Keeping cool when the heat’s on:aquatic insects in rainforest streams

August 2002

Freshwater insects are ectotherms -cold-blooded animals whose tempera-ture balance is regulated by externalheat sources. Because of its directeffects on physiological functions likerespiration and digestion, temperaturehas a influence on where and whendifferent species of freshwater insectsare found. In Europe and NorthAmerica, temperature alters predict-ably with altitude and the seasons, andaquatic insects there can easilyoptimise thermal responses to fit thesepredictable patterns. The Australianenvironment presents more of a prob-lem to freshwater insects though,because stream temperatures fluctuateunpredictably from year to year.

A number of factors contribute to this,the strongest being the frequency ofdroughts in Australia, when streamsmay be reduced to little more than aseries of shallow pools. Stream tem-peratures generally rise higher in drythan wet years because smallervolumes of water gain temperaturemore rapidly. Cyclones and bushfiresalso contribute as their effects reduceshade over forested streams causingsolar heating to intensify until thecanopy regrows. To deal with suchfluctuation, Australian freshwaterinsects could logically be expected toevolve more flexible, broad-rangingtemperature responses than thosefrom more predictable climates inEurope and North America.

The ChironomidaeRainforest CRC PhD student BrendanMcKie recently studied the distribu-tion of aquatic Chironomidae in 36streams throughout the Wet Tropics.Known commonly as non-bitingmidges, Chironomid larvae inhabitfreshwater habitats, feeding on organicdetritus in the sediment at the bottomin the same way that earthworms doin soil. Adults superficially resemblemosquitos but do not feed on blood.

Brendan also examined how well theytolerate extreme temperatures in thestreams of the Wet Tropics and thetemperate subalpine streams nearCanberra.

Broader tolerance in ancient faunaRecent discovery in the Wet Tropics ofspecies previously thought to be re-stricted to cooler regions of southeastAustralia suggests these midges havevery broad temperature tolerancesindeed. Laboratory testing confirmsthis, although Brendan discoveredvariation linked to the evolutionaryhistory of midges. Species that evolved150-90 million years ago whenAustralia was part of the Gondwanansupercontinent (with Antarctica andSouth America), and was colder andwetter than now, have narrower toler-ances than those that evolved morerecently, during drier periods inAustralia’s history.

Right: a rainforest stream typical of

Chironomid habitats in the Wet Tropics

The Gondwanan chironomids how-ever, are still more temperaturetolerant than typical species fromEurope and North America.

Brendan found no differences in thetolerances of midges from theCanberra streams compared withthose from the Wet Tropics. Eventhough the Canberra streams aresubstantially cooler, during hot drysummers water temperatures can besimilar to those of tropical streams. Theenvironmental fluctuations thatcharacterise Australian streamsappears to have maintained broadtemperature tolerances among midgesAustralia-wide, contrasting with thenorthern hemisphere where insects atdifferent latitudes typically havedifferent temperature tolerances.

This contrast continues in the distri-bution patterns of species found. In thenorthern hemisphere, midge speciesare generally restricted to narrowaltitudinal zones, and exhibit markeddifferences in the type of communitiesfound at high, middle and low alti-tudes. No such zonation occurs in theWet Tropics. Although Gondwananspecies are more common at highaltitudes, they occur in well-shaded,cooler lowland streams also. No zona-tion was evident with latitude either,with Gondwanan species occuringthroughout the entire Wet Tropicsbioregion. Similarly, at the continen-tal scale several species range downthe entire eastern seaboard, and evenin the alpine regions of southeastAustralia, there are no unique speciesabove the snowline.

The trouble with too muchtoleranceAustralia has fewer chironomid spe-cies than comparably sized areas inEurope and North America. Here,between 15 and 50 species are typicallyfound at single stream sites, whereasnorthern hemisphere sites frequentlyyield hundreds of species, reflectingrelative degrees of specialisation inthese animals. Temperature can be re-garded as a resource since ectothermsexploit the heat energy available intheir environment to power their dailyactivities. The predictability of north-ern hemisphere seasonal temperaturecycles enables its chironomids tonarrowly optimise their temperatureresponses. This allows the temperatureresource to be more finely divided, anadvantage which enhances speciation- or the evolution of new species. Incontrast, the unpredictability of theAustralian stream temperature cyclehas favoured chironomids withbroader tolerances, preventing fine-scale niche differentiation and possi-bly, retarding speciation.

Management ImplicationsOther Rainforest CRC researchershave discovered similarly high levelsof tolerance to both low oxygen con-centrations and high sediment loadsin the Wet Tropics freshwater fauna. Itappears that broad environmental

tolerances may be a general feature ofAustralian aquatic insects. However,the very adaptation of the fauna to hy-drological unpredictability makes itpeculiarly vulnerable to changescaused by the construction of damsand weirs that impose greater regular-ity on stream and river flows. Suchstructures dampen environmentalfluctuations and favour introducednorthern hemisphere species betteradapted to more constant conditions,many of which are pests. With increas-ing pressure on water supplies from agrowing human population in the WetTropics region, care is needed to avoidexcessive impacts on the rainforeststreams. Like the terrestrial forests,these streams support unique animalspecies found nowhere else in Aus-tralia. Among these are some endemicGondwanan midges that do not occurin warmer north Queensland streamsoutside the Wet Tropics. Any alterationto stream temperature regimes result-ing from dam construction or otherdisturbances could compromise theexistence of this ancient fauna.

For further information contact:Professor Richard PearsonSchool of Tropical BiologyJames Cook University, TownsvillePhone: (07)4781 5466Facsimile: (07)4781 5589Email: richard.pearson@jcu.edu.au

Midge larva, living inside a tube constructed from silk and detrital particles. The mid-grey

colour is due to a blood-like chemical, possessed by many chironomids to aid in oxygen

uptake. Such species are known to fishermen as “bloodworms” (illustration: Linda Davis).