As animal seed dispersers go the way of the dodo, forest plants are at risk
As animal seed dispersers go the way of the dodo, forest plants are at risk
Sharon Guynup, Mongabay
April 28, 2022
See link
https://news.mongabay.com/2022/04/as-animal-seed-dispersers-go-the-way-of-the-dodo-forest-plants-are-at-risk/
for photos, video & illustration.
The few remaining rainforests on Mauritius whisper with the memory of
former residents, now ghosts.
This once-lush Indian Ocean island remained mostly uninhabited by humans
until 1598, when it became a Dutch port of call. By the time the French
government took over some 150 years later and imported slaves to start
sugar plantations, the ecological damage was already irreparable. The dodo
disappeared, and in the years since would become an icon for extinction.
Meanwhile, as jungles fell and humans and domestic animals streamed in,
tortoises, parrots, pigeons, fruit bats and giant lizards also dwindled and
vanished.
Other losses on the island were harder to notice: trees and other plants
that relied on those vanished animal species to spread their nuts and
seeds. Changes in the plant kingdom are not easily perceived because “we’re
just not seeing the trees within the forest,” says Tammy Mildenstein, a
wildlife biologist at Iowa’s Cornell College. But the absence of flora also
reverberates, further reducing fauna that rely on those plants for food and
homes.
In some woodlands, jungles, marshes and grasslands, “vegetation is
relatively lush, but you hear only silence,” says Evan Fricke, an ecologist
at Rice University. It’s a condition sometimes called empty forest
syndrome. “If it doesn’t have birds and mammals, what happens to those
forests?” Fricke asks.
In a recent study, his team reported that seed-disbursing creatures have
“steeply declined” globally. Their findings wave a red flag, highlighting
growing concern about the ability of plant communities to reproduce and
survive long-term in a changing climate, on altered landscapes, with
diminished wildlife.
Now, researchers are delving into the many unstudied mysteries of
plant-animal relationships, to unravel the ways that flora and fauna
mutually support each other. Conservationists are also beginning to
incorporate seed dispersers into rewilding and forest regeneration projects
to restore healthy ecosystems worldwide.
Spreading Seeds
Some seeds, like dandelions and maples, fly on the breeze. Some fruits
explode, forcibly ejecting seeds. But many plants and trees evolved
relationships with wildlife that feed on their fruits, berries, seeds or
gather their nuts — and help them reproduce.
It’s an effective strategy that’s been adopted by up to 90% of tree species
in the tropics and half of those in temperate zones, says Haldre Rogers, a
community ecologist at Iowa State University.
There are many, many animals that disperse seeds: birds, bats, rodents,
primates, insects, and an army of small and large mammals. With some 73,000
known tree species and perhaps 435,000 plant species overall, wildlife has
a lot of work to do.
Animals provide seeds with the legs or wings they need to survive, Rogers
says. Plants’ future progeny travel inside mouths, beaks and stomachs;
hitchhike on legs and fur; and are carried off, dropped, regurgitated or
excreted some distance away from the parent plant. In new territory, there
are better odds that the seeds won’t sprout in big, competitive clumps, be
deprived of light, infected by pathogens or eaten by seed predators.
These natural systems are well designed: Digestion, for example, offers
power-washing services, with acids stripping away pulp and neutralizing
compounds that attract fungus and pathogens, a process that may improve
germination. Animals then defecate seeds, depositing them within their own
dung fertilizer packet.
But human activities are disrupting these systems: Hunting, logging,
expanding agriculture, development, and now, climate change, are driving
animal extinctions, with many species now in a precipitous worldwide
decline. Wildlife losses ripple throughout ecosystems that have evolved in
synchrony over millennia. When animals no longer distribute seeds, plant
communities change and some species disappear. Then, with fewer fruiting
trees and bushes, animal species from insects to elephants go hungry,
sparking the next wave of declines or local extinctions.
This “trophic cascade,” or domino effect, is easily triggered in tropical
forests, which are home to four-fifths of the world’s biodiversity, Rogers
says. Ecuador’s Yasuní National Park offers an example, with 670 tree
species found in just a single hectare (2.5 acres) of old-growth rainforest.
Rare plants living in small or specialized habitats face the highest risk
when they lose their seed dispersers. Depending on the species, there may
be a substantial time lag before they disappear entirely, since some trees
live for hundreds of years — long after the demise of the animals that
helped them reproduce.
Naturalist John Muir described the process well. “When we try to pick out
anything by itself, we find it hitched to everything else in the Universe.”
When one domino falls, it takes down another, and another.
From Bats and Birds to Lemurs and Elephants
There’s a long history of species losses sparked by humans — losses we can
learn from. The giant lemur was an ancient casualty. This Madagascar
primate went extinct some 2,000 years ago after people arrived on that
African island. The giant lemur once transported seeds in its belly and
helped keep endemic forests healthy.
Now, amid Earth’s sixth mass extinction, there are many recent declines,
including elephants poached for their ivory in Africa; birds decimated by
the invasive brown tree snake in Guam; tapirs, monkeys and other mammals
hunted out of Brazil’s Atlantic Forest; and the many species gone from
Canada’s boreal forests, leveled for biomass energy, tar sands oil and sand
mining.
Globally, thousands of species help keep flora alive. A single fleshy fruit
may be a staple food for a particular animal. Other species have broad
palates. Any bird that eats fruit plays a role: they are the workhorses of
the seed-dispersing world. Bats may play a similar role, “bigger than I
ever would have guessed,” says Mildenstein. She and colleagues tallied
1,072 plant species eaten by 75 species of fruit bat, an animal that
reseeds a huge swath of the world.
Some seeds hitch rides with many creatures. But certain plants, such as
Asia’s strangler fig, coevolved with a specific animal that may be crucial
for its long-term survival. Mildenstein explained that birds, monkeys, wild
pigs and others gorge on the figs, but it’s the flying foxes that best
propagate this tree. The foxes soar high above the canopy, defecating seeds
that fall onto top branches. Shadowed on the ground in dense tropical
undergrowth, fig seeds fail, but in full sunlight, the stranglers sprout
and extend their braided roots to the forest floor. Figs are an important
community member, available year-round to feed animals when little other
food is available.
For some plants, the distance carried is critical. Large bats travel long
distances, up to 88 kilometers (55 miles) in a night, or hundreds when
migrating. Hornbills — the so-called “farmers of the forest” in Africa and
Asia — move more than 700 plant species up to 11 km (7 mi) in a day.
Chimpanzees spread huge amounts of seed across miles in Senegal’s savannas
and woodlands.
“But the king of all dispersers is the elephant,” says John Poulson, a
tropical ecologist at Duke University’s Nicholas School of the Environment.
At 3.5 tons, they’re the largest fruit-eating mammal in Central African
forests, carrying vast numbers and types of seeds in their gut for up to
100 km (60 mi). They also ingest fruit too large for other animals to
swallow.
But poaching has made it difficult for fruit-bearing trees and bushes to
survive in places like Gabon’s Minkébé National Park, where more than
25,000 forest elephants were slaughtered for their ivory from 2004 to 2014.
Fricke highlighted the crux of the problem. “Unfortunately, the large
animals that are most important for seed dispersal are the types of species
that are often the first to disappear from our ecosystems.”
The Need to Move
Because plants are firmly rooted immobile organisms, they need to move
their seeds to regenerate and spread, especially amid major environmental
disturbances and increasing climate change impacts.
But that’s growing harder as animals decline and landscapes fragment into
what conservation biologist George Powell dubbed “green measles” ––
disconnected scraps of habitat, broken up by fences, cultivated fields,
pastures, roads, and settlements.
Overall, deforestation is the greatest threat to both plants and animals,
with land cleared for timber and industrial-scale agriculture: cattle, soy,
rubber, palm oil and other commodities. From 2001 to 2020, the planet lost
4.1 million square kilometers (1.6 million square miles) of tree cover,
according to the nonprofit Global Forest Watch.
Many of nature’s architects need large areas of habitat to survive. In
patchy, subdivided landscapes, both plant and animal communities contract
in size and diversity. Only a subset of creatures remain in scraps of
habitat, severing interactions between former residents that helped keep
all alive.
A Changing Climate
The need for mobility is rising alongside warming temperatures and more
frequent extreme weather events. Transported elsewhere, plants may be able
to “outrun” warming climate, says Beatriz Rumeu, an island ecologist at the
University of Oviedo in Spain.
Some organisms will need to move upslope, toward the poles, or closer to
water, says Fricke. But in flat areas, species may need to migrate tens of
kilometers yearly to maintain constant environmental conditions. “That’s a
super tall order [for plants], especially in places that don’t have animals
to transport seeds over long distances,” Fricke adds.
He led a study showing that in habitats with reduced birds and mammals, the
risk of plants being stranded in inhospitable environments rises to at
least 60%. Trapped without their animal associates, they face wildfire,
flood, heat, drought and more frequent disease outbreaks.
California’s huckleberry bush offers one example. It’s been a century since
grizzly bears wandered the state, gorging on berries and reseeding this
bush. Robins, foxes and others still eat huckleberries, but they distribute
fewer seeds over much shorter distances that may not keep pace with climate
change.
Researchers have identified similar cascading impacts linked to climate
change in Gabon’s Lopé National Park. In 1993, ecologist Caroline Tutin
discovered that some Gabon trees only flowered and produced fruit if nights
cooled below 18° Celsius (66° Fahrenheit). Temperatures have since warmed,
and now, there’s a fruit famine. In the 1980s, one of every 10 trees held
ripe fruit. Now, it’s one in 50, with the physical health of the region’s
remaining forest elephants visibly deteriorating as a result. Elephant
poachers compound the trees’ seeding problems, says Poulson. Long-distance
dispersal, once carried out by elephants, is now mostly left to guenons,
mangabeys, chimps and gorillas that foray over shorter distances.
Unraveling the Mysteries of Plant-Animal Relationships
Much remains unknown about individual plant-animal disperser relationships,
says Lilisbeth Rodríguez, a botanist at the Smithsonian Tropical Research
Institute in Panama.
To reveal the hidden natural history of one of these unstudied plants,
Zamia pseudoparasitica, Rodríguez clipped into climbing ropes and hoisted
herself aloft –– sometimes up to 30 meters (100 feet) into trees –– to
install camera traps. This rare epiphyte, a “living fossil” that appeared
some 34 million years ago, lives attached to trees in Panama’s Atlantic
lowland and cloud forests — but no one knew who came to eat its large,
inch-long seeds.
The cameras were left in the forest for three months. Rodríguez and her
biologist colleagues, Claudio Monteza-Moreno and Pedro Castillo-Caballero,
retrieved them just as the world was locking down at the start of the
COVID-19 pandemic, and so were forced to sleep in their car as they
traveled between study sites.
The photos and video they captured revealed a hidden world, showing a bird
(a toucanet) and seven mammals (including opossums, kinkajous, squirrels
and monkeys) visiting the plant’s seed cones. But just one animal, the
northern olingo, was captured actually taking and moving seeds.
This single study points out the challenges of such investigations. But
even as science is exploring specific plant-animal relationships, many
interactions are changing. Much of what we know about shifting
relationships comes from studies on islands, which are exceptionally
vulnerable to ecological change. With constrained boundaries, quick
lifecycles, unique and diverse species, and sometimes intense human
pressures, islands act as living laboratories.
The Galápagos Islands are among those laboratories, a place where
fleshy-fruited plant species dominate. Beatriz Rumeu joined expeditions
there that highlighted the archipelago’s precarious plant-seed disperser
balance. The key fruit eaters are the San Cristóbal tortoise, two species
of lava lizards, two mockingbirds, and a flycatcher. However, the loss of
just one species, the Santa Cruz lava lizard, would prove disastrous, as it
spreads more than half the community’s plant species. The giant tortoise
also plays a unique role on these islands as the only creature big enough
to eat large fruits. Meanwhile, invasive fire ants, parasitic nest flies,
feral dogs, cats — and extreme weather events — put the entire system at
risk.
Haldre Rogers shares an extreme scenario. On the island of Guam, the
accidentally introduced exotic brown tree snake wiped out the island’s seed
dispersers: its birds and most of the bats, too. A small number of bats
remain, and feral pigs also munch on fallen fruit, but the ecosystem is
irrevocably altered. The quick-growing “pioneer plant species” that need
well-lit areas “just drop right out,” Rogers says. “I don’t think people
recognize that animal dispersers are incredibly important for keeping our
systems running.” Guam offers a dire warning, showing what happens when
dispersers disappear, she says.
A World Without Seed Dispersers
By disbursing seeds, animals help facilitate all the benefits plants
provide for us: storing carbon, controlling floods, and providing water,
food, timber, fiber, fuel and medicine, Fricke says. Without seed
dispersers, humanity would face serious challenges.
But there are growing efforts to restore this critical ecological process.
If successful, such projects could offer a bonanza of benefits: preventing
extinctions, helping bringing back endangered plants and wildlife,
recovering wild lands, and curbing climate change. Some of this work is
being facilitated under the United Nations Decade on Ecosystem Restoration.
Protecting key animal species will help — especially toucans, elephants,
apes and others that travel far or carry large seeds. Because bats are so
important, there are global efforts to protect roosting sites.
Rewilding is another relatively low-cost solution, although difficult to
implement due to the need for permits, quarantines and careful monitoring.
One success story comes from Brazil’s Atlantic Forest, where most mammals
were hunted out, threatening 45 types of native palm. Researchers, reserve
managers and animal keepers reintroduced red-humped agoutis and brown
howler monkeys to Tijuca National Park, near Rio de Janeiro.
These seed-dispersing animals filled an important ecological void, sparking
a “disproportional effect on forest regeneration.” Reintroducing
heavily-hunted giant tortoises to the Galápagos also proved successful,
offering promise for other islands. Tropical forests can regrow relatively
quickly after being burned or logged, especially if seed dispersers are
available to aid regrowth.
In some areas, reforestation could restore destroyed habitat, but only if
important native species are planted — not exotic monocultures, says
Monteza-Moreno. Reconnecting fragmented habitat and protecting wild, intact
landscapes is also critical. The is now paying countries to protect forests.
Poulson mentioned another important initiative happening in Gabon, a
country that previously protected 13% of its land in national parks.
President Ali Bongo Ondimba signed a policy in 2020 that takes effect this
year, requiring logging concessions to operate sustainably. It’s an
important model, one that needs similar versions for agriculture and other
deforesting industries,
Positive things are happening, Poulson says. “The question is whether the
positive things can outrace climate change, unsustainable hunting,
poaching, industrial agriculture and deforestation.”
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