Christopher Henderson

The ecological impacts of landscape modification and urbanisation have transformed the composition of plant and animal assemblages, and altered the condition of ecosystems globally. Landscape transformation influences the spatial distribution of species and ecological functions by selecting for generalist species with wide ecological niches, which can adapt to opportunities in highly-modified environments. These effects of landscape modification can shape functional diversity on land, but it is not clear whether they have similar functional consequences in the sea. We used estuaries as a model system to test how landscape transformation alters functional diversity in coastal seascapes, and measured how variation in level of urbanisation, catchment modification and habitat loss influenced fish diversity across thirty-nine estuaries in eastern Australia. Fish were surveyed with baited remote underwater video stations and functional diversity was indexed with three metrics that describe variation in the functional traits and niche space of assemblages. The extent of landscape transformation in the catchment of each estuary was associated with variation in the functional diversity of estuarine fish assemblages. These effects were, however, not what we expected as functional diversity was highest in modified estuaries that supported a large area of both urban and grazing land in their catchments, were bordered by a small area of natural terrestrial vegetation and that contained a moderate area of mangroves. Zoobenthivores and omnivores dominated assemblages in highly-modified estuaries, and piscivorous fishes were common in natural waterways. Our results demonstrate, that the modification and urbanisation of ecosystems on land can alter functional diversity in the sea. Intense landscape transformation appears to select for abundant generalists with wide trophic niches, and against species with specialised diets, and we suggest that these changes might have fundamental consequences for ecosystem functioning in estuaries, and other highly modified seascapes.

1.Ecosystem functioning is positively linked to biodiversity on land and in the sea. In high diversity systems (e.g. coral reefs), species coexist by sharing resources and providing similar functions at different temporal or spatial scales. How species combine to deliver the ecological function they provide is pivotal for maintaining the structure, functioning and resilience of some ecosystems, but the significance of this is rarely examined in low-diversity systems such as estuaries. 2.We tested whether an ecological function is shaped by biodiversity in a low-diversity ecosystem by measuring the consumption of carrion by estuarine scavengers. Carrion (e.g. decaying animal flesh) is opportunistically fed on by a large number of species across numerous ecosystems. Estuaries were chosen as the model system because carrion consumption is a pivotal ecological function in coastal seascapes, and estuaries are thought to support diverse scavenger assemblages, which are modified by changes in water quality and the urbanisation of estuarine shorelines. 3.We used baited underwater video arrays to record scavengers and measure the rate at which carrion was consumed by fish in 39 estuaries across 1,000 km of coastline in eastern Australia. 4.Carrion consumption was positively correlated with the abundance of only one species, yellowfin bream (Acanthopagrus australis), which consumed 58% of all deployed carrion. The consumption of carrion by yellowfin bream was greatest in urban estuaries with moderately hardened shorelines (20%-60%) and relatively large subtidal rock bars (> 0.1 km2). 5.Our findings demonstrate that an ecological function can be maintained across estuarine seascapes despite both limited redundancy (i.e. dominated by one species) and complementarity (i.e. there is no spatial context where the function is delivered significantly when yellowfin bream are not present) in the functional traits of animal assemblages. The continued functioning of estuaries, and other low-diversity ecosystems, might therefore not be tightly linked to biodiversity, and we suggest that the preservation of functionally dominant species that maintain functions in these systems could help to improve conservation outcomes for coastal seascapes.

Coastal seascapes are composed of a diversity of habitats that are linked in space and time by the movement of organisms. The context and configuration of coastal ecosystems shapes many important properties of animal assemblages, but potential seascape effects of natural and artificial habitats on nearby habitats are typically considered in isolation. We test whether, and how, the seascape context of natural and urban habitats modified fish assemblages across estuaries. Fish were sampled with underwater videos in five habitat types (mangroves, rock bars, log snags, unvegetated sediments, armoured shorelines) in 17 estuaries in eastern Australia. Different habitats supported distinct fish assemblages, but the spatial context of mangroves and armoured shorelines had pervasive ecological effects that extended across entire estuaries. In most estuarine habitats, fish diversity and abundance was greatest when they were in close proximity of mangroves, and decreased due to the proximity of armoured shorelines. Many cities are centred on estuaries, and urban expansion is often associated with the fragmentation of mangrove forests. Our findings emphasize that these transformations of urban estuarine landscapes are likely to propagate to broader ecological impacts detectable in multiple habitats beyond mangrove forests.

Animals are central to numerous ecological processes that shape the structure and function of ecosystems. It follows that species that are strongly linked to specific functions can represent these functions spatially and hence be useful in conservation planning. Here we test this notion of 'functional species surrogacy' for the conservation of seagrass meadows that have been impacted by stressors. We measured algal herbivory and herbivorous fish assemblages across a range of seagrass meadows in the Moreton Bay Marine Park, Queensland, Australia. We determined the suitability of herbivorous fish to act as a surrogate for the function of algal herbivory and modelled the abundance of this surrogate, and thus herbivory, in seagrass meadows to compare the spatial distribution of this function within existing reserves. We used underwater video systems to determine the abundance of all herbivorous fish species in seagrass meadows. The abundance of the dusky rabbitfish (Siganus fuscescens) was the best predictor of algal herbivory in seagrass meadows, supporting the suitability of this species as a functional surrogate. The distribution of dusky rabbitfish, and therefore the ecological function of herbivory, was not well represented in the Moreton Bay Marine Park protected areas. Only 7% of the equivalent area of seagrass meadows protected in marine reserves were found to have high abundances of dusky rabbitfish. We demonstrate that the abundance of functionally important herbivores can be suitable as a surrogate for herbivory in seagrass conservation. Our findings show that data on the spatial distribution of ecological functions can alter priorities for reserve design, and we suggest that our functional approach to species surrogacy is likely to improve conservation performance in seagrass ecosystems.