The Role of Host Plant Specificity and Spatial Dynamics in the Biodiversity of a Tropical Herbivore Guild (Lepidoptera)

Abstract
Research on the network of forest dynamics plots (FDPs) managed by CTFS has enabled significant progress toward understanding the general principles governing the dynamics of tropical plant communities. Unfortunately, major observational lacunae prevent similar advances in the study of higher trophic levels in tropical forests. We have begun a long-term, spatially explicit study of host plant use by lepidopteran herbivores in the Khao Chong FDP in Trang , Thailand . The first 18 months of this project will assess the diversity, phenology, and food web interconnections of a lepidopteran herbivore guild and their insect parasitoids. These data will guide the design of a long term study in 3 FDPs along a latitudinal transect to investigate a new suite of ecological and biogeographic questions. This tractable investigation will provide a clear picture of an important tropical food web and inform ongoing plant demographic studies.

Introduction
With over 160,000 species in 121 families distributed world-wide [1, 2] , the Lepidoptera (butterflies and moths) offer an excellent opportunity not only for ecological sampling, but also for comparative studies into the evolution of tropical diversity. Over 99% of caterpillars are herbivorous, and they constitute 95% of all leaf feeders in neotropical communities [3, 4] . Contrary to expectation [5] , the diets of tropical, leaf chewing Orthoptera and Coleoptera (crickets/katydids/grasshoppers and beetles, respectively) are much less specialized than their temperate relatives, while lepidopteran caterpillars are frequently monophagous, feeding on only one host plant species [6] . This inability of lepidopteran larvae to use alternative hosts may intensify the impacts of herbivory on particular host trees. Our data on caterpillar phenology and distribution will complement existing floristic data to enable a better understanding of plant demography and phenology in our research sites.

Community ecology studies of tropical plants and their herbivores typically follow one of two paradigms [3, 5] : "phyto-centric" studies sample focal plant species to understand the diversity of their herbivore fauna [7-11] , while "entomo-centric" investigations survey vegetation in a predefined area for a focal herbivore taxon [3] . Although several hypotheses for the generation and maintenance of tropical diversity predict that herbivores and pathogens will disproportionately attack aggregated or common tree species/taxa, few studies have examined patterns of herbivory in relation to the spatial distribution of trees in a mature forest [12-14] . We seek support to assist the initial phase of a long-term project that combines these two research models to assess the diversity and spatial dynamics of the largest guild of leaf-chewing tropical herbivores, lepidopteran caterpillars, and their insect parasitoids.

The first 18 months of the proposed project (Phase 1) will focus on the 24-hectare Khao Chong (KC) FDP plot in Trang, Thailand. During this initial period, we will refine our research techniques and collect sufficient data to apply for continued funding from the National Science Foundation (NSF) for an expanded, two-year study of three sites (Phase 2) along a latitudinal transect: Pasoh Forest Preserve, Malaysia ; KC; and Huai Kha Khaeng Wildlife Sanctuary, Thailand .

Research Hypotheses: Phase 1
By coupling plant demographic data with lepidopteran host plant use information, we will be able to address a suite of ecological hypotheses, phrased below as questions:

Each of these questions will also be asked in relation to insect parasitoids (flies and wasps) feeding on their caterpillar prey.

Methods
Our sampling protocols are adapted from techniques first developed by Dan Janzen and colleagues [15] in the Guanacaste Conservation Area of northern Costa Rica . These alterations to Janzen's methodology are designed to minimize impacts on trees and seedlings within the FDP. A small team of Thai entomologists and assistants inspect the understory foliage of each tree and herb within defined areas of the FDP so that each plant is inspected every week. Upon discovering a caterpillar, the date, tree tag number of its host, and a unique serial number are recorded. The caterpillar is then be reared to adulthood by one of two methods. If possible, a fine mesh bag is placed around the caterpillar and a whorl of terminal foliage and tightly tied to the branch. If this method is not feasible, a small amount of the host plant foliage is excised, placed in a clear plastic bag with the caterpillar, and taken to a covered rearing shed. Care is taken to remove no more foliage than the caterpillar would have eaten on the plant. Each bagged caterpillar is inspected daily. Pupae are placed in screened pupation containers and euthanized shortly after eclosion. Three legs of each specimen are placed in 100% ethanol to preserve its DNA, and the specimen is spread and dried. Parasitoids emerging from caterpillars or pupae are also euthanized and preserved for identification.

The resulting moth, butterfly, and parasitoid specimens are identified to species or described as new by a collaborating taxonomic specialist. Lepidopteran larvae, pupae, and adults are photographed at every stage, and a database comprising these photographs and all records of plant-caterpillar-parasitoid associations will be made available on this website to provide almost immediate access to the widest possible audience.

To address questions about lepidopteran diversity and host specificity, a series of small, carefully chosen sub-plots have been demarcated within the main FDP and surveyed for caterpillars weekly.

To investigate the potential role of conspecific tree distance on the intensity of herbivore attack, we have identified several tree species that have both isolated individuals and clumps of conspecifics within the FDP. To simplify this investigation, we do not consider trees to be “isolated” if they are within 10 meters of a congeneric individual. Lepidopteran herbivores from each focal individual are reared in the manner described above.

Finally, to assess the relationship between the taxonomic composition of reared individuals to adults in the study site, we sample the moth fauna on a weekly basis with light traps and sheets, sample the butterfly fauna with bait traps and by netting individuals along a transect, and sample the hymenopteran and dipteran (parasitoid) fauna with Malaise intercept traps [16-18] . Whenever positive identification is possible without a voucher specimen, we mark and release individuals. In addition, directed mark-recapture studies of focal lepidopteran species may also permit estimation of local population sizes.

Research Output
In addition to a unique dataset on the diversity and spatial dynamics of a tropical insect herbivore guild, this work will generate a number of additional tools for assessing the insect diversity of the Thai-Malay peninsula.

Long-term Goals: Phase 2 Research Hypotheses
The proposed study site in Trang is situated at the junction of the Indo-Burma and Sundaland biodiversity hotspots and at the latitudinal border between seasonal (deciduous) and aseasonal (evergreen) forest. The flora and fauna to the north and south of this narrow zone are distinct, and the transition is abrupt [19, 20] . Distributional data for South East Asian Lepidoptera are generally poor, but available data for butterflies indicate that many pairs of subspecies flank this transition zone [21-26] , emphasizing its importance in insect zoogeography.

In later years, comparisons among the three FDP sites positioned along a latitudinal gradient—Pasoh, KC , and Huai Kha Khaeng—will enable us to understand better the zoological implications of this unique transition zone. Our unique, long-term sampling scheme will enable us to answer many more questions, including the following:

Significance to CTFS and broader relevance
Lepidopteran herbivores are the most abundant herbivores in tropical plant communities [3] , and are therefore likely to be a crucial biotic factor affecting tree dynamics in tropical ecosystems. By conducting this research on CTFS plots, we are accumulating information about this key guild of herbivores from the second trophic layer (primary consumers) and the principal enemies of that guild—insect parasitoids. While it is impractical to sample the entire plot repeatedly for insect herbivores, it is hoped that this carefully focused study will provide sufficient information to inform ongoing botanical studies at these sites.

Although several other studies use a similar approach to assess questions relating to tropical insect ecology [3, 6] , to our knowledge, the proposed study is the first in Asia and the first to incorporate plant spatial information. A study of this type is only possible on a large plot in which every tree has been identified, a factor that also effectively eliminates the burden of having to identify every tree eaten by each insect in the study.

Perhaps the most significant aspect of these data is that, with rigorous sampling of the focal sub-plots, we will not only know that is being eaten, we will know what is not being eaten. As pointed out by Janzen [27] , “The ecologist traditionally poses a question about interspecific and/or intraspecific interactions focused on a specific habitat or specific taxon. Here, the positive statements are many, but ‘absence' statements are impoverished.” Our data on caterpillar dynamics will enable these powerful ‘absence' statements to be made about the most significant guild of tropical herbivores and their most important enemies at three sites along a latitudinal transect, and thereby inform studies of forest dynamics and tropical food webs in a novel way.

References

1. Common, I.F.B. 1990. Moths of Australia . Melbourne, Melbourne University Press. 535 pp.

2. Carter, D.J. & N.P. Kristensen. 1998. Classification and keys to higher taxa. In Handbuch der Zoologie, N.P. Kristensen, eds. Walter de Gruyter GmbH & Co.

3. Janzen, D.H. 1988. Ecological characterization of a Costa Rican dry forest caterillar fauna. Biotropica 20 : 120-135.

4. Janzen, D.H. 1981. Pattern of herbivory in a tropical deciduous forest. Biotropica 13 : 271-282.

5. Janzen, D.H. 1973. Comments on host-specificity of tropical herbivores and its relevance to species richness. Pp. 201-211 in Taxonomy and Ecology, V.H. Heywood, eds. London, Academic Press.

6. Novotny, V., Y. Basset, S.E. Miller, G.D. Weiblen, B. Bremer, L. Cizek, & P. Drozd. 2002. Low host specificity of herbivorous insects in a tropical forest. Nature 416 : 841-844.

7. Basset, Y. 1991. The taxonomic composition of the arthropod fauna associated with an Australian rain-forest tree. Australian Journal of Zoology 39 : 171-190.

8. Basset, Y. 1999. Diversity and abundance of insect herbivores collected on Castanopsis acuminatissima (Fagaceae) in New Guinea: Relationships with leaf production and surrounding vegetation. European Journal of Entomology 96 : 381-391.

9. Novotny, V. & Y. Basset. 1998. Seasonality of sap sucking insects (Auchenorrhyncha, Hemiptera) feeding on Ficus (Moraceae) in a lowland rain forest in New Guinea. Oecologia 115 : 514-522.

10. Novotny, V. & Y. Basset. 2000. Rare species in communities of tropical insect herbivores: Pondering the mystery of singletons. Oikos 89 : 564-572.

11. Novotny, V., Y. Basset, S.E. Miller, P. Drozd, & L. Cizek. 2002. Host specialization of leaf-chewing insects in a New Guinea rainforest. Journal of Animal Ecology 71 : 400-412.

12. Basset, Y. 2000. Insect herbivores foraging on seedlings in an unlogged rain forest in Guyana: Spatial and temporal considerations. Studies on Neotropical Fauna and Environment 35 : 115-129.

13. Wong, M., S.J. Wright, S.P. Hubbell, & R.B. Foster. 1990. The spatial pattern and reproductive consequences of outbreak defoliation in Quararibea asterolepis , a tropical tree. The Journal of Ecology 78 : 579-588.

14. Arnold, A.E. & N.M. Asquith. 2002. Herbivory in a fragmented tropical forest: Patterns from islands at Lago Gatun, Panama. Biodiversity and Conservation 11 : 1663-1680.

15. Janzen, D.H. & W. Hallwachs. Area de Conservacion Guanacaste (ACG), northwestern Costa Rica: Caterpillars, pupae, butterflies & moths. http://janzen.sas.upenn.edu/caterpillars/database.lasso (Last accessed 22 February 2004).

16. DeVries, P.J., D. Murray, & R. Lande. 1997. Species diversity in vertical, horizontal, and temporal dimensions of a fruit-feeding butterfly community in an Ecuadorian rainforest. Biological Journal of the Linnean Society 62 : 343-364.

17. Fonseca, G. & et al. Butterfly Monitoring Protocol - Conservation International TEAM Initiative .

18. Kitching, R.L. 2000. Moth assemblages as indicators of environmental quality in remnants of upland Australian rain forest. Journal of Applied Ecology 37 : 248-297.

19. Hughes, J.B., P.D. Round, & D.S. Woodruff. 2003. The Indochinese-Sundaic faunal transition at the Isthmus of Kra: An analysis of resident forest bird species distributions. Journal of Biogeography 30 : 569-580.

20. Woodruff, D.S. 2003. Neogene marine transgressions, palaogeography and biogeographic transitions on the Thai-Malay Peninsula. Journal of Biogeography 30 : 551-567.

21. Pinratana, A. & J.N. Eliot. 1996. Butterflies of Thailand, Volume 3: Nymphalidae . Bangkok, St. Gabriel's College. 140 pp. + 84 pl.

22. Pinratana, A. 1981. Butterflies of Thailand, Volume 4: Lycaenidae . Bangkok, St. Gabriel's College. 215 pp. + 36 pl.

23. Pinratana, A. 1983. Butterflies of Thailand, Volume 2: Pieridae and Amathusiidae . Bangkok, St. Gabriel's College. 71 pp. + 48 pl.

24. Pinratana, A. 1985. Butterflies of Thailand, Volume 5: Hesperiidae . Bangkok, St. Gabriel's College. 150 pp. + 40 pl.

25. Pinratana, A. 1988. Butterflies of Thailand, Volume 6: Satyridae, Libytheidae and Riodinidae . Bangkok, St. Gabriel's College. 60 pp. + 44 pl.

26. Pinratana, A. & J.N. Eliot. 1992. Butterflies of Thailand, Volume 1: Papilliondiae . Bangkok, St. Gabriel's College. 81 pp. + 92 pl.

27. Janzen, D.H. & I.D. Gauld. 1997. Patterns of use of large moth caterpillars (Lepidoptera: Saturniidae and Sphingidae) by ichneumonid parasitoids (Hymenoptera) in Costa Rican dry forest. Pp. 251-271 in Forests and Insects, A.D. Watt, N.E. Stork, & M.D. Hunter, eds. London, Chapman & Hall.

 

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