Abstract:
This thesis investigates interactions between small (1 to 8 mm) mobile invertebrates and coralline turfs in shallow coastal rocky reefs in northeastern New Zealand. The effects of host species identity and spatial variability were examined on animals inhabiting subtidal coralline algal turfs, by comparing the assemblages of small mobile invertebrates associated with five coralline turf species across a number of subtidal rocky reefs in northeastern New Zealand. Coralline turf fauna were abundant (average of 16,000 to 80,000 ind.m−2) and diverse (129 taxa in total), with assemblages dominated by arthropods, gastropods and polychaetes. Despite substantial differences in the morphologies of the coralline turf species, host identity had little effect on total abundance and richness of the fauna, and a moderate effect on taxonomic composition. Spatial variation at the scale of 102–103 m had a stronger influence than host identity on all three assemblage-level properties, with wave exposure and depth having the most explanatory power of the environmental factors measured. Hostspecificity was low, probably due to the close taxonomic relatedness of the host algal species and their inedibility (with consequent lack of dietary specialisation by fauna). These results justify the common practice of lumping coralline turf species in ecological studies with regard to the description of the total abundance and richness of the associated fauna, but not the taxonomic composition of the fauna. The principle that biological traits relating to the behaviour, morphology and lifehistory of organisms can influence ecological functioning is becoming increasingly used to examine the functional response of communities to natural and anthropogenic variation in environmental factors. The taxonomic composition and biological traits of fauna were examined along an environmental gradient ranging from (1) relatively deep, wave-exposed sites with short turf containing a low proportion of fine sediment to (2) shallow wave-sheltered sites with taller turf containing a higher proportion of fine sediment. The most common traits of fauna were small size, globose shape, calcareous exterior, detritus/deposit feeding, omnivorous diet and low larval mobility. Total abundances of animals were higher at the wave-exposed sites. The animal assemblage as a whole more clearly reflected the environmental gradient when the assemblage was described in terms of taxonomic composition than in terms of biological traits. Only 3 out of 23 traits (suspension feeding, detritus/deposit feeding and vermiform shape) were at least moderately correlated with the overall gradient, although stronger relationships were present between some of the traits and individual environmental variables. Overall, the biological traits of turf-dwelling faunal assemblages were less sensitive to an environmental gradient than taxonomic composition, indicating that functioning can persist despite taxonomic change. Benthic primary producers such as seaweeds and seagrasses are often inhabited by high densities of small arthropods, but little is known about the impact of these animals on their hosts, which could be positive if they keep them free of fouling epiphytes. The impact of small arthropods on algal epiphytes growing on coralline algal turf in a shallow, waveexposed, rocky reef in warm temperate northeastern New Zealand were assessed. Plaster blocks impregnated with the insecticide carbaryl were used to reduce arthropod densities (by 96% relative to unmanipulated controls in the case of amphipods). By the end of the ~3.5 mo experiment total epiphyte cover was 88% when arthropods were excluded compared to 38% in unmanipulated controls, a 2.3-fold increase. Brown (Colpomenia spp.), green (Ulva spp.) and ‘filamentous and microscopic’ algal epiphyte taxa were responsible for the increase, while the cover of red foliose algal epiphytes decreased slightly. Hyalid amphipods (Protohyale spp.) were likely responsible for suppressing the epiphytes, as they were the most abundant arthropod mesograzer taxon and ate both Colpomenia spp. and Ulva spp. in a laboratory no-choice feeding assay. During the field experiment 2 large storms removed most of the epiphytes that had grown in the mesograzer exclusion treatment. Overall, these results indicate that the amphipods prevented overgrowth of the turf by epiphytes during calm periods. These results provide the first in situ cageless example of arthropod mesograzer-exerted control on the abundance and composition of primary producers on a subtidal rocky reef.