Resulting Patch Dynamics

The spatial and temporal dynamics of seagrass patches is strongly influenced by the magnitude and frequency of physical disturbances in a given area and by the capacity of the species involved to persist and recover from disturbances. Some seagrass populations experience continuous patch extinction and replacement, which maintains the vegetation in a permanent state of colonization and promotes the development of a mosaic of patches of different age and developmental stages (Duarte and Sand-Jensen, 1990a; Olesen and Sand-Jensen, 1994b; Vidondo et al., 1997). When in balance, such populations will maintain a dynamic equilibrium with a uniformity of patch distribution in time and space such that an overall landscape equilibrium of patches applies. This has been demonstrated for Cymodocea nodosa growing on highly mobile sediments where the time interval between the passage of consecutive sub-aquatic sand dunes allowed a close balance between loss of vegetation caused by erosion and burial and the formation and development of new patches (Marba and Duarte, 1995).

The dynamic properties of seagrass patch formation and subsequent growth and survival are essential to the recolonization process in denuded areas. The more than 10-fold span across species in rhizome elongation rates and reproductive effort, defining an upper limit for patch formation from seed, suggests contrasting capacities to recover from disturbances (Duarte et al., 1997b; Marba and Duarte, 1998; Marba and Walker, 1999). Small seagrass species exhibit potential fast patch growth, and clonal growth of these species is held responsible for much of the temporal dynamics observed following small-scale disturbances (Williams, 1990; Duarte etal., 1997b). Sexual reproduction is, however, still essential for the recovery of small seagrasses (e.g. Kenworthy, 2000). Nevertheless, some of the larger seagrass species (e.g. Zostera marina) with slow elongation rates can achieve high colonization potential by having high reproductive effort (Verhagen and Nien-huis, 1983). In contrast the combination of very slow clonal growth and poor ability to set seeds in other large species (e.g. Posiodonia oceanica and P. sinuosa) suggests that these are to slow patch growth and an extremely slow recovery process (Duarte, 1995).

Small seagrass species also tend to produce more seeds per ground area than large species and have the ability to build up persistent seed banks whereas large species typically produce seed with no or limited dormancy (Kuo and den Hartog, Chapter 3). However, the rate of patch formation from seeds does not necessarily bear a simple relationship to seed production but is also influenced by loss processes acting on seeds and seedlings and by the seed dispersal capacity (Orth et al., Chapter 5). In a recent study (Olesen et al., 2004), the importance of contrasting reproductive strategies to recovery dynamics was studied over 2.5 years on a mixed-species Philippine seagrass meadow by following patch formation, growth, and mortality in a disturbed gap area (1200 m2). Different species were involved in sexual vs. colonization as the large species Tha-lassia hemprichii and Enhalus acoroides with slow clonal growth but relatively high production of large, broadly dispersed seeds were the major contributors to colonization in areas devoid of vegetation. Although seedling turnover was rapid the high frequency of sexual recruitment (T. hempricii 0.0521.31 m-2 year-1and E. acoroides 0.043-0.081 m-2

year-1) allowed the successful formation and development of new patches and subsequent patch extension through clonal growth. In contrast the small fast-growing species Cymodocea rotundata and Halodule uninervis with limited seed dispersal ensured rapid clonal extension (>1.5 m year-1) of surviving patches in areas where disturbances had only removed part of the existing flora. Hence, where species of both strategies are present, the scale of area affected by disturbance and its interaction with the reproductive strategy of the contrasting species is fundamental to the recovery dynamics of seagrass communities.

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