Invasion ecology, the study of how organisms spread in habitats to which they are not native, divides itself into three topics:
- Invasiveness of Species: Which species are most likely to become invasive?
- Invasibility of Habitats: Which habitats are most susceptible to invasion?
- Impacts: How are communities and underlying ecosystems effected?
Definition (med): “The spreading of pathogenic microorganisms or malignent cells that are already in the body to new sites” is transferable into an ecological context but more recent definitions including the “element of harm to species already in a newly occupied habitat” are now in use (Pysek 1995; Randall 1997).
To consider these questions with regard to plants, it is suggested to distinguish natives, non-invasive non-natives and invasive non-natives. Non-invasive, non-natives introduced into a habitat may turn out to be invasive in an connected habitat. This may be triggered by:
- Unusual rainfall or temperature (physical, climatic, geological trigger)
- Arrival of a non-native mutualist
Natives are considered invasive when they spread into human-made habitats and landscapes. In a selection of grasslands (Smith & Knapp 1999) the abundance of natives is positively correlated with the species richness of non-natives while the correlation to the abundance of non-natives is negative.
Alpert, P., Bone, A., Holzapfel, C. (2000) Invasiveness, invasibility and the role of environmental stress in the spread of non-native plants. Perspectives in Plant Ecology, Evolution and Systematics, Vol. 3/1, pp. 52–66. Urban & Fischer Verlag
Pysek, P. (1995) On the terminology used in plant invasion studies. Plant Invasions: General Aspects and Special Problems (eds. P. Pysˇek, K. Prach, M. Rejmánek & M. Wade), pp. 71–81. Academic Publishing, Amsterdam.
Randall, J.M. (1997) Defining weeds of natural areas. Assessment and Management of Plant Invasions (eds. J.O. Luken & J.W. Thieret), pp. 18–25. Springer, New York.