When wrens can't make rent

Urbanization fractures populations and displaces Southern California natives

By Amelia Taylor-Hochberg

Los Angeles is often stereotyped as a city of desperation – of struggling narcissists, westward aiming loners, and thirsty desert-dwellers.

And it's not just the humans who are getting desperate. As more people flock to California, and the state struggles with a historic drought, urban development strives to keep up with demand, sprawling further and further into previously wild land. It fragments the landscape into islands of undeveloped habitat– often to the detriment of species that have been here all along.

One animal antihero of this urban-splintering phenomenon is P-22. Arguably California’s most famous mountain lion, P-22 is the only puma living in Los Angeles’ Griffith Park – a “rugged” urban park cast over more than 4,300 acres. His fame is sealed by his solitude. Fenced in by multiple freeways, he is the park's lone resident mountain lion, isolated in his territory and now – collared with a tracking device – photographed constantly. He even has a glamour shot in National Geographic, with the Hollywood sign in the background.

While P-22 appears to live (more or less) normally within the freeway-bound borders of Griffith Park, his genetic heritage is, effectively, already dead. With no accessible females, he stands no chance of reproducing and passing on his genes. P-22's lineage will end when he dies, his genetic legacy snuffed by the terms of his celebrity.

While P-22 is something of a cautionary tale of urbanization's impact on genetic diversity, his individual situation is far less dire than the threat to other California species. When entire populations become fragmented by urban sprawl, the effects can be felt over many generations, and can even lead to localized extinctions.

Urbanization is directly destructive – pouring concrete or razing forests – but it is also indirectly destructive, constricting local populations to smaller and smaller islands of habitat in perpetuity. Entire populations can be split into “ghettos” by urban development, incapable of re-establishing migration and breeding patterns across the fragmented landscape– cut off from the larger genetic pool. Even species endemic to urbanized areas are threatened – species like the cactus wren.

The cactus wren (Campylorhynchus brunneicapillus) is a gregarious and sprightly bird, flecked with black speckles on a white and brown body. Its narrow beak and modest tail balance out a pleasantly-plump chest, making it an easily cute member of the genus. Native to the Southwestern US and northern Mexico, its name is derived from its preferred nesting spot: in cactus plants.

Cactus wrens generally stay put in a given area, and do not migrate. They are characterized as a curious species, and will defend their territory with a “monogamous” partner (meaning they form permanent pair bonds) throughout the year. In Southern California in particular, the birds occupy coastal sage scrub habitats – 85-90% of which has been destroyed by urban and agricultural development, leaving them little space left to defend.

The wrens may not be literally fenced in by infrastructure like P-22, but their Southern Californian populations are physically limited by urban areas. According to a study by the US Geological Survey, cactus wrens now are located mostly on the periphery of what historically would have been their habitat – wedged into the dreary “suburban” coastal sage scrub between urban developments and mountain ranges. And it doesn’t take much to split a population of wrens irrevocably apart – a 16-lane highway (one of many in Southern California) was completely impassable to the birds, isolating populations that fell on either side, and making breeding between them impossible.

When a community of wrens is split into isolated populations, the consequences can be severe. By cutting off connections, the wren population's regional genetic diversity is splintered into pockets of isolated breeding pools. Time goes on, generations pass, and the north-of-the-freeway wrens turn inward to their own small population when looking for a mate. Inbreeding over time leads to decreased genetic diversity, and less genetic variation means less resilience – the population of birds won't be as well equipped to deal with disease, famine, a changing landscape–whatever nature, or more urbanization, may throw at it. As the cycle continues and urbanized spaces expand, the isolation is exacerbated. At some point, the populations collapse, and the survivors become P-22s.

The Jerusalem cricket (Stenopelmatus ‘mahogani’) is in a similar situation. Another species endemic to the region, it faces a similar case of atomization by urbanization in California, followed by compromised genetic diversity. A 2006 study conducted by the US Geological Survey found that urbanization in California had severely limited the habitat available to the flightless insect, resulting in the same ghettoization seen in the cactus wren.

Without taking into account any ripple effects that these compromised populations might have on other species, it's likely that the crickets and wrens aren't the only victims of urbanization in Southern California. Other species are probably fragmented too. Worldwide, it's a tragically common story.

Isolated in their atomized ghettos, the wrens will likely become vulnerable to local extinction (if current trends continue). As populations dwindle and collapse, genetic diversity will be compromised across the region, making it even more difficult for the wrens and other species to adapt to future anthropogenic interventions – which, given California’s housing crisis, ongoing drought and continually growing population, are unlikely to stop.

That urbanization contributes to this type of local extinction is tragic in and of itself– but as one ecologically critical species collapses, others also begin to decline. As the ecological web unravels, species–like dominoes– begin to fall.

But in a way, this is also a remarkable opportunity– seeing the biological impact of human behavior on neighboring species, all within the timescale of a human life– gives us the chance to change. Distilled cases of anthropogenic consequences – mountain lion, wren or cricket – give us a shorthand for understanding and conservation in the future. The anthropocene is here, and we’re all living in it.

Amelia Taylor-Hochberg:

Species: Homo sapiens

Habitat: Los Angeles, California

Diet: Omnivorous – Sandwiches, open-faced mostly.

Occupation: Journalist / editor

Contact: longhyphen@gmail.com