Reading: Ecological Community Description

Ecological community description using the food web, species abundance, and body size
Cohen, Joel E., Thomas Jonsson, and Stephen R. Carpenter

This is a summary of this math-heavy reading for Lecture 16 on November 9. Complicated equations will be left out, but relationships between variables described.

This study focuses on the near inverse relationship between body mass and numerical abundance, tied in to energy flow in a community. The paper attempts to show the connectedness of ecological patterns normally shown as independent variables. The research was conducted at Tuesday Lake in the UP.

They note the tale of the blind men and the elephant, where the blind men cannot agree because they are experiencing different parts of the animal. This is like studying the food web, M, and N separately, when an integrated approach will give you a more comprehensive picture of the ecosystem.

Variables of note:
M: average body mass of individual
N: numerical abundance of the species
B: biomass abundance of species
H: trophic height (place on the food web

Trophic position: placement of a species in the food chain, one plus the number of species below it. (autotrophic species with no species below are in position 1)
Trophic vulnerability (V): number of consumers the species has
Trophic generality (G): number of prey the species has

Then some calculations and statements of relationships are made:
• From previous studies, it is know B= MN
• Now the study moves on to connect H. If larger species on average are found higher up than smaller species in the food web, the trophic height H (remember that trophic pyramid?) can be related to M-rank
• The predator’s M is generally larger than the prey’s M
• Small, numerically abundant species occur at low H and less abundant species occur at higher H
• An increase in M normally means a decrease in N
• Most of the time, predator M is greater than prey M, and predator N is less than prey N
• Consumer species’ N should be greater than expected if the consumer has more species of prey and less than expected if the consumer shares its prey with other consumer species.
• Decreasing numbers of species with increasing H.

The food web makes it possible to refine the relationship between N and M. After plotting numerical abundance vs. body mass on logarithmic chart, the plotted slope show that consumer biomass abundance increases from the bottom to the top of the food web.

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