Temperature Data Logging - Declining Bat Populations
Project Overview
As a result of declining population sizes, many species of bats are receiving federal or state protection under the Endangered Species Act. Most biologists believe that habitat loss and degradation is a primary cause of these population declines. Bat researchers have spent the past decade working to determine what aspects of a species' habitat are preferred by individuals and allow for successful reproduction.
Biologists have known for some time that fetal gestation rate and developmental rate of juvenile bats is influenced by temperature. If a bat's roost (where it spends its daytime hours) is warm, then fetal and juvenile bats develop faster than if temperatures are relatively cool. Biologists hypothesize that reproductive females choose warmer roost sites, allowing for earlier birth and weaning of their young. By minimizing gestation and weaning time, reproductive females provide their young with greater time during the remaining summer months to become successful foragers and gain adequate fat reserves. Without them, young bats entering their first winter of hibernation may experience relatively high mortality rates due to energy reserves being depleted prior to the onset of spring.
Recent research has provided indirect evidence that reproductive female bats do indeed choose the warmer of available roost sites. For example, for species that roost in trees (such as in tree hollows or under loose bark), females often choose relatively large trees, that are taller than surrounding trees and that likely allow for increased exposure to solar insulation. Although available evidence offers valuable information about habitat requirements, it does not directly test whether the bats are choosing the warmest of available roost sites (assuming roost sites do not get too hot).
System Description
To help answer this question, NexSens micro-T temperature sensors are being used to generate a thermal map of areas inhabited by one of the rarest bats in North America, the eastern small-footed bat.
Dr. Jacques Veilleux of Franklin Pierce College in Rindge, NH began a research project during the summer of 2005 that aimed to document the habitat preferences of eastern small-footed bats (state endangered) at the Surry Mountain Lake Dam, located in southwestern New Hampshire.
Eastern small-footed bats were found roosting in rock crevices of the crushed rock “riprap” that forms the upper layer of the north- and south-facing dam slopes. On most occasions, pregnant females chose to roost in rock crevices on the far western side of the south-facing dam slope — the side that receives sunlight earliest in the morning. It seemed reasonable to hypothesize that female bats were choosing to roost at the dam location that warmed earliest and became warmest during the daytime.
The two slopes are identical in nearly all respects except for their direction and, therefore, exposure to the sun. This presents the opportunity for a natural experiment to test the hypothesis and provide the first direct evidence of habitat choice based on temperature differences.
A total of 36 micro-T sensors were installed, 18 on each slope. Sensors were placed at approximately 100-meter intervals along the east/west and at 20-meter intervals along the north/south gradients of each slope. Temperature data will be collected once per hour, for approximately three months, generating a thermal map of the dam slopes.
It is predicted that: 1) the south-facing slope will be significantly warmer than the north-facing slope, and 2) the west side of the south-facing slope will be significantly warmer (especially early in the day) than the east side.
If the female bats are shown to prefer roost sites that are warmer than other available roost sites, biologists will have a much better understanding of what components of the bat's habitat are critical when considering conservation and management actions. In addition, knowing the thermal preferences of a particular species may allow researchers to examine additional habitat areas that may offer similar beneficial microclimates, in order to locate additional populations of such rare and endangered species.
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