Demographic effects of fragmentation history in modular organisms: illustrated using the bryozoan Mucropetraliella ellerii (MacGillivray)

There is a well-established body of literature that deals with population models for modular organisms but that ignores the fragmentation history of individuals. Processes that cause colony damage, such as fragmentation, can result in reduced growth and fecundity and increased mortality of fragments, leading to two classes of colonies—undamaged and damaged. If these classes differ in demographic performance, models ignoring fragmentation history may be inaccurate. We present a general population model for modular organisms, which addresses this issue by including fragmentation history. The model is illustrated using the colonial bryozoan Mucropetraliella ellerii (MacGillivray). We used the model to investigate the impact of additional disturbance of a kind that would damage colonies. We used existing data for two populations living under boulders in the low intertidal zone to parameterise the model. Despite similar population size structures, the population at Point Addis was predicted to be at risk only after very large increases in disturbance, while the Flinders population had a high risk of extinction over 10–15 years, with only a small increase in disturbance. The results suggest that site-specific demographic models may be required when establishing and managing marine protected areas that include this species.