Article alert: Don't judge species on their origins
Every once in a while, a group of seminal ecologists gets together to kick around an idea. Sometimes that idea is a fundamental component of ecological practice, natural resource management, or environmental policy, and sometimes their conclusion gets people all sorts of riled up. For example, this last week a commentary by Mark Davis et al. ran in Nature that criticized how we manage non-native and invasive species. This paper received press from a variety of sources, including the Scientific American podcast, and the Ecological Society of America before making it onto the ESA's email list, Ecolog. They argue that most non-native species control is based in tradition and eco-ecultural bias within the conservation community.
The analysis and backlash have jumped across the internet faster than the front of the almost entirely uncontained fires in Arizona.* Commentary grew quickly and an author defended his work and the peer-review process of Nature on Ecolog, providing the debate further fuel.
Now that the semantic tinder box is fully ablaze, what exactly did the authors say, conclude and recommend? Well, like most things in ecology, it's complicated. My interpretation of their arguments is as follows:
1. Ecosystems are changing at an unprecedented rate due to human pressures including development, resource use and climate change, driving shifts in communities of organisms. With these changes, ecological communities more commonly consist of "non-native" species that are then eradicated based on their origin rather than their ability to thrive in new places at the expense of other endemic species.
2. Non-native species can increase local biodiversity, not always at the expense of the local ecology.
3. Native species expanding their ranges and population size, like the mountain pine beetle (Dendroctonus ponderosae), adversely affect ecosystems, but we don't try to eradicate them.
4. Invaders like tamarisk (Tamarix sp.) don't degrade ecosystems, they take advantage of modified hydrologic (or more broadly, environmental) conditions that have already shifted ecosystem structure and function. Therefore, tamarisk and species like it should get a free pass.
5. Because some introduced non-native species are merely showing up or expanding after human disturbance, these species may merit less attention, especially if they do less damage than 3. and increase diversity like in 2.
6. Invasion control is expensive and uses resources that could be spent better elsewhere--besides in some examples (USDA and honeysuckle), the introduced species benefit other species!
My personal take is that this over-authored piece (19 people for two pages!?) is a bit of a no-brainer. It builds on already exhausted arguments without regard for the implications of their premise on those actually performing invasion control. While I could write this off as a piece by academics for academics, I'll do my best to stray from being dismissive, instead hitting their points one by one with a foot in application and a foot in research.
1. Unprecedented changes are undoubtedly occurring in the world's ecosystems due to numerous pressures. With these changes there have been large shifts in species' biogeography, including range expansions and contraction, biological invasions and shifts in community composition, among species native or otherwise. The implications of this are less well understood than Davis et al. imply. Biotic homogenization, the combination of functional homogenization and taxonomic homogenization, is occurring as specialist species are displaced, often by native generalists (Clavel et al 2011, Olden et al. 2004), but just as likely by invaders. To accept this inertia as both largely harmless and unstoppable seems a bit premature, especially in systems that will be indefinitely managed to provide targeted ecosystem goods and services e.g. agricultural systems, forests, wetlands, etc. At what point have we scuttled the idea of control too soon or too late?
2. Non-native species may have benefits in some systems. Schlaepfer et al. (2011) made a nice list of circumstances in which non-native species can provide benefits to local biodiversity and ecosystem services. Several years ago, I also worked first-hand on nurse-cropping forest species using invasive gorse (Ulex europaeus) in New Zealand. Therefore gorse is desirable, because it facilitates native Manuka (Leptospermum scoparium)? This is a slippery slope. The next logical conclusion is perhaps that we plant gorse in abandoned pastures to facilitate other species. Where does a temporary, novel successional pathway become a new norm? If we argue as ecologists, that non-native species can be used in restoration, reclamation and conservation, our sentiment may be misinterpreted as non-native species should be used in these projects and that we shouldn't bother managing invasive non-native species, or preferentially using native species (much less native species based on targeted gene pools...).
3 and 4. These point are a bit of a straw wo/man because they ignore the ecosystem processes that drive and respond to the species they cite. The mountain pine beetle is expanding due to the reduced intensity and duration of winter across its range, allowing it to expand its population and geographic limits. To decouple an outcome from the process that drives it is a bit sloppy, even in a commentary piece. Tamarisk may be responding to altered hydrologic regimes, so why accept that severely modified hydrology and therefore tamarisk are inevitable. As a society, we have choices in what resources we use and conserve, so to slam the door on hydrologic modification as the norm now and forever is short-sighted.
Embedded in this argument is that restoration always heads towards some historic, pre-human state. The work of Bill Mitsch and Steve Whisenant and their many students stands contrary to this. Both scientists argue towards process-based restoration rather than stable, historic endpoints based on community composition and native-status. The idea of historic endpoints has largely been discarded, so to bring it up as a guiding tenet of ecological restoration is either naive, lazy or misleading.
5. Schlaepfer et al. (2011) wrote a full-length piece in Conservation Biology that more clearly lays out an argument for how non-native species might fit into conservation frameworks. While Schlaepfer's case had significantly more room to build the concept than the paper in question, it still fails to draw a firm line between invasive species and mere non-natives. If biodiversity is the end goal--a goal that is perhaps as antiquated as historic endpoints as restoration goals--then yes, non-natives may serve benefits as they add to alpha and beta diversity or in some cases, facilitate diverse local assemblages. I suppose it depends what the conservation and/or restoration goals are in a given system, but I doubt they're always as the authors claim/assume.
6. This is perhaps the most dangerous point that Davis makes: ecologists are wasting money on non-native species control and eradication. As an individual who has worked for a variety of entities, I am convince that the species that receive specific targeting are generally the worst of the worst in a given locale. Davis implies that society is trapped in an expensive boondoggle to pull every pasture grass out of native prairies, or cut down every locust tree that's out of range. I can assure him that in a day and age of limited money, personnel and resources, the natural resources community is spending their time on invasive knotweeds that cause trophic ripples in riparia and streams (see Urgenson et al. 2009), Phragmites that clog formerly open waterfowl habitat and weeds that generally cause reduced agricultural producivity, physical harm and/or economic losses elsewhere.
To suggest that practitioners are actively pulling every dandelion at great expense is not only disingenuous, but dangerous. Some people might not read the authors' conclusions very closely, and it could easily be misinterpreted as "cease and desist on controlling non-native species." With current NGO, local, state and national budgets being what they are, if we plant that seed, we might be left with even less resources for invasive species and noxious weed programs.
Now that the storm has brewed and is blowing in, I look forward to seeing what follows in the newly invigorated debate over non-native species.
Citation: Davis, M.A. et al. 2011 Don't judge species on their origin. Nature 474. 153-154.
*Unrelated sidenote: The NY Times and Here and Now consulted Wallace Covington of NAU whose research has shown that diverse changes have occurred in forest stands due to grazing and other human activities - all of which can drive more intense, stand-replacing fires. The work of the Ecological Restoration institute and Dr. Covington are both timely and important reads as this fire rips across the American Southwest.