An introduction to trophic cascades using sea otters as an example

What is a keystone species? Keystone species are species that are critical to the vitality of other species in a given ecosystem (Bond 1994).

WATCH this to see how a sea otters eat sea urchins à

http://www.arkive.org/sea-otter/enhydra-lutris/video-ne08a.html

(viewed 8 March 2014)

Sea otters and sea urchins live together on the coasts of California and Alaska. These waters are home to abundant kelp forests, which provide protection for the otters and food for the urchins (Silliman & Angelini 2012). These ecosystems display a significant evolutionary interrelationship that is vital for maintaining a healthy ecosystem (Los Padres ForestWatch 2013).

Did you know? A sea otter is one among the few animals on earth who uses tools. They use rocks to dislodge crustaceans from the sea floor and to open shells for the flesh inside (Los Padres ForestWatch 2013).

Sea otters lack the layer of fat that most marine mammals use for insulation. In its place, they have evolved an extremely dense fur coat (in fact the densest coat of any animal on earth). It was these thick coats that drew the attention of humans for the first time. In the early 20^th century, humans began heavily hunting otters for their coats (Los Padres ForestWatch 2013).

A raft of sea otters (Michael Jay 2013)

The otter population rapidly declined, eventually leaving only one colony still functioning (Silliman & Angelini 2012).

This is where trophic cascades come in.

A trophic cascade occurs when a change in the number of top predators (organisms at the top of the food chain) /keystone species affects the density and/or behaviour of their prey, thus shaping the survival of the next lower trophic level (Silliman & Angelini 2012).

The sea otter, sea urchin and kelp story is an example of a three level trophic cascade. As the otter population began to dwindle, urchin numbers escalated. As a result, urchins ate away at the kelp forests with nothing to stop them, and the ecosystems that once flourished with life were seen to be deteriorating (Kowalewski 2012). Clearly, the sea otter is a vital evolutionary component to maintain a stable and healthy ecosystem.

Understanding trophic cascades has implications for conservation and management (Silliman & Angelini 2012). The trophic cascade theory implies that in the absence of a vital top predator, ecosystems become unhealthy. Whole ecosystems can be restored to a healthy state by re-establishing the keystone species in the ecosystems, restoring the balance.

Sea otters are now a protected species, and in areas where populations have been restored, ecosystems are coming back to life. 

Follow me for more cool examples of the effect one species has on a whole ecosystem – including the physical aspects!

Image source: 

viewed 8 March 2014 <http://lpfw.org/our-regionwildlifesouthern-sea-otter/>

Other sources:

Kowalewski, D 2012, ‘Howling about trophic cascades’, Cambridge University Press, vol. 28, no. 1, pp. 17-26,  <http://search.informit.com.au>

Los Padres ForestWatch 2013, Los Padres ForestWatch, Santa Barbara, viewed 8 March 2014, <http://lpfw.org/our-regionwildlifesouthern-sea-otter/>

Silliman, B. R & Angelini, C 2012, Trophic cascades across diverse plant ecosystems, Nature Education Knowledge, <http://www.nature.com/scitable/knowledge/library/trophic-cascades-across-diverse-plant-ecosystems-80060347>