Studies of several species of vertebrates are beginning to clarify the relationships between wildlife and habitat structure, making it possible to evaluate the influence of management programs on wildlife populations. One important wildlife group that has been neglected in these studies, however, is bats. In this study we hope to provide the scientific basis to evaluate forest management alternatives that may influence bat populations in and adjacent to riparian areas.
Bats are extremely important to the region's biodiversity and ecology. Nearly 20 percent of the mammal species occurring in the Oregon Coast Range are bats. Of the 11 species of bats occurring in the Coast Range and Siskiyous, three are listed as sensitive species by the Oregon Department of Fish and Wildlife, and one of these is a candidate for federal threatened or endangered status. Bats are significant predators of nocturnal insects, some of which are forest pests, and bats are prey to other mammals and birds.
Only one study to date has examined bat communities in the Oregon Coast Range (Thomas 1988). It showed that the abundance and diversity of bats are closely tied to stand age of forested upland areas. Upland habitat is used by many species of bats primarily as roosting habitat. Riparian zones are used by bats for both roosting and foraging. Although there is some evidence that bat populations may be influenced by habitat structure in riparian zones, no definitive studies have examined this relationship.
The goals of this study are: 1) to identify the bat species in the Oregon Coast Range that utilize riparian zones, 2) to determine the importance of riparian zones to these species, and 3) to determine the influence of landscape structure, forest structure, and stream and riparian characteristics on patterns of habitat utilization by bats.
Before we could determine the most appropriate experimental design, we first needed to gather some basic information. Our preliminary studies have focused on two basic questions: 1) what are the echolocation signatures of different species of bats in the Oregon Coast Range, and 2) how much temporal variation in activity occurs among nights along riparian zones in the Coast Range?
Initial research has relied primarily on monitoring echolocation calls to determine patterns of use of riparian areas. The various species of bats emit different frequencies of sound while echolocating during their foraging and orientation activity. In an attempt to identify echolocation signatures of different species of bats, we caught bats with mist nets and recorded their calls when the bats were released. From these recordings we are building a catalogue of bat calls, with each call depicted as a time-frequency curve (Figure 7). We will use this catalogue to identify species on the basis of the shape, duration, maximum frequency, and minimum frequency of the curves.
For example, our preliminary work suggests that the echolocation pulse of the Yuma myotis (Myotis yumanensis) in the central Oregon Coast Range is relatively steep, has a sharp inflection point, lasts 3-7 milliseconds, and has a maximum frequency around 100 KHz and a minimum frequency between 40 and 47 KHz (Figure 7A). In contrast, the echolocation pulse of the long-eared myotis (Myotis evotis) has no sharp inflection point, lasts only 1-3 milliseconds, and is lower in frequency, with minimum frequencies between 30 and 35 KHz (Figure 7B). Although we may not be able to determine differences in echolocation patterns between all species of bats, we are confident that we will be able to identify all calls to the genus level and many to the species level.
To determine temporal patterns of variation, we monitored bat activity at two streams, Bark Creek and Buttermilk Creek, on 30 nights from July through September of 1993. We found that Bark Creek experienced about five times as much activity as Buttermilk Creek. There was also substantial temporal variation in activity during this time, with nightly activity varying more than tenfold at each stream (Figure 8). However, the temporal variation was not random; a "good night" for bats at one location was generally also a "good night" at the other. This temporal pattern of variation appears to be closely associated with minimum nightly temperatures. As a result of these findings, we will structure our activity monitoring program so that comparisons are paired by night, thereby minimizing extraneous variation due to weather conditions. We will try also to identify other factors that affect levels of bat activity so that these influences can be accounted for in the experimental design.
Our analyses of echolocation signatures and patterns of temporal variation should be completed by early 1995. After that, we will conduct a series of studies examining the influence of active riparian-zone management on bat populations, begin looking at the influence of thinning operations on bats, and compare patterns of use by bats of alder-dominated and conifer-dominated riparian zones. The results of these studies will provide a scientific basis for appraisal of management alternatives that may influence bat populations in and adjacent to riparian areas.
Thomas, D.W. 1988. The distribution of bats in different ages of Douglas-fir forests. Journal of Wildlife Management 52:619-624.
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