What happens to a plant’s seeds when its seed dispersers have dispersed?

Iriomote Island, Japan, might seem like a sub-tropical paradise with lush green forests, palm trees and golden beaches. A closer look reveals something different. Iriomote is the last refuge for a few species. Things aren’t quite that bad for Monoon liukiuense yet, but a study by Ryo Furumoto published in the Journal of Tropical Ecology has found this endangered tree may not be doing as well as it looks. The trees produce plenty of fruit, but Furumoto’s study shows that the fruit isn’t getting dispersed as it should.

A whopping 82% of these seeds, rather than embarking on grand travels, simply tumble down from their mother trees. Furomoro points out that in Panama, it’s been observed that only 38% of wild nutmeg fruits – comparable in size to Monoon liukiuense seeds – meet a similar fate. This discrepancy suggests that Monoon liukiuense lacks the helping hand – or rather, the beak, paw, or snout – of sufficient primary seed dispersers.

On the ground, 90% of the seeds remained at the drop site, even though various creatures feasted upon the fruit pulp. Typically, terrestrial animals act as movers and shakers of the seed world, transporting these potential new trees from their birthplace. This notable lack of relocation indicates a distinct shortage of secondary seed dispersers on Iriomote Island.

Furomoto saw four animals taking an interest in Monoon liukiuense fruits:

• the brown-eared bulbul, a bird that biologists mistakenly put in the blackbird genus for a while, 
• the Yaeyama flying fox, which is actually a bat, 
• the yellow-margined box turtle, which is mainly dark brown,
• the large-billed crow, which is a crow with a large bill.

Although bulbuls are known to be important fruit spreaders in the region, it seems the fruits of Monoon liukiuense are too large for them to handle. The turtles, too, were more likely stumbling upon the fruit rather than intentionally moving it. The story for the Yaeyama flying fox and the large-billed crow is more intriguing.

The flying foxes were seen flying away with fruits in their mouths, hinting at potential seed spreading. However, a plant wouldn’t want to rely on the flying foxes as distributors as their population is declining. The animals that looked like the best opportunity for Monoon liukiuense were the crows. They had no trouble handling the larger fruit – but the crows are also a problem. The flying foxes eat the fruit and drop the seeds. The crows, in contrast, are happy to eat the seeds too.

The fate of the seeds was determined by embarking on a year-long stakeout, observing the tree’s fruiting seasons and what happened to its fruits. Furomoto affixed time-lapse cameras to the trees’ branches, capturing photos every few seconds. These cameras were watching over the fruits from when they were still greenish-yellow until every last one was gone.

The cameras recorded each fruit’s fate: Was it visited by an animal? Eaten? Dropped from the tree? Or carried away by some creature who fancied a fruity snack? With the help of these constant photographic records, Furomoto could categorize each event, weaving a detailed story of the fruits’ journey from tree to ground and beyond.

However, the ground below the trees was equally important. Furomoto also placed cameras there to see what happened to the fruits that had fallen naturally or had been dropped by animals. Furomoto observed how often these grounded fruits were visited, eaten, moved around, or carried away by animals.

Of the 358 fruits observed in the tree canopies, almost half were knocked off by animals, a third dropped naturally, and a small percentage were carried away by creatures. The average fruit didn’t travel far, with 82% of them ending up under their mother tree.

And who are these fruit thieves? The most frequent visitor was the brown-eared bulbul. However, the most common fruit dropper was the Yaeyama flying fox, followed by the bulbul and the large-billed crow. These airborne visitors were the main agents of fruit travel, though the large-billed crow was particularly significant, carrying off more fruit per visit than the others.

Furomoto also watched what happened to fallen fruits on the ground. Of 222 fruits, most were consumed on the spot, with their seeds left behind. The main ground-level visitor was the yellow-margined box turtle. They munched on the fruit pulp and largely left the seeds where they were. The large-billed crow made a few appearances on the ground too, carrying away fruits each time.

These results reveal that while the tree’s fruits are visited by various animals, they don’t usually travel far, mainly due to the behaviour of the animals involved. The lack of travel may be a big problem waiting to reveal itself. 

Monoon liukiuense is found only in small patches of regenerating forests scattered among rice paddies and sugarcane fields. It’s a bit like having all your eggs in one basket. If a disease or other disaster strikes, the trees are all in the same spot, and the entire population could be wiped out. Seed dispersal helps to avoid this by spreading young trees around, increasing their chances of survival. That’s why Monoon liukiuense needs animal seed dispersers.

Compared to close relatives of Monoon liukiuense that are served by an array of diverse fruit-dispersing agents like civets, hornbills, and cassowaries, the Iriomote Island’s tree seems to have a rather simple, and perhaps, insufficient seed dispersal process. The absence of any ground-based fruit movers was particularly notable. In most forests, multiple agents work to move seeds around, ensuring wider dispersal. But for Monoon liukiuense, the story seems to end with the bat and the crow.

That may not be enough.

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Furumoto, R. (2023) “Potential seed dispersal agents of Monoon liukiuense on Iriomote Island, Japan,” Journal of Tropical Ecology, 39(e18), p. e18. Available at: https://doi.org/10.1017/s0266467423000056.