Posted by Delayn Fritz @WildOptic

My previous blog post focused on global trade and the invasion process as a whole, but what happens to an exotic species after it becomes established? Post-establishment spread and dispersal is the next step for an exotic species transforming into an invasive species. Of particular interest are the causes of that spread, both natural and non-natural.

The simplest way that a species spreads is through its competitive edge. A study of the competitive nature of the Argentine ant (Linepithema humile) showed that they completely overcame native ants in North America in terms of resource gathering. Being able to displace native species allows the exotic invading species to take over their turf and spread locally. In fact displacing or removing native species may even lead to an ecological meltdown which can cause other invasive species to establish and cause a positive feedback loop.

An Argentine ant (Linepithema humile) specimen. Photo Credit: April Nobile / © AntWeb.org / CC BY-SA 3.0

Another way for an exotic species to spread is environmental shift. This can be through habitat fragmentation and disturbance wherein a gap in the normal functioning of an environment displaces the native competitors to a point where the exotic species become dominant. One form of this that has been documented repeatedly is the increased proportion of exotic plants along roadsides, which act as a corridor for exotic species. Climate change may play another role in spread, as one study showed that increased rainfall had a positive effect on range expansion while drier years actually decreased the range of both Argentine ants and native ants in North America.

Long range jump dispersal patterns are also a key factor in spread, but this is often achieved by non-natural means. This jump dispersal usually occurs by the exotic species hitching a ride on internal trade routes or just regular vehicles. This leads to species achieving spread rates a whole order of magnitude  higher than the distance travelled by normal dispersal. It has also been shown that when this jump dispersal occurs it can actually increase the spread rate by up to three orders of magnitude.

For my MSc, I will be trying to analyse some of the data on currently established exotic ant species in New Zealand, and using taxonomic collections and their historic data to try and figure out the spread rates of different species and see if this is related to any of the aforementioned spread processes.

Delayn Fritz is an MSc student in the Centre of Biodiversity  and Biosecurity, School of Biological Sciences, University of Auckland. He is interested in the invasion process of ants (Hymenoptera: Formicidae) in New Zealand. He is supervised by Darren Ward and Eckehard Brockerhoff (Scion, B3).