Microclimate and resource quality determine resource use in a range-expanding herbivore

Abstract

The consequences of climate change for biogeographic range dynamics depend on the spatial scales at which climate influences focal species directly and indirectly via biotic interactions. An overlooked question concerns the extent to which microclimates modify specialist biotic interactions, with emergent properties for communities and range dynamics. Here, we use an in-field experiment to assess egg-laying behaviour of a range-expanding herbivore across a range of natural microclimatic conditions. We show that variation in microclimate, resource condition and individual fecundity can generate differences in egg-laying rates of almost two orders of magnitude in an exemplar species, the brown argus butterfly ( Aricia agestis ). This within-site variation in fecundity dwarfs variation resulting from differences in average ambient temperatures among populations. Although higher temperatures did not reduce female selection for host plants in good condition, the thermal sensitivities of egg-laying behaviours have the potential to accelerate climate-driven range expansion by increasing egg-laying encounters with novel hosts in increasingly suitable microclimates. Understanding the sensitivity of specialist biotic interactions to microclimatic variation is, therefore, critical to predict the outcomes of climate change across species' geographical ranges, and the resilience of ecological communities.

Climate-driven variation in biotic interactions provides a narrow and variable window of opportunity for an insect herbivore at its ecological margin

“…Host condition and phenological synchrony in biotic interactions appear to be mediated by local (micro-)climatic variation, and are major determinants of spatiotemporal variation in fecundity and population size of host-specialist herbivores such as the brown argus [19,22,26,46,47]. By reducing temporal overlap between suitable resources and key herbivore life stages (e.g.…”

“…Brown argus butterflies prefer to lay their eggs on Geraniaceae, on which larvae grow 10% larger and faster, than on Helianthemum (the ancestral host at the range margin in Britain), and prefer to lay on better condition leaves regardless of host species [19,21,24–26,28,29]. However, the annual, more ephemeral growth form of the Geraniaceae host plants may make them less reliable as a food source than the evergreen perennial Helianthemum, especially under more variable climatic conditions at the range margin [16].…”

“…Using an average of 33.7 (0.25 m 2 ) quadrats per survey (SI-1.3 for quadrat placement), we recorded percentage cover (SI-1.3.2), phenophase (phenological stage) and condition of each host species. Host phenophase and condition underpin brown argus egg-laying site choice in the field [19,30]. Phenophase was estimated on a four-point scale (in leaf L, bud B, flower F, or had set seed S).…”

“…Phenophase was estimated on a four-point scale (in leaf L, bud B, flower F, or had set seed S). Condition was visually assessed on a scale of 0–3 (poor–high quality for egg-laying, following [19,26]; see SI-1.3.3 for details and justification). We also measured mean sward height and percentage cover of bare ground (SI-1.3) which can alter local microclimates [31,32].…”

“…Robust evidence is therefore required on the drivers and vulnerability of host-herbivore phenological synchrony (which may scale up to emergent patterns of herbivore range and host shifts [10,19,21]), including the role of microclimate in consumer persistence by potentially buffering asynchrony in biotic interactions [18,20]. Here we address this knowledge gap, using as an exemplar the brown argus butterfly (Aricia agestis; Lepidoptera: Lycaenidae) at its range margin in the UK.…”