Urban Wildlife Podcast on synanthropes and urban island castaways

Urban Wildlife Podcast “Episode 3: Timbers on a Boston Island”

I love this episode because it captures the two major human impacts of urban ecology: the creation of commensal niches for some species while other species are isolated to islands of small remaining habitat. We learn about a variety of organisms from birds to bugs that like to live in the habitats that we create. We also learn about timber rattlesnakes, whose few remaining habitats are defined by places where humans have difficulty building urbanity. The specific discussion here was about the Blue Hills of Massachusetts, but I was reminded of San Diego, where I spent a lot of time as a kid. If not for the canyon landscape of greater San Diego, it would be one continuous urban/suburban city. But many areas of the city (I am most familiar with Clairemont, where my mother grew up) the settlements are on mesas around which the canyons interweave. Snakes, rodents, and coyotes thrive in these areas where humans do not like to live. So one way of being a successful urban species is to thrive in habitats and terrains where people do not like to build.

The other strategy of course is to thrive in places where humans do build. This is the episode where the Urban Wildlife Podcast crew introduces the critical idea of “synanthropy”, giving birth to their “synanthropic organism of the week”. Lots of species do better where we provide them with food, living space, or areas that are free from predators and competitors. I think that it is important to recognize that synanthropy encompasses three of the five fundamental ecological interactions: commensalism, mutualism, and parasitism. Basically synanthropy is symbiosis, but where you define symbiosis pretty broadly (synanthropic organisms live around but not necessarily in very close proximity to humans… they are really symbiotic with our civilization, not necessarily us as individuals) and where humans are always the host. Whether a synanthropic organism is a parasite, a commensal, or a mutualist depends on what effect that species has on us. We provide resources to commensals as a byproduct of our normal activities, so we neither benefit nor are harmed by their presence. Mutualistic species provide us with some benefit, even if what we give them is just a byproduct as well. Parasites cause us harm, attracted to the resources we provide while despoiling some aspect of our environment, stealing resources, or causing risk to our health.

Deciding whether a given synanthropic species is parasitic, mutualistic, or commensal is not always so easy, because doing so requires estimating the net effect of a species’ presence in human-dominated environments. Squirrels seem like a pretty good candidate for being strictly commensal, which is pretty difficult to do because “zero effect” is much less likely ecologically than a mild positive or negative effect. I am not aware of any measurable benefit of the presence of squirrels, so I tend to call them commensal. Clearly species like bees are potentially mutualistic because serve as pollinators, although I guess the magnitude of that benefit really depends on whether or not there are valuable foods being pollinator in or near urban areas. There are some classic parasites such as mosquitos (which we foster by creating habitat for their larvae, and then suck our blood in thanks) or zebra mussels (which thrive in our human-mediated waterways and then thank us by gumming them up).

But for a lot of synanthropic species the relationship is not so clear. Take pigeons, who we allow to be synanthropic through a combination of food provision (some intentional, most probably a byproduct of our activities) and habitat provision (they are cliff-dwellers, and they love the little cliffs we have created for them in our buildings, bridges, and other infrastructure). What is the net effect of pigeons on humans? Ask most people and they would say that their lives are unaffected by pigeons, so perhaps they are commensal. But we also expend a lot of time and other resources dealing with the undesirable effects of pigeons: lots of plastic thorns are installed in public places to prevent pigeons from “roostin’ and poopin'” (that’s a technical term). This “cost” of having pigeons live synanthropically is borne more by the whole society than by individuals (although being pooped on is emotionally traumatic for some). So maybe pigeons are a kind of parasite on our society? But then there are those who intentionally feed pigeons and seem to derive pleasure from them.. perhaps to these folks, who get emotional pleasure from pigeons, they are a mutualistic species. Humans and what we call “costs” and what we call “benefits” are complex, so that leads to ambiguous relationships with a lot of species.

There is also an interesting discussion in this episode about the fact that timber rattlesnakes can reproduce parthenogenetically, which basically involves female snakes making clonal offspring without the need for mating. Is this a characteristic that would make a species more resilient in the face of habitat loss due to human activities? I guess that depends on whether the value of being able to reproduce without a mate (which might allow for a very isolated population to remain viable) outweighs the eventual effects of lack of genetic diversity (one could imagine clonal island populations emerging).