Influence of habitat drying on New Zealand stream macroinvertebrates

Abstract

A better understanding of the responses by stream macroinvertebrates to habitat drying is needed as the pressure put on freshwater resources and hydrological regimes through land-use change, groundwater takes, and irrigation increases. To further understand the influences of abiotic and biotic factors on macroinvertebrate emergence, development, and mortality during habitat drying, I designed a field study and a mesocosm experiment. Three distinct drying regimes were measured in scour pools in the Selwyn River, Canterbury, New Zealand, over the austral summer of 2008/09. Pool hydroperiod (time from pool disconnection to drying) restricted the number of macroinvertebrate individuals that escaped desiccation. Fast, flexible development and multiple generations per year were more important invertebrate life history traits than metamorphosis type, with the highest emergence rates found in Diptera, and very low emergence rates in Trichoptera, both of which undergo complex metamorphosis. Emergence rates of Ephemeroptera, which undergoes less complex metamorphosis, decreased as water levels declined, indicating time restrictions were too great for a response to rapid drying. To further study the responses of macroinvertebrates with differing life history development (Deleatidium spp., hemimetabolous life history, and Olinga feredayi, holometabolous life history) to biotic and abiotic factors occurring during habitat drying, responses of macroinvertebrates to the three drying speeds, in the presence and absence of predator cues, were assessed in a mesocosm experiment. Total emergence of Deleatidium spp. was highest in slow-dry mesocosms with scent cues from predatory bullies present, and a three-way interaction was present between flow treatment, predator treatment, and time, in daily Deleatidium spp. emergence rates. Olinga feredayi had no emergence in any treatment but their survival was highest, possibly due to a trade-off between predation resistance and flexible life history. Thus responses by macroinvertebrates were highest in the highest risk situation, and were dependent on lifecycle type allowing rapid development in a highly time constrained habitat.