Water chestnut can quickly form dense mats that disrupt recreation and negatively affect other aquatic flora and fauna. Once a population is established, tools such as the mechanical harvester seen above, may need to be employed to provide temporary relief from this plant's deleterious effects.

Invasive species change the composition of native ecosystems. Native plants and wildlife may not have evolved defense mechanisms against the invader; alternatively, native species may not be able to compete for needed resources with a species that has no predators. This leads an invader to spread quickly and perhaps take over an area.

Invasive species change not only the way natural systems look but also the way they function. Infestations can disrupt forest succession, species composition, water absorption and circulation, nutrient cycling, or even create toxic growing conditions for other plants and animals.

The direct threats of invasive species:

• out-competing native species for food or other resources
• preying on native species
• causing or carrying disease
• preventing native species from reproducing or killing their young

Invasive species can be harmful to human health as well. Research suggests that loss of trees to emerald ash borer is associated with an increase in mortality related to cardiovascular and lower respiratory tract illness. Invasive shrubs can increase populations of Lyme disease-carrying ticks. Research is finding that heavy infestations of barberry – a thorny, multi-branching invasive shrub – can lead to an increased prevalence of ticks infected with Lyme disease-causing bacteria. This seems to be because ticks utilize the habitat created by barberry for part of their life cycle. The dense thickets of barberry make a refuge for the white-footed mouse. These mice are an alternate host for Lyme disease, and a food source for larval ticks. So with more mice and ticks crossing paths, you see a higher level of Lyme disease-carrying ticks in the tick population.

Many of our commercial, agricultural, and recreational activities also depend on healthy native ecosystems.