Philip Chapman comments on recent article, Contrasting impacts of land-use change on phylogenetic and functional diversity of tropical forest birds.

It is increasingly recognised that biodiversity conservation needs to look beyond the goal of preserving species richness, to maintaining healthy functioning ecosystems and their evolutionary resilience in the face of environmental change. Species vary in evolutionary distinctness and the uniqueness of traits contributing to key ecological processes, such as pollination or seed dispersal. Therefore communities harbour different amounts of phylogenetic diversity (PD), which can help them adapt to future threats; and functional diversity (FD) which in theory influences the health and stability of ecosystem functions.

Scarlet-Rumped Trogon (Harpactes duvaucelli): a beautiful and functionally important bird of primary forest which can persist in logged forest, but which is lost from oil palm plantations

In recent years, ecologists have devised many indices quantifying different facets of FD and PD including the total amount of diversity represented by communities. However, the inter-dependence between FD and PD indices is unclear. Increasingly, PD is becoming easier to measure with advances in genetic technology such as next-generation DNA sequencing, while FD remains time consuming, requiring long hours collecting trait data from organisms in the field, laboratory, museum collections, or published literature. So, if PD was proven to be a good proxy for FD, ecologists could reliably prioritise communities and whole ecosystems based on their evolutionary diversity, knowing that selected sites were also likely to be optimal in their ecosystem functioning and resilience.

Birds are one of the few major taxa with available phylogenetic and functional trait data needed to investigate these relationships. We used bird communities from the megadiverse Bornean tropical rainforest, and explored the PD-FD relationship by comparing three pairs of indices calculated from conceptually identical functional and phylogenetic trees for each community. Firstly we compared ‘traditional’ FD and PD to get a measure of the total functional and phylogenetic ‘breadth’ of each community. Then, we compared functional and phylogenetic measures of how clustered species are on the tree.  Crucially, we wanted to see how the PD-FD relationships vary across a habitat gradient from primary forest, through selectively logged forest, to oil palm plantation.

Birds decline markedly in species richness and PD over this land-use gradient – mostly when logged forest is cleared for oil palm plantation. However, while FD is also lowest in oil palm, it declines less precipitously than the other two metrics. Nevertheless, we found that FD and PD were broadly correlated across the gradient: communities with the same species richness tend to be more functionally diverse if they are more phylogenetically diverse. However, the patterns of clustering are less congruent between phylogeny and function, particularly after controlling for species richness. The land use gradient also has different effects on these fine-scale structuring metrics. Therefore, while the broad PD-FD correlation is a positive result for conservation planners, we suggest that it’s easy to over-emphasise the value of PD metrics as a proxy for FD metrics. Indeed, the association between metrics is not so tight that assessments based on phylogenetic metrics alone will ensure the protection of the most functionally diverse and resilient communities. Overall, our results suggest that it is worth using a combined PD-FD approach to prioritise communities for conservation, whenever possible.

While our FD-PD relationships remained similar in logged and unlogged forest, the slope was invariably steeper in oil palm, indicating a more rapid loss of functionally similar or ‘redundant’ species. The net result is that oil palm supports only a handful of species and low PD, however these birds provide a disproportionately wide range of function – which includes commercially valuable ecosystem services such as pest control. It’s therefore important for oil palm plantation managers to try and support these few species in the landscape – if they are lost then there’s unlikely to be an equivalent species to take on the same role.

Read the full Open Access article, Contrasting impacts of land-use change on phylogenetic and functional diversity of tropical forest birds in Journal of Applied Ecology.