Food webs and global change: a random sampling

Here is a semi-coherent blob of ideas and things I wrote down from the speakers from yesterday’s symposium

Shurin:  Various pieces of “received wisdom” are not necessarily as they seem, such as the shape of trophic pyramids, the idea that trophic cascades are stronger in aquatic systems, and the role of food web complexity in FW vs marine systems.  Meta-analysis shows that H/A ratios are a strong function of autotroph (A) nutrient content but not of productivity.  Also it might not be the case that FW foods are actually simpler than marine ones (vis a vis role of omnivory).

Sommer:  Warming potentially produces a mismatch between nauplii growth / emergence and timing of algal bloom.

Gruner:  Top-down / bottom up meta-analysis shows that nutrient effects and herbivore removal effects are widespread and similar in all ecosystem types (FW, marine, terrestrial).  Generally nutrient x herbivore interaction effects are weak.

Elser:  blah blah blah.

Diehl:  In mesocosm study, warming treatment led to runaway green algal bloom with very high seston C:P ratio leading to collapse of Daphnia.  Matches stoichiometric model very nicely.

Tranvik: Meta-analysis of 7514 lakes worldwide has median 5.71 mg C / L.    “Geomolecules” generated by chaotic polymerization.  DOC concentration positively correlated with pCO2 supersaturation.  Typically <20% of DOC is “available”.  %DOC lost in a lake is a directly proportional to water residence time. Half-life ca. 3.5 years.  Large-scale studies suggest 80% of DOC is degradable. Degradation rate is the same across lakes and not affected by N&P.  DOC lost is not a simple one-pool kinetic; more like a “reactive continuum”.
Should reject “passive pipe” model of C handling by inland waters.  0.9 Pg of C exported by land, 0.9 Pg received by ocean.  More accurate:  2.9 Pg into inland water.  1.4 Pg via degassing, 0.6 Pg to sediments.  Therefore 0.9 Pg to ocean.  For comparison, ocean C uptake is 2.6 Pg.

Bronkowski:   Pathways of C in soils. Higher CO2 increased yield and net primary production, reduces evapotranspiration. Leads to wetter soils and stimulates soil microbial respiration?  Microbial respiration stimulated by N/P addition, including P.   N sequestered by fungi and bacteria when C:N>30. When C:N <12.5 N is released by fungi but sequestered by bacteria. Between 12.5 and 30, only released by fungi. Enhanced CO2 will increase root exudation. Competition for nutrients between plant roots and microbes? N release dependent upon release by protozoa. 50% of OM of litter is lost within the first month. Rapid initial phase of decomposition, then slow later phase.

Wolkovich:  14% of global soil pool is in the Arctic.  Warm soil leads to increased plant uptake (good) but nearly all of that is released as exudation that stimulates microbial respiration plus stimulates metabolism of stored soil C pool (“priming” effect).

Meta-analysis shows median of % fixed C exuded of 10%.  Max of 60%  Min approaching 0%.

Hillebrand:  Effects of humans on biodiversity patterns from a meta-community perspective.  Habitat fragmentation, species invasion, global warming.   Warming generally predicted to result in lower diversity (by meta-analysis of tundra experiments; Walker et al. PNAS).   Arctic study showing strong temp x N interaction in amplifying negative effects on diversity.

Olff:  Parallel interaction networks in ecosystems.  Interaction networks, not just trophic networks.  Plus effects of environmental conditions on species and effects of species on environmental conditions.

“Airplane” shape of food webs largely driven by C:nutrient of resource base.  As C:X ratio  gets higher, you need to be bigger to burn off the extra C. Algae and microbes on the left (low C:N), grass in the middle,  trees on right with highest (C:N).   High C:X food base means short food web on that end.  Low C:X in food base can support higher trophic level.


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