Does tropical tree diversity explain the diversity of soil arthropods?


Strumigenys rogeri in the leaf litter

In 1982, a small journal called The Coleopterists Bulletin carried a two page note by beetle expert Terry Erwin that increased- by an order of magnitude- the estimated number of species on the planet. Erwin crunched some back-of-the-napkin numbers based on the tree specificity of arthropods he'd collected in Panamanian tree canopies and the richness of tropical tree species worldwide to surmise that the earth should hold 30 million species. An impressive bump from the 1 to 2 million that was the going estimate.

Later research on canopy arthropods (For example, Novotny et al 2006) lent some support to Erwin's notion that tree diversity was responsible, at least in part, for the nearly numbing diversity of tropical insect species.

But tree canopies aren't the only species-rich habitat. Tropical forest leaf litter, the rich compost of decaying leaves and branches covering the forest floor, contains a similarly astounding diversity. As way of an example, check out Antweb's Strumigenys page. Any given patch of tropical forest can hold several Strumigenys species, and that's just one genus of litter specialist ants among many genera. And no one knows why there are so many species.

Is the diversity of litter arthropods, like their arboreal counterparts, tied to tree diversity?

Apparently not, according to a new study in the journal Oecologia. David Donoso, Mary Johnston, and Mike Kaspari collected litter samples from under ten species of Panamanian trees at varying distances and sorted through more than 5,000 mite specimens and 7,000 ant specimens. (I'm sure that wasn't at all tedious. But I digress). While they catalogued an impressive diversity of species, the identity of the overhanging tree species was not a good predictor of the composition of the litter fauna.

I find this result surprising. After all, the properties of litter are partly a function of leaf chemistry and physical structure of the associated tree species- an observation that was confirmed in this study. Something must be behind the richness of litter species, though. But what?


Incidentally, if you'd like to read more about Terry Erwin's canopy-fogging work pick up a copy of Rob Dunn's Every Living Thing, which covers this and other stories related to biological discovery.

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Something must be behind the richness of litter species, though. But what?

A population becomes vicariously fragmented, speciation happens, populations expand and ranges overlap, there's so much to eat that competition isn't a problem and/or the species have evolved trophic specializations such that they don't directly compete, and all this happens in an environment sufficiently lush that extinction is a rare event. Thus: diversity. To my mind, it isn't tropical biodiversity that needs an explanation, it's the depauperate condition elsewhere that needs to be explained.

By darwinsdog (not verified) on 19 Apr 2010 #permalink

That's a excellent point. That many of the biologists seeking to explain biodiversity live in post-glaciated regions must introduce biases into how we perceive what needs to be explained.

Still, whether litter-dwelling populations fragment based on patterns of tree diversity is a relevant question.

Is there any chance that the low nutrient levels found in tropical soil could lead to increased competition, which, of course, leads to an increase in diversity? Or is nutrient level in the soil not really an issue for most terrestrial species?

By FormicidaeFantasy (not verified) on 19 Apr 2010 #permalink

Darwin's Dog mostly nailed it, imo.

Substitute the words 'stable and productive' for 'lush' and you have all you need to know.

I am sure the complex of co-evolved myco-heterotrophy, commensal fungi and bacteria, detritovores, etc are all affected by allelopathic interactions of the varied plant species to some extent. It would seem likely though that interspecific trophic and microhabitat segmentation//specialization becomes a larger selective force in a largely unchanging environment.

Conversely, in a wildly variable environment typified by northern latitudes, such specialization more often results in extinction and the wide distribution of species with generalist and opportunistic evolutionary strategies.

I'd say the discussion here so far is off the mark. Iâve worked in a range of litter systems from tropical rainforest to recently post-glacial boreal forests. The striking differences in diversity between the two regions isnât the diversity at any particular site: boreal systems have quite impressive soil arthropod diversities.

For example, of the 176 species of mites Iâve identified so far at my aspen forest reference site in central Alberta (ice went ~9,500 ya), 77 are in the suborder Oribatida and 42 are in the Mesostigmata (of which 39 are in the Gamasida). Donoso et al. seem to be acarologically challenged, since their âOribatidaeâ and âGamasidaeâ are not valid names, havenât been in use since the early 1900âs, and refer to large collections of families, but they claimed 35 oribatid species (I would have expected more than 100 species) and 62 gamasids in the first paragraph of the Results. Ants do seem a bit too wimpy for boreal forests: 93 âspecies/morphospeciesâ vs a few Myrmeca(?), Lasius, Camponotus, and a heap of Formica, but Iâd guess only 10 species in total.

In any case, the really striking difference between tropical and temperate diversity is how much change there is between sites: go a kilometer down the road and the mites are mostly different in the tropics, but pretty much the same here. Generally, there are more species per site in the tropics, but it is how they change over the landscape that makes the impressive difference.

Also, tropical litter disappears almost as fast as it falls and rainforest âsoilâ is more tree roots than dirt. Litter resources are ephemeral in the tropics but accumulate into deep and structured layers in the boreal forests (less so in most temperate deciduous forests, but still deeper than the tropics). Stability and resource availability appear to be more predictable in the boreal forest.

In any case, Iâm not sure much can be concluded from this study. Why would oribatid mites (mostly feeding on fungi, not the leaves themselves), âgamasidsâ (almost entirely predatory), or ants (presumably mostly predators [some on mites] or scavenger/predators) be expected to show much relationship to tree species litter diversity? Now, if they actually knew something about the mites and had looked at a subset of the Oribatida that do tend to burrow in fallen leaves (e.g. the box mites), then maybe they would have had a valid test. As it is, Iâm not sure this paper tells us much of anything.

Very very interesting issues are discussed here.
¡What a mistery! ¿why are there so many different soil mites? anywhere?
I think you should check the information on the mites themselves, morphology, taxonomy, and mainly .... there may be is the real explanation, because ecology seems to tells us nothing definitely clear about it:
no competition, no food troubles, no habitat real preference, no indicators, no KEY SPECIES, unexplained redundancy, bla bla bla.
I love mites!

By Patagonian bot… (not verified) on 17 Jun 2010 #permalink