I'm trying to explore the consequences of just removing humans from the ecosystem. How could I kill off all of humanity without affecting plants, other animals, etc?
Your question is broad and depends heavily on the degree of realism you are striving for as well as the agent of this destruction. For degrees of realism, I will break it down into three categories: realistic, plausible, and fantastical. For the agent of destruction I will go with just two categories: internal and external.
Realistic scenarios are ones that could actually take place here in reality.
Plausible scenarios are ones that would take extraordinary circumstances but are still fall within known natural laws.
Fantastical scenarios are ones involving the supernatural or technology so advanced from our prospective that it may as well be.
Internal agents of destruction would be ourselves, our technology, and the Earth itself.
External agents of destruction include anything not of this world or dimension.
With all that being said, here is a rough outline of scenarios:
Internal/Realistic: There is no scenario I can conceive of now, or in the foreseeable future, that would allow this to happen. Any attempt would have to involve hundreds, if not thousands of people, working in secret to set their plan up. This would be true regardless of whether you're talking about bio-weapons, or straightforward genocide via extermination camps. These people would ALL have to be so fanatically devoted to your cause that they'd be willing to murder their families and friends. Remember, there are more than 7 billion people alive today and all it would take is for a few hundred of them to slip through the cracks for any extinction plan to fail.
External/Realistic: I still cannot think of a scenario that will satisfy the requirement of "not affecting plants, other animals, etc". Any cosmic event (meteor, gamma-ray burst, sun exploding, etc) of a magnitude sufficient to exterminate all of mankind would take the majority of plant and animal species with us.
Internal/Plausible: An ecological terrorist hatches up a scheme to exterminate mankind. He is a wealthy multi-billionaire so is able to independently hire crack teams of geneticists, biochemists, and other scientists necessary to create a gene-knockout virus that would render humanity sterile. He carefully compartmentalizes the teams so that none of them can share information with each other so nobody knows what the "big picture" ultimately is. He leads top lieutenants to believe that he is going to wipe out everyone else and they, and their families, will be spared. When the virus is ready, he devises some means of near-universal dispersal to disseminate the virus discretely. He would have approximately one, maybe two years at most to do this before people caught on to the fact that nobody was able to have children. At this point, it'd be a race to see if the combined efforts of every health organization on Earth could undo the efforts of his scientists before the last woman hits menopause. This plan also gambles on the virus not mutating sufficiently to become benign or having a tiny percentage of the population that is immune (even if it's 99.9999% effective there would still be more than 7,000 people remaining who could repopulate).
External/Plausible: Intelligent alien life, if it exists, could either do the internal scenario described before. Or they could dispatch millions of tiny, insanely fast drones to relentlessly hunt down every human they run across. Alternatively, if they're more benevolent and have the means to do so, they could persuade or coerce humans to leave on massive evacuation shuttles. They could use their drones to make sure there were no straggling bushmen in Papua New Guinea or wherever else.
Internal/Fantastical: We're entirely outside the bounds of reality so do whatever you like. Here are some ideas: a mad cultist of an alien god uses an artifact of doom, humanity learns the secrets of ultimate enlightenment and ascends to a higher plane of existence, a mad scientists cooks up the ultimate bio weapon against humanity and has his faithful robots disperse it.
External/Fantastical: Again, we're outside of reality here so do as you will. Here are some ideas: humanity is pulled into another dimension by some mysterious entity, a cosmic anomaly that emits plot-waves swings by Earth and wipes out all sapient life, a powerful entity wishes us out of existence.
Regardless of the path you take, there are some key points that need to be addressed:
Anything, or anyone, seeking to exterminate humanity is going to have a war on their hands. If governments think there is a serious, existential threat they will almost certainly use any and all means at their disposal to stop it. The continued existence of every species besides us cannot plausibly be guaranteed in that scenario.
There are a number of species that depend on us for survival. These include domesticated species and critically endangered species that are currently kept alive by human effort. Anything that wiped us out would also have to take our roles as caretakers for these species.
If the plot relies on people to work, it is almost certain to fail in any even remotely plausible scenario.
Humans are insanely hard to kill as a species. Any expedient means of doing so(meteor, nukes, etc) will certainly have collateral damage. Any methodical means of doing so will result in #1 happening.
Finally, in summary, you need something that will satisfy all three of these requirements:
- Is specific to humans.
- There is no protection from it or, if there is, it is so difficult that not enough humans will remain to repopulate.
- Acts swiftly enough that species under our care won't go extinct before replacement caretakers are put in place.
- Is global in scope and, moreover, is capable of affecting people living in extremely remote areas.
Voluntarily. Assume that current demographic trends continue, and the average birth rate eventually falls to 1 child per woman and then stabilises. From a starting point of 8 billion, with a generation length of 30 years, we’re extinct in 33 generations or a thousand years.