Chief Science Correspondent
Rising global temperatures may be magnifying the threat of invasive species, according to researchers from Dartmouth College and the University of Wisconsin, Madison.
In a study published in Ecology earlier this month, Samuel Fey and Christina Herron examined the effect of elevated temperature on population dynamics in a system inhabited by a invasive species.
Invasive species tend to thrive in ecosystems by competing with similar native species for resources. Because invasive species have not evolved alongside the system's top predators (a phenomenon known as antagonistic evolution), they are generally not recognized as particularly attractive prey. Thus, these species grow unchecked, draining resources, crowding out competitors and offsetting the ecosystem.
The U.S. Fish and Wildlife Services predicts that there are approximately 50,000 non-native species in the United States. Up to 19 percent of these species are believed to be invasive.
This is the type of border control we should be worrying about. These are the "immigrants" that pose the biggest threat to American life and industry. Invasive species have contributed to the decline of numerous American species including wild salmon, honeybees and even possibly the Maryland blue crab. These species are critical, both biologically and economically.
The depletion of wild salmon requires a greater reliance on farmed varieties. The honeybees are one of our most vital pollinators, aiding the production of over 90 U.S. crops. The blue crab industry generates $600 million to Maryland's economy each year.
So how could changes in temperature potentially make these problems even worse? Temperature has an enormous impact on species interaction, influencing factors such as feeding rate, growth rate and reproduction. Rising temperatures will increase the pressure of predation (also known as the top-down effect) on prey species. If the growth and predation rates of the predators outweigh those of their prey, they will seek their prey in greater numbers. The invasive species, competing with native species for resources, are less vulnerable to predation. Therefore, they are not harmed by the increased demand for predation, but benefit from the potential for higher growth rates. This is known as enemy release. Over time, this process could lead to the complete decimation of the native prey species.
After constructing a mathematical model to track these changes, Fey and Herron conducted a temperature experiment involving juvenile pumpkinseed fish (Lepomis gibbosus), a predator to native zooplankter (Daphnia pulex) and the non-native competitor, Daphnia lumholtzi. The researchers found that the non-native species did thrive as temperature impacted the delicate balance between these species.
Fey and Herron's model heightens enemy release when the predator has a higher thermal sensitivity than its prey. Essentially, the predator species would increase at a faster rate than the prey. This won't necessarily always be the case. However, the model also suggests that if the prey are more impacted by the thermal changes, the invasive species will continue to increase, depleting available resources and negatively impacting their competitors. The paper notes that this effect will appear most often in situations where the non-native prey experience greater thermal sensitivity than their native competitors.
It is important to remember that these interactions are very complex. Fey and Herron note that many factors have not been accounted for in this model. Non-native species have many competitors and potential predators, and the model only accounted for a singular relationship. The non-native species' food source will also be impacted by temperature. Additionally, thermal sensitivity could impact the native competitor to a greater degree, leading to a regulation of the non-native invasion.
As the environment changes, it is more important than ever that we work to minimize our impact on our local ecosystems. This is largely a large-scale concern. However, the USFWS maintains that we, as citizens, can do our part to help mitigate the problem. The organization offers these suggestions:
1. Always clean recreation gear (especially hiking boots) after use. Always clean and drain a boat after use.
2. Always clean dirt off of vehicles before leaving a site. This is especially important for construction sites.
3. Avoid disturbing natural sites whenever possible.
4. When gardening, research plants that are appropriate for your region. Avoid using non-local plants whenever possible.
5. Never release unwanted pets (including the contents of an aquarium) into the wild.
6. Do not bring foreign species to the U.S. Never lie on customs forms!
7. Research common invasive species in your area and look out for any organisms you see that appear to match their descriptions.
8. If you find an organism that you believe to be invasive, contact your State Department of Natural Resources office or your local U.S. Fish and Wildlife Service Office. Other resources could include your local zoo, aquarium, university or nature center.
(Paraphrased from the U.S. Fish and Wildlife Service's website. For more information, visit http://www.fws.gov/invasives/what-you-can-do.html.)