As described in Harry Potter and the Chamber of Secrets, there are a lot of really big spiders in the forest, and they are all carnivorous. I know "what does X eat?" is a kind of obnoxious form of poking holes in fantasy universes, but I can't help wondering if this was considered.
Imagine a "den" of 150 lions. You wouldn't expect there to be many deer left in that forest after too long, would you? What exactly are those spiders eating?
The official info from Fantastic Beasts and Where to Find Them mentions that they enjoy 'large prey'
The Acromantula is a monstrous eight-eyed spider capable of human speech. It originated in Borneo, where it inhabits dense jungle. Its distinctive features include the thick black hair that covers its body; its legspan, which may reach up to fifteen feet; its pincers, which produce a distinctive clicking sound when the Acromantula is excited or angry; and a poisonous secretion. The Acromantula is carnivorous and prefers large prey.
We know (based on mentions) that there are a considerable number of larger creatures living in the forest that they could be eating including deer, stoats, birds, foxes and sucking bugbears(?) as well as the odd thestral, hippogriff, centaur and unicorn. It would also, presumably include mundane forest creatures indigenous to Scotland such as badgers, pine martens, hares, foxes and wildcats, all of which would be potential fodder for an Acromantula colony.
Note that Hagrid doesn't think Grawp will have much trouble in the food department
"He can get his own food, no problem. Birds an' deer an' stuff"
In general, web-building spiders save a lot of energy by not moving, most of the time. Their metabolic rate drops and can stay that way for days, weeks, months even. They tend to only expend energy when building a web to assist in catching food or when moving to actually catch food, or else when disturbed. They also tend to then store the food for later feeding, in handily-spun, juicy webbed packages. This could be consumed some time after capture, giving the spider more than just the stores of its own body to survive on. Think of your average daddy long-legs. How long might it take an acromantula to consume a deer, and for how long might this supply it with energy?
And it may be that the breeding rates of fantastical prey creatures are higher than natural ones.
This is totally up to our speculation. Remember: artistic license applies!
According to the Pottermore site, the magical creatures that inhabit the forest include:
Non magical creatures include deer and foxes.
While of lesser canon status, the wikia adds the following:
The wikia also mentions possible giants, and blast ended skrewts, but the skrewts wouldn't have been introduced until the events of HP4:GOF at the earliest. (Additionally, the skrewt in the maze I believe is addressed in the book as the last surviving one).
Since acromantulas are carnivorous and prefer large prey, all of the above would qualify.
Spiders also tend to be cannibalistic in nature, it is possible that the colony was feeding on itself to sustain the lives of its older and larger members.
