DEFINING BIOLOGICAL INVASIONS is challenging due in part to the proliferation of terms, especially among biological disciplines, that usually describe a set of ranging and different concepts. Depending on the author, a species in the invasion might be referred to as: alien, exotic, invasive, non-indigenous, imported, weedy, introduced, nonnative versus naturalized, endemic or indigenous. Some of the terms employed evoke anthropocentric concepts such as aggression, assault, and attack, which have normative implications. These implications and the lack of consistent uses of terms contribute to confusion.
Invasive species in general are defined as species that occupy or are in the process of occurring in regions where they have not been present historically. Specifically, invasive species are defined by their origin and distribution. An invasive species might spread into native plant communities and cause environmental harm by developing self-sustaining populations and disrupting the structure and functioning of the system. What characterizes invasive organisms is the ability to take hold of a habitat and become aggressive and dominant. It could be that either invasive species have extraordinarily wide distributions around the world, or that they are distributed locally with very high population densities affecting the endemic biota of specific regions. Invasive species are usually alien species, meaning they are able to reproduce outside their native ecosystems, and whose introduction is more likely to cause environmental harm.
Plants and insects are the most common orders in terms of their invisibility. From a population biology point of view, invasive plants tend to produce reproductive offspring, often in very large numbers (e.g., seeds, spores) and disperse them at considerable distances from parent plants. Invasive species also spread successfully through the use of roots or rhizomes (for example, more than 2 meters/year for taxa spreading), and such strategies enable them to spread over a considerable area. This definition also concerns species that have spread previously but not currently because of competition. These species are still considered invasive species because once local competition disappear, it may lead to re-invasion.
Research questions on invasive species range with discipline with extremes in population biology and ecology to economics. Ecological approaches center on how the biological aspects of invasive species related to the biophysical environment. Due to the complexities of such relation, the human linkages are less explicit and only mediated through disturbance processes. Concepts such as competition, disturbance, homogenization of habitats and species capacity are at the core of ecological research. Among social scientists, economists have attempted to understand the linkages between human activities and the spread of invasive species. Economic theory tends to be helpful in diagnosing human sources of invasion problems, to provide information on the risks associated with invasions, and to evaluate the damaging effects of invasions on public goods like biodiversity or common-property lands. Economic analyses are useful to prescribe when, where, and how to control invasions; at estimating the expected benefits of various control programs; and in minimizing the costs of controlling invasions that have already taken place. The links between invasion and effects on ecosystem services is usually descriptive and not normative, and its links with economic activities are through monetary value. Such utilitarian perspective is problematic, especially from a conservation perspective; improvements, however, could be made if nonmarket valuations of the ecological and social processes are included in such analyses.
A cultural and political ecological approach to invasive species considers human aspects to be central to the understanding of the patterns and process of biological invasions. Humans alter habitat conditions where exotic species can succeed, and the mobility that allows larger and faster movement of species across the world. The process of invasion is not only an ecological process, but a social process that needs to consider not only the current cost–benefit analysis or tradeoffs in economic terms, but also recount of the humanized natural history. Under this perspective, invasive species benefit as well as harm societies. Nonnative species are so much part of human livelihoods and traditions that is difficult to picture such societies never experiencing them.
For example, foods like potatoes and tomatoes in Europe are not native to these environments, but from South America; however, they are very much an integral part of certain European cultures. On the other hand, invasive species could create dire political and economic consequences. Classic Ecological Imperialism shows how disease and other species brought from Europe to America factored in destroying native populations and transforming the landscape in dramatic ways. Understanding the biological character of invasions is critical to understanding the political ecology of such process, which involves an understanding of the cultural complexities and the potential uneven impacts of invasion in society.
Cane Toads; Insects; Zebra Mussels.
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