Presentation on theme: "Did predation cause prey population declines on pro-wildlife ranches in Laikipia District? RATIONALE Conservation in African drylands will entail the persistence."— Presentation transcript:
Did predation cause prey population declines on pro-wildlife ranches in Laikipia District? RATIONALE Conservation in African drylands will entail the persistence of wildlife in human-occupied landscapes, and management intervention that is guided by an understanding of how anthropogenic factors influence large-scale ecological processes. In Laikipia District, a dry savanna region in central Kenya, wild ungulates share the landscape with humans and livestock. We examined why five of nine abundant wild ungulate species (waterbuck, Thomson’s gazelle, buffalo, eland and hartebeest) declined in abundance for more than a decade on properties where wildlife was actively conserved and highly valued (the ‘pro-wildlife properties featured in the previous poster). Declines were substantial, amounting to 37% of total wild herbivore biomass (excluding elephants) between 1990 and 2005. Declines were unintended, unwelcome, and not understood. Of additional interest was why other species declined only slightly or not at all (plains zebra, Grant’s gazelle, impala, and giraffe). THE APPROACH Focusing on hartebeest where appropriate (the species that declined the most), but generalizing to other species, we addressed twelve possible causes of decline for which data permitted a direct test, or available information permitted a reasoned evaluation. The first five are human-induced: displacement by livestock, habitation, or cultivation, and over-consumption, either due to illegal hunting or sanctioned harvesting. The remaining seven are ostensibly ‘natural’: exceptional rainfall patterns, inter- and intra-specific competition, parasitism, low birth rates, low survival rates, and predation. Acknowledgements This study was supported by Mpala Wildlife Foundation. We thank G. Grant, J. Ruggieri and P. Valentine, O. de Beaumont, and E. Parfett for permission to work on their ranches, and C. Mortensen for contributing such sound ecological observations. G. Powys, T. Dyer, M. Littlewood, J. Wreford-Smith, and J. Evans shared their valuable recollections of changing wildlife and livestock management in Laikipia over recent decades. Research assistance from F. Lomojo was greatly appreciated. Fig. 2. ‘Bottom-up’ to ‘Top-down’ control: Hartebeest population dynamics in Laikipia District, 1985-2005 (circles), divided into an initial phase during which the population was rainfall-dependent (filled circles), and a latter phase when the population declined steadily (open circles). The first phase was modeled by Δ = 483.4R–438.4 (r=0.83, n=6, P<0.05), were Δ is the change in hartebeest numbers on pro-wildlife properties, and R is the Rainfall Deficit Index from one census to the next. The solid black line depicting the rainfall-dependent model is projected beyond 1997 as a dotted black line, showing how the observed population trajectory deviated radically from expectations based on rainfall. The declining phase was modeled by a best-fit simulation (solid grey line), in which the number of calves and sub-adults reaching adulthood was only 5-10% per year. RESULTS Of a dozen factors potentially contributing to observed herbivore declines on pro-wildlife ranches in Laikipia District, the following could not be ruled out: illegal hunting of eland when they move off pro-wildlife properties; short-term effects of disease associated with an exceptionally wet El Niño event in 1997-1998; short-term effects of drought associated with an exceptionally dry La Niña event in 1999-2000, possibly interacting with lungworm in hartebeest; low survival in hartebeest, especially among juveniles and adult males, and possibly low birth rate; and predation. Each may have contributed, but only predation is consistent with the timing, synchrony, duration and species composition of the observed declines (Figs. 1 and 2). Fig. 1. Among hartebeest, there were striking differences in age structure (shown at right) between Solio ranch, where predators have been absent from for several decades, and four core ranches (Mpala, Segera, Ngorare and El Karama), where predators are abundant (map shown at left). A further predator removal comparison was afforded when hartebeest were enclosed within a predator-proof sanctuary on Mugie ranch in 2003. Their numbers doubled within two years, and all calves survived. OTHER WILD HERBIVORES PREDATORS CATTLE ZEBRAS RAINFALL + + - - - + 1 2 4 3 - - 5 6 HUMANS THE PRINCIPAL CAUSE OF INCREASING PREDATION The principal factor causing predators to increase in Laikipia was a change in land use from cattle ranching to wildlife conservation and eco-tourism that began in the early 1990’s. Under ranching, plains zebras and predators were severely suppressed. The switch to eco-tourism allowed a 5-fold increase in plains zebra abundance, and created a demand for living predators. Although not limited by predators, plains zebras supported a substantial predator community. We infer that increasing predator pressure caused predation-susceptible prey species to decline. DO CATTLE AFFECT PREDATOR-PREY DYNAMICS VIA COMPETITION WITH ZEBRAS? Adding cattle to the landscape may affect predator-prey dynamics in both direct and indirect ways, depending on rainfall: in extreme years (floods or drought), episodic die-offs of livestock directly subsidize predators; in below-average rainfall years, competition between cattle and wild grazers (primarily zebras in Laikipia) may indirectly limit predator carrying capacity. As a consequence, removal of cattle to favor wildlife may actually depress predator-susceptible prey species (Fig. 4). SYNTHESIS AND APPLICATIONS Herbivore dynamics in Laikipia shared features with previously reported responses by herbivores to predator manipulation in Kruger and Serengeti National Parks. All revealed profound impacts of predators on only a subset of prey species that typically are not apparent under unperturbed conditions. And all featured one or a few numerically dominant herbivore species, which were primarily limited by rainfall or density, supporting a predator community that in turn limited the abundance of other prey species (Fig. 3). Among ungulate prey species in Laikipia, plains zebras and giraffes, the dominant grazers and browsers, are not limited by predators, but by rainfall and density-dependence. By contrast, eland, hartebeest, and waterbuck seem to be limited by predation. Fig. 3. Ranked contributions to total biomass by dominant prey species in Serengeti, and in Laikipia before (centre) and after (right) the restoration of predators. Where known, species are coloured according to whether they are limited by rainfall (blue), density (green), or predation (red). Changes in prey community structure in Laikipia reflect the switch from ‘bottom-up’ to ‘top-down’ control. Wildebeest Zebra Giraffe Hartebeest Zebra Giraffe Hartebeest Eland SERENGETI NP LAIKIPIA 1990 LAIKIPIA 2005 Fig. 4. This hypothesis is expounded as follows: Plains zebras and cattle are rainfall-limited in Laikipia (1), and both are moderate-sized grazers. These are necessary and may be sufficient conditions for them to be competing, at least in dry years (2). Zebras form the majority of the prey biomass, and support a large predator community, but are not limited by predators (3). Other wild herbivores, however, are predator-limited (4). Removal of cattle may therefore result not only in an increase in zebras, but also in predators, causing further declines in other prey species. The principal elements of this hypothesis require confirmation, particularly the relative extent to which humans in the landscape affect predators and prey (5 and 6). a a a a b a a a a b b aa a a Nicholas Georgiadis, Festus Ihwagi, and J. G. Nasser Olwero Mpala Research Centre
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