Woody Weeds researchers Brian van Wilgen, Jaco Le Roux and Maria Loreto Castillo assessing Prosopis invasions in Ethiopia.
Some introduced plant species are more prone to become invasive than others, and finding the reasons for this is one of the key questions in invasion ecology. Woody Weeds researchers Maria Loreto Castillo, Jaco Le Roux, Urs Schaffner and Brian van Wilgen conducted a series of experiments to investigate the reasons why Prosopis juliflora has become invasive in Kenya, while the closely-related P. pallida has not.
These experiments were possible because the original founder plantations of both species, planted in the early 1980s, were still in place. This provided a unique opportunity to study the establishment and spread of the offspring of two congeneric species introduced to the same new environment at the same time, as up to now invasion ecologists have not been able to reliably locate founder populations in new environments or because these populations usually no longer exist.
The study investigated two possible explanations for the difference in invasiveness between the two species. The first was that Prosopis juliflora became invasive because it improved its ability to cope with its new environment through rapid evolution. In other words, rapid trait differentiation between founding and invasive populations of P. juliflora, driven by either deterministic (i.e. natural selection) or stochastic processes (e.g., drift or spatial sorting), may have made them progressively more fit to survive and spread in their new environment. The second explanation could be that P. juliflora was already inherently more plastic than P. pallida, and this plasticity allowed it to cope with a broader range of environmental conditions than P. pallida. The study found evidence for both explanations, suggesting that P. juliflora has higher levels of phenotypic plasticity in response to resource availability (water and nitrogen) than P. pallida, and that the species has also evolved rapidly over a few generations, resulting in it becoming an aggressive invader. Interestingly, these two Prosopis species also hybridise, but no evidence was found that these hybrids became invasive (as has been observed in other parts of the world for other Prosopis species). What remains to be tested is whether this trait divergence between founder and invasive genotypes was a response to selective pressures.
“We were very lucky to be able to build on the rare opportunity presented by the co-occurrence of founder genotypes of both species in the field, one of which has become invasive,” said Maria Loreto Castillo, “and we are also excited to be able to demonstrate that this evolution took place in as few as 10 generations”. Maria has subsequently been awarded a PhD degree at Stellenbosch University in South Africa based on this work. “If Prosopis juliflora has undergone rapid evolution due to selective pressures, the control of the species should focus on the leading edge of the invasion to counteract these adaptive forces, more than on the core areas of invasion” she added.
Woody Weeds researcher Maria Loreto Castillo collecting data from transplanted Prosopis seedlings.
The paper describing this work is under review in the Journal of Ecology, with the title “The contribution of phenotypic traits, their plasticity and rapid evolution to invasion success: insights from an extraordinary natural experiment in Eastern Africa”.