DePaul University
Environmental Science Program and Public Policy Studies Program
Illinois Invaded:
Ecology, Impacts and Policy Approaches to Illinois Invasive Species Problem


Biology of Invasions

Determining the driving force behind the success of invaders can be a difficult task given the intricacies of the systems they invade. However, in order to make predictions of invasion impacts and create policy regarding those invasions, it is useful to generalize some characteristics that mark ideal invaders. David Lodge (1993) has compiled a list of widely-cited generalizations of common characteristics of biological invasions/invaders. These include r-selected traits, high dispersal rate, single-parent reproduction, vegetative reproduction, high genetic variablility, phenotypic plasticity, large native range, eurotropy, polyphagy and human commensalisms. In addition, a lack of evolutionary history with native flora or fauna may provide an avenue for exotics to launch successful invasions.


Many invaders have r-selected characteristics which means that in general they mature quickly, rapidly produce a large amount of offspring and put a smaller amount of energy (compared to k-selected species) to the upbringing of their offspring. These r-selected species also often have shorter life-spans than k-selected species. K-selected species often have opposite traits of those listed for r-selected species. For invasives that are r-selected species, one or more of these traits may prove to be advantageous in a new environment. The zebra mussel, an aggressive invader in the great lakes is a good example of an r-selected species as a single female may produce more than 5 million eggs per year (Bright 1998).

The mode of reproduction can also have many implications for the ability of an organism to adapt and establish itself in a new environment. Single-parent and vegetative reproduction which both refer to asexual reproduction may offer energetic benefits. With asexual reproduction, concerns of finding a mate are obsolete to species who employ this method of reproduction. However, it should be noted that genetic diversity is often low in populations that reproduce asexually which can have negatively impact the survival of the population. In populations with more genetic variability, there may be individuals within that population with advantageous characteristics in their new environment. These individuals may survive and reproduce more than individuals without that trait and become more likely to establish themselves in this new environment.

Adaptability in a new environment is also key to the success of many invaders. Phenotypic plasticity refers to the ability of organisms to adjust developmental patterns or phenotypes according to varying environmental conditions. This ability can prove essential in environments where one of those developmental patterns or phenotypes proves advantageous. The term eutropic also refers to organisms that can tolerate a wide variety of environmental conditions while polyphagous refers specifically to the ability to feed on different kinds of food. The ability to utilize a wide variety of food resources has driven the success of the brown tree snake on Guam. On Guam this invasive snake has contributed to the extinction of 12 of the island's 14 land bird species and is now working its way through other lizards and birds on the island (Bright 1998).

Many of the above mentioned characteristics contribute to the success of invaders. However, more importantly, these characteristics evolved in light of selection pressures in their native environment. In a new environment with different selection pressures, these characteristics may confer great advantages. This is why some species may prove to be particularly potent in foreign environments. The system in which they invade including all the organisms that inhabit it have not coevolved with this new invader. Thus a system which an alien invades is rendered evolutionarily helpless against those traits the invader has which makes it successful.

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