The food web was a major focus of this module

25 Jun The food web was a major focus of this module

Question
Directions: The food web was a major focus of this module. There is an active food web in our oceans as well, and it has a very prominent “microbial loop.” Please read the following article:

Pomeroy, L.R. (2001). Caught in the food web: complexity made simple? Scientia Marina 65 (Suppl. 2): 31-40.

Note: You can find the above article in your iBoard course under Homework Assignment Instructions.

This paper will introduce you to many of the themes covered in your lecture, but from the perspective of an oceanographer. The paper is now over 10 years old and further advancements have been made.

Write a three to five page paperwhich describes the Pomeroy article and search for more recent findings on the topic (e.g., recent advances in nitrification in our ocean).

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SCI. MAR., 65 (Suppl. 2): 31-40

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SCIENTIA MARINA

2001

A MARINE SCIENCE ODYSSEY INTO THE 21st CENTURY. J.M. GILI, J.L. PRETUS and T.T. PACKARD (eds.)

Caught in the food web: complexity made simple?*
LAWRENCE R. POMEROY
Institute of Ecology, University of Georgia, Athens GA 30602-2202, USA. E-mail: [email protected].edu

SUMMARY: Several historically separate lines of food-web research are merging into a unified approach. Connections
between microbial and metazoan food webs are significant. Interactions of control by predators, defenses against predation,
and availability of organic and inorganic nutrition, not any one of these, shape food webs. The same principles of population ecology apply to metazoans and microorganisms, but microorganisms dominate the flux of energy in both marine and
terrestrial systems. Microbial biomass often is a major fraction of total biomass, and very small organisms have a very large
ratio of production and respiration to biomass. Assimilation efficiency of bacteria in natural systems is often not as high as
in experimental systems, so more primary production is lost to microbial respiration than had been thought. Simulation has
been a highly useful adjunct to experiments in both population theory and in studies of biogeochemical mass balance, but it
does not fully encompass the complexity of real systems. A major challenge for the future is to find better ways to deal with
the real complexity of food webs, both in modeling and in empirical observations, and to do a better job of bringing together conceptually the dynamics of population processes and biogeochemistry.
Key words: food web, microbial food web, energy flux, community structure.

INTRODUCTION
“The classic marine food chain –algae, zooplankton, fish– can now be considered as a variable phenomenon in a sea of microbes (Karl 1999).”
The food web is one of the earliest and most fundamental concepts in ecology. Darwin (1845) recognized the existence of a pelagic food chain. Elton is
credited with first appreciating the importance of food
chain and food web concepts (Lawton 1989), but
major antecedents include Petersen’s (1918) quantitative conceptual model of the food web that is supported by eel grass, and Hardy’s (1924) conceptual model
of the herring food web. Elton, and later Hutchinson
and his students, developed both population and mate*Received March 29, 2001. Accepted April 30, 2001.

rials-flux approaches to food webs (Hagen 1992), but
the two approaches quickly diverged. Paine (1980)
showed that population interactions do not equate with
energy flux, and asserted that energy flux was unimportant and “has generated few insights into ecological processes.” This put studies of population biology
and energy-flux on separate paths that only recently
have shown signs of beginning to merge into a unified
paradigm (e. g. McQueen, et al., 1986; Hunter and
Price 1992; Polis and Winemiller 1996). Ecologists
recognize that energy input matters, although they
agree less on when and how it matters. What clearly
matters is formulating tractable hypotheses about
ecosystem structure and function. Repeated attempts
to simplify the inherent complexity of food webs have
led to Sisyphus-like progress in which investigators
have made generalizations and then have been forced
to qualify them.
FOOD WEBS 31

A step not yet firmly taken is the assimilation of
microorganisms into conceptual food webs and food
web models in spite of evidence that the ocean’s
food web, like most others, is primarily microbial (e.
g. Fuhrman et al., 1989; Li et al., 1992; Sommer
1993; Karl, 1999). This review views microbial food
webs and metazoan food webs as a single, continuous entity and examines evidence that energy flux
can influence community structure. Of 113 food
web descriptions compiled by Cohen et al., (1990),
20 include bacteria, 9 protozoa, and 8 fungi. Virtually every one of those fails to include 2 or more
microbial links. It is equally true that studies of
microbial food webs have rarely included interactions with metazoans except as donors of organic
matter, sometimes on grounds that the large metazoans constitute such a small fraction of energy flux.

FOOD WEB STRUCTURE
Recognizing microbial food webs
A very early suggestion of the significance of
microorganisms in the sea came from Lohmann
(1911), whose observations were imaginatively
extrapolated by Vernadskii (1926). At that time,
microbiology lacked the technology to enumerate
bacteria or to estimate their production. During
most of the 20th Century, microorganisms were
thought to be significant in regenerating nitrogen
and phosphorus but not to be significant components of the flux of carbon in the marine food web
(e. g. Steele, 1974; Cohen et al.,, 1982). Because
their size is similar to the wavelengths of visible
light, most marine bacterioplankton are invisible to
conventional microscopy and could not be counted
directly until the development of epifluorescence
microscopy (Francisco et al., 1973; Hobbie et al.,
1977). Their metabolic impact on ecosystems was
underestimated until the development of tracer
methods (Azam and Hodson 1977; Fuhrman and
Azam 1980). The view that the ocean is a microbial
system has not been shared by many fisheries scientists (e. g. Steele 1974; Cohen et al., 1982;
Boudreau and Dickie 1992; Cury et al., 2000).
They acknowledge the existence of the microbial
food web but dismiss its significance to the food
webs of fishes. A different view has been proposed
by oceanographers, based on the work of Moloney
and Field (1990) and Painting, et al., (1993) which
will be discussed in section 4.
32 L.R. POMEROY

Significant differences exist between terrestrial
and aquatic food webs (e g. Cohen 1994; Fenchel
1994) but there is not complete agreement on what
the differences are. It is probably important that,
unlike single-celled phytoplankton, entire terrestrial
plants are rarely consumed completely by grazers,
and the standing biomass of terrestrial plants is
rarely depleted by them (Hairston et al., 1960),
while in the sea only transient phytoplankton
blooms temporarily outrun consumption. What is
sometimes overlooked is that in terrestrial ecosystems most of the microbial food web is situated in
soils, which are not treated as a part of the system in
most population models or analyses of terrestrial
food webs, but are modeled separately by different
investigators. Connections from microbes to consumer populations exist through soil invertebrates
and their predators. A significant connection of
microbes to higher organisms is through symbiotic
mycorrhizae that sequester inorganic nutrients for
trees while receiving organic nutrients. Mycorrhizae
are consumed by the metazoan food web in the soil,
and their sporocarps are eaten copiously by vertebrates, including H. sapiens. Much of the biodiversity and many physically small organisms in terrestrial systems are in the soil, so terrestrial systems
may not be as depauperate in those respects as
Cohen (1994) suggests.
Size versus biomass
The total amount of prokaryotic carbon in the
biosphere is at least 60% of that in plants and probably exceeds their metabolically active tissue (Whitman et al., 1998). In terrestrial soils, 94-97% of the
non-root biomass is bacteria and fungi, present in
quantities of tons/ha (Coleman, 1994), similar in
magnitude to root biomass (Schlesinger, 1991). In
abyssal benthic systems, 87% of the biomass consists of bacteria, present in quantities of kg/ha (Tietjen 1992). In oligotrophic temperate lakes, bacteria
comprise up to 58% of the biomass (Biddanda in
press). In the oligotrophic Sargasso Sea, bacterioplankton constitute 70-80% of the biomass, and protozoa constitute 7-17% (Fuhrman et al., 1989; Li et
al., 1992). In the North Atlantic spring bloom (Table
1), bacterioplankton constitute 15-50% of total biomass and 64-75% of heterotrophic biomass, with
mesozooplankton being 0.5-2% (Harrison et al.,
1993; Li et al., 1993). The distribution of biomass
among different decades of size is not as regular as
some earlier work suggested (e. g. Sheldon et al.,

TABLE 1. – Comparison of the biomass, mg C m-3, in a coastal marine system, an oceanic central gyre, and oceanic frontal region. Integrated
to 100 m off New Zealand (Bradford-Grieve et al., 1999), and to 200 m in the Sargasso Sea (Fishes: Angel, 1989; Net zooplankton: Ortner
et al., 1980 and Ashjian et al., 1994; Protozoa: Caron et al., 1995; autotrophs and bacterioplankton: Li et al., 1992). English Channel
sampled only at 10 m (Rodruíguez et al., 2000, except fishes Cohen et al., 1982).
English Channel off Plymouth
Season
Fishes and squid
Zooplankton
Protozoa
Bacterioplankton
Autotrophs

spring

summer

autumn

winter

770
500
50
10
45

770
1200
150
20
160

770
1200
25
20
28

Sargasso
Sea
spring

770
300
15
15
10

1
1.3
4.3
5.6
5

Waters East of New Zealand
Subtropical
Subtropical front
Subantarctic
spring
winter
spring
winter
spring
winter

1.2
1.2
5.1
11.2

29
10.1
22.4
9.7

1.9
1.3
5.7
22.2

5.5
4.7
14.2
43.5

3.1
1
7.9
3.4

3.9
5.9
11.5
10.8

1972). Bacterial biomass in the ocean varies much
less than the biomass of phytoplankton and other
microorganisms (Table 1), and bacteria tend to be
the dominant biomass in extremely oligotrophic
environments, such as the central ocean gyres
(Fuhrman et al., 1989; Li et al., 1992). Where seasonal studies have been performed, they tend to
show major changes in biomass of other components (Donali et al., 1999; Rodruíguez et al., 2000).

system size, but not system productivity (Post et al.,
2000). To make matters worse for investigators, a
large fraction of the micro- and picoplankton is
composed of mixotrophic organisms and internalized mutualistic associations (Safi and Hall, 1999;
Caron, 2000; Dolan and Pérez, 2000). Also,
chemolithotrophic bacteria have evolved associations with organisms in many phyla (Cavanaugh,
1994; Polz et al., 2000).

Chain length and web complexity

Predator or donor control of structure?

Although not everyone agrees (e. g. Morin,
1999), probably neither the amount of primary production nor the assimilation efficiency of consumers
is a major determinant of the length of food chains or
diversity of most food webs. Fretwell (1977), while
observing terrestrial metazoan food chains, proposed
that high productivity produced longer food chains,
although Ryther (1969), observing the sea, proposed
that the high fish production of coastal upwellings
was the result of high primary productivity and a
short food chain. We now know that the food web of
coastal upwellings contains most of the links present
in the food webs of central gyres (Vinogradov and
Shushkina, 1978; Moloney and Field, 1990; Painting
et al., 1993; Carr, 1998). The emergence of short
chains in blooms and upwellings may, nevertheless,
be significant for terminal-consumer fishes, as
Ryther (1969) suggested (Legendre, 1990).
The number of trophic transfers is limited ultimately by dissipation of energy, but simple conclusions may be confounded by re-utilization of organic matter (Strayer, 1988) and by variable effects of
scales of time and space. The scales on which system productivity affects system function and structure may be different from those of predator-prey
effects (Menge, 2000), and empirical evidence from
lakes indicates that food chain length correlates with

The proposition of Hairston et al., (1960) that
“the world is green,” because terrestrial predators
control the size of grazer populations, instigated a
cascade of research and theory which has trickled
down to the ocean over four decades. Fretwell
(1977) refined the proposition that the number of
links in food chains determines whether primary
producers were controlled by grazers. In systems
with an even number of links, primary producers
were grazer-controlled while in systems with an odd
number of links primary producers were resource
controlled. As Fretwell pointed out, this was most
likely to happen in simple food chains with little or
no branching. Under the rubric, exploitation ecosystems, this was formalized in models (Oksanen et al.,
1981). Based primarily on terrestrial examples, the
principles found useful application in lakes (Carpenter and Kitchell, 1993). Further research in fresh
water (McQueen et al., 1986), in terrestrial systems
(Hunter and Price, 1992), and in microbial microcosms (Lawler, 1998) has revealed a complex interplay of top-down (predator) and bottom-up
(resource) effects. A general theory of food web
dynamics has to take both into account, as well as
other factors. The terrestrial world is green as much
because plants have evolved defenses as because
predators control herbivores (Polis and Strong,
FOOD WEBS 33

1996). Most of the examples of predominant control
by predators are from simple webs, such as rocky
intertidal communities, or even more sparse experimental ones. More complex systems reveal more
complex interactions (Polis, 1991; Polis and Strong,
1996; Polis and Winemiller, 1996). Wiegert and
Owen (1971) pointed out that organic matter does
not accumulate in ecosystems, despite the low utilization of plant resources, because it is being utilized along detrital-microbial pathways.
We are approaching a level of understanding
where controls from top and bottom can be seen
interacting. A modeling study suggests that the ability of copepods to control phytoplankton abundance
may be limited by nitrogen-limited phytoplankton
cells (Roelke, 2000). Also, Thingstad (2000) developed a marine planktonic food web model incorporating both resource limitation and grazer-predator
controls. The model suggests that the control of relative population sizes is top-down but that the total
amount of limiting element (C, N, P) in the entire
food web mediates changes in the size of each population.
While empirical evidence in the ocean indicates
that bacteria are partially resource-limited (Ducklow, 1992), their nearly constant abundance also
suggests limitation by predators. Marine bacterioplankton approach the minimum size possible for an
organism, having even reduced their water content
to a minimum (Simon and Azam, 1989), possibly as
a result of grazer selection of the largest prey. However, some of those very small bacteria transform
into larger rods in the presence of added C, N. and P.
Heterotrophic bacteria attached to detritus, where
presumably there is a more favorable microenvironment, are large (2 µm) rods (Boenigk and Arndt,
2000). Their transformation from 0.3 µm spheres to
rods and back again has been documented (Wiebe
and Pomeroy, 1972; Novitsky and Morita, 1976;
Jacobsen and Azam, 1984). These hypotheses of
strong control on marine bacterioplankton by both
resources and predators have been difficult to test,
but some progress has been made in lakes (Pace and
Cole, 1996; Langenheder and Jürgens, 2001).
Stability: an hierarchical mismatch?
Much theoretical work on food web stability has
relied upon models, microcosms, and simple, accessible communities, such as the rocky intertidal or
upon food chains condensed into trophic levels.
Theory, thus derived, has often been extrapolated to
34 L.R. POMEROY

the larger world, although the problems inherent in
doing this have been pointed out (Allen and Starr,
1982; Lawton, 1989; Polis and Strong, 1996). Models of food webs are necessarily structurally simple
and lack complexity, redundancy, and much of the
reticulate nature of real food webs (Polis, 1991;
Polis and Strong, 1996). The empirical food web
descriptions assembled by Cohen et al., (1990) frequently contain “trophic species,” which are condensations of a guild of species or sometimes much
more. Such caricatures of real food webs have been
criticized as lacking reality (Lawton, 1989). Cohen
et al., (1990) wisely label these simplifying abstractions “spherical horses.”
Sometimes, as Cohen et al., say, you can ride a
spherical horse, but don’t count on doing so. How
effective condensation may be will depend on how
similar the combined species are and what questions we ask about them. Suppose, for example, that
in a model of an upwelling system we create a
trophic species called dinoflagellates, a lesser condensation than the typical trophic species. Dinoflagellates are important in the diet of newly-hatched
anchovy larvae (Lasker, 1979). If the dominant
dinoflagellate is Gymnodinium, the larvae eat them
and grow. If the dominant dinoflagellate is
Gonyaulax, the larvae eat them, but cannot digest
them, and die. Similar food-source problems have
been demonstrated for oysters (Ryther, 1954a) and
for Daphnia (Ryther, 1954b). Communities are
filled with this sort of minute complexity. The obvious danger is in too casually transferring to the rain
forest, or marine plankton, principles shown to
work in three-level, five species models. Caswell
(1988) quotes Lewontin: “It is not the function of
theory to describe what has happened in a particular instance.” But investigators sometimes do that,
even though the real world is more complex.
Lawton (1989) pointed out the lack of ecological studies encompassing both population structure
and energy flux. Pickett et al., (1994) say,
“…unboxing organism features and behaviors and
examining the reciprocal effects of organisms and
ecosystems is a frontier for integration.” An inherent difficulty in this is that populations, communities, and ecosystems are on hierarchically different
levels of organization (Allen, 1987) and have different return times (Slobodkin, 1961). A scheme of
population structure and energy flux interactions
was described for marine planktonic systems by
Legendre and Rassoulzadegan (1995) who classified planktonic communities based on relative

fluxes of energy and nutrients. In the most impoverished central regions of oceans, a microbial-loop
dominates biomass and energy flux. All community components are present, including large fishes
and cetaceans, but the latter are rare. In upwelling
systems, eukaryotic autotrophs and consumers are
more abundant, but the microbial components are
present and their abundance and energy flux is
greater in absolute terms than it is in the central
gyres (Carr, 1998).
The importance of species redundancy for
ecosystem function continues to be debated (Hart et
al., 2001). By creating “trophic species,” modelers
implicitly acknowledge species redundancy,
although Polis and Strong (1996), among others,
question the realism and usefulness of both trophic
species and trophic levels. Systems that appear stable energetically over time may experience catastrophic changes in species interactions. Yet, experiments suggest interaction between biodiversity and
system function, although most such experiments
are small in scale and may not be representative of
larger-scale processes (Moore and Keddy, 1989). In
comparing the stability of different systems, it is
also essential to consider the generation times of the
dominant organisms. Forests appear to be more stable than phytoplankton until we normalize for generation time (Allen 1987).

FOOD WEB FUNCTION
Size versus metabolic rate
We have seen that microorganisms –especially
bacteria– constitute a major fraction of the biomass in both marine and terrestrial ecosystems.
While this alone should indicate a major role in
energy flux for microorganisms, the case is made
stronger by the relation of production per unit biomass (P/B) to body size. Both metabolic rate and
P/B increase by a factor of approximately 1.75
with each order of magnitude decrease in body
weight, although this is variable, with a normally
distributed range having SD = 0.11 (Peters, 1983).
Most compendia of allometric relationships do not
include bacteria. Including them extends the range
of P/B upward by an order of magnitude while
extending the biomass range downward several
orders of magnitude (Fig. 1). Bacterial production
and bacterial respiration are the dominant processes in ocean waters (Sherr and Sherr, 1996) and in

FIG. 1. – Ranges of annual production per unit biomass (P/B) for
organisms of various sizes and taxa. Data from Banse and Mosher
(1980) and Ducklow (1992).

most soils and sediments (Coleman and Crossley,
1996). Combining the ranges of bacterial production reported by Ducklow (1992) and the ranges of
bacterial growth efficiencies reported by Jahnke
and Craven (1995) and del Giorgio and Cole
(2000), it is evident that most marine primary production is utilized in the microbial loop.
Because analyses of food webs that omit bacterial processes are usually missing most of the flux of
energy or carbon, most theoretical population ecology involves a small fraction of the flux of materials.
Microbial processes that dissipate large amounts of
the energy sequestered by autotrophs have major,
quantitative consequences for the production of biomass by terminal consumers (Pomeroy, 2000).
While differences in energy flux may not change the
length of food chains in predictable ways, they will
affect the amount of biomass at the terminal levels,
i. e. the rarity of large, terminal consumers such as
fishes. To the above, Lenz (1992) adds temperature,
pointing out that as temperature increases with the
usual Q10 of 2-3, small organisms will have a greater
increment in production than large ones. While this
is true, we should note that temperature effects are
relative and regional. The optimum temperature
ranges of both bacteria and phytoplankton vary with
latitude, and just because water is “cold” in polar
regions does not imply that organisms are growing
more slowly than in low latitudes, where water is
“warm” (Pomeroy and Wiebe, 2001).
FOOD WEBS 35

Assimilation efficiency
Only recently has assimilation efficiency
received the attention it deserves (del Giorgio and
Cole, 2000). Early assumptions of a 10% “ecological efficiency” of each “trophic level” and later
assumptions of a generally high growth efficiency of
heterotrophic bacteria of around 50%, have been
shown to be poor generalizations (Jahnke and
Craven, 1995). Modelers conventionally put a constant in their equations to correct for energy losses
owing to assimilation efficiency, but they rarely
explore the sensitivity to variations in that constant.
The efficiency of poikilotherms is generally
assumed to be higher than that of homeotherms,
with bacteria highest of all. However, these assumptions come largely from laboratory experiments that
do not always duplicate natural conditions. In the
real world, parasitized, starving beasts and bacteria
often are shown to have low assimilation efficiency.
Indeed, organisms sometimes subsist on stored
materials, including structural proteins, but this is
not usually captured in models, although simple sensitivity tests can show the importance of assimilation efficiency (Pomeroy, 2000).
An example of the importance of assimilation
efficiency can be seen in the study of a Phaeocystis bloom in the North Sea. Rousseau et al., (2000)
found that 75% of the carbon sources of the mesozooplankton were diatoms, although diatoms were
responsible for only 30% of primary production.
Mesozooplankton obtained the other 25% of
organic carbon from a microbial food web which
originated from the 70% of primary production by
Phaeocystis. Trophic efficiency of the food chain
from diatoms to mesozooplankton was estimated to
be 34%, while that from Phaeocystis via a detritus
food chain was estimated to be 1.5%. A detritus
food web, such as that originating from Phaeocystis, involves branching and looping food chains in
which the passage of materials through bacteria, in
particular, may be very inefficient (del Giorgio and
Cole, 2000). If, however, as is the case in most
oligotrophic waters, the primary producers are not
diatoms but primarily a mixture of microflagellates
and autotrophic bacteria being eaten by protozoans, the trophic efficiency of the microbial food
web can be significantly higher. In both cases, the
connections between the microbial chain and the
metazoan chain are an intergral part of a coherent
food web that is the principal support of larger
metazoans.
36 L.R. POMEROY

Although the length of food chains does not correlate well with primary productivity, variable
assimilation efficiency can conspire with primary
production to render chains beyond a given length
energetically small and the terminal consumers rare.
This would seem to work against the effectiveness
of detritus food chains which tend to be long and to
involve food sources of low quality (e. g. high C:N
or C:P and a preponderance of aromatic rather than
aliphatic organic compounds). But efficiency is not
everything in life (Odum and Pinkerton, 1955).
Abundant, low-quality food can be as satisfactory as
rare, high quality food –ask any ruminant. Although
early food web theory suggested that detritus food
webs should be unstable and therefore rare in nature
(Cohen et al., 1990), empirical work suggests the
reverse. Detritus food webs, and their in…