In south-west Western Australia, an incredible diversity of plants sustains an incredible diversity of pollinators. So what happens when both are under threat?
Take a walk through the mallee heath on Bush Heritage’s Fitz-Stirling reserves at any time of year and you’re guaranteed to come across flowers. From spiky hakea balls, to spidery grevillea tendrils and conical banksia candles, there is always something to stick your nose in here – whether you’re a bird, a butterfly or a bushwalker.
There aren’t many places on Earth where such a guarantee can be made; in most landscapes, a handful of dominant plant species will bloom and fade in synchronicity.
But in south-west Western Australia, where these reserves are located on Noongar country, north-east of Albany, there are over 8000 species, many of which occur nowhere else.
Scientists believe this diversity has to do with the region’s geographic isolation (hemmed in as it is by oceans and deserts), ancient past and subsequently impoverished soils.
“It’s had no glaciation, no volcanoes, for millions and millions of years. So the plants have had a long time to evolve in situ,” explains Bush Heritage ecologist Angela Sanders, who helps look after the Fitz-Stirling reserves.
“It seems counterintuitive but when you have really rich soils, a few plants can dominate, whereas when you’ve got really poor soils, it is a lot more difficult for one particular plant group to dominate.”
Each plant species in the southwest operates on a different flowering schedule, with the end result being that there’s a year-round supply of nectar and pollen. Because of this, the region is able to support one of the highest concentrations of pollinators in Australia.
Birds, mammals, insects, even some reptiles, get essential sugars, proteins, salts, vitamins and healthy fats from these flowers. Many, like the Honey Possum – the world’s only marsupial to subsist entirely on nectar and pollen – are found nowhere else.
“Southwest WA is one of the only places on Earth that could have supported the evolution of a Honey Possum; it’s the only place that has enough nectar and pollen available for them 12 months of the year,” says Angela.
In exchange, these pollinators spread their powdery loads far and wide, enabling regeneration, helping to prevent inbreeding, and playing a fundamental role in the conservation of the south-west’s diversity.
At least 15% of the south-west’s plants are pollinated by birds – more than anywhere else in the world. But, with the exception of some orchids, no plant here relies exclusively on one species for its pollination. Honey Possums, Western Pygmy Possums, skinks, butterflies, bees, honeyeaters – all have a role to play.
It’s hard to say which came first – the diversity of plants or pollinators – but one thing is for sure: neither can persist now without the other, and both are under threat.
Botanist Libby Sandiford has been methodically surveying Bush Heritage’s Fitz-Stirling reserves for eight of the 20 years that they’ve been under the organisation’s care. Over that time, she’s seen some sobering changes in the surrounding landscape.
“If you take, for example, the top of the Stirling Ranges – there was once this incredibly diverse heath there that you could hardly walk through. Now, it’s virtually disappeared as a result of two things, really: Phytopthera dieback and too many fires… that’s actually incredibly disheartening,” she says.
It’s not just Phytopthera, a soil-borne water mould, and bushfires (forecast to worsen with climate change) that threaten this region though. Due to widespread land clearing, less than 30% of the south-west’s original native vegetation remains today, and much of that remnant bushland exists in fragmented pockets.
Angela and her colleagues work in a 70-kilometre stretch of land between the Stirling Range and Fitzgerald River national parks. For the past 20 years they’ve been diligently protecting, restoring and, where necessary, revegetating the land here.
What started as one reserve has grown to ten and now they have an almost-connected nature corridor totalling about 10,000 hectares. Through her surveys, Libby has discovered an astonishing diversity of species (980 and counting) between these reserves, despite them being at times only kilometres apart.
“What we’ve clearly demonstrated is that neither plant species nor vegetation types are evenly distributed across the landscape. And the implications of that are that if you only have one reserve, you’re not necessarily preserving all the local flora and vegetation,” says Libby.
Research into the genetics of plant species in the revegetated areas indicates that some, such as the hakeas and acacias, are doing well, while others, such as banksias, are showing signs of in-breeding. According to Angela, this could indicate they’re not getting pollen from far enough afield.
“If you’ve got lots of plants in one small area, and you’ve just got mammals, which don’t travel very far, pollinating them, then you can end up with in-breeding,” she says.
As more pollinators return to the revegetation they will naturally help to diversify the genetics, just as they have for millions of years. In the meantime, the team is planting banksias and other proteaceous species further apart in the landscape to spread the pollen around.
So complex is the ecological web of life in the south-west that Angela says it’s impossible to answer the question of exactly how the loss of any species might impact others. Hopefully, she won’t have to find out.
“One of the best things for me sitting here in my office today, is knowing that those reserves out there are just doing their thing and will continue to as long as we can look after them. So it’s a difficult question to answer, but I think the alternative is unthinkable.”