Population Density and Patch Size: A Field Study of within and between Generation Variability

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ABSTRACT.-

How population density scales with habitat area is an important consideration for spatial population dynamics and community patterns. I examined the relationship between local population density and habitat area for an herbivorous beede Tetraopes tetraophthalmus (Forster) inhabiting patches of its host plant Asclepias syriaca L. Field observations over 4 y (1992, 1995-97) at the same site revealed that the relationship was variable during the adult fight season. Highest densities occurred on large patches early in the season, but as the season progressed, densities tended to equilibrate among patches of various sizes. Among years the relationship also showed considerable variability, but similar within generation patterns. Considering entire generations, increasing, decreasing and constant density with patch size were observed over the four years of observation. The results of this study indicate that there is considerable within and between generation variability in the density-area relationship. How population density varies with habitat area affects conservation reserve design and underlies spatial population and community theory. Variability in density-area relationships will introduce greater uncertainty for predictions of metapopulation persistence as well as estimates of community structure. Given the degree of variability seen in this system, any single estimate of a density-area relationship would be misleading. Multiple estimates, taken both within and between generations, should be performed before applying density-area relationships in studies of spatial population dynamics, community patterns and reserve design.

INTRODUCTION

A basic area of inquiry for spatially segregated populations is how local population density varies with habitat area. This relationship, called the individuals-area relationship (Schoener, 1986) or the density-area relationship, underlies ecological dieory (MacArthur and Wilson, 1967; MacArdiur et aL, 1972; Schoener, 1986; Hanski, 1994; Connor el aL, 2000; Matter, 2000) and is pertinent to conservation issues (Haila, 1988; Simberloff, 1988; Andren, 1994; Hanski, 1994; Connor et aL, 2000; Gaston and Matter, 2001) and pest control (Root, 1973; Denno et al, 1981; Kareiva, 1983). How population density varies with habitat area affects single-species population and metapopulation dynamics. Within a network of populations, increasing or decreasing density with area clusters individuals into large or small patches, respectively. This clustering of individuals changes the impact of density-dependent processes and the relative importance of different sized patches for spatial population dynamics and persistence (Hanski, 1994; Matter, 2000; 2001a). Similarly, local populations of the same size, but within networks of patches with different density-area relationships can show different dynamics (Matter, 1999; 2001a). Clustering of individuals into large or small patches can also affect community-level patterns, such as the species-area relationship and nestedness, which assume that die population density of component species does not vary with site area (Connor et aL, 2000; Matter, 2000).

Density-area relationships have been considered a species specific trait, resulting from social or behavioral constraints or patch size dependent dispersal patterns specific to a species (Connor et aL, 2000). Lending credence to diis belief, examinations of densityarea relationships generally have found consistency, at least in terms of sign, for species for which multiple estimates have been made (Bowers and Matter, 1997; Connor et aL, 2000). Relatively litde attention has been given to variability in the relationship. Examining and understanding this variability is critical if density-area relationships are to be of any practical use. For example, metapopulation models assume that the risk of population extinction decreases with increasing population abundance. To limit the amount of data needed, population abundance is often assumed to increase with habitat area and extinction risk is then related to habitat area dirough an exponential relationship, radier than directly to population abundance (Hanski, 1994). …