Realising the direct impact humans have on animal species is easy - it is obvious and intended. Changing one's actions so as to not impact these animals is therefore easy as well, one just stops doing this or that.
We do, however, affect animals in other, indirect ways as well, at times maybe even to a bigger extent than with the direct impact. I would therefore like to know which animal species is the most negatively affected by which indirect human activities. By indirect I mean anything where the harm or exploitation of animal is not the primary reason for an action, but merely a byproduct of it. Suggestion of alternatives is a welcomed addition to an answer.
EDIT: Seeing the answers this question inspired, I wanted to clarify what I would like to see in an answer so as it is not based on opinions but rather on data/reasearch.
- Answer is about an animal specie or genus, not some very broad taxonomical unit such as class.
- Answer provides some quantitative information about the impact, eg. number of animals killed/harmed/impacted or perhaps a % of population affected.
- Answer tries, where possible, to work with the indirect impact. While this can certainly be hard to do, I believe there is possibility of at least making an educated guess.