Interdependency between organsims on our planet and its relation to ecosystem?
The problem is not really that we don't understand some stuff (lthough this is certainly true) but that an ecosystem, or the biosphere is a highly complex network of interactions. This network is continuously displaying some chaotic (determinist but not foreseeable) behavior. In such a system it becomes very hard or practically impossible to predict what the impact of a change (even a minor one) might be. To illustrate, one could say that killing half of the world's lions might cause the population of rabbits in Norway (!) to increase but killing two thirds might cause the same population of rabbits to decrease. Note: Stuart Kauffman wrote some articles where he argues that life is a system which is always balancing between chaotic dynamic and non-chaotic dynamic. Today, many conservation biologists use the tools of graph theory (network analysis) in order to get a better understanding of how things are influenced by a minor change.

But defending an endangered species like the pandas or elephants is certainly much more a matter of ethics than of human security faced with environmental shifts. I would guess (a reference on how the world's money is spent on conservation action would be interesting) that an important part of the money  spent on programs for the defense of endangered species are for ethical reasons, just because these species are big, clever, sing well or display nice colors.

There are many false reasons in ecology (in the political sense of the word) where people act for their ethics rather than to defend humans from environmental change. (I don't mean that ethics is stupid, or that only defending the environment for the benefit of the human species should be considered). You may already have heard "the amazonian forest is the lung of our planet" suggesting that this forest produces lots of $O_2$. This is completely wrong! Cutting down this forest would not change much in terms of either $O_2$ or $CO_2$ levels (except if the wood is burned!).

So to summarize I would say:


We spend quite a lot of money defending species for ethical reasons rather than to protect humans from environmental shifts.
Changing a little thing might affect the world-wide biosphere in a way that is practically impossible to predict.