Fact is, mechanical mowing is nothing at all like grazing in terms of its impacts upon a plant community.
Grazing animals are selective in how they forage, eating plants they find tastier and avoiding plants they don't like. In this way, the power of a highly competitive plant could be nerfed because that plant is yummy and gets eaten more.
Grazing animals also don't graze uniformly over a whole pastureāthey have areas they prefer to hang out and areas they hang out in less. This creates a broad range of specific levels of pressure that the plant communities face
Grazing animals chop and churn up soil with their hooves. They jostle seeds loose and bury them in mud. They compact some areas of soil but not uniformly.
Grazing animals poop. Ruminants like cattle, bison, goats, and deer use symbiotic bacteria that live in their guts to digest their food. The bacteria break down the plants into nutrients. When the animal defecates, it provides a feast of nutrients and moisture for plants and insects.
Finally, grazing animals are significantly slower and less violent than lawn mowers, and thus less likely to run over critters and shred them to bits.
I doubt it would be possible to comprehensively measure how many insects get shredded to bits by lawn mowers, or what impact this has on the overall population, but running a Creature-Shredding Machine regularly over your entire yard can't be good for them. You're chopping up a lot of caterpillars and other flightless larval forms of bugs, which birds need a LOT of in spring to feed to baby birds, among literally everything else.
Our turf grasses evolved to have large animals munching them down, which is why we have to cut them with machines. However, the machines fail entirely to fulfill their part of the symbiosis.
Most importantly, the machines spew shredded plant material all over the ground, where the mess dries out and essentially does nothing. Lawns are too short to hold in the ground's moisture, and perpetually sun-drenched because of the scalped plant cover. Ruminants chew their cud to help their bacteria break down the incredibly tough plant material into a form that releases the absorbable nutrients. Lawn mowers are not meticulous like that. The shreds take forever to be decomposedāassuming they are decomposed on-site at all instead of being blown or washed away.
The spewing of shredded plant gunk is something that certainly has unique impacts on the plant life. I have particularly noticed that grass clippings get stuck to leaves of nearby plants (often causing them to rot) or even pierce through leaves, injuring or killing the leaf. The death of a single leaf on a single plant is tiny, but tiny stresses like these can determine which plants are capable of thriving in an environment and which die out. A plant might happen to respond badly to frequent micro-injuries.
Here's an interesting fact: There is a highly endangered plant found in Kentucky called running buffalo-clover. Why do you think it is called that?
The running part means it forms long, vine-like stems that sprawl out and run along the ground, growing new roots along their length.
The buffalo part refers to the plant's association with bison. When bison trample on it, they chop the running stems with their sharp hooves. Since the stems are rooted into the ground, they can live on their own without being linked to the main plant, and in this way, the bison are chopping the plants up into more plants!
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