Spatial variation in functional group diversity in a sandflat benthic community: implications for ecosystem resilience

Abstract

The resilience of macrobenthic communities to environmental stress depends upon the vulnerability, adaptability and connectivity of species. Recent studies suggest that the function(s) species perform may be more influential in driving community response to change than the more traditional measures of abundance and occurrence. Species that perform similar functions within a community theoretically give rise to redundancy, an important attribute of resilience. This thesis assesses the potential for functional redundancy in coastal macrobenthic communities by comparing the degree of sharing of specific functional traits, patterns of abundance and spatial distribution to provide insight into the potential for resilience. The research is focused on the functional diversity of a species rich macrobenthic community from a large intertidal area in the Kaipara Harbour, New Zealand. 400 macrofaunal and 360 sediment cores were collected using a newly developed hierarchical sampling grid covering an area of 300,000 m². This resulted in a data set consisting of 115 taxa and 23,682 individuals and 360 observations of sediment grain size and chlorophyll a. Linking species attributes such as body type, size, feeding mode, and living depth, produced 26 species functional groups that characterised important functional attributes of the macrobenthic community. These attributes relate to ecosystem functions associated with sediment biogeochemistry, stability and resilience to disturbance. Redundancy was assessed within these functional groups (ranging from 1-13 species per functional group) considering both occurrence and abundance in their spatial distribution. Various levels of redundancy were identified for different functional groups, for example, functional groups characterised by small deposit-feeding polychaetes encompassed high redundancy, whilst functional groups consisting of large suspension-feeding, highly mobile bivalves maintained low redundancy. Nevertheless, the latter functional group does contribute considerably to abundance despite its low redundancy. The spatial patterns exhibited by different functional groups (identified by correlograms using Moran’s I) were used to provide insights to connectivity and exposure of the functional group to localised disturbance. A range of spatial patterns were apparent, reflecting small-scale homogeneity to large-scale heterogeneity with spatial arrangements including gradients and distinct patches. Density maps showed that some functional groups, such as tube worms and large mobile suspension-feeding bivalves, showed strong and opposing spatial distributions, separated by clear boundaries. Canonical correspondence analyses indicated that the measured environmental variables were not important drivers of the spatial distribution of functional groups. Thus, either biological interactions between functional groups are the driving force of spatial diversity or this sampling strategy failed to measure relevant environmental parameters. These findings emphasise a role for spatial variation in functional diversity and species redundancy in structuring community resilience. Understanding the functional roles of species, the diversity of these functions and associated biological interactions, is essential for evaluating biodiversity and resilience.