Seasonal cycles of diversity and similarity in a Central American rainforest butterfly community

V. Grøtan, R. Lande, I. A. Chacon and P. J. DeVries

Lowland tropical rainforests harbor the greatest insect diversity on earth, and ecologists have repeatedly shown that wet and dry seasons are correlated with insect abundances. Despite its importance for understanding ecological diversity, it has long remained unclear whether tropical insect community composition exhibits seasonal cycles in species diversity and community similarity. This has largely been due to the difficulty in conducting standardized long-term studies beyond a few years, coupled with the problem of utilizing a taxonomically well-known group that can be sampled in a way that minimizes human collector bias. A serious challenge also exists concerning the development of robust analytical methods to directly compare diverse tropical communities composed of many rare species. In this study we were able to address these issues by trapping fruit-feeding nymphalid butterflies and employing new statistical methods.

Costa Rican field biologists servicing a canopy trap during the butterfly seasonality study at the Tirimbina Biological Reserve in Costa Rica. Photo by P. J. DeVries.

Butterflies in the family Nymphalidae that feed on the juices of rotting fruit as their major adult food resource comprise a feeding guild known as fruit-feeding nymphalids. These butterflies are easily sampled with fruit-baited traps, and account for a significant proportion of the total nymphalid richness in tropical forests. In this study we deployed sixty traps within 150 ha of rainforest at the Tirimbina Biological Reserve, Heredia Province, Costa Rica. Traps were baited with mashed banana, and we sampled butterflies for five days each month for 104 consecutive months. The samples were pooled to provide a time series of monthly species abundance distributions to compare with local weather data.

A Costa Rican fruit-feeding nymphalid butterfly Agrias amydon philatelica (Charaxinae) feeding on mashed banana on the rainforest floor. This uncommon butterfly typically inhabits the forest canopy. Photo by P. J. DeVries.

Our sampling recovered monthly abundance distributions for 106 fruit-feeding butterfly species, demonstrating biannual cycles in species diversity, but that community similarity occurred as an annual cycle that peaked in the driest months. We found that community similarity did not tend to decline with increasing time lag through the years, which we attributed to a lack of long-term changes in species abundances. These findings differed from our similar study on fruit-feeding butterflies in Amazonian Ecuador where cycles in weather and community diversity were both distinctly annual (Grøtan, et al. 2012). Together these studies showed that even when tropical butterfly communities have many taxa in common, ecologically their composition may respond to seasonal weather patterns in different ways. These studies also highlight that intensive long-term sampling and appropriate statistical methods were required to visualize the dynamical diversity of these tropical forest insect communities.

The Costa Rican fruit-feeding nymphalid butterfly Cithaerias pireta (Satyrinae: Haeterini) feeding on a fallen fruit of the rainforest tree Dipteryx panamensis (Fabaceae). Rodents often gnaw the thin pulp covering the seed, thus making fermenting juices available to these butterflies. Photo by P. J. DeVries.

Finally, our work suggests that intensive long-term monitoring, and robust statistical analysis of changes in community composition with respect to environmental factors have the potential to illuminate the dynamics of communities, and help predict their response to climate change.

References

Grøtan, V. et al. 2012. Seasonal cycles of species diversity and similarity in a tropical butterfly community. – J. Anim. Ecol. 81: 714–723.

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