Macroinvertebrate community responses to duration, intensity and timing of annual dry events in intermittent forested and pasture streams

Intermittent streams comprise the majority of stream length in many parts of the world, particularly dry regions. Many dry regions are expected to experience longer and more intense dry periods as the global climate changes. The response of benthic macroinvertebrate communities in intermittent streams to current variability in dry-period duration and intensity may predict their future response to climate change, but such responses require quantification in different stream types and climate zones. I compared the macroinvertebrate community among drier and wetter years from 2008 to 2012 in intermittent forested and pasture streams in a relatively warm, dry region of New Zealand. I predicted that macroinvertebrate communities would decline in density and taxonomic richness with increasing dryness, and that drought-sensitive Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa would show a stronger response than Diptera and non-insects. EPT richness and density did decline with increasing dry period duration, whereas total macroinvertebrate density and richness, and Diptera richness, showed weak and/or non-significant relationships. I predicted that loss of remnant pools (a potential dry-season refuge) would cause a decline in macroinvertebrate richness and density, but found no significant difference between years with and without pools. I predicted that years when flow resumed late in the autumn (when temperatures are too cool for insect flight) would have lower EPT richness and density than years when flow resumed early, and results confirmed this. Intermittent pasture stream communities showed a weaker response to dry period duration than communities in intermittent forested streams, as taxa relatively tolerant of agricultural stressors are also relatively tolerant of drying. Results suggest that global climate change will cause a loss of drought-sensitive species from intermittent streams in this region.