Interaction of Press and Pulse Disturbance on Crayfish Populations: Flood Impacts in Pasture and Forest Streams

Interactions between press and pulse disturbances can significantly impact benthic macroinvertebrate (crayfish) populations. Press disturbances such as land-use change can make crayfish more vulnerable to the detrimental effects of pulse disturbances, by changing the habitats available to crayfish. The impact of a pulse disturbance, a major flood (1 in 28 years return period), on crayfish (Paranephrops planifrons) was significantly greater in pasture than native forest streams. Population densities of crayfish in three forest and three pasture streams had been measured seasonally for 2 years prior to the flood and many crayfish had been marked with individually identifiable tags. Low numbers of marked crayfish were recaptured after the flood in the forest streams, but no marked crayfish were identified in any pasture stream. Crayfish densities in one pasture stream declined from an average of 5 m^–2 prior to the flood to <1 m^–2 soon after the flood and it took 3 years for the population to show evidence of recovery. Macrophytes and cobbles, the dominant habitats of crayfish in pasture streams, did not appear to provide stable refugia during the flood. Habitat stability was linked to the riparian zone in forest streams where undercut banks, tree roots, and pools were important habitats for crayfish. Frequent pulse disturbance could affect population persistence of refugia-dependent species when the pressure of land-use change affects the stability of habitats, but this may only be evident over long time scales.