We already learned in Chapter 3 that invasive species can endanger populations of native species. However, the impacts of invasive species can extend to entire ecosystems. The Fynbos vegetation of South Africa is one of Earth’s biodiversity hotspots, where rolling green meadows border the southern Indian Ocean. Unfortunately, invasive pine and acacia species in the Fynbos have sucked so much water out of the ground that they have reduced water flow in nearby river basins by nearly one-third. These invasive plants have also impaired a wide range of other valuable services: They displace plants used for traditional medicines and tea, and invade coastal dunes and riparian areas, which restricts access for tourism and fishing.

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Woody invasive plants have also increased the intensity of fires by increasing the amount of flammable plant biomass in the ecosystem. In the southwest United States, a thirsty tree called saltcedar, Tamarix species, has dried up streams and changed the fire regime by increasing the frequency and intensity of fires in these ecosystems. Similarly, cheat grass, Bromus tectorum, which has invaded vast areas in western North America, not only increases fire frequencies but also is a poor source of forage for livestock or native animals. Prior to the cheat grass invasion, wildfires occurred every 60 to 110 years; now they happen every 3 to 5 years (Figure 4.23).

The impact of invasive species is not limited to terrestrial environments. Invasive aquatic species have been spread around the world with devastating consequences to ecosystem goods and services. Countries across Africa spend millions of dollars each year to control invasive aquatic weeds that choke waterways important for fishing and commerce. The introduction of the Nile perch into Lake Victoria, one of the great lakes of Africa, has resulted in hundreds of native fish species going extinct (Figure 4.24), but it has also had consequences for the ecosystem as a whole. For instance, local people once caught smaller native fish, which they could sun-dry on racks. The larger Nile perch must be smoked, so people need to cut down trees for fire. This has increased erosion and led to increases in pollution by agricultural runoff.

In Asia, the golden apple snail has ravaged rooted aquatic plants, reducing their biomass and releasing nutrients into the ecosystem. As a result, previously clear waters are now turbid and choked with algae. In the United States and Canada, zebra mussels have become a nuisance by attaching to nearly any submerged structure, including the intakes to water supply systems.

The executive director of the United Nations Environmental Programme estimated that the costs of invasive species to the global economy exceeded $1.4 trillion in 2010. The economic costs of controlling just the saltcedar invasion of western North America have been estimated at $127 to $291 million annually for control and estimated water losses. Meanwhile, zebra mussel control associated with water systems around the Great Lakes approaches $70 million each year. However, some invasive species exact even greater costs. For instance, the water losses resulting from invasive species in the Fynbos region are valued at $1.4 billion, and the golden apple snail, introduced from South America, causes over a $1 billion in annual losses in rice cultivation in the Philippines. David Pimentel and colleagues from the College of Agriculture and Life Sciences at Cornell University estimated that as of 2005, invasive species have caused over $120 billion in damages annually in the United States alone.

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