Disturbance initiates diversity in recruitment of canopy-forming algae: interactive effects of canopy-thinning and substratum availability

Abstract

We assessed whether disturbance within monospecific canopies (Ecklonia radiata, Phaeophyta) increases diversity of macroalgal recruits by creating gaps in the canopy and by liberating space on the substratum. We tested the hypothesis that a greater number of species of canopy-forming algae recruit to areas where canopies of monospecific E. radiata are thinned and the availability of barer substratum is increased (i.e. fewer holdfasts without thallus remain attached to the substratum). A significant interaction between canopy-cover and substrate-availability was detected; losses to canopy caused greater diversity of recruits only where there were few holdfasts remaining on the substratum. Interestingly, we found large numbers of species under thinned canopies where remnant holdfasts were sparse, which were also found under dense canopies where remnant holdfasts were dense. Residual holdfasts may increase the likelihood of survival of recruits under dense canopies through the provision of habitat rather than displacing appropriate habitat for recruitment. Nevertheless, these results indicate that disturbance events not only initiate diversity of canopy-forming algae by thinning monospecific canopies, but through the liberation of available substratum.