Since then, these beds have partly recovered, but doubts remain as to what level compared to the pristine situation (Dankers and others 2001). As such, they have a high conservation value, and conservationists were worried to see that in the late 1980s, in a period with low spatfall and ongoing fisheries on mussels on these beds, hardly any intertidal bed remained (Ens 2006 Herlyn and Millat 2000). These intertidal bivalve beds are the habitat of many benthic and epibenthic species and attract numerous birds and fishes, for which these beds provide a rich food source (Goss-Custard and others 1982 Waser and others 2016). Our interest is in the dynamics of intertidal bed-forming bivalve populations living on the soft sediments of the Dutch Wadden Sea, such as the blue mussel Mytilus edulis and the recently introduced Pacific oyster Crassostrea gigas. ![]() It is much easier and it requires less data to describe the fate of patches, communities or populations than that of individual organisms. In fact, the same argument is used by students of colonial insects, subtidal epifauna and island populations. He acknowledges that ‘the ultimate parts of the community are the individual plants,’ but adds ’but a description of it in terms of the characters of these units … is impracticable …’ (Watt 1947). It is interesting to note that the choice of Watt was not so much based on theoretical, but merely on practical considerations. That is, the birth and death of connected populations are studied. ![]() Metapopulation ecology, introduced by Hanski, makes a further step by using entire populations, connected into a metapopulation, as the main unit of interest (Hanski and Gilpin 1991). Eventually these ideas evolved to a more general theory of patch dynamics, where the birth, growth or shrinkage, and death of patches rather than of individual organisms are the central issue (Levin and others 1993 Levin and Paine 1974 Picket and White 1985). The recolonization of such clearings will then, just as in the case of forest gaps, depend upon size and the characteristics of the surroundings (Connell and Keough 1985). Clearings are regularly made either by physical forces or by predators. These hard substrata can almost entirely be covered by epifaunal invertebrates such as sponges, tunicates and mussels. Similar thoughts are developed in coastal ecology, where hard substrata are discontinuous pieces of habitat surrounded by sand and mud. The regeneration rate of the gap is then related to the size of the gap and characteristics of the surroundings (van der Maarel 1996). In forests, gaps appear as a result of, for example, storms or of the fall of a single canopy individual. In plant ecology, Watt’s seminal paper on the dynamics of plant communities resulted in the idea of gap-phase dynamics, which still plays a central role in forest ecology (Watt 1947 Gratzer and others 2004 Hunter and others 2015). But, as queen supersedure within a colony occurs in many species, such is not necessarily true. ![]() ![]() The life time of an ant colony may coincide with that of the queen, in which case one might argue that work is still done at the level of the individual organism. Students of colonial insects such as ants and termites often focus on the birth and death of colonies. Sometimes, however, it might be more convenient to work at a higher level of biological organization (Boughton and Malvadkar 2002). Obviously, most attention has been paid to the birth and death rates of individual organisms. On the contrary, the observed high survival rate of mixed and oyster beds and the expectation that such beds will predominate in the near future can easily result in larger future bed coverage than what has been measured before.īirth and death processes are at the core of population ecology and demography. Claims that bivalve bed recovery is impossible without restoration efforts are premature and not supported by our analysis. Simulation studies, using the observed recruitment series, which was very variable, and the estimated survival curves, showed large variability in total bed area, implying that the present area, though lower than before, does not point to a systematic deviation from the pre-1990 situation, that is, the situation before intensive fisheries on these intertidal beds and the disappearance of them around 1990. Yet the most striking result was that oyster and mixed beds have a much lower hazard rate than pure mussel beds. Cox’s proportional hazard rate model with covariates such as orbital speed, exposure time and bed size and type showed that large, low-lying beds that experience a low orbital speed live longer. Recruitment and fate of all 1436 mussel and oyster beds of the Dutch Wadden Sea were studied for the period 1999–2013.
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