Bats and OLD-GROWTH FORESTS: Are Both Vanishing?

Although spotted owls may be more well-known, they are not the only animals that rely on the dwindling old-growth forests of the Pacific Northwest…

By Donald W. Thomas

WHAT DO BATS AND NORTHERN spotted owls have in common? Both are nocturnal and secretive and both depend on old-growth forests for their survival in the Pacific Northwest. Bats and spotted owls are in good company. At least 14 species of vascular plants, 16 species of birds, 6 species of non-bat mammals, and 11 amphibians either depend upon or reach their peak abundances in old-growth forests of the West Coast.

Yet old-growth forests are disappearing. Place yourself on just about any mountain top from California to British Columbia and look out over the facing slopes. The odds are that any forest you see will be a patchwork of clearcuts and regrowth less than 100 years old. The original stands of massive 200-year-old Douglas fir, 5 to 10 feet in diameter and stretching up over 100 feet, are mostly gone. One hundred years ago there were about 30 million acres of old-growth forest in California, Oregon, and Washington. Today, only about 17 percent remains, and almost all of it is on public lands controlled by the U.S. Forest Service.

In the early 1980s old-growth was disappearing so rapidly that biologists predicted it would be gone by the turn of the century. Many questions were raised. What would happen if old-growth simply ceased to exist? Was old-growth a specific wildlife habitat or simply an age category of forest? How many species depend on old-growth as critical habitat in the rich plant and animal communities of the western slopes of the Cascades, Coast Ranges, and Sierra Nevadas? Would we witness widespread extinctions with the removal of old-growth? At that time, no one could be sure.

Given our lack of any fundamental knowledge about the relationships between plants and animals and their forest habitats, the answers would only come from careful study. Lobbying by concerned biologists stimulated the Forest Service to propose, and Congress to fund, a study specifically focusing on old-growth habitat. Beginning in 1983, the Old-Growth Forest Wildlife Habitat Program (OGFWHP) was faced with the daunting task of determining just how plants and animals made use of Douglas-fir forests of different ages and whether they required undisturbed old-growth to ensure their long-term survival. Fortunately, far-sighted planners included bats in the study.

When I was called in to run the OGFWHP bat study in 1984, I was struck by how little we knew about bats in natural habitats. Bats are widespread and often seen, but along with most bat biologists I would first look for them in buildings. Virtually nothing was known about the types of roosts that bats typically use in undeveloped forest habitats as opposed to rural landscapes. For the common and widespread little brown bat (Myotis lucifugus), only two descriptions of natural roosts had ever been published. And even less was known about most of the 11 other species that we were likely to encounter.

MY FIRST CHALLENGE WAS to develop a sampling method that would allow us to observe and identify the various bat species in forest stands, regardless of their roosting preferences or the age and structure of the forest. Searching for roosts would be difficult and it seemed inefficient. Direct observations were obviously out. Capturing bats either with specialized harp or Tuttle traps or with fine nylon bird nets* was also fraught with problems. What if the bats flew in or above the forest canopy? With their sophisticated echolocation system, would all bats be equally prone to capture? Would feeding bats pay more attention and be less likely to be caught? If so, we could possibly overlook important feeding habitats.

A possible means of detecting and even identifying bats was to eavesdrop on their echolocation calls. Most bats continually betray their presence by emitting relatively loud echolocation calls as they navigate through forests or hunt for small insects. If moths and certain other nocturnal insects can use their specialized ears to detect bats at great distances (and thus avoid becoming a meal), why couldn’t we? With microphones sensitive to the high-frequency calls of bats, and electronic circuitry to reduce frequencies to the range audible to the human ear, we could simply listen in on bats as they went about their normal activities.

The passive detecting system of a bat detector had several features that lent itself to the type of habitat-use survey that the OGFWHP study required. First, most insectivorous bats echolocate continuously as they commute or hunt, so if they are present they will be heard. Bat detectors sample a volume of air 30 or more feet in radius and are likely to pick up the sounds of far more bats than traps or nets would ever catch.

Microphones can also be raised high above the ground to listen deep into, and even above, the forest canopy where nets or traps could never be hoisted. And unlike nets or traps, detectors do not rely on bats making navigational errors and so are less affected by the bats’ attentiveness or agility. Finally, bat detectors can provide information not only on the presence of a bat, but also on its identity and what it is doing.

Many species have their own specific patterns of echolocation calls that allow us to identify them much as bird-watchers do with bird calls [BATS, Summer 1991]. For example, little brown bats sweep from over 70 kHz (70,000 cycles per second) down to almost exactly 40 kHz and they do so in 5-7 milliseconds. Big brown bats (Eptesicus fuscus) sweep from about 40 kHz to 25 kHz, stretching this low-frequency part of the call out over 8-10 milliseconds.

By capturing bats, recording their calls, and then analyzing the calls in the laboratory we found that we could easily recognize the echolocation signatures of some bats, but that others could not be separated. For instance, the California, northern long-eared, western long-eared, and western small-footed bats (Myotis californicus, M. evotis, M. ciliolabrum) had similar echolocation calls. Despite these limitations, we were able to assign the 12 species of bats present in the Pacific Northwest to one of seven groups.

When bats are simply commuting, say from their day roost to a feeding site, they send out calls at a relatively low rate of about 1-2 pulses per second. This allows them to emit their signals, receive the echo, and have sufficient information to allow them to navigate down paths or through forests, avoiding large obstacles.

When bats are hunting, however, they require considerably more information. They must be able to capture small insects when the combined speed of both hunter and prey covers a distance 6-11 yards per second. Once an insect is detected, most bats dramatically increase their pulse repetition rate to over 100 per second in order to get a more precise bearing on the insect and determine its position, relative speed, direction, and maybe even size and surface features. This high repetition rate “feeding buzz” may last less than a half second, but when we hear it over a bat detector, it is a solid indication that a bat is finding insects and trying to capture them.

So, by eavesdropping on bat echolocation calls I could determine not only that a bat was present, but also what kind it was and whether or not it was trying to feed. This was all the information that I needed to determine whether old-growth forests were important for bats. To get the answer, my first step was to build a series of automated bat detectors that would turn on in the evening, record both bat calls and the time on a small cassette recorder, and then turn off in the morning to save battery power.

OVER THE SUMMERS OF 1984 and 1985, we sampled bat activity in 90 different Douglas-fir forest stands in the Cascade Mountains and Coast Ranges of Washington and Oregon. From the 3,000 bats we detected in Washington and the 6,000 that we detected in Oregon, several important trends became clear.

Bats were far more common in old-growth forests than they were in forests that had been disturbed either by logging or by fires. In Washington, all seven Myotis species were three to six times more abundant in old-growth than they were in disturbed forests. The same pattern held in Oregon where the Myotis species were three to four times and silver-haired bats (Lasionycteris noctivagans) were 10 times more common in old-growth forests.

The fact that all nine bat species we commonly encountered showed a clear association with old-growth forests in Oregon or Washington is a strong indication that old-growth is important habitat for bats. But what does old-growth offer that disturbed and younger forest can’t? When I examined the data carefully I found that most bats didn’t remain in the forests to feed.

There was a peak of activity for 15 minutes as bats left their day roosts, but through the rest of the night the stands were almost quiet. Feeding buzzes were concentrated elsewhere; they were over 10 times more common above streams and ponds than they were in forests. This made sense because in a parallel study we showed that the small insects that most bats hunt were far more abundant over water than they were inside forest stands.

The pattern that we observed indicates that old-growth forests offer critical roosting habitat for most of the bat species that inhabit the Pacific Northwest. We know that apart from the impressive size of the trees, old-growth forests are characterized by an abundance of old or dead trees that have had the time to develop the broken tops, cracks, hollows, and scaling bark that can serve as roosting sites for bats. Without old-growth forests, I believe that we would witness a dramatic decline in populations of not just one species of bat, but of almost all of the species currently found in the Pacific Northwest.

There is reason for both pessimism and optimism when considering the future of old-growth forests and bats on the West Coast. On the pessimistic side, old-growth harvesting continues, albeit at a reduced pace. Harvesting in the National Forests and on land controlled by the Bureau of Land Management is slowly reducing old-growth to isolated tiny patches. In Oregon and southern Washington, 39 to 50 percent of the old-growth patches are 30 acres or less. Wind penetrates to the center of patches this small and, over time, will blow down the majority of damaged and dead trees. Because these are precisely the trees that bats are likely to use as roosting sites, this fragmentation of old-growth forests may dramatically reduce the value of remaining old-growth as bat habitat.

On the optimistic side, the rate of old-growth harvest has dropped dramatically over the past few years. One of the reasons is that the Forest Service and Bureau of Land Management, who control approximately 80 percent of the remaining old-growth, are obliged by law to ensure that adequate habitat remains for the conservation of all plant and animal species.

Although bats still may not be able to stimulate the public pressure required to set aside large tracts of valuable old-growth, concern for the conservation of the northern spotted owl has done just that. Pressure from environmental groups and good government planning has resulted in the protection of sizeable tracts of old-growth Douglas-fir forests throughout Washington and Oregon to ensure adequate breeding habitat for spotted owls.

While bats and other forest species can undoubtedly benefit from efforts to ensure the survival of the spotted owl, it is unfortunate that our perception of conservation issues is so often limited to protection of single high-visibility species. It is tempting to believe that low-visibility species, such as bats, can ride on the coat-tails of high-profile conservation movements.

There are two problems, however, with this thinking. When we focus our attention on a single species like the northern spotted owl, we risk becoming complacent. If future studies show owl populations to be stable and healthy, what arguments will we use to push for continued monitoring of the low-visibility species like bats? We must also remember that bats are not owls and they almost certainly have different requirements. Setting aside tracts of forest that have been identified as good owl habitat does not necessarily mean that we have acted wisely to ensure the well-being of bat populations into the 21st century.

The Old-Growth Forest Wildlife Habitat Program is an example of good management planning, but it is just the beginning. We showed that old-growth forests are critical habitat for many other species of animals in the Pacific Northwest, but we know little about why this is so. It will be up to future studies to answer the remaining questions and provide a sound foundation for long-term planning and management.

NOTE:
*Two methods are commonly used to capture and study bats, the harp or Tuttle trap and mist nets. The trap, perfected by Merlin Tuttle, resembles an upright bedspring strung vertically with monofilament fishing line. When bats strike the line, they fall into special collecting bag below from which they cannot escape. Mist nets made of extremely fine nylon mesh have long been used to capture flying birds, but they are also excellent to capture bats. Both methods allow biologists to capture, study, and release bats without harming them.

[Bio]Donald Thomas is director of the Groupe de Recherche en Écologie, Nutrition, et Énergétique and Professeur Agrégé in the Département de Biologie, Université de Sherbrooke in Québec. His research on bat and bird ecology and energetics has taken him to Central and South America, Africa, Polynesia, and the rainy Pacific Northwest.