Environmental filtering in the Anthropocene: Investigating community assembly processes at multiple scales

Abstract

Environmental change in the Anthropocene is driving the redistribution of species and restructuring of communities globally. In marine systems, anthropogenic impacts, including the deliberate and accidental movement of species (i.e., non-native and invasive species) and construction of artificial structures (i.e., ocean sprawl), are creating novel opportunities for range expansion, altering the abiotic environment and causing global biodiversity loss. As the geographic distribution and community assembly of species depends on dispersal ability, abiotic tolerance and interactions with other species, understanding the effects that anthropogenic impacts will have on these processes is critical to predicting how ecosystem structure and function is likely to change in future. This thesis explores the effects of two pervasive anthropogenic impacts to marine systems: non-native species (NNS) and ocean sprawl, on the roles of dispersal limitation, abiotic conditions and interactions between species as processes governing species distribution and community assembly using two model species: the non-native Pacific oyster, Magallana gigas, and the common limpet, Patella vulgata.

Anthropogenic impacts in marine systems are creating novel opportunities for range expansion to areas outside of a species’ native range through vectors such as aquaculture, ballast water and ships’ hulls. The effects of these impacts on the distribution and spread of species are obvious in the case of M. gigas, which has undergone rapid range expansion over the last 56 years, at a rate of 23.1 km y-1 (Chapter 2). Patterns in the spatio-temporal spread of M. gigas revealed a combination of short- and long-range dispersal is likely to have driven range expansion, representing both assisted dispersal from anthropogenic vectors of spread and a lack of barriers to, and therefore multi-generational stability in, larval dispersal pathways (Chapter 3). Consequently, dispersal limitation is considered to have a weak role in changing the distribution and spread of M. gigas.

Artificial structures in marine environments are contributing to the spread of NNS by acting as ‘stepping-stones’ to distant locations and/or altering the suitability of habitats for recruitment through modified surface complexity. Assessing the effects of change in surface complexity revealed lower M. gigas abundance on ‘eco-engineered’ higher topographic complexity surfaces than on lower complexity ‘flat’ structures, typically characteristic of marine artificial structures in intertidal habitats worldwide. Conversely, native taxon richness and abundance increased with increasing topographic complexity (Chapter 4). Intermediate levels of topographic complexity produced increased movement distances by a key intertidal gastropod, P. vulgata, and may indicate the potential importance of surface complexity in modifying the spatial distribution of consumer pressure and influencing biotic processes – a concern given the increased presence of ‘low complexity’ artificial structures in intertidal environments (Chapter 5). Given the ever-increasing prevalence of homogeneous artificial structures in coastal environments, and the clear roles of both abiotic filtering and biotic interactions in forming species assemblies, reintroducing complexity to these structures through eco-engineering may provide a promising means by which to begin reversing biodiversity loss typically associated with the human modification of coastal environments.

It is clear that anthropogenic impacts in marine systems are fundamentally changing dispersal patterns, abiotic tolerance and interactions between species, resulting in changes to species’ distribution and the processes that govern community assembly. The methodological frameworks and results of this thesis make significant advancements towards our understanding of the assembly processes responsible for NNS spread and their modification under ocean sprawl. These can be applied to other species, spatial contexts and anthropogenic impacts to further understand and mitigate the negative effects of global environmental change.