With the widespread effects of the changing climate on biological communities and landscapes across the world, it has become increasingly important for ecologists to identify indicator species among these ecosystems that can indirectly relate information about environmental changes that are not apparent or easily accessible. So it is in the west, the Rocky Mountains and in particular the Colorado River Basin, where temperatures have increased more than anywhere else in the contiguous United States, an average 1.2° C higher than the 20th century averages. The biggest increases in temperature happens at the highest elevations, which is
With warming temperatures comes less precipitation and less snowpack, which means during the summer months, the breeding season for most species, there is significant reductions of stream discharge, which has reduced the flow of the Colorado River. Thirty million people rely on the water provided by the Colorado River, and the Basin is foundational to all life in such a dry environment. Bats, as this article in Ecology explains, are particularly sensitive to these changes and, due to their enormous numbers, are integral to food webs as predator and prey. They may be that indicator ecologists are looking for.
Using capture and environmental data from over 12 years - 1996 to 2008 - Rick Adams from the University of Colorado has demonstrated dramatic correlations between the reduced availability of water and declines in the reproductive success of certain species of bats in the west. Bats are particularly sensitive to evaporative loss because of their small size, large surface area to volume ratio and uninsulated wings. Reproductive females are particularly sensitive considering that 76 percent of their milk is water. Lactating fringed myotis bats have been demonstrated to drink 13 times more often than non-reproductive females from nearby sources like streams or pools.
The study area was in the foothills of the Rockies, between 1650 m and 2250 m, a mix of montane meadows, shrubland, pine woods, riparian woodland and mixed coniferous forest, the habitats of nine species of bats; data was collected on the five most common: small-footed myotis (M. ciliolabrum), little brown myotis (Myotis lucifugus), big brown bat (Eptesicus fuscus), long-eared myotis (M. evotis), and fringed myotis (M. thysanodes). The 2,329 bats captured were put into one of four categories: Non-reproductive, Pregnant, Lactating or Post-lactating.
The reproductive output of these bats has declined, especially when stream discharge dipped below 7 cubic meters per second. During the hottest and driest years, 2007 and 2008, Adams captured more non-reproductive females. Among two species, M. thysanodes and M. lucifugus, the percentage of non-reproductive females was remarkably high, 56 percent and 64 percent respectively.
Both of these species use maternity sites having south or southeast aspects that promote highest solar gains throughout the diurnal roosting period (Adams and Thibault 2006; Adams and Hayes 2008), maintaining internal temperatures between 27° C and 36° C (Adams unpubl. data). Such microenvironmental conditions within roost sites promote high evaporative water loss and consequently a greater need for water intake, especially during the lactation period.
The other myotis species are more likely to roost in cooler, more humid microclimes, closer to the ground.
So if bats - mammals with high mobility* - are facing difficulties from a reduction of water availability, what about other animals more restricted to certain areas? How is this aspect of climate change affecting them? Bats, Adams says, are good bioindicators, organisms that can help scientists predict similar, indirect effects of climate change in other regional animal populations.
Current predictions from the IPCC tell us that this is just the beginning; it's "very likely" (90 percent confidence) that ecosystems will be significantly affected if the warming trend continues. In the next century, due to continued average temperature increases and an increase in the frequency of heat waves and drought, the Colorado River is facing a potential 8 - 11 percent reduction of flow. This will certainly exacerbate the bats' reproductive problems, but perhaps the continuance will afford ecologists the opportunity to transpose data to study similar problems among other animals and propose meaningful, sensible solutions - even if they are bandaids, like providing artificial water sources for vulnerable populations, temporary but viable, buying much needed time for more comprehensive applications.
*Bats are mobile, but they stick to their traditional maternity sites, still focused in a local area.
Adams, R. (2010). BAT REPRODUCTION DECLINES WHEN CONDITIONS MIMIC CLIMATE CHANGE PROJECTIONS FOR WESTERN NORTH AMERICA Ecology DOI: 10.1890/09-0091