ANSWER 1

Written response:

Many tropical species expand their ranges in part thanks to boundary ocean currents, which push larvae and other propagules from the tropics and towards the poles. In the past, these tropical larvae or propagules may have perished when they got to higher latitudes, especially during winter months, because sea temperatures were too cold. However in recent decades warming waters now allow the tropical larvae and propagules of many species to survive, establish themselves as juveniles and then eventually become reproductive. This is for example what is happening with many tropical fish species in the east and west coast of Australia, which are now settling in places like Sydney or Rottnest where they can overwinter and in time become reproductive.

Tropical species are expanding their ranges southward as Australia’s oceans warm at up to four times the global average, with almost 90% of documented shifts moving poleward. A continent-wide review has already identified 198 species from nine Phyla shifting their distributions since 2003, most commonly in coastal fish and temperate systems, though tropical and equatorward range limits remain underexplored. To build on this, state-based and national Redmap report cards (WA, NSW and Tas) have been developed to assess species potentially undergoing range extensions into new southern areas. These report cards draw on citizen science databases and apply a robust decision-tree analysis to estimate confidence in species’ historical range boundaries and new observations further south. This means we can now more reliably identify which species are on the move, and with what certainty.

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