Climate change and invasive species are often listed as two of the greatest threats to biodiversity. Is it possible that changing climate can itself cause more frequent and damaging invasions, such that the two factors have synergistic effects? A recent article in Frontiers in Ecology and the Environment by Jeffrey M. Diez et al. approaches this question by examining how extreme climatic events might influence species’ invasions. This kind of question seems especially important as the United States faces the largest drought in 50 years.
Extreme climatic events (ECEs) are events such as droughts, floods, and storms that throughout historical records are typically quite rare, but may now occur more frequently as global climate changes. Importantly for biological invasions, these events are not only defined by our own human perspective (e.g., what we call 100 year floods), but they are also defined by the organisms affected by them. An ECE therefore might be an event that exceeds a species’ physiological limits for heat, salt, etc.
To understand how ECEs might affect invasive species, one must examine each part of the invasion pathway: introduction, establishment, geographic spread, and growth to levels that cause damage. ECEs such as storms or floods may increase introduction rates by moving more propogules further distances or with more regularity. This is especially well known in the case of floods that have released species from aquaculture, such as black carp. Establishment and growth of invasives may be affecting whether ECEs kill native species (that would be their competitors or predators), provide a pulse resource opportunity (such as light in a forest gap), or stress native species (making them less competitive). ECEs may have indirect effects, such as storms that transport diseases or their vectors long distances, the arrival of which may stress native species that are not directly affected by the ECE itself.
Diez et al. present many examples of ECE-invasion synergisms including: hurricanes which buried native seagrass, allowing for establishment of invasive seagrass; heat waves that led to higher mortality of native than non-native mussel species; drought and salt stress that favor Tamarix over native species; and invasive Bromus grasses that are able to recover quickly from drought, allowing them to invade areas where forests experienced a die-off.
Diez et al. stress that not all ECEs facilitate invasion. Examples of cases where ECEs actually prevent invasion are also numerous, including native fish that are more favored than invaders in the face of floods and Hawaiian grasses that are more drought tolerant than invasive counterparts. Then there are more complex interactions. One interesting example is that even though invasive Taramix perform better than natives in drought, the native Populus species can recolonize faster after extreme floods, so the outcome in this system depends on which ECE occurs more often! Another interaction is when an invasive species such as cogongrass are more prone to fire, which if combined with dry, hot ECEs, can increase the intensity and frequency of fire, causing cyclical damage to native species.
The article closes by stressing that although there are now numerous case studies of invasive species interacting with ECEs, it is not possible to make general conclusions or predictions. An important and interesting question is, what makes native ecosystems resilient (able to recover from an ECE)? It is possible, for example, that communities or species that have evolved under highly variable environmental conditions will be especially resilient. Another interesting possibility is that invasive species benefit from singular ECEs, but not repeated one. For example, while invasive plants may respond well to rare drought events, due to fast growth and high leaf area, they will not prosper under sustained or repeated events.
In any case, ecosystem managers must consider that invasion probabilities will change with ECEs. Some species will be more likely to invade, and some ecosystems will be more susceptible than others. Fortunately, it is likely that targeted management efforts can increase the resilience of native communities, and, with more research, predict which species are most likely to establish or expand after an ECE.