Ecosystem equivalence and the ability to generalise: insights from global consistencies in mangrove fish assemblages

Abstract

It is common practice in ecology to extrapolate understanding of processes and functions from one example of an ecosystem to another. Valid extrapolation requires an assumption of ecosystem equivalence, i.e. that the different examples of the ecosystem are actually similar in regard to the understanding being extrapolated. I use the example of mangrove fish assemblages to assess ecosystem equivalence using data compiled from 76 studies from around the world. Although there were distinct fish faunal groups (FGs) in different areas of the world, there was evidence of underlying faunal equivalence. Mangrove fish comprised a restricted pool of 170 families, with 41 of those occurring in all 4 FGs. Whether studies only considered fish actually entering mangrove forests or included associated habitats was not an important factor for differentiating assemblages. This suggests that from a fish perspective, the mangrove ecosystem is defined at a whole-of-mangrove system level rather than the scale of the mangrove forest. There were notable differences among families that distinguished the different FGs, but these differences did not translate into functional differences. Although the results appear to provide a degree of validity for extrapolating understanding of processes and functions from one example of an ecosystem to another, this implied support must be set in the context of the limitations of the available data. In-depth evaluation of ecosystem equivalence is urgently needed because this untested assumption is central to the validity of every model or interpretation based on data from a distant location.