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I am not a biologist but I just wonder if all species are interdependent. I mean if the tiger becomes extinct does it affect the ecosystem? If the mosquito or mouse become extinct does it affect the environment and us?

I am asking because this question was raised by one of my friends when I was in favor of protecting tigers, lions but I had no answer to "tiger extinction will not affect humans".

So, is it true that the extinction of any species will affect all other species?

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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.
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Could you elaborate on your point about the amazon? While it is probably true that little of the world's oxygen comes from the Amazon, your sentence seems to be equating O2 to CO2. –  terdon Nov 28 '13 at 16:08
    
@terdon I have no reference for the following! The amazon forest is an ecosystem that reached its climax, meaning that trees are not growing more that what they're dying. Or in other words, trees don't fix more CO2 than what they free up to the atmosphere. So yes, $V_{CO2}=V_{O2}$ in the amazon. Therefore, in terms of CO2 there is no cost of cutting down this forest (except for the fuel to be consumed for the work and except if the wood got burnt). And globally speaking, indeed most of the oxygen is produced of several species of plankton. –  Remi.b Nov 28 '13 at 16:15
    
I know, my point is that your phrasing is confusing: "Cutting forest != decrease in $O_2$, therefore no difference in $CO_2$ production. I have edited to show what I mean. –  terdon Nov 28 '13 at 16:49
    
@terdon Ok, Thanks for editing! –  Remi.b Nov 28 '13 at 16:53

Remi.b's answer is spot on - predicting the effects of losing a given species is nigh on impossible. However, I do think that there are some general trends that can give us some intuition, and make the campaign to protect certain species more than just an ethical issue.

Biodiversity (i.e. having a wide range of different organisms (and genetically diverse organisms)) has a number of potential benefits (for references see the work of David Tilman, among others):

  1. Improved resilience to catastrophic events.
  2. Improved temporal stability of populations within an ecosystem (which kind of dovetails with (1))
  3. Improved ecosystem productivity (this has mostly been studied in terms of the amount of biomass grown by plants in an ecosystem, but may generalize to other kinds of ecosystem functioning)

Ecosystem services are effects that various ecosystems have on the world that makes it more hospitable to humans. While some may be, as Remi.b suggested, overhyped, some are very real, such as bioremediation (the removal of compounds toxic to humans from the air and water), the continued presence of pollinators and nutrient-rich soil in which to grow crops, and carbon sequestration. Given that the continuation of these ecosystem services is necessary for our long-term survival as a species, it is in our interest to ensure that ecosystems remain intact to provide them. Increasing resilience and temporal stability of ecosystems reduces the chance that we will lose ecosystems and the services they provide. Increasing the productivity of an ecosystem in many cases increases the extent to which it can perform that service (this is most obvious in the case of carbon sequestration). (NOTE: I'm phrasing this all in terms of human survival because I think that's the most clear cut argument. I think there are other arguments, but they're murkier)

Of course, this raises the question of how much of an impact the loss of a single species (such as a tiger) is likely to have on the benefits associated with biodiversity. There are a few hypotheses here, but in any specific case the impacts will be fairly dependent on the role of the extinct species in the ecosystem. The loss of some species (keystone species) will have a disproportionate impact on the rest of the environment. While it is hard to predict which species will turn out to be keystone species until it is too late, apex predators (like tigers) are more likely than average to be keystone species, or to have an important but not disproportionate role in the ecosystem. This makes sense, because such predators are fairly often the only species at their trophic level in an ecosystem, and having more trophic levels dramatically increases the biodiversity possible in an ecosystem.

Even if there isn't reason to think that a certain species is relatively important to an environment, there can sometimes be benefit in trying to save it via what's called "the umbrella effect." This boils down to the idea that saving one species generally involves saving the rest of the ecosystem that it lives in. So sometimes, from a funding perspective, it makes the most sense to choose the most charismatic species from a given ecosystem and attempt to get public investment in saving that species. An attempt to save this species can then result in preserving a great deal of biodiversity in the rest of the ecosystem.

So, ultimately, no, we cannot know how important a given organism is to an ecosystem or to human well-being. But we can make some fairly well-informed guesses.

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+1 Indeed the notion of resilience is important to add to this discussion. The danger does not only come from in unpredictable ways which is what my answer suggested. –  Remi.b Dec 3 '13 at 9:02

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