Marine benthic ecological functioning over decreasing taxonomic richness Anna Törnroos a, ⁎, Erik Bonsdorff a , Julie Bremner b , Mats Blomqvist c , Alf B. Josefson d , Clement Garcia b , Jan Warzocha e a Åbo Akademi University, Department of Biosciences, Environmental and Marine Biology, Artillerigatan 6, FI-20520 Turku, Finland b Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, United Kingdom c Hafok AB, SE-179 61 Stenhamra, Sweden d Department of Bioscience, Aarhus University, 4000 Roskilde, Denmark e National Marine Fisheries Research Institute, Kollataja 1, 81-332 Gdynia, Poland abstract article info Article history: Received 15 November 2013 Received in revised form 24 April 2014 Accepted 26 April 2014 Available online xxxx Keywords: Biodiversity Species Richness Functional Diversity (FD) Functional Change Trait Analysis Zoobenthos Alterations to ecosystem function due to reductions in species richness are predicted to increase as humans continue to affect the marine environment, especially in coastal areas, which serve as the interface between land and sea. The potential functional consequences due to reductions in species diversity have attracted considerable attention recently but little is known about the consequence of such loss in natural communities. We examined how the potential for function is affected by natural reductions in taxon richness using empirical (non-simulated) coastal marine benthic macrofaunal data from the Skagerrak-Baltic Sea region (N. Europe), where taxon richness decreases 25-fold, from 151 to 6 taxa. To estimate functional changes we defined multiple traits (10 traits and 51 categories) on which trait category richness, functional diversity (FD) and number of taxa per trait category were calculated. Our results show that decrease in taxon richness leads to an overall reduction in function but functional richness remains comparatively high even at the lowest level of taxon richness. Although the taxonomic reduction was sharp, up to 96% of total taxon richness, we identified both potential thresholds in functioning and subtler changes where function was maintained along the gradient. The functional changes were not only caused by reductions in taxa per trait category, some categories were maintained or even increased. Primarily, the reduction in species richness altered trait categories related to feeding, living and move- ment and thus potentially could have an effect on various ecosystem processes. This highlights the importance of recognising ecosystem multifunctionality, especially at low taxonomic richness. We also found that in this system rare species (singletons) did not stand for the functional complexities and changes. Our findings were consistent with theoretical and experimental predictions and suggest that a large proportion of the information about alterations of function is found in measures such as functional diversity and number of taxa per trait category. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Biodiversity is often perceived as species richness, and a reduction in the number of species (or taxa) is interpreted as a general indicator of impoverished ecosystem quality (Loreau et al., 2001; Worm et al., 2006). Concerns over how a rapidly increasing loss of biodiversity will affect the functioning and stability of the systems have occupied essen- tially all fields of ecological research, from theoretical to experimental and observational studies over the last few decades (Naeem et al., 2012). The intuitive understanding is that as species richness is reduced, so is ecosystem function (Hooper et al., 2005; Worm et al., 2006). Function, defined as a combination of the biological and chemical processes operating in an ecosystem, must therefore be determined by the species present and their functional abilities or traits in the system. Theory also suggests that if there is evidence of functional redundancy, systems may be less susceptible to changes in ecosystem function caused by a species loss (Gamfeldt et al., 2008). Indeed, the redundancy hypothesis states that if several species perform the same functions and thus express similar trait values, in case of one being eliminated, others remain to continue or potentially even expand function (Yachi and Loreau, 1999). Based on the current knowledge there are both direct and indi- rect effects on ecosystem function due to species removal (Balvanera et al., 2006; Cardinale et al., 2006; Schmid et al., 2009; Solan et al., 2009). Direct effects may act on the functional composition in a community (sampling effects), reduce the level of resource partitioning (differences in traits) and lower the probability that interspecific inter- actions will facilitate or inhibit function (Solan et al., 2012). Indirect effects are related to the trait composition in the community after Journal of Sea Research xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +358 41 434 1741. E-mail address: anna.m.tornroos@abo.fi (A. Törnroos). SEARES-01247; No of Pages 8 http://dx.doi.org/10.1016/j.seares.2014.04.010 1385-1101/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Sea Research journal homepage: www.elsevier.com/locate/seares Please cite this article as: Törnroos, A., et al., Marine benthic ecological functioning over decreasing taxonomic richness, J. Sea Res. (2014), http:// dx.doi.org/10.1016/j.seares.2014.04.010