Kill off the animals and you change the forest — fast

Last year I brought you the story of Lambir Hills National Park, a Bornean forest in which I used to live and work, where hunting and other pressures have forced into extinction much of the biggest wildlife species (see The near empty forest that proves conservation is failing).

It describes how recent surveys had failed to find 20 percent of the park’s resident bird species and 22 percent of its mammal species. The forest is emptying fast. The losses include half of the park’s primate species and six out of seven hornbill species —  all important dispersers of rainforest seeds. Sun bears and gibbons, bearded pigs and flying foxes all once called Lambir Hills home. Today it is hard to find an animal that weighs more than a kilogram in the national park.

Now researchers have shown what these extinctions mean for the forest itself. Rhett Harrison and colleagues tracked the fates of over 470,000 trees of more than 1,100 species for a 15-year period since intense hunting began there.

In a new study published in Ecology Letters, they have shown that the forest has changed markedly. There are far more trees now — the density of saplings increased by over 25 per cent between 1992 and 2008 — probably because there are fewer deer and other mammals to eat the young plants. But overall the diversity of trees has fallen. And compared to species that rely on gravity or wind to spread their seeds, there has been a relative decline in the number of new trees from species that depend on animals to disperse their seeds.

Species with animal-dispersed seeds — especially those with bigger seeds — are also more clustered than they were before hunting took off. This is probably because the loss of large fruit-eating animals means that seeds, on average, now travel shorter distances. There was no increase in clustering among species that need no animal assistance to spread their seeds.

The authors write: “Fruit that would formerly have been eaten by hornbills, gibbons or fruit pigeons, all of which are efficient long-distance seed dispersers, are now unlikely to be fed on by anything larger than a bulbul or a barbet”. For those of you who don’t know the birds of Borneo, members of the latter two types are both small enough to fit in a trouser pocket.

The researchers could draw their conclusions because Lambir is home to one of the world’s longest running forest studies. In 1992, scientists marked out a 52 hectare patch of the forest and then tagged, measured, mapped and identified every tree bigger than 1 cm diameter at breast height. In 1997, 2003 and 2008 they went back and repeated the exercise, each time taking several months to complete the task.

Their massive datasets, which track the identity and positions of around half a million trees every 5-6 years can animate the forest’s history. Like the photographs that form time-lapse videos, these periodic census snapshots reveal the patterns of life over time.  The next census of the 52-hectare plot, which is due to take place soon, will add a critical fifth image that further refines the picture of a forest in flux.

The results are already striking but, as the authors note: “the full impacts of defaunation at Lambir are only likely to be realised over several plant generations.” So far, none of the species that depends on big animals to disperse its seeds has gone extinct. That’s just a matter of time.

Reference:

Harrison, R. D. et al. 2013. Consequences of defaunation for a tropical tree community. Ecology Letters. Article first published online: 12 MAR 2013 DOI: 10.1111/ele.12102