The Plant Diversity Hypothesis Explains Insect Gall Diversity in Sipaliwini, Suriname

Undergraduate #16
Discipline: Biological Sciences
Subcategory: Ecology

Alyssia Velez - Virginia State University
Co-Author(s): Sabria Greiner and Sarah Melissa Witiak, Virginia State University, Petersburg, VA Gisla Giram-Joerga, University of Suriname, Paramaribo, Suriname

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Galls are growths on plants that develop when an insect applies a chemical stimulus to plant tissue. Galls are not usually harmful to the plant, but provide shelter, protection and nutrition to the insect. Galling is evolutionarily beneficial to insects and has evolved in many plant and insect lineages (Stone and Schonrogge, 2003). Two hypotheses have been proposed to explain gall diversity (Lara et al. 2002). One hypothesis states that habitats with higher plant diversity would have greater gall diversity. Habitats with higher plant diversity, such as rainforests, would have greater gall diversity because more host plant options are available to galling insects. The second hypothesis states that higher gall diversity should be found in harsh environments. A harsher habitat such as a savannah would have greater gall diversity because galls would provide protection to insects from environmental stresses. To test these two hypotheses, we surveyed savannah and interspersed rainforest areas in the Sipaliwini region of Suriname. We surveyed for galls using two approaches: active searching and vegetation transects. Three researchers actively searched for 10 hours in 2 savannah sites and 11 hours in 5 rainforest sites. Samples of galls and host plants were pressed for identification and voucher specimens. In addition, two 100 meter long transects were taken in savannah and two in rainforest. For each transect, gall number, individual plant heights and plant types were recorded for 1×1 meter plot every 10 meters, for a total of 11 plots per transect. Similar durations of active searching yielded a total of 51 galls on host plants from 18 families in the rainforest, and 5 galls on 4 families of host plants in the savannah. Chi square tests showed that there was a strong relationship between environment and gall number (Chi square= 500. 6, DF=20, p-value < 0.001). Plant type diversity tested with Simpson’s index confirmed that rainforest had a much higher plant diversity (0.2673) than savannah (0.9793). This first survey ever for insect galls in Sipaliwini supports the hypothesis that greater gall diversity occurs in habitats with greater plant diversity. In addition, all galls were found on shrubs and trees, suggesting that plant form may influence galling diversity in addition to plant diversity. Future research will address this aspect of gall diversity.

Funder Acknowledgement(s): I thank S. Greiner, A. White, D. Davidson, A. Harris, D. Hardy for help in the field and G. Jiram-Doerga (University of Suriname) for help with identification and herbarium preparation. I also thank A. Ansari and P. Kaseloo for help with funding and statistics. This study was supported by a grant from HBCU-UP (NSF) awarded to Sarah Melissa Witiak.

Faculty Advisor: Sarah Melissa Witiak,