Ecological Characterization of a Costa Rican Dry Forest Caterpillar Fauna

Abstract

The dry forests of Costa Rica's Santa Rosa National Park (lION lat., 0-350 m elevation, 6 mo without rain, 900-2300 mm rainl yr) range from deciduous and 2 m tall to nearly evergreen and 30 m tall. This vegetation and a complex mosaic of secondary succession in old fields and pastures supports at least 3140 species of caterpillars. Butterfly caterpillars constitute only 11 percent of the species and less than 1 percent of the biomass. Although the moth caterpillars are very species-rich, each of the 72 5 species of vascular plants is attacked by less than 20 species of caterpillars. From the viewpoint of a species of plant, the caterpillar species richness in Sanra Rosa is therefore substantially less than from the viewpoint of any of a number of extratropical tree species; they are fed on by many more species of caterpillars at a single location. Although Santa Rosa does have some species of caterpillars that feed on seeds, dung, detritus, bark, etc., more than 95 percent of the species eat green leaves. Of these, about 37 percent feed exposed on the leaf surface. A Santa Rosa woody plant commonly loses 1-20 percent of its leaf area to defoliators during the first half of the rainy season and very little after that, but on rare occasions plants are totally defoliated. These defoliations are almost always committed by a single generation of 1 or 2 species of caterpillars feeding on their 1 or 2 sole host plants. Nearly all the species of Santa Rosa plants are not fed on by any given species of caterpillar either because the plants contain chemicals that render them unacceptable or because the caterpillar refuses to eat foliage that lacks a feeding stimulant. Simultaneously, it is the negative impact of inclement weather and carnivores on Santa Rosa caterpillar populations that prevents any given species of caterpillar from usually defoliating the few species of plants that it can eat. The Sanra Rosa carnivore fauna ranges from absolutely monophagous to highly polyphagous. Irs caterpillar food supply fluctuates enormously with year, season, position in a season, and microsite. This favors generalized and flexible carnivores on the one hand, and strongly specialized carnivores on the other hand. Many caterpillars are not available to the carnivores because they are too cryptic to be found, personally well-defended by urticating spines or chemicals (and are aposematic), mimics of distasteful or dangerous caterpillars, or participating in predator-satiating synchronization of life-stages. Probably no carnivore recognizes more than about three kinds of prey or host: those that are perceived and rejected, those that are eaten or used directly when encountered, and those that require some special treatment. No two species of carnivores divide the many species of caterpillars equally among these three kinds of prey or hosts. Certainly no carnivore views the Santa Rosa caterpillar fauna as consisting of 3140 species. Vittually all Santa Rosa caterpillar species share their host plants with less than 20 other caterpillar species (the average may be less than 5). This means that from the direct perspective of anyone species of caterpillar, the Santa Rosa caterpillar fauna is quite small and potential competitive interactions are very limited in ecological time. Furthermore, actual direct competitive interactions are rare (if they occur at all) owing to the generally low density of caterpillars at any given point in space and time. However, there are many potential opportunities for indirect intra- and interspecific caterpillar interactions that are mediated through shared carnivores (such as parasitoid Hymenoptera and Diptera, diseases, vettebrates, spiders, etc.).