Via The Economist, recent research published by Evan Preisser and Joseph Elkinton yields an interesting result to those concerned with the conservation of Eastern Hemlock (Tsuga canadensis) trees. From Virginia to Connecticut, the species has been getting clobbered by an invasive hemipteran, Hemlock Woolly Adelgid (Adelges tsugae), native to Asia. However, comes another sap-sucker, Elongate Hemlock Scale (Fiorinia externa), also invasive, to feed on the hemlock. According to the paper, in experimental infestations, trees inoculated with both bug species fare better than those inoculated with just the adelgid.
Recent publications by Steven D. Gaines and Dov Sax challenge the conventional wisdom that invasive, exotic species inevitably lead to extinctions and loss of biodiversity, as reported by Carl Zimmer.
Emma Marris reviews the range of schemes for making choices in conservation biology and even uses the charged word “triage” for Nature‘s 8 November 2007 issue (paywall-protected link).
The EDGE program (Evolutionarily Distinct and Globally Endangered) of the Zoological Society of London gives priority to species that are taxonomically distinct, “far out on their own on the tree of life,” if you will. The reasoning is that a distinct taxon, now endangered, one that branched tens of millions of years ago from the tree, represents a unique chapter of evolutionary history that can’t be rewritten once lost. Priority amphibians include Chinese Giant Salamander (Andrias davidianus) (up to 1.8 meters long!), Sagalla Caecilian (Boulengerula niedeni), a worm-like burrower with an extremely restricted range in Kenya, and Purple Frog (Nasikabatrachus sahyadrensis), only described in 2003; top mammals are Yangtze River dolphin (Lipotes vexillifer) (perhaps already extinct), Long-beaked Echidna (Zaglossus bruijni), and Riverine Rabbit (Bunolagus monticularis) of South Africa. The system doesn’t appear to have been applied to other orders. A paper by Isaac et al., “Mammals on the EDGE: Conservation Priorities Based on Threat and Phylogeny” documents the EDGE metrics as applied to mammals.
The majority opinion among conservation biologists today is that they still understand too little about ecosystem functions to say for sure which species are the ‘load-bearing’ ones whose presence keeps a complex, multi-tiered ecosystem from collapsing into some worst case dull scenario of rats, roaches and invasive grass. “We are so fundamentally ignorant,” says Norman Myers, a fellow of the University of Oxford, UK, and adjunct professor at Duke University in Durham, North Carolina. “We cannot afford, by a long, long way, to say which species are dispensable.”
Thus Myers pioneered the concept of habitat hotspots, and a number of overlapping hotspot maps have proliferated. Birders may be familiar with catalogues of Important Bird Areas, Birdlife International’s Endemic Bird Areas, or Conservation International’s biodiversity hotspots. The problem for conservation biology is that each hotspot schema starts with different assumptions, chief among them the metric that is to be optimized. Do we seek to minimize extinctions of species or taxa, maximize land area preserved, maximize taxonomic diversity, or optimize some other measure? Marris writes that the work of Hugh Possingham of the University of Queensland in this area is getting a lot of attention: Possingham seeks to maximize the number of species conserved (vascular plants and vertebrates, in the paper cited below), trading off against the real-world costs of conservation efforts—land acquisition, invasive predator extirpation on islands, fire management, replanting, what have you. A paper by Kerrie A. Wilson et al., “Conserving Biodiversity Efficiently: What to Do, Where, and When,” explains the methodology and applies it as an example to 17 of the world’s 39 Mediterranean ecoregions.
What I find notable about the paper’s approach are the tools of economic analysis that are brought to bear on the problem. An expenditure in conservation activity is modelled as a financial investment. Different activities (“ecoactions”) show different expenditure streams: compare the one-time cost of land acquisition, for example, to the ongoing cost of fire management. The paper uses standard discounting methods and Net Present Value calculations to make investment choices comparable. The model reflects that the impact on species preservation will show diminishing marginal returns as investment is increased. The investment allocation algorithm is dynamic over time: it accounts for positive effects in the ecosystem as investments are made, and adjusts allocations year by year in response.
Wilson et al. acknowledge that the methodology does not yet account for uncertainty, a keystone of modern financial analysis. Also, it would be fruitful—albeit computationally more complicated—to consider the interaction effects of various conservation activities, rather than assuming that each activity acts independently of others.
A worked example chooses between three ecoactions in the Swan Coastal Plain region of Australia: revegetation to counteract habitat fragmentation, invasive predator control, and management of a soil-borne pseudo-fungus, Phytophthora cinnamomi). Even though Phytophthora management is the most expensive per square kilometer ($514K versus $301K for replanting and $7K for predator control), it is nevertheless the most cost-effective: a marginal $2 million spent controlling the pseudo-fungus, in this computation, will protect 49 species, versus 4 for predator management and effectively zero for revegetation.