In an age when endangered species are often recovered just as much by force of legislation, á la the Endangered Species Act, as they are by scientific principles, I often find myself weighing the Big Picture of ecological effectiveness against the minutae of things like genes and mere numbers.
Let me explain. I’m not knocking genetic diversity by any means, it’s just that genes and population size aren’t the only things we need to consider when recovering imperiled species. And yet, these are often the elements that we get so caught up on, almost myopically. As readers of Wild Muse know, I often think a lot about wolves and large mammalian predators. And when it comes to recovery of species that have been exterminated — or pushed out of — most of their range, you inevitably run into issues of genetic diversity and how many of these imperiled animals are where. Which is important and has a prominent place in species recovery arguments.
But what about species that create strong interactions among other species in the communities within which they make a living? Does recovering X-number of them in Y-populations truly = recovery? For example, if Montana has 300 gray wolves and 10 breeding pairs within their state lines at all times, can we really walk away from this and say that gray wolves in Montana are recovered? Legally, yes — they would be. But ecologically, we know this would not be true.
At the time the Endangered Species Act was written, my guess is that the state of scientific knowledge hadn’t yet fully uncovered the nature of strongly interacting species. Michael Soule coined this term to replace the more commonly-recognized term, keystone species, because it more accurately captures the concept that species have a spectrum of interactions: from weak to strong. Soule defines the term broadly by saying that when a strongly interacting species is removed (effectively or virtually) from it’s ecosystem, then some feature or component of that ecosystem is significantly changed.
For example, some species like gopher tortoises or beavers exert strong influences on the other species that share their habitat by engineering the types of micro-habitats these other species need to live (think gopher tortoise burrow) or by creating entire temporal communities (think beaver dams and ponds). If you take gopher tortoises out of an ecosystem, then other creatures that rely on these burrows are also affected, like cotton mice, indigo snakes, gopher frogs and many invertebrates. But it’s not always about habitat, sometimes it’s about changing behavior. Recent papers have shown that elk in areas where wolves have been reintroduced out West alter their behavior by spending less time in open valleys near streams where they are more open to attack. This translates to a reprieve on browsing and has altered the growth of aspen stands — allowing them to regrow after decades of over-browsing by elk when wolves were absent. Important species interactions may take the form of pollination, seed or spore dispersal, defoliation, predation, parasitism, and cavity, burrow or wetland creation. Strongly interacting species may fall under previous typologies such as mutualists, consumers or ecosystem engineers.
But the ESA doesn’t legislate protection of these interactions, which are the same ones that generally keep ecosystems ticking along. Hence the dichotomy of my thoughts between gearing recovery efforts to the big picture of ecological effectiveness versus restoring a certain minimal number of animals within a minimal number of populations in their former historic range. To better think through this issue, I turned to a conceptual paper published by Michael Soule, James Estes, Joel Berger and Carlos Martinez Del Rios in 2003 in Conservation Biology.
The authors explored case studies of sea otters and wolves, both species that exert very strong influences upon their ecological communities, and argue that our societal efforts to recover endangered species need to focus on the big picture of ecologically effective densities. Otherwise, we’re just maintaining limited numbers of isolated animals in a way that fulfills the letter of the law, but does little to address the biodiversity crisis, which the ESA initially sought to address in spirit. To borrow from the paper’s introduction: “Although the first-order effects of the biodiversity crisis— the loss of species—are dire, the second-order conse- quences—the loss of species interactions—may be more ominous.”
Ominous indeed. Can you imagine a world in which our oceans are devoid of sharks? Or one where we lose all of our significant pollinators?
To prevent the loss of strong species interactions, in addition to the loss of the species themselves, the authors propose that two goals be embedded within species recovery management plans:
The first is the goal of geographic representation of interactions, which calls for extensive geographic persistence of highly interac- tive species. Conservation plans and objectives (design, management, and recovery) should provide for the main- tenance, recovery, or restoration of species interactions in as many places as feasible, both within the historic range of highly interactive species or in other sites where the consideration of climate change and other factors is appropriate.
The second goal concerns ecological effectiveness within ecosystems, communities, or landscapes. Conser- vation plans should contain a requirement for ecologi- cally effective population densities; these are densities that maintain critical interactions and help ensure against ecosystem degradation. This goal replaces the de facto nonecological practice of requiring only the attainment of minimum viable populations.
The first goal speaks widely to the importance of geography, but the second gets at the effectiveness of species interactions in maintaining the places where they live. In my mind, the functional aspect of “ecological effectiveness” — in at least a portion of a species’ former range — is what the ESA needs to be doing. And this is where there is a profound and disturbing disconnect between the legal recovery goals of species like Mexican gray wolves, gray wolves and Florida panthers, and the ecological meaning that these goals have in light of what we know about these animals and how they affect their ecosystems, communities and landscapes. Minimum viable populations — the smallest populations necessary to allow for the continued existence of that population for a hundred years or more — don’t even begin to address this idea of ecological effectiveness. And yet, many species recovery plans are based on the idea of minimum viable populations.
Although the authors discuss both wolves and sea otters as examples, I’ll give my wolf blogging a break here and just summarize the sea otter case scenario. Basically, sea otters feed on sea urchins and sea urchins feed on the bottom “roots” that hold large stalks of kelp in place on the ocean floor. When the sea urchin munch through these roots, the kelp float free. When sea otters were nearly wiped out of the Aleutian island chain geographic area due to maritime fur traders, sea urchin numbers exploded (released from their predation by sea otters) and went crazy eating kelp roots. They denuded entire areas of their kelp forests, which robbed many other species of the kelp forests they used as a home or to find food within. This is, of course, highly simplified. But you get the drift re: the species interactions getting knocked out of their normal modes.
But because the functional relationship between how much kelp is present and the density of sea urchins is not a linear one (I won’t get into here, it gets really complex!), the authors describe a term called breakpoint density. This term pinpoints the minimum density of the strongly interacting species — sea otters — needed to affect a phase shift in the kelp-sea urchin relationship. “Strong nonlinearities in the na- ture of plant-herbivore interactions at the base of the food web result in distinct breakpoints in the number of sea otters required to maintain the kelp forest ecosystem,” they write.
To think of it another way, it might be the minimum density of wolves necessary to affect a phase shift between newly-growing aspen stands and elk; or bees and a certain species of flower they pollinate. You get the idea. You might also get the idea that calculating this breakpoint is a really tough task that is highly contextual. A lot of study has to go into figuring out the base relationship of plant-primary consumer interactions in the case of predator-herbivous prey examples like sea otters and wolves.
But it makes a whole heck of a lot of sense. It makes maintaining minimum viable populations look more akin to grooming living zoos in the wild, versus truly recovering species. Staving off genetic stagnation is important, of course, but so is guarding against the simplification and degradation of entire ecosystems. They conclude:
In the end, our success in rehabilitating an ecologically degraded world will be judged more on the persistence of interspecies interactions than on the geographically limited persistence of populations based only on causing the least economic burden and ensuring only symbolic survival.
The pragmatist in me knows there are many problems with the current ESA. But the idealist in me hopes that one day soon we can incorporate Soule et al.’s ideas of ecological effectiveness into the recovery goals of strongly interacting species. To do any less seems like we’d only be spinning our wheels.
Soule, M., Estes, J., Berger, J., & Del Rio, C. (2003). Ecological Effectiveness: Conservation Goals for Interactive Species Conservation Biology, 17 (5), 1238-1250 DOI: 10.1046/j.1523-1739.2003.01599.x