A key question in invasion biology is why some regions have more alien species than others. This is likely to depend on
features of the local environment, but how humans influence the invasion process is likely to matter as well. To help our understanding
of the determinants of alien species richness, Richard Duncan from the University of Canberra, Phill Cassey from the University of Adelaide,
Alex Pigot from University College London (UCL), and C·I·B Associate Tim Blackburn (UCL), developed a general theoretical
model of the process.

The alien species richness of an area depends on how many species were introduced there (colonization pressure), and how
many of those introduced species managed to establish a viable population. Establishment success in turn depends on two broad factors:
1) how many times a species is introduced, and how many individuals are introduced in total (together, these comprise the species’ propagule
pressure); and 2) the probability that a founding individual leaves a surviving lineage (lineage survival probability). We therefore derived
a simple general model of an area’s alien species richness in terms of just these three key factors: colonization pressure, propagule pressure
and lineage survival probability. This model helps us to understand how changes in these factors can influence the number of alien species that
an area ends up with.

We started by using our model to calculate how many alien species you would expect an area to be home to if you introduce
100 individuals (10 separate introductions, each of 10 individuals) of each of 100 species, with each individual having just a 1/200 chance of
leaving a surviving lineage. The answer was 39. We then varied each of these values in turn to see what effect each factor had on our predictions
of alien species — the graph below shows the results.

What is immediately obvious is how many alien species are introduced is the key factor — you end up with many more alien
species in an area if you increase colonisation pressure, relative to increasing either propagule pressure or lineage survival probability. This
is because alien species richness ultimately must be limited by how many species get introduced. Overall, therefore, the model shows that what
people do (in terms of introducing more or fewer species) is the key driver of alien species richness.

*“The results of the model make perfect sense”*, says Tim Blackburn. *“Whatever else varies, how many
alien species you get in area is basically limited by how many you introduce. It’s then obvious that the best way to avoid ending up with lots of
alien species anywhere is not to introduce many.”*

**Read the paper in ***Biological Invasions*

Duncan, R.P., Cassey, P., Pigot, A.L. & Blackburn, T.M. (2019). A general model for alien species richness.
*Biological Invasions*, in press. https://doi.org/10.1007/s10530-019-02003-y

Analysis to show how alien species richness changes with proportional increases in colonization pressure (*S*),
propagule pressure (*I* and *N*), and lineage survival probability (*p*). We specified initial values of number of species
introduced to a location *S* = 100, the number of introduction events per species *I* = 10, the number of individuals introduced
per per event *N* = 10, and lineage survival probability *p* = 0.005. our model predicts that 39 alien species richness will be
present in the area. We then measured the effect on alien species richness of increasing each parameter (*S*, *I*, *N*
and *p*), in turn, by up to 5 times the initial value, while keeping the other parameters fixed at their initial values.

(*Graphic from Duncan et al.*)