Niche is "space occupied" in the ecosystem - a space mostly determined NOT by the environment but by the other species in the ecosystem. In essence, a species' niche is a dynamic, shifting space in a "meta" consisting of all the other organisms in the ecosystem.
what this means practically is that a species' niche is not an intrinsic property of the species, but rather the outcome of the species in its environment, and is different according to its environment.
it's hard to envision niche space since it would be multidimensional and we're bad at thinking in more than 3 dimensions (for obvious reasons)
Leafcutter ants (which are a large group of hundreds of species of ants), along with their fungus-growing (but not leaf-cutting) relatives are all extremely important herbivores in their environment, and relatively safe from most predators. more generally, right, insects are the dominant consumers of plant material (by total volume consumed) in basically all terrestrial ecosystems, so that's unsurprising.
leafcutter ants do not alter the ecosystem in the same way that, say, elephants do, though. They play a much less substantial role in affecting, say, light levels, disturbing the soil, or dispersing seeds compared to what you see in most large herbivores.(likewise, Eciton army ants are among the most important and largest (by biomass consumed) predators in their ecosystems, and display similar requirements to those of large vertebrate predators, such as needing large territories.)