Module 14: Exotic Species Introduction

Exotic Species and Their Impact on the Ecosystem

By Dr. James A. Danoff-Burg, Columbia University

Impacts

When species are introduced into a novel habitat, they often explode in population size and disrupt the local ecological balance. These introduced species are referred to as exotics, invaders, alien, nonindigenous, or nonnative species and are often either directly introduced or indirectly so by human activity. In both terrestrial and marine environments, most exotic species are present in areas where human traffic and shipping is greatest.

The extreme and novel selective pressure imposed on the local species by the exotics often leads to the local extirpation and global extinction of locally endemic species. Approximately 68% of all the extinction events where the main causative factors are known involve the introduction of an exotic species.

Mechanisms for Success

Exotics tend to be most successful on isolated islands and can comprise surprisingly large percentages of the biota and then cause the extinction of native species. The introduction of the domestic cat alone into Hawaii has been estimated to be responsible for the extinction of more than twenty species of birds found only there. There are 44 species of ants in Hawaii, all of which were introduced and are rapidly eliminating an unestimated number of vertebrate and invertebrate species. All told, as much as 50 to 70% of the flora in Hawaii may be exotic species, which primarily became established by displacing indigenous plant species.

Successful introduced species are successful because they are introduced. As a consequence of being taken from their original location, they are isolated from the predators, parasites, and pathogens that evolved to interact with them. Therefore, in their new habitat the introduced species experience “ecological release” and are often able to grow without most of their normal ecological restraints. Consequently, population sizes of these species tend to explode and decimate their host species and competitors. To make matters worse, when they explode in population size, exotic species disrupt the stable state of the community and thereby make it easier for other exotics to invade the community who in turn further disturb the local ecology.

Widespread exotics are often more efficient competitors than the local species. This is because widespread exotic species have only become widespread after they have prevailed against other native species and across highly heterogeneous habitats. Therefore only those species that are rigorous competitors in a diversity of habitat types will be geographically widespread.

Long-Term Impact of Exotics

The ecological disruptions caused by exotics usually ramify to effect more than just the species with which they directly interact. Often, the influence of exotics will cascade across the entire food web, affecting species that are separated from the exotic by three or four trophic levels. After several generations of ecological counter-adaptation to the initially novel selective forces of the exotic species, the now irrevocably altered local community will reach an alternative stable state. Some species will be eliminated, most relationships will be altered, and the population sizes of nearly every species in the ecosystem will be changed.

When they have the strongest impact, exotic species can facilitate the creation of unnatural communities and permanently damage an ecosystem. These changes are established quickly and are based on the preexisting local ecological idiosyncrasies. Introduced species often irrevocably change the basic ecosystem structure and the ways in which it functions. If a key species or key interaction is disrupted and a major organismal die-off occurs, changes in runoff, erosion, sedimentation, pH, standing biomass, ionic activity, productivity, and frequency of disturbances are all possible. However quickly formed, these novel interactions and ecosystem changes have evolutionary consequences for the ensuing millennia.

These altered ecosystem processes can also combine to alter the gross geomorphology of an ecosystem. Because they affect not only the community composition but major ecosystem components as well, these changes are often the most damaging and longest lasting effects of exotics.

Exotics have many and diverse impacts on an ecosystem. We will study some of these effects and attempt to answer the general question of how one introduced species has impacted other species and ecological processes in your local ecosystem.
 

Additional Relevant Online Resources

The US National Park Service has a page discussing how exotic species impact the ecology of the US National Park system.

The abstract of an excellent article by Peter M. Vitousek, Carla M. D'Antonio, Lloyd L. Loope and Randy Westbrooks entitled "Biological Invasions as Global Environmental Change" is available from American Scientist, published by Sigma Xi. This article is the source of the statistic that 68% of extinctions are caused by exotics.

Information on Exotic Plant Species of the Southwest is available from the USGS, including exotic plant species lists for many of the Southwestern US states.

The Hawaii Cat Foundation website discusses the magnitude of the feral cat problem in Hawaii.

The Hawaiian Ant Group, allied with the Hawaiian Ecosystems at Risk Project has an extensive set of online references discussing ants in Hawaii.

Daniel Simberloff has an excellent review article on the Impacts of Introduced Species in the United States available from the online journal Consequences.

An interesting discussion of the impact of marine bioinvaders and a large listing of links to pages discussing Marine Bioinvaders is available from the Massachusetts Bay Information Server from the MIT Sea Grant.

Nonindigenous Aquatic Species in a United States Estuary: A Case Study of the Biological Invasions of the San Francisco Bay and Delta by Andrew N. Cohen and James T. Carlton is available from the USGS, USFWS, and Connecticut Sea Grant. The San Francisco Estuary may be the most invaded aquatic ecosystem in North America - this report summarizes to what extent this is the case.

Background information on Exotic species is available from Living Landscapes of the Columbia Basin, British Columbia.

A clearinghouse of websites discussing the effects of free-roaming and feral cats on biodiversity is available from the California Department of Fish and Game.

All Materials Copyright © 2000 by James Danoff-Burg

All Rights Reserved.