Grazers as Sources and Sinks for Nutrients:Conclusions, Limitations. and Speculations
211
The positive relationship between algal growth rate and nutrient supply
rate in the simple two-level food chains considered here implies that
eutrophication can also be considered a gradient in loss rates from the
phytoplankton community. This means that in the stable equilibrium
situation, the outcome of algal resource competition and the sequence of
species replacements in an eutrophication gradient are directly predictable
from the Monod curves for the individual species (Figs. 3.6 and 6.2). When
the system has a stationary state with persistent (periodic or aperiodic)
oscillations, the zooplankton can change temporally from being a net sink,
when the population is increasing, to a net source when it is declining. In a
situation with several competing prey populations, a fluctuating grazer
biomass can alternate between being a net source and a net sink for nutrients to different prey populations. This creates temporal niches that can be
exploited by different algal species, allowing coexistence of up to three
species on a single limiting resource (Fig. 6.4).
Such coexistence, mediated by fluctuations in grazer abundance, is
apparently only possible when the grazers feed nonselectively on the available prey species: introducing either differential grazing or differential
recycling of essential resources leads to the competitive exclusion of all but
one species (Sects.6.2 and 6.3). In such a competitive situation, grazers will
generally constitute a net source for nutrients to the superior species, and a
net sink to the excluded species, irrespective of whether the grazer population is stable or fluctuating.
Since nutrients located in inedible algae will be unavailable to grazers,
such species will represent a net sink for nutrients from the herbivore point
of view. The accumulation of inedible algae will reduce food-chain transfer
efficiency of energy and nutrients, and eventually destroy food-chain integrity; as such, it is often considered a major cause of biomanipulation failure. Since edible algae suffer extra losses in the presence of grazers, foodchain integrity can be maintained if edible algae are competitively superior
to inedible algae in the absence of grazers. In other words, food chains can
be resistant to invasion by inedible species if strong tradeoffs between
predator defence and competitive ability for limiting nutrients can be
found. The survey of algal growth and nutrient uptake parameters in
Chapter 3 revealed only one such tradeoff; algae classified as resistant to
grazing had significantly lower maximal specific growth rates. The model
predictions in Figs. 6.9 and 6.10 indicate that a growth rate advantage is
insufficient to make food chains invasion-resistant, and that uninvadability
to completely inedible algae will only be possible if the invaders have additional handicaps, such as high sinking loss rates or high nutrient efflux
rates.
Due to the lack of systematic studies with simultaneous measurement of
relevant model parameters, individual parameters have mostly been
treated as independent, random variables. The demonstration of evolutionary tradeoffs between different model parameters would impose a
211
The positive relationship between algal growth rate and nutrient supply
rate in the simple two-level food chains considered here implies that
eutrophication can also be considered a gradient in loss rates from the
phytoplankton community. This means that in the stable equilibrium
situation, the outcome of algal resource competition and the sequence of
species replacements in an eutrophication gradient are directly predictable
from the Monod curves for the individual species (Figs. 3.6 and 6.2). When
the system has a stationary state with persistent (periodic or aperiodic)
oscillations, the zooplankton can change temporally from being a net sink,
when the population is increasing, to a net source when it is declining. In a
situation with several competing prey populations, a fluctuating grazer
biomass can alternate between being a net source and a net sink for nutrients to different prey populations. This creates temporal niches that can be
exploited by different algal species, allowing coexistence of up to three
species on a single limiting resource (Fig. 6.4).
Such coexistence, mediated by fluctuations in grazer abundance, is
apparently only possible when the grazers feed nonselectively on the available prey species: introducing either differential grazing or differential
recycling of essential resources leads to the competitive exclusion of all but
one species (Sects.6.2 and 6.3). In such a competitive situation, grazers will
generally constitute a net source for nutrients to the superior species, and a
net sink to the excluded species, irrespective of whether the grazer population is stable or fluctuating.
Since nutrients located in inedible algae will be unavailable to grazers,
such species will represent a net sink for nutrients from the herbivore point
of view. The accumulation of inedible algae will reduce food-chain transfer
efficiency of energy and nutrients, and eventually destroy food-chain integrity; as such, it is often considered a major cause of biomanipulation failure. Since edible algae suffer extra losses in the presence of grazers, foodchain integrity can be maintained if edible algae are competitively superior
to inedible algae in the absence of grazers. In other words, food chains can
be resistant to invasion by inedible species if strong tradeoffs between
predator defence and competitive ability for limiting nutrients can be
found. The survey of algal growth and nutrient uptake parameters in
Chapter 3 revealed only one such tradeoff; algae classified as resistant to
grazing had significantly lower maximal specific growth rates. The model
predictions in Figs. 6.9 and 6.10 indicate that a growth rate advantage is
insufficient to make food chains invasion-resistant, and that uninvadability
to completely inedible algae will only be possible if the invaders have additional handicaps, such as high sinking loss rates or high nutrient efflux
rates.
Due to the lack of systematic studies with simultaneous measurement of
relevant model parameters, individual parameters have mostly been
treated as independent, random variables. The demonstration of evolutionary tradeoffs between different model parameters would impose a