Since Humboldt, evolutionary biologists and ecologists have been interested in the geographical distributions of plants. Prior to the Modern Synthesis, ecological and geographical (ecogeographical) differences between related species provided key evidence for plant evolution, as differences in ecogeographic conditions serve as a primary pre-mating barrier. Indeed, although he did not expand on the topic in the Origin of the Species (1859), Darwin was aware of the importance of understanding species distributions when he wrote to his friend CJF Bunbury in April 1856 that, “I have lately been especially attending to Geographical Distribution, & [a] most splendid sport it is, – a grand game of chess with the world for a Board”. As early as 1908 David Starr Jordan formalized the importance of ecogeographic isolation (EI) in speciation as: “the nearest related species is not to be found in the same region nor in a remote region, but in a neighbouring district separated from the first by a barrier of some kind” (Jordan, 1908). This later became known as Jordan’s Rule.
Nowadays, one of the major challenges in ecology is to understand how plant distributions will respond to accelerating climate change. Plants are often adapted to, or at least have a preference for, particular abiotic and biotic conditions. However, the distributions of plants will likely change as a result of shifting temperature and precipitation regimes. Understanding these changes in the context of EI is particularly important in plant groups that rapidly hybridize. This is because any change in the overlap of their distributions will modify the primary barrier to potential hybridization (spatial isolation); shifts in such a barrier may then influence evolutionary trajectories of plants, for example, by increasing hybridization. Members of Pulmonaria (Fig.1) are a good example of such species – they occur throughout Europe, they have different geographical ranges which often overlap, and because they share bee pollinators, they hybridize frequently in sympatry (much to the ire of taxonomists!).
Fortunately, unlike researchers of previous eras, we have access to large databases and powerful statistical and GIS tools to better understand the distributions of plants across large geographical scales. In our present study, we used distribution data from the Global Biodiversity Information Facility (GBIF) for each of these species. We mapped the distribution of each species to a 10 x 10 km resolution and quantified the geographical niche of each species using a Maxent framework. We then compared the niche of each species under different future climate scenarios with present-day ranges. The climate change scenarios represent scenarios that predict a carbon emission peak around 2040 followed by a steady decline, (RCP 4.5) and a “business as usual” strategy with carbon emissions rising throughout the 21st century (RCP 8.5).
Our key finding was that EI between Pulmonaria species will increase under all future climate change scenarios. This will be mainly due to increasing temperature over the next 50 years. This was surprising to us as, due to species-specific responses to climate change and differences in the species present-day ranges, we initially predicted that climate change would actually lower EI between the species, or at least have no discernible effect. Indeed, we found a net decrease in EI when we did not account for the fact that Pulmonaria seeds are ant dispersed (highlighting the importance of understanding species biology and performing field studies). While we have no experimental data to show whether different ant species move Pulmonaria seeds at different rates, or whether there is specificity in their interaction, we used a conservative approximation of 10 km buffer around each presence point for each species to account for dispersal limitation. The net result is that there will be increased EI under all climate change scenarios compared to the present day (Fig.2).
As this is a prediction over a large geographical scale, the next steps in this research would involve experimentally testing how Pulmonaria seeds are dispersed and understanding climatic variables that control range overlap at finer geographical scales using temperature controlled manipulative experiments and experimental translocations. Nonetheless, these results provide us with a useful prediction of the influence of climate change – that it will bring about a relatively rapid change in species distributions, which may influence the evolutionary trajectory of these species. The idea that human-mediated shifts in atmospheric temperature may cause species range shifts might be a startling revelation to biogeographic researchers, who were only beginning to understand factors that underlie plant distributions merely 100 years ago.
Karl J. Duffy, University of Naples Federico II, Italy
Read the full paper online: Climate change increases ecogeographical isolation between closely related plants