Australian rainforest trees with large fleshy fruits are rare because few animals are capable of dispersing them, a new study suggests.
Especially in sub-tropical areas, such trees have smaller ranges and fewer connected populations, than those with smaller fruit, say researchers today in the Royal Society journal Proceedings B.
The animals that feed on fruit and disperse seed that way are fairly limited in Australia …
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“We found that species with fruits larger than 3 centimetres have significantly smaller ranges throughout the continent,” said lead author, Dr Maurizio Rossetto, principal research scientist at the National Herbarium of New South Wales.
The survival of such species may rely on human-assistant transplantation to establish new populations, or even possibly introduction of animals such as cassowaries to help ensure their dispersal, said the researchers.
Rainforest trees that bear fleshy fruit, such as the native ‘black apple’ Planchonella australis (pictured below), rely on animals to disperse their seeds and establish a network of interconnected populations that can replenish ailing populations and are a source of genetic diversity.
Without good dispersal, a plant species is more vulnerable to such threats as climate change, disease or extinction.
Dr Rossetto and his colleagues looked at the distribution of over 1000 species and fleshy-fruited rainforest trees and vines across Australia.
Not only did they find that large-seeded trees were rarer than smaller ones, genetic analysis showed they tended to have fewer and less well-connected populations.
Dr Rossetto and colleagues concluded the best available explanation for this finding was a lack of animals that were capable of picking up and dispersing the larger fruit.
“The animals that feed on fruit and disperse seed that way are fairly limited in Australia compared to in South America, Asia, and Africa, which have rhinoceroses, monkeys, squirrels and all sorts of birds to do this,” he said.
Importance of animals in rainforests
In Australia, the best dispersers of big seeds are a handful of large birds, including the cassowary and Torresian pigeon, which only live in northern Australia.
In line with their hypothesis, the researchers found the greatest number of big-seeded trees was greater in the north.
They also found areas of rainforest that had been disturbed had no species with large fruit.
“If an area becomes devoid of rainforest and is then recolonised and there are no animals capable of dispersing large-fruited species, it becomes recolonised only by small-fruited species,” Dr Rossetto said.
“So you lose a whole component of your biodiversity because there’s no mechanism to bring the large-fruited species back.”
Dr Rossetto said that past ice ages resulted in repeated contraction and fragmentation of rainforests in Australia and that while plant species may have survived this, many large-fruit dispersers were lost in this process.
He said that recent clearing of rainforest areas like the Big Scrub in northern New South Wales was exacerbating the problem.
“The more disturbance you get, the more local populations you lose, and the more these species become rare,” he said.
By contrast, places with a large number of large-fruited species like Nightcap National Park in Northern New South Wales (see video below) had a lot of biodiversity that needed protecting.