Observed Variations in Snake Diet More Likely Due to Local Effects Than Climate Change

By analyzing changes in the diets of opportunistic predator species, it is possible to observe the effects of climate change on prey populations. The asp viper, a small snake found in a variety of habitats in Central and Southern Europe, is prime for analysis due to the prevalence of data on its feeding ecology and the wide range of its inhabitance. In their study, Rugiero et al. (2012) hypothesized that if climate change had significant effects on asp viper diets, they would increase consumption of Mediterranean climate species, decrease consumption of temperate climate species, and decrease in diversity. However, their results found that despite decreased rainfall and increased temperatures as a result of climate change, asp vipers maintained relatively consistent diets. Only two prey species showed consistent trends of change, with the consumption of bank voles increasing and the consumption of shrews decreasing. However, as these are temperate climate species and thus both predicted to decline, these observations are more likely due to a local cessation of logging than climate change.—Katie Huang
Rugiero, L., Milana, G., Capula, M., Amori, G., Luiselli, L., 2012. Long term variations in small mammal composition of a snake diet do not mirror climate change trends. Acta Oecologica 43, 158–164.

                  Rugiero et al. studied field data from 1987–2010 on the diets of the asp viper (V. aspis), a small snake found in Central and Southern Europe. The asp viper is able to survive in a wide variety of habitats, including those with Mediterranean and temperate climates, and its diet consists mainly of rodents and species in the order Soricomorpha. The authors hypothesized that if asp viper diets were affected by global warming, they should increase in Mediterranean climate prey species consumed and decrease in diversity. If they were adversely affected by logging, species sensitive to forest loss would decrease. The authors collected data in the Tolfa Hills of central Italy, a region that was affected by a 13% decrease in rainfall and significant increases in annual mean temperature, likely due to climate change. In the region, they observed 11 prey species of rodents and eight species of soricomorphes, six of which lived in Mediterranean climates and two in temperate climates. To analyze snake diets, they randomly captured free-living individuals, marked and sexed them, and palpated their abdomens until the ingested prey were regurgitated or defecated. In order to determine the abundance of mammals in the field to compare those frequencies with those that occurred in the snake diets, Rugiero et al. conducted trapping sessions using live traps in the spring and autumn of 1992, 1993, 1997, 1999, and 2006.
                  The authors found that fluctuations in the composition of the snake diet reflected changes in the relative abundances of their prey, reinforcing studies showing that the asp viper is an opportunistic feeder. In general, asp vipers preferred rodents to soricomorphes, except for the period from 1987–1989 when the two groups has similar species abundances. There was also a significant decrease of soricomorphes over time. However, asp vipers tended to consume similar proportions of both rodents and soricomorphes despite annual fluctuations in species abundance, suggesting that the asp vipers’ opportunistic behavior creates a compensation effect. Rugiero et al. also found a significantly positive relationship between the abundance of small mammals in the field and the frequency of occurrence in viper diet. However, contrary to their hypotheses, the authors did not find that the composition of snake diets changed significantly over time. Asp vipers consistently preferred the same three types of prey, and only two species of prey showed consistent trends over time. The frequency of bank vole consumed increased significantly and the frequency of shrews consumed declined linearly. For all other species, there were no significant trends. According to the authors’ predictions, the bank vole, which lives in a temperate climate, should have declined in population, but its increase may signify that climate change did not significantly affect asp viper diets. Rugiero et al. instead suggest that the cessation of logging may explain these observations, as it likely affected the separate regions inhabited by the two groups differently. They conclude that changes in the viper diet are more attributed to local disturbances than climate change.

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