• Climate change appears to be disrupting the yield of fruit trees, a critical food source for many large mammals in Central Africa.
• A new study warns that endangered forest elephants and other keystone species in Lopé National Park in central Gabon — such as western lowland gorillas, chimpanzees, and mandrills — could be facing famine.
• “The changes are drastic,” says Emma Bush, co-lead author of the study. “The massive collapse in fruiting may be due to missing the environmental cue to bear fruit.”
• Some tropical trees depend on a drop in temperature to trigger flowering, but since the 1980s, the region recorded less rainfall and a temperature increase of 1°C.

The rainforest of Lopé National Park in central Gabon is one of the last safe havens for the endangered forest elephant (Loxodonta cyclotis). But in a new study in Science, researchers warn that elephants and other keystone species in the park, such as western lowland gorillas, chimpanzees, and mandrills, could be facing famine.

Climate change is disrupting the yield of fruit trees, a critical food source for many larger mammals. Scientists from Gabon’s national parks agency (ANPN) and the University of Stirling in Scotland, found a staggering 81% decline in fruit production in the protected area between 1986 and 2018.

Their analysis of an extensive photographic database covering a twenty-year period showed a critical decline in weight and body fat of forest elephants. An 11% decline in the body condition of forest elephants since 2008 is primarily attributed to decreased fruit consumption.

The researchers draw on a rare longitudinal data set funded by the EU’s ECOFAC-6 programme and collected by the ANPN. Every month for 34 years, botanists observed the crowns of more than 2,000 tropical trees and scanned the canopy for flowers, leaves, and both ripe and unripe fruits of 73 species important to the diet of elephants, gorillas and chimpanzees.

“The changes are drastic,” says Emma Bush, co-lead author of the study. “The massive collapse in fruiting may be due to missing the environmental cue to bear fruit.” Some tropical trees depend on a drop in temperature to trigger flowering, but since the 1980s, the region recorded less rainfall and a temperature increase of 1°C.

“Less fruit in the ecosystem will have huge impacts on rainforest dynamics such as seed dispersal, plant reproduction and food availability for wildlife,” says the conservation scientist. Where 30 years ago, an elephant would have found ripe fruits on almost one in every ten trees, today it needs to search more than 50 trees.

“Central Gabon’s forests are undergoing rapid, ecosystem-wide change,” says Aurélie Flore Koumba Pambo, scientific advisor to the National Park Authority. Over time, she says, reduced fruiting will impact on the forest structure and tree diversity, and this could further affect local and global climate.

Forests can’t flourish without fruit

Fruit scarcity may also have far-reaching consequences for conservation, says Chris Thouless, Save the Elephants’ Head of Research and Director of the Elephant Crisis Fund. The seasonal fruiting period could be critical for elephant reproduction: “If they are unable to put on enough weight during the fruiting season, they may not come into estrus and thus lose the opportunity to become pregnant,” he explains.

Forest elephants have an extraordinary low birth rate: they don’t reach sexual maturity until the age of 23 and only give birth every 5 or 6 years. “If they are getting poorer quality food, then the population’s ability to recover from depleted numbers will be compromised because of the reduced population growth rate, even with good protection,” says Thouless.

Since the 1970s, forest elephant populations across Central Africa have substantially declined both in absolute numbers and in range. Between 2010 and 2012, 40,000 elephants died: killed for their large tusks with the distinctive ‘pink ivory’ hue, the smaller and genetically distinct relative of the savanna elephant is heading towards extinction. Whilst overall, African elephants are listed as ‘vulnerable’ on the IUCN Red List, Central Africa’s forest elephants have been classified ‘endangered’.

In Gabon, large tracts of forests have historically led to relatively lower levels of poaching, making the Central African country a unique refuge for forest elephants in the region. An estimated 50,000 individuals roam its woodlands – that is 60% of all remaining forest elephants in the world. Now climate change poses an additional threat.

“If important protected areas like Lopé National Park in Gabon can no longer support them because there is not enough food, then we may see further population declines. This jeopardises their survival in the long-term,” says Robin Whytock, an ecologist at the University of Stirling and co-lead author of the study on fruit scarcity.

Megaherbivores such as elephants are considered critical species in the rainforest ecology, explains Whytock. “We know that large-bodied animals, like elephants, are disproportionately important for the healthy functioning of ecosystems. Their loss could result in broad changes to forest systems and even reduce the amount of carbon stored there.”

The ‘megagardeners of the forest’ significantly shape and maintain the composition and structure of the Congo Basin’s rainforest, the planet’s second-largest after the Amazon.

Elephants play a pivotal role in seed dispersal of tropical fruit and canopy tree species within Central African forests – they can travel more than 50 km before depositing seeds in nutritious dung heaps, covering farther distances than any other animal.

Many plant species, including wild mango and spiky breadfruit, are not able to survive without the mammal, and the composition and genetic diversity of forest areas are likely to change as the animals disappear. This loss will have a devastating impact on other animals such as monkeys, birds and insects – many of them frugivores and fully fruit-dependent for their survival.

“With the relationship between species disturbed,” says Bush, “we are going to lose the unique complexity of this ecosystem.” In areas like West Africa, where no elephants spread their seeds, the breadfruit tree has already become functionally extinct: “It is just standing there waiting to die out because the elephants have gone,” she explains.

The steep decline in megafauna in tropical forest ecosystems can bring about unforeseen long-term consequences, agrees Fabio Berzaghi, who was not involved in the study. The ecologist at the Laboratory of Climate and Environmental Sciences in France has researched the effects of elephants’ ecological engineering in rainforests.

Elephants weed out the understory of vegetation and by doing so, alter light and water availability for other trees and plants. They also prefer to eat fast-growing softwood saplings, making room for slow-growing hardwood trees. These tall and dense trees can capture much more carbon, says Berzaghi. He estimates that if forest elephants were to disappear from the Congo Basin, it would result in a three billion ton loss of carbon storage into the atmosphere.

System-wide changes needed

As our planet plunges into its sixth mass extinction, biodiversity is declining at an unprecedented rate. Bush is working with a network of African phenologists and other scientists across the Congo Basin, to further explore the impact of climate change on tropical trees and the correlation between plant extinction and animal starvation.

To protect the remaining megafauna and flora of the African tropics and their unique role in our biosphere, coordinated international efforts to relieve direct human pressures and to halt and reverse climate change will be critical, write Bush and her co-authors in Science.

“In the longer term, we are concerned about habitat change and transformation by human settlements. The increase in palm oil plantations is a particular concern,” says Thouless. Industrial cultivation of palm provides a critical livelihood to some communities but threatens Gabon’s forest, which covers over 80% of the country.

With humans encroaching on their natural habitat and fruit becoming more scarce, the ‘mega gardeners’ of the tropics may no longer be in a position to repopulate protected forest areas in Gabon, says Thouless. He reiterates that poaching remains the most urgent threat.

“We have been partially successful against poaching, but we haven’t won that war yet,” says Professor Lee White, Gabon’s Minister of Water, Forests, Seas and the Environment. “Poaching at the border with Cameroon has been our biggest problem for a long time.”

Forest elephants are squeezed further towards the centre of Gabon and are not inhabiting their actual potential range, confirms Whytock. “If we reduce the hunting pressure and elephants can have their space back, it would give some short term reprieve.” It may enable elephants to migrate out of hunger zones, he says. “We must protect the ecosystem because, without the forest, there will be no more elephants.”

“These trends appear to be driven by global climate drivers,” says Pambo from ANPN, “and only international agreement can reduce carbon emissions. For Gabon, this warns us that we need to prepare for system-wide change in response to the global climate crisis.”

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Citation

Bush, E. R., Whytock, R. C., Bahaa-el-din, L., Bourgeois, S., Bunnefeld, N., Cardoso, A. W., … Abernethy, K. (2020). Long-term collapse in fruit availability threatens central African forest megafauna. Science, eabc7791. doi:10.1126/science.abc7791