Why is the floral biodiversity of grazed grassland higher than that of mown grassland?

Question

I have collected some data to compare the biodiversity of a field in which the plagioclimax is maintained by machine mowing with a field in which the plagioclimax is maintained by sheep grazing. What I found was that there was a significant increase in floral biodiversity in the field which was managed by agricultural grazing when compared to the floral biodiversity of the machine-mown field. I am trying to explain my findings, and I believe that it is due to the comparitively short amount of time that machine-mowing has been a grassland management technique, so there has been little time for natural selection to take place and for species to develop adaptations to machine-mowing (such as being low-lying, or extremely fast-growing). Whereas the field which is managed through sheep-grazing, contains multiple competing species which have adapted over time to grazing sheep, such as *Cirsium arvense*. Moreover, the field which is managed by grazing has nutrients returned to it through defecation of the grazing animals, allowing for more favourable conditions for growth. **Is my explanation valid? Are there any other reasons why the floral biodiversity of a field managed by sheep grazing would be higher than one managed by machine-mowing?** Thanks in advance!

Answer 1

It's largely the bottom-up effect that you have cited, that being: the grazing animals urinate and defecate as they eat, subsidizing the field with nitrogen waste, water, the undigested fecal matter, and bacteria from their gut. This would likely have a cascading effect upwards through the soil and plant communities. (You can see this effect in places where dogs have urinated, as those patches tend to taller in growth).

In addition to this, there is also a tilling effect of the animals trampling the ground as they eat and walk, which aerates the soil.

I work at a prairie, and the manager considered renting a flock of goats last year because the benefits were significantly better than the machine mower (except that the cost ended up being prohibitive).

Good luck.