By Beyhan Maybach | October 25, 2008
Current invasion models designate invasive species as the cause of diversity loss in many ecosystems. However, there is a new paradigm that suggests that in some instances, the root cause of diversity loss could be other than invasive species themselves.
Much of the evidence for the “direct-cause model” is based on simple correlations between invasive dominance and native species decline in altered ecosystems. The main question that should be asked here is whether the positive correlation between native species decline and invasive species dominance necessarily means that invasive species are the drivers of the observed modification (Didham et al, 2005).
There is an alternative hypothesis which suggests that invasion could be the direct consequence of environmental changes which could be; reduced dispersal due to fragmentation, altered disturbance regimes and climate change (MacDougall and Turkington, 2005). MacDougall and Turkington offer the first direct experiments of whether invasive species are the ‘drivers‘ of community change, or merely ‘passengers’ along for the ride.
They tested models of interactive vs. non-interactive dominance in a heavily invaded oak savanna in southwestern British Columbia which was dominated by two perennial grass invaders (Poa and Dactylis).
MacDougall and Turkington (2005) reasoned that, “if interactive processes are responsible for native species decline, then removal of invasive species should result in a direct increase in the richness and relative abundance of native species (the ‘driver’ model). Conversely, if invasive species are not the limiting factor for native species, then eradication should have minimal impact (the ‘passenger’ model)” (Didham et al. 2005).
MacDougall and Turkington (2005) posed the question “whether some dominant grasses were driving community structure by competition or if non-interactive factors better explain relative abundance.” Their results suggest that invasive species abundance was determined more by environmental conditions, such as long-term fire suppression, than by traits associated with competition.
MacDougall, A.S. and Turkington, R. (2005). Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86(1):42-55.
Didham, R.K., Tylianakis, J.M, Hutchison, M.A., Ewers, R.M. and Gemmell, N.J. 2005. Are invasive species the drivers of ecological change? Trends in Ecology and Evolution. Vol.20 (9) 470-474