The changing climate of plant distribution – Karl J Duffy

Dr. Karl J Duffy traces the history of plant distribution science and shares recent research on the impact of climate change on plant distribution

Botanists have long been interested in the distributions of plants. Going back to the early 1800s the great biogeographer, Alexander von Humboldt, was the first to seriously quantify plant distributions, particularly in central and South America (1799-1804) and in Europe. He was well trained in geology, mineralogy, and meteorology, and during his travels, in the Americas, he collected reams of data, from local temperature, air pressure, information on geological substrates, and made vast collections of plants. Indeed, he developed isotherms and isobars, aligning regions with similar temperatures and air pressures, which laid the foundations for the field of climatology. The results of his voyages and the 30 volumes he wrote about them inspired future biologists to think more broadly about the environmental differences over large spatial scales and the limits to species distributions.

“I have lately been especially attending to Geographical Distribution, and a most splendid sport it is – a grand game of chess with the world for a Board”

One admirer of von Humboldt was Alfred Wallace, who along with Charles Darwin, proposed the theory of evolution by natural selection. Wallace was a Victorian collector and biogeographer who spent eight years (1854-1862) in the Malay Archipelago. He knew the importance of biogeography in evolution when in 1855 he wrote, “Every species has come into existence coincident both in space and time with a pre-existing closely allied species.” The key word here is “space” – geographical differences were at the forefront of his mind when he was thinking about evolution. He based this observation on what he saw in his collections of thousands of plant and animal specimens from both South America and the Malay Archipelago. Interestingly, Darwin himself did not expand on the geography of plants in the Origin of the Species. However, he seemed to be 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, and a most splendid sport it is – a grand game of chess with the world for a Board”.

Prior to our understanding of genetics (Darwin and Wallace had developed the theory of evolution by natural selection without knowing about Mendel’s pioneering work on genetic traits in pea plants), geographical differences between species were seen as key evidence for evolution. In the USA, in 1908 the biologist David Starr Jordan formalised the importance of the combination of ecological and geographical isolation – 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 DS. 1908. The law of geminate species. American Naturalist, 42, 73–80). This later became known as Jordan’s Rule.

Nowadays, estimating the influence of environmental factors on species distributions is back en vogue among botanists. Indeed, a major botanical challenge is to understand how plant distributions will respond to accelerating climate change.

Plants often have a preference for particular abiotic (e.g. temperature, soil nutrients) and biotic (e.g. pollinators, seed dispersers) conditions. However, human-induced climate change will shift temperature and precipitation regimes (i.e. the abiotic conditions) which will result in shifts in the distributions of plants. Understanding these changes in the context of EI is particularly important in plant groups that rapidly hybridise. This is because any change in the overlap of their distributions will modify the primary barrier to potential hybridisation (spatial isolation); shifts in such a barrier may then influence evolutionary trajectories of plants, for example, by increasing hybridisation. Members of the Boraginaceae, the genus Pulmonaria, 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 hybridise frequently in sympatry (much to the ire of taxonomists).

Fortunately, we have access to large databases and powerful statistical and Geographic Information System (GIS) tools to better understand the distributions of plants across large geographical scales. To test whether EI might shift under climate change, my colleague and I mapped the distribution and quantified the niche (the absolute physiological tolerance of species) of each of nine Pulmonaria species. We then compared the niche of each species under different future climate scenarios with present-day ranges.

The climate change scenarios we used represent scenarios that predict a carbon emission peak around 2040 followed by a steady decline, (called ‘RCP 4.5’) and a “business as usual” strategy with carbon emissions rising throughout the 21st century (called ‘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 the 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 10km 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. 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.

“It will bring about a relatively rapid change in species distributions, which may influence the evolutionary trajectory of these species”

Quantifying the impact of climate change on species distributions is a hot topic. There are now more studies that show that climate change will result in a separation of populations of species along elevational gradients, thereby increasing their isolation. From a horticultural perspective, the influence of climate change on ornamental plants, i.e. invasive alien plant species that were originally introduced to regions for horticultural purposes, was recently examined (Haeuser et al, J Appl Ecol. 2018;55:2386–2395).

This is because many regions now have many ornamental alien species which have not yet naturalised. Hence, climate change represents a threat, as it may lower the barriers to naturalisation for some alien species. The authors of this research found that by 2050, naturalisation probability will increase by more than 10% for 41 species, and only decrease by more than 10% for one species. This indicates that climate change will not just reduce potential hybridisation in plants in their native range such as Pulmonaria, but may also increase the frequency of ornamental plants becoming naturalised and potentially invasive within the next 30-50 years. That human-mediated shifts in atmospheric temperature may cause species range shifts, might be a startling revelation to pioneering biogeographic researchers, who were only beginning to understand factors that underlie plant distributions just over 100 years ago. ✽