Effects of River Otter Activity on Terrestrial Plants in Trophically Altered Yellowstone Lake

Abstract

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.

Density of river otters (Lontra canadensis) in relation to energy development in the Green River Basin, Wyoming

“…First, our observed site-specific habitat measurements have shown that when establishing a latrine in all river reaches, river otters selected for similar features, namely high overstory and grass cover, moderately sloping banks, and relatively deep pools. These features have been previously described as suitable for river otters in western streams and lakes (Crait and Ben-David, 2007;DePue and Ben-David, 2010). As indicated by our habitat modeling, the NF had the highest availability of overstory and grass cover at the landscape level compared with the other two reaches.…”

“…Errors using the TEV method were generally higher, but showed similar trends (Table B1). These error rates are higher than other studies in this region (Hayes et al, 2014), but adequate for quantification of habitat features important for river otters (i.e., overstory cover; Crait and Ben-David, 2007;DePue and Ben-David, 2010).…”

Indirect effects of bioinvasions in Yellowstone Lake: The response of river otters to declines in native cutthroat trout

“…Less often has this debate addressed the consequences of mitigating biological invasions in one ecosystem on linked ecological processes in an adjacent one. For example, river otters transfer aquatically-derived nutrients to terrestrial habitats via their scent-marking behavior at latrines and create "hot spots" of nutrient heterogeneity at the waterland interface (Ben-David et al, 2005;Crait and Ben-David, 2007;Roe et al, 2010). Our finding of behavioral changes in latrine use suggests that the spatial distribution of these hot spots may have shifted away from tributary streams.…”

Differential tree and shrub production in response to fertilization and disturbance by coastal river otters in Alaska

“…Several studies in temperate forests have shown increased nitrogen (N) concentration in plant tissue or increased plant production in response to such animal-mediated nutrient transfers (Ben-David et al 1998a, b, Hilderbrand et al 1999, Helfield and Naiman 2001, 2002, Crait and Ben-David 2007.…”