Organisms as cooperative ecosystem engineers in intertidal flats

The importance of facilitative interactions and organismal ecosystem engineering for establishing the structure of communities is increasingly being recognised for many different ecosystems. For example, soft-bottom tidal flats host a wide range of ecosystem engineers, probably because the harsh physico-chemical environmental conditions render these species of particular importance for community structure and function. These environments are therefore interesting when focusing on how ecosystem engineers interact and the consequences of these interactions on community dynamics. In this review, we initially detail the influence on benthic systems of two kinds of ecosystem engineers that are particularly common in tidal flats. Firstly, we examine species providing biogenic structures, which are often the only source of habitat complexity in these environments. Secondly, we focus on species whose activities alter sediment stability, which is a crucial feature structuring the dynamics of communities in tidal flats. The impacts of these engineers on both environment and communities were assessed but in addition the interaction between ecosystem engineers was examined. Habitat cascades occur when one engineer favours the development of another, which in turn creates or modifies and improves habitat for other species. Non-hierarchical interactions have often been shown to display non-additive effects, so that the effects of the association cannot be predicted from the effects of individual organisms. Here we propose the term of "cooperative ecosystem engineering" when two species interact in a way which enhances habitat suitability as a result of a combined engineering effect. Finally, we conclude by describing the potential threats for ecosystem engineers in intertidal areas, potential effects on their interactions and their influence on communities and ecosystem function. (C) 2013 Elsevier B.V. All rights reserved.