Stanford Report, March 12, 2001
|Survival in the city: Researcher studies how bats behave
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BY KATHARINE S. MILLER
To study how cutting down forests affects endangered wildlife, you could hardly choose a better species than bats. That's what Michelle Evelyn decided six years ago when she began her research. And now, her many hours of bat surveys are teaching us how to protect bats in developed areas.
Evelyn is a biological sciences graduate student associated with Stanford's Center for Conservation Biology. With her collaborator and husband, David Stiles, she studies bats' foraging and roosting behavior in Mexico and at Jasper Ridge Biological Preserve. A 1,200-acre Stanford preserve in the eastern foothills of the Santa Cruz Mountains near campus, Jasper Ridge serves as a natural laboratory for Stanford researchers as well as for scientists from all over the world.
Evelyn's work at Jasper Ridge has revealed that some bats use very large trees for their roosts. "Protecting veteran trees may be vital to survival of bat populations," she says.
Yuma bats that forage in the preserve travel several miles to roost in large trees in Portola Valley and Woodside, suburban communities on the San Francisco Peninsula. The average diameter of the bats' chosen trees is about a yard across -- more than three times wider than the average tree in those areas.
Researcher Michelle Evelyn and field assistant Demetrio Alvarez remove a fruit-eating bat from a mist net. COURTESY: Ron Evelyn
Evelyn also discovered that Searsville Lake, located in the preserve, is the most important year-round feeding area for bats at Jasper Ridge. But the artificial lake is gradually filling in with sediment. "When the lake disappears, " Evelyn says, "the bats will need an alternative foraging source."
Evelyn cares about bat survival. When she accidentally refers to bats as "people," she laughs and says, "they're my friends." But her concern for the bats is not just personal. "Bats are both vulnerable and ecologically important," she says. "Bats matter."
Insect-eating bats, like those at Jasper Ridge, are the primary predators of nocturnal flying insects. They consume not only pesky mosquitoes but also moths, reducing the number of moth eggs and larvae that can damage agricultural crops.
The number one agricultural pest in the United States is the corn earworm moth. Mexican free-tailed bats reduce their numbers by following the moths' migration north from Mexico.
Despite their economic importance, more than half of the bat species in the United States are either on the endangered species list or are candidates for it -- that's a higher proportion than any other group of animals.
Bats also typically have one baby per year, so it's hard for them to recover from a population decline, Evelyn says. And they roost together, so damage to one roost site can have a large impact on the entire population.
For her research, Evelyn asked the question: "What can we do to keep animals around in a human-dominated landscape?" Bats have been studied mostly in protected and managed forests, but rarely in residential areas, as Evelyn's project does. The preserve, with its many types of vegetation, offers a "virtual smorgasbord" of foraging opportunities for bats, she observes.
To see where bats actually forage, Evelyn used a machine called Anabat that listens to the high-frequency calls the animals make to navigate and find prey. A bat's echolocation ability is so precise, it can detect and avoid the fine nets that researchers commonly used to catch bats before Anabat came along. Using Anabat, Evelyn says, "We can listen in on their calls and detect them without having to catch them."
The Anabat listening system can distinguish the calls of different types of bats by plotting their frequency and time on a laptop computer. It's not perfect: All species are not detected equally. "We never heard calls from any Townsend's big-eared bats [a threatened species], though we know they live in the preserve," Evelyn says. And it is often difficult to distinguish similar calls among species, she adds.
Still, the Anabat detected more than 10,000 bat calls in 2,300 hours of survey at a variety of sites within the preserve. Evelyn then used those data to plot the frequency of calls at each site during each season of the year.
For purposes of her research, Evelyn divided the Jasper Ridge bats into two broad categories easily distinguished by the Anabat: Myotids, which emit a higher frequency sound; and non-Myotids, which emit a lower frequency sound. The two groups also differ in wing length, which Evelyn predicted might affect their ability to forage in forests. Myotids, with their short, broad wings, should maneuver easily in forests, while non-Myotids' longer, narrower wings appear ill suited to forests.
The survey confirmed Evelyn's expectations: Non-Myotids forage in less cluttered habitats, while Myotids feed in the woods. Both bat types are common year-round near Searsville Lake, "a hotspot of insect activity," Evelyn notes. The creeks are popular foraging sites in the spring, but are less commonly used during the summer dry season and in winter when water flow is too fast and noisy for the bats.
To study where bats roost, Stiles, Evelyn's collaborator, put radio locators on 16 Yuma bats and watched where they went. It turned out that the animals commuted from the preserve to roosts several miles away in the suburban towns of Woodside and Portola Valley. And their roosts of choice were enormous trees. Indeed, the bats only roosted in the five tree species with the highest average diameters -- their two favorites being coast redwoods and California live oaks. Half the sites were in second-growth redwoods, suggesting the need to preserve such trees. In absolute numbers Douglas fir and valley oak trees were less common roost choices, but they were clearly favored by the bats, who chose them in far greater numbers than one would expect given how rare they are in this area, Evelyn says.
Curiously, the Yuma bats in this survey used much bigger trees than bats in any other North American research study published to date, comments Evelyn. She offers two possible explanations: Either Yuma bats prefer larger trees, "or we simply have bigger trees available for roosting."
In recent years, land managers have started becoming aware of the need to preserve old dead trees as roosting and nesting sites. But Evelyn's research shows that land managers should protect big live trees as well. Evelyn also recommends maintaining forests along creeks and preserving larger bodies of water.
According to Evelyn, all of the communities around Jasper Ridge should become involved in bat preservation because bats roost in a number of suburban neighborhoods outside the preserve.
But Evelyn's research goes beyond local forest management. For her doctorate, she studied two species of fruit-eating bats in Mexico. She found that one roosts in the cavities of big old trees while the other hangs from branches in tall trees in areas of high forest cover. Evelyn also discovered that Mexican fruit-eating bats spread more seeds in clearings than birds do. For this research, she collected droppings twice a day in an open field and counted the seeds. The result: About 90 percent of animal-dispersed seeds arriving in the clearings came from bats.
Bats in deforested parts of Mexico and in the Jasper Ridge Biological Preserve have very specific habitat requirements that are threatened by nearby development, Evelyn maintains. In the part of Mexico where she did her research, the threat comes from slash-and-burn agriculture. In the San Francisco Bay Area, it's urban sprawl.
Evelyn believes that more bat research is needed because bats are an extremely important component of global biodiversity. They account for almost a quarter of all mammal species on Earth, and they play vital ecologic roles. "By pollinating plants, dispersing seeds and controlling insect pests, bats provide great benefits to humanity," Evelyn argues. "It is to our advantage to work to understand how best to increase their chance of survival in human-dominated landscapes."
Katharine Miller is a
science writing intern with Stanford News
Researcher Michelle Evelyn at Jasper Ridge Biological Preserve with a "bat box." The box contains an Anabat detector and laptop for recording bat calls at night. Every morning, Evelyn collects the data, charges the battery, and moves the box to a new site. COURTESY: Ron Evelyn