Ecological Impact of Invasive Species: Why We Should Care

Ecological Impact of Invasive Species: Why We Should Care

As this blog comes to a close, it is time to reiterate the impact of invasive species on ecosystems.  The regional distinctiveness of the earth’s biota developed over millions of years of evolution.  Biodiversity was maintained by the isolation of the continents.  While invasions may have occurred occasionally, human activities have greatly accelerated introduction of non-native species.  As a single phenomenon, biological invasions have had a greater impact on the world’s biota than many of the other aspects of global environmental change such as rising CO₂ levels, climate change, and decreasing ozone layer.  As seen in previous blogs, non-native species have displaced or replaced native plants and animals, disrupted nutrient and fire cycles, and changed the pattern of plant succession (Bhowmik 2005).  Invasions by non-native plants, animals, fungi, and microbes are believed to be responsible for greater losses of biological diversity than any other factor except habitat loss and direct exploitation of organisms by humans (Bhowmik 2005).

Invasive plants tend to have many similar biological attributes relating to high reproductive potential and stress tolerance.  These traits include: 1) rapid seedling growth and maturation, 2) ability to reproduce at an early stage, 3) ability to reproduce by vegetative propagules as well as by seeds, 4) ability to produce viable seeds, 5) seed dormancy ensuring periodic germination, 6) diverse dispersal mechanisms and high dispersal rate, 7) high rates of photosynthesis, respiration, transpiration, and growth, 8) high acclimation capability, 9) tolerance of high habitat disturbance, and 10) rapid and generalistic response to resource availability (Bazzaz 1986).

                                                                          

In previous blogs, I discussed the mechanisms that invasive species use to gain a survival advantage over competition.  The diagram in figure 1 summarizes some of these mechanisms (Wolfe 2005).

Figure 1

 

As is seen above in Figure 1: One, greater litter production can lead to greater fire potential.  Two, root exudate can alter resource availability for belowground communities.  Three, invasive species can release novel chemicals with antimicrobial activities.  Four, novel nutrient acquisition strategies, such as nitrogen fixation, can alter biogeochemical processes.  Five, invasive plant roots can induce differences in the local soil environment.

 

Figure 2

 

As previously discussed in blog 9 and seen in Figure 2, an altered fire regime with the grass-fire positive feedback is another mechanism used by invasive plants to alter ecosystems and replace normal woody plant succession with permanent grasslands (Vitousek 1996).

While some advocate control of invasive plants with herbicides and physical removal, biological control with natural enemies (herbivorous insects and fungal pathogens) from the plants’ native range has been more successful.  In order to be used safely, the agents should be genus or species specific for the targeted plant and proper risk analysis procedures should be followed.  The advantages are that these organisms are self-producing and self-sustaining and can be used against weeds that are difficult to control or occur in areas that are ecologically sensitive and prone to harm if chemical or mechanical means are used for their control.  The best results are seen with weeds that are not closely related to native species.

Figure 3

While it would be desirable to test the potential efficacy of the agent before release (Figure3, Step3), how biological control agents perform in nature will depend on a complex set of population processes that cannot be tested in the laboratory and include the effects of parasitoids, predators, competitors, and climate (Van Driesche 2013).  Ecosystem management must be determined individually for each type of habitat. 

The consequences of biological invasions on ecosystems demonstrate the importance of species properties in controlling ecosystem properties.  Plants are most capable of altering ecosystem characteristics when they differ in life form from native plants.  Invasive plants can establish positive feedbacks that magnify the ecosystem-level impact of biological invasions (Vitousek 1986).

Why should we care about the impact of invasive species?  Almost half of the native species in the US are endangered due to invasive species, which is the second greatest threat to biodiversity.  They alter ecosystem processes, as is seen in the change in the frequency and severity of a fire regime caused by invasive grasses.  It is estimated that invasive species cause $120 billion per year in economic losses in the US as a result of loss of crop and infestation management.   Human activities (intentional and inadvertent) are responsible for the spread of invasive species.  The longer the problem is ignored, the greater the severity.  Collectively we have to be much more cautious when introducing anything non-native into an ecosystem; they do not recognize manmade boundaries.  We must be informed; most states have an exotic or invasive species council. The bottom line: invasive species change ecosystems, and these changes make it difficult or impossible for native plants and animals to survive in these affected ecosystems (What Are Invasive Species and Why Should We Be Concerned About Them 2013).

References

1. Bazzaz, F. A. (1986). Life History of Colonizing Plants: Some Demographic, Genetic, and Physiologic Features. Ecology of Biological Invasions of North America and Hawaii. Springer-Verlag, New York.

2. Bhowmik, Prasanta C. (2005). Characteristics, significance, and human dimension of global

invasive weeds. Invasive plants : ecological and agricultural aspects. Birkhäuser Verlag, Berlin.

3. Van Driesche, Roy and Center, Ted. (2013).Biological Control of Invasive Plants in Protected Areas. Plant Invasions in Protected Areas. Springer, New York

4. Vitousek, Peter M. (1986). Biological Invasions and Ecosystem Properties. Ecology of Biological Invasions of North America and Hawaii. Springer-Verlag, New York.

5. Vitousek, Peter M., et al. (1996). Biological Invasions as Global Environmental Change. American Scientist. 84:468-478.

6. What Are Invasive Species and Why Should We Be Concerned About Them. (2013). Extension. http://www.extension.org/pages/62270/what-are-invasive-species-and-why-should-we-be-concerned-about-them#.VHkBTzHF8YE.

7. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.