Latitudinal gradient of host breadths in herbivorous butterflies

The combination of accumulated occurrence data and host use records across Japan revealed that the fundamental resource specialization of butterfly communities becomes more specialized toward higher latitudes.

Above: Japonica lutea is a butterfly species widely distributed in Japan. Photograph by Ryosuke Nakadai.

Are host breadths of herbivorous insects more specialized in the tropics compared to higher latitudes? This question is based on MacArthur’s (1972) latitude–niche breadth hypothesis, in which niche breadth is positively associated with latitude. To answer the long-standing question, many previous studies have compared host breadth patterns between temperate regions and the tropics but the results have been mixed and gradually the hypothesis has become highly controversial. The original hypothesis and following empirical studies have targeted “realized” host breadths, although processes constructing realized patterns in nature can be very complex due to local interspecific interactions. Here, we aimed to focus on the underlying “fundamental” host breadth, hoping it could facilitate our understanding of the processes driving variation in the degree of resource specialization without considering the complicated effects of local interspecific interactions.

Cover image article: (Free to read online for a year.)
Nakadai, R., Nyman, T., Hashimoto, K., Iwasaki, T., & Valtonen, A. (2021). Fundamental resource specialization of herbivorous butterflies decreases towards lower latitudes. Journal of Biogeography, 48, 2524–2537.

To evaluate the fundamental host breadth, we used two types of accumulated datasets across Japan; occurrence data and host use records. Based on those datasets, the fundamental host breadth was calculated in each grid cell (about 10 km × 10 km). In addition, we considered several factors which potentially affect the host breadth pattern, specifically climate, geography, and butterfly body size. To our surprise, we found that the fundamental host breadths of butterflies increase toward lower latitudes, which is opposed to the classical prediction based on the latitudinal gradient of realized host breadths. Also, the pattern seems to be mainly driven by climate, especially annual mean temperature.

In the article, we only focused on the fundamental host breadths as the pattern of realized host breadths were unknown in this region. The pattern of realized host breadth could correlate positively, negatively, or it could not correlate with the latitude. In theory, the patterns of realized host breadth could simply reflect the pattern observed in the fundamental host breadths, or the local processes could construct a ­reverse latitudinal trend against fundamental host breadth. To test these possible outcomes is one of the missing pieces in this article. Furthermore, we emphasize that the approach used in the evaluation of fundamental host breadth is applicable to many other areas and taxa for which reliable information on species occurrences and niches is available. The continuous improvement in open-access databases on species distributions and host use will hopefully eventually allow the testing of patterns of fundamental resource specialization on a global scale.

MacArthur, R. H. (1972). Geographical ecology: Patterns in the distribution of species. Princeton University Press.

Written by:
Ryosuke Nakadai (1), Anu Valtonen (2)
(1) Research Associate, Biodiversity Division, National Institute for Environmental Studies
(2) Senior Researcher, Faculty of Science and Forestry, Department of Environmental and Biological Sciences, University of Eastern Finland

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