Seasonal and Annual Variation of Insect Abundance in the Nairobi National Park, Kenya

Abstract

A Malaise trap erected at the interface of grassland and forest in the Nairobi National Park was used to monitor insect abundance during a five-year period. Annual rainfall patterns during the study were extremely variable and included periods of major drought. Increases in insect density that followed the rains were largely due to an increase in the number of individuals per species, but the number of species also increased as much as threefold. The timing of peak seasonal abundance of phytophagous insects (grasshoppers and leafhoppers) and parasitic Hymenoptera was variable from year to year, thus suggesting dependence on environmental variables such as rainfall or host density. By contrast, several insect groups less dependent upon plants showed distinct seasonal meaks of abundance that were quite consistent each year. One annual peak of abundance was observed among Chrysopidae, Psocidae, Tabanidae, and Sarcophagidae. Two peaks were recorded for Asilidae and Satyridae. SEASONALITY IS A CONSPICUOUS FEATURE in the life history of many organisms in temperate zones, but information for tropical organisms remains sketchy. In the tropics, temperature changes are slight, and seasonal changes in rainfall exert the most dramatic effect on the environment. Rainfall patterns, however, can vary greatly from year to year, and for some organisms low rainfall is not likely to present as severe an obstacle as winter in a temperate zone. Thus, for some tropical species development appears to be aseasonal (e.g., Owen and Chanter 1972, Ehrlich and Gilbert 1973, Leuthold and Leuthold 1975) or to continue over a prolonged period (MacArthur 1972). Yet, many other tropical species are highly seasonal, perhaps as a response to close packing of competitors (MacArthur 1972) and to the selection of seasons that are best suited for active development. Distinct seasonal cycles have been observed in tropical plants (Janzen 1967, Burger 1974), fish (Kirschbaum 1975), birds (Snow and Snow 1964), rodents (Field 1975), some large mammals (Leuthold and Leuthold 1975), and several groups of insects from the Neotropics (Fairchild 1942, Galindo et al. 1956, Ricklefs' 1975) and Africa (Owen and Chanter 1970, Owen 1972, Duviard and Pollet 1973, Bigger 1976, Leston 1977). This-report examines the temporal distribution of insects sampled with a Malaise trap in the Nairobi National Park, Kenya, during a five-year period from August 1972 to July 1977. Marked seasonality was observed among several insect taxa, and the overall abundance of insects fluctuated greatly, especially in response to several periods of severe drought. METHODS STUDY AREA.-The trap was erected in grassland 2 m from a forest edge (fig. 1) in the western portion of Nairobi National Park (1?21'S, elevation 1800 m) within 2 km of the Langata gate. According to the Holdridge classification system (Holdridge et at. 1971) the forest would be considered a tropical premontane dry forest. It is dominated by Brachylaena hutchinsii and Croton megalocarpus while the grassland, which is derived from forest, is characterized by Themeda triandra: and Pennisetum clandestinum (Verdcourt 1962, Lind and Morrison 1974). Characteristically, the region has two rainy seasons (fi. 2): the long rains from mid-March to the end of May bring about 450 mm of rainfall and the short rains from mid-October to mid-December yield around 250 mm. Precipitation at other times brings the annual total to 900 mm (50-year average). Mean monthly temperature (fig. 2) ranges from 17?C during July and August to 20?C in March. Monthly rainfall data used in figure 3 were based on records of the East African Meteorological Department for Wilson Airport, a station located 4 km from the study