Abstract

Insect herbivores can be limited by host plants in two ways: density-dependent competition for food resources or density-independent search time limitation. Our understanding of density-dependent host plant limitation is relatively well developed and well integrated into conservation plans for at-risk insects. Search time limitation, a density-independent process, is much less well developed. Here, we explore both mechanisms using empirically based models of monarch butterfly population dynamics. These mechanisms differ fundamentally in their predictions: resource competition leads to matching of herbivore densities to host plant densities, and visible competition via consumption of host plants. Search time limitation leads to changes in population growth rate that can cause herbivore numbers to decline when host plant densities are constant. Search time limitation also implies that host plants can limit herbivores, even when many individual plants are uneaten. For monarch butterflies, our calculations suggest that many parts of North America have host plant densities below the threshold for search time limitation, which contrasts with the typical assumption of resource competition. More generally, incorporating search time limitation into conservation plans is important for reframing our understanding of how host plants limit insect herbivores in highly fragmented landscapes.

Highlights

  • The assumption that resource limitation drives population declines is threaded throughout applied ecology

  • Time limitation model of monarch butterflies, based on integrodifference equation models for insect populations in patchy landscapes (Crone et al 2019; Musgrave and Lutscher 2014). We explore both models numerically to identify host plant densities under which population dynamics are likely to be limited by resource competition vs. search time limitation

  • This method of projecting population dynamics assumes that host plant densities are reset each generation, which could be caused by phenological turnover of host plant species (Yang and Cenzer 2020) or, in the case of monarch butterflies, movement to other parts of the breeding range

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Summary

Introduction

The assumption that resource limitation drives population declines is threaded throughout applied ecology. Time limitation model of monarch butterflies, based on integrodifference equation models for insect populations in patchy landscapes (Crone et al 2019; Musgrave and Lutscher 2014). We explore both models numerically to identify host plant densities under which population dynamics are likely to be limited by resource competition vs search time limitation.

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