The longer, wider and higher trees might cause the higher number of animal population, the higher diversity index, and the more spreading distribution of foliage-roosting animals.
Is that comparison structure coherent?
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The verb is correct, however the sentence is not quite coherent; you have a few other errors.
First, if you are speaking in general, I would change the first "the" in the sentence to a null article, or no article. If you are talking about a specific group of trees then keep the "the," but I will remove it in this example.
Next I would consider changing "longer, wider, and higher" to "larger." You are essentially repeating the same information three times. Unless you are trying to stress each of those dimensions, I would change it. (And if you keep the three dimensions by the way, we would not use "longer" to describe one of the dimensions of a tree. I think you mean "taller" in this case, and that means you should remove "higher" at the end, because they are synonyms here.)
Third, the predicate "might cause" is correct.
Then, I advise you to change "the higher number of animal population" to "a higher animal population." "Population" in this sense actually is like a number, or a count of something, in this case the animals. So to say "a higher number of a population" is redundant. Notice I also changed the definite article "the" into the indefinite "a" in my recommendation.
Now, with "the higher diversity index." We should use the indefinite article here again. "A higher diversity index."
Lastly, you have an error with "the more spreading distribution of foliage-roosting animals." "The more spreading" is incorrect. It is correct without any article: "more spreading."
So, if you combine all of these, I would make the final sentence:
Larger trees might cause a higher animal population, a higher diversity index, and more spreading distribution of foliage-roosting animals.