Encrusting algae are one of the major occupiers of space on hard marine substrata and are thought to influence the patterns of distribution and abundance of other organisms in intertidal areas of rocky seashores. However, little is known about their ecology and the mechanisms which may affect their distribution and abundance in space and time. Pseudolithoderma sp., a brown encrusting alga common on intertidal rocky shores in northeastern New Zealand, occurs in discrete patches on sheets of the honeycomb barnacle Chamaesipho columna. Patches of the alga change their size and shape over a wide range of spatial and temporal scales. To identify potential mechanisms that may influence the life history of this alga, patterns of colonisation and persistence of patches of Pseudolithoderma were monitored for 1 year by measuring the colonisation of spores of Pseudolithoderma on settlement plates in relation to existing patches of the alga, and measuring the amount of lateral expansion and contraction of established patches. Colonisation of Pseudolithoderma occurred at a variable rate through the year, but was consistently greater on plates placed on patches of Pseudolithoderma than those placed 1 m away from patches and was rare on plates placed 10 m away from established patches. Established patches of Pseudolithoderma had a much faster tale of lateral expansion (m2per year) than those previously measured for other species of crustose algae, and declined in area unpredictably. The rapidity and lack of seasonality in the changes of the patches is hypothesised to be due to a variable age structure of the patches of Pseudolithoderma, along with localised interactions with small grazers and the underlying barnacles on the rocky shore. This suggests that processes operating at a very localised scale may be equally or even more important in determining the demography of fleshy encrusting algae, such as Pseudolithoderma, than processes operating at larger scales.
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