The importance of context on the prevalence of individual diet specialization: the interaction between intra-specific competition and habitat

Background/Question/Methods

The recognition and quantification of individuality is now a common research theme in the fields of behavioral, evolutionary, community, and population ecology. Individual level differences in behavior, prey choice, and movement have been documented over a wide variety of taxa and ecosystems. The potential for individuality in a population is likely context dependent, and the influence of habitat characteristics, which can be strongly correlated with (prey) diversity, on the prevalence of individuality has received less attention than competition. We examined individual diet specialization in ~15 sea otter (Enhydra lutris) populations across the entire northeast Pacific margin from southern California to the central Aleutian Islands, Alaska. Because exploitation/management histories and general habitat characteristics vary among these sites, we could examine the effects of prey diversity/availability and intra-specific competition on the prevalence of individual diet specialization. We used observational dietary data to quantify prey diversity and characterize relative habitat availability. We primarily relied on stable isotope data to quantify population and individual dietary variation and individuality. As in previous isotope-based studies of individuality, we defined the within-individual component (WIC) as the amount of intra-individual isotopic variation and the total isotopic niche width as the sum of the WIC and between-individual components (BIC).

Results/Conclusions

Using observational diet data, we classified half of our sites as rocky-substrate habitats, while the others contained a mixture of rocky substrate and soft-sediment habitats. Previous studies have shown that in rocky substrate habitats in California, individuality increases with intra-specific competition as measured by sea otter density. Our results, which include data for 8 rocky substrate sites from southern California to the central Aleutians, support previous findings. Specifically, the slope of the relationship between the within-individual component (WIC) and total isotopic niche width (TINW) was low (0.24) but significant (P<0.001, R²=0.90). Furthermore, population dietary diversity (TINW) was positively correlated with sea otter density. In contrast, the slope of the significant relationship (P<0.001, R²=0.95) between WIC and TINW for otter populations inhabiting mixed substrate sites was much higher (0.50), suggesting a low degree of individuality irrespective of competition (i.e., sea otter density). Our results show that the potential for individuality to occur as a result of increasing intra-specific competition is context dependent and that habitat characteristics, which ultimately influence prey diversity and abundance, are important in determining when and where individual specialization occurs in nature.