Community dynamics of the bethnic fauna in a West Coast stream ecosystem
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Benthic macroinvertebrate community structure and function were investigated at two open and two forested sites along a stream continuum near Reefton on the West Coast of the South Island, New Zealand. Benthic sampling was semi-quantitative and was largely carried out between April 1976 and March 1977, although subsidiary sampling continued until October 1978. At all four sites the stream was physically unstable, and susceptible to unpredictable and severe floods. A regression analysis of flood peak records for nearby, very large and very small catchments allowed the estimation of flood discharge peaks: specific discharges of over 1 m³.km⁻².s⁻¹ were probably exceeded at least ten times during the period April 1967 – October 1978. A total of 182 aquatic and water associated invertebrate taxa were recognised, the most recorded from a New Zealand stream. Diptera (notably Chironomidae) and Trichoptera were represented by the largest numbers of species, although Ephemeroptera and Plecoptera were numerically dominant at all sites. Spatial changes in community structure were analysed using rank correlation methods and a community similarity index. Species of Trichoptera and Diptera were generally stenotopic, whereas species of Plecoptera and Ephemeroptera were frequently eurytopic: these findings are related to New Zealand’s geological history, the holometabolous development of the former groups, and their ability to utilise silk. The life histories of six stonefly species were investigated in detail. All but one species had poorly synchronised seasonal cycles and long flight periods, which is attributed to the historically oceanic and unpredictable nature of New Zealand’s climate, and the general lack of pulsed autumnal leaf inputs into New Zealand streams. Statistically significant differences in stonefly population size frequency distributions were found between different sites in several species, notably Zelandoperla agnetis: possible factors contributing to these are discussed. The fauna at all sites was very resilient to flooding. A first order forested tributary supported the most resident species and was the site with the most temporally stable community changes very little along the continuum considered. Animals that appear to feed primarily on the stone surface organic layer dominated the fauna at all sites, and shredders were very scarce, which is attributes to the poor debris retention characteristics of the study streams. Little evidence was found of food resource partitioning or temporal segregation of larval growth periods amongst closely related spatial cohabitants: coexisting species were probably segregated primarily on the basis of microhabitat preferences. A preliminary investigation into the composition and formation of the stone surface organic layer indicated that it should provide a reliable food resource throughout the year. The uptake of leaf leachate derived dissolved organic matter by the micro-organisms and their associated sediments is postulated to be of major importance in providing this food resource for the macrobenthic communities. A conceptual model of energy flow in forested and open site communities is developed for systems where shredders are scarce or absent. It is concluded that abiotic, rather than biotic factors are the most important influence upon benthic macroinvertebrate population in the study area.