Reacción a "Reactions: birds lose morphological diversity due to climate change"

Species extinction is an irreversible event that not only reduces global biodiversity, but also contributes to increased biotic homogenisation and altered ecosystem functioning. In this new study, Emma Hughes and colleagues ask what would happen if currently threatened bird species become extinct: would functional and phylogenetic diversity be reduced, and would biotic homogenisation increase?  

Answering these questions is, in theory, straightforward; it only requires estimating the different components of biodiversity for all species and comparing it with what would be expected if the species that are currently most threatened become extinct. In practice, the task is far from easy, starting with the need to obtain information on functional traits for a sufficiently representative number of species. 

The present study is based on morphological and phylogenetic information for more than 8,400 bird species, representing about 85% of the species currently living on the planet. By analysing this huge amount of data, the study finds intriguing results. On the one hand, no loss of phylogenetic diversity, i.e. evolutionary history, is detected. This result is, at first sight, unexpected, since we know that some properties that protect species from extinction - such as life strategy or behavioural plasticity - are evolutionarily conserved. And there is also evidence that drastic alterations to natural habitats tend to eliminate the most evolutionarily unique species and favour those belonging to more evolutionarily successful groups. 

On the other hand, the new study reveals that the extinction of threatened species would lead to a greater loss of morphological diversity than we would expect by chance. The impact is expected to be greater in some regions of the planet than in others, but the prediction is that the morphology of species in different regions will become increasingly similar. In other words, the extinction of threatened species would further enhance the ongoing process of biotic homogenisation, a process that is also related to biological invasions and has been extensively documented in previous studies.  

The importance of Hughes et al.'s contribution lies in demonstrating that human activities not only reduce the number of species, but also have great potential to eliminate unique life forms. The work is not the first to demonstrate this, but it is the first to quantify it at an unprecedented scale and resolution. The study also identifies regions particularly vulnerable to losing biodiversity as a consequence of impending extinctions. 

The paper does not delve into the causes of biodiversity loss, even though we know that the traits that favour the response to habitat destruction do not necessarily help respond to climate change or overexploitation. It is perhaps because of the omission of these causes that the study has not detected any loss of phylogenetic diversity. While there is a possibility that loss of morphological diversity may alter ecosystem resilience and resistance, I also question the extent to which the traits studied have implications for ecosystem functioning.