Bottom‐up and top‐down processes in African ungulate communities: resources and predation acting on the relative abundance of zebra and grazing bovids

Abstract

African ungulate populations appear to be limited principally by their food resources. Within ungulate communities, plains zebras coexist with grazing bovids of similar body size, but rarely are the dominant species. Given the highly effective nutritional strategy of the equids and the resistance of zebras to drought, this is unexpected and suggests that zebra populations may commonly be limited by other mechanisms. Long‐term research in the Serengeti ecosystem and in the Kruger National Park suggests that zebra could be less sensitive to food shortage, and more sensitive to predation, than grazing bovids: if this is a general principle, then, at a larger scale, resource availability should have a weaker effect on the abundance of zebra than on grazing ruminants of similar body size (wildebeest and buffalo), and zebras should be relatively more abundant in ecosystems where predators are rare or absent. We test these expectations using data on 23 near‐natural ecosystems in east and southern Africa. The abundance of wildebeest is more closely related to resources than is that of zebra; buffalo are intermediate. We show that hyena densities are closely correlated with those of lions, and use the abundance of lions as an index of predation by large predators. The numerical response of lions to increases in the abundance of their prey was linear for mesoherbivores, and apparently so for the three species alone. Finally, the abundance of zebra relative to grazing bovids is lower in ecosystems with high biomasses of lions. These results indicate that zebras may commonly be more sensitive to top‐down processes than grazing bovids: the mechanism(s) have not been demonstrated, but predation could play a role. If it is true, then when numbers of the large mammalian predators decline, zebra populations should increase faster than buffalo and wildebeest.

Ecological rules governing helminth–microparasite coinfection

“…Ecological methods can also identify which population control mechanism is most important for each species in an ecosystem. Zebras, for instance, appear to be primarily regulated by predation, whereas coherding wildebeest are regulated by the availability of food (18). Would analogous studies of parasite populations reveal that different, even closely related, parasite species are controlled within the host by distinct ecological mechanisms?…”

“…In some senses, this is unsurprising; microparasites are organisms, after all, and there is no reason to suspect that they are exempt from ecological processes that shape the abundance of organisms in macroecosystems (e.g., refs. [17][18][19][20]. Furthermore, IFN-␥ is central to the clearance of virtually all microparasites, despite differences in the details of which cell types or anatomical location might be involved (27,30).…”

“…However, the complexity of within-host interactions (12)(13)(14)(15)(16) can make it difficult to predict how one infection will affect the course of others. Disentangling such complexity is a core activity of community ecologists, who in recent years have successfully identified trophic rules that determine, for example, the abundance of organisms in marine (17) as well as tropical (18,19) and temperate (20) terrestrial ecosystems. These trophic rules include ''bottom-up'' control of population size via resource limitation and ''top-down'' control via predation (21).…”

“…These trophic rules include ''bottom-up'' control of population size via resource limitation and ''top-down'' control via predation (21). Only by considering regulation by both resources and consumers have ecologists been able to explain the abundance of organisms across such diverse communities (17)(18)(19)(20). One major finding is that the relative importance of top-down and bottom-up control mechanisms varies among species and ecosystems; a charismatic example of this is provided by African savannah systems, in which zebra populations are primarily controlled by predators, whereas wildebeest are more strongly regulated by the quality and quantity of plant matter available for them to eat (18).…”

“…Only by considering regulation by both resources and consumers have ecologists been able to explain the abundance of organisms across such diverse communities (17)(18)(19)(20). One major finding is that the relative importance of top-down and bottom-up control mechanisms varies among species and ecosystems; a charismatic example of this is provided by African savannah systems, in which zebra populations are primarily controlled by predators, whereas wildebeest are more strongly regulated by the quality and quantity of plant matter available for them to eat (18). I applied these principles on a different scale in the present study, to investigate whether defined within-host ecological interactions, namely, competition for resources (bottom-up population control) and/or predation by the immune system (top-down control) (22), could explain changes in microparasite population size caused by underlying helminth coinfection.…”