Posted by Melissa Kirk @MGKir_04

Evolution and adaptation

Exotic species have the potential to adapt and rapidly evolve in their new introduced ranges. This can have multiple consequences including changes to their host preference, defence mechanisms, growth rates and biomass, climate tolerance, fecundity and phenology. These changes can lead to an increase in abundance, range expansion, and a difference in their overall impacts.

Such adaptations are highly likely due to the multiple new selective pressures they may encounter. For example, new selection pressures may occur as they encounter new competitors, new climates and new habitats. These adaptations and trait shift changes can occur in relatively short time periods, within a few generations. The absence of competitors and natural enemies can lead to relaxed selection, and thus a change may occur through a non-adaptive shift. A non-adaptive shift may not translate to a genetic shift initially; however, such shifts can lead to reproductive isolation and subsequently speciation.

Adaptations in plant-herbivore systems

There are two key theories behind why many plant species become weeds: the ‘enemy release hypothesis’ and the ‘novel weapon hypothesis’. The theories state that either the lack of natural enemies in the new introduced area or the presence of novel defence mechanisms which allows no or low herbivory to occur in the new environment. Thus the role of coevolution between weeds and specialists herbivores has also been attributed to plants invasiveness. If a plant has escaped its specialised herbivores, there is no need to produce costly defence mechanisms and this energy and resources can be used for growth and increased competitiveness in its new introduced range. An example of this is when the wild parsnip, Pastinaca sativa invaded the US, and after many generations without its coevolved enemy the webworm, Depressaria pastinacella its levels of defence chemicals reduced. However, after the webworm became established within the US and the plant-herbivore system were reunited; rapid evolution resulted in increased levels of defence chemicals (Zangerl & Berenbaum, 2005).

Re-association: wild parsnips and webworms in New Zealand

Wild parsnips also occur within New Zealand, like in the US, parsnip populations went many years without the webworms. It wasn’t until over 150 years after the establishment of the wild parsnips that the parsnip webworms were accidentally introduced into New Zealand. A previous study found that for wild parsnips in NZ, the re-association with their natural enemy the webworm did not result in an increase of defence chemicals, rather an increase in plant size (Jogesh, Stanley & Berenbaum, 2014). Therefore a switch in strategies seems to have occurred from resistance using chemical defence to tolerance via the plants size, however, whether this change is a true adaptive shift needs to be investigated.

As part of my PhD research on the ‘rapid evolution of exotic species’ I am planning on researching exotic herbivores and weed interactions, and how they can influence each other’s evolution. For this I am planning on investigating the wild parsnip and webworm interaction in NZ. I am also planning on investigating the interaction between nodding thistle and its exotic herbivores in NZ, like that of the wild parsnip it is thought that the presence of the nodding thistles natural enemies has influenced the traits and evolution of the plants growth and reproduction.

Key references:

Blossey, B., & Notzold, R. (1995). Evolution of increased competitive ability in invasive nonindigenous plants: a hypothesis. Journal of Ecology, 83(5), 887-889.

Callaway, R. M., & Ridenour, W. M. (2004). Novel weapons: invasive success and the evolution of increased competitive ability. Frontiers in Ecology and the Environment, 2(8), 436-443.

Jogesh, T., Stanley, M. C., & Berenbaum, M. R. (2014). Evolution of tolerance in an invasive weed after reassociation with its specialist herbivore. Journal of evolutionary biology, 27(11), 2334-2346.

Müller-Schärer, H., & Steinger, T. (2004). Predicting evolutionary change in invasive, exotic plants   and its consequences for plant–herbivore interactions. Genetics, evolution and biological control, 137-162.

Zangerl, A. R., & Berenbaum, M. R. (2005). Increase in toxicity of an invasive weed after    reassociation with its coevolved herbivore. Proceedings of the National Academy of Sciences, 102(43), 15529-15532.

Melissa is a PhD candidate within the Centre for Biodiversity and Biosecurity, School of Biological Sciences at the University of Auckland. She is studying rapid evolution in exotic species, and is supervised by Darren Ward, Thomas Buckley and Quentin Paynter. Email: mkir508@aucklanduni.ac.nz