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South Africa, Our Burning Planet

How citizen science is uncovering animal adaptations to climate change

How citizen science is uncovering animal adaptations to climate change
Holes made by African black oystercatchrs foraging for food on the sand on top of the rocky intertidal zone, Goukamma, March 2023. (Photo: Mark Dixon)
Julia Evans
By Julia Evans
28 Jul 2024
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Recent annual sand inundations at Goukamma and Gericke’s Point on the Cape south coast have smothered mussel beds, which is what oystercatchers target as their primary food source.

While walking along the beach near his home, Mark Dixon, a passionate citizen scientist from Sedgefield on the Western Cape Garden Route, counted 84 African black oystercatchers along a 7.5km strip of coastline in the Goukamma Marine Protected Area.

This was a striking contrast to the 12 birds he observed in a similar stretch at Sandbaai, near Hermanus. What explained this dramatic difference?

Speaking at the Plett Marine Science Symposium last month, Dixon shared his observations and research as a citizen scientist into the African black oystercatchers.

animal adaptations climate change oystercatchers African black oystercatchers are endemic to southern Africa and are found along the coastlines of South Africa and Namibia. (Photo: Mark Dixon)



Dixon founded a volunteer citizen science organisation, the Strandloper Project, in 2018 – originally wanting to look at the negative impact of fishing and plastic on birds. Over five years, Dixon and his team have hiked nearly 1,000km of the southern Cape coastline, documenting 1,801 oystercatcher individuals, and has been able to observe what factors have an impact on the population densities of this species in the region.

For oystercatchers, which are endemic to southern Africa (found along the coastlines of South Africa and Namibia) and can live for more than 29 years, the citizen scientists found that oystercatcher populations fluctuate between one pair per 2,000 metres to one pair every 135m. The factors that influence this density variation include food type, aspects of the intertidal substrate and the geology in the intertidal zone.

 

For example, in Goukamma’s rocky intertidal shelves, mussels are more abundant due to the aspect of the intertidal shelf – and as oystercatchers prey on mussels, their populations are more dense in these areas.

In sandy sections, such as Kleinkrans, the density is lower, with only two birds per 2km, while the Gamtoos River Dune Fields support a massive 124 birds over 18km due to higher densities of prey species.

Dixon’s team uses the CyberTracker app to record geo-referenced data on oystercatchers, noting their numbers, activities, and age. This data has provided invaluable insights into how climate change is altering the foraging behaviour of these birds, revealing a story of resilience and adaptation.

As a former Convention for the Conservation of Antarctic Marine Living Resources scientific observer, on long-liner fishing vessels in Antarctica, he has seen first-hand how a changing climate has knock-on effects on ecosystems. 

A changing environment


While rocky intertidal shelves are usually the best place for oystercatchers to look for food, Dixon has noted that for the past three seasons, those intertidal shelves have been covered in sand during the summer seasons. These annual sand inundations at Goukamma and Gericke’s Point have smothered mussel beds, which is what oystercatchers target as their primary food source.

 

The Intergovernmental Panel on Climate Change, which asseses thousands of peer-reviewed studies, found that “sea level rise, storminess, wave energy and weathering regimes will continue to erode coastal shorelines and affect the soil accretion [increase] and land-based ecosystems.”

African black oystercatchers target mussels (brown mussels, Mediterranean mussels, sand mussels) as well as limpets, polychaetes, and crustaceans – which they usually find in the intertidal zone, the rocky shelf that appears during low tide. 

But with the sand covering the intertidal shelf, the birds are forced to look for food elsewhere, going to the sandy shores to forage for the less abundant white mussels. With not enough to sustain themselves, has led to low recruitment and high chick mortality.

animal adaptations climate change oystercatchers African black oystercatchers have had to move to sandy beaches to forage for white mussels and polychaetas (blood worms), now that their typical foraging site is covered in sand. Sandy beach in Goukamma, May 2024. (Photo: Mark Dixon)



animal adaptations climate change oystercatchers Holes made by African black oystercatchers foraging for food on the sand on top of the rocky intertidal zone, Goukamma, March 2023. (Photo: Mark Dixon)



“They start pecking away in the sand, looking for sand mussels and other invertebrates,” said Dixon during his talk. “And this unfortunately has happened in December, January, and February – which is their prime time for breeding and chick rearing. So there’s not enough nutrition for the adults to get for their chicks.”

How oystercatchers are trying to adapt


Despite these challenges, oystercatchers have demonstrated remarkable adaptability. In the 2023/2024 summer season, Dixon observed a pair of oystercatchers in Goukamma MPA using sound to locate prey buried under the sand, a behaviour akin to echolocation. This adaptation highlights their resilience in the face of rapidly changing environmental conditions.

Dixon explained watching an oystercatcher on top of a mussel bed inundated with sand, saying, “I thought it was looking for white mussels or trying to get something, and it stopped, and it cocked its head and it listened,” said Dixon.

“And then it just spun around, stabbed its beak into the sand and pulled out about a 20cm worm.”

That worm, a polychaete, typically lives in the mussel bed, but caught under the covering layer of sand, was trying to move out of it.

“As it was going through the sand, that movement was enough for it to hear and target its prey accurately. It wasn’t stabbing a couple of times, it was one stab and up came the worm, [it] flew off and went and ate elsewhere,” said Dixon, who said he observed this behaviour a few times, on the inundated mussel bed on the rocky intertidal zone, and on the sandy beaches were oystercatchers were trying to forage.

However, human interference also remains a significant threat. Beachgoers and fishermen disturb nesting sites, leading to clutch failures and chick mortality.

What’s also making it difficult for the birds is that their prey is affected by climate change.

The IPCC reported with high confidence that “changes in ocean temperature will also indirectly impact marine mammals, seabirds and reptiles by changing the abundance and distribution of their prey”.

This impact on prey availability compounds the birds’ struggle for survival. 

The Strandloper Project is beginning its second series of expeditions in October which will give an insight into whether there has been any change in the population over the past five years. DM

This article was factually updated on 29 July at 4.40pm.

https://www.youtube.com/watch?v=REeWvTRUpMk

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