The Role of Ants and Mammalian Herbivores on the Structure and Composition of Insect Communities Found on Canopies of Acacia Drepanolobium
Total Page：16
File Type：pdf, Size：1020Kb
African Journal of Agricultural Research Vol. 7(38), pp. 5317-5331, 2 October, 2012 Available online at http://www.academicjournals.org/AJAR DOI: 10.5897/AJAR12.059 ISSN 1991-637X ©2012 Academic Journals Full Length Research Paper The role of ants and mammalian herbivores on the structure and composition of insect communities found on canopies of Acacia drepanolobium S. K. Kuria 1* and M. H. Villet 2 1Department of Biological Sciences, Walter Sisulu University, P/B X1 Mthatha 5117, South Africa. 2Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa. Accepted 4 June, 2012 Acacia drepanolobium Sjøstedt (Fabaceae) constitutes about 99% of the woody vegetation in the cotton soil ecosystem of Laikipia, Kenya. The tree has symbiotic association with four ant species that discourage large mammalian herbivores from feeding on it. However, there is no information as to whether these ants affect the community of canopy insects. Therefore, this study investigated the effect of the four ant species and differential vertebrate grazing and browsing pressures on the insect community inhabiting canopies of A. drepanolobium trees. Insect samples were collected using standard fogging and beating methods and identified to family and morphospecies. At the morphospecies level, the insect communities separated into two distinct groups, one comprised of samples collected from trees occupied by Crematogaster mimosae and Crematogaster nigriceps , and the other of samples obtained from trees inhabited by C. sjostedti and Tetraponera penzigi . However, differential vertebrate grazing and browsing patterns did not show any significant effect on the insect community occupying canopies of A. drepanolobium . Key words: Community structure, coexistence, diversity, herbivory, Mpala, Kenya. INTRODUCTION The interactions of ants and other arthropods on plant some effect on the species richness of phytophagous canopies are complex and require proper evaluation to insect community in the Eastern Cape, South Africa. elucidate how ants affect or are affected by other Crematogaster liengmei was also shown to prey on the arthropods (Fagundes et al., 2005). Previous studies eggs of the biological control agent Cactoblastis have shown that ants influence the arthropod community cactorum (Lepidoptera: Pyralidae) in the Eastern Cape, that inhabits plant canopies in a wide variety of locations which led to the local failure of this insect to control the and habitats. For instance, the presence of ants on tree invasive prickly pear cactus, Opuntia ficus-indica canopies in northern England resulted in a significant (Robertson, 1988; Robertson and Hoffmann, 1989). increase of Periphyllus testudinaceus (Hemiptera: These examples illustrate how intricate ants’ associations Drepanosiphidae), while their removal resulted in a with other animals and plants can be. If properly decline (Skinner and Whittaker, 1981). The species understood and documented, interactions between ants richness of canopy ants in a semi-deciduous humid forest and other organisms could be used to predict ecological correlated positively with richness of butterflies and conditions within a given habitat by the presence of a canopy beetles (Lawton et al., 1998). Ross (1994) found particular ant species (Agosti et al., 2000). that the presence of ants on Ficus burtt-davyi Hutch had The black cotton soil ecosystem of Laikipia District, Kenya, is heavily used for ranching of both game and cattle (Young et al., 1997), and supports a wooded grassland technically named ‘Acacia bushed grassland’ * Corresponding author. E-mail: [email protected]. Tel: +27 (Young et al., 1995), in which more than 99% of the 5022170. woody vegetation is Acacia drepanolobium (Harms) 5318 Afr. J. Agric. Res. Sjøstedt. At Mpala Research Centre, Kenya, four ant MATERIALS AND METHODS species that are usually mutually hostile compete for A. drepanolobium trees in which they nest (Palmer et al., Study area 2000). The smaller trees are generally inhabited by Fieldwork was conducted between September 2003 and June 2005 Crematogaster nigriceps and Tetraponera penzigi at the Kenya Long-term Exclosure Experiment (KLEE) plots (Young (subordinate species), while bigger trees are occupied by et al., 1998) at Mpala Research Centre, Laikipia District, Kenya Crematogaster sjostedti and Crematogaster mimosae (0°17 ′ N, 36° 56 ′ E; 1800 m elevation). The study site had nine (dominant species) (Palmer et al., 2000). C. nigriceps experimental plots, six exclosures in the KLEE and three non- sterilises A. drepanolobium trees by chewing off the floral exclosed plots next to the KLEE plots. The KLEE exclosures were buds (Stanton et al., 1999), while T. penzigi destroys the initiated in September 1995 to investigate the effects of herbivores on the savannah ecosystem (Young et al., 1998). The average foliar nectaries of its tree, in effect lowering the chance of annual rainfall at Mpala Research Centre is 580 mm, while the having more aggressive dominant ants take over the vegetation is generally composed of a woody layer which is trees it inhabits (Young et al., 1997; Palmer et al., 2002). composed mainly of A. drepanolobium (> 99%) interspersed with Apart from these direct effects on the host plant, there several grass species (Young et al., 1997). A detailed description of may be direct and indirect effects on the community of the KLEE exclosures was published by Young et al. (1998). The KLEE exclosure plots were 200 m × 200 m, while the three non- other insects inhabiting these trees, and it is important to exclosed plots measured 100 m × 200 m. Two of the KLEE understand the whole ecosystem to see what kind of exclosure treatments, 0 (no large herbivores including cattle were insect community is associated with these four ant allowed), and C (only cattle were allowed) were sampled along with species. a treatment E (all herbivores including cattle were allowed) that lay Ecological pressure from vertebrate grazing and outside the KLEE exclosures but adjacent to C plots. The browsing sometimes have positive or negative effects on treatments were replicated three times in what is referred to as the “North”, “Central” and “South” blocks (Young et al., 1998: Figure 1). invertebrate communities (O’Neill et al., 2003). Apart from It was felt that these three treatments would give a general the potential effects of the ants, the black cotton soil representation of the various insects found on the canopies of A. ecosystem is measurably affected by the vertebrate drepanolobium and how they interact with ants in the face of herbivores inhabiting it, as shown by the Kenya Long- various degrees of vertebrate herbivory. It was also assumed that term Exclosure Experiment (KLEE) (Young et al., 1998, nine years was sufficient time that if excluding large mammals had 2005; McCauley et al., 2006; Goheen et al., 2007; Odadi any effect on the canopy insects, it should have become measurable. et al., 2007; 2011; Riginos and Young, 2007; Maclean et al., 2011), which is designed to distinguish between the effects of six combinations of cattle (grazers); very large, Sampling indigenous browsing herbivores (giraffes and elephants); and other, primarily grazing, indigenous herbivores Four sampling sessions were carried out with intervals of three (buffalo, smaller ungulates and rodents). The effects of months between consecutive sessions, as follows: first sampling these herbivores was reflected in the community of (27 October – 13 November 2003), second sampling (11 – 28 February 2004), third sampling (26 May – 12 June 2004) and fourth spiders found in the herb layer of the vegetation (Warui et sampling (10 – 27 September 2004). To minimise method-induced al., 2005), but their effects on the community of variation and bias, the insects inhabiting A. drepanolobium invertebrates in the canopy has not been published. canopies were collected using fogging and beating. Five trees Since the four ant species associated with A. occupied by each of the four acacia ant species were sampled drepanolobium behave and modify the tree canopies using each method, making a total of 20 trees during each sampling differently, the current study was undertaken to determine session in each of the three herbivory treatments. The two sampling methods combined sampled a total of 40 trees in each of the if this effect influenced the insect communities inhabiting herbivory treatment during one sampling session. In total 1440 A. drepanolobium . Another possibility was that the trees were sampled during four sampling sessions. herbivores might modify the influence of the ants, so this effect was incorporated by conducting sampling at the KLEE site. The knowledge of the abundance and Beating samples diversity of canopy insects was intended to provide an insight to understanding the interactions that may exist Eighty trees were semi-randomly marked using aluminium tags by between these insect community and the four acacia- following a compass direction on a straight line, and tagging trees ants, taking into account a major land use, the types of within a 20 m range, in each of the herbivory treatments. The trees incorporated twenty trees occupied by each of the four acacia-ants vertebrate herbivores in the ecosystem. The aims of this (C. sjostedti, C. mimosae, C. nigriceps and T. penzigi ). Only trees study were therefore: 1) to establish a checklist of the with heights ranging between 1.0 to 2.5 m were tagged, since these insect species that coexisted with the four obligate