Chris Thomas writes that recent gains in species numbers associated with climate warming could more than balance species losses (Nature 502, 7; 2013). But conservation is not just about total species richness — it is also about functioning ecosystems.
Unlike ecosystems that have resulted from millennia of competition and predation, we are much less sure about the stability of new, unfamiliar ones created by invasive species, which we know can wreak havoc.
Thomas also extols the virtues of hybrid vigour generated by crosses between native and immigrant species. But again, we have little idea of these newcomers' long-term future because they have yet to be winnowed by selection — in contrast to their contemporary, non-hybrid counterparts. Furthermore, species hybridization flies in the face of conservation, which aims to preserve extant species and their genomes.
We are still ill-equipped to predict the biological effects of climate change. It would therefore be foolish from the standpoint of both ecology and evolution to stop protecting pre-Anthropocene ecosystems and species from the onslaught of climate-driven newcomers.