Extinctions on the island of the dodo are pushing plants towards extinction

Almost a third of Mauritius' native fruits are no longer being dispersed as no animals are big enough to swallow their seeds.

This dietary dilemma is the result of 400 years of extinctions on the island, which have left 28% of the island's native fruits and 7% of seeds simply too big to fit in the mouths of the smaller fruit-eating animals (frugivores) that are left. Introduced species are also too small, so they can't take over the role of spreading seeds.

Many of these introduced species, such as macaques and pigs, are also more likely to break seeds open while eating them rather than spreading them. With seed predation increasing by over a half since the time of the dodo, this means many of the island's unique plants face an uncertain future.

Dr Julia Heinen, the lead author of a new study revealing this phenomenon, says, 'Animals that arrived on Mauritius with sailors 400 years ago, such as rats and pigs, are unlikely to replace extinct animals in their crucial ecological function of dispersing the seeds of plants when eating fruits.'

'This has led to concern that Mauritius' plants, most of which are critically endangered, may have lost the capacity for seed dispersal via animals, and ultimately increasing the risk that they will go extinct.'

'The extinction of native plants could ignite a cascade of extinctions, taking other species with them in the downfall.'

The study, which was co-authored by Museum scientific associate Dr Julian Hume, was published in Nature Communications.

The threats facing Mauritian plants

Like many islands, Mauritius is home to hundreds of unique species, but introduced animals and habitat destruction puts their survival in peril. This makes Mauritian plants some of the most threatened in the world.

It is estimated that just 4.4% of the island's original habitats are left, with many plants now at risk of disappearing forever. For instance, 57% of Mauritius' tree species are at risk of extinction, which is the second highest threat level in the world after the neighbouring island of Madagascar.

The high risk of extinction is, in part, linked to the loss of native species that used to spread the plants' seeds. As more than half of plant species rely on the assistance of animals to disperse, coextinction is a significant threat.

To examine this issue more closely, the researchers looked at how the relationships between Mauritius' plants and frugivores have changed over time. While the number of fruit-eating species living on the island has actually increased slightly over the past four centuries, the ability of these animals to spread seeds has significantly declined.

One of the main reasons for this is size. Mauritius used to be home to large fruit-eating species such as giant tortoises and the dodo, but following their extinction only a handful remain. The surviving native species are much smaller, while most  of the animals that have been introduced also aren't that big.

As a result, Mauritian frugivores are around half as big than they used to be. This means that while small-seeded plants are still being spread, species with bigger seeds only have one or two animal dispersers that they rely on.

In total 203 fruit feeding interactions have been lost on the island, while 173 have been gained through introductions. These gains, however, tend to lean towards seed predation rather than dispersal meaning that the seeds are often being killed rather than spread.

Some of the worst culprits of seed predation are the crab-eating macaques, which can destroy all the seeds they eat. Meanwhile, pigs generally kill around 86%, while the huge number of rats on Mauritius damage around 65% of the seeds they eat.

Would reintroducing extinct species help Mauritian plants?

In light of the threats facing Mauritian biodiversity, a variety of reintroduction schemes have been proposed. These include bringing over Aldabra giant tortoises from the Seychelles to replace two extinct species.

While the paper notes that these species could help to spread large seeds and so help some plants recover, they might also aid the spread of invasive species as well.

Another high-profile project has proposed the 'de-extinction' of the dodo, followed by its reintroduction to the island. Proponents of the project have said this would help to restore Mauritian ecosystems, but any benefits are uncertain.

In the 1970s, for instance, it was suggested that the tambalacoque tree was dependent on the bird to spread its seeds, but this has since been debunked. While the dodo may have eaten the seeds, there's simply not enough evidence to know whether it acted as a seed disperser or a seed predator.

Instead of returning lost species, the paper suggests that conservation efforts should be focused on supporting the surviving Mauritian flying fox. The bat is the island's last native animal capable of spreading larger seeds, but has become Endangered as a result of hunting, culls and habitat destruction.

'The Mauritian flying fox is a unique and endangered animal that plays a vital role in the ecosystem,' Julia says. 'It is one of the few remaining animals that can disperse the plants over the island.'

'However, the bats are being culled because of concerns over noise and their impact on the fruit industry. This is the last place on Earth that the bat lives, but the science is being ignored.'

Together with the Mauritian bulbul, the researchers found that the flying fox is now responsible for 90% of seed dispersal interactions. If it becomes extinct, then many plants would lose the ability to spread using animals, putting them on a slow path towards extinction as they become more vulnerable to damage, disease, and harvesting.

'These native plants can be saved,' Julia adds. 'This study makes it clear that by protecting species such as the Telfair's skink, the Mauritian bulbul and the flying fox, these plants have a much better chance of survival.'