Mammal carcasses, migration, and drought in Amboseli National Park, Kenya
Research on the recent bones of Amboseli Park originally was aimed at understanding how carcass destruction and preservation affect biological information preserved in fossilized samples of vertebrate communities. These data also can be used to examine carcass input to this tropical ecosystem, and here we compare patterns of mortality in migratory versus resident large (>15 kg) herbivores. Data on the modern surface bone assemblages in Amboseli's 600km2 central basin have been collected periodically between 1975 and 2010 using survey plots in 6 major habitats. Each survey recorded thousands of bones from hundreds of individuals. Skeletal remains range from fresh to highly weathered, and carcasses with known times of death are used to calibrate years post-mortem for 6 sequential weathering stages (WS 0-5, fresh to extremely weathered). Aggregating skeletal records from these surveys into WS 0-2 and WS 3-5 allows binning of the data into ~6-year samples. Live census data from air and ground surveys are available for the same species and time intervals, and the combined live-dead data provide insights about spatial and temporal patterns of mortality and how these contribute to ecosystem dynamics. Wildebeest and zebra, the dominant grazing herbivores, migrate outside of the park during rainy seasons but return to the permanent swamps in the central basin for dry-season foraging. Other species such as impala, Grant’s gazelle, and giraffe are resident in the basin year-round.
Four time bins, 1964-69, 1970-76, 1993-98, and 1999-2004, represent ecosystem processes across four decades, when loss of woody cover increased the area of grasslands in the central basin. The bone data in these bins show that carcasses of migratory species are increasingly abundant in the swamp and plains habitats over time. In their normal diurnal cycle, grazing species concentrate in the swamps by day and move to the plains at night, where predation distributes carcasses (and nutrients) away from the dry-season foraging habitat. During droughts, the migratory populations of grazers deplete forage in the swamps, and in the extreme drought of 2009, 95% percent of wildebeest and 65% of zebra populations died in an area representing less than 1% of the total ecosystem. The browser populations, in contrast, show little seasonal movement or change through time in carcass distributions. In this grassland-dominated savanna, herbivore mortality is concentrated in the migrants’ late season range, contributing a large proportion of carcass-derived nutrients to a relatively small area of the ecosystem.