Invasive Species

Introduction

Invasive alien species are defined as all organisms, animals, or plants that have a negative effect on “the local ecosystem and species” because humans have introduced them to an area that is outside of “their natural range,” and they then establish themselves and become broadly distributed in these areas. It is usually because of their impacts that invasive species are of concern—and sometimes generate controversy. This entry follows the IUCN's ecological emphasis, although impacts on the economy or society may also be of concern.

Historically, species have often shifted their ranges naturally in response to changed environmental conditions. However, documented species invasions have overwhelmingly originated from purposeful or accidental introductions by humans. In Australia, a review led by Malcolm Nairn found that more than 70% of the 290 plant species naturalized during 1971 to 1995 were introduced through human activities. However, not all species that are introduced become pests. One rule of thumb suggests that 10% of species imported are introduced to ecosystems in the receiving area, of which 10% become established, of which 10% become pests (Darling & Blum, pp. 751-765).

Characteristics of Successful Invasions

Models describing the life histories (that is, the patterns of development, reproduction, and mortality) of species have been used to identify traits that may favor invasiveness. Invasive species are often categorized as having high potential rates of population growth. These are enabled by such characteristics as short life cycles and preferential allocation of resources to reproduction. Other correlated traits can include small propagule size (that is, the small size of seed, egg, or other part capable of giving rise to a different individual) and effective dispersal, often over long distances. A species may also become invasive in a new area through being free of the natural enemies that were present in its native range.

Many of this model the process of succession by which one ecological community gives rise to another. In general, disturbance of communities is considered to facilitate invasion by new species, and late-succession or species-rich communities are thought to resist invasions (Deagle & Bax, pp. 709-719).

Predicting Invasion Risk

Most predictive approaches identify the pathways (such as shipping containers or transport by air passengers) by which species may arrive; assess the likelihood of the species entering, becoming established, and spreading in a new area; and assess the consequences that would result. Uncertainty often pervades these predictions and needs to be considered in making decisions. Where the risks posed are considered unacceptable, the risk may be managed by preventing or controlling the entry and the establishment or spread of species and by limiting their impact. Defining an acceptable level of risk is important and is ultimately a social decision, involving tradeoffs between preventing undesirable impacts and interference with worthwhile activities. Consultation with stakeholder is important at all stages of the risk assessment; an interactive exchange provides ownership of the solution proposed and can enhance the acceptance of decisions (Hellman & Byers, pp. 534-543).

The probability of success at different stages of invasion is influenced by various ...