Density-dependent resource selection by a terrestrial herbivore in response to sea-to-land nutrient transfer by seals.

This research article investigates how sea-to-land nutrient transfers influenced by seal populations can modify the feeding behavior of a land-based creature, in this case, feral horses. The study suggests that these nutrient transfers impact the horses’ selective foraging habits and concludes that the horse population’s sustainability on Sable Island may be directly connected to these nutrient transfers.

Key Research Purpose and Methods

• The study sought to understand how sea-to-land nutrient transfers—the nutrients carried from sea to land by seals—impact terrestrial food webs and influence the behavior of opportunistic species.
• To explore this, the research team traced the movement of nitrogen, brought to land by gray seals, through stable isotope measurements (δ N) in marram grass. They then studied whether this availability of nitrogen affects the foraging behavior of feral horses on Sable Island, Canada.

Findings

• The research team discovered that δ N levels in the grass had direct correlations with the grass’s protein content which, in turn, had a strong link with densities of seal pups.
• Such varying protein contents in the marram grass led to a significant influence on the horses’ selective foraging habits, with these behaviors being notably density-dependent, in line with optimal foraging theory. Essentially, where there are more nutrients (due to higher seal populating hence more nitrogen), the horses would preferentially forage.
• The researchers hypothesize that the enduring presence of horses on Sable Island, far removed from typical food sources, is facilitated by these nitrogen subsidies.

Implications

• These findings present the first proof of the effects of sea-to-land nutrient transfers on the behavioral process of resource selection—that is, how an animal chooses what resources to use relative to what is available—in terrestrial herbivores.
• From the perspective of ecology and biogeography, these findings could inform further understanding of trophic dynamics, island biogeography, and metaecosystem ecology where ecosystems interact with each other.