Best Student Oral Presentation
Brian E. Sedio
PhD Candidate, Department of Ecology and Evolutionary Biology, University of Michigan, USA
Divergence in anti-herbivore defense among sympatric Psychotria shrubs and implications for plant-insect interactions in phylogenetically-clustered assemblages
by Brian E. Sedio1, Christopher W. Dick1, and S. Joseph Wright2
1 University of Michigan, 2 Smithsonian Tropical Research Institute
Presented at Wantilan Convention Center (Wantilan rear, 10:45 am, July 22, 2010) during the Symposium "Herbivory in phylogenetically-clustered assemblages in phylogenetics in the tropics: building trees to understand community structure and tropical biodiversity" organized by Nathan Swenson, Vinita Gowda and W. J. Kress
Abstract. Many tropical forests contain diverse assemblages of congeneric trees, which raises the question of how ecologically similar species can avoid competitive exclusion. Negative density-dependent fitness imposed by specialized natural enemies (i.e. the Janzen-Connell hypothesis) is a broadly accepted mechanism of species coexistence. Specialist herbivores, however, often specialize on a particular genus, not species, of plants. On the other hand, plant defenses can be quite divergent among closely related sympatric species. Herbivore-driven spatial overdispersion of such defense traits might allow a Janzen-Connell-like mechanism to facilitate coexistence within local assemblages. We examine the specificity of interactions between herbivorous insects and their host-plants among 21 sympatric species of the diverse tropical shrub genus Psychotria (Rubiaceae) on Barro Colorado Island (BCI), Panama. Data on plant-insect associations based on DNA barcode analysis of insect gut contents are combined with profiles of host plant alkaloid secondary compounds to address three principal questions: i) how specialized are the insect herbivores of a single, diverse plant genus? ii) do Psychotria alkaloid defenses show evidence of evolutionary divergence? and iii) do Psychotria species assemblages reveal limited similarity in defense? Results indicate that the insect herbivores of Psychotria on BCI are not strict specialists. Phylogeny is a poor predictor of species alkaloid similarity in general, though particular principal components of alkaloid variation retain phylogenetic signal. While previous results indicate that species of Psychotria that co-occur within 28 m2 plots are more closely related than by chance relative to the BCI species pool (i.e. are phylogenetically clustered), co-occurring Psychotria are less similar chemically than by chance (i.e. are chemically overdispersed). Evolutionary divergence in defense may facilitate coexistence within such assemblages by reducing herbivore overlap between close relatives.
More about Brian Sedio's study
There are more tree species in a half-hectare of tropical forest in Ecuador than in all of temperate Europe, Asia, and North America combined. This raises the question of how large numbers of ecologically similar species manage to coexist despite intense competition for light, water, space, and nutrients. One widely-accepted idea proposes that insects may give an advantage to rare plant species by attacking successful plants when and where they become abundant. Yet this mechanism assumes that insects have specialized diets, and that may not be the case. Tropical forest plants, however, are known to express a multitude of chemicals thought to function in defense. Do species differences in these so-called secondary compounds influence insect herbivores in a way that might limit their diet breadth and thereby favor rare plants with distinct chemistry?
Brian Sedio examines the diet specialization of the insects that are found to eat tropical forest understory shrubs in the genus Psychotria and their relatives in the Coffee Family. With the help of several undergraduates, Sedio is busy collecting several thousand beetles, caterpillars, grasshoppers, and other insect herbivores of Psychotria in the forest of Barro Colorado Island (BCI), Panama. In order to identify the plants that a particular insect has eaten, DNA is isolated from the insect’s stomach and the plant from which it came identified using “DNA barcodes” unique to each species of plant. Sedio has also investigated species differences in alkaloid compounds thought to protect Psychotria from insect pests.
Preliminary results indicate that insect herbivores of Psychotria are not strict specialists. Nevertheless, species of Psychotria that live within 3 m of each other on BCI are less chemically similar than by chance. This suggests that similarity in defense, and possibly insect herbivores, plays a role in the community assembly of these understory plants. It is possible that even generalist natural enemies may facilitate plant coexistence if plants reduce the overlap in their herbivore communities through evolutionary character displacement of defensive traits. Illuminating the role of insects in maintaining the tree diversity of tropical rainforests will shed light on one of the persistent mysteries of earth’s most diverse ecosystems.
Illustrations (from top to bottom): Brian Sedio; characteristic inflorescence of Psychotria poeppigiana; Psychotria herbivore: Chrysomelidae subfamily Eumolpinae; first author collecting insects from P. acuminata on Barro Colorado Island at night; leaves and fruit of P. limonensis. (All photos © Brian E. Sedio.)
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