Factors controlling the distribution and abundance of a filter-feeding mayfly, Coloburiscus humeralis, in New Zealand streams.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The influence of abiotic and biotic factors on the distribution and abundance of filter-feeding invertebrates has not been well studied. This study examined the effect of predatory fish and invertebrates, food supply and abiotic factors on the distribution of Coloburiscus humeralis, a filter feeding mayfly, in streams near Cass, Hanmer and on Banks Peninsula, New Zealand. Observations of the feeding behaviour and examinations of mouthpart morphology using scanning electron microscopy together with a distribution survey, channel experiment and behavioural studies were used to elucidate patterns. Stability was the major factor controlling the distribution and abundance of Coloburiscus in streams and the density of nymphs was reduced by simulated bed disturbance in stream channels. Higher abundances of nymphs were associated with more stable streams. Other abiotic factors affecting the distribution of Coloburiscus were also connected with stream stability (i.e. current velocity and substrate size). Predatory fish did not influence the distribution or abundance of Coloburiscus in streams or stream channels and no nymphs were found in the gut contents of trout or galaxiids in the channel experiment. However, native galaxiids affected the feeding behaviour of nymphs, by decreasing the amount found in the guts of Coloburiscus. Anti-predator defences, such as the cerci, may reduce the risk of predation but predatory galaxiids still affect the feeding behaviour of Coloburiscus nymphs. The results of this study imply there is a trade-off between vulnerability of Coloburiscus to predation and susceptibility to disturbance. Predatory fish had no effect on the distribution or abundance of Coloburiscus probably because they are morphologically defended and move very little but Coloburiscus nymphs were not found in unstable streams. The high cost of investing in anti-predator defences results in increased vulnerability to disturbance because they are not mobile enough to seek refuge.