Effects of kelp forests Macrocystis pyrifera on the larval distribution and settlement of red and purple sea urchins Strongylocentrotus franciscanus and S. purpuratus

Abstract

It has often been observed that the abundance of sea urchins is lower inside kelp forests and along their nearshore edges than outside their offshore boundaries. We tested the hypothesis that this distributional pattern is a reflection of settlement patterns by monitoring settlement on artificial surfaces at 3 kelp forests near San Diego, California. USA. We estimated settlement at paired sites under the canopy and outside the offshore edge of the kelp forests and at paired sites located 20 m outside the canopy near the inshore and offshore boundaries of each kelp forest. There was no evidence of an effect of dense stands of giant kelp on the settlement of purple sea urchins. The results for red sea urchins were ambiguous; in 3 of 7 comparisons, average settlement was h~g h e r or observations of higher settlement were more frequent offshore from the kelp forest than elsewhere. Although this could result from different effects on the 2 species, we think ~t probably was due to accidents of sampling We conclude that the distributional patterns of purple and red sea urchins relative to kelp forests are unllkely to reflect larval availability or settlement, but are more probably a function of post-settlement events We qualify our concl.usions because of the tremendous temporal variability in the physical and biological factors that can, potentially affect the local distnbution of sea urchin larvae. The 2 years of this study may be short relative to natural cycles.

Predators of the destructive sea urchin Strongylocentrotus droebachiensis on the Norwegian coast

“…A higher predation pressure is therefore also to be expected inside kelp forests under natural conditions (Hereu et al 2005, Bonaviri et al 2012. Sea urchin larvae tend to settle within both barren areas and kelp forests (Rowley 1989, Schroeter et al 1996, McNaught 1999, but adult sea urchins are typi cally very rare inside the kelp forests (Skadsheim et al 1995, Steneck et al 2004. A number of studies imply that greater predation pressure within kelp forests may explain this pattern (Rowley 1989, McNaught 1999.…”

The effects of seascape pattern on algal patch structure, sea urchin barrens, and ecological processes

“…Smaller seascape scales may be important for algal patch structure in smaller forests. Scaling of edge to area ratios affects the hydrography within kelp forests including current magnitudes which are damped in large forests (Jackson and Winant, 1983), and ecological processes such as larval penetration (Schroeter et al, 1996) and susceptibility to grazing disturbances (Rosenthal et al, 1974). These processes must also affect the scales and resilience of algal patch structure.…”

At what spatial scales are alternative stable states relevant in highly interconnected ecosystems?

“…Urchin larvae can then experience stochasticity in the number of dispersing larvae that end up in a population r U ðs K ; xÞ, which captures presettlement larval survival and movement by water currents (Hart and Scheibling 1988), settlement success dependent on factors such as local cues, and post-settlement survival dependent on predation on recently settled juveniles (Schroeter et al 1996), where our distinction between "presettlement", "settlement", and "post-settlement" phases follows Keough and Downes (1982) and Benton and Bowler (2012). Urchin larvae can then experience stochasticity in the number of dispersing larvae that end up in a population r U ðs K ; xÞ, which captures presettlement larval survival and movement by water currents (Hart and Scheibling 1988), settlement success dependent on factors such as local cues, and post-settlement survival dependent on predation on recently settled juveniles (Schroeter et al 1996), where our distinction between "presettlement", "settlement", and "post-settlement" phases follows Keough and Downes (1982) and Benton and Bowler (2012).…”