Tuesday, October 25, 2011

Ecosystem function in a brown world

ResearchBlogging.org    I highlighted this paper a few days ago as looking interesting, but after reading it, I felt like expanding a bit.  The paper is by Rubbo et al (2011, Aquatic Sciences) and is entitled "Species loss in the brown world:  are heterotrophic systems inherently stable?"
   Before I get into this paper, I think it is probably worthwhile to put this in some sort of perspective.  There are a few fundamental, objective units in biology.  One of these is the cell, one is the individual.  Within a particular ecosystem, in a particular point in time, another might be the species.  Species definitions get confusing when you start thinking about them over evolutionary time-scales or across huge ranges.  In most settings, it is pretty clear that individual cricket frogs form a distinct and unified group when compared to (for instance) Daphnia magna.  In the minds of old-school natural history biologists, and the general public, species are important.  The whole idea of biodiversity is rooted in the idea that more species is somehow better.
   However, when people started thinking about ecosystem functions (nutrient cycles, primary production) and ecosystem services (carbon sequestration, food production, waste disposal), it was trickier than you might imagine to connect biodiversity to these ecosystem processes and products.  Some research suggested it is particular species, rather than diversity that drove ecosystem function.
   The impression I get (although I'm certainly not an expert) is that certain species are thought to be more likely to have big effects on ecosystem processes than others:  Species that dominate overall biomass, top-predators and other 'keystone' species.
   So what did they do in this paper?  Well, essentially they were extending these relationships between ecosystem processes and community composition into very heterotrophic ecosystems:  Forested vernal ponds. These ponds probably don't get a lot of sunlight, so not a lot of primary production.  Instead, most of the system's energy comes from material that falls in from the nearby terrestrial system.
   The authors have done quite a bit of work on these systems.  They decided to see what would happen if they removed the top predators out of these vernal ponds.  So the authors removed all the frogs and salamanders from 12 ponds, and then added them back in to a randomly selected subset of those ponds.
  The really surprising thing is that nothing seemed to happen.  The "ecosystem processes" that they measured (essentially production and respiration) varied a lot, but didn't vary in response to the presence of these frogs and salamanders.  The authors speculated that because this is a heterotrophic system, the base of the food web (being microbial) has such high turnover that changing grazing pressure wasn't able to affect the overall movement of energy up the food web.  Hence the title suggesting the "Brown world" is more resilient.  That sounds good, but doesn't that suggest algae based systems would be very resilient?  Have these kinds of studies been done in streams (which are also heterotrophic)?
   This is a perfect example of a nice study that is simple, easy-to-understand and extremely well-written.  I don't usually think of Aquatic Science as a top-tier journal, but if this is the caliber of paper that is routinely published there, then I feel like I need to change my thinking.

  Michael J. Rubbo, Lisa K. Belden, Sara I. Storrs-Mendez, Jonathan J. Cole, & Joseph M. Kiesecker (2011). Species loss in the brown world: are heterotrophic systems inherently stable? Aquatic Sciences

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