Disturbance-related heterogeneity in the seed banks of a marine angiosperm

1 Patterns in the distribution of seeds of the marine angiosperm Halodule uninervis were described over spatial scales ranging from a few centimetres to meadows separated by more than 120 km along the north-eastern coast of Queensland, Australia. Separate surveys examined how seed bank densities were related to: (i) the distribution of male and female plants (primary dispersal) and (ii) disturbances caused by turbulent water movement and the foraging of large marine vertebrates. 2 The distribution of seeds was extremely heterogeneous at very small scales of sampling (centimetres to tens of centimetres). Variation in seed abundance within individual meadows (87% of total variation in seed densities) was at least an order of magnitude greater than that among separate intertidal populations (13%). Densities of seeds were greatest in micro-topographic depressions where the overlying vegetation had been removed by directional water currents and the feeding activities of dugongs. Smaller disturbances that did not disrupt the cover of vegetation had no effect on seed abundance. Detailed mapping of the distributions of seeds and male and female reproductive shoots showed only a weak relationship between primary seed production and the seed bank, but revealed a pattern of seed distribution that was consistent with accumulation in linear depressions in the meadow. 3 The seed banks of intertidal sediments exhibit many similarities to those of physically dynamic desert sands. In these unstable environments, seeds are most abundant in sheltered micro-habitats where sediment mobility is impeded. Plants that inhabit desert and marine sands appear to have responded similarly to strong selective pressures on seedling recruitment by developing mechanisms that restrict primary seed dispersal to take advantage of micro-sites provided by established plants.