I don't think that total in your particular case is an adjective — instead, I will argue below that it's an adverb after all.
First of all, if total was an adjective, it should be possible to find other similar adjectives that could go into the same slot in your sentence. But let's try with some of the synonymous adjectives suggested by Merriam-Webster for total (note that I only chose synonyms that aren't listed as adverbs to avoid the same ambiguity as we have with total):
the five prairie-dog species inhabit as little as one to two million acres absolute
the five prairie-dog species inhabit as little as one to two million acres complete
the five prairie-dog species inhabit as little as one to two million acres unconditional
the five prairie-dog species inhabit as little as one to two million acres utter
None of these sentences appear to be grammatical with the adjective after acres — but they would be grammatical if the adjectives preceded acres (even if the meaning might be somewhat obscure):
the five prairie-dog species inhabit as little as one to two million absolute acres
the five prairie-dog species inhabit as little as one to two million complete acres
the five prairie-dog species inhabit as little as one to two million unconditional acres
the five prairie-dog species inhabit as little as one to two million utter acres
This observation is important because one of the other answers uses the fact that total can move to this attributive position in front of acres as an argument that total in the sentence-final position has to be an adjective as well. But if that's really true — why can't the other adjectives moved to the sentence-final position as well?
One easy explanation would be this: total can be used both as an adjective and an adverb, but absolute, complete, unconditional, and utter aren't used as adverbs, only as adjectives. As the latter can't occur in the position after acres, but the former can, this slot may be reserved for adverbs, but blocked for adjectives.
We can test this explanation by trying out some synonyms of total that are also used both as adjectives and adverbs:
the five prairie-dog species inhabit as little as one to two million acres all-out
the five prairie-dog species inhabit as little as one to two million acres downright
the five prairie-dog species inhabit as little as one to two million acres flat
the five prairie-dog species inhabit as little as one to two million acres thorough
To be honest, I'm not really happy with all of these examples, but at least flat and to some degree, all-out seem kind of okay to me — much more so than all the examples above with absolute, complete, unconditional, and utter which I'd just reject as ungrammatical.
This test, then, seems to support my hypothesis that total is an adverb. But you don't have to agree with this deduction — there's also support in the literature, specifically in the Cambridge Grammar of the English Language (CGEL, 2002). But we'll need to do a small structural analysis of your sentence first. Regardless of whether it's an adjective or adverb, it's clear that alone is part of the noun phrase that acts as the object to inhabit:
the five prairie-dog species inhabit [as little as one to two million acres total]
We know this since we can replace the whole group of words by a single pronoun such as this or them, and we can turn the words into the subject of a passive sentence:
the five prairie-dog species inhabit [this]
[as little as one to two million acres total] are inhabited by the five prairie-dog species
But that's not all: this noun phrase contains another noun phrase [one to two million acres] in a nested structure, like so: [as little as [one to two million acres] total] (we know this because again, we could substitute one to two million acres by a pronoun:
the five prairie-dog species inhabit [as little as [this] total]
So, what we're dealing with is what CGEL calls a noun phrase with external modifiers, i.e. with two elements that flank the noun phrase to the left (as little as) and to the right (total) to form a new, more complex noun phrase.
More to the point, CGEL analyses total as a "peripheral modifier" (CGEL, ch. 5.2), and it provides a list of three forms that can occur in this particular position: (1) adverbs, (2) prepositional phrases, (3) reflexive pronouns. Here are modifications of your sentence that illustrate each type (although admittedly, the last one is a bit weird):
- Adverb: the five prairie-dog species inhabit [as little as one to two million acres too]
- Prepositional phrases: the five prairie-dog species inhabit [as little as one to two million acres across the board]
- Reflexives: the five prairie-dog species inhabit [as little as one of two million acres themselves]
Notably, adjectives are not considered by CGEL as peripheral modifiers, which agrees with our test using synonymous unambiguous adjectives above, and which seems to be fair if we look at adjective-adverb pairs that are morphologically distinct such as true/truly:
the five prairie-dog species inhabit [as little as one to two million acres true] ← NOT POSSIBLE
the five prairie-dog species inhabit [as little as one to two million acres truly] ← POSSIBLE
TL;DR:
So, to conclude: Since total can be an adverb (as supported by your Merriam-Webster link), since it's been argued (by CGEL) that adverbs (but not adjectives) can be used as peripheral modifiers, and since total behaves differently from other synonymous adjectives that cannot be mistaken as adverbs, it seems to me that the most plausible interpretation is that total is an adverb after all.
