Quantifying mortality of modular organisms: a comparison of partial and whole‐colony mortality in a colonial bryozoan

Abstract Comprehensive studies on the population dynamics of colonial organisms require estimates of mortality from the level of the individual module (partial mortality) to the entire colony (whole‐colony mortality), as well as determining the factors that affect mortality at each level of organization. However, accurate measurements of whole‐colony and partial mortality of modular organisms can be difficult to obtain, and few studies involve concurrent measurements of modular and colony mortality. We implemented multiple approaches to measure whole‐colony and partial mortality of modular species using the colonial bryozoan Membranipora membranacea as a model organism. M. membranacea is a cosmopolitan species and of particular ecological significance in the northwest Atlantic, where it is the dominant epiphyte on laminarian kelps and the main driver for the defoliation of kelp beds. Rates of whole‐colony mortality were measured in the field (1) indirectly through repeated subsampling cohort analyses and (2) directly by tagging colonies in situ. Partial mortality of colonies was measured (1) in the field by quantifying the proportion of degenerated zooids per colony and (2) in the laboratory by monitoring loss of colony surface area during seasonal senescence of colonies over winter. Temporal patterns differed substantially between partial and whole‐colony mortality, suggesting that factors affecting mortality of individual modules differ from those affecting whole colonies. Our study indicated that the accuracy of common methods for measuring mortality of modular organisms depends on the species‐specific life‐history characteristics. However, we suggest that rates and mechanisms of whole‐colony and partial mortality can be most accurately quantified by revisiting tagged individuals in situ and recording loss of entire colonies (whole‐colony mortality) and loss of living colony area (partial mortality) between successive sampling times. For M. membranacea , measuring whole‐colony or partial mortality in isolation would overestimate important demographic rates and underestimate the influence of temperature on mortality, respectively. This study demonstrates the need to include mortality measurements from the level of the individual module to the entire colony when quantifying population dynamics of colonial organisms.