Climate change was faster: researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) have reconstructed the distribution history of black pepper over the past 21,000 years in an international study. Using a new approach, they discovered, among other things, that the plant migrated too slowly after the last Ice Age to take advantage of its potential distribution area.

• WSL researchers have shown that the results of their dynamic species distribution model can be improved by incorporating genetic data.
• To do this, they reconstructed the dispersal history of the pepper plant (Piper nigrum) since the peak of the last Ice Age.
• Among other things, they found that the plant was likely much more widespread in the past than it is today, although it did not colonise all suitable areas quickly enough.

"Go where the pepper grows!" The message of this German saying is clear: go far, far away. But how far exactly? Today, this could mean Vietnam, Brazil, Indonesia, Burkina Faso or India. Yet the wild form of the pepper plant only occurs in the Western Ghats, a mountain range in south-western India. And it was from India that pepper, a valuable and important commodity, first came to Europe in the Middle Ages. The earliest mentions of the place where pepper grows also date from this period.

The climbing plant, whose fruits provide the spice, scientifically known as Piper nigrum, colonises evergreen forests in the Western Ghats. WSL biologist Michael Nobis, together with researchers from India and Japan, has investigated how its distribution has changed since the peak of the last Ice Age using a new approach.

Part of the team analysed the genetic material of wild pepper plants across their entire distribution range for clues as to how the species has migrated overtime. For example, higher genetic diversity indicates areas that were colonised earlier. Initially, independent of this population genetics data, Nobis reconstructed the distribution history of pepper using ‘KISSMig’, a dynamic distribution model he developed. This model simulated the former distribution of pepper based on its current occurrence and climate change since the last Ice Age. However, the results varied depending on how quickly the species could migrate in the model and what climate the researchers assumed to be suitable for it.

The researchers then combined both analyses: they integrated population genetics data into their distribution modelling approach. "This is not fundamentally new," says Nobis. "But we tried it for the first time with KISSMig, a very simple model." Thanks to its simplicity, this model runs very quickly , allowing the researchers to carry out many different simulations and to select the one which is best supported by the genetic data. "This optimisation greatly reduced the uncertainties of the assumed migration speed and climate suitability."

Around 15,000 years ago, the climate changed abruptly and the potential distribution area rapidly expanded northwards. The plant followed suit. However, it did not manage to colonise the entire suitable area – its actual distribution lagged behind its potential. "Evidently, climate change at that time was faster than the pepper's migration speed," explains Nobis. Over the last 5,000 years, the population ultimately split into several smaller ones, which still inhabit the Western Ghats today.

"The distribution model with incorporated genetics provided significantly more reliable results than the one without," says Nobis. However, when both methods were carried out separately and only the final results were compared, they often did not match well. The team chose pepper because genetic data from the entire distribution area was available, and a comparatively simple distribution history was to be expected.

Following this successful test, the researchers now plan to study other species in Europe and Switzerland using this method. The improved approach can provide important information: the species' previous migration speed can be used to better predict the effects of future climate change. Such reconstructions also reveal long-colonised areas, which are often characterised by high genetic diversity. For example, the researchers identified areas that could serve as genetic reservoirs for this important spice plant.