Complex interplays between limpets and alien species in South Africa: multispecies interactions, zonation and size effects

Abstract

Integrating observations and experiments, we address the progressive effects of three alien species, the musselsMytilus galloprovincialisandSemimytilus patagonicus, and the barnacleBalanus glandula, on limpet species in South Africa. We describe four aspects: (1) Interactions among algae, the limpetScutellastra granularisandM. galloprovincialis. (2) The influences ofB. glandulaonS.granularisand on the periwinkleAfrolittorina knysnaensis. (3) Transformation of the zonation ofS. granularisby the successive arrival of these three aliens. (4) Assessment of how effective the published predictors of the effects ofM. galloprovincialishave been when applied toS. patagonicus. We conclude: (a)Scutellastra granularisimproves mussel survival and condition by regulating algae that would otherwise overgrow and smother the mussels. (b)Balanus glandulahas largely occupied the upper half of rocky shores, and at high densities depletes or eliminates suitable bare-rock habitat forS. granularis. However, it has positive effects on another gastropod,A. knysnaensis.(c)Mytilus galloprovincialisdominates the midshore and has positive effects on the recruitment ofS. granularis, but negatively influences adults of this limpet, so that its size composition, density, reproductive output and zonation are all altered by this mussel. (d)Semimytilus patagonicuspresents different challenges to those created byM. galloprovincialis, settling at much greater densities, lower down the shore, and reaching smaller maximum sizes. Rather than generating a favourable habitat for epizooticS. granularisrecruits and juveniles, it almost completely excludes that limpet. We conclude that the influences of alien species are not readily predictable, depending on the nature of the invader and recipient species, environmental conditions and complex interactions among species. Collectively, the three alien species now cover almost all zones on wave-exposed rocky shores, completing the ‘zonation squeeze’ on limpets, but are less influential on wave-sheltered shores and in years when recruitment is low.