Aliens Vs Climate Change

Could invasive species be driven by climate change?

In 2006 a team of scientists studied the distribution of terrestrial and aquatic organisms in Great Britain between 1960-2000. The researchers found that 12 of the 16 taxonomic groups studied shifted their range to high latitudes or altitudes (Hickling et al., 2006). Hickling et al., (2006) reasoned that during this period global warming was occurring and was likely to be a factor driving changes in species range.  More recently, a meta-analysis has been published revealing that species are migrating north of the equator at an average rate of 16.9 km per decade, or to a higher elevation at a rate of 11 m per decade (Chen et al., 2011). This is at least twice as fast as was previously predicted. Moreover, Chen et al. (2011) provided statistical evidence that these rates of migration are correlated to temperature change, indicating a direct link between species migration and climate change. 

Why move north? 

Generally speaking, as you move north or south of the equator there is a decline in temperature, similarly at higher altitudes the temperature cools. In other words, the warmest temperatures that put populations under the highest level of stress are nearer the equator. Adaptation to climatic stress takes place over many generations and if species cannot evolve fast enough then global warming may result in population crashes (Pounds et al., 1999). 

The Edith's Checkerspot is a particularly well studied butterfly species which excellently represents this change in geographic range over time. Parmesan (1996) found that net extinctions were greater for populations at lower latitudes than at higher latitudes (Fig. 1). This extinction gradient is what was expected if climate change was causing the range shift, as populations closer to the equator suffer from hotter temperatures. 

Figure 1. Left: Edith's Checkerspot. Photo credit: H. Stroo.
Right: Map of Edith's Checkerspot populations monitored between 1992-1996. Red indicates extinct populations. Blue indicates present populations (Parmesan, 1996)

Previously I defined invasive alien species as those that are introduced to a new environment and have a negative effect. Therefore if migrating species compete with exiting communities, climate change has the potential to directly facilitate the introduction of invasives! 

Until now attributing the cause of invasives to either human activity or natural change has been fairly straight forward. Climate change poses a slightly harder question. I think that the findings from the Firth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) are critical to this debate. Although climate change is affected by natural internal variation, one of the key focuses in this report was the increased certainty that humans are the main cause of recent global warming (Fig. 2). 

Figure 2. IPCC (2013)

With this in mind I'll leave you with one final thought. Do we have the right to prevent species from naturally migrating and invading new locations in order to survive global warming when humans are the dominant cause? If we are currently experiencing a 6th major extinction (Barnosky et al., 2011), should we in fact be letting these species establish in new ranges if it minimises the total number of species extinctions?

Until next time!
Elena