The terrestrial and semi-aquatic invertebrates of intermittent rivers and ephemeral streams

Abstract

Intermittent rivers and ephemeral streams (IRES), which cease flow and/or dry at some point, are the most abundant waterways on earth, and are found on every continent. They can support a diverse, and often abundant, terrestrial and semi-aquatic invertebrate (TSAI) fauna, which has been poorly explored due to its position at the fringe between aquatic and terrestrial disciplines. TSAIs can inhabit a variety of habitat types, including the shoreline, the surface of exposed gravel bars, unsaturated gravels, dry riverbeds, riparian zones, and floodplains. Much less is known about the species composition and ecological roles of TSAIs of IRES than their aquatic counterparts, with TSAIs being largely overlooked in conceptual models, legislation, policy, and ecological monitoring. Herein we review the TSAI literature that has increased substantially over the last decade and present conceptual models describing how TSAIs respond to hydrological changes in IRES. Then, we test these models with data collected during wet and dry phases in IRES from Australia and France. These generic models can be utilised by water managers and policy makers, ensuring that both wet and dry phases are considered in the management and protection of IRES. IRES should be viewed as a habitat continuum through time, with taxa from a pool of aquatic, semi-aquatic and terrestrial invertebrates inhabiting at any hydrological stage. We call for collaboration among terrestrial and aquatic ecologists to explore these invertebrates and ecosystems further.

Keywords: Carabidae; Coleoptera; Formicidae; Hebridae; Lycosidae; Staphylinidae; exposed riverine sediments; taxonomic diversity; terrestrial arthropods; unsaturated sediments.

Fig. 1

The intermittent rivers and ephemeral streams (IRES) habitat continuum model. The aquatic invertebrates plot (A) represents taxa that inhabit flowing, cessation of flow, and drying pool habitats (modified from Boulton, 2003), as well as taxa that persist by utilising drought refuges in the riverbed and hyporheic zone and/or as desiccation‐resistant forms (eggs, cysts, larvae, pupae, adults). The terrestrial and semi‐aquatic invertebrates plot (B) includes terrestrial shoreline invertebrate taxa, with an increase in terrestrial and semi‐aquatic invertebrate taxa once surface water disappears, due to the addition of opportunistic predatory and scavenging taxa (the ‘clean‐up crew’). Once the aquatic resources of dead and dying fish, insects, algae, etc., have been consumed, the assemblage is dominated by dry riverbed specialists. Dashed lines indicate when cessation of flow and the loss of surface water occurs.

Fig. 2

Examples of terrestrial and semi‐aquatic invertebrates (TSAIs). (A) Specimens from a sample collected from a dry channel in the Tagliamento River, Italy, with ants (Formicidae), beetles (Coleoptera), and spiders (Araneae) among others. (B) Beetle from the genus Mecynotarsus, showing the pronotal horn used for digging in sand. (C) Beetle (Staphylinidae) at the shoreline of the Tagliamento River, Italy. (D) Ant dragging a recently emerged mayfly from Wild Cattle Creek, Australia. Photographs by Alisha Steward.

Fig. 3

Examples of terrestrial and semi‐aquatic invertebrate (TSAI) habitats in intermittent rivers and ephemeral streams (IRES). (A) The shoreline of Warrannee Waterhole in Cooper Creek, Australia, showing stranded algae. (B) Dried algal mat, leaf litter, and cobbles in the dry riverbed of Oaky Creek, Australia. (C) Cracking clay around an isolated pool in Cooper Creek, Australia. (D) Dry, sandy riverbed in the Cloncurry River, Australia. Photographs A and C by Jonathan Marshall; photographs B and D by Alisha Steward.

Fig. 4

Conceptual diagrams of aquatic‐dominated stages of intermittent rivers and ephemeral streams (IRES): (A) an overbank flood; (B) a flow event; (C) cessation of flow; (D) loss of surface water; (E) short‐term dry; and (F) long‐term dry. Note that the transition between stages can be gradual through time.

Fig. 5

Ants, spiders, and other terrestrial biota seeking higher ground during a flood in the Bokhara River, an intermittent river in north‐western New South Wales, Australia. Photograph used under licence © Stuart McEvoy/Newspix.

Fig. 6

Taxon richness and abundance of terrestrial and semi‐aquatic invertebrates in riverbeds (solid lines) and riparian zones (dashed lines) over time during drying and rewetting in (A, B) Oaky Creek, Australia (A, taxon richness; B, abundance), and (C, D) Albarine River, France (C, taxon richness; D, abundance). Error bars are ±1 standard error.