Worldwide, marine rocky shores are being modified by alien species, but their successive impacts are rarely recorded. We documented sequential invasions of Marcus Island on the west coast of South Africa by comparing communities from 1980 (preinvasion), 2001 (after invasion by the mussel Mytilus galloprovincialis) and 2012 (following invasions by another mussel, Semimytilus algosus, and the barnacle Balanus glandula). Their influence on habitat complexity was measured with a novel technique enabling retrospective calculation of historical complexity. In 1980, habitat complexity, invertebrate abundance and species richness decreased from the low-shore to the high-shore, but homogenised in 2001 after M. galloprovincialis elevated habitat complexity across most of the shore. In 2012, these variables returned to preinvasion patterns, after M. galloprovincialis declined in the high-shore and was replaced there by B. glandula. With the first mussel invasion, several indigenous species extended up the intertidal, but retreated once M. galloprovincialis receded. Community composition differed significantly among nearly all years and zones, irrespective of whether the alien species were included in the analyses or not. Some once-dominant native species were negatively affected by the invasions: one indigenous mussel, Choromytilus meridionalis, disappeared by 2012, and another, Aulacomya atra, declined. The abundance of recruits of the limpet Scutellastra granularis rose and fell with the arrival and recession of M. galloprovincialis, but adults were adversely affected. Changes to habitat complexity induced by sequential invasions supported hypothesised changes in invertebrate abundance and species richness, but could not alone predict changes in community composition, which were also influenced by zonation.

Of the 89 alien species recorded from South African marine waters (Robinson et al. 2016), three have become widespread along the open coast: the Mediterranean mussel Mytilus galloprovincialis, from the Mediterranean Sea and eastern Atlantic; the Pacific barnacle Balanus glandula from the American west coast; and the bisexual mussel Semimytilus algosus from Chile (Branch and Steffani 2004;Laird and Griffiths 2008;de Greef et al. 2013;Assis et al. 2015;Robinson et al. 2015). In 1980, prior to any marine invasions, a study of intertidal rocky-shore community composition was undertaken on Marcus Island, Saldanha Bay, to assess prey availability for the endemic African black oystercatcher Haematopus moquini (Robinson et al. 2007). These data were later used as a historical baseline against which to consider changes in community composition associated with the introduction of M. galloprovincialis (Robinson et al. 2007) and the later arrival of B. glandula and S. algosus (Sadchatheeswaran et al. 2015). Adult M. galloprovincialis are recognised as an example of 'ecosystem engineers': species that can transform environments and modulate resource availability to an ecosystem, particularly the benthic communities (sensu Jones et al. 1994;McQuaid and Arenas 2009). Because of limitations inherent in the historical data, there were restrictions on the analyses presented by Robinson et al. (2007) and Sadchatheeswaran et al. (2015). The most notable limitation was that the studies were all snapshots in time, set 10 to 20 years apart, and thus failed to consider either interannual or seasonal changes. Therefore, changes in community composition ascribed to the arrival of these alien invasive species could not be distinguished from natural shifts that might have arisen seasonally or from year to year, such as those recorded by Dye (1998). An assessment of the magnitude of seasonal and interannual changes at Marcus Island was thus required. The second challenge associated with the historical data was that the datasets considered only invertebrates, because the original 1980 survey was focused on the availability of prey for H. moquini (Robinson et al. 2007). Furthermore, the exclusion of algae is problematic, as multiple studies have demonstrated a variety of roles for

No 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. the Pacific barnacle Balanus glandula (Harley, 2006;Harley and O'Riley, 2011). Besides changes to native biodiversity due to modulation of access to resources, ecosystem engineers may also alter (1) the amount of surface area or substrate (

Ecosystem engineers often affect structural complexity of habitats. There are multiple methods of quantifying complexity, variously measuring topography, surface area, volume, fractal dimension, or rugosity. We compared eight methods, four employing the 3D modelling program 'Blender' to estimate total surface area, top surface area, their ratio, and interstitial volume; and four empirically measuring interstitial volume, fractals and two indices of rugosity. We compared these using seven metrics: 1) correlations among comparable measures; 2) consistency; 3) accuracy; 4) precision; 5) discrimination among configurations of objects; 6) discernment of complexities among zones on rocky shores; and 7) practicality. Of the eight methods, the virtual volumetric method, Blender interstitial volume, performed the best. Direct measurements of three-dimensional space related more closely to patterns in biodiversity than did measurements of two-dimensional space or indirect measures of complexity like fractals. Blender interstitial volume is thus the recommended means of measuring structural complexity of benthic environments.