Ecological Dynamics on Pebbles, Cobbles, and Boulders

Pebble-size and larger mobile rocks in aquatic environments may be colonized by a variety of sessile benthic organisms. These diverse encrusting, boring, and nestling communities have a long fossil record that can be used to test many paleoecological and evolutionary hypotheses. The frequent rolling or overturning of rocks during the residence time of their colonizing fauna represents a distinct physical disturbance that can be observed in the fossil record. This disturbance may vary in intensity; some substrata are overturned continually, while others are rarely moved at all. Increased disturbance is generally correlated with increased taxonomic diversity, unless the substrate is overturned so frequently that few colonizers are retained. Intermediate levels of disturbance enhance the diversity of mobile rock faunas by limiting the dominance of late successional taxa. The diversity gradient produced in a variable disturbance regime is often a reflection of the ecological succession in the rocks. Diversity is also dependent upon available niche space. A dramatic increase in rock-boring during the Mesozoic produced a variety of holes in mobile rocks, expanding the niche space available to encrusting and other coelobitic organisms. Thus the preserved faunas associated with Mesozoic and Cenozoic mobile rocks are, on the average, far more diverse than their Paleozoic counterparts. Organisms persisting on mobile rocks have developed three strategies in response to the physical disturbance: 1) some encrusters, notably late successional bryozoans, produce a mature colony shape that stabilizes the mobile substrate and thus reduces the potential for disturbance; 2) encrusters on mobile rocks in a sand or gravel matrix tend to develop morphologies that resist physical abrasion; and, 3) if suitable holes are present in the rock, encrusters may avoid the hazardous surface and live as cavity-dwellers (coelobites).