A significant Nature paper co-authored by Owen Lewis - my tutor at uni and lead researcher on the Panama project - has just been making waves, attracting attention all the way from Radio 4 to some slightly more improbable quarters. (I’m currently using the free wifi on a train from Durham, and one of the paper’s news reports, in VICE magazine, is being blocked on the grounds of ‘nudity and lingerie content’… Ooookkkkaaaaaayyyyyyy.........)
In my last post, I mentioned the idea that as a species becomes more abundant locally, its members will find it increasingly difficult to survive. This means that there’s a feedback process favouring rare species, allowing diversity to be maintained. This friendly, egalitarian mechanism is known to function in rainforests, but whether it is the mystery cause of tropical hyper-diversity remains unknown.
What did they do?
For the current Nature paper, the authors plunged into Belize, and sprayed fungicide in the name of science. Lots of fungicide. And lots of insecticide. The upshot of all this rainforest blitzkrieg was the finding that, if you smother an area in fungicide, rare tree species lose their advantage. And, importantly, abundant species are no longer restrained. This is important news, adding to earlier evidence that fungal pathogens drive the Janzen-Connell mechanism. It implies that tropical hyper-diversity is maintained by, of all things, the fungal diseases of plants (Fig. 2). Who’d ‘av funk it?
In Fig. 2, by the way, you’ll notice that two different fungicides are used, and they produce pretty different results. This is interesting, because Ridomil -– the one that doesn't seem to affect diversity -– targets oomycetes (which aren’t quite fungi, but are fairly similar). Oomycetes are responsible for a fair amount of human suffering, having caused the catastrophic potato famine in Ireland in the 1800s, and were up till now strongly suspected as drivers of negative density-dependence. Turns out, then, that it’s your run-of-the-mill fungi, rather than oomycetes, that underpin the J-C mechanism.
Are insects important?
How about the insecticides? Does removing insect herbivores also undermine the J-C process? The mechanism does, of course, allow for pests as well as pathogens to be the drivers of diversity. Strangely, the answer is that killing off the insects doesn’t really do much to the diversity of trees (in terms of seedlings; Fig. 2), although it does dramatically affect their sheer abundance (Fig. 3).
To diagnose negative density-dependence, you can look at the ‘seed-to-seedling transition’, which is just what it says on the tin. Have a look at Fig. 5, which shows this transition for a single (representative) species. The helpful dotted line in the top left corner (a slope of 1 in a log-log plot) shows the relationship you’d expect if density-dependence isn’t in action. In other words, a dense population of seeds will translate into a dense population of seedlings, in just the same proportions as a sparse population of seeds would. You’ll notice that the ‘control’ slope - in which no fungicide or insecticide was sprayed - is shallower than the dotted line, which means that a more abundant population of seeds faces increased difficulties in the ‘seed-to-seedling transition’. So much so, in fact, that even plots with over 5,000 seeds per m2 end up with only a slightly woeful 3 or so seedlings per m2. The good-ol' negative density-dependent process is bubbling away happily in normal forests.
If you apply fungicide, the slope loses its shallow gradient, and starts to look a bit more like the helpful dotted line. In other words, the intensity of negative density-dependence drops away when the fungi are removed: the culprits are identified.
Closing in on the mystery of rainforests?
This is a significant study, teasing apart the contributions of specific natural enemies and analysing the community-wide force of the Janzen-Connell mechanism. It adds to our confidence in the J-C mechanism as a crucial determinant of species coexistence in the tropics, and even has quirkily deep implications for conservation. As Owen remarked in one of his (many!) recent interviews, "a group of organisms that aren’t normally a target of conventional conservation efforts - fungal diseases of plants - might actually be critical in maintaining the diversity of the system" (see the Nature podcast for 23rd February 2014).
Before we go, here's a cliffhanger. One intriguing result in Bagchi et al. (2014) is that the results don’t appear to fit an important pattern previously found in two high-profile papers - and I'll explore that interesting topic in an upcoming blog post...
"This is the first study to explicitly link a particular group of natural enemies to negative density dependence and the maintenance of species diversity in tropical forest plants."
If you want to know more:
Bagchi, R., Gallery, R., Gripenberg, S., Gurr, S., Narayan, L., Addis, C., Freckleton, R., & Lewis, O. (2014). Pathogens and insect herbivores drive rainforest plant diversity and composition. Nature [advance online publication], doi:10.1038/nature12911
The Nature Podcast: Diversity drivers
Nature podcast, 22/01/2014:
Fungi crucial for biodiversity in rainforests
Planet Earth, 22/01/2014, Harriet Jarlett
Rainforests Can Thank Fungi for Their Good Looks
Vice (US), 22/01/2013, Jason Koebler
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