Effects of modification of habitat on kelp epifauna

Ezequiel "Ziggy" Marzinelli (Supervisors: Ass. Prof. Ross Coleman, Prof. Tony Underwood and Dr David Blockley)


Background
Bryozoan living on kelp

Loss and degradation of natural habitats are amongst the major causes for the current loss of biodiversity. Urbanization of coastal habitats, particularly increased numbers of pier-pilings, jetties and seawalls along shorelines, is a global issue. Such modifications of habitats have important consequences to abundances of biota. The addition of artificial structures can affect biota directly or indirectly, i.e. by altering some aspect of the behaviour or ecology of some species, which, in turn, alter abundances of other species. Epibiota in some modified habitats in Sydney Harbour provided experimental opportunities to examine models about direct and indirect processes and test hypotheses to distinguish between them.

Aims

The main objective of this thesis was to study the effects of anthropogenic modification of habitats by the addition of artificial structures on subtidal kelp epifauna and to determine which processes regulate patterns of diversity and abundances of kelp epifauna in modified habitats.

Importance
Kelp growing on pilings

Manipulative experiments such as those done in this thesis to unconfound multiple components of habitats influencing disturbances to biota are needed to understand human impacts on natural systems. Understanding how these components of habitats affect ecological processes is necessary to allow sensible prediction of the effects of modifying habitats on the ecology of organisms.

Results

In Sydney Harbour, the kelp Ecklonia radiate on pier-pilings supported different assemblages of bryozoans from those on rocky reefs. Abundances of bryozoans, in particular of the invasive species Membranipora membranacea, were much greater on kelps on pilings than on those on rocky reefs. This difference was consistent in time and space. The difference in abundances could be caused directly by the pilings or indirectly by differences in the type of kelp found on each habitat (or both). Kelps were experimentally transplanted from pilings to rocky reefs and from rocky reefs to pilings to examine these models. Properties of the habitat affected the abundance of bryozoans, not the type of kelp that grows on pilings or rocky reefs. Models about differences between these habitats on the effects of physical factors and/or biological interactions on ecological processes were subsequently investigated.

Pilings support piers that often shade the organisms that inhabit them. It was observed that kelps were shaded to different extents on pilings compared with rocky reefs. Greater shading on kelps on pilings could thus influence covers of bryozoans. Kelps on rocky reefs were experimentally shaded to examine the model that the observed differences in abundances of bryozoans on kelps between pilings and reefs were due to greater shading on pilings. Experimental shading of kelps caused an increase in covers of bryozoans, but these did not match covers on undisturbed kelps on pilings, suggesting that other factors influenced covers of bryozoans.

Abundances of the sea-urchin Holopneustes purpurescens, which lives on the canopy of E. radiata and feeds on their fronds and on epibiota, were much smaller on kelps on pilings than on those on reefs. It was therefore proposed that the observed differences in covers of bryozoans were caused by this sea-urchin. Experimental exclusion of urchins from kelps on reefs caused an increase in covers of bryozoans, but these did not match covers on undisturbed kelps on pilings. The addition of urchins in similar densities as they occur on reefs to kelps on pilings resulted in a decrease in covers of bryozoans, but these did not match covers on undisturbed kelps on reefs. Models about the effects of shade and sea-urchins on covers of bryozoans were then examined simultaneously doing an orthogonal experiment. Both factors (shade and urchins) influenced covers of bryozoans, but there was no interaction. The effect of urchins appeared to be greater than that of shade. Thus, pilings affected abundances of bryozoans directly (through shade) and indirectly (by influencing abundances of sea-urchins).

Finally, recruitment and growth of bryozoans on kelps on pilings and on those on reefs were investigated. It was proposed that the differences in covers of bryozoans, in particular M. membranacea, were due to greater recruitment and/or growth on kelps on pilings than on those on reefs. The number of colonies of bryozoans and M. membranacea that recruited to kelps and the growth of colonies of M. Membranacea were much greater on pilings than on reefs. Pilings therefore affect both processes influencing covers of bryozoans on kelps.