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Bats Northwest

"helping bats in Washington State"

Census of the Bats of the UW Arboretum

By Scott Pedersen and Jay Glover

We've all heard that bats are blind, rabies-infected, flying vermin that infested the Governor's Mansion in 1997. But forget what you "think" you know! Bats are fascinating creatures that are critical components of a balanced ecosystem. They are not blind and they are more closely related to primates than to rodents. Despite the paranoia spread by press during the summer of 1997, the incidence of disease (rabies) in wild bats is exceedingly low. These small mammals are unmatched predators of night-flying insects (many of which are human pest-species), and are prey for higher-level carnivores (e.g., owls) thereby providing an important link in the food-web.

The two of us spent the summer of 1997 documenting which species of bats are present, the availability of roost sites, and foraging patterns of bats in the Arboretum. Such studies concerning the status of bat populations in our area are critical-of the eight species thought to occur in UW Arboretum, one is Federally listed as Threatened/Endangered, while three others are being monitored by the State of Washington.

Our research involved the capture of bats using large (45' long x 12' high) mist nets stretched over ponds, streams, and walking trails where bats forage for insects. Nets were erected approximately an hour before dark during each evening of the study. Captured individuals were identified to species, weighed, sexed, checked for reproductive condition and overall health (ectoparasites, etc.), and released. No animals were injured during the study; all captured animals were in good physical condition; none were pregnant or lactating; all were immediately released.

We were struck by the low levels of activity and the singular lack of species diversity. We were only able to capture a single species in the Arboretum - the Little Brown Myotis (Myotis lucifugus). It is quite possible that we may also have caught several members of its sister species, the Yuma Myotis (Myotis yumanensis), but these species are extraordinarily difficult to distinguish from each other in this part of their range, and it is thought that they may even interbreed. Though we did not capture them, we noted several Big Brown Bats (Eptesicus fuscus) foraging high overhead above the canopy on several evenings. There was no question as to their identification as Eptesicus as their flight pattern and body size are unmistakable.

Both Big Brown and Little Brown bats are common neighborhood-bats and are found throughout the United States. Upon leaving their day-roosts, bats stream towards a water source to rehydrate. Many of our early captures were of bats heading towards the "Lagoons" area for a drink. The bats would then fly around within a meter of the water's surface picking up small insects in mid-air. After the Lagoons had been cleared of insects, the bats moved on to forage throughout the Arboretum, or would go hang-up and digest their meal of 200-300 small insects. A second round of activity over the Lagoons would occur several hours after the initial feeding activity - i.e., the time it would have taken for the bats to digest their first stomach-full of insects.

We would often capture bats deep in the wooded areas of the Arboretum, but this activity was always quite low in terms of the numbers of bats captured. The patterns that we noticed can be influenced by a great number of factors including: competition with other species of bats, competition among age-groups within a species, temporal distribution of insects, nutritional demands/variety, weather patterns, or even predator activity (though we saw no owls).

Fortunately for the bats, fragmentation of habitat - the urban mosaic - has affected biodiversity in bat populations in at least one positive manner. As opposed to large tracts of uninterrupted forest monoculture, fragmented habitats tend to concentrate insects along edges, thereby benefiting foraging bats. Another positive effect of humans on bat populations is the addition of lights. Lights also tend to concentrate insects, namely moths. It has been shown that bats tend to roost near light sources and exhibit great lamp-post-fidelity in their nightly foraging bouts. (Note, we found it unusual that Arboretum bats were not utilizing the street lights where insects congregated in great numbers).

Dead trees provide tremendous wildlife habitat under loose bark, in hollows, and under fallen woody debris. Because roost sites are the most important ecological limiting factors to bats, the identification of roost-site locations (possible colonies) within the Arboretum was an essential first step in understanding the Arboretum's bat population.

Given the highly manicured nature of the Washington Park Arboretum, the area is relatively poor habitat for bats and many other cavity-nesting species because of the relative lack of large woody debris - standing or otherwise. In fact, we found no trees on the property that could function as permanent maternity roosts. Similarly, we were unable to document bat occupation of the few snag-trees in the Lagoons area that might have sufficed as temporary night-roosts. It is our understanding that the Arboretum's bats are simply not living in the Arboretum, rather they are living in surrounding neighborhoods and commute each evening into the Arboretum for a bite to eat.

As mentioned previously, bats are important components of an ecosystem. Because the predominant cause of decreasing bat populations worldwide is human disturbance and the destruction of roost sites, the establishment of wildlife habitat within living trees, and/or the construction of low-impact/low-visibility roosting habitat would greatly enrich the habitat and biodiversity of all animals within the Arboretum.

If you are interested in observing the Arboretum's bats, we encourage you to come down to the Lagoons around dusk (July and August are best) and position yourself so that you can see a patch of sky and its reflection in the water. Against this backdrop, bats can easily be seen flying across the water's surface for about 30 minutes. You can count the numbers of insects eaten simply by counting the number of zigs and zigs each bat makes as it flies along!

If you would like to learn more about bats, Bats Northwest is a non-profit bat conservation group based in Seattle, Washington. Like other small conservation groups, their primary goals are to educate the general public about bats and to protect bats and their habitat in the Pacific Northwest. BNW contributes information to local schools and community groups, and conducts public bat-walks throughout the summer around Green Lake Park in Seattle. Bats Northwest can be contacted by writing PO Box 18735 Seattle, Washington 98118, or calling 206-256-0406.

For further information about bats, check your favorite bookstore or library: