Chapter 5: Ecosystem Effects of Sea Otters

SHORT SUMMARY:

Having focused on species-specific biological considerations for sea otters in the first few chapters, Chapter 5 broadens the perspective to consider ecosystem-level implications of a sea otter reintroduction. Sea otters are often considered to be a textbook example of a “keystone species”, defined as a species that has disproportionately large effects on its ecosystem relative to its abundance (Paine 1969). As sea otters have been reintroduced or naturally recovered to other coastal areas in North America, they have caused substantial perturbations to the structure and dynamics of nearshore food webs. Some of these effects are perceived as beneficial for people and some are perceived as negative. After providing a brief primer on the ecological concepts necessary to interpret the direct and indirect effects of sea otter recovery, the chapter provides a review of these effects and discusses their implications for nearshore ecosystems and human communities in coastal Oregon.

Figure 5.2. Some of the known or suspected linkages between sea otters and coastal marine ecosystems.

The influence of sea otters on coastal ecosystems is one of the most well-known and well-documented examples of a trophic cascade (the complex network of consumer-prey interactions, from apex predators at the top of the food web to plants at the bottom). The top-down effects of sea otters on coastal ecosystems result mostly from the direct limiting influences of sea otters on their macroinvertebrate prey and through the indirect knock-on effects on other species and ecological processes. The most extensively studied and well-known knock-on effect of sea otters occurs through their limiting influence on herbivorous sea urchins and the resulting enhancement of  kelps and other groups of macroalgae, which in turn benefits numerous other species and ecological processes. This chapter summarizes these known direct and indirect ecological effects and their likely evolutionary consequences and draws particular attention to many others that are either less well studied or entirely unknown. Although many of the patterns and processes described have been well documented, the details can vary substantially from place to place and through time. The sea otter’s powerful and diverse ecological influences result in both costs and benefits to human societies.