Victorian Subtidal Reef Monitoring Program - Parks Victoria

More documents

Recommendations

Info

Parks Victoria Technical Series No. 67PPB Sanctuaries Subtidal Reef Monitoring2.3 DATA ANALYSIS2.3.1 Community StructureCommunity structure is a function of both the species present and the abundance of eachspecies. The difference in community structure between pairs of samples was describedusing The Bray-Curtis Dissimilarity Coefficient. This index compares the abundance of eachspecies between two samples to give a single value of the difference between the samples,expressed as a percentage (Faith et al. 1987; Clarke 1993).Prior to analysis, the data were log transformed to weight down the influence of highlyabundant species in describing community structure, giving a more even weighting betweenabundant and rarer species (following count and points abundance transformations bySweatman et al. 2000).The Bray-Curtis Dissimilarity Index was calculated for all possible combinations of sites. Thisresulted in a matrix of pair-wise comparisons known as a dissimilarity matrix. Thedissimilarity matrix is also termed a distance matrix as it effectively represents distancesbetween samples in hyper-dimensional space. The dissimilarity matrix was used for allanalyses of community structure in this study.2.3.2 Depiction of Community DifferencesThe hyper-dimensional information in the dissimilarity matrix was simplified and depictedusing non-metric multidimensional scaling (MDS; Clarke 1993). This ordination method findsthe representation in fewer dimensions that best depicts the actual patterns in the hyperdimensionaldata (reduces the number of dimensions while depicting the salient relationshipsbetween the samples). The MDS results were then depicted graphically to show differencesbetween the replicates at each location. The distance between points on the MDS plot isrepresentative of the relative difference in community structure.Kruskall stress is an indicator statistic calculated during the ordination process and indicatesthe degree of disparity between the reduced dimensional data set and the original hyperdimensionaldata set. A guide to interpreting the Kruskall stress indicator is given by Clarke(1993): (< 0.1) a good ordination with no real risk of drawing false inferences; (< 0.2) canlead to a usable picture, although for values at the upper end of this range there is potentialto mislead; and (> 0.2) likely to yield plots which can be dangerous to interpret. Theseguidelines are simplistic and increasing stress is correlated with increasing numbers ofsamples. Where high stress was encountered with a two-dimensional data set, threedimensionalsolutions were sought to ensure an adequate representation of the higherdimensionalpatterns.18
Parks Victoria Technical Series No. 67PPB Sanctuaries Subtidal Reef Monitoring2.3.3 Species DiversitySpecies diversity involves the consideration of two components: species richness andheterogeneity. Species richness is the number of species present in the community whileheterogeneity related to the distribution of abundances between species. Species diversity isa combination of species richness and the relative abundance of each species, and is oftenreferred to as species heterogeneity. Measures of diversity give an indication of the likelihoodthat two individuals selected at random from a community are different species.Species richness (S) was enumerated by the total species count per site. This value wasused for calculation of evenness and heterogeneity statistics. Species diversity (i.e.heterogeneity among species) was described using the reciprocal of Simpson’s Index(1/D Simpson = Hill’s N 2 ). This value describes species diversity as a combination of speciesrichness (i.e. the number of species) and species evenness (i.e. the equitability of theabundances of the species). The value varies between 1 and s (i.e. the total number ofspecies in the sample) with higher values indicating higher diversity. In general, Hills N 2gives an indication of the number of dominant species within a community. Hills N 2 providesmore weighting for common species, in contrast to indices such as the Shannon-WeinerIndex (Krebs 1999), which weights the rarer species. The weighting of common species wasconsidered more appropriate for this study because the sampling regime is designed totarget the more common species.2.3.5 Species PopulationsThe abundance of each species was summarised by calculating total counts of fish andinvertebrates and total percentage cover of macroalgae, for each site. The population sizestructure for blacklip abalone Haliotis rubra was assessed by calculating median lengths andthe inter-quartile range for each site and time.19
Page 4 and 5: PARKS VICTORIA TECHNICAL SERIESNUMB
Page 6 and 7: Parks Victoria Technical Series No.
Page 29: Parks Victoria Technical Series No.
Page 43: Parks Victoria Technical Series No.

Victorian Subtidal Reef Monitoring Program - Parks Victoria

Create successful ePaper yourself

Delete template?

Save as template?