VI
Foreword
choice for modelling purposes, not only because it is a good mediator of high
trophic efficiency, but also because there is a long tradition of ecophysiological studies on these organisms that provide reliable input parameters. In
logical model development, the foundation in terms of the basic lifeand
death parameters for the central actors, Daphnia, algae, and bacteria, is set
before the first act is played. The main theme is then elaborated further
through increasing model complexity in the subsequent acts.
The play metaphor is in the spirit of A. J. Lotka, who in his pioneering
work from 1925, first attempted to merge nutrient cycling and stoichiometric
arguments into basic ecology. Actors and the scene as well are made of the
same stuff that recycle over and over again in the great theatre of life. This
early attempt was largely neglected, due partly to purely practical reasons. It
this book, however, the author pays an enlightening revisit to Lotka's ideas by
incorporating stoichiometric concepts in his models, clearly demonstrating
how vital they are to productivity and foodweb structure. Recent models
incorporating stoichiometric considerations in food-web studies show that
the transfer of energy in terms of carbon from autotrophs to higher trophic
levels may be governed by food quality in terms of element ratios. When food
is high in C relative to other essential elements like nitrogen and phosphorus,
excess C simply "goes to waste". The application of the Liebig minimum principle, also for the heterotrophs, is a new way of predicting "trophic efficiency"
in food webs. In a prey-predator scheme, these stoichiometric applications,
when isocline analysis is carried out, show a new, paradoxically stable state
with zero predator (grazer) biomass at high algal biomass, when the algae are
too P-deprived to support zooplankton growth. Such qualitative predator
defences should attract attention from all those working with prey-predator
systems.
However, intriguing they may be to a theoretical ecologist, we should, as
always, ask whether these models have any bearing on predictions at the
ecosystem level. They have indeed. In this book, these aspects are explored,
not merely as a performance of single actors, but as an examination of the
play in a more complete context. With a stepwise logical argumentation for
parameter inputs, the author creates a toolbox of submodels, allowing him to
judge Rosenzweig's axiom on less subjective grounds than most other people.
The data do indeed provide evidence for reduced grazer (Daphnia) success,
instability and chaotic ecosystem behaviour beyond a certain enrichment
threshold; enrichment thus decreases predictability. There is a lesson to be
learned from these exercises, not only for aquatic ecologists but for all those
dealing with herbivore impact on ecosystems, and the impact of enrichment.
Through a clear line of reasoning, we are guided on a very visible path all the
way from large-scale processes such as phosphorus loading, lake hydrology
and dilution rate, via species' physiology, to cellular processes and back to a
better understanding of other large-scale processes like foodweb "efficiency"
and ecosystem stability.
Oslo, January 1997
Dag O. Hessen
Foreword
choice for modelling purposes, not only because it is a good mediator of high
trophic efficiency, but also because there is a long tradition of ecophysiological studies on these organisms that provide reliable input parameters. In
logical model development, the foundation in terms of the basic lifeand
death parameters for the central actors, Daphnia, algae, and bacteria, is set
before the first act is played. The main theme is then elaborated further
through increasing model complexity in the subsequent acts.
The play metaphor is in the spirit of A. J. Lotka, who in his pioneering
work from 1925, first attempted to merge nutrient cycling and stoichiometric
arguments into basic ecology. Actors and the scene as well are made of the
same stuff that recycle over and over again in the great theatre of life. This
early attempt was largely neglected, due partly to purely practical reasons. It
this book, however, the author pays an enlightening revisit to Lotka's ideas by
incorporating stoichiometric concepts in his models, clearly demonstrating
how vital they are to productivity and foodweb structure. Recent models
incorporating stoichiometric considerations in food-web studies show that
the transfer of energy in terms of carbon from autotrophs to higher trophic
levels may be governed by food quality in terms of element ratios. When food
is high in C relative to other essential elements like nitrogen and phosphorus,
excess C simply "goes to waste". The application of the Liebig minimum principle, also for the heterotrophs, is a new way of predicting "trophic efficiency"
in food webs. In a prey-predator scheme, these stoichiometric applications,
when isocline analysis is carried out, show a new, paradoxically stable state
with zero predator (grazer) biomass at high algal biomass, when the algae are
too P-deprived to support zooplankton growth. Such qualitative predator
defences should attract attention from all those working with prey-predator
systems.
However, intriguing they may be to a theoretical ecologist, we should, as
always, ask whether these models have any bearing on predictions at the
ecosystem level. They have indeed. In this book, these aspects are explored,
not merely as a performance of single actors, but as an examination of the
play in a more complete context. With a stepwise logical argumentation for
parameter inputs, the author creates a toolbox of submodels, allowing him to
judge Rosenzweig's axiom on less subjective grounds than most other people.
The data do indeed provide evidence for reduced grazer (Daphnia) success,
instability and chaotic ecosystem behaviour beyond a certain enrichment
threshold; enrichment thus decreases predictability. There is a lesson to be
learned from these exercises, not only for aquatic ecologists but for all those
dealing with herbivore impact on ecosystems, and the impact of enrichment.
Through a clear line of reasoning, we are guided on a very visible path all the
way from large-scale processes such as phosphorus loading, lake hydrology
and dilution rate, via species' physiology, to cellular processes and back to a
better understanding of other large-scale processes like foodweb "efficiency"
and ecosystem stability.
Oslo, January 1997
Dag O. Hessen