Continuing trophic cascade effects after 25 years of no-take marine reserve protection

Abstract

Between 1978 and 1996 benthic communities in the Leigh Marine Reserve shifted from being dominated by sea urchins to being dominated by macroalgae.This was a result of a trophic cascade thought to be an indirect effect of increased predator abundance.We assessed further changes in communities from 1996 to 2000, differences in benthic communities between reserve and adjacent unprotected sites, and the stability of these patterns from 1999 to 2001.Since 1996, densities of sea urchins Evechinus chloroticus have continued to decline in shallow areas of the reserve (< 8 m), and all sites classified as urchin barrens in 1978 are now dominated by large brown algae.Comparisons between reserve and non-reserve sites revealed differences consistent with a trophic cascade at reserve sites.The greatest differences in algal communities between reserve and non-reserve sites occurred at depths where E. chloroticus was most abundant (4 to 6 m).Reserve sites had lower urchin densities and reduced extent of urchin barrens habitat with higher biomass of the 2 dominant algal species (Ecklonia radiata and Carpophyllum maschalocarpum).At reserve sites densities of exposed E. chloroticus (openly grazing the substratum) declined so that urchin barrens were completely absent by 2001.Lower density of the limpet Cellana stellifera and higher densities of the turbinid gastropod Cookia sulcata at reserve sites are thought to be responses to changes in habitat structure, representing additional indirect effects of increased predators.The overall difference in community types between reserve and non-reserve sites remained stable between 1999 and 2001.Localised urchin mortality events due to an unknown agent were recorded at some sites adjacent to the marine reserve.Only at 1 of these sites did exposed urchins decline below the critical density of 1 m -2 , which resulted in the total replacement of urchin barrens with macroalgae-dominated habitats.At other sites urchin barrens have remained stable.Declines in the limpet C. stellifera occurred across all sites between 1999 and 2001 and may be indirectly associated with urchin declines.Long-term changes in benthic communities in the Leigh reserve and the stability of differences between reserve and nonreserve sites over time are consistent with gradual declines in urchin densities due to increased predation on urchins, thus providing further evidence for a trophic cascade in this system.The rapid declines in urchin numbers at some unprotected sites, however, demonstrate how short-term disturbances, such as disease, may result in shifts in community types over much shorter time frames.