American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“A spatial perspective on the phenological distribution of the spring woodland caterpillar peak”

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Jack D. Shutt, Malcolm D. Burgess, and Albert B. Phillimore (Nov 2019)

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Winter moth (<i>Operophtera brumata</i>) caterpillar foraging on young sessile oak (<i>Quercus patraea</i>) leaves.<br/>(Photographer: Gergana Daskalova, 2017)
Winter moth (Operophtera brumata) caterpillar foraging on young sessile oak (Quercus patraea) leaves.
(Photographer: Gergana Daskalova, 2017)

In temperate regions, warmer springs cause a whole suite of ecological events to occur earlier, from trees coming into leaf to birds breeding. A cause for concern is whether species higher up the food chain are able to shift their timings forward by as much as those species below them. In deciduous forests there is a peak in moth caterpillar abundance during the spring, coinciding with the new leaves on the trees. It is well known that many bird species, including tits and flycatchers, rely heavily upon this spring food peak to raise their nestlings. However, little is known about how this caterpillar peak varies among the particular deciduous tree species involved, or geographically (e.g. by elevation or latitude), or which caterpillar species are most important in creating this food peak and whether their identity varies from one location or tree species to another. Drs. Shutt, Burgess, and Phillimore set about answering these questions by collecting caterpillars from 40 woodlands spread over 220 km in Scotland. They identified 62 different caterpillar species based on DNA; however, just three species made up over half of all caterpillars collected, with the most common species (the winter moth) representing a third of the total, showing the importance of key species in the food chain. However, different tree species had different overall numbers of caterpillars, with oak and willow having the most, making these tree species of more value to the birds in spring. In addition, the timing of the peak in caterpillars was considerably later at higher elevations but didn’t change across latitude and was similar across the different tree species within a given area. These results are important as they show that poor breeding timing for the birds cannot be buffered by having a variety of tree species present.


Abstract

A classic system for studying trophic mismatch focuses on the timing of the spring caterpillar peak in relation to the breeding time and productivity of woodland passerine birds. Most work has been conducted in single-site oak woodlands and little is known about how insights generalize to other woodland types or across space. Here we present the results of a three-year study on the species composition and temporal distribution of the spring caterpillar peak on different tree taxa across 40 woodland sites spanning two degrees of latitude in Scotland. We used molecular barcoding to identify 62 caterpillar species, with winter moth (Operophtera brumata) the most abundant, comprising a third of the sample. Oak (Quercus sp.) and willow (Salix sp.) hosted significantly higher caterpillar abundances than other tree taxa, with winter moth exhibiting similar trends and invariantly proportionate across tree taxa. Caterpillar peak phenology was broadly similar between tree taxa. While latitude had little effect, increasing elevation increased the height of the caterpillar peak and retarded timing by 3.7 days/100 m. These findings extend our understanding of how mismatch may play out spatially, with caterpillar peak date varying with elevation, and tree taxa varying in the caterpillar resource that they host.