Does functional diversity follow changes in communities?

Submitted by editor on 26 January 2016.Get the paper!

Ecological studies based on time-series often investigate community changes centered on species abundance or biomass but rarely expose the consequential functional aspects underlying such changes. Various measures of functional diversity (FD) gained much attention in the field of ecology over the past decades, and ecosystems with high FD are generally assumed to display broader ecosystem functioning.

In our recent article, we investigated the relationship between long-term compositional changes in zoobenthic communities, their functional diversity and their functional identity (i.e. composition and expression of selected traits).

We studied a coastal system in the northern Baltic Sea, the Åland islands, where the coastal zone comprises over 6000 islands, forming a highly heterogeneous land- and seascape, ranging from shallow, sheltered bays to exposed open-sea areas.  

There, long-term changes in zoobenthic communities have been observed over a 40-year time frame with contrasting developments in sheltered and exposed areas. In sheltered areas, zoobenthic communities have undergone pronounced changes in species composition in addition to continuously decreasing biomasses, while exposed sites show increasing biomasses and higher overall species richness. Furthermore, the studied system also encompasses the large-scale invasion of a non-native species, the polychaete Marenzelleria spp., which became dominant over the past decade. However, the functional role of this new addition to the existing species complex still remains uncertain.

 

This background, including a species invasion and the dissimilar progression of the communities over four decades, created a suitable case study to demonstrate how changing community patterns may result in altered functional properties.

Benthic fauna plays an indispensable role in various processes of a healthy and functioning ecosystem, especially in coastal areas where the vast majority of fish rely on zoobenthos as food. Differential expressions of particular traits may alter predator-prey dynamics, remineralisation of nutrients or organic matter, sediment oxygenation and system productivity.

In our study we included traits that focus on two functional aspects within coastal systems, namely; (1) the quality of zoobenthos as food resource for higher trophic levels, with traits related to production, susceptibility to predation and palatability and (2) the role of zoobenthos as ecosystem engineers, including traits related to habitat modification, bioturbation, spatial resource dynamics and elemental cycling.

We show that two zoobenthic communities with diverging development (species abundance, biomass, diversity) in sheltered and exposed areas, traditionally suggesting respectively lower and higher ecological quality status, maintained equal functional diversities in terms of their functional dispersion (FDis), pointing toward functionally complex communities in both areas. Interestingly, the invasion of Marenzelleria seemed to have promoted a generally higher FDis. In contrast to the maintained diversity of functions, the communities’ functional identity, measured as community weighted means of trait expression (CWM), changed significantly over space and time, highlighting shifts in qualitative services for ecosystem functioning, e.g. alterations in palatability proxies such as size, protection, energy content and environmental position. These changes suggest altered food quality and functionality of zoobenthos in the studied system.

We propose a framework of coupling traditional community-based analytical tools with novel approaches to determine multi-functional changes of communities by highlighting the specific alterations in trait composition and their functional implications for the studied ecosystem. We exemplify the usefulness of combining the two functional measures, FDis and CWM, as both reveal complimenting aspects of community functionality. While FDis serves as a proxy for the pure diversity of expressed functional traits, CWM is able to showcase the underlying qualitative features of the present trait expressions and their implications for ecosystem functioning.

This approach increases the understanding of functional developments of communities in dynamic ecosystems and can help reveal alterations in qualitative services for the ecosystem at hand.

Benjamin Weigel

@Benjamin_Weigel

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