Freshwater ecosystems have been modified by the creation of dams and the withdrawal of water for human use, which have changed the flow of many large river systems. This in turn has had other effects such as reducing sediment flows, the main source of nutrients for estuary ecosystems.
Within terrestrial ecosystems, more than half of the original area of many types of grasslands and forests has been converted into farmland. The only types of land ecosystems which have been changed relatively little are tundra and boreal forests, but climate change has begun to affect them.
The distribution of species on Earth is becoming more homogenous. By homogenous, we mean that the differences between the set of species at one location on the planet and the set at another location are, on average, diminishing. The natural process of evolution, and particularly the combination of natural barriers to migration and local adaptation of species, led to significant differences in the types of species in ecosystems in different regions. But these regional differences in the planet’s biota are now being diminished.
Non-native species
Figure 1.7 Non-native species
Two factors are responsible for this trend. First, the extinction of species or the loss of populations results in the loss of the presence of species that had been unique to particular regions. Second, the rate of invasion or introduction of species into new ranges is already high and continues to accelerate apace with growing trade and faster transportation. (See Figure 1.7.) For example, a high proportion of the roughly 100 nonnative species in the Baltic Sea are native to the North American Great Lakes, and 75% of the recent arrivals of about 170 nonnative species in the Great Lakes are native to the Baltic Sea.
When species decline or go extinct as a result of human activities, they are replaced by a much smaller number of expanding species that thrive in human-altered environments. One effect is that in some regions where diversity has been low, the biotic diversity may actually increase—a result of invasions of nonnative forms. (This is true in continental areas such as the Netherlands as well as on oceanic islands.)
hope this helps im not sure it will
