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The loss of mammal and bird species threatens to have devastating impacts on the survival of plants as the climate changes.
Animal-dispersed plants are on average 60% less able to spread their seeds than before the arrival of humanity, leaving them unable to keep up with changing global temperatures.
The historic loss of large mammals such as wolves and aurochs means that plants are much less able to keep pace with modern climate change.
Researchers from the USA and Denmark found that the ability of plants to move into suitable areas, as the climate changed, had declined by as much as 95% as the animals which spread them declined, moved away, or went extinct.
The most severe losses are found across Europe, the Americas and Australia. Plants in these places, on average, don't move into new areas fast enough to keep up with a changing climate. The loss of large animals which roam great distances has been blamed in particular for the increasing inability of plants to disperse.
Dr Evan Fricke, the study's lead author, says, 'When we lose mammals and birds from ecosystems, we don’t just lose species. Extinction and habitat loss damage complex ecological networks. This study shows animal declines can disrupt ecological networks in ways that threaten the climate resilience of entire ecosystems that people rely upon.'
The findings of the study were published in Science.
While plants have many unique innovations compared to animals, one ability they haven't gained is the ability to relocate themselves from one place to another. Though there are some suggestions that so-called 'walking palms' can move themselves after being knocked over, a number of studies have since disproven this.
Instead, plants are locked to the location their seeds germinate in. Populations of plant species can only grow elsewhere with assistance, with wind, water and animals being among the ways plants spread their seeds far and wide.
The use of animals to move seeds is the most popular method, with over half of plant species relying on it. The plants use a variety of ways to do this, from tempting nutrient-rich fruits to hooks that catch on passing animal fur.
In addition to dispersing seeds away from the parent plant, animal dispersal offers other benefits. Ants carrying a lipid-rich seed back to their colony leave it underground to grow once they have finished with it, while fruits like blackberries are much more likely to germinate after passing through the animal's gut.
As a result of these relationships, some plants become dependent on specific animals to spread their seeds. For instance, Prunus javanica, a plant found in southeast Asia, can be pollinated by a number of species, but gibbons give seeds the best chance of survival by carrying them long distances after eating the fruits.
However, all gibbons are threatened with extinction, including five Critically Endangered species and 14 Endangered species. Scientists, therefore, wanted to assess how the loss of seed-spreading animals like the gibbons could impact the plants which depend upon them.
To quantify this impact, the researchers gathered information from thousands of studies, as well as historic records, to work out how animals spread seeds in the past and present, as well as predicting values for the future, based on machine learning.
They found that a loss of only 10% of species from levels before humanity's impact resulted in declines of seed dispersal approaching 75% in some cases. The loss of large mammals and birds has had the most significant impact, as these organisms are capable of carrying seeds the furthest distances.
With fewer animals available to move their seeds, plants are more vulnerable to climate change. A variety of studies have shown that animals are moving as the planet warms to stay in areas that meet their temperature needs, but plants are unable to make these same moves without assistance.
Climate change is also exacerbating other pressures, with the researchers in this study finding that the loss of seed-spreading animal species classed as Vulnerable or Endangered would reduce the ability of plants to keep up with the climate from 40% to around 25%.
Plants in southeast Asia and Madagascar are most at risk from the loss of these threatened animals, leaving the future of plants which depend on them in doubt.
Prof Jens-Christian Svenning, who co-authored the paper, called on governments around the world to restore the planet's ecosystems to help stop these species from being lost.
'Large mammals and birds are particularly important as long-distance seed dispersers and have been widely lost from natural ecosystems,' he says. 'The research highlights the need to restore faunas to ensure effective dispersal in the face of rapid climate change.'