How do humans affect species diversity?
Well, we're hurting it through climate change, development, hunting, pollution, habitat loss and degradation, and invasive species.
Currently, we're in the sixth mass extinction. Since humans established a dominant presence on the planet, the rate of species extinction has risen to between 1,000 and 10,000 times the normal rate.
Here are a few causes:
Climate Change : Change in temperatures and precipitation globally (including and especially in oceans) have led many species, who are are unable to keep up with the rapidly changing conditions, to go extinct.
Structures and industrial development : In the last 50-100 years, the US has built dams in almost every place it could. Dams prevent natural migrations of fish, build up sediment in places where it shouldn't be, increase flooding, and decrease soil nutrition content in areas downstream of it.
This has led to the extinctions of a plethora of species, many of which we haven't even been able to identify. However, a lot of them have a large impact on the ecosystem's health and the diversity of species.
Hunting : The number of animals we've hunted to extinction or near extinction is astonishing. Even in the many cases where measures were taken to prevent the extinction of a species, such as with many of animals in Africa (lions, cheetahs, rhinos, elephants, etc.), the decreased population of these animals has proven to be very detrimental to their ecosystems, greatly decreasing their species diversity, and thus the health of the ecosystem.
Pollution : Between the oil spills, mining runoff and byproducts, fertilizer and agricultural runoff, industrial waste, not to mention the coal, natural gas, and oil we have been burning since the industrial revolution, this is a significant way we have impacted species diversity. The number of species that have been severely damaged or forced to extinction by the pollutants we emit in the air and water is pretty staggering.
Habitat loss and degradation : While connected to impacts mentioned above, humans have substantially altered the surface of the earth through urbanization, building of infrastructure, expansion of agriculture, and resource consumption. This has lead to major changes and loss of habitat. Habitat is degraded when roads cut through forests, when light pollution from cities invades natural darkness, when selective logging occurs, and so forth.
introduction of invasive species : Whether purposefully or accidentally, humans have introduced non-native species to new habitats and at times this has had drastic consequences. Invasive species often have no predators or controlling population effects and thus out compete natural species. For example, cane toads are linked to species extinctions and the Guam broadbill is now extinct in part due to invasives.