[I wrote this this morning, but was afraid to post it because, well, I’m dumb and not very knowledgable - so I had to wait]
When reading Nybakken’s and Berntess’ Marine biology: An ecological approach I came across this on page 320:
“The fact that most sandy beach animals are opportunistic feeders also suggests that competition is not important. Similarly, there are relatively few indigenous invertebrate predators, and it seems unlikely that they are responsible for any major distributional pattern. This observation is supported by the fact that, on the few occasions that predators have been experimentally excluded from sediments, diversity has increased, not decreased, as on rocky shores. The effect of large vertebrate predators, such as birds and fishes, remains to be investigated.”
I would say the statement on the importance of invertebrate predation on the diversity isn’t as solid as it would seem at the first glance. It argues that excluding invertebrate predators on rocky shores decreases diversity - which is undisputable. Many researchers have excluded various animals, including animals concidered “key stone” species, which, in effect, made the shore line go boom, meaning a certain species was able to take root, which gave it leverage to dominate. As mentioned, on sandy shores, when excluding invertebrate predators (that are relatively sparse to begin with), the diversity increases. I would argue that since sand works as 3D matrix, giving organisms much more room for food, living space, the number of possible interactions also increases.
[My reading in the evening produced this next exerp]
And 57 pages later, my suspicion is confirmed:
bla bla… exclusion of large predators from soft-bottom communities (not exactly sand, but close)… bla bla… the density of benthic organisms increases, but competitive exclusion leading to dominance by a competative dominant does not occur. This situation is in contrast to that on rocky shores, where the removal of a predator commonly leads to a competative dominant emerging, occupying all space, and forcing other species out. Why should such different results occur here? (drum roll) Peterson (1979) has suggested that interference competition, which operates on the rocky intertidal zone, cannot operate in soft muds because the organisms living there cannot obtain enough purchase to push away or crush another organism and because the organisms can exploit the third dimension (depth) of the soft substrate.
The only weak link here is that the upper paragraph talks about sand, and this last one is about mud. No biggie, except a difference in grain size in a few orders of magnitude. Sand is… well, sand, and mud is silts and clay - indeed, very tiny particles!