Modelling changes in trophic and structural impacts of alien ecosystem engineers on a rocky-shore island

Abstract

Between 1980 and 2012, successive arrivals by three alien ecosystem engineers on a rocky shore community at Marcus Island on the west coast of South Africa led to substantial changes in species composition and diversity. An ecosystem analysis of this open intertidal system was developed using Ecopath with Ecosim to determine the impacts of these aliens and the services they provide on the native community. A baseline Ecopath model of the community in 2015 was generated using values of biomass, production/biomass, consumption/biomass and the dietary composition of 30 functional groups. Ecosim, a time-dynamic modelling routine, was then used to simulate the changes in biomass of native species. A 1980 model (pre-invasion) was constructed and 22 simulations were run up to 2015 by systematically adding (1) biomass time series for non-native species; (2) relative biomass time series for native species; (3) mediation functions that mimicked biomass impacts due to changes in substrate, shelter and feeding grounds created by the alien ecosystem engineers; and (4) the effects of wave action as a source of mortality. Positive or negative influences of these ecological processes on diversity and the final biomasses of all groups in 2015 were assessed. Trophic impacts by the alien species affected diversity and biomass at the end of all simulations, but the addition of shelter or a combination of all three ecosystem services provided by ecosystem engineers (shelter, substrate and feeding grounds) resulted in 2015 model ecosystems that most closely matched the diversity and individual group biomasses empirically measured on Marcus Island in 2015. Wave action had only a minor impact. Marcus Island's rocky shore community was therefore driven mainly by the fixed input of alien species biomass and made more realistic by the incorporation of their ecosystem services. However, structural complexity and zonation, explored in a follow-up paper invoking spatial modelling, need to be represented for a more complete realisation of the ecosystem.