. 76. Verlag Goecke & Evers, Keltern, 520pp. Work, T.T. and McCullough, D.G. 2000. Lepidopteran communities in two forest ecosystems during the first gypsy moth outbreaks in northern Michigan. Environmental Entomology. 29: 884 - 900 74 Table 3.1: Experimental design for one set of replicates within each stand type. Four replicates were used for each treatment, with samples taken on two dates from the Morgan Arboretum. Beech Maple Treatment 6 cm control 6 cm disturbance 3 cm control 3 cm disturbance Code 6 Ctrl 6 dist 3 ctrl 3 dist Sample taken litter/soil litter/soil litter/soil litter/soil Treatment 3 cm reduction litter removal 6 cm addition litter removal Code red bO add mO Sample taken litter/soil soil alone litter/soil soil alone Table 3.2: ANOVA comparing oribatid species richness and oribatid abundance in soil and litter samples collected from beech stands. Data were log transformed prior to analysis. Data Set Treatment Model MS ± SE F, D.F. F P-value Oribatid richness Litter Litter depth 0.078 ±0.106 F2.20 0.86 0.439 Sample date 2.3E-5 ± 0.087 Fl,20 0.00 0.988 Oribatid abundance Litter depth 0.757 ± 0.274 F2,20 1.26 0.306 Sample date 0.009 ± 0.224 Fl,20 0.01 0.906 Oribatid richness Soil Litter depth 0.411 ±0.384 F3.17 0.55 0.652 Sample date 0.009 ± 0.275 Fi,17 0.01 0.914 Oribatid abundance Litter depth 0.211 ±0.179 F3,17 0.81 0.508 Sample date 0.102 ±0.163 Fl,17 0.39 0.540 Table 3.3: ANOVA comparing oribatid species richness and oribatid abundance in soil and litter samples collected from maple stands. Data were log transformed prior to analysis. Data Set Treatment Model MS ± SE F, D.F. F P-value Oribatid richness Litter Litter depth 0.141 ±0.080 F2,20 2.77 0.087 Sample date 0.004 ± 0.065 Fl,20 0.08 0.778 Oribatid abundance Litter depth 0.305 ±0.181 F2.20 1.16 0.332 Sample date 0.035 ±0.148 Fl,20 0.13 0.719 Oribatid richness Soil Litter depth 0.156 ±0.081 F3,12 1.48 0.270 Sample date 1.04 ±0.217 Fl,12 4.74 0.050 Oribatid abundance Litter depth 0.217 ±0.299 F342 0.69 0.576 Sample date 1.04 ±0.217 F1.12 3.31 0.094 75 Table 3.4: Results of two-tailed Student's t-test comparing mean oribatid abundance and raw species richness of control treatments from beech and maple stands (a = 0.05). Comparison Comparison t Critical t Calculated t Litter abundance to.05(2),14 2.145 0.802 Litter richness to.05(2),14 2.145 0.748 Soil abundance to.05(2),ll 2.201 1.09 Soil richness to.05(2),ll 2.201 0.832 Table 3.5: MRPP of oribatid assemblage composition in litter treatments collected from maple and beech stands at the Morgan Arboretum. Differences between groups (A) were evaluated with the Sorenson (Bray-Curtis) distance measure. Data were med prior to analysis, an d bold type den<3te s significant ef Grouping variable A P-value red vs. 6 ctrl -0.060 0.888 red vs. 6 dist -0.009 0.496 red vs. 3 Ctrl 0.324 0.010 red vs. 3 dist 0.245 0.011 red vs. add 0.106 0.043 6 ctrl vs. 6 dist -0.032 0.695 6 ctrl vs. 3 ctrl 0.259 0.030 6 ctrl vs. 3 dist 0.181 0.052 6 ctrl vs. add 0.079 0.114 6 dist vs. 3 ctrl 0.347 0.010 6 dist vs. 3 dist 0.324 0.005 6 dist vs. add 0.162 0.006 3 ctrl vs. 3 dist -0.032 0.571 3 ctrl vs. add -0.046 0.780 3 dist vs. add -0.079 0.918 76 Table 3.6: Indicator species analysis reporting significant indicator values (a < 0.05) for oribatid mites by litter depth. Data were pooled among sampling dates. Monte Carlo test of 1000 runs used to test significance of maximum indicator value (IV). Species Family Genus Species Author Max treatment Mean Std. rv Code dev. Adrmag Gymnodamaeidae Adrodamaeus magnisetosus (Ewing) Beech — 3 cm reduction 23.0 8.07 41.7 Nanele Nanhermanniidae Nanhermannia elegantula Berlese Beech - 6 cm disturbance 23.5 9.12 49.9 Oppnov Oppiidae Oppiella nova (Oudemans) Beech - 6 cm disturbance 23.8 6.53 37.6 Opprig Oppiidae Oppia nr. rigida (Ewing) Beech - 6 cm disturbance 24.5 4.65 34.1 Trhame Trhypochthoniidae Trhypochthonius Americanus Ewing Beech - 3 cm reduction 23.2 9.95 51.8 77 Fig. 3.1: Site locations within the Morgan Arboretum of southwestern Quebec. Solid line indicates border of the Arboretum. 78 1000 L J Total abundance V/////A Beech abundance Maple abundance 800 o 600 A CO T3 C =3 < 400 200 A m v E. ^vvy * Fig. 3.2: Rank abundance plot for the fourteen species most commonly collected (>100 individuals from all samples), separated by stand type (beech and maple). Species codes are found in Appendix 2.1. a) 24 22 Q 20 w* CO T— 18 + V) 16 ic h 14 i_ (n d) o 12 100 200 300 400 500 600 Individuals b) 35 30 Q CO 25 -\ 0) c 20 -oC V) g> 'o 15 H 0) D. CO 10 H 6 cm disturbance 3 cm reduction 6 cm control 200 400 600 800 1000 1200 1400 1600 Individuals Fig. 3.3: Rarefaction curves showing species richness (±S.D.) of oribatid mites collected from leaf litter in a) maple stands and b) beech stands, separated by litter depth treatment. 80 4xis 2. • Beech - reduction : 35. o 6 /o o Beech • control T Beech - disturbance A Maple - control • Maple - disturbance D Maple - addition Fig. 3.4: Three-dimensional non-metric multidimensional scaling (NMS) ordinations of log-transformed oribatid mite data pooled by date. Percent variation by axis shown in figure (Final stress = 9.22, axis 1: p = 0.020, axis 2: p = 0.020, axis 3: p = 0.020). 81 io o oS 3 » O 3 3 3 3 3 3K Sfif B* •3 t3 -§ "S o Q O oQn o o 5 ft L.. 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S § 1 § 5 »- f. •3 I J § 'o s f £ .a s •s§« Hi a a ft So Ifll s J s a a •S -S -S '5 '5 a a a o o -o *o o ^^ fulfill ooo ri" ^ ^ d* « ttesi oS- 7>S3 " g; a, a, a, a, a, a. 03 ^1 >i cS « i-5-Bil§« T) V. | 13 | O £ &>> U CHAPTER 4: GENERAL CONCLUSION Through my investigations on the effects of habitat type and litter depth on the composition of oribatid assemblages, new insights have been gained with regards to possible determinants of oribatid abundance, diversity and distribution. I have demonstrated that the importance of general characters such as habitat type cannot be ignored when considering the factors structuring mite assemblages. This work also contributes to understanding the diversity and distribution of oribatid assemblages in forest systems of southwestern Quebec, thus my work provides an important base line biodiversity survey of oribatid mites. The relative abundances of oribatid mites differed among forest stand types, and species richness was higher in beech dominated stands and coniferous plantations (chapter 2). Habitat type appeared to structure oribatid assemblages, as there were significantly different assemblage compositions within each habitat type. This finding is contrary to other studies that haye found habitat identity to be unimportant in structuring oribatid assemblages (Kaneko et ah, 2005; Migge et ah, 1998), although other findings suggest that increased plant species richness (a factor varying with habitat identity) is important in supporting diverse oribatid assemblages (Hansen, 2000; St. John et ah, 2006). The greater diversity and abundance of oribatids in coniferous plantations consisting of only two tree species compared with maple dominated or mixed deciduous stands does, however, raise questions about whether it is specifically plant species richness or habitat complexity in general that is responsible for supporting more complex oribatid assemblages. Litter in 84 coniferous plantations was thicker than that of the two previously mentioned deciduous stands, and may have provided a complexity not afforded by the thinner (but more species rich) litters of the other two stand types. My experimental study in Chapter 3 was unique and designed to elucidate potential effects of litter depth on mite assemblages. However, no effect of changes in litter depth was found on the relative abundance and species richness of oribatid mites. This is in contrast with Osier et al. (2006), who found that oribatid densities decreased as litter depth increased. Additionally, no effect of the single disturbance event in establishing depth treatments was noted on oribatid assemblages, confirming the findings of Maraun et al. (2003) demonstrating that low levels of disturbance do not reduce the complexity of oribatid assemblages. Ordination analysis further confirmed the lack of effect of litter depth changes on oribatid assemblages, but did reinforce findings from Chapter 2 regarding the importance of habitat type in structuring oribatid assemblages. These results suggest that while there are likely to be specific factors covarying with habitat types that serve to determine the structure of oribatid assemblages, litter depth is not among them given the scale, time frame, and forest site in which I did the experiment. Several other environmental factors covary with habitat type, such as persistence of litter (as demonstrated by beech litter, see Sariyildiz and Anderson, 2003), fungal biomass, soil depth and organic matter composition of soils, and these maybe important determinants of oribatid diversity and assemblage structure, but this remains to be verified in future studies. 85 LITERATURE CITED Hansen, R.A. 2000. Effects of litter habitat complexity and composition on a diverse litter microarthropod assemblage. Ecology. 81: 1120-1132 Kaneko, N., Sugawara, Y., Miyamoto, T., Hasegawa, M. and Hiura, T. 2005. Oribatid mite community structure and tree species diversity: a link? Pedobiologia. 49: 521 - 528 Maraun, M., Salamon, J-A., Schneider, K., Schaefer, M. and Scheu, S. 2003. Oribatid mite and collembolan diversity, density and community structure in a moder beech forest (Fagus sylvatica): effects of mechanical perturbations. Soil Biology & Biochemistry. 35: 1387-1394 Migge, S., Maraun, M., Scheu, S. and Schaefer, M. 1998. The oribatid mite community (Acarina) of pure and mixed stands of beech (Fagus sylvatica) and spruce (Picea abies) of different age. Applied Soil Ecology. 9:115-121 Osier, G.H.R., Korycinska, A. and Cole, L. 2006. Difference in litter mass change mite assemblage structure on a deciduous forest floor. Ecography. 29: 811-818 St. John, M.G., Wall, D.H. and Behan-Pelletier, V. 2006. Does plant species co occurrence influence soil mite diversity? Ecology. 87: 625 - 633 Sariyildiz, T. and Anderson, J.M. 2003. Decomposition of sun and shade leaves from three deciduous tree species, as affected by their chemical composition. Biology and Fertility of Soils. 37: 137-146 86