Invasive Species' Effect on Ecosystems

Invasive Species’ Effect on Ecosystems

When we think of global environmental changes, we should consider, along with the effects of rising CO₂ levels and global warming, the dramatic changes brought about by invasive plant and animal species.  The terms invasive species, exotics, and non-indigenous species are used interchangeably to indicate non-native species.  Human travel, exploration, migration, and commerce between the continents and islands have been accompanied by the intentional and non-intentional introduction of non-native plants and animals to all these lands (Mack et al 2000).  Some non-indigenous plants (corn, wheat, and rice) and animals (cattle and poultry) are beneficial to humans as food and economic profit, while many others adversely affect human health, economics, and the environment (Pimentel et al 2000).  Consider the damage done by kudzu, introduced as a forage plant.  In 1932, it was said kudzu would not be a pest, but it now covers 2-3 million hectares in the southeast (Perry et al 2008).

While most introduced non-indigenous plants will not survive or spread, some do, especially when aided by natural or man-made disturbances (Mack et al 2000).  Invasive species affect humans by acting as disease vectors (Asian tiger mosquito), by weeds reducing crop yields, by diseases and parasites destroying forests (chestnut, elm, beech, dogwood, and hemlock), and by reducing biodiversity.  Some authors call the increased rate of invasive introductions everywhere, with extinctions of native organisms, the “homogenization of the planet” (Vitousek et al 1996).

Although not always seen when an invasive plant displaces native plants, exotic plants sometimes can affect function of the entire ecosystem.  An ecosystem is viewed as the collective total of all its organisms, by examining the combined functions and species interactions, we can distinguish the ecosystem’s properties.  In theory, invasive species can be so dominant in abundance or activity that they alter the ecosystem’s defining processes.  Authors have defined ecosystem-level impacts when a collective functional ecosystem property such as productivity, consumption, decomposition, water fluxes (balance between evapotranspiration and runoff), nutrient cycling, soil fertility, erosion, and disturbance frequency is altered significantly (Ecology and Biological Invasions of North America and Hawaii 1984).  Some well-known examples of invasive species alter multiple ecosystem properties.  To this list of functional ecosystem properties, some authors have added structural or engineering changes caused by the sheer abundance or size of the invasive plants or animals as ways ecosystems are significantly altered.  For example, a large canopy can alter an ecosystem by affecting the amount of light and moisture that reach the ground (Ehrenfeld 2010).

Productivity can be affected by an invasive plant’s ability to supply soil nutrients (e.g. nitrogen-fixation) not available to native plants.  Another example of productivity is deep rooted invasive plants that can acquire water and nutrients out of reach of native plants.  Some plants accumulate salt, decrease fertility, and lower productivity (Vitousek 1990, Wolfe 2005).  Consumption by invaders can certainly affect the ecosystem if the consumer is a plant pathogen or herbivore in a new environment without natural enemies (Ecology and Biological Invasions of North America and Hawaii 1984).  Litter quantity, quality, and decomposition rate varies.  Some non-indigenous pines produce thick acidic litter of poor quality that is slow to decompose, but most invaders produce high quality litter that decomposes rapidly (Ehrenfeld 2010).  These plants often have means to recover the released nutrients better than competing native plants.  The effects on water flux can be seen in invasive plants with inefficient control of evapotranspiration, but efficient root systems for acquiring water at the expense of native plants without such root adaptations (Vitousek 1990).  Invasive pine tree stands have less runoff than broadleaf forests.  Nutrient cycling has been studied extensively, and there are many examples of non-indigenous plants that alone or with mutualistic organisms are more efficient at acquiring, storing, and recycling carbon, nitrogen, and phosphorus.  Soil fertility is related to nutrient cycling, and some invasive plants are better able to acquire and concentrate nutrients to improve the quality of otherwise nutrient-poor soil (Simberloff 2010).  There are examples of plants that accumulate salt, which is associated with increased soil erosion.  Also, there are some good examples of exotic grasses that have increased the frequency of disturbance by wildfires, following which they have adapted to recover sooner and have now dominated large areas of western states (Wolfe 2005).  Regarding the structural or engineering effect of the abundance and size of exotic plants on the ecosystem, it is regarded independently, and also in combination with other functional ecosystem disturbances (Simberloff 2010).

Invasive species should concern us because as disease vectors, they can harm our health; they can be expensive to control if they have no natural enemies; they can alter ecosystems; they can reduce biodiversity, and they can cause extinctions of native species.  The many ways that non-indigenous species can establish themselves, invade, and completely alter ecosystems is particularly interesting to me, and with my next blog entries, I will describe in more detail the manner by which exotic species change the ecosystem by affecting productivity, consumption, decomposition, water fluxes, nutrient cycling, soil fertility, changes in land use, and altered disturbance regimes.

References

1. Ehrenfeld, Joan G. (2010). Ecosystem Consequences of Biological Invasions. Annual Review of Ecology, Evolution, and Systematics. 41:59-80.

2. Ecology of Biological Invasions of North America and Hawaii. (1984). Springer-Verlag: New York.  p 163-173.

3. Mack, Richard N. et al. (2000). Biotic Invasions: Causes, Epidemiology, Global Consequences, and Control. Ecological Applications. Vol. 10. No. 3:689-710.

4. Perry, David A., Ram Oren, and Stephen C. Hart. (2008). Forest Ecosystems 2^nd Edition. The Johns Hopkins University Press: Baltimore.  p 111-117.

5. Pimentel, David et al. (2000). Environmental and Economic Costs of Nonindigenous Species in the United States. BioScience. Vol. 50. No. 1:53-65.

6. Simberloff, Daniel. (2010). How common are invasion-induced ecosystem impacts? Biological Invasions. 13:1255-1268.

7. Vitousek, Peter M. (1990). Biological invasions and ecosystem processes: towards an integration of population biology and ecosystem studies. Oikos. Vol. 57, Fasc. 1:7-13.

8. Vitousek, Peter M. et al. (1996). Biological Invasions as Global Environmental Change. American Scientist. Vol. 84 No.5:468-478.

9. Wolfe, Benjamin E. and John N. Klironomos. (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. Vol. 53 No. 6:477-487.