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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.

17 pages, 6 figures, 5 tables, supplementary materials https://doi.org/10.1016/j.ecolmodel.2020.109227, 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, Financial contributions from the University of Cape Town, the Andrew Mellon Foundation, the South African Research Chair Initiative (funded through the South African Department of Science and Innovation (DSI) and administered by the South African National Research Foundation (NRF)), and the DSI-NRF Centre of Excellence for Invasion Biology are gratefully acknowledged. [...]. MC acknowledges partial funding from the European Union's Horizon research program grant agreement No 689518 for the MERCES project, With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

Graphical abstract, Blender interstitial volume is a novel method that utilizes 3D modeling techniques to accurately and efficiently quantify the volume of interstitial gaps in marine benthic habitats, as well as the space provided by substrate rugosity. This method builds upon the analog methods routinely used on rocky shores and intertidal habitats, including those that measure rugosity, topography, fractals and volume. The method provides a direct Euclidean measurement and uniquely allows retrospective analysis if historical data on species composition are available. Blender interstitial volume allows users to quickly build and measure a large number of samples at no extra cost. • The program for Blender is free and opensource, and requires no extra equipment. • Once 3D models of species are made, the entire method takes less than ten minutes to complete. • Blender interstitial volume is as accurate as Fractal analysis in determining structural complexity on rocky shores, but is more consistent and precise, and better at discerning differences.

The South African coastline has been invaded by numerous alien species. Rare pre-invasion (1980) and post-invasion datasets (2001 and 2012) exist for Marcus Island, a small land-tied island in Saldanha Bay, South Africa. These snapshot datasets of the island’s intertidal invertebrate community were complemented with monitoring across seasons, from 2014 to 2016. Invertebrate communities were compared among the summers of 1980, 2001, 2012, 2014, 2015 and 2016 to assess interannual differences, while invertebrates and algae were monitored quarter-annually to assess seasonal changes. In addition, the population dynamics of the alien mussel Mytilus galloprovincialis were monitored. Differences in invertebrate communities between consecutive summers were significant but much smaller than changes induced by the arrival of alien species. Invertebrate and seaweed communities differed among years and shore zones but not among seasons, whereas species diversity differed among years, seasons and shore zones, with zones having the strongest influence. The invasion by M. galloprovincialis, and ensuing spatial and temporal variability in its recruitment, emerged as the most important factor influencing community composition, overshadowing interannual and seasonal changes. This work highlights that the impacts of alien species can be distinguished from natural variability by combining long-term monitoring with surveys at finer temporal scales. This is an important step in extending our understanding of the impacts of marine alien species.Keywords: Balanus glandula, biomonitoring, historical data, invasive species, Mytilus galloprovincialis, Saldanha Bay, Semimytilus algosus

11 pages, 7 figures, supplementary materials https://doi.org/10.1016/j.ecolmodel.2021.109731, In a prequel to this paper, we used non-spatial temporal modelling to investigate the impact of non-native ecosystem engineers on a small-scale, intertidal rocky shore in Saldanha Bay, on the west coast of South Africa, where invasive species have changed the physical environment between 1980 and 2015. However, we considered this approach incomplete without the direct inclusion of spatial modelling and zonation. To address this, we compared multiple, layered simulations employing the food-web approach of Ecospace, the spatial-temporal module of Ecopath with Ecosim (EwE). Our simulations included a control; a simulation that restricted drivers to depth and habitat preferences; two simulations to account for structural complexity as a function of the biomass of alien ecosystem engineers – the first indirectly via mediation, and the second via a novel plug-in ‘Ecoengineer’ – and lastly the inclusion of wave action to replicate its effects. Only the simulation that included the Ecoengineer routine matched empirical observations of species diversity indices and the exclusion of the native mussel Choromytilus meridionalis by the arriving alien Mytilus galloprovincialis. Inclusion of mediation did not differ from the model simulation that used only habitat preference and depth to drive the model, and the addition of wave action did not improve model fits. Our results emphasise that when analysing intertidal ecosystems, they should be modelled with an explicit representation of structural habitat complexity over time and space, and we consider that the application of our Ecoengineer plug-in is an effective and novel way of accomplishing this, MC acknowledges partial funding from the European Union's Horizon research program grant agreement No 689518 for the MERCES project, the Institute of Research for Development (IRD, France) and the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019–000928-S) to the Institute of Marine Science (ICM-CSIC)