Mountain multitudes
Why do the Blue Mountains have so many species (10 minute read)
As the rain pelted down outside, I was glad I had packed my waterproof clothing and carried my camera gear in a dry bag. For the previous week, the Blue Mountains, which are inland from Sydney, had been enjoying perfect winter weather, with crisp mornings and clear sunny days. When I arrived, it was damp and cold. The weather cycled between heavy rain, drizzle and feeble attempts by the sun to break through the low cloud. But I own a dog bred for Scottish winters. Donna expects to get a walk whatever the Wellington weather. I barely give the rain a second thought, unless it’s also blowing a gale and therefore the rain is travelling sideways. Even then, Donna and I still take our walk.
So I’m not bothered by a bit of rain in the Blue Mountains. Wrapped up in merino wool and waterproofs, I put an extra layer of clothing in my backpack, just in case, and head out.
I’ve only been in the Blue Mountains a couple of hours, and I can’t wait to see the forest. I’m heading for a short trail which goes to a waterfall known as the Minnehaha Falls. I like waterfalls as much as the next person, but I’m more interested in the plants. At this point, I don’t know what to expect, beyond knowing that I’m going to see gum trees, wattles and banksias, since these plant groups dominate the Australian forest.
It takes me half an hour of walking through the outskirts of Katoomba, the town where I’m staying, before I reach the start of the track. As I walk, I’ve got one eye on the gum trees, because every ten metres or so there seems to be a different type. The other eye is on the slope of the road. For the full half hour, I’m walking downhill. Getting back to my accommodation is going to be hard work. After that, I will have to walk to the shops, in the other direction, to get food for the next few days. But I want to get fitter, so I can’t complain.
Once I reach the forest, I encounter is something I hadn’t expected; water is pouring across the track and I’m likely to get wet feet passing through it. But after the effort to get here, I want to walk in the forest, so I walk on. I’ve done plenty of hiking in wet boots and I’ve got dry socks in my suitcase.
This isn’t the first place I encounter water. In fact, for the first fifteen minutes the track alternates between stream and swamp. The water seems to have decided that the track is the perfect place to flow. Here’s a picture to show you – at first glance it might appear to show a stream flowing down the middle of the image. But this isn’t a stream, it’s the track I walked along.
In fact, Katoomba in the Blue Mountains gets considerably more rain each year than most of
Wellington (with the exception of parts of Wainuiomata). This is one of a number of facts about the Blue Mountains that I learned only after my visit. One I found particularly fascinating is that the Blue Mountains are home to nearly 100 species of gum tree. It explains why I struggled to identify any of them when I was there.
The Blue Mountains are what is known as a biodiversity hotspot. They cover around one million hectares, considerably less than 1% of Australia’s land area, yet more than 10% of Australia’s native plants are found there. The area is also home to a multitude of mammals, birds, insects and other animals. Even a short walk like the track to Minnehaha Falls, which isn’t even in the national park, dazzled me with its diversity.
Biodiversity hotspots are broadly defined as areas with particularly large numbers of species according to their area. Different sources define them differently – for example one of the early publications on the subject considered the whole of New Zealand a biodiversity hotspot, while the only hotspot in Australia was the southern tip of Western Australia. However, since then the whole Blue Mountains region has been named as a UNESCO World Heritage Area because it is considered a biodiversity hotspot.
We don’t know how many species of plant, animal and microbe there are in the world, for many reasons. Mostly, it’s because we simply haven’t done the research to find out. But there are also legitimate scientific debates about how a species is defined (I’ll come back to this issue in a couple of weeks). The current best estimate is that there are around 8.7 million species in the world, but there are some indications that the number could exceed 100 million.
What we do know for certain is that these species aren’t spread evenly around the world. For example, scientists have identified around 64,000 tree species, and estimate that there are another 9,000 still to be discovered. Of the species that are known, South America has 27,000, Eurasia (Europe and Asia together) have 14,000, Africa has 10,000, North America has 8,600 and Oceania has 6,7001. But Euraisa covers 55 million square kilometres, and Oceania only 8.5 million. Taking area into account, South America has twice the number of tree species that Oceania has, while North America, Africa and Eurasia each have less than half the number that Oceania has.
Why do some places have more species than others? It sounds like a simple question, but it doesn’t have an easy answer. The first point is that the overwhelming majority of species are tropical. Not only that, but wet tropical areas have more species than dry tropical areas. South America has so many tree species largely because it has more wet tropical forest than anywhere else. This rule isn’t just seen with trees, it has held true for most types of living thing, on land and in the sea, for tens of millions of years. However, it isn’t universal. Parasites don’t follow this rule, for example. Nor do seaweeds.
Scientists have recognised for many years that there are more species in tropical areas than temperate, but they still aren’t sure why this should be. Warmer, wetter areas are more productive, meaning that they generate more biological material in a given area than cooler or drier areas, regardless of the fertility of the soil. So it’s thought that this higher productivity results in more new species. However, there’s also the fact that in the Northern Hemisphere, the temperate regions (Europe, Asia and North America) are all more or less connected. Tropical regions are less connected to each other, and areas which are isolated from each other evolve different species. It’s telling that there are more temperate biodiversity hotspots in the Southern Hemisphere, where the temperate regions are isolated from one another, than the Northern Hemisphere.
But this doesn’t explain the Blue Mountains. It’s certainly wet there, but it’s far from tropical, as I experienced. Why does this tiny area hold 10% of Australia’s plants? And what about New Zealand? Why do we have around 10% of the number of plant species Australia has, when we cover only 3% of the land area, and are much colder as well?
Different biodiversity hotspots can result from different factors, but there are three important contributors. The first is variation in environmental conditions. Different environmental conditions create opportunities for different species. Topographic variation – such as sharp changes in altitude over a small area – is important for some biodiversity hotspots. There’s no doubt that the Blue Mountains have this. Take any walk there and you’ll find yourself going up and down lots of stairs. As you’ll see from my photos, the views are spectacular, but some of the drops are precipitous. Soil variation is important in some hotspots. Again, this is a factor in the Blue Mountains and it’s also important in New Zealand.
The second factor is isolation. A major reason that New Zealand has so many unique species is because we are miles from any other land. But this is a factor for the Blue Mountains too. They are a pocket of high rainfall in a mostly dry region, so they are effectively isolated from other wet areas of Australia, such as the coast north of Port Macquarie.
The third factor is the rate at which species are going extinct. If there is a low rate of extinction, as there is in areas which have a more stable geology and climate, then a greater number of total species can accumulate. Australia is geologically stable, so that is a third factor contributing to the Blue Mountains being a hotspot. It’s not so much of a factor for New Zealand, which is certainly not geologically stable. It also means that New Zealand isn’t quite the biodiversity hotspot it once was. More than 40% of our land bird species have become extinct since humans arrived. But human-caused destruction aside, geological and climate stability can contribute to low extinction rates.
As I kept walking, I came to a fence which ran along the side of the track. On the other side of the fence there was a sheer drop. Across from me, there was another equally sheer cliff. Huge slabs of bare rock hung over the spindly gum trees which grew at the bottom of the gully. I wondered whether they were young trees, or whether that was as large as they got. I don’t know enough about the forest to identify whether it is healthy, recovering or declining – something I easily see when I walk in the forest in New Zealand. But I know that many of Australia’s unique species are in as much trouble as ours.
The track turned into a series of metal steps, going down, down, down. I knew it would be hard work going the other way, but I kept going down, as the air got cooler and the sound of water louder. By the time I reached the pool at the base of the waterfall, the sun was trying to break through. But it didn’t dispel the gloom of the gully. This place, I was told, was once a sacred place for indigenous women. I assume that it probably still is – I’m sure they won’t have forgotten. But their ability to use it in their traditional way is gone. I imagine people clambering down the cliff, long before the metal stairs were installed. The older women teach the girls the way, generation after generation, for thousands and thousands of years. They probably know every stone, every foothold, every plant.
Even they are newcomers to this ancient land, though. The extraordinary plant diversity I see around me has been evolving for more than 100 million years. But it’s in trouble, just as it is in New Zealand. Australia has lost 37 plant species since scientists started counting, and plants are declining at a much greater rate than mammals and birds.
Yet Australia’s doing well compared to some hotspots. Going back to the number of species of trees in the world – of the 9000 estimated to be unknown to science, around 40% are thought to be in the South America, particularly places like the Amazon. Will they be cut down before scientists even know that they exist? What about the extraordinary hotspot of Madagascar, somewhere I would dearly love to see although I doubt I ever will. It has more than 3000 tree species, most found nowhere else, and two thirds are threatened with extinction.
And then there’s climate change. Biodiversity hotspots developed, in part, due to climate stability. But we have thrown out the rule book. Australia (well, technically the Torres Strait) has already lost one species, a type of rodent called the Bramble Cay melomys, to climate change. What about Australia’s alpine plants? Where will they go when the climate warms?
I’m depressing myself now. I realise that although I’ve talked about extinction in New Zealand, I haven’t looked at the wider loss of biodiversity around the world. Because I’m lucky enough to live in one of the few cities in the world where native biodiversity is increasing, I sometimes forget what is going on outside. So I’m going to take a closer look – but I won’t do that now. Instead, I’m going to climb up those steps and trudge back up the hill. I’m going to exclaim with delight over tiny plants growing in cracks and crevices. I’m going to hug a couple of those gum trees and be grateful that I’m here to see them.
Thanks for taking us on your blue mountain walk with you Melanie. Felt like I was there.
A really interesting account of your walk, thanks Melanie. I'm looking forward to your post on how a species is defined.