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Modeling the Size Spectrum for Macroinvertebrates and Fishes in Stream Ecosystems

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The May threshold and life-history allometry.

Lev R Ginzburg¹, Oskar Burger, John Damuth

¹Department of Ecology and Evolution, Stony Brook University, NY 11794, USA.

Biology Letters

|July 2, 2010

Summary

Population reproductive rates exceeding the May threshold can lead to extinction. This ecological elimination may explain why lifetime reproductive rates are independent of body size across diverse animal taxa.

Area of Science:

Ecology
Population Dynamics
Evolutionary Biology

Background:

Robert May's work established that chaotic population dynamics arise when average lifetime reproductive rates surpass a critical threshold.
Populations exceeding this 'May threshold' are prone to extinction, with the threshold varying based on density-dependence curves.
Species with similar density-dependence curves share a May threshold, irrespective of size or generation time.

Purpose of the Study:

To propose that the ecological elimination of unstable populations explains the observed allometric independence of lifetime reproductive rate from body size.
To challenge the conventional explanation of this allometry as solely due to physiological life-history trade-offs.

Main Methods:

Theoretical analysis linking population stability thresholds to life-history traits.

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Comparative analysis across animal taxa to assess the relationship between body size and lifetime reproductive rate.

Main Results:

The study argues that a shared May threshold among species with similar density-dependence curves predicts a lack of allometric scaling between lifetime reproductive rate and body size.
This ecological mechanism provides an alternative explanation for the observed independence of lifetime reproductive rate from body size across diverse taxa.

Conclusions:

The ecological elimination of populations pushed into chaotic dynamics by high reproductive rates is a significant factor shaping life-history allometries.
This provides a compelling ecological explanation for the widespread observation of lifetime reproductive rate independence from body size in the animal kingdom.