Question

Are endangered species caused by human actions or a natural cycle?

Answer 1

Both.

Explanation:

Species numbers which get dangerously low can occur for a variety of reasons, however the reasons lately have been more human than natural.
Human Actions such as deforestation, over-fishing, poaching, poisoning of our soils, war, desertification, the enhanced greenhouse effect and many other actions all lead to some impact that has reduced the numbers of many species.

The reduction in the numbers of individuals in a species makes them more unstable, and often, such as in the case of Tigers in parts of South East Asia, interbreed, producing less genetically diverse young which are more likely to have genetic defects. This usually occurs when there is a lack of habitat and organisms cannot move effectively enough to find unrelated mates.

When Natural Cycles do have a part to play, would be in what we could call natural disasters, in which large numbers of a species could be at threat of being killed off due to the reduction in genetic diversity.

There is also the alternative which is mainly that of natural selection, in which if the environment the creature is finding itself in is changing (naturally, such as a volcanic eruption releases enough emissions to cause an enhanced greenhouse effect, or there is freak weather of sorts like drought, storm surge or monsoon,) and they are unable to adapt or migrate elsewhere, they can go extinct.

There is also the aspect of how they can have a reduction in individuals if a new, better adapted species spreads to their habitat (naturally) and out-competes the original species or predates on them without giving enough time for them to adapt to this new pressure.

On the whole, the reduction of a species occurs naturally and with human action, what we need to do however is begin to reduce our actions on the environment and prevent our actions from causing species to become endangered and ultimately extinct in that select area/region.

If the natural cycle occurs, usually if a species disappears in an ecosystem, another species will take its place, in which the ecosystem will be able to support itself or at least give itself enough time to adapt to the new species.

Anyway, hope this helps!
-Charlie .P