400
interaction and generate alternative community
states.
The alternative states generated by changes in
light and/or nutrient supplies appear to be qualitatively different worlds of interaction. The first state,
which I will call the "quantity world," is in the vicinity of equilibrium point Bl (see Fig. 26.7B) and
is a world where actively growing algae are kept in
check by high grazing rates; grazers are constrained
by low food quantity and regenerate P at high rates.
This is the world where most conventional food
web theory holds and where most of our intuition
about trophic interactions has been developed.
However, there appears to be a second world of
interaction that may be lurking in many ecosystems, generating confusion in food web ecologists.
I refer to this world as the "quality world," the part
of the phase plane at or near the locally unstable
equilibrium point B2 and influenced by the locally
stable point B3. In the quality world, algae are
growing at low, P-stressed rates and are fed upon
by low biomasses of zooplankton grazers whose
growth rates are strongly limited by food quality
and whose rates of P-release are extremely low.
Furthermore, the entire system is at risk of collapse
to the locally stable grazer extinction point. In this
world, much of our ecological intuition is strangely
inaccurate. Here, high grazer growth capacity is
disadvantageous to the grazer as high body P composition increases the likelihood of extinction.
Grazers are no longer a source of nutrients for algae
via P recycling but instead are a sink. In the quality
world, grazers are not in competition with each
other for food and instead intraspecific (Sommer
1992) or even interspecific facilitation (Urabe
1995) are possibilities. Facilitation occurs because
increased cropping of algae potentially raises per
capita algal nutrient supply and thus improves food
quality and therefore growth for each grazer. The
situation in the quality world involves unconventional interactions but appears consistent with observations from P-stressed lakes on the Canadian
Shield (Elser et al. 1998; Hassett et al. 1997;
Sterner and others 1997). It seems imperative to
better understand what environmental factors affect
algal nutritional status and thus shift the pelagic
food web between the quantity and quality worlds.
The balance of light and nutrient supplies discussed
earlier (Sterner et al. 1997) may be a place to start.
James J. Elser
New theoretical work incorporating stoichiometric
constraints seems critical as well.
What About the Microbes?
At this point of the chapter many of my
pelagic colleagues may be tapping their fingers,
impatient with my emphasis on phytoplanktonmacrozooplankton interactions and waiting for a
discussion of how the critically important transformations mediated by microbes (bacteria, protozoa)
are incorporated into this way of thinking. Unfortunately, I have little room left here to describe
exciting work by which similar principles have
been elucidated for processes operating in the microbial realm, but in this section I will briefly summarize some of the most important findings in this
area. Much of this work supports the idea that
supplies of energy and key materials jointly affect
the microbial food web. Thus, stoichiometric thinking may help illuminate the general principles
of energy and material cycling that are common to
the microbial and conventional food webs, and
thus help to unify these sometimes divergent
approaches.
First, as for the autotrophic algae, bacterial elemental composition is not fixed and instead varies
considerably with environmental conditions and
growth rate (Vadstein et al. 1988; Chrzanowski and
Kyle 1996). Here, the basic principles are roughly
the same: under P-limited conditions, bacterial CIP
ratio is inversely proportional to growth rate
(Chrzanowski and Kyle 1996). Stoichiometry also
appears important in the bacteria-flagellate interaction, as rates of nutrient release are strongly
linked to both the elemental composition of the bacteria and of the bacterivore (Eccleston-Parry and
Leadbeater 1995; Jurgens and Gude 1990; Caron et
al. 1988; Goldman et al. 1985, 1987; Nakano
1994). Furthermore, mixotrophic bacterivores appear to respond adaptively to changes in their nutritional environment. For example, studies by Nygaard and Tobiesen (1993) and Rothhaupt (1996)
show that mixotrophs shift between bacterivorous
and autotrophic modes of nutrition depending on
relative supplies of light, inorganic P, and prey
bacteria.
Microbial processing of detrital materials is also
strongly dependent on the elemental composition
interaction and generate alternative community
states.
The alternative states generated by changes in
light and/or nutrient supplies appear to be qualitatively different worlds of interaction. The first state,
which I will call the "quantity world," is in the vicinity of equilibrium point Bl (see Fig. 26.7B) and
is a world where actively growing algae are kept in
check by high grazing rates; grazers are constrained
by low food quantity and regenerate P at high rates.
This is the world where most conventional food
web theory holds and where most of our intuition
about trophic interactions has been developed.
However, there appears to be a second world of
interaction that may be lurking in many ecosystems, generating confusion in food web ecologists.
I refer to this world as the "quality world," the part
of the phase plane at or near the locally unstable
equilibrium point B2 and influenced by the locally
stable point B3. In the quality world, algae are
growing at low, P-stressed rates and are fed upon
by low biomasses of zooplankton grazers whose
growth rates are strongly limited by food quality
and whose rates of P-release are extremely low.
Furthermore, the entire system is at risk of collapse
to the locally stable grazer extinction point. In this
world, much of our ecological intuition is strangely
inaccurate. Here, high grazer growth capacity is
disadvantageous to the grazer as high body P composition increases the likelihood of extinction.
Grazers are no longer a source of nutrients for algae
via P recycling but instead are a sink. In the quality
world, grazers are not in competition with each
other for food and instead intraspecific (Sommer
1992) or even interspecific facilitation (Urabe
1995) are possibilities. Facilitation occurs because
increased cropping of algae potentially raises per
capita algal nutrient supply and thus improves food
quality and therefore growth for each grazer. The
situation in the quality world involves unconventional interactions but appears consistent with observations from P-stressed lakes on the Canadian
Shield (Elser et al. 1998; Hassett et al. 1997;
Sterner and others 1997). It seems imperative to
better understand what environmental factors affect
algal nutritional status and thus shift the pelagic
food web between the quantity and quality worlds.
The balance of light and nutrient supplies discussed
earlier (Sterner et al. 1997) may be a place to start.
James J. Elser
New theoretical work incorporating stoichiometric
constraints seems critical as well.
What About the Microbes?
At this point of the chapter many of my
pelagic colleagues may be tapping their fingers,
impatient with my emphasis on phytoplanktonmacrozooplankton interactions and waiting for a
discussion of how the critically important transformations mediated by microbes (bacteria, protozoa)
are incorporated into this way of thinking. Unfortunately, I have little room left here to describe
exciting work by which similar principles have
been elucidated for processes operating in the microbial realm, but in this section I will briefly summarize some of the most important findings in this
area. Much of this work supports the idea that
supplies of energy and key materials jointly affect
the microbial food web. Thus, stoichiometric thinking may help illuminate the general principles
of energy and material cycling that are common to
the microbial and conventional food webs, and
thus help to unify these sometimes divergent
approaches.
First, as for the autotrophic algae, bacterial elemental composition is not fixed and instead varies
considerably with environmental conditions and
growth rate (Vadstein et al. 1988; Chrzanowski and
Kyle 1996). Here, the basic principles are roughly
the same: under P-limited conditions, bacterial CIP
ratio is inversely proportional to growth rate
(Chrzanowski and Kyle 1996). Stoichiometry also
appears important in the bacteria-flagellate interaction, as rates of nutrient release are strongly
linked to both the elemental composition of the bacteria and of the bacterivore (Eccleston-Parry and
Leadbeater 1995; Jurgens and Gude 1990; Caron et
al. 1988; Goldman et al. 1985, 1987; Nakano
1994). Furthermore, mixotrophic bacterivores appear to respond adaptively to changes in their nutritional environment. For example, studies by Nygaard and Tobiesen (1993) and Rothhaupt (1996)
show that mixotrophs shift between bacterivorous
and autotrophic modes of nutrition depending on
relative supplies of light, inorganic P, and prey
bacteria.
Microbial processing of detrital materials is also
strongly dependent on the elemental composition