A lot of media attention continues to be focused on the question of whether and how wolves trigger trophic cascades in ecosystems, by suppression of herbivory by ungulates and consequent release of vegetation and species such as birds that are dependent on the vegetation for their habitat needs. Trophic cascades can be caused by numeric effects (declines in ungulate populations), behavioral effects (prey foraging differently and avoiding areas of high predation risk), or a combination of the two. Behavioral or non-consumptive effects can be linked to numeric effects when altered behavior leads to poorer nutrition and lower pregnancy rates.
This last effect has been proposed as a cause of elk declines in the Greater Yellowstone Ecosystem, but a new paper by Arthur Middleton and coauthors in Ecology Letters challenges that hypothesis, and finds that changes in elk behavior due to encounters with wolves have little effect on elk body fat or pregnancy rate, probably because elk encounter wolves infrequently (about every 9 days) in this area. This leaves open the possibility that wolves are contributing to elk population declines directly via predation. This last point has at times been missed by the media, resulting in headlines such as “wolves not to blame for elk decline”.
Another paper by the same research team found that a factor contributing to elk population declines in Yellowstone National Park was the decline in native cutthroat trout in Yellowstone Lake after nonnative lake trout were introduced. Because spawning cutthroat trout, but not lake trout, form an important food source for grizzly bears, the invasive speceis indirectly caused grizzly bears to shift their diet towards increased predation on elk calves, contributing to a decline in elk that had previously been attributed primarily to wolf predation (figure below).
While most research of wolf-induced trophic cascades has taken place in Yellowstone, two new papers test the trophic cascade hypothesis in the northcentral US and Poland. Both take a correlative approach that compares vegetation in areas with vs. without wolves. In Wisconsin, Ramana Callan and coauthors found that species richness of both forbs and shrubs was significantly higher in areas with high wolf use. This supports the hypothesis that wolves, by reducing the intensity of browsing by white-tailed deer, are reversing the biotic impoverishment of understory plant communities caused by decades of overabundant deer populations. Similar contrasts between areas of high and low wolf use were found by DPJ Kuijper and coauthors in Poland, where browsing intensity of tree saplings was lower inside wolf core areas. At a finer scale within wolf core areas, sites with more coarse-woody debris, which is an impediment to escape from wolf predation, had even lower browsing rates, supporting the conclusion that at least a portion of the effects on vegetation are behaviorally-mediated rather than solely due to lower numbers of ungulates.
From Middleton et al. 2013
An interesting new paper by Brad McRae and colleagues has been published in the journal PLoS One. Entitled “Where to Restore Ecological Connectivity? Detecting Barriers and Quantifying Restoration Benefits” (download link), the research uses techniques to identify connectivity barriers that are similar to those used in microchip design, where simulated voltage levels reveal areas with strong voltage gradients where electrical connectivity must be enhanced. Software to implement the method is freely available (download link).
In a new paper in the journal Frontiers in Ecology and the Environment, Chris Wilmers and colleagues examine how much carbon is stored (in the form of kelp) in North Pacific coastal ecosystems which either have or lack sea otters. In the absence of sea otters, sea urchin populations increase and graze kelp forest to form urchin barrens. Wilmers’ results suggest that sea otters can substantially alter ecosystem carbon budgets through their indirect effects on plants. They hypothesize that predators can strongly influence the carbon cycle in general and atmospheric carbon dioxide through top-down forcing and trophic cascades. Although the authors caution that the extent to which these effects can be extrapolated across species and global ecosystems remains to be determined, they suggest that, because predators exert strong indirect effects on plants in many ecosystems, these effects might appreciably influence the concentration of atmospheric carbon. The effects of trophic cascades on carbon flux and storage also have an economic dimension, given that the value of increased kelp carbon standing stock due to sea otters would be valued at between 205 and 408 million dollars on world markets for carbon credits.
The authors suggest that the degree to which predator effects in other ecosystems would substantially influence atmospheric carbon dioxide concentration will depend on three factors: the overall influence of predators on autotrophs through trophic cascades across global ecosystems; food chain length and the resulting degree to which the trophic cascades have a positive or negative influence on associated plant populations; and the standing plant biomass and NPP for each particular ecosystem. They propose that we should expect predators in food webs with odd numbers of trophic levels to reduce atmospheric carbon (via increased sequestration by plants), while predators in food webs with even numbers of trophic levels might increase atmospheric carbon. They conclude “This influence alone complicates the assessment of predator effects on carbon in aquatic systems because food chain length varies considerably among aquatic systems. Large predators in most terrestrial ecosystems occupy the third trophic level, thus implying a more consistent sequestering effect of predators on C for the terrestrial realm. However, terrestrial ecosystems are rife with other complexities such as predator interference, omnivory, and defended plant tissue that make it difficult to form general conclusions about the magnitude of such effects.”
This new study is important in focusing attention on the ecosystem effects of predator restoration. However, proposals to monetize the effects of predator-prey dynamics on ecosystem carbon storage raise ethical questions. For example, although in parts of the developed world, ungulates are superabundant due to predator removal, in other regions, ungulate abundance and distribution has been reduced below historic levels by over-exploitation. Would recovery of these prey populations be opposed due to the effects on carbon storage?
Figure from Wilmers et al. 2012. When occurring at ecologically effective densities, sea otters reduce sea urchins, resulting in large kelp standing stocks and high net primary productivity (NPP). (b) When sea otters are absent, urchins decimate kelp stands, resulting in small kelp standing stocks and low NPP.
The Society for Conservation Biology recently completed a major overhaul of the SCB website.
The new website provides a wealth of information on recent issues in conservation policy. You can access regular updates on conservation policy news by subscribing to the Policy RSS feed.
Other sections of the website provide information on SCB’s regional sections and working groups. The most popular section of the website is the board listing job openings in the field of conservation biology.
A new study published in Science by Brosi and Biber compares species listed under the US Endangered Species Act (ESA) in response to citizen petitions versus initiatives from within the agencies (FWS and NMFS). The authors asked whether citizen involvement, as some claim, diverts scarce conservation resources to species which are at lower risk than those identified by the agencies. The authors found, on the contrary, that species listed in response to citizen petitions were at least as threatened as those proposed by the agencies. These findings support the wisdom of the drafters of the ESA, who included the ability of citizens to petition for species’ listing to help ensure that species are not overlooked in the listing process due to political concerns or other reasons.
As the New York Times notes, “These impressive statistical results also help restate — and re-ratify — the reason the authors of the Endangered Species Act included the public in the first place. There are a lot more of us than there are Fish and Wildlife Service scientists. And the petitioning public isn’t merely an amorphous cross section of Americans. It includes scientists, local specialists, committed conservationists and passionate defenders of nature, who, in many cases, can keep a closer eye on the ground than the Fish and Wildlife Service.”
Science Daily also noted “The public brings diffuse and specialized expertise to the table, from devoted nature enthusiasts to scientists who have spent their whole careers studying one particular animal, insect or plant. Public involvement can also help counter the political pressure inherent in large development projects. The FWS, however, is unlikely to approve the listing of a species that is not truly threatened or endangered, so some petitions are filtered out. “You could compare it to the trend of crowdsourcing that the Internet has spawned,” Brosi says. “It’s sort of like crowdsourcing what species need to be protected.”
A new paper by Levi and Wilmers in the journal Ecology uses a 30-year time series of wolf, coyote, and fox relative abundance from the state of Minnesota, USA, to show that wolves suppress coyote populations, which in turn releases foxes from top-down control by coyotes. The authors conclude “Mesopredator release theory has often considered the consequence of top predator removal in a three species interaction chain (i.e., coyote–fox–prey) where the coyote was considered the top predator (Ritchie and Johnson 2009). However, the historical interaction chain before the extirpation of wolves had four links. In a four-link system, the top predator releases the smaller predator. The implication is that a world where prey species are heavily predated by abundant small predators (mesopredator release) may be similar to the historical ecosystem.” The study’s findings suggest that “among-guild interaction chains with even numbers of species will result in the smallest competitor being suppressed while among-guild interaction chains with odd numbers of species will result in the smallest competitor being released.” These findings have important implications for efforts to predict the consequences of removal or restoration of top predators.
A new paper in PLoS Biology by Levi and colleagues (here) describes a new approach for assessing trade-offs between economic and ecological goals in “Ecosystem Based Management” (EBM). The paper concludes:
“Commercial fisheries that harvest salmon for human consumption can end up diverting nutrients that would normally be directed to terrestrial and aquatic ecosystems. We examined this problem for Pacific salmon fisheries by using grizzly bears as indicators of salmon ecosystem function. Bear densities vary enormously depending on salmon availability, and by leaving uneaten salmon carcass remains beside spawning streams, bears play an important role in dispersing marine nutrients to plants, invertebrates, and other wildlife. By relating the number of spawning fish to bear diet and density, we developed a model to quantify ‘‘ecosystem-harvest’’ tradeoffs; i.e., how bear density changes with the amount of fish harvested (fishery yields). We estimated this tradeoff between yields and bear density for six sockeye salmon stocks in Alaska and British Columbia (BC) across a range of management options that varied the number of salmon allowed to escape from the fishery. Our model shows that bear densities will increase substantially with more spawning fish at all sites. Notably, in most study systems, fishery yields are also expected to increase as the number of spawning fish increases. There is one exception, however, in the Fraser River (BC), where bears are threatened and sockeye salmon are nearly the only species of salmon available. Here, releasing more salmon to spawn would result in lower fishery yields. To resolve such conflicts in this and other systems, we propose a generalizable ecosystem-based fisheries management framework, which allows decision-makers (such as fisheries managers and conservation scientists) to evaluate different allocation options between fisheries and other ecosystem recipients.”
In a news story from California (here), the study’s authors suggest that their conclusions are also relevant to areas where grizzly bears are extinct: “Levi argues that having more salmon in streams would also have economic benefits from better wildlife viewing opportunities. Increased salmon abundance would surely help California’s bald eagles. More salmon would be a boon to the state’s recovering eagle population. More salmon would also increase black bear populations, he says, which is good for both hunters and wildlife observers.”
Conservation biologists have long debated whether and how it is appropriate for scientists to influence policy decisions. A pair of essays in the journal Conservation Biology (one published, another in press) asks whether it’s appropriate for scientists to review and critique recovery goals for endangered species. Wilhere (2012) argues that because recovery criteria are inherently normative (values driven), scientists are engaging in “inadvertent advocacy” when they criticize such criteria. In a response, myself and coauthors agree with Wilhere that recovery criteria represent an interaction of science and values, but provide a different view on the appropriate role of individual scientists and scientific societies in reviewing recovery criteria and recovery plans. This debate is central to recovery planning for many species, and we suggest a way forward for the agencies to more clearly separate the normative and scientific elements of recovery criteria. We call on the agencies to develop an explicit decision framework that would provide the flexibility needed to address the unique biological circumstances faced by different species but would limit the abuse of discretion that has allowed political interference to drive many listing and recovery decisions.
A new paper in the journal Nature Climate Change finds evidence that declining snowfall in the southwestern US indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk. Abundances of deciduous trees and associated songbirds have declined with decreasing snowfall over 22 years of study in montane Arizona. The researchers experimentally tested the hypothesis that declining snowfall indirectly influences plants and associated birds by allowing greater over-winter herbivory by elk, by excluding elk from one of two paired snowmelt drainages and replicating this paired experiment across three distant canyons. Over six years, the exclosures reversed multi-decade declines in plant and bird populations by experimentally inhibiting heavy winter herbivory associated with declining snowfall. Predation rates on songbird nests decreased in exclosures, despite higher abundances of nest predators, demonstrating the over-riding importance of habitat quality to avian recruitment.
The Connectivity Analysis Toolkit is a software interface that provides conservation planners with tools for both linkage mapping and landscape-level ‘centrality’ analysis. 450 people from around the world have downloaded the CAT since it became available in 2010.
We have just released Version 1.2 with the following changes:
• Approximate shortest-path betweenness centrality allows faster computation of this metric
• Approximate current flow betweenness centrality allows faster computation of this metric; function also uses sparse matrices for lower RAM requirements
• Network flow functions updated to LEMON version 1.2.2
• Updates to manual and tutorial dataset
These are major updates which speed computation in some cases by an order of magnitude. Thanks to Aric Hagberg for his work adding these new functions to NetworkX and thus making them available for the CAT.
The software is freely available at www.connectivitytools.org (a link is also posted on this blog site).