The Upper Clutha River is a favourite spot for anglers, rafters and kayakers. However, this still water hides the presence of didymo – an introduced algal pest – that threatens to spread to all our rivers. Rock snot, is it’s also known, can form massive algal blooms that choke the river and threaten marine life. Just a single drop of contaminated water can spread the algae.

Ecologists are concerned that introduced environmental weeds threaten to overrun our native fauna. It’s a problem that will get worse as more species arrive and escape into the environment. The greatest fear is catastrophic system collapse – that tipping point where forests become silent and rivers run still.

– Fraser Crichton