If scientists want to understand how ecosystems will respond to a changing climate, they might need to look beyond how hot and wet the future becomes and ask how windy it will be too.
Research by scientists at the University of Pretoria on a remote island in the Southern Ocean suggests that wind could be an overlooked force shaping plants, ecosystems and even seabirds.
Marion Island lies in the latitudes known as the “Roaring Forties” and is one of the windiest environments in the world. Gale-force winds are part of daily life, making the roughly 290km² sub-Antarctic island an extraordinary natural laboratory for understanding how persistent wind shapes an ecosystem – from where plants grow to how birds fly.
“Temperature and rainfall are fundamental to understanding climate change but they are not the whole climate system,” said professor Peter Le Roux of the university’s department of plant and soil sciences. “Our work on Marion Island is showing that wind can influence ecological patterns at a remarkably fine scale.”
Le Roux said that understanding how wind regimes changed was important for determining what happens to organisms living at the limits of what they could tolerate.
“Our work shows that wind conditions really matter for plants and animals in the sub-Antarctic. For example, two sites that are close together but have different wind exposure can have very different vegetation, both in terms of which types of plants occur at the site and in terms of how those plants grow.
“This impact of spatial variation in wind conditions suggests that temporal variation will also matter. In other words, if two sites that differ in wind exposure have different vegetation characteristics now.
“This provides a basis for forecasting how changes in wind speed and turbulence at one site through time will affect the ecosystem there — and this is the basis for our prediction that shifts in wind patterns driven by global climate change will have substantial consequences for sub-Antarctic ecosystems.
Refining climate forecasts
This means that some of the climate change impact forecasts might need refining, especially in areas where there could be the largest changes in wind conditions in the future.
Le Roux pointed to earlier research by Timm Hoffman and colleagues at the University of Cape Town as an example of why wind could be an overlooked climate variable.
Their 2011 study found that although temperatures in the Western Cape had risen over three decades, which would normally be expected to increase evaporation, evaporation had declined. The researchers linked the counterintuitive finding partly to a reduction in wind speeds, he said.
The finding matters because moisture availability is critical for ecosystems, agriculture and fire conditions. Without considering wind, an important part of the picture can be missed.
Marion Island’s relatively limited direct human disturbance makes it particularly useful for isolating the ecological effects of wind. The team’s research has linked wind conditions to the distribution of plant species and vegetation types, where wandering albatrosses choose to nest and locations where grey-headed albatrosses crash.
Le Roux cautioned against directly applying findings from Marion Island to mainland South Africa. He said the research demonstrated that changing wind conditions could not be ignored when assessing how ecosystems responded to climate change.
He pointed out that average and minimum wind conditions on Marion Island had increased over several decades, while other sub-Antarctic islands had also experienced shifts in wind conditions.
Globally, wind speeds declined through much of the late 20th century before beginning to increase again around 2010.
Wind direction was changing too. Although the sub-Antarctic was characterised by strong, predominantly westerly winds, Marion Island had experienced a shift in its dominant wind direction, with average winds moving northwards.
“We’re living in a world of changing wind conditions and should make sure that we have a better idea of what these changes will mean for the ecosystems that support us,” he said.
Seeing the wind in fine detail
To understand the changes at an ecological scale, researchers combined field measurements with engineering models.
The team installed wind-logging stations across Marion Island, while mechanical and aeronautical engineering master’s student Kyle Goddard used computational fluid dynamics to simulate wind flowing across the entire island.
The model simulated wind from 16 different directions, from 0.2m above the surface to 800m high. Crucially for ecologists, it produced estimates of wind speed and direction at a horizontal resolution of about 30m.
“Engineering allows us to make the invisible visible,” said professor Ken Craig of the mechanical and aeronautical engineering department. “We cannot put a weather station on every few metres of an island. Computational fluid dynamics allows us to investigate how the terrain shapes wind across that landscape.
“What makes this project particularly powerful is that those engineering insights can then be connected to biological observations on the ground.”
The connection has revealed effects that would be difficult to see from conventional weather records.
Research by Dr Mia Momberg showed that wind strongly affects fine-scale vegetation patterns on Marion Island, with wind speed having a greater influence than temperature in some cases. Wind variation was also associated with the distribution of plant communities and where wandering albatrosses, the world’s largest seabirds, choose to nest.
Dr Janine Schoombie investigated how airflow around breeding areas affects grey-headed albatrosses and whether wind conditions contribute to the mortality of breeding adults.
“By modelling the airflow around breeding areas, we can start asking much more precise questions about where flying becomes energetically demanding or potentially dangerous,” Schoombie said. “That gives us a different way of thinking about how environmental change could affect these birds.”
The same wind dataset is also helping researchers investigate seed dispersal.
Master’s student Nompilo Mazibuko combined the wind simulations with a mechanistic dispersal model and information about seed characteristics to examine how alien and indigenous plants spread across Marion Island.
Despite the island’s famously strong winds, the research showed that the low-growing nature of its sub-Antarctic vegetation generally restricts windblown seeds to relatively short distances. Living in an exceptionally windy environment, in other words, does not necessarily mean seeds travel exceptionally far.
Research by master’s student Sinethemba Msibi is probing whether Marion Island’s strongly directional winds influence the growth and mortality of one of its most widespread plants. Preliminary findings suggest that wind patterns can drive uneven growth and die-back and may even alter how the species interacts with neighbouring plants.
Honours student Nicola Marneweck approached the question at an even smaller scale, comparing vegetation on the exposed and sheltered sides of natural windbreaks such as large boulders.
Plants in more exposed locations were significantly shorter, although many other characteristics showed surprisingly limited responses to increased wind exposure.
Together, the studies show why knowing the average wind speed recorded at a weather station might not be enough to understand what an organism experiences. A ridge, slope, cliff or boulder can radically alter airflow over a relatively short distance. The local differences can matter to a nesting bird, low-growing plant or wind-carried seed.
For Le Roux, one of the biggest surprises was that wind conditions appeared to be more important than soil temperature or moisture in determining where different plant species occur on Marion Island.
He had expected wind to influence vegetation but was surprised by the strength of the relationship. The finding suggests that future changes in wind patterns could significantly alter vegetation and the ecosystem processes associated with it.
Another unexpected finding came from Schoombie’s research into grey-headed albatrosses, revealing how local wind conditions seem to cause a hotspot for fatal greyheaded albatross collisions on Marion Island.
“These are birds that are masters of flight, spending long periods out at sea in incredibly windy conditions but under certain wind conditions they appear to be particularly vulnerable to crashing into the ground when visiting their nests on the island.
“This doesn’t seem to be related to extreme winds but rather linked to an area of more variable local windflow where birds may unexpectedly experience a sudden loss of lift.
“To me this highlights the importance of understanding windflow patterns at biologically-relevant scales – this is something that was initially entirely invisible to us, but which research has now been able to describe and explain.”
Le Roux said that illustrated why wind needed to be understood at the scale experienced by individual organisms. Patterns that were previously invisible could have significant biological consequences.
“Climate change projections ultimately need to tell us something about the conditions organisms actually experience,” he added. “This work shows what becomes possible when ecologists and engineers approach the same environmental problem from different directions.”
The team has also made its wind observations and island-scale simulations available to other researchers, allowing them to use the data to investigate new questions and inform planning and logistics on the neighbouring Prince Edward Islands.