Trophic rank and the species-area relationship

Ecology, July, 1999 by Robert D. Holt, John H. Lawton, Gary A. Polis, Neo D. Martinez

INTRODUCTION

The tendency for species richness to increase with area (the "species - area relationship") is one of the most robust empirical generalizations in ecology (May 1975, Rosenzweig 1995). Most studies of species - area patterns have focused on particular taxa, guilds, or functional groups, rather than broader comparisons within entire communities. Yet, comparisons of species - area relationships among taxa or functional groups can highlight essential differences in their spatial dynamics and responses to spatial heterogeneity (Kareiva 1994). For instance, biogeographic studies of West Indies vertebrates reveal stronger species - area relationships for nonflying mammals than for bats or birds, consistent with the likely greater importance of mobility for determining local community composition in the latter groups (Wright 1981).

A familiar way to characterize the structure of entire communities is to construct food webs, which are interlinked chains of trophic interactions that define energy and material flows among species (Pimm 1982, Cohen et al. 1990). An enormous amount of work has been devoted to empirical and theoretical food web analyses (e.g., Martinez 1991, Pimm et al. 1991, Havens 1993, Polis and Winemiller 1996), with a growing interest in spatial aspects of food web ecology (e.g., Briand and Cohen 1987, Schoener 1989, Warren 1989, Winemiller 1990, Martinez and Lawton 1995, Holt 1996a, b, Polis et al. 1996, 1997, Harte and Kinzig 1997). A simple descriptor of a species position in a food web is its "trophic rank." Our specific purpose in this paper is to explore the proposition that trophic rank may systematically influence the strength of the species - area relationship. Our more general aim is to highlight the importance of linking studies of food web structure with spatial and landscape ecology.

There are various ways to define trophic rank (Yodzis 1989:209). For instance, if for each quantum of energy consumed by an individual in a focal species, one were to back-calculate the number of species through which that quantum had passed before being consumed, the "trophic rank" of the species might be the average length of all such energetic pathways. Ambiguities in assignment of species to trophic ranks arise principally because of trophic generalization (e.g., omnivores feed at multiple levels). If food webs were comprised entirely of specialists, with specialist carnivores consuming specialist herbivores feeding on single plant species, there would be no ambiguity in trophic rank assignment.

We first briefly review salient aspects of species - area theory. We then present a simple model that predicts that the species - area relationship should be stronger at higher trophic levels, when most consumers are trophic specialists. Next, we sketch empirical examples in which species - area relations were assessed for taxa differing in trophic rank. The theoretical predictions match some, but not all, patterns in these systems. In the Discussion, we examine alternative reasons why trophic rank might influence the species - area relation, as well as factors obscuring such an influence. In particular, we argue that the effect of trophic rank on the species - area relationship may often be weaker (or even reversed) in webs characterized by trophic generalists.

A PRECIS OF THE SPECIES - AREA RELATIONSHIP

There are three basic kinds of species - area relationships (Holt 1992, Rosenzweig 1995): (1) species richness vs. sample area in nested samples, within a defined habitat or region (a "type-1" species - area relationship); (2) total species richness vs. total area, among habitats or regions differing in area (e.g., islands in an archipelago; a "type-2" relationship); (3) local species richness in a sample of defined size, among habitats or regions differing in area (a "type-3" relationship). Types 1 and 2 have received the most attention in the literature (Rosenzweig 1995). Often, a power law S = c[A.sup.z], or, equivalently, log(S) = log(c) + z log(A), provides a reasonable statistical summary for the increase of species richness with area (Rosenzweig 1995), where S is the number of species and A is area. The quantity z describes the strength of the scaling of species richness with area; z is independent of the units used to measure area (Rosenzweig 1995:21).

There are three explanations for species - area relationships (Connor and McCoy 1979, Williamson 1981): sampling, habitat heterogeneity, and colonization-extinction dynamics.

Sampling. - Consider a type-1 species - area relationship, for instance nested quadrats used to sample a plant community. Very small quadrats necessarily contain few individuals; at small spatial scales, an increase in species richness with increasing quadrat size almost surely reflects merely an increase in sample size (Rosenzweig 1995). Sampling effects may also explain some type-2 relationships. Type-3 species - area relationships, however, automatically control for sample area, and so are less prone to sampling effects (Holt 1992).