Sea stars play an important role in kelp forest regime shifts
This research involved a unique ‘natural experiment’ which revealed that size-specific predation of sunflower sea stars (Pycnopodia helianthoides) on sea urchins is complementary to the size-specific urchin predation by sea otters. Using field data and a population model (accounting for changes in space and time), we show that Pycnopodia can enhance the resilience of kelp forests and facilitate their collapse by consuming small and medium sea urchin prey otherwise left unconsumed by sea otters. When Pycnopodia populations collapsed after Sea Star Wasting Disease, this corresponded to a 311% increase in medium urchins and a 30% decline in kelp densities. This highlights a previously under-appreciated species interaction occurring within a ‘classic’ trophic cascade, emphasizing the critical role of middle-level predators in mediating rocky reef regime shifts.
Experiments to Ask: What Influences Sea Urchin Grazing Rates?
This research combines experimental and survey methods to ask: 1) What is the functional relationship between urchin density and the per capita urchin grazing rate? and 2) What measurable factors best predict in situ urchin grazing rates? In June 2014 I conducted subtidal grazing experiments at eight rocky reef sites on the central coast of BC to quantify how the abundance, size and behaviour of red sea urchins (Mesocentrotus franciscanus), as well as drift abundance, wave exposure, and the presence of other algal species influence the rate of kelp loss. Here’s a little video that describes the work:
Kelp Forest Monitoring on B.C.’s Central Coast
Myself, members of SFU’s Coastal Marine Ecology and Conservation Lab, and the Hakai Institute have been working together since 2013 to monitor the ecological changes occurring on rocky reefs along B.C.’s central coast. We conduct annual dive surveys at 11 reef sites, recording fish species, invertebrate species, and kelp communities, and combine these efforts with other annual projects focused on sea otter foraging, fish capture surveys, measuring kelp productivity, and kelp mapping. Here’s a video I made in 2013 highlighting this work:
and another video I made in 2015 highlighting some preliminary results.