Invasive plants: the process within wetland ecosystems - Invasive Plants: Ecological and Agricultural Aspects - page 96

Invasive plants: the process within wetland
ecosystems
Robert G. Wetzel
Department of Environmental Sciences and Engineering, School of Public Health, University of
North Carolina, Chapel Hill, North Carolina 27599-7431, USA
Invasion as a process
Expansion of the biogeographic range of organisms is a fundamental repro-
ductive strategy. Clearly expansion occurs where essential energetic resources
and environmental conditions and their dynamics lie within the physiological
ranges of the organisms and their capacities to adapt and reproduce. An imme-
diate consequence of such spatial expansion is competition for resources
among other organisms. Obviously interactions among invasive species can
lead to various degrees of competition, depending on the niche requirements
of the organisms, and result in displacement of indigenous organisms.
Such diffusion of biological propagules, as Hengeweld [1] puts it, can be
radically altered by the activities of humans. Certainly the rates of natural inva-
sion have been advanced spatially and accelerated temporally by human activ-
ities. Although the rates and efficacy of introduction of invasive species are
altered by human activities, invasions are an integral part of evolutionary
processes and will continue regardless of fortuitous or intentional interven-
tions of animals, including humans.
Invasive interactions and competition
The acceleration of invasive species with human activities has raised great con-
cerns because of a multitude of negative biological and economic effects
among many different types of ecosystems (see reviews of Cronk and Fuller
[2]; Vitousek et al. [3]; Pimental et al. [4, 5]; Mack et al. [6]; and Ehrenfeld
[7]). Some 20-30% of introduced species are economic pests and cause major
environmental problems. Many accidental and deliberate introductions of agri-
cultural species were imported and cultivated by human immigrants for food,
forage, seasonings, medicine, or ornamentals (e.g., [8-10]). Many species
escaped from areas of intended use. Growth of naturalized flora and the num-
ber of invasive taxa were facilitated by a marked acceleration and increased
efficiency of human transportation both inter- and intra-nationally.