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I went through a line in my textbook which read:

"But for predators, prey species could achieve very high population densities and cause ecosystem instability."

I was not able to understand the meaning 'but for predators'. Can anyone please help me to interpret it's meaning?link to page where this line is mentioned

Edit: In terms of biology, I was unable to understand the meaning of the sentence, and I wanted to make sure that I don't misunderstand things... And this is why I posted the question.. I feel that the answer given is correct and in case, you find better explanation, please do post.

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closed as unclear what you're asking by fileunderwater, March Ho, rg255, AliceD, WYSIWYG Jun 27 '16 at 10:37

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Can you provide a longer quote and what book it is from? To me, this isn't really answerable at the moment. $\endgroup$ – fileunderwater Jun 26 '16 at 10:40
  • $\begingroup$ @fileunderwater - This is the text I was able to find. Third paragraph under the table. $\endgroup$ – GForce Jun 26 '16 at 11:49
  • $\begingroup$ @GForce yes... This is where I read from... Ncert 12th biology.. Organism and population.. To be precise $\endgroup$ – Deepali Jun 26 '16 at 13:45
  • $\begingroup$ I'm voting to close this question as off-topic because it's about English language. $\endgroup$ – rg255 Jun 26 '16 at 16:13
  • $\begingroup$ @user24693 You should add this info to the actual question (title, author, and preferably a longer quote. $\endgroup$ – fileunderwater Jun 26 '16 at 21:19
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I disagree with GForce's explanation; the meaning is not that growth of prey populations causes instability in predator species.

The sentence is merely saying that without predation, prey population growth is more likely to be at a level which leads to ecosystem instability. The term "but for predation" means "if it wasn't for the effects of predation". In other words:

"Ecosystem instability can occur when population growth of some species goes unchecked by predation."

See here for more explanation, where this example comes from in which it says that running a red light caused a crash:

"but for running the red light, the collision would not have occurred"

Biologically this makes sense in the sentence you show; without predators a species is limited by its supply of resources, and it can use these resources at an unsustainable level, whereas if you add predators to the mix there is additional extrinsic effects on population size, not determined by ecosystem properties such as space or nutrients.

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  • $\begingroup$ I agree with this explanation and a practical example of this is the cascading effect of the wolf species canis lupus. Their reintroduction to the Yellowstone national park in the late 90s saw the proliferation of many life forms and also the change in the geological nature of the area, this effect was simply a result of the wolves changing the pathways of the moose species which were initially along the stream banks. $\endgroup$ – Chimango Chisuwo Jun 27 '16 at 5:55
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It's essentially saying that high densities of prey species can cause ecosystem instability not only for themselves but also for the predators which prey upon them.

In other words, high densities of prey species not only cause ecosystem instability by competing with each other, but this instability can move up the food chain and affect predators at higher trophic levels. One example would be a high density prey species having a population crash and having a corresponding adverse effect on predators which feed upon them. Without prey to feed on, predator populations would crash as well.

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