Food-web structure and ecosystem services: insights from the Serengeti

Abstract

The central organizing theme of this paper is to discuss the dynamics of the Serengeti grassland ecosystem from the perspective of recent developments in food-web theory. The seasonal rainfall patterns that characterize the East African climate create an annually oscillating, large-scale, spatial mosaic of feeding opportunities for the larger ungulates in the Serengeti; this in turn creates a significant annual variation in the food available for their predators. At a smaller spatial scale, periodic fires during the dry season create patches of highly nutritious grazing that are eaten in preference to the surrounding older patches of less palatable vegetation. The species interactions between herbivores and plants, and carnivores and herbivores, are hierarchically nested in the Serengeti food web, with the largest bodied consumers on each trophic level having the broadest diets that include species from a large variety of different habitats in the ecosystem. The different major habitats of the Serengeti are also used in a nested fashion; the highly nutritious forage of the short grass plains is available only to the larger migratory species for a few months each year. The longer grass areas, the woodlands and kopjes (large partially wooded rocky islands in the surrounding mosaic of grassland) contain species that are resident throughout the year; these species often have smaller body size and more specialized diets than the migratory species. Only the larger herbivores and carnivores obtain their nutrition from all the different major habitat types in the ecosystem. The net effect of this is to create a nested hierarchy of subchains of energy flow within the larger Serengeti food web; these flows are seasonally forced by rainfall and operate at different rates in different major branches of the web. The nested structure that couples sequential trophic levels together interacts with annual seasonal variation in the fast and slow chains of nutrient flow in a way that is likely to be central to the stability of the whole web. If the Serengeti is to be successfully conserved as a fully functioning ecosystem, then it is essential that the full diversity of natural habitats be maintained within the greater Serengeti ecosystem. The best way to do this is by controlling the external forces that threaten the boundaries of the ecosystem and by balancing the economic services the park provides between local, national and international needs. I conclude by discussing how the ecosystem services provided by the Serengeti are driven by species on different trophic levels. Tourism provides the largest financial revenue to the national economy, but it could be better organized to provide more sustained revenue to the park. Ultimately, ecotourism needs to be developed in ways that take lessons from the structure of the Serengeti food webs, and in ways that provide tangible benefits to people living around the park while also improving the experience of all visitors.

Figure 1

Illustration of the nested relationship between Serengeti carnivores and their prey (data adapted from Sinclair et al. 2003b). The figure in the top left illustrates the prey species on the vertical access and the predators on the horizontal access. The figures to the right and bottom illustrate the degree to which each species of prey and predator fail to fit the underlying pattern of nestedness. The histogram at the bottom compares the probability of obtaining the observed pattern of nestedness (vertical line) with 200 random webs with the same number of predator–prey links, but with prey randomly assigned to each predator.

Figure 2

Illustration of the nested relationship between Serengeti herbivores and the plant species they consumed in the (a) wet and (b) dry seasons. The figures are organized in the same way as for figure 1. The wet-season network is significantly nested, the dry-season network is not significantly different from distribution of randomly assembled networks.

Figure 3

The relative abundance of different grass species in the 17 plant communities recognized by McNaughton (1983). The communities are listed along a rough gradient that runs from the dry south-eastern boundary of the park, north and west across the plains and woodlands to Lake Victoria and the Kenyan border. This spatial change is matched by an increase in annual rainfall and a decrease in altitude and soil quality. The communities do not follow a distinct well-defined gradient, but instead form a graded mosaic of habitats that interdigitate in ways that reflect local aspects of the topology and underlying soil characteristics.

Figure 4

Bifurcation diagram of the relationship between rates of interspecific transmission and the number of infected hosts in each of four host species that potentially share the same pathogen (Dobson 2004b). The abundance (carrying capacity) and birth and death rates of each host species are allometrically scaled to their body size; the smallest species has the fastest birth and death rates and settles to the highest carrying capacity (DeLeo & Dobson 1996). The other three species each illustrate a doubling of body mass and this determines their abundance, birth and death rates, and within-species rates of transmission. Between-species transmission is scaled along the x-axis to be a proportional value of the averaged rates of within-species transmission. When between-species rates of transmission are low, each species exhibits epidemic cycles whose amplitude and frequency are determined by its vital rates (small species have faster larger cycles than larger species). As rates of interspecific transmission increase, the dynamics of all species become coupled and they all settle to relatively constant abundance. As rates of interspecific transmission approach rates of intraspecific transmission, the species that recovers fastest from the disease outbreaks can use the pathogen to drive the other hosts extinct; the surviving species then exhibit the epidemic cycles they exhibited in the absence of between-species transmission.

Figure 5

Sketch of the Serengeti web for the larger species of vertebrates. The x-axis attempts to organize the species along a geographical access that runs from the short grass plains in the south of the park through the long grassland to the woodlands in the north and west; effectively, the plant communities described by McNaughton (1983; figure 3) run along this axis. The y-axis provides an indication or relative trophic level for the vertebrate species that derive the majority of the annual nutritional requirements from the habitats of the x-axis.

Figure 6

The proportional economic value of ecosystem services provided by species on each trophic level. The original data are provided in the paper by Costanza et al. (1997); the study represents data from a variety of different ecosystems where annual economic benefit to the human economy has been estimated. For each study, I have divided the services into which trophic level dominates their production and then divided the revenue accordingly. I then determine the proportion of revenue provided by each trophic level for each study and determine an average figure across all studies.