Chironomus larvae (midge) commonly found in geothermal areas. Photo Credit: Ian Hogg.

Few invertebrate species live only in thermal areas. Most are pre-adapted to the high temperatures and low pH, or are found in geothermal waters as temporary visitors. Some proliferate in high abundance because there is less competition and fewer predators.

The factors regulating distribution are temperature, pH and food availability. Fewer invertebrates occur in close proximity to geothermal areas than in cooler neighbouring areas. The resident communities are also more diverse and abundant where different habitats are available, for example, sinter terraces support less life than areas populated by vegetation.

Fly grazing on mat

Fly grazing on geothermal microbial mat. Photo Credit: Duncan Graham.

Invertebrate fauna includes:

  • Residents – specialist species found only in geothermal ecosystems that do not occur in neighbouring colder areas.
  • Adapters – species also resident in non- geothermal areas that are tolerant of geothermal conditions.
  • Migrants – temporary visitors who transit areas influenced by geothermal heat and steam; some regularly, others only in winter.
  • Foragers – these predatory insects do not reside in geothermal ecosystems, but visit to raid the carcasses of other invertebrates that have been killed or knocked down by rising steam near hot springs and streams.
insect larvae

Insect larvae living on a microbial mat in a geothermal seep at Ngatamariki. Photo Credit: Duncan Graham.

Diptera (flies) and Coleoptera (beetles) have the greatest tolerance to high temperatures with individuals having been found in waters of up to 55°C. The most common geothermal specialists are the Ephydridae (the shore fly), chironomid midge (Tanytarsus), the endemic mosquito (Culex rotoruae) larve and the larvae and adults of hydrophilid and dytiscid beetles. Acclimatised macroinvertebrates include Odonata (damselflies), Chironomus (midges), Hemiptera (swimming bugs) as well as a range of other flies, beetles and a few snails.

Acid vs Alkaline Environments

Alkali-chloride and/or bicarbonate environments tend to have a community dominated by midges, ephydrid flies and cyanobacteria. Acid-sulfate and/or chloride waters tend to have communities dominated by Chironomus flies along with various other Diptera flies and diatoms.