Rainfall and Seasonal Changes in Arthropod Abundance on a Tropical Oceanic Island

Abstract

Weekly sweeps for arthropods were taken and compared for an entire year (1977) on Grenada, West Indies, where there is a marked wet and dry season. The average wet-season abundance of arthropods was 2.3 times greater, and the wet-season biomass (grams) was 3.1 times greater, than that for the dry season. A significant correlation between biomass and abundance existed for the entire year. There was no change in arthropod size between wet and dry season. Intra-seasonal variations in arthropod abundance occurred, and were significantly correlated with seven-day accumulated rainfall following a three-week interval. Abundance of seven of 10 taxa showed the same three-week response to rainfall. Coleoptera were identified to species, so that species diversity, species overlap, and feeding guilds for the entire year could be compared to Central American Coleoptera. The results indicate the Coleoptera fauna on Grenada are more generalized compared to the tropical mainland, and composed of the same species whose abundance fluctuates seasonally throughout the year. MOST STUDIES OF SEASON ALITY in insect assemblages have been conducted on the tropical mainland (Bigger 1976; Buskirk and Buskirk 1976; Janzen 1973a, b 1976; Janzen and Pond 1975; Janzen et al. 1976; Janzen and Schoener 1968; Ricklefs 1975; Robinson and Robinson 1970; Smythe 1974; Wolda 1977, 1978). In contrast, there are only a few such studies of seasonality on tropical islands (Allan et al. 1973; Frith 1975; Janzen 1973a, b; Pipkin 1953). Data collected on a reguar basis throughout the wet and dry seasons have not been gathered for any oL the neotropical island insects. Such a study would be useful in detailing the basic difference between island and mainland ecology, as well as in adding new information to the subject of tropical seasonality. The purpose of this paper is to, investigate the seasonal changes that occur in tropical arthropods on an oceanic Caribbean island with a marked wet and dry season. Species diversity, overlap, and feeding guilds of the Coleoptera are compared to similar data from Central American studies.