What can nestedness metrics tell us about birds?

The prevalence of nestedness in ecological communities has been studied and debated for over twenty years. These studies have all dealt with the concept of compositional nestedness, i.e. the ordered composition of species assemblages. However, this traditional approach overlooks the fact that species which share the majority of their ecological characteristics will often be functionally redundant in a given ecosystem. A recent paper introduced a new nestedness metric (treeNODF) which allows workers to test for functional nestedness, i.e. it can be used to test for nestedness in ecological samples, whilst taking into consideration how similar species are in terms of their ecological traits. As it has only been recently introduced, the treeNODF metric has not been used outside of the original paper and thus the prevalence of functional nestedness in ecological datasets is unknown.

Our study, now published in Early View in Oikos, "Assessing functional nestedness patterns in habitat islands: an analysis of multiple bird datasets", aimed to determine the prevalence of functional nestedness, and the role of island area in driving functional nestedness,  in 18 bird-habitat-island datasets from a variety of different countries. We collected measurements of eight continuous functional traits from over 1000 bird species, and used these data in conjunction with presence-absence matrices to calculate treeNODF. We found that significant functional nestedness was relatively common when presence–absence matrices were ordered according to island area and a permutation null model approach was used, but was much less prevalent when a fixed-fixed null model was used.

Our study demonstrates that treeNODF can reveal interesting functional patterns that are masked when one focuses purely on species composition, and our findings are consistent with previous work that has found species traits to provide valuable information in modelling species at risk of extinction due to habitat loss.

The authors through Tom Matthews