Rivets or bolts? When single species count in the function of temperate rocky reef communities

Abstract

There is considerable controversy about the role of individual species in ecosystem functioning. Most models stress the role of species richness and diversity in ecosystem function, but it is also recognised that individual species or functionally similar species can play prominent roles in assessments of function. There have been relatively few tests of functional replacement by similar species in the marine environment. On intertidal reef platforms in southern New Zealand, six species of fucoid algae co-occur. Two of these, Cystophora torulosa and Hormosira banksii, have extensive cover in the mid-tidal zone. These species were removed from their areas of dominance in the mid-tidal zone at two sites and followed for several years to determine how resilient they are to disturbance, what processes determine functional recovery, the role of the dominant grazer, the gastropod Turbo smaragdus, in the recovery process, and whether there is functional replacement of species. In the C. torulosa removal experiment, H. banksii recruited rapidly into removal treatments within the first year at one site, but this took a year longer at the other site. A press removal of C. torulosa developed almost 100% cover of H. banksii after 3 years, but only at one site. In the pulse treatment, there was a mixed stand of the two species after 3–4 years. In the H. banksii removal experiment, there was no functional replacement by other species. The rate of recovery for H. banksii was variable. Bare space increased following canopy removal, particularly on the higher shore treatment at one site, because of the burn-off of newly exposed turfing coralline algae, which had not fully recovered by the end of the experiment. After 10 months, there were 20–40% fewer species in the removal plots in both experiments than in controls. T. smaragdus grazing had great effects on ephemeral algae, but not on the fucoids. The conclusion is that these fucoid species have no functional equivalents in their areas of dominance. They are the autogenic engineers or foundation species on which most other species in their communities rely. There is little buffering overall in these mid-shore zones should these species be lost or severely reduced in abundance. They are, therefore, key species in the function of the intertidal system and must be understood if the functioning of these areas is to be managed effectively. These results are discussed in the spirit of this festschrift for AJ Underwood.