In the 1940s, a cargo ship likely brought
a stowaway to Guam—the brown tree snake.
Since its arrival, the snake’s population has exploded,
and eliminated almost all native bird species on the island.
That’s had a domino effect on the ecosystem—
many tree species relied on birds consuming
their fruits and nuts to disperse their seeds.
But this problem exceeds the small island of Guam:
widespread losses of birds and mammals throughout the world
have put this crucial ecosystem service at risk.
U.S. and Danish researchers set out to measure how these declines
impact plants that rely on animals to move, especially as the climate changes.
First, they had to create models that could predict two interactions
between plants and fruit-eating animals:
germination and how far seeds were dispersed.
Previous models estimated how far an animal can circulate a seed
using a simple relationship: the larger the animal,
the further it can transport a seed.
But there’s more nuance to these interactions.
Take two birds of a similar mass:
one might travel far distances high above the forest canopy,
while the other flies shorter distances below the canopy,
leading to different dispersal lengths.
The researchers trained a model using data
from thousands of plant-animal interactions.
This helped predict germination benefits and dispersal distance
for any pair of animals and plants, based on their traits.
The scientists also modeled how plants and animals
might interact with each other as they expand to new areas.
Most data on plant-animal interactions are between species
that have shared the same habitats for hundreds of thousands,
if not millions, of years.
But climate change is shifting these ranges,
causing species that never evolved together to occupy the same space.
Using data from native species, researchers sought to predict the
closest stand in for these future groups:
species introduced to new areas by humans.
They found that species without a shared history still interact
with each other if they share similar traits to native species.
Next, they used these models to estimate how human activity
has impacted the ability for mammals to disperse seeds long distances.
They found that most regions of the world
lack mammals or birds that can transport seeds long ranges.
Under a different scenario, without human-caused extinctions or species introductions,
most regions of the world would have animals
that can transport seeds far greater.
The scientists then estimated how species losses
have changed the ability for animal-dispersed plants
to keep pace with climate change.
They found that on average animal-dispersed plants plants
experienced a 60% reduction in the ability for their seeds to
move to new, more suitable environments.
The largest declines were in temperate regions,
where large mammals that are strong seed dispersers have disappeared.
Some large fruit-eaters, such as elephants,
still exist in other parts of the world.
Maintaining these species are crucial.
In addition to conservation, re-establishing seed-dispersers
can help strengthen resilience to climate change.
From 2010 to 2017, conservationists in Brazil
re-introduced the red-rumped agouti and the brown howler monkey
to the Tijuca National Park.
Building wildlife corridors in fragmented habitats
could also make it easier for animals
to disperse seeds more widely.
The researchers hope their study will inspire others
to evaluate the ripple effect that species losses have
on highly connected ecosystems.