Degrade ecosystem functions
Like the term biodiversity, "ecosystem functions" needs to be defined to help you understand the impacts of invasive species. Biologists define this phrase as..."the collective life activities (e.g., feeding, growing, moving, excreting waste) of organisms in an ecosystem and the corresponding effects these natural activities have on the physical and chemical conditions of their environment." For example, wetlands store water, transform biological nutrients, promote the growth of living matter, and enhance the diversity of wetland plants. Invasive species reduce the benefits that man derives from all of these functions and this translates into decreased recreational opportunities, poor flood control and greater water damage and the decline of functional natural systems. (Restoration, Creation, and Recovery of Wetlands -- Wetland Functions, Values, and Assessment, R. P. Novitzki, R. D. Smith and J. D. Fretwell, U.S. Geological Survey Water-Supply Paper 2425)
"In our natural world, everything is connected to everything else." The expansion of invasive species as a global challenge has affected many of these natural processes, which provide positive benefits to humans and our society. In our natural world, everything is connected to everything else. When one aspect of an ecosystem is affected, it creates a domino affect resulting in many unforeseen changes. Even though zebra mussels have nothing to do with the aquarium, water garden or pond hobby, they provide a good example of how an introduced nonnative species can cause pronounced ecological changes. In the Great Lakes, the zebra mussel's rapid reproduction, coupled with its consumption of microscopic plants and animals, has affected the fragility of this system's entire aquatic food web.
Nuisance plant invasions also trigger several domino affects. Water hyacinth is an example of a nuisance plant that degrades water quality by blocking photosynthesis, which greatly reduces oxygen levels in the water. This creates a cascading effect by reducing other underwater life such as fish and other plants. Water hyacinth also reduces biological diversity, impacts native submersed plants, alter immersed plant communities by pushing away and crushing them, and also alter animal communities by blocking access to the water and/or eliminating plants the animals depend on for shelter and nesting.