Limiting resources in sessile systems: Food enhances diversity and growth of suspension feeders despite available space

Background/Question/Methods

Much of our understanding of competition comes from observations in sessile systems, such as rainforests and marine invertebrate communities. In terrestrial systems, sessile species often compete for multiple limiting resources (such as light, nutrients, water and space) but in marine systems space is viewed as the primary or sole limiting resource. Competition theory, on the other hand, predict the number of species in a community to be equal to the number of limiting resources. Thus, competition for a single limiting resource is unlikely to maintain high species diversity, but manipulative tests of competition for other resources in marine benthic systems are exceedingly rare. Here, we manipulate the availability of food for a classic system – marine sessile invertebrate communities. We modified an existing slow-release method to create four types of food blocks with different compositions of  microalgae, zooplankton and rotifers, which each released an average of 5x10⁵ cells when fully dissolved. The effects of food availability were tested on experimental assemblages on settlement plates as well as laboratory-settled bryozoans deployed into the field in Melbourne, Australia

Results/Conclusions

We found the number of species to be greater, available space to be lower and the community composition to be different in assemblages subjected to increased food availability compared to controls, but there were no differences among types of food for either the uni- or multivariate analyses. Similarly, laboratory-settled bryozoans deployed into the field grew more in the presence of enhanced food. Our results suggest that food can act as a limiting resource, affecting both diversity and abundance, even when bare space is still available in hard-substratum communities. This is the first manipulative experiment on food as a limiting resource, but non-contact intraspecific competition has previously been shown in marine sessile species and oxygen depletion can prevent settlement on free space. Similarly, the flow-velocity of currents can affect growth and diversity of hard-substratum assemblages regardless of available space, indicating that other resources can limit sessile species. Consequently, broadening the view of resource limitation beyond solely space may increase our understanding and predictability of marine sessile systems.