Document: KUR-3-88-10

Modeling the effects of spatial heterogeneity of resource subsidies on stream food webs.

ANDERSON, K.E.*, R.M.NISBET and S.D.COOPER

University of California, Santa Barbara, Santa Barbara, CA 93106 USA ¹

Abstract:
Streams provide a powerful framework for examining the effects of spatial variation in resource subsidies on complex food webs. Here, a model was examined to investigate how spatial heterogeneity in extrinsic resource inputs interacted with the behavioral and developmental responses of consumers to resource levels and predation risk to shape patterns of stream organism abundance and distribution. This model described a linear sequence of well-mixed stream habitat patches, with each patch containing an independent carrying capacity and tri-trophic food chain. Patches were connected by unidirectional flow; thus, organisms emigrating from one patch were immigrants to the next patch downstream. Producer, consumer, and predator equilibrium densities all tracked spatial variation in carrying capacity when the responses of consumers and predators were limited to unidirectional, downstream movement. This pattern emerged regardless of the spatial scale at which the variation occurred. The sensitivity of consumers to variation in resource levels and predation risk and the scale of movement that they could undertake in response affected the equilibrium densities of all members of the food web. When predators were allowed to move upstream as well as downstream, resulting densities approximated an ideal-free distribution, which tended to homogenize consumer densities. More complex patterns of organism distribution occurred when consumers and predators responded to variation in habitat quality both behaviorally and developmentally (e.g., through growth and/or mortality), leading to clumping of consumers at either the initial or terminal ends of the stream or in complete extinction depending on the range of parameter values. Taken in full, these results suggest that complex spatial patterns of stream organism abundance are not due to the scale or magnitude of spatial heterogeneity in extrinsic resource inputs per se; but are rather a result of the mode which consumers and predators respond to such variation. Of particular importance is the extent to which consumers and predators can move upstream, which has far-reaching impacts for their persistence and distribution.

Keywords: spatial hetergeneity, food webs, persistence, movement behavior, spatially explicit lattice simulations, well-mixed models

This abstract is being presented at: 8:45 AM in session:
Oral Session #55: Invertebrates in Streams: Foodwebs.