Any change in a habit will introduce selective pressure.
As a concrete example, consider the activity of beavers in North America. In a savanna or in a forest, beavers will construct a dam across a small creek or stream, and thereby create a small pond.
The populations in this ecosystem will change almost immediately. The numbers and types of fish, waterfowl, and trees will change.
As the beaver’s pond ages, it can gradually turn into a larger lake, or it an fill in with sediment and become a swamp. In either case, the populations of flora and fauna will change again.
The changes brought about by the beavers will mean that numerous individuals belonging to these various species will die.
This is merely one example of the many different ways in which a habitat can change, and thereby exert selective pressure on the populations in that habitat. But not only can numerous individuals die when habitats change. When changes are large enough and numerous enough, entire species can become extinct.
Aside from beaver activities, there are other ways in which habitat change: solar activity, volcanic activity, changes in the earth’s magnetic field, etc.
These factors, and many others, cause extinction, as David Wallace-Wells writes:
The earth has experienced five mass extinctions before the one we are living through now, each so complete a wiping of the fossil record that it functioned as an evolutionary reset, the planet’s phylogenetic tree first expanding, then collapsing, at intervals, like a lung: 86 percent of all species dead, 450 million years ago; 70 million years later, 75 percent; 125 million years later, 96 percent; 50 million years later, 80 percent; 135 million years after that, 75 percent again.
A small cluster of microbes, which can travel hundreds of miles by clinging to a bird or to a bit of driftwood, can introduce diseases into regions, and quickly wipe out an entire species — or several species.
These forces are non-anthropogenic. The vast majority of extinctions happen without any human influence. The natural forces of selective pressure are much more ruthless than the effects of human activity.
The earth’s atmosphere changes spontaneously, again without any human intervention. Both the climatic temperature and the carbon dioxide levels have demonstrated their ability to vary wildly. Of the mass extinction events which have regularly happened throughout the planet’s history,
all but the one that killed the dinosaurs involved climate change produced by greenhouse gas. The most notorious was 250 million years ago; it began when carbon dioxide warmed the planet by five degrees Celsius, accelerated when that warming triggered the release of methane, another greenhouse gas, and ended with all but a sliver of life on Earth dead.
The direction of causation between extinction events and climate change is not always clear: in some cases, an extinction event may have caused the climate change; in other cases, climate change may have caused the extinction event.
In any event, these events were non-anthropogenic.
The chances of any one species becoming extinct are high; the chances of it surviving over the long run are low. Survival should be seen as an exception; extinction is the norm.
Viewing earth’s past on a geological time scale, it is clear that climate change, CO2 levels, and extinction events happen spontaneously and sometimes independently over the millennia. It is clear that in the past and in the present, they can and often do happen non-anthropogenically.
