Variation in the recruitment behaviour of seagrass seeds: implications for population dynamics and resource management

Abstract

Effective conservation of marine organisms requires an understanding of the processes that affect the establishment, persistence and extinction of local populations. Our knowledge of the recruitment of seagrasses comes largely from studies done at small spatial and temporal scales within extant meadows. Descriptions of the demography of local populations, therefore, typically emphasize prolific ramet production and only a minor role for sexual propagules. Recent genetic and field studies, however, have shown greater variation in recruitment behaviour than previously suspected. In this paper, I review what is known about the seeds of seagrasses - including their dormancy, dispersability and requirements for germination and establishment - and examine the utility of recent conceptual models, developed for terrestrial clonal plants, to explain the long-term dynamics of seagrass populations. Sizable variation among species in seed size and dispersal strategy appears to be related predictably to variation in life-history and rates of recruitment. Species with small, poorly-dispersed fruits (e.g., Halophila, Halodule) are more likely to form persistent seed reserves and be rapid colonizers of disturbances within established meadows. Genera with large, buoyant fruits, capable of moderate dispersal (e.g., Thalassia, Posidonia), in contrast, appear to recruit rarely within existing meadows of conspecifics. Our ability to model long-term changes in demography and community structure is likely to benefit from a better knowledge of the importance of seed supply and microsite availability to recruitment.