Composition and Functional Change
Ecologists consider invasive species to be a serious threat to the biodiversity of natural areas. In some cases, invasive species may outcompete native species, for example,
Oenothera deltoides declines in native communities with invasive grass species because of the thatch and associated shade created by the grasses. Subsequently, the germination of O. deltoides is reduced, and eventually the population numbers of the species decline. However, invasive species may not always eliminate native species. Recent studies of L. salicaria in North American wetlands suggest that the presence of this invasive species does not eliminate native species, but instead, reduces the sizes of native species.
Emerging research suggests that the function of ecosys tems dominated by invasive species differs from those dominated by equivalent native species (Table 1). One study of riparian floodplain function in the southwestern United States showed that after invasion by Tamarix chinensis, half of the functional traits of importance in ripar ian floodplain function differed from native Populus fremontii floodplain forests. The impacts of invasive species on eco system L. salicaria function may be subtle, and yet have far reaching effects for the function of the system. If dominant species that are important as food sources in the system are replaced by less useful invasive species, the secondary pro duction of animals higher on the food chain may be affected. For example, L. salicaria did not support fish populations because the species decomposed at a different rate than a native sedge species (Carex lyngbyei) that it replaced in the Fraser River Estuary of the northwestern United States. Here, a shift of dominant species caused a loss of the productivity of the fisheries, because the invasive L. salicaria did not provide a seasonally equivalent food base for the invertebrate species that support the fish populations. Shifts in function due to invasive species may be profound, but we understand little about the threats of invasive species from this perspective.
The majority of invasive species invade native commu nities after disturbance, so that invasive species are common in early old field succession and also in restora tion sites. However, some invasive species can invade natural areas without the help of human disturbance. In wilderness canyons of southern Illinois, D. oppositifolia (Chinese yam) can invade floodplains of streams via bul bils (asexually produced disseminules). Lygodium microphyllum (Old World climbing fern) invades tree islands in the Everglades of Florida. Also, Impatiens glan dulifera is dispersed along rivers in Central Europe. These three examples are situations where flood pulsing along stream channels spreads the disseminules of invasive spe cies. Invasives that are maintained by natural disturbances are particularly problematic from a management perspec tive, because the same natural disturbances, which are critical in maintaining plant community types, also main tain these invasive species.
Table 1 Potential ecosystem function shifts in areas with invasive species
Specific shift in function
Soil/water/salinity/ geomorphological relationships
Decomposition dynamics Secondary production
Size of animals reduced, for example, bog turtles (Clemmys muhlenbergii) smaller in wetlands with Lythrum salicaria Surface/ground water levels changed Erosion/sedimentation levels changed Soil content (silt/clay/sand proportion) changed Organic matter dynamics in long-term succession changed
Organism function in stream channel, bank stabilization, or island-building capacity changed Water infiltration of ground is changed (repelled or absorbed more easily than native species) Species richness of native species lowered
Early succession dominated by invasives, so that maturation or establishment of later-successional species is impeded Displaces native species Fire frequency changed
Differs from native species in its ability to tolerate disturbance/perturbations such as wind, flooding, herbivory/insectivory/parasitism, animal foraging or digging Nutrient availability lowered, for example, pH lowered by adding salinity or acidity so that nutrients become less available Adds nutrients, for example, leguminous invader and associated bacteria fixes nitrogen Rate of decomposition shifts, so that litter or nutrient dynamics are altered Species at the top of food chain changed, for example, invasive species does not provide the same level of food support to specific higher-order organisms Habitat structure differs from that of native species in plant height, density, openness, or light characteristics so that animal foraging, nesting, or burrowing is impacted
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