Central-place forager effects on food web dynamics and spatial pattern in Northern California meadows

Ecology, June, 1998 by Jonathan M. Chase

INTRODUCTION

A central-place forager is a consumer that has a place from which it forays, but to which it must return after a given foraging bout (Orians and Pearson 1979, Schoener 1979, Stephens and Krebs 1986). Consumers can be confined to a central place for a number of reasons. Animals that provide food for offspring on a nest, animals that hunt from a particular spot, and animals that defend their territories from a central area are all examples of central-place foragers. Another scenario in which consumers are restricted to a central place occurs when animals remain in or near refugia in order to reduce exposure to predators, but must emerge periodically to forage (e.g., Sih et al. 1988, Abramsky et al. 1990, 1992).

As a central-place forager ventures from its refuge, its costs increase with distance from the central place due to travel time (and predation risk if the forager is restricted to refuges from predation). By definition, a forager always returns to its central place after a given foraging bout. Thus, an optimal central-place forager should concentrate its efforts near refugia, and decrease its foraging effort with increasing distance from that refuge. Although not explicitly considered in foraging behavior models, the variability of foraging effort with distance from the central place necessarily affects the population dynamics of the forager's prey. Thus, central-place foragers can have cascading effects on the rest of the food web, but those effects should vary spatially in relation to the proximity of the central place. The impacts of such foragers should be strongest directly adjacent to the central place, diminishing with distance. Regardless of the mechanisms that restrict a consumer to a central place (e.g., predation risk, territoriality, nests), patterns of food web dynamics and trophic abundances should vary in space relative to central places and refuges provided by the external environment (see also Power 1984, Power et al. 1989, Abramsky et al. 1990, 1992).

In this study, I examined the direct effects of a predator, the western fence lizard, Sceloporus occidentalis (Squamata: Iguanidae), on its insect prey (primarily herbivorous grasshoppers; Orthoptera: Acrididae and Tettigonidae), and the indirect effects extended to meadow plants. Western fence lizards, as their common name suggests, are generally found on or near various structures such as fences, rock and brush piles, and various other structures (Van Denburgh 1922, Smith 1946, Marcelini and Mackey 1970, Davis and Verbeek 1972, Leviton 1972, Stebbins 1985). I conducted observational studies to examine the extent and possible cause of the lizard's restriction to structures. In addition, I took advantage of a "natural experiment" in which lizards resided on most, but not all such structures, and tested for relationships between the abundances of grasshoppers or plants and distance from structures with and without lizards. Finally, I conducted an exclosure experiment to test hypotheses about the causes of the observed patterns. I conclude that the impact of the western fence lizard on the meadow food chains varies considerably over small spatial scales, as a result of the lizard's restriction to a central place.

STUDY SYSTEM

Sceloporus occidentalis (hereafter, western fence lizards) are common and conspicuous diurnal insectivores that occur throughout much of western North America (Stebbins 1985). Western fence lizards are habitat generalists that occur in several different types of communities, including grasslands and meadows (e.g., Marcelini and Mackey 1970, Davis and Verbeek 1972, Rose 1976, Stebbins 1985). This study was conducted on the Angelo Coast Range Reserve in northern California (Mendocino County, California, United States; 39 [degrees] 44[minutes] N, 123 [degrees] 39[minutes] W), June-September 1995. Several small meadows (size range 0.5-8 ha) occur as islands within the old-growth Douglas fir (Pseudotsuga menziesii) and coastal redwood (Sequoia sempervirens) forest along the terrace of the South Fork of the Eel River. These meadows have been present at least as long as the earliest European settlements (Johnson 1979).

Within the meadows, there are a variety of small structures on which western fence lizards appear to reside, including tree stumps, brush piles, and rock outcrops. These structures vary in size and location within the meadows, but appear to be central to the activity and residence of these lizards (Van Denburg 1922, Smith 1946, Davis and Verbeek 1972, Leviton 1972, Stebbins 1985). Grasshoppers (Orthoptera: Arididae and Tettigonidae), dominated by Melanoplus devastator, composed [greater than]85% (by biomass) of the grassland herbivores (invertebrate and vertebrate) at this site. Western fence lizards are generalist insectivore predators that readily catch and eat grasshoppers (Smith 1946, Rose 1976, Stebbins 1985). In these meadows, grasshoppers composed [greater than]80% of the lizard's diet (J. M. Chase, unpublished data), suggesting that this might be an important food web link.