Our world is undergoing a massive change, induced by humans. There is no debate about this among scientists. There is debate, however, about the consequences of this change.
Like many other organisms we actively alter our environment and become ecosystem engineers (http://en.wikipedia.org/wiki/Ecosystem_engineer). A classical example for this process is beavers that build dams and create lakes they can use as habitat. Of course this comes at the expense of the existing habitat. The plants in the now flooded area are obviously going to drown. Termites build massive structures that house their colonies. Another example is the evolution of photosynthetic algae that led to the release of large amounts of oxygen, causing the extinction of unique biota that were intolerant of oxygen. Our elixir is another organisms toxicant.
Most importantly, the activities of ecosystem engineers create new habitats not just for themselves, but for many other species that can use them, but at a cost.
Clearly, major changes to the environment by organisms are neither uniquely anthropogenic nor new. While ecosystem engineering always has an impact on the environment, sometimes even at a large scale, I would argue that the human impact on the planet in the Anthropocene is unique in tempo (see also the post by Lynn Soreghan). I have seen unique elements of the natural world that my grandchildren will not be able to see because they have been altered.
Biologists debate what the consequences of our actions are. Models of what may happen to species and ecosystems as climate change progresses are abundant and often offer a bleak outlook: species that are already vulnerable are likely to disappear forever. Some scientists argue that we are in the middle of what they call “Anthropocene defaunation” (Dirzo et al. http://www.sciencemag.org/content/345/6195/401.short) or the Sixth Extinction (see e.g. the book by Elizabeth Kolbert, The Sixth Extinction). Dirzo et al. are especially alarmed by the loss of range for many species and the resulting reduction in abundance of species. This trend is found across all taxa studied, but the exact mechanisms need further investigation.
Now, why should the extinction of a nocturnal mammal or a shift in the range of an obscure insect interest us? Apart from an ongoing, and loud, debate about the value of nature, ecosystems, and species, all of these changes we induce are also impacting us humans in countless ways. The prospect of a future ecological disaster is haunting and nightmarish, and indeed the changes also impact us directly. For example, many human diseases are transmitted by insects, and humans often help insects colonize new habitats—as with the fast sweep of Chikungunya, which is transmitted by mosquitoes. Chikungunya, is a debilitating disease, which causes high fevers and strong joint pain (http://www.cdc.gov/chikungunya/geo/united-states.html). This import from Africa has recently swept through the Caribbean, and has now reached the US in Florida. Travelers help this disease to spread, but areas of the continental USA made habitable to the mosquitoes by climate change give it a permanent foothold the continental USA.
It seems that altering your own habitat can also backfire, and clearly this is the case with humans.
Inhabiting the Anthropocene 
Thanks, Ingo, for introducing the idea of ecological engineering–which is obviously related to the niche construction concept, and is therefore key to understanding habitability. I think you have pointed to the two key ethical considerations these notions all raise: that what one species does to enhance its own habitat can be bad for others (e.g. the plants that drown in a beaver pond), and that a species’ effort to enhance its habitat can backfire, producing a worse life for itself (something humans seem unfortunately good at!). As many have argued, the complexity of the systems involved, with the probability of unforeseen consequences, should probably make us more modest in our niche construction efforts!