The Middle Way

Small-Time Predators with Big-Time Impacts

by on July 01, 2011

 

Skunks are adaptable insectivores that do quite well in urban environments, where lawns and other landscaping make fine feeding grounds.

Photo by Sebastian Kennerknecht, pumapix.com.

 

 

The first encounter is likely to be olfactory rather than visual: You’re driving along and suddenly the car fills with an odor so rank, so palpable, it seems like a physical presence, a creature with black claws raking at your sinuses. Only one thing smells like that, of course–as the old Loudon Wainwright song put it so eloquently, it’s a “dead skunk in the middle of the road, stinking to high heaven!”

And for most Bay Area residents, that’s where it ends. Road-killed skunks are generally nothing more than minor if unpleasant distractions, gory little reminders that nature occasionally intrudes on our well-ordered suburban lives. But these highway casualties point to something larger. They are reminders of a great flux taking place at the edges of our awareness, one shaping our parklands, open spaces–even our backyards.

Striped skunks are mesopredators: carnivores of intermediate size. So are raccoons, opossums, gray and red foxes, feral cats, and coyotes–all Bay Area inhabitants. When considered singly, the impact of mesopredators may seem minimal. They don’t have the cachet of cougars or black bears. TV cameras don’t show up when a raccoon or opossum raids a garbage can. But what they lack in size, they make up in sheer abundance. They shape basic ecological processes by what they eat and by where and how they live. They demand our attention–even our respect and admiration–for their resilience and toughness. This, after all, is their time: In an increasingly urbanized world, the mesopredator reigns supreme.

TV cameras don’t show up when a raccoon or opossum raids a garbage can. But what they lack in size, they make up in sheer abundance.

In a 2009 article published in the journal BioScience entitled “The Rise of the Mesopredator,” the authors describe a phenomenon known as “mesopredator release”–the rapid population growth of intermediate-size mammalian carnivores in the absence of larger top predators. The Bay Area is an epicenter of mesopredator release. Mesopredators are happy to live with–and off–us. Our homes are often, quite literally, their homes. They thrive on our garbage, on the pet food we leave on our back porches, on the mice and rats our refuse attracts. For many mesopredator species, we are indulgent benefactors. For others, we are easily circumvented obstacles.

“One thing’s for sure,” observes Doug Bell, the wildlife program manager for the East Bay Regional Park District. “We have healthy numbers of them. In general, they are superb culture followers–we provide them everything they need. They flourish in urban interface areas, in our buildings and gardens.”

Raccoon takes a step
Raccoons are omnivores that have adapted to eat pet food and other leavings of humanity, though this one in Mount Diablo State Park could be getting by the old-fashioned way. Photo (c)2009 Yamin Bilal, chochenyonature.com.

Of all the region’s mesopredator species, none is thriving more extravagantly than the raccoon. This is pretty evident to anyone who has driven Bay Area roads, lived in a Bay Area suburb, or even left a bowl of dog food on the back porch.

“We get lots of raccoons–about 150 a year,” says Susan Heckly, wildlife rehabilitation director for Walnut Creek’s Lindsay Wildlife Museum. “That’s way more than any other species.”

Reginald Barrett, professor of wildlife management at UC Berkeley, says the research on Bay Area raccoons is scant: Most of the work has been done in the Midwest, focusing on the species’ impacts on waterfowl.

Raccoon taking bread from trash can
A raccoon takes advantage of another park visitor’s leftover baguette, undeterred by the cover on this trash can. Photo (c)2010 Yamin Bilal, chochenyonature.com.

But Barrett does recall a recent senior thesis that at least sketches the general dimensions of the local raccoon situation. “The student gathered several years’ worth of information on raccoon roadkills in Berkeley’s city limits and combined it with mapping and field research,” he says. “The bottom line is that Berkeley needed a minimum population of around 800 raccoons to support the annual mortality rate. The student discovered many if not most of them were living in storm drains. He’d shine his flashlight in a drain, and there’d be all these beady little eyes looking back at him.”

Heckley says our neighborhoods might be even more raccoon-friendly than open lands. “Subdivisions are simply an expansion of their preferred habitat,” she says. “Sheds and crawl spaces give them the dark, quiet places they need to hole up. They get food from pet dishes, vegetable gardens, and fruit trees, water from swimming pools and birdbaths. I think we have higher raccoon density in the suburbs than in our local parks and wildlands.”

And many Bay Area residents are active–or at least complicit–in raccoon care and feeding. Most people suffer a few raccoons feeding opportunistically on pet food left on the back porch. But other folks purposefully spread a groaning board for raccoons; they are besotted with the little masked predators.

“One woman who lived near Tilden Park was feeding about 30 raccoons a night,” Barrett says with a sigh. “That would have to be considered an artificially high population under any circumstances.”

Opossum passing in the night
For opossums like this one, there is no old-fashioned way in the Bay Area—they were introduced here in about 1910. Photo by Sebastian Kennerknecht, pumapix.com.

Raccoons have a couple of peers in the Mesopredator Success Sweepstakes: opossums and the aforementioned striped skunks. The only North American marsupial, Virginia opossums–as their name suggests–are not native to California. Introduced to the state in the early 20th century, they have since spread to all but alpine and desert habitats. They are prolific breeders and feed on everything from carrion to small mammals, insects, and fresh fruit. They also dote on garbage and pet food and find crawl spaces, outbuildings, and old cars congenial habitations. Not surprisingly, they are abundant in the Bay Area.

Like raccoons and opossums, skunks are relatively fecund breeders, with sexually mature females producing one litter a year of one to five kits. They are solitary animals by nature–which is just fine with everything else that shares their habitat. Virtually all mammals give them a wide berth. Barrett has been involved in a remote camera project around water sources on San Benito County ranchlands for several years, and he notes it has been particularly instructive when it comes to striped skunks and their interactions with other animals.

“We’ve recorded virtually all the region’s major [mammalian] predators, including mountain lions, coyotes, and badgers,” Barrett says. “But when a skunk comes down to water, everything else disappears. A skunk with its tail up is a signal everything understands.” (The exceptions to this rule, Barrett adds, are great horned owls, which prey on skunks with enthusiasm.)

While striped skunks will eat pet food occasionally, they are largely insectivorous. But habitat disruption can play to their favor; suburban landscaping constitutes a convenient buffet. “Lawns are ideal foraging habitat for skunks,” Heckley says. “They really like to dig around in them for grubs and beetles.”

Even those mesopredators that require environments somewhat more expansive than a suburban backyard are thriving in the Bay Area, thanks to the region’s abundant open spaces–national, regional, county, and state parks and protected watersheds. Among these are two species that were de facto apex predators here 20 years ago: bobcats and coyotes. They have been “demoted” to mesopredator status in the past couple of decades only because of the reappearance of mountain lions.

Two bobcats at Sunol
This pair of bobcats (male on left, female on right) was seen at Sunol Regional Wilderness, not far from the parking area. Photo by Joseph E. DiDonato, Wildlife Consulting and Photography.

Of the two, bobcats are the more elusive, but their populations are healthy, particularly in the East Bay and North Bay. “They’re seen fairly often,” says the park district’s Doug Bell. “In some areas, they seem unalarmed by humans who aren’t acting in a threatening way.”

If bobcats are reasonably numerous, coyotes are positively ubiquitous. They’re common in most of the large parks and open spaces of the East Bay, North Bay, and Peninsula and are seen with increasing frequency in the Bay Area’s smaller parks, particularly those with riparian zones. They are often spotted foraging for voles, pocket gophers, mice, and ground squirrels on grassy hillsides, in croplands, even along highway medians.

“When I was with the [East Bay] parks, most of our [wildlife] complaints came from two species,” recalls Joe DiDonato, a consulting conservation biologist who served nearly 20 years as a wildlife biologist with the East Bay Regional Park District. “First were black-tailed deer. Then came coyotes. I certainly don’t consider them a big public threat. But they’d follow hikers, chase dogs–and they’re especially hard on cats. We had one in particular, a flop-eared guy that worked the parklands around El Cerrito. He killed at least 30 cats–and those were just the ones we knew about.”

Gray fox looking alert
Gray foxes, unlike nonnative red foxes, prefer uplands, and are even known to climb trees. Photo by Bruce Finocchio, dreamcatcherimages.net.

Indeed, cats–feral cats–also make up a significant portion of the Bay Area’s mesopredator biomass. This is worrisome for biologists, given that Felis catus is an introduced nonnative species and public enemy number one for songbirds, particularly ground-nesting varieties. “Cats have a significant impact [on native wildlife]” says DiDonato, citing a University of Wisconsin study that concluded feral and domestic cats kill at least eight million wild birds in the state annually. The issue is further complicated by the emotional bond many people share with cats, domestic or feral. House cats are often dumped at parks by owners unable or unwilling to care for them, and some feline advocates feed feral cats despite specific proscriptions against the practice in many jurisdictions. Attempts to eliminate feral cats on public lands are often met with fierce resistance by animal rights activists.

Gray fox drinking
Gray foxes are among many mesopredators, from common raccoons to elusive badgers, that live in nearby wildlands and even in our backyards. This fox drinks from a pond where photographer Bruce Finocchio had set up a photo blind. Read about how he captured this remarkably intimate image. Photo by Bruce Finocchio, dreamcatcherimages.net.

The Audubon Society has long been at forefront of the feral cat battle, but these days it’s emphasizing education more than confrontation. “It’s a basic fact–people are deeply concerned about their pets,” says Gary Langham, the director of bird conservation for California Audubon. “Are feral cats a problem? Yes. But in suburban and urban environments, mesopredators in general, introduced or not, can have significant impacts on birds. Proportionately, there’re more of them than there would be in a ‘natural’ system, and you see the impacts on their prey species. So to achieve more balanced ecosystems in areas top-heavy with mesopredators, more apex predators are needed.”

The Bay Area supports two fox species: the indigenous gray fox and the larger nonnative “lowland” red fox. The latter was introduced from the eastern United States for fur farming and sport hunting in the late 19th century and was well established in the Bay Area by the late 1970s. (The Sierra Nevada red fox is native to California and is endangered across its range.)

Grays and reds vary markedly in genetics, appearance, and behavior. The gray fox is smaller and prefers to forage in uplands. It is the only North American canid that can climb trees, often in search of fruit and nuts. The red fox is more aggressive and more generalized in its habitat requirements. Unlike gray foxes, it will happily hunt in marshes. This has made the species deeply problematic for wildlife managers; red foxes have been linked to the decline of the listed California clapper rail and salt-marsh harvest mouse. And though red fox numbers may have begun declining in recent years, they remain a concern.

“One showed up on Brooks Island near Richmond not too long ago,” says Bell. “There’s a large Caspian tern colony there, and biologists were really worried until the fox was [eliminated]. That one fox could have destroyed hundreds of nests.”

A rich array of other mesopredators are found in the nine Bay Area counties, especially mustelids (weasels and their cousins). Though rarely seen, long-tailed weasels–perhaps ounce for ounce the most voracious of the state’s predators–are very common, particularly near wetlands. Out in marshlands around West Pittsburg, freelance biologist Jim Hale captured six weasels on a single set of live traps meant for salt-marsh harvest mice.

“I decided to pick one up to examine it more closely,” Hale recalls with a laugh. “They’re as stinky as skunks, and when he released his musk–well, it was really something. And I couldn’t catch it–it was like trying to grab greased lightning.”

Weasel in a culvert
A long-tailed weasel in a culvert near a residential area in Danville. Weasels are ferocious predators—one biologist who tried to handle them described them as “greased lightning.” Though not commonly seen, they are relatively plentiful near wetlands. Photo by David Jesus, davidjesus-naturephotography.com.

The weasel’s bigger aquatic cousins, river otters, also appear to be thriving. These sleek and playful mammals are formidable predators with catholic appetites. They’ll gorge on fish of all species, shellfish, reptiles, amphibians, and small mammals.

“Unfortunately, there hasn’t been much research on them, but you can say that about [California] mesopredators in general,” Barrett says. “Anecdotally, they seem to be doing well. I saw one in the Berkeley yacht harbor not long ago, which you wouldn’t normally consider river otter habitat. And I was up along Cache Creek recently, and otter tracks were everywhere.” Hale also thinks river otters are more than holding their own. “Virtually all the reservoirs, city and county ponds, and creeks have them,” he says. “Not long ago, I heard them crunching crayfish in the creek behind my house in Concord.”

Mink are making a big comeback in the Bay Area after a long absence, Hale observes: “We haven’t seen mink for a long time, but they seem to be returning, particularly in the Delta, and especially in Suisun Marsh. Recently I was in the Delta and a male mink chased a female right across my boot tips. They were so focused on each other they didn’t even notice me.”

A related mesopredator, however, is struggling. The American badger inhabits grasslands throughout the state, including the open range of eastern Alameda and Contra Costa counties. Unlike long-tailed weasels, minks, and red foxes, badgers are not suited to marsh habitats and thus are not benefiting from the extensive wetland restoration projects now under way in the Bay Area. Nor are they generalists like striped skunks, opossums, and raccoons, able to profit from our garbage and infrastructure. Their needs are specific: large expanses of grassland that contain an abundance of burrowing rodents, particularly ground squirrels. Though such rangelands do exist in the eastern and southern Bay Area counties, they have been shrinking steadily for more than 50 years and remain under long-term development threat.

Relatively slow-moving and slow to reproduce, badgers are especially vulnerable around motor vehicles. In a 2008 study on Bay Area badgers, San Jose State University graduate student Chris Lay noted that badgers disperse less readily across wildlands than most other predators due to their difficulty in negotiating barriers–for example, highways. When they encounter roads, they tend to get run over and killed.

Badger walking in the grass
Low-slung and hard to see, nocturnally active badgers seem especially susceptible to becoming roadkill. Photo by Bruce Finocchio, dreamcatcherimages.net.

“Badgers are nocturnal,” says DiDonato. “They’re moving around when visibility is poor. Plus, they’re low to the ground and hard to see.” Certain roads, Vasco Road for example, cross some of the best remaining badger habitat in the Bay Area. Considered from a larger perspective, then, badger mortality on Vasco Road may be an issue for regional badger populations.

“Last year on Vasco Road, we confirmed 16 dead badgers,” says Hale. “The year before that it was 14, and the year before that it was 12. That kind of annual mortality is devastating. I think it shows we really need to incorporate more [under-road] wildlife passages, especially in areas with critical habitat.”

Despite these sobering statistics, one heartening fact remains: There are tens of thousands of acres of protected land in the East Bay, including some prime badger habitat and plenty of space where skunks and raccoons thrive far from the nearest dish of dog kibble. Overall, the Bay Area’s mesopredator complex remains as Bell described it: healthy. And disease, habitat loss, and highway encounters are unlikely to change that. Nature is vulnerable in many ways, but in other respects it is implacable, a force ready and able to fill any cracks that appear in our fragile civilization. Those sounds of crayfish being crunched in a neighborhood creek, of garbage cans assaulted and tipped over in the wee hours? Get used to them. They’re only going to grow louder.

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one comment:

P K Peirce on January 20th, 2012 at 12:00 am

Hi Glen,

We went to your natural history series talk and found it interesting so we looked up the article. We found it very interesting to learn about intra-predator competition too.

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