Why are scientists concerned about the loss of species as a result of human activities?

I mean there are many reasons why this happens. One, if one species on the bottom of the food chain dies, it would cause a chain reaction and predators would die too. Also, take an example of bees, there is something called CCD that is causing colonies of bees to disappear completely, and this is very serious because they pollinate like all our plants (besides wheat, which is wind-pollinated) and we need them. And all the cows, pigs, chickens, etcetera need to eat those plants so technically, we are next in the list.

The loss of species, also known as extinction, would decrease the biodiversity existing within the biomes of the Earth, and this would generally be, in informal terms, very bad for the humans.

The decrease of the number of species generally affects the web of organisms that live on the spheres of our planet, and losing species means generally impacting all of the ecosystems that are on the Earth. These specific species play vital roles in their own ecosystem: no animal (unless its an invasive species) is put in their habitat without a general reason or adaptation that will benefit or at least affect the ecosystem some way or somehow of being efficient. With decrease in biodiversity, these species will no longer be able to help these ecosystems out, making the rest more vulnerable to things that could wipe out other species, such as disease or pests. This can affect humans greatly, as this would mean deaths and extinctions of more species (even without the intervention of humans), dwindling food supply (food chain is messed up and stuff), and more vulnerability of humans towards again, diseases, pests, or other things.