Hydrology, hydraulics and scale influence macroinvertebrate responses to disturbance in tropical streams

Abstract

Despite much research on the influence of disturbance on stream macroinvertebrates, comparative information on tropical streams is limited. We assessed disturbance influence on assemblages under different conditions of hydrology (aseasonal and seasonal) and hydraulics (pools, riffles and stream morphology) by sampling a stream in the Australian Wet Tropics (AWT) pre-and post-flood, and by undertaking experiments at whole-riffle and single-stone scales in several streams in the AWT and Papua New Guinea (PNG). The fauna was very mobile in all streams, facilitating recolonisation. Resilience (recovery) was strongly related to scalerapid in single-stone experiments (2-4 d) and least following whole-stream disturbance (»45 d, comparable with some temperate systems), indicating the importance of scale in understanding disturbance. Local undisturbed patches and pools acted as refugia and provided colonists at least partly via drift. The presence of all life stages of most species through the year and rapid development would facilitate recovery. Multiple disturbances had similar impact to single events, resulting in a reduced but resilient fauna. Resilience was not greatly different in more regularly disturbed sites, but richness was lower in PNG than AWT possibly due to their different disturbance regimes. Disturbance selects a sustained fauna that is resilient (or tuned) to the prevailing disturbance regime.

Colonisation, emigration and equilibrium of stream invertebrates in patchy habitats

“…However, the total numbers drifting over the 5‐day period were only about 40% greater than the standing stock of invertebrates, in contrast to observations that numbers drifting over a unit area of streambed could be several times greater than the standing stock (Benson & Pearson, ; Waters, ). The drift density into the channels (24.1 animals/m 3 ) was higher than base rates recorded elsewhere in the tropics, but lower than catastrophic rates (Benson & Pearson, ; Rosser & Pearson, ); the higher base rate was probably due to much lower mesh size in the present study (63 μm) retaining many small individuals (Supporting information Figure S1). Several species were more numerous in the drift than the benthos, but passed through the channels without settling.…”

“…In our channels, colonisation of litter packs was much greater than that of the bare cobbles, presumably because the more complex litter packs provided greater protection or greater surface area for feeding on the leaves or their biofilm. The rate of recolonisation may also depend on season, interacting with patterns of flow and the scale of the preceding disturbance: For example, in Wet Tropics and New Guinea streams, following simulation of minor floods, recolonisation occurred in 2–4 days, whereas, following large wet‐season floods, recolonisation was accomplished in about 45 days (Rosser & Pearson, ). In these streams, drift appeared to be the major source of recolonisation, with densities much higher following the simulated floods (abundance, 68‐fold in New Guinea and 32‐fold in the Wet Tropics; richness, eightfold at both sites).…”

“…Understanding the links between ecosystem functioning, species' traits and the mechanisms that determine species diversity gradients requires description of ecological processes at small scales (Baird et al, 2007;Willig, Kaufman, & Stevens, 2003). In streams, local biotic diversity is influenced by many factors, including both niche and neutral processes (Brown, Wahl, & Swan, 2018;Thompson & Townsend, 2006; Valente-Neto, Durães, Siqueira, & Roque, 2018) such as habitat characteristics, species' traits and interactions, disturbance and dispersal (Astorga, Heino, Luoto, & Muotka, 2011;Pearson et al, 2017;Rosser & Pearson, 2018). An ecological issue that is unresolved generally is the nature of the link between diversity and productivity (Grace et al, 2016;Gross, 2016;Pierce, 2014), which, in streams, is frequently enhanced by anthropogenic nutrient inputs that alter biotic assemblages (Connolly & Pearson, 2013;Dudgeon et al, 2006;Malmqvist & Rundle, 2002).…”

“…Continual colonisation and emigration enable faunas to recover from the disturbances that are characteristic of stream environments (Downes, 1990), at scales of substratum (e.g., cobbles or litter packs), and stream reach or whole stream (Mackay, 1992;Rosser & Pearson, 2018;Sheldon, 1984). Observed rates of colonisation and drift by stream invertebrates imply substantial turnover of individuals (Downes, 1990;Downes, Bellgrove, & Street, 2005;Rosser & Pearson, 2018), and the residence of an individual in a habitat patch may be short, influencing the frequency and intensity of encounters with competitors or predators in that patch (Allan, 1978;Charnov, Orians, & Hyatt, 1976).…”

“…Continual colonisation and emigration enable faunas to recover from the disturbances that are characteristic of stream environments (Downes, 1990), at scales of substratum (e.g., cobbles or litter packs), and stream reach or whole stream (Mackay, 1992;Rosser & Pearson, 2018;Sheldon, 1984). Observed rates of colonisation and drift by stream invertebrates imply substantial turnover of individuals (Downes, 1990;Downes, Bellgrove, & Street, 2005;Rosser & Pearson, 2018), and the residence of an individual in a habitat patch may be short, influencing the frequency and intensity of encounters with competitors or predators in that patch (Allan, 1978;Charnov, Orians, & Hyatt, 1976). Benthic invertebrates move upstream, downstream and vertically in the substratum (Pearson & Jones, 1987;Stubbington, 2012), but downstream drift, in which individuals are transported by the stream flow, is typically the most important pathway for redistribution (Benson & Pearson, 1987a;Brittain & Eikland, 1988;Lancaster, Hildrew, & Gjerlov, 1996;Townsend & Hildrew, 1976;Waters, 1972).…”