A novel approach to the analysis of survey data is casting new light on the processes that shape marine communities.

NERP Marine Biodiversity Hub scientists from the University of Tasmania and CSIRO have used the statistical approach, developed by the CERF Marine Biodiversity Hub, to analyse data collected at rocky reefs off Southern Australia.

Their findings suggest that marine communities may be collections of independent species, questioning the long held view of communities as discrete, integrated units that are relatively constant in time and place.

‘A fundamental challenge in ecology is to understand how species are associated in a community,’ Piers Dunstan of CSIRO says. ‘Are communities highly structured units, or coincidental collections of individualistic species?’

‘We explored this question using a statistical modelling approach – species archetype models or SAMs – that clusters species based on the environmental conditions they live in, (such as wave exposure, depth and temperature), rather than on species composition.’

The team analysed records from an extensive survey of demersal fish and macro-invertebrates at 600 sites in a rocky, subtidal system south of continental Australia and north of Tasmania. Reefs in this region are subject to strong environmental gradients and support a rich and geographically varied flora and fauna.

The analysis produced six demersal fish archetypes, and eight macro-invertebrate archetypes, with clear spatial patterns and considerable overlaps.

One fish archetype – which included the black spotted wrasse, and the rough headed and yellow headed bulls-eyes – had a strong affinity to the South Australian gulfs.

Two macro-invertebrate archetypes included species that were ubiquitous and common in the region, (a pattern not seen for demersal fish), such as the cart-rut shell, green and black lip abalone, and the orange feather star.

Communities may be fluid

‘We found that the same species can be present in many different communities, suggesting that the community is a fluid concept,’ Piers says.

‘While some groups of species (archetypes) consistently are highly correlated, these groups are unlikely to form consistent, complex webs of interacting species because species identity changes between locations.

‘Rather, particular species are likely to be present at a site because the environment suits them, (according to the available species pool), up to a limit at which resources are insufficient for more species to exist. And the distribution of these species will change if environmental conditions change.’

Similar patterns of overlapping archetypes have been reported in other marine systems such as inter-reefs in the Great Barrier Reef.

‘We feel confident that these patterns are typical of marine systems in diverse, low nutrient seas around Australia,’ Piers says. ‘We are unsure whether they occur in more nutrient rich seas or terrestrial systems.’

What are the implications for management?

This ‘weakening’ of the concept of a community implies that communities may not form naturally self-sustaining networks. Management may therefore be required throughout the range of many species groups to support their contribution to the local assemblage of species.

‘Our findings highlight the need for caution when making decisions based on community structure, to ensure that all species are considered appropriately,’ Piers says.

This work is providing important tools for estimating cumulative impacts on other groups of species around Australia. Whether or not communities are self sustaining or otherwise may have important implications for decisions made to protect or conserve biodiversity, for example using marine reserves or legislative listing mechanisms for threatened species and communities.

The lead author of the paper that described this research, Dr Rebecca Leaper of the University of Tasmania, passed away last year (before publication). The manuscript is dedicated to her memory.

Further reading

R. Leaper; P.K. Dunstan; S.D. Foster; N.S. Barrett; G.J. Edgar. Do communities exist? Complex patterns of overlapping marine species distributions Ecology (2014, in press)

Images

Gunns leatherjacket.  Image Neville Barrett, University of Tasmania
Striped trumpeter.  Image Neville Barrett, University of Tasmania

Contact

Piers Dunstan, CSIRO