Chapter 17 Food Webs
The ocean around
The krill-feeding fishes and squid are eaten
by predaceous species, including emperor penguins, larger fish, and
Weddell and Ross seals. Leopard seals, a highly carnivorous species,
feed on penguins and the smaller seals. Finally, the ultimate predators
in this community are the killer whales. which
eat seals, including leopard seals, and even attack and consume baleen
whales. Huge populations of organisms live in the oceans surrounding
How can we go beyond a confusing verbal description to a useful and easily understood summary of the feeding relationships within communities? One of the earliest approaches to the study of communities was to describe who eats whom. Since the beginning of the twentieth century, ecologists have meticulously described the feeding relationships in hundreds of communities. The resulting tangles of relationships came to be called food webs. lfwe define a community as an association of interacting species, a moment's reflection will show that a food web, a summary of the feeding interactions within a community, is one of the most basic and revealing descriptions of community structure. A food web is, essentially, a community portrait (fig. 17.2).
FIGURE 17.2 The Antarctic pelagic food web.
l A food web summarizes the feeding relations in a community.
l The feeding activities of a few keystone species may control the structure of communities.
l Exotic predators can collapse and simplify the structure of food webs.
CASE HISTORIES: community webs
A food web summarizes the feeding relations in a community.
The earliest work on food webs concentrated
on simplified communities. In 1927, Charles Elton pointed out that the
number of well-described food webs, which he called "food cycles,"
could be counted on the fingers of one hand. One of the first of those
food webs described the feeding relations on
FIGURE 17.3 Simple food web of an Arctic island.
Summerhayes and Elton used a food web to present the
feeding relations on
The work of Summerhayes
and Elton revealed that even in these "impoverished faunas,"
feeding relations are complex and difficult to study. For instance,
they failed to document several probable feeding relations in their
Detailed Food Webs Reveal Great Complexity
Now let's go from the high
Winemiller represented the food webs at his study sites in various ways. In some he only included the "common" fish species whose aggregate abundance comprised 95% of the individuals in his collections. These common-fish webs excluded many rare species. He also drew "top-predator sink webs," food webs consisting of all prey consumed by the top predator in a community, all items consumed by the prey of the top predator, and so on down to the base of the food web. Third, Winemiller constructed food webs that excluded the weakest trophic links, those comprising less than 1% of the diet.
Let's look at the results from Cano Volcan, the simplest fish community. Figure 17.4 shows that even when only the 10 most common fish are included in the food web, it remains remarkably complex. The most comprehensible of Winemiller's food webs were those that focused on the strongest trophic links.
Food web representing the feeding relations of the 10 most
common fish species at
Strong Interactions and Food Web Structure
Robert Paine (1980) suggested that, in many cases, the feeding activities of a few species have a dominant influence on community structure. He called these influential trophic relations strong interactions. Paine also suggested that the defining criterion for a strong interaction is not necessarily quantity of energy flow but rather degree of influence on community structure. We will revisit this topic later in the Case Histories section on keystone species, but for now, let's look at how recognizing interaction strength can simplify depictions of food webs.
Paine's distinction between strong and weak
interactions within food webs has been used to model the interactions
within at least one terrestrial food web. Teja
Tscharntke (1992) has worked intensively on a food web associated
with the wetland reed Phragmites australis. This reed grows in large stands along the shores
of rivers and other wetlands. Tscharntke's study site was along the
Tscharntke discovered that at least 14 species of parasitoid wasps attack G. inclusa. How can so many species attack a single host species and continue to coexist? Does this seem to violate the competitive exclusion principle (see chapter 13)? Tscharntke explains this apparent paradox by pointing out that each parasitoid species appears to specialize on attacking G. inclusa at different times and on different parts of Phragmites. In winter, blue tits, Parus caeruleus, move into stands of Phragmites, where they peck open the galls formed by G. inclusa and eat the larvae, causing mortality in this population as well as in its parasitoids.
Tscharntke represented these trophic interactions with a food web that captures the essential interactions among species in this community (fig. 17.5). Even though there are fewer interactions than in Winemiller's tropical fish webs (see fig.17.4), Tscharntke's web still contains plenty of complexity. However, figure 17.5 focuses the reader on the most important interactions in the community by distinguishing between strong, weaker, and weakest interactions by representing this gradient in interaction strength by red (strong), blue (weak), or green (weakest) lines.
FIGURE 17.5 Food web associated with Phragmites australis (data from Tscharntke 1992).
Figure 17.5 suggests that feeding by blue tits strongly influences the parasitoids Aprostocetus calamarius and Torymus arundinis and their host, G. inclusa, in large gall clusters on main shoots. The other series of strong interactions involves the parasitoids Aprostocetus gratus and Platygaster quadrifarius, which attack the G. inclusa that inhabit small gall clusters in side shoots of Phragmites. These side shoots are in turn stimulated by the stem-boring larvae of the moth A. geminipuncta. Notice that blue tits only weakly influence populations on this side of the web.
By distinguishing between weak and strong interactions, Tscharntke produced an easily understood food web to represent the study community. Identifying strong interactions allows us to determine which species may have the most significant influences on community structure. Those with substantial influence we now call keystone species.
CASE HISTORIES:keystone species
The feeding activities of a few keystone species may control the structure of communities.
Robert Paine (1966, 1969) proposed that the feeding activities of a few species have inordinate influences on community structure. He called these keystone species. Paine's keystone species hypothesis emerged from a chain of reasoning. First, he proposed that predators might keep prey populations below their carrying capacity. Next, he reasoned the potential for competitive exclusion would be low in populations kept below carrying capacity. Finally, he concluded that if keystone species reduce the likelihood of competitive exclusion, their activities would increase the number of species that could coexist in communities. In other words, Paine predicted that some predators may increase species diversity.
Food Web Structure and Species Diversity
Paine began his studies by examining the relationship
between overall species diversity within food webs and the proportion
of the community represented by predators. He cited studies that demonstrated
that as the number of species in marine zooplankton communities increases,
the proportion that are predators also increases.
For instance, the zooplankton community in the
Paine described a food web from the intertidal zone at
Paine also described a subtropical food web
(31°N) from the northern
FIGURE 17.6 Roots of the keystone species hypothesis: does a higher proportion of predators in diverse communities indicate that predators contribute to higher species diversity?
Paine found that as the number of species
in his intertidal food webs increased, the
proportion of the web represented by predators also increased, a
pattern similar to that described by G. Grice and A. Hart (1962) when
they compared zooplankton communities. As Paine went from
Does this pattern confirm Paine's predation hypothesis? No, it does not. First, Paine studied a small number of webs--not enough to make broad generalizations. Second, while the patterns described by Paine are consistent, with his hypothesis, they may be consistent with a number of other hypotheses. To evaluate the keystone species hypotheses, Paine needed a direct experimental test.
Experimental Removal of Starfish
For his first experiment, Paine removed the
top predator from the intertidal food web
response of the community. He chose two study sites
in the middle intertidal zone that extended
Paine followed the response of the intertidal community for 2 years. Over this interval, the diversity of intertidal invertebrates in the control plot remained constant at 15, while the diversity within the experimental plot declined from 15 to 8, a loss of 7 species. This reduction in species diversity supported Paine's keystone species hypothesis. However, if this reduction was due to competitive exclusion, what was the resource over which species competed?
As we saw in chapter 11, the most common limiting resource in the rocky intertidal zone is space. Within 3 months of removing Pisaster from the experimental plot, the barnacle Balanus glandula occupied 60% to 80% of the available space. One year after Paine removed Pisaster, B. glandula was crowded out by mussels, Mytilus californianus, and goose-neck barnacles, Pollicipes polymerus. Benthic algal populations also declined because of a lack of space for attachment. The herbivorous chitons and limpets also left, due to a lack of space and a shortage of food. Sponges were also crowded out and a nudibranch that feeds on sponges also left. After 5 years, the Pisaster removal plot was dominated by two species: the mussel, M. californianus, and the goose-neck barnacle, P. polymerus.
This experiment showed that Pisaster is a keystone species. When Paine removed it from
his study plot, the community collapsed. However, did this one experiment
demonstrate the general importance of keystone species in nature? To
demonstrate this we need more experiments and observations across a
wide variety of communities. Paine followed his work at
The intertidal community
along the west coast of
These results show that intertidal
communities thousands of kilometers apart that do not share any species
of algae or genera of invertebrates are influenced by similar biological
processes (fig. 17.7). This is reassurance to ecologists seeking general
ecological principles. However, the two communities are not identical.
FIGURE 17.7 The effect of removing a top predator from two intertidal food webs (data from Paine 1966, 1971).
In a second removal experiment, Paine removed
both the starfish Stichaster and the large
brown alga Durvillea from two different study
plots. The result was far more dramatic than when Paine had removed
the starfish only. After only 15 months, Perna
dominated the study area and excluded nearly all other flora and fauna,
covering 68% to 78% of the space in the two removal sites. Paine's studies
in North America and
Consumers' Effects on Local Diversity
Jane Lubchenko (1978) observed that previous studies had indicated that herbivores sometimes increase plant diversity, sometimes decrease plant diversity, and sometimes seem to do both. She proposed that to resolve these apparently conflicting results it would be necessary to understand (1) the food preferences of herbivores, (2) the competitive relationships among plant species in the local community, and (3) how competitive relationships and feeding preferences vary across environments. Lubchenko used these criteria to guide her study of the influences of an intertidal snail, Littorina littorea, on the structure of an algal community.
Lubchenko studied the feeding preferences of Littorina in the laboratory. Her experiments indicated that algae fell into low, medium, or high preference categories. Generally, highly preferred algae were small, ephemeral, and tender like the green algae, Enteromorpha spp., while most tough, perennial species like the red alga Chondrus crispus were never eaten or eaten only if the snail was given no other choice.
Lubchenko also studied variation in the abundance of algae and Littorina in tide pools. She found that tide pools with high densities of Enteromorpha, one of the snail's favorite foods, contained low densities (4/m2) of snails. In contrast, pools with high densities of Littorina (233-267/m2) were dominated by Chondrus, a species for which the snail shows low preference. Lubchenko reasoned that in the absence of Littorina, Enteromorpha competitively displaces Chondrus. She tested this idea by removing the Linorina from one of the pools in which they were present in high density and introducing them to a pool in which Enteromorpha was dominant. Lubchenko monitored a third pool with a high density of the snails as a control.
FIGURE 17.8 Effect of Littorina littorea on algal communities in tide pools (data from Lubchenko 1978).
The results of Lubchenko's removal experiment were clear (fig.17.8). While the relative densities of Chondrus, Enteromorpha, and other ephemeral algae remained relatively constant in the control pool, the density of Enteromorpha declined with the introduction of Littorina. Meanwhile, Enteromorpha quickly increased in density and came to dominate the pool from which Lubchenko had removed the snails. In addition, as the Enteromorpha population in this pool increased, the population of Chondrus declined. Lubehenko began another addition and removal experiment in two other pools in the fall to check for seasonal effects on feeding and competitive relations. This second removal experiment produced results almost identical to the first. Where Littorina were added, the Enteromorpha population declined, while the Chondrus population increased. Where the snails were removed, the Chondrus population declined, while the Enteromorpha population increased.
How can we explain Lubchenko's results? Littorina prefers to feed on Enteromorpha, a species that can outcompete Chondrus in tide pools. So, in the absence of the snails, Chondrus is competitively displaced by Enterornorpha. However, where it is present in high densities, Littorina grazes down the Enteromorpha population, releasing Chondrus from competition with Enteromorpha.
What controls the local population density of Littorina ? Apparently, the green crab, Carcinus maenus, which lives in the canopy of Enteromorpha, preys upon young snails and can prevent the juveniles from colonizing tide pools. Adult Littorina are much less vulnerable to Carcinus but rarely move to new tide pools. Populations of Carcinus are in turn controlled by seagulls. Here again, we begin to see the complexity of a local food web and the influences that trophic interactions within webs can have on community structure.
So, within tide pools Enteromorpha can outcompete the other tide pool algae for space and Enteromorpha is the preferred food of Littorina. How might feeding by the snails affect the diversity of algae within tide pools? The relationship between the snails and the algal species they exploit is similar to the situation studied by Paine, where mussels were the competitively dominant species and one of the major foods of the starfish Pisaster.
Lubchenko examined the influence of Littorina on algal diversity by observing the number of algal species living in tide pools occupied by various densities of snails (fig. 17.9). As the density increased from low to medium, the number of algal species increased. Then, as the density increased further, from medium to high, the number of algal species declined.
How would you explain these results? At low density, the feeding activity by Littorina is not sufficient to prevent Enteromorpha from dominating a tide pool and crowding out some other species. At medium densities, the snail's feeding, which concentrates on the competitively dominant species, prevents competitive exclusion and so increases algal diversity. However, at high densities the feeding requirements of the population are so high that the snails eat their preferred algae as well as less preferred species. Consequently, intense grazing by snails at high density reduces algal diversity.
What would happen if Littorina preferred to eat competitively inferior species of algae? This is precisely the circumstance that occurs on emergent substrata, rock surfaces that are not submerged in tide pools during low tide. On these emergent habitats the competitively dominant algae are species in the genera Fucus and Ascophyllum, algae for which the snails show low preference. On emergent substrata, the snails continue to feed on ephemeral, tender algae such as Enteromorpha, largely ignoring Fucus and Ascophyllum. In this circumstance, Lubchenko found that algal diversity was highest when Littorina densities were low (fig.17.9).
FIGURE 17.9 Effect ofLittorina littorea on algal species richness in tide pools and emergent habitats (data from Lubchenko 1978).
What produces this reduction in diversity? Let's think first about the effect of competition by Fucus and Ascophyllum. In the absence of disturbance, these two algae will gradually cover all emergent substrata, crowding out other algal species in the process. Feeding by snails accelerates elimination of the competitively subordinant species. In this case, competition by Fucus and Ascophyllum and exploitation by Littorina are both pushing the community toward reduced species richness.
Lubchenko's research improved our understanding of how trophic interactions can affect community structure. Her work demonstrated that the influence of consumers upon the structure of food webs depends upon their feeding preferences, the density of local consumer populations, and the relative competitive abilities of prey species. While Lubchenko moved the field well beyond the conceptual view held by ecologists when Paine first proposed the keystone species hypothesis, one basic element of the original hypothesis remained: Consumers can exert substantial control over food web structure; they can act as keystones.
Can predators act as keystone species in environments other than the intertidal zone? The next two examples concern keystone species in riverine and terrestrial environments.
Fish as Keystone Species in River Food Webs
Mary Power (1990) tested the possibility that
fish can significantly alter the structure of food webs in rivers. She
conducted her research on the
In early summer, the boulders and bedrock
Seasonal changes in hiomass and growth form
of benthic algae in the
Chironomids are eaten by predatory insects and the young
(known as fry) of two species of fish: a minnow called the
17.11 Food web associated with algal turf during the summer
Power asked whether or not the two top predators
Significant differences between the exclosures and enclosures soon emerged. Algal densities were initially similar; however, enclosing fish over an area of streambed significantly reduced algal biomass (fig.17.12). In addition, the Cladophora within cages with fish had the same ropy, webbed appearance as Cladophora in the open river.
17.12 The influence of juvenile steelhead and
How do predatory fish decrease algal densities?
The key to answering this question lies with the
FIGURE 17.13 Effect of juvenile steelhead and roach on numbers of insects and young (fry) roach and sticklebacks (data from Power 1990).
All of the examples that we have discussed so far have been aquatic. Do terrestrial communities also contain keystone species? An increasing body of evidence indicates that they do.
The Effects of Predation by Birds on Herbivory
Let's move now from the Mediterranean climate
Atlegrim studied the food web associated with the
bilberry, Vaccinium myrtillus,
which is a dominant understory shrub in many
boreal forests in northern
Atlegrim's study sites were located approximately
Atlegrim took care to ensure that he could attribute any experimental effects to the exclusion of birds. His exclosures excluded birds but allowed small predaceous mammals, such as shrews, and predaceous invertebrates to move freely into and out of the study plots. He also kept track of the densities of these alternative predators by periodically sampling them with pitfall traps. Why was this an important aspect of Atlegrim's study? In the absence of predation by birds, higher densities of herbivorous insects might have attracted higher numbers of other predators, that is, produced a localized numerical response (see chapter 10). Atlegrim also measured the intensity of sunlight within his exclosures and in adjacent control plots. Why was this aspect of the study necessary? If the exclosures created significant shading, physical effects alone (see chapters 4-6) could have affected the distributions of herbivorous insects. Finally, Atlegrim measured the density of Vaccinium shoots to verify similar densities in exclosure and control plots. His measurements showed that levels of light, densities of nouavian predators, and densities of Vaccinium shoots were similar on exclosure and control plots.
Exclosures increased larval insect density an average of 63% across all study sites (fig. 17.14). So, the answer to Atlegrim's first question is yes. Insectivorous birds reduce the densities of herbivorous insect larvae feeding on Vacciniurn. However, as Atlegrim predicted, some herbivorous larvae are more vulnerable to insectivorous birds than are others. Sawfly and geometrid larvae, which feed in exposed positions, were significantly higher within exclosures, while the densities of tortricid larvae, which feed in their constructed shelters, showed no effects of bird exclusion. Higher densities of herbivorous insect larvae translated directly into higher levels of damage to Vacciniurn.
FIGURE 17.14 Effect of insectivorous birds on herbivorous insect populations on Vaccinium myrtiIlus (data from Atlegrim 1989).
What other piece of information might increase our confidence that the differences between exclosure and control plots were due to bird predation? One of the most significant bits of evidence would be direct observations of birds feeding on the control plots. Atlegrim observed three bird species feeding on control plots: Hazel hen chicks, Tetrastes bonasia, great tits, Parus major, and pied flycatcher, Ficedula hypoleuca.
Insectivorous birds also reduce insect populations
and insect damage on plants in midlatitude
The researchers sprayed another set of 30 white oak saplings each week with a pyrethroid insecticide. They also handpicked any remaining herbivorous insects from these trees. A third set of white oaks, the control, was not manipulated.
Marquis and Whelan's caged plants were populated by larger numbers of herbivorous insects and experienced significantly greater insect damage (fig. 17.15). These results are consistent with those of Atlegrim's earlier study. Marquis and Wbelan also measured the biomass of each of their trees in 1990 and 1991. They found that the biomasses of sprayed and uncaged white oaks were significantly higher than the biomass of caged white oaks. In other words, the higher densities of herbivorous insects on the trees from which birds were excluded reduced their growth rates. One implication of these results is that insectivorous birds increase the growth rates of temperate forest trees.
FIGURE 17.15 Effect of insectivorous birds on herbivorous inaect populations, leaf damage, and sapling growth in white oaks (data from Marquis and Whelan 1994).
Many studies of food webs and keystone species
have been done since Robert Paine's classic study of the intertidal food web at
FIGURE 17.16 What is a keystone species (data from Power et al. 1996)?
CASE HISTORIES: exotic predators
Exotic predators can collapse and simplify the structure of food webs.
Introduced Fish: Predators That Simplify Aquatic Food Webs
People have moved all sorts of species around
the planet, but one of the most commonly introduced groups of organisms
is fish. Introduced fish often substantially change the food webs of
the water bodies where they are introduced. For instance, introduced
fishes have devastated the native fishes of
Today we are witnessing what may be the greatest
devastation ever wrought by an introduced predator. That predator is
the Nile perch, Lates nilotica,
and the aquatic system is Lake Victoria, one of the great lakes of
Lake Victoria harbors one of the greatest
concentrations of fish species in the world, and the
Hundreds of fish species appear on their way
to extinction because humans introduced one more fish species into a
lake already containing over 400 species. The Nile perch is a predaceous
fish native to
As the population of
FIGURE 17.17 Influence
of an exotic predator. Nile perch, on the food web of
The Nile perch has had a major effect on the
food web of
Depletion of oxygen has produced massive fish
kills. Biologists had observed some fish kills at the surface of
Maybe the Nile perch has not caused all of
the changes in
APPLICATIONS AND TOOLS: humans as keystone species
People have long manipulated food webs both as a consequence of their own feeding activities and by introducing or deleting species from existing webs. In addition, many of these manipulations have focused on keystone species. Consequently, either consciously or unwittingly, people have, themselves, acted as keystone species in communities.
The current plight of the tropical rain forest
is well known. However, Kent Redford (1992) points out that with few
exceptions, most studies of human impact on the tropical rain forest
have concentrated on direct effects of humans on vegetation, mainly
Hunters generally concentrate on a small percentage
of larger bird and mammal species, howeven
For instance, Redford estimated that at Cocha
Cashu Biological Station in
17.18 Highly selective hunting by Amazonian natives (data
As impressive as all these numbers are, there
remains a critical question: Do hunters reduce the local densities of
the birds and mammals they hunt? The answer is yes.
There may be cause for concern, however, that goes beyond the losses of these immense numbers of animals. As you might expect, many large rain forest mammals and birds may act as keystone species (fig.17.19). If so, their decimation will have effects that ripple through the entire community. The first to suggest a keystone role for the large animals preferred by rain forest hunters was John Terborgh (1988), who presented his hypothesis in a provocative essay titled, "The Big Things That Run the World."
FIGURE 17.19 Large predators such as this jaguar may act as keystone species in tropical rain forests.
Terborgh's hypothesis has been supported by a variety
of studies. He observed that in the absence of pumas and jaguars on
Ants and Agriculture:
In 1982, Stephen Riseh
and Ronald Carroll published a paper describing how the predaceous fire
ant, Solenopsis gerninata, acts as a
keystone predator in the food web of the corn-squash agroecosystem
FIGURE 17.20 Effect ofSolenopsis geminata on the arthropod populations on corn (data from Risch and Carroll 1982).
The conceptual breakthrough represented by
the work of Risch and Carroll is impressive.
However, their work had been anticipated, 1,700 years earlier, by farmers
The Gan (mandarin orange) is a kind of orange with an exceptionally sweet and delicious taste...In the market, the natives of Jiao-zhi [southeastern China and North Viemam] sell ants stored in bags of rush mats. The nests are like thin silk. The bags are all attached to twigs and leaves, which, with the ants inside the nests, are for sale. The ants are red- dish-yellow in color, bigger than ordinary ants. In the south, if the Gan n'ees do not have this kind of ant, the fruits will be damaged by many harmful insects and not a single fruit will be perfect.
Now, 17 centuries after the observations of
Ji Han, we know this ant as the citrus ant,
OecophyUa smaragdina. The use of
this ant to control herbivorous insects in citrus orchards was unknown
Oecophylla is one of the weaver ants, which use silk to construct a nest by binding leaves and twigs together. These ants spend the night in their nest. During the day, the ants spread out over the home tree as they forage for insects. Farmers place a nest in a tree and then run bamboo strips between trees so that the ants can have access to more than one tree. The ants will eventually build nests in adjacent trees and can colonize an entire orchard.
The ants harvest protein and fats when they gather insects from their home tree, but they have other needs as well. They also need a source of liquid and carbohydrates, and they get these materials by cultivating Homoptera, known as soft-scale insects or mealy bugs, which produce nectar. The ants and soft-scale insects have a mutualistic relationship in which the ants transport the insects from tree to tree and protect them from predators. In return the ants consume the nectar produced by the soft-scale insects. Because of this mutualism with the soft-scale insect, which can itself be a serious pest of citrus, several early agricultural scientists expressed skepticism that Oecophylla would be an effective agent for pest control in citrus. They suggested that the use of this ant could produce infestations by soft-scale insects.
Despite these criticisms, all Chinese citrus growers interviewed insisted that OecophyUa is effective at pest con trol and that the damage caused by soft-scale insects is minor. Research done by Yang appears to have solved this apparent contradiction. Comparing orange trees treated with chemical insecticides to those protected by Oecophylla, Yang recorded higher numbers of soft-scale insects in the trees tended by ants. However, these higher numbers did not appear to cause serious damage to the orange trees. When Yang inspected the soft-scale insects closely, he found that they were heavily infested with the larvae of parasitic wasps. He also found that the ants did not reduce populations of lacewing larvae and ladybird beetles, predators that feed on soft-scale insects. Huang and Yang concluded that Oecophylla is effective at pest control because while it attacks the principal, larger pests of citrus, it does not reduce populations of other predators that attack the smaller pests of citrus, such as soft-scale insects, aphids, and mites (fig. 17.21).
FIGURE 17.21 While
pests in this North American orange orchard are controlled mainly by
chemical insecticides, weaver ants have been used to control insect
pests of orange orchards in
The association between Oecophylla
and citrus trees seems similar to that between ants and acacias (see
chapter 15). There is a difference, however. Humans maintain Oecophylla
as a substantial component of the food web in citrus orchards. Not only
have specialized farmers historically cultivated and distributed the
ants, Oecophylla must also be protected from the winter cold. The
ant cannot survive the winter in southeast
The labor and expense of maintaining these
ants through the winter may be reduced by mixed plantings of orchard
trees. Farmers in Shajian village in the Huaan
district of southeast
The farmers of southeast
A food web summarizes the feeding relations in a community. The earliest work on food webs concentrated on simplified communities in areas such as the Arctic islands. However, researchers such as Charles Elton (1927) soon found that even these so-called simple communities included very complex feeding relations. The level of food web complexity increased substantially, however, as researchers began to study complex communities. Studies of the food webs of tropical freshwater fish communities revealed highly complex networks of trophic interaction that persisted even in the face of various simplifications. A focus on strong interactions can simplify food web structure and identify those interactions responsible for most of the energy flow in communities.
The feeding activities of a few keystone species may control the structure of communities. Robert Paine (1966) proposed that the feeding activities of a few species have inordinate influences on community structure. He predicted that some predators may increase species diversity by reducing the probability of competitive exclusion. Manipulative studies of predaceous species have identified many keystone species, including starfish and snails in the marine intertidal zone and fish in rivers. On land, birds exert substantial influences on communities of their arthropod prey. Jane Lubchenko (1978) demonstrated that the influence of consumers on community structure depends upon their feeding preferences, their local population density, and the relative competitive abilities of prey species. Keystone species are those that, despite low biomass, exert strong effects on the structure of the communities they inhabit.
predators can collapse and simplify the structure of food webs. Introduced fishes have devastated the native
Humans have acted as keystone species in communities.
People have long manipulated food webs both as a consequence of their
own feeding activities and by introducing or deleting species from existing
food webs. In addition, many of these manipulations have been focused
on keystone species. Hunters in tropical rain forests have been responsible
for removing keystone animal species from large areas of the rain forests
of Central and
1. You could argue that the classical food
2. Winemiller (1990)
deleted "weak" trophic links from
one set of food webs that he described for fish communities in
3. What is a keystone species? Paine (1966,
1969) experimented with two starfish that act as keystone species in
their intertidal communities along the west coast of North America
4. Explain how the experiments of Lubchenko (1978) showed that feeding preferences, population density, and competitive relations among food species all potentially contribute to the influences of "keystone" consumers on the structure of communities. What refinements did the work of Lubehenko add to the keystone species hypothesis?
5. When Power (1990) excluded predaceous fish from her river sites, the density of herbivorous insect larvae (chironomids) decreased. Use the food web described by Power to explain this response.
6. Using Tscharntke's food web (1992) shown in figure 17.5, predict which species would be most affected if you excluded the bird at the top of the web. Parus caeruleus. What species would be affected less? Assume that P.caeruleus is a keystone species in this community.
7. Atlegrim (1989) and also Marquis and Whelan (1994) showed that birds in high latitudes and temperate forests reduce insect populations. The results of this research suggests that birds act as keystone species in some communities. What else would we need to know about the birds in these communities before we could conclude that they are keystones in the strict sense? (Hint: Consider figure 17.16.)
8. Notice that in the study by Marquis and Whelan (1994) the biomass of uncaged Q. alba was as great as that of sprayed individuals. In other words, spraying protected oak seedlings as well as birds. If spraying can control herbivorous forest insects, why rely on birds to improve tree growth? What advantages does predation by birds have over spraying?
9. Some paleontologists have proposed that overhunting caused the extinction of many large North American mammals at the end of the Pleistocene about 11,000 and 10,000 years ago. The hunters implicated by paleontologists were a newly arrived predatory species, Homo sapiens. Offer arguments for and against this hypothesis.
10. All the keystone species work we have discussed in this chapter has concerned the influences of animals on the structure of communities. Can other groups of organisms act as keystones? What about parasites and pathogens?