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 ofthe 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 ecosystems 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 chinen-sis, half of the functional traits of importance in riparian floodplain function differed from native Populus fremontii floodplain forests. The impacts of invasive species on ecosystem 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 production 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.

Table 1 Potential ecosystem function shifts in areas with invasive species

Function

Specific shift in function

Productivity

Soil/water/salinity/ geomorphological relationships

Community attributes

Disturbance attributes

Nutrient dynamics

Decomposition dynamics Secondary production

Habitat structure

Biomass of native species reduced

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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