Cicada Ecology in a Costa Rican Tropical Rain Forest

Abstract

The seasonal patterns of adult emergence in several co-occurring species of cicadas were determined in Costa Rican lowland tropical rain forest through exhaustive census of nymphal skins in three major habitats: primary-growth forest understory and floor, banks of forest streams, and old cultivated second-growth bordering forest. Both seasonal and horizontal habitat selection were demonstrated for the cicadas studied. Vertical habitat selection of adults in forest and micro-habitat selection in the forms of oviposition site-selectivity and eclosion substrates were also seen. Three species show strong seasonal peak adult emergence, two of these emerging in the dry season, and a third at the beginning of the wet season. These cicadas are periodical. Other non-periodical species emerge continually in low numbers throughout the year. Of the two emerging during the dry season, there is character displacement for annual alternation of peak adult emergence: both species appear to be very similar morphologically and occupy the same habitat but one species emerges one dry season and the other emerges the following dry season. This character displacement is hypothesized to be a unique mechanism of habitat selection made possible by the multi-year life cycles of cicadas, and functions to reduce competitive interactions between two sympatric congeneric species. For another pair of congeneric species in the same habitat, micro-habitat selection was seen in oviposition and eclosion sites, another mechanism reducing interspecies competition in closely related sympatric species of cicadas. TERRESTRIAL AND MARINE tropical communities are characterized by the co-occurrence of many closely related species or groups of species (Klopfer and MacArthur 1961, Pianka 1966, MacArthur 1969, Kohn 1971). Despite the presumably high specialization of interactions between herbivorous insects and their host plants in the tropics (Ehrlich and Raven 1965, Janzen 1970), there have been relatively few studies on the population biology of tropical insects in general, excluding, of course, those species which transmit diseases. Clearly such studies are needed to test various hypotheses regarding the structure and complexity of tropical communities. Long-term studies on the seasonal emergence of insects with multi-year life cycles and their mechanisms for habitat selection (Cody 1968) should give an insight into the maintenance of high local species densities in tropical communities. Cicadas, a group of herbivorous insects known for their multi-year life cycles, are very diverse in the Central American tropics. Metcalf (1963) lists seven genera and 14 species from Costa Rica, and more recent collections by myself and others (Dr. Thomas E. Moore) have brought this latter figure up to about 30 species. This is approximately the same taxonomic complexity found in all of North America east of the Mississippi River. Despite the wealth of population data on temperate cicadas (Beamer 1928, Marlatt 1907, Moore and Alexander 1958, Alexander and Moore 1962, Dybas and Davis 1962, Dybas and Lloyd 1962), tropical cicadas have been neglected in this respect. Most studies of cicadas in tropical regions of the world have been qualitative notes on outbreaks (e.g., Champion 1911, Innes 1915, Finlayson 1934). This paper comprises the first report on the seasonal population emergence pattern and habitat selection for several co-occurring cicadas in a Costa Rican lowland tropical wet forest (Holdridge 1967).