Spatio-temporal patterns in the diversity of demersal fish communities off the south coast of South Africa |
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Authors: | Dawit Yemane John G Field Rob W Leslie |
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Institution: | (1) MArine REsearch (MA-RE) Institute, University of Cape Town, Private Bag X3, Rondebosch 7701, Cape Town, South Africa;(2) Department of Zoology, University of Cape Town, Private Bag X3, Rondebosch 7701, Cape Town, South Africa;(3) Present address: Department of Environmental Affairs and Tourism, Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa |
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Abstract: | The diversity of ecological communities has been the focus of many studies. Because biodiversity provides several indicators
used in an Ecosystem Approach to Fisheries (EAF) to track changes in fish communities, we investigated the spatial and temporal
patterns in the diversity of some demersal fish communities subjected to varying fishing pressure. Depth and catch rate were
the most important predictors in explaining changes in diversity followed by longitude and survey year. Diversity, as measured
by the various indices except for taxonomic distinctness (∆*), initially declined with increasing depth to about a depth of
80 m, then increased to about 150 m after which it declined. Taxonomic distinctness index (∆*) showed an increase in the taxonomic
heterogeneity of the demersal community below the 300-m isobath. Diversity remained relatively constant with increase in longitude
to around 24°E (which has the lowest diversity) after which it increased. The assessment of the temporal trend in diversity
indicates that survey year has a significant effect on all diversity indices except for ∆*. Diversity increased and dominance
declined with time. This may be result of a decline in the abundance of dominant species or an increase in the abundance less
dominant species, or a combination of both effects. Multivariate analysis of the set of diversity indices showed three groups
of indices: those reflecting species richness (S, Margalef’s d), those measuring mainly taxonomic relatedness (∆*), and those balancing the richness and evenness components of diversity
(J′, H′, λ, ∆, Hill’s N1, and Hill’s N2). The relationship between evenness, catch rate, and size was also investigated. Size classes with highest evenness were
found to have lowest catch rate and vice versa. This highlights the need to consider the size and trophic level of species
when linking diversity to the functioning of ecosystems. |
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