Cape Town - Many of the hikers and mountain bikers who enjoy the tranquil beauty of the Jonkershoek Nature Reserve near Stellenbosch are oblivious to 12 slightly darker, even-sided patches in the cloak of near-pristine fynbos that covers much of the mountainside.
Yet these patches – actually old fishing nets strung tautly over wooden posts, six on either side of the valley – are the sites of a major field experiment, the latest in a research series in Jonkershoek stretching over decades that has produced some of our most important knowledge about the globally significant fynbos ecosystem.
This particular experiment is designed to measure the ecological impact of removing birds from the system – how it will affect vital functions like pollination, seed production, seed predation by other species and potentially increased herbivory on the plants by insects, perhaps all culminating in what might be a significantly different plant and animal community after fire.
This research is the work of Stellenbosch University masters degree student Pieter Botha, supervised by Associate Professor Anton Pauw of the department of botany and zoology, who is an evolutionary ecologist specialising in animal interactions.
Much of our current knowledge about fynbos has derived from long-term studies in the Jonkershoek valley, Pauw notes. These include research into the impact of fire and invasive alien plants on fynbos vegetation and on fynbos water catchment areas, for example. What’s been missing is detailed information about the impact of birds on fynbos, and specifically at a community level.
There have been previous studies that have focused on specific plant-animal interactions, but nothing that has really looked at the “big picture” of birds and fynbos, including their role both as pollinators and predators of insect herbivores.
“So our focus is very much on the community level in this study – this is the gap that we’re hoping to fill,” Pauw says.
So how does one set about simulating a “world without birds”? Well, you actually have to physically exclude the birds, and this is where the dark patches come in.
Botha and Pauw have constructed a netted cage 20 metres square so that birds are totally excluded from a surface area of 400m2 of fynbos, at each of six carefully selected sites around the valley dominated by bird-pollinated plants.
The mesh of the net had to be fine enough to keep out small juvenile birds, like the endemic Orange-breasted Sunbirds, but big enough to allow insects through. It also had to be UV stable to deal with the long exposure to the sun – the experiment will be running for three years – and strong enough to keep baboons out, Botha explains. It was a delicate balance to achieve, but after “quite a search” they eventually settled on very strong fishing nets with a 2cm x 2cm mesh.
Then, to ensure that the experiment wasn’t contaminated by the shading effect of this netting on the plants, they also set up a controlled shaded area at each site that was open on all sides to allow birds in. These are slightly smaller, 10m x 7m, so that birds can fly freely right through and not feel intimidated in the centre area.
Botha started his masters at the beginning of last year and the cages were constructed last August after a lot of hard preparatory work.
“It all took some time – getting the permissions, designing the experiment, finding the contractors and so on,” he remembers.
Fortunately Stellenbosch is a major hub of the viticulture industry, meaning there are contractors available who are very experienced at putting trellises in vineyards.
“We worked closely with these contractors and the design of the cages was done with them – they have lots of experience in putting in poles and getting wires taut and so on. And it was relatively challenging. The material is heavy and bulky and it all had to be carried up to the sites that are rocky.”
Three of the bird exclusion cages and their complementary shade-exclusion “roofs” – everything will be removed at the end of the experiment – were constructed on the warmer north-facing slopes where the dominant bird-pollinated plant species are three Proteas: the waboom or wagon tree (P.nitida), the narrow-leaf sugarbush or blackbeard sugarbush (P.neriifolia) and the common sugarbush (P.repens).
On the cooler south-facing slopes of the valley, the green sugarbush (P.coronata), common pagoda or rooistompie (Mimetes cucullatus) and the needle-leaf pincushion (Leucospermum lineare) are in the majority.
“These are all quite dominant bird-pollinated plants, and there are also other bird-pollinated species like the ericas and the irises,” Pauw says.
The major bird pollinators involved are the Orange-breasted Sunbird, the spectacular Malachite Sunbird, and the Cape Sugarbird that is also endemic to fynbos. There are lots of nests in the area – sunbirds and sugarbirds, and a few nests have been established right next to the cages, so the birds have clearly accepted their slightly altered landscape, says Botha.
He gathers data from the sites by standing at each one observing for 20 minutes at a time, five or six times a day.
“I just stand and look at the birds, and I’ve learnt now to stand very still for a long time!”
But it also means he has a chance to observe other elements hikers and mountain bikers might miss – “It’s been great, I’ve seen lots of things, klipspringers. When you spend a lot of time in the field, you see lots of things you don’t normally see.”
And it’s not a hardship for him. Now 25, he grew up in Stellenbosch and went to the local school, Paul Roos Gymnasium, before starting at the university where he did a BSc degree followed by an honours specialising in biodiversity and ecology.
“I grew up hiking and mountain biking here in Jonkershoek, spending time in the mountains – I love this valley,” he says.
He has two field assistants who monitor another cage at exactly the same time as he does so he can compare results and avoid biases of weather impacts or different times of the day. They also monitor insects at the sites, looking at the same plant species, and are also doing seed counts.
Last month, Pauw attended a United Nations Environment Programme meeting in Bonn, Germany where the focus was on pollination.
“It’s a big issue and especially important from an agricultural perspective – is there a global decline in pollinators?” he says.
“But it’s even more important in fynbos because of fires every 15 years or so. Fynbos plants have these very short life cycles (compared to plants in other ecosystems) and then they have to start all over again from seed, so they need quick and efficient pollinators. And it’s very important to know how significant birds are in this process, because we do lose them to things like fires, pesticides and habitat loss.”
There could be “cascading effects” of such losses, particularly because birds have a dual role as pollinators and also as predators of insects – they feed on a lot of insects, particularly when they’re raising chicks, Pauw explains.
“If the lack of birds caused a meaningful reduction in seed production (because of less pollination), we expect to see that the new veld will lack bird-pollinated plants and will instead be dominated by insect and wind-pollinated plants, including invasive aliens.
“So all-in-all, we hope to be able to give a definitive answer to the ecological question of whether birds really matter.”