All of them. Humanity is just one of many species. Do you think 1 human is worth more than 1 dolphin?
There is also the issue of climate change, will a mass reduction in human population stop growth in global temperatures? The rise of global temperatures could kill many (billions??), if it causes large crop failures or make large areas of arable land non-productive. Disputes over dwindling resources is seeing conflict around the world. The population change would affect many things.
Note: The world had a population of 3.7Billion in 1970, not that long ago.