Data and code from: The effect of resource concentration on consumer population densities depends on spatial scale and diet breadth

Le résumé fourni par la source

Herbivore responses to host plant concentration and plant community diversity are predicted to depend on diet breadth, yet most studies to date have focused on dietary specialists. The few that have included generalist herbivores have largely been conducted in agricultural settings or measured resources at a single spatial scale, leaving it unclear whether these predictions hold in natural systems across multiple spatial scales. Here, we examine how host plant concentration and plant community diversity shape the population densities of dietary specialist and generalist larval Lepidoptera (caterpillars) in secondary forests of New England. We measured host plant assemblages and densities at scales from 3–400 m to assess the patch size-specific effects of host concentration and plant diversity. To accurately characterize the resource landscape for herbivores that consume >1 host species, we weighted host plant concentration by electivity (a measure of each species' observed frequency of association with each host plant relative to its availability) and demonstrate that this weighting is necessary to detect responses that are otherwise obscured. We found that dietary generalists’ population densities correlated positively with their host plant concentrations at scales up to 25 m: a pattern that was only detectable when electivity was considered. In contrast, dietary specialists’ population densities correlated positively with their host plant densities at scales < 3 m and negatively at scales 25–50 m. This suggests that specialists aggregate in small, dense patches of resources, while their density saturates or declines at intermediate scales, consistent with resource dilution, not concentration. Neither generalist nor specialist caterpillar populations responded strongly to plant community diversity at any scale. These findings demonstrate that herbivore responses to resource concentration are contingent on diet breadth and spatial scale, that detecting generalist responses requires diet- breadth-appropriate measurement, and that the influence of plant diversity on herbivore density documented in agroecosystems may not generalize to structurally complex natural forests.

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