Rising Temperature Effects on Small Mammal Populations in North-Central Chile

Although temperature increase is often the most discussed factor in the conversation about global climate change, precipitation is another important factor. In arid and semi-arid areas, such as western South America, changes in rainfall affect the flora and fauna in different ways. This area of the world is also particularly affected by the El Niño Southern Oscillation (ENSO), which causes periods of increased precipitation and flooding. Meserve et al. (2011) studied the effects of precipitation changes as a result of ENSO and climate change on small mammal populations in north-central Chile. They found that permanent resident small mammals went through large population changes during and after times of heavy rainfall, while temporary resident small mammal species abandoned the thorn scrub habitat for some periods of time. All small mammal species were able to survive in nearby mesic habitats. Species which had inhabited the area for a long time were also found to be more plentiful than more recent species. The authors predict that in the future, ENSO and precipitation affected by global climate change will contribute to changes in species distribution and richness in this area. —Isabelle Heilman
            Meserve, P.L., Kelt, D.A., Previtali, M.A., Milstead, B., Gutierrez, J.R. 2011. Global climate change and small mammal populations in north-central Chile. Journal of Mammology, 93 1223–1235.

            Precipitation is a major factor in the survival of flora and fauna in arid and semi-arid habitats. In areas such as western South America, changes in precipitation are seen due to the El Niño Southern Oscillation (ENSO). The ENSO causes periods of elevated rainfall and flooding, which can have positive or negative effects on the wildlife in these habitats. Meserve et al. (2011) studied the effects of increased rainfall due to ENSO on small mammal populations in north-central Chile. The results of their study show that abiotic factors linked with ENSO periods are significantly contributing to changes in species richness and distribution in these semiarid habitats. In the future, the researchers expect the populations of small mammals with long life spans to increase and outnumber the small mammals with shorter life spans, which will contribute to changes in species composition in these areas.
            Meserve et al. conducted their research in the Bosque Fray Jorge National Park in north-central Chile. The park is described as containing semiarid thorn scrub vegetation and fog forests with a Mediterranean semiarid climate, producing 90% of precipitation from May to September and its warm and dry period from December to March. Research for this study began in 1989, when the authors created a complex of 75 x 75 m small mammal live-trapping grids to assess the minimum number of small mammals living in each area. Small mammals were trapped for four days per month per grid. Perennial shrub cover, annual cover, geophyte cover, soil seed densities, and predator diet and activity were monitored in addition to the mammals. For each year of study, the rainfall was categorized as either less than the long term annual mean of 131 mm, or in the first, second, or third year of a rainy cycle. The precipitation and small mammal trapping data were analyzed using a mixed-model analysis of variance. Rainfall data between the first and second decades of experimentation were different, so the researchers also performed a Shannon-Weiner diversity index test to see if the difference in rainfall had an effect on species diversity.
            Both the responses of small mammals and plants to increased rainfall were variable with individual species responses influenced by residency status and life-history traits. Annual shrub cover in the area oscillated between a low of 0% during drought-ridden years of La Niña and a high of 86% during wet El Niño years. Changes in small mammal population were categorized by either permanent resident species or temporary resident species. The permanent resident species Octodon degus showed small reactions to heavy rainfall, while the populations of another permanent resident, Phyllotis darwani, appeared to fluctuate independent of rainfall. These differences could be explained by differences in life-history characteristics. Life-history of O. degus shows that variation is usually produced by long term patterns, while in P. darwani variation is quicker, possibly due to larger litters and more movement during times of heavy precipitation. Temporary residents Abrothrix longipilis and Oligoryzomys longicaudataus also acted differently from each other. A. longipilis had erratic population increases and decreases, usually during the wet periods, including some time of complete absence. O. longicaudataus had slow population increases during times of heavy precipitation and delayed decreases in dryer times.
            Despite the differences within the permanent resident species and the temporary resident species, there was a general pattern. Permanent resident species with high variability in population established long-term residence and temporary residents showed less variability but still were prone to absence from some areas completely. As global climate change continues, these semiarid areas are subject to increased ENSO events, which could intensify the current patterns and change the current species composition. 

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