Can diverse habitats cope with disturbance better than monospecific habitats?

Dr Paris Goodsell


Background
Example of a diverse assemblage of intertidal algae

We need to understand the consequences of losses in biodiversity for the normal functioning of biological systems. There are many 'functions' of a biological system, but generally we talk about processes such as productivity, nutrient cycling, decomposition, etc. It is suspected that when biodiversity declines or is lost, processes that are necessary for a system to function normally are damaged or stopped. Ecological models have proposed that diverse sets of species are more 'stable', that is their abundances and distribution over time are more constant than is the case for less diverse assemblages. In coastal environments, disturbances (trampling, wave-action, herbivory) are a constant threat to seaweeds (algae). Despite this, we have limited knowledge of the relationships between diversity and stability for assemblages on intertidal rocky shores.





Aims

I compared the responses of algae to trampling (measured as the extent of reduction/variation in cover) in diverse assemblages to that of algae in less diverse assemblages. Some algal taxa, such as those with soft,fleshy fronds are more susceptible to trampling than are seaweeds with tougher bodies. They may therfore have different responses to disturbance when intermingled with tougher species. I compared the response to trampling of three algae with different structures: Corallina officinalis, Hormosira banksii and [[i||Sargassum sp]., eachwhen growing by itself, in 2-species combinations with each of the other algae and in a 3-species combination. Because people do not walk in constant and singular directions or paths on a shore, I compared the response of algae to the effects of the spatial extent of trampling (number of paths trampled) and the localised intensity of trampling (number of travels per path).

Importance
Three common algae on temperate Australian rocky shores have different structures

Much research done in the Centre evaluates factors that influence patterns of diversity. Assessing the role of diversity itself on the functioning of systems is a central part of this goal. All systems on earth experience disturbance, so it is critical to understand whether losses in diversity may make ecological assemblages of species less able to cope with disturbance

Results

For the most part, the cover of each algae in plots which were trampled was less than those in the un-trampled plots 2 and 4 weeks after being disturbed. The number of paths trampled had more impact on each alga than the number of times paths were travelled. H. banksii, as predicted, was the most affected by trampling at each level than the coarser Sargassum sp. and C. officinalis. Generally, each alga was more inert to trampling when in the presence of the other two species than it was when in by itself. The response of H. banksii and Sargassum, to disturbance seemed, in many cases, to be due to the presence of a particular Corallina rather than being due to te number of species present. These results suggest that the relationship between diversity and resistance is complex in intertidal habitats, depending on the particular species present. I am still monitoring these plots to assess the time it takes to fully recover and whether this is also influenced by the initial diversity.