Fungi drive rainforest biodiversity
Rainforests are hot spots of biodiversity, with up to 300 plant species sharing the same hectare. An international research team has now shed light on what keeps dominant plant species in check, giving rarer species a chance to thrive.
Over 40 years ago, American ecologists Daniel Janzen and Joseph Connell proposed that plant pathogens and herbivorous insects in the rain forest keep dominant plant species in check, keeping them from elbowing out all other plants. A research team led by Owen Lewis of Oxford University and Robert Bagchi, who started the study at Oxford and concluded it at ETH Zurich, has now tested this hypothesis for the first time on the plant community as a whole. Their study reveals that fungi, usually implicated in a negative context as pests, are actually at the centre of rainforest biodiversity.
“In the plant world, close relatives make for bad neighbours,” explains Lewis. “Seedlings growing near plants of the same species are more likely to die and we now know why.” Plant pathogens like fungi spread quickly among individuals of the same species that grow in close proximity, thus causing density dependent mortality. By limiting the number of individuals of a dominant species, fungi thus level the playing field for a greater variety of plant species to thrive and prosper.
Disease and diversity
“The fact that very common species are more prone to disease may not be that surprising and has been fairly well established. But the next stage on from that – providing experimental evidence that this in fact leads to higher diversity among rainforest plant communities – that is an exciting step forward,” says Bagchi, first author of the study now published in Nature.
In order to test the Janzen-Connell hypothesis and to distinguish the effect of insects and pathogens on plant communities, the researchers assessed sampling plots in the Chiquibul Forest Reserve in Belize after spraying the plots either with water, one of two fungicides or an insecticide. Notably, one of the two fungicides caused a marked drop in the number of plant species. The insecticide on the other hand did not reduce the diversity of the plant community but changed its composition.
At ETH Zurich, Bagchi used computer models to compare the case where eliminating fungi merely affects the survival rate of plant species regardless of their density to an alternative one where it disproportionately affects species growing at high density. The computational model that simulated the latter produced similar results to those observed in the field, confirming fungi to be at the centre of density dependent mortality and therefore allowing the high plant diversity in tropical forests.
Implications for ecosystem preservation
“We need to be cautious about over-interpreting these results as we only assessed the effect over a relatively short period of time, 17 months, and in one forest”, says Bagchi. The effect may be different in other rainforests with a different climate. The rainforest in Belize for instance has a pronounced dry season which limits the spread of fungi. “We suspect that the effect of fungi – causing density dependent mortality – will be strongest in wetter, warmer areas because this is where they do best.”
The findings also add a potential new dimension to the consequences of climate change. A warming climate is predicted to reduce overall rainfall and thus could limit the spread of pathogens like fungi. “One may think ‘Great, less pathogens!’ but less pathogens also means less diversity. And the diversity of our rainforests is something we certainly want to preserve,” says Bagchi.
Bagchi R, Gallery RE, Gripenberg S, Gurr SJ, Narayan L, Addis CE, Freckleton RP, Lewis OT: Pathogens and insect herbivores drive rainforest plant diversity and composition. Nature, 22nd January 2014, doi: 10.1038/nature12911