"You often see caterpillars covered with between five and 20 ants – it looks like its being attacked, but it's actually being protected."

Believed extinct until its rediscovery in the 1980s, the Eltham copper butterfly is found in just three localities, all of which are in Victoria.

The reason it is so restricted is a combination of habitat loss and the butterfly's relationship with the sweet bursaria shrub and the Notoncus genus of ants.

The butterflies lay their eggs in the base of the bursaria bush – the only plant on which they feed.

The ants build "satellite nests" around the base of the bursaria to house the caterpillars as they develop over winter and emerge in spring.

Not only do they guard the caterpillars, the ants also keep them clean. As they feed, the Eltham copper's excrete a sugary byproduct from their backs. Attempts at raising the butterflies in captivity have failed because – without the ants removing it – this viscous liquid turns mouldy, killing the caterpillars.

And this is what the ants get out of the arrangement: the caterpillar nectar forms a reliable and nutritious part of the colony's food supply.

But the relationship has its drawbacks. Much of the butterfly's habitat has been cleared for agriculture or swallowed up by urban growth. Because of its total reliance on a single plant species and one genus of ants, the Eltham copper butterfly clings on in just a couple of dozen colonies, mainly upon islands of remnant bush, surrounded by a sea of houses or farms.

Of their 25 known colonies, perhaps the most isolated is the Andrew Yandell Habitat Reserve, a six hectare pocket of box-ironbark forest and sanctuary for rare orchids, lillies and herbs.

Banyule City Council park ranger Megan Lowe said it was totally cut-off from other colonies by shopping centres, busy roads and houses.

With the butterflies unable to move to find suitable habitat, land managers have to engineer conditions so they are just right for the Eltham copper.

This crucial role that people now play was evidenced in the aftermath of the Millennium drought.

When it broke around 2010, larvae numbers soared. But so too did wattles and cassinia bushes. This new, tangled understory did not suit the butterflies which need sun and room to fly.

So in 2011, caterpillar numbers plummeted. In the Greensborough reserve that year, Ms Lowe and her team counted 45 larvae, down from 189 in 2010. The butterfly fluttered closer to oblivion.

In response, land managers began thinning understory vegetation. The result has been a gradual but steady increase in caterpillars.

But with its fate now firmly entwined with yet another species – humans – Ms Lowe hopes people will learn to love the little-known insect. And not only for the butterfly's sake.

"Any species that's threatened or endangered is a sign of decline," Ms Lowe said.

"It's not just about that particular species, it's about the whole ecosystem, because everything works in harmony and once one thing disappears than another disappears ... until you're left with nothing that naturally occurred there."