Coastal river otter effects on terrestrial plant productivity of Prince William Sound, Alaska: The competing effects of marine-derived nutrient deposition and disturbance

Background/Question/Methods Spatial coupling of marine and terrestrial ecosystems through animal-mediated transport of matter, energy, and disturbance can alter ecosystems in unpredictable ways. Coastal river otters (Lontra canadensis), ubiquitous along marine shores in Alaska, mediate such transport by feeding in the marine environment, depositing nutrients in communal terrestrial latrines, and trampling vegetation on the sites. Because anthropogenic impacts, including global warming, may reduce otter abundance, it is important to determine if the hotspots of nutrient inputs by otters play a key role in changing the heterogeneity of coastal vegetation communities. We studied the uptake of marine-derived nitrogen in the vegetation of latrines using stable isotopes and tested whether uptake resulted in increased production of dominant shrubs and trees, given the potential negative effects of trampling. On 83 latrines and 30 similar, coastal randomly-located non-latrine sites, productivity was compared for the dominant species using indices such as leaf area index (LAI) measured indirectly with a ceptometer and/or annual mass of shrub leaf and fruit production. Such variables were compared not only between latrine and non-use coastal sites but also within a local site by contrasting areas on the latrine with adjacent areas off the latrine.

Results/Conclusions The d15N of blueberries (Vaccinium sp.) and western hemlock (Tsuga heterophylla) from latrines were significantly greater with an enrichment of 3.31 and 9.50 ‰ relative to non-use sites, respectively. These species also showed a significant increase in d15N on the latrine compared to adjacent areas off the latrine. Western hemlock did not differ in density between latrine and non-latrine sites but showed a 26% increase in the effective LAI on the latrines relative to non-use sites, possibly due to fertilization. In contrast, blueberry shrub density and aboveground non-woody production was reduced on latrines relative to the adjacent off portion of the site (62 and 86% decline respectively), consistent with the effects of heavy trampling. Nonetheless, after accounting for differences in plant morphology, individual shrubs showed a fertilization effect. Shrubs that had survived trampling on the latrines had 229 times more growth than individuals found on non-use sites, after plant size was taken into account. Otters appear to be increasing the heterogeneity of the forested coastal landscape by opening up the dense understory and increasing aboveground productivity of the trees and remaining shrubs.