Monday, October 17, 2011

Caddisflies through time
I recently had a paper from a co-worker (Jason Veldboom) come across my semi-ridiculous RSS feed.  This paper does something a little bit different than any other publication I've seen:  Follow the elemental composition of a population and its (presumably) primary food resource through time.  
The study is straight-forward, in that the authors simply sampled a filter feeding caddisfly larvae and seston through time.  There's interesting stuff all over this paper.  First off, there are 4 streams sampled here, and initially, the caddisflies from those 4 streams have very different nutrient ratios.  As the caddisflies prepare to emerge, however, the nutrient content in all the streams converges.  The paper can't really pinpoint why this is (are all the larvae without this nutrient ratio dying?), a lot of the difference seems to be driven by carbon content (i.e., fat and energy reserves).  
Secondly, the sheer difference in mass among sites is pretty astonishing.  At the beginning of the study, one of the streams has caddisflies that are >3 times bigger than the other streams!  These are streams with virtually identical thermal regimes, in close geographic proximity and we're talking about a single species of caddisfly.  Why would one stream have individuals who start out their life-cycle with such a massive size difference?  This study really can't even address that, but it is definitely interesting.
Third:  A handful of previous studies have sampled particular species in different locations and generally found minimal differences in elemental composition.  This is really the primary form of evidence in favor of a homeostatic model when it comes to elemental composition in invertebrate consumers.  I've never been a fan of these studies being cited as compelling evidence for homeostasis, because although the presumption is that food quality differs among locations, there's no guarantee that's true.  Further, the evidence from experimental manipulations almost always finds biota are more flexible in their elemental composition.  This study actually suggests even finding a lack of variation among sites might be luck all the way around, since only at the very end of their aquatic life-stage (just at pupation) did these caddisflies have similar elemental composition.
Finally, the whole point of the paper is to look at whether growth in these caddisflies is affected by the imbalance between consumer demand and the nutrient composition of the available food resources.  The surrogate for this imbalance is the imbalance between elemental composition of consumer and food (seston), and it does appear to be related to growth rates (at least for P).  However, these relationships are week (R2 ~0.16-0.25).  I imagine this is because this estimate of elemental imbalance is probably only a crude reflection of the imbalance between demand and available nutrient ratios.  
Overall, this paper was excellent.  I'm interested to see other studies on how elemental composition changes though different life-history stages.  I'd be particularly interested to see how tropical and temperate species compare, since energy storage for low-productivity winter months seems likely to strongly affect elemental composition in temperate biota.

Veldboom, J.A., & Haro, R.J. (2011). Stoichiometric relationship between suspension-feeding caddisfly (Trichoptera: Brachycentridae) and seston Hydrobiologia

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