Southern African caterpillar that feeds millions may be next climate casualty

Climate change will severely reduce the distribution of a caterpillar that’s harvested, sold and eaten by millions of people across Southern Africa, with some regions projected to lose nearly 100% of suitable habitat for the insects even under a moderate climate change scenario, a new study suggests.

The mopane worm (Gonimbrasia belina) feeds mostly on the butterfly-shaped leaves of the tree of the same name, Colophospermum mopane, a deciduous species that grows in mixed woodland or forms vast pure stands. But rising temperatures will severely restrict the places where mopane trees can grow, and, consequently, where the mopane worms can flourish, according to the study published in Frontiers of Biogeography.

The scientists behind the work devised species distribution models to predict the future range of the caterpillars in Botswana, Zimbabwe and South Africa, where mopane worms are most abundant, under two different climate change scenarios.

Even under the less-extreme scenario, known as RCP4.5, in which carbon emissions peak in 2040 and then decline, suitable habitat of the caterpillars will shrink overall by 56% within the next 60 years, the study says. This represents a decline from the current 649,000 square kilometers to 285,000 km2 (251,000-110,000 square miles).

The biggest impact will be felt in Botswana and southern Zimbabwe, where the researchers predict almost all suitable caterpillar habitat will be lost by the 2080s. In Botswana, the impact will be especially pronounced, with 99% of the mopane worm’s current habitat forecast to be lost.

“When we predict that there are going to be big contractions in distributions of the mopane worm, it’s likely to do with the fact that the trees are no longer present where they should be, or maybe changing climate or changing distributions of predatory birds means there is much higher predatory pressure [on the caterpillars],” says study lead author David Shen, an ecologist and spatial scientist at University College London.

The researchers incorporated “biotic factors” into their models, such as increased predation of the caterpillars by birds, and loss of their mopane leaf forage to competitors like elephants, to gain a more accurate picture of the future. “They might struggle to survive with more predator pressure, or maybe that the climate has changed so much that it’s not suitable for the mopane worm.”

Loss of a vital protein source

For centuries, the speckled black, red, white and yellow mopane worms, which are the caterpillars of the short-lived, slow-flying emperor moth, have been harvested from the wild by local communities. They do this at the height of the Southern African summer — in December and January — and again at the end of the summer rains in April.

That is also the region’s lean season, a time when the previous year’s harvest of staple maize grain is depleted, and the new season’s harvest is not yet in. At this time, the mopane worms provide an important food supplement.

Studies  have shown, for instance, that 80% of households in Limpopo, South Africa’s northernmost province and the only place in the country where mopane trees grow, eat the caterpillars three to five times per week during the lean season. A handful of dried mopane worms can provide a person with 76% of their daily protein needs, further studies  show.

The loss of suitable habitat for the caterpillars in a hotter world would severely impact these food provisioning services, the latest study warns. It will leave on-the-brink communities without an alternative food source when crops fail, something expected to become more frequent under climate change.

“It’s a very scary situation,” says study co-author Caswell Munyai, a senior lecturer in invertebrate biology at South Africa’s University of KwaZulu-Natal (UKZN).

Munyai was in Limpopo province in April to conduct research on the caterpillars. He met traders from Botswana bringing their late summer harvest of dried mopane worms by the sackful for sale to South African buyers. Traders from Zimbabwe, one of South Africa’s northern neighbors, also bring their harvests to sell in Limpopo. The loss of the mopane worm would have far-reaching consequences for this cross-border industry, Munyai says.

“Should they decline, it will mean more and more challenges for the whole community.”

It could also lead to negative impacts on other species. With few insect alternatives deemed palatable by communities in the region, “what will people do for protein?” Shen says. “Will they increase bushmeat hunting? That would equally have knock-on effects.”

Shen and his team carried out their study as part of a global collaboration known as the Sustainable and Healthy Food Systems (SHEFS) project supported by the Wellcome Trust’s Our Planet, Our Health program.

Trees move just once a generation

There are already signs that mopane trees, which are adapted to grow in fairly hot, arid regions, won’t be able to adapt quickly enough to the extreme pressures exerted by human-induced climate change.

A study published in 2007 on quiver trees (Aloidendron dichotomum) in Namibia showed that the distribution of these giant, desert-adapted succulents was shrinking as average temperatures rose and rainfall decreased.

The scientists behind that study revealed that the population of quiver trees at the trailing edge of their distribution, where critical climate limits had likely been exceeded, were declining while those at the leading edge were not expanding into new territory.

The distribution of mopane trees could shrink in a similar way.

The inability of many long-lived plants to move into newly available habitat to compensate for losses elsewhere is definitely a realistic concern, says Richard Corlett, emeritus professor at the Xishuangbanna Tropical Botanical Garden, of the Chinese Academy of Sciences, who was not involved with the mopane worm study.

“Trees move once a generation and many species don’t move far,” he says.

While the fossil record suggests that plants adjusted to climate changes in the past by moving, projected rates of climate change over the next few decades are higher than any known to date.

“For most woody plants it is safest to assume little or no habitat gain in the next several decades,” Corlett says. “They will survive, or not, where they are now.”

And where they are now, mopane trees and the worms that feed on them are vulnerable to more immediate threats, including deforestation and overharvesting by humans. These two human pressures weren’t included in the species distribution models by Shen and his colleagues, who limited their assessment to non-human interactions with the trees and the caterpillars.

“Deforestation is probably the biggest driver, and the second-biggest driver of the loss of mopane worms is just overharvesting,” Shen says. “There’s a lot in the literature about where the caterpillars have been overharvested, their population has declined by around 90%.”

In the communal areas of Limpopo province, beyond the protection of national parks and game reserves, the trees are being overharvested for the excellent quality of their fuelwood, says UKZN’s Munyai.

He says that browsing by large hungry mammals does have a major impact on mopane trees — many of which occur in protected areas such as Kruger National Park, where elephants are abundant — but that this pales in comparison to the impact of humans on trees growing outside the reserves.

Commercial harvesting of mopane timber for fuelwood is “big business,” he tells Mongabay. “It will hammer the population of the tree much more seriously than what elephants and other browsers can do.”

Munyai’s own family roots are in Limpopo. He says overharvesting of the trees there runs counter to local cultural norms, which state that only mopane trees larger than a certain diameter may be felled in order to give successor trees the chance to reach maturity.

“In deep rural communities no one polices anyone, but they know exactly what not to do, and how to sustainably work with the system,” he says. “It’s only when commercial harvesting comes in that it becomes devastating.”

Whether the extirpation of mopane trees and their caterpillars comes first from deforestation, overharvesting or climate change, the authors of the study say their projections on the climate-related impacts should be seen for what they are: a call to protect the trees to bolster regional food security and save biodiversity.

“There’s a whole cascade of bad things that come with climate change and biodiversity loss,” Shen says. “This is both of them.”

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Banner image: The mopane worm (Gonimbrasia belina) feeds mostly on the butterfly-shaped leaves of the tree of the same name, Colophospermum mopane. Image by SAplants via Wikimedia Commons (CC BY-SA 4.0).

Citations:

Shen, D. Y., Ferguson-Gow, H., Groner, V., Munyai, T. C., Slotow, R., & Pearson, R. (2023). Potential decline in the distribution and food provisioning services of the mopane worm (Gonimbrasia belina) in Southern Africa. Frontiers of Biogeography, 15(2). doi:10.21425/f5fbg59408

Baiyegunhi, L. J., Oppong, B. B., & Senyolo, G. M. (2016). Mopane worm (Imbrasia belina) and rural household food security in Limpopo province, South Africa. Food Security, 8(1), 153-165. doi:10.1007/s12571-015-0536-8

Foden, W., Midgley, G. F., Hughes, G., Bond, W. J., Thuiller, W., Hoffman, M. T., … Hannah, L. (2007). A changing climate is eroding the geographical range of the Namib Desert tree Aloe through population declines and dispersal lags. Diversity and Distributions, 13(5), 645-653. doi:10.1111/j.1472-4642.2007.00391.x

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