Animals that deposit aquatically derived nutrients on terrestrial landscapes link food webs and affect a variety of in situ processes. This phenomenon, however, is poorly documented in freshwater habitats, especially where species introductions have drastically changed an ecosystem's trophic structure. In this study, we used stable isotopes to document water-to-land nutrient transport by river otters (Lontra canadensis) around Yellowstone Lake, an ecosystem recently altered by nonnative species invasions. We then investigated the effects of otter fertilization on plant growth and prevalence at latrine (scent-marking) sites and evaluated how the recent changes to the lake's food web could influence these plant responses. Values of delta15N were higher on latrines compared to non-latrine sites in five of seven sample plant taxa. Additionally, latrine grasses had higher percentage N than those from non-latrines. Foliar delta15N positively related to fecal deposition rate for some plants, indicating that increased otter scent-marking led to a rise in these N values. Logistic regression models indicated that otters selected for well-shaded latrines with access to foraging. Atypical latrines, misclassified as non-latrines by the regression models, had values of delta15N similar to correctly classified latrines, suggesting that site effects alone cannot explain elevated N values at otter latrine sites. No difference in plant diversity or percent cover of N-fixing taxa occurred between latrine and nonlatrine sites, though specific genera did differ between site types. Measurements of shoot lengths indicated increased growth of some latrine currants (Ribes sp.). In Yellowstone Lake, a twofold reduction in otter numbers could result in an even greater decline in nutrient deposition at latrines, as otters may become less social in a system with decreased prey availability. Our results highlight the role of animals in linking aquatic and terrestrial habitats in inland freshwater systems and suggest that ongoing changes in the trophic structure of Yellowstone Lake could have unexpected ramifications well beyond the lake itself.
We tested the behavioral and demographic responses of American kestrels, Falco sparverius, and gray-tailed voles, Microtus canicaudus, to vegetation height and addition of perches. We conducted our experiment in sixteen 0.2-ha rodent enclosures with four replicates assigned to each of the following treatments: tall vegetation without perches, tall vegetation with perches, short vegetation without perches, and short vegetation with perches. The enclosures were stocked with 20 gray-tailed voles in early November 1998. Before perches were erected during the 12th week of the experiment, kestrels showed a preference for short-vegetation enclosures (P < 0.05). After perches were erected, kestrels used enclosures with perches, showing the greatest preference for short-vegetation enclosures with a perch. Vole populations and recruitment rates were higher in tall-vegetation enclosures than in short-vegetation enclosures, but supplemental perches did not affect vole populations or recruitment. In many agricultural areas where perches are not available, providing supplemental perches may increase accessibility to prey species that cause crop damage. Facilitating predation by raptors may reduce vole populations and reduce the need to use potentially harmful chemicals in pest population management.
Nonnative species threaten ecosystems throughout the world -including protected reserves. In Yellowstone National Park, river otters Lontra canadensis depend on native cutthroat trout as prey. However, nonnative lake trout and whirling disease have significantly reduced the abundance of these native fish in the park's largest body of water, Yellowstone Lake. We studied the demographic and behavioral responses of otters to declining cutthroat trout on Yellowstone Lake and its tributaries. From 2002-2008, we monitored otter activity at latrine (scentmarking) sites, collected scat for prey identification, and used individual genotypes from scat and hair samples to evaluate survival and abundance with capture-recapture methods. Otter activity at latrines decreased with declines in cutthroat trout, and the prevalence of these fish in otter scat declined from 73% to 53%. Cutthroat trout numbers were the best predictor of temporal variation in apparent survival, and mean annual survival for otters was low (0.72). The density of otters in our study area (1 otter per 13.4 km of shoreline) was also low, and evidence of a recent genetic bottleneck suggests that otter abundance might have declined prior to our study. River otters in and around Yellowstone Lake appear to be responding to reductions in cutthroat trout via changes in distribution, diet, and possibly survival and abundance. Our results provide a baseline estimate for monitoring the broader outcome of management efforts to conserve native cutthroat trout and emphasize the indirect ecosystem consequences of invasive species.
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