Humans transport many organisms around the world. This can initially result in at least a localized increase in biodiversity, because any species you add to an ecosystem that wasn’t there before increases the species richness of that ecosystem. This is especially true in cities and gardens where biodiversity might initially be low, but humans alter the natural conditions in order to make them more favorable to supporting more species.
However, depending on the species, this could ultimately lead to the replacement of native species from natural areas. This can happen because introduced species may be faster growing, shading out other species. Or they may be toxic to the local herbivores, and so the herbivores eat everything except that introduces species. Or the new species may actually be toxic to other plants, releasing what are called allelopathic secondary compounds from their roots that only they are immune to the effects of. Over the long term, this replacement of native species can either have no effect, or a negative effect on the total biodiversity of a local area. If the number of species introduced is less than the number of species eliminated, you end up with a reduction in biodiversity.
However, if the number of species introduced is the same as the number of species eliminated, then you still have the same number of species.But this is just on the local level. If you scale up to a regional, or world-wide level, then overall you see a reduction in the total number of species. This is because those species that we move around are often the same ones. For example, suppose you have three ecosystems, and in them you have only one species each, species A, B, or C. If species A is moved from region 1 to region 2, and there it replaces species B, then region 2 still only has a single species, and biodiversity in region 2 has therefore not changed at all. If you also move species A from region 1 to region 3, and there species A also replaces species C, again, the biodiversity of region 3 has not changed because it still only has 1 species. Now, if you look at all three regions at once, instead of having 3 species, A, B, and C, instead there is only species A.
Biodiversity at the higher scale has gone from 3 species to 1.Cities are a great example of this. They can have a considerable amount of biodiversity, sometimes even more than the natural areas they replaced. For instance, you can have a large number of different bird species in a city that all take advantage of human trash and architecture – birds like house finches, house sparrows, peregrine falcons, crows, ravens, magpies, rock pigeons, mourning doves, blue jays, etc etc. But the problem is that every city in America has the exact same set of birds.
So, that’s a rather simplified look at how species introductions can affect biodiversity, but humans do a lot of other things too. Human activities that might be detrimental to biodiversity include (but are not limited to):-Land-use alteration, such as the clearing of old-growth forest for agricultural development, logging, or urban development. This is most-likely the biggest threat to biodiveristy.-Over-foraging and over-hunting of particular species, especially of keystone species that themselves have effects that increase biodiversity-Warming and polluting of the planet beyond tolerance levels of some species. This is already apparent in alpine ecosystems and elsewhere.
-Landscape fragmentation, such as the construction of a 8-lane highway that bisects a forest and disconnects populations of the same species that occasionally interbreed, reducing the stability to the meta-population as a whole.However, humans also actively increase and preserve biodiversity. Scientists and environmentalists work very hard to identify places where they can make positive impacts to restore ecosystems where biodiversity has been reduced, and then make efforts to re-introduce species (or functionally-similar species) or genetic diversity to struggling populations.