Chapter 17: Species Interactions and Community Structure
Why is the fact that hunters reduce local densities of birds and mammals important to community structures?
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. 17.22). If so, their decimation will have effects that ripple through the entire community
Do terrestrial communities also contain keystone species?
An increasing body of evidence indicates that they do, particularly in tropical forests (see fig. 17.22), where they create a "trophic cascade" In contrast to foundation species, keystone species are those that, despite low biomass, exert strong effects on the structure of the communities they inhabit. As we shall see in the following discussion, those strong effects are not always positive, particularly where they involve invasive species.
indirect commensalism
Commensalism is an interaction between two species in which one species is benefited and the other is neither benefited nor harmed. Indirect commensalism occurs when the activities of one species indirectly—that is, through an intermediary species—benefit another species without itself being helped or harmed.
Ants and Agriculture: Keystone Predators for Pest Control In 1982, Stephen Risch and Ronald Carroll published a paper describing how the predaceous fire ant, Solenopsis geminata, acts as a keystone predator in the food web of the corn and squash agroecosystem in southern Mexico.
In their own experiments, Risch and Carroll demonstrated how predation by Solenopsis in the corn and squash agro-ecosystem reduces the number of arthropods and the arthropod diversity (fig. 17.23). This study showed how Solenopsis could act as a keystone species to the benefit of the agriculturist.
Snail (Littorina) Effects on Algal Diversity -Jane Lubchenco (1978) -what 3 criteria did she state impacts the study of the relationship between snails and algae?
- increased our understanding of the ecological details on which the influences of a keystone species can depend. Lubchenco 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.
keystone species
A species that influences the survival of many other species in an ecosystem; The feeding activities of a few keystone species may control the structure of communities. In contrast to foundation species, keystone species are those that, despite low biomass, exert strong effects on the structure of the communities they inhabit. As we shall see in the following discussion, those strong effects are not always positive, particularly where they involve invasive species.
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.
How would you explain these results? (from previous notecard?)
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 is apparent competition?
Ecologists generally infer interspecific competition, where interactions between the individuals of two species have a negative impact on their fitness. However, ecologists are discovering that in some cases these competition-like effects are the result of apparent competition, in which negative impacts are the result of two species sharing a predator or herbivore or by one species facilitating populations of a predator or herbivore of the second species. For example, if two prey species share a predator, increases in one of the prey populations may lead to increased numbers of the predator, which feed on and depress populations of the second prey species.
Paine and experimental removal of sea stars
For his first experiment, Paine removed the top predator from the intertidal food web at Mukkaw Bay and monitored the response of the community. He chose two study sites in the middle intertidal zone that extended 8 m along the shore and 2 m vertically. One site was designated as a control and the other as an experimental site. He removed Pisaster from the experimental site and relocated them in another portion of the intertidal zone. Paine followed the response of the intertidal community to sea star removal 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 richness supported Paine's keystone species hypothesis
Hunters generally concentrate on a small percentage of larger bird and mammal species. What effect does this have? Do hunters reduce the local densities of the birds and mammals they hunt?
For instance, Redford estimated that at Cocha Cashu Biological Station in Manu National Park, located in the Amazon River basin in eastern Peru, hunters concentrate on 9% of the 319 bird species and 18% of the 67 mammal species. Because hunters generally concentrate on the larger species, this small portion of the total species pool makes up about 52% of the total bird biomass and approximately 75% of the total mammalian biomass around Manu National Park. 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. Redford estimated that moderate to heavy hunting pressure in rain forests reduces mammalian biomass by about 80% to 93% and bird biomass by about 70% to 94%.
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."
He observed that in the absence of pumas and jaguars on Barro Colorado Island, Panama, medium-sized mammal species are over 10 times more abundant than in areas still supporting populations of these large cats. In other studies, Terborgh and colleagues (Terborgh et al. 2001, 2006) documented changes in community structure following the isolation of small tropical forest fragments on islands in a Venezuelan hydroelectric impoundment that excluded large predators. The consequent changes in community structure were so massive that Terborgh and colleagues characterized them as "ecological meltdown." Such observations prompted Redford to warn, "We must not let a forest full of trees fool us into believing all is well." Tropical rain forest conservation must also include the large, and potentially keystone, animal species that are vulnerable to hunting by humans.
Snail (Littorina) Effects on Algal Diversity -which algae were highly preferred by snails? -which were not?
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.
summary of what Lubchenco's work revealed
Her work demonstrated that the influence of consumers on the structure of food webs depends on their feeding preferences, the density of local consumer populations, and the relative competitive abilities of prey species. While Lubchenco 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.
direct vs indirect interactions -difference in definitions -4 main examples of direct -4 main examples of indirect
Indirect interactions between species are fundamental to communities. Food webs emphasize direct trophic interactions between species. Direct interactions between two species, including competition, predation, herbivory, and mutualism (see chapters 13, 14, and 15), involve positive or negative effects of one species on another without the involvement of an intermediary species. However, direct interspecific interactions can also result in ecologically significant indirect interactions between species. In indirect interactions, one species affects another through a third, intermediary species. Indirect interactions include trophic cascades, which we discuss in detail in chapter 18 (section 18.4), apparent competition, and indirect mutualism or commensalism.
Applications: Redford and "ecologically extinct" species
Kent Redford (1992) pointed 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 on deforestation. Redford expands our view by examining the effects of humans on animals. The picture that emerges from this analysis is that humans have so reduced the population densities of rain forest animals in many areas that they no longer play their keystone roles in the system, a situation Redford calls "ecologically extinct."
abundance of each type of algae relation to snail density
Lubchenco 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. Lubchenco reasoned that in the absence of Littorina, Enteromorpha competitively displaces Chondrus. She tested this idea by removing the Littorina from one of the pools in which they were present in high density and introducing them to a pool in which Enteromorpha was dominant. She monitored a third pool with a high density of the snails as a control. The results of Lubchenco's removal experiment were clear (fig. 17.11). 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 Lubchenco had removed the snails. In addition, as the Enteromorpha population in this pool increased, the population of Chondrus declined. *summary: Where Littorina were added, the Enteromorpha population declined, whereas the Chondrus population increased. Where the snails were removed, the Chondrus population declined, while the Enteromorpha population increased.*
Oecophylla smaragdina (citrus ant) and citrus plants
Now, 17 centuries after the observations of Ji Han, we know this ant as the citrus ant, Oecophylla smaragdina. The use of this ant to control herbivorous insects in citrus orchards was unknown outside of China until 1915. Oecophylla, a weaver ant, uses 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. Page 379 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. Despite these criticisms, all Chinese citrus growers interviewed insisted that Oecophylla is effective at pest control 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. 17.24).
robert paine and strong interactions -what defines a strong interactions
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.
Power asked whether or not the two top predators in the Eel River food web, roach and steelhead, significantly influence web structure.
She tested the effects of these fish on food web structure by using 3 mm mesh to cage off 12 areas 6 m2 in the riverbed. The mesh size of these cages prevented the passage of large fish but allowed free movement of aquatic insects and stickleback and roach fry. Power excluded fish from six of her cages and placed 20 juvenile steelhead and 40 large roach in each of the other six cages. These fish densities were within the range observed around boulders in the open river. 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. 17.15). In addition, the Cladophora within cages with fish had the same ropy, webbed appearance as Cladophora in the open river.
How do predatory fish decrease algal densities?
Steelhead and large roach feed heavily on predatory insects, young roach, and sticklebacks. Lower densities of these young roach and sticklebacks within the enclosures decreased predation on chironomids. Higher densities of the herbivorous chironomids increased the feeding pressure on algal populations. This explanation is supported by the results of Power's experiment in which enclosures contained lower densities of predatory insects and fish fry and higher densities of chironomids (fig. 17.16). By enclosing and excluding fish from sections of the Eel River, Power, like Paine and Lubchenco, who worked in the intertidal zone, demonstrated that fish act as keystone species in the Eel River food web.
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 Eel River in northern California, where most precipitation falls from October to April, sometimes producing torrential winter flooding. During the summer, however, the flow of the Eel River averages less than 1 m3 per second. -this notecard just explains the food web she was studying
Summary of food web in pic: In early summer, the boulders and bedrock of the Eel River are covered by a turf of the filamentous alga Cladophora (fig. 17.13a). However, the biomass of the algae declines by midsummer and what remains has a ropy, prostrate growth form and a "webbed" appearance (fig. 17.13b). These mats of Cladophora support dense populations of larval midges in the fly family Chironomidae. One chironomid, Pseudochironomus richardsoni, is particularly abundant. Pseudochironomus feeds on Cladophora and other algae and weaves the algae into retreats, altering their appearance in the process. Chironomids are eaten by predatory insects and the young (known as fry) of two species of fish: a minnow called the California roach, Hesperoleucas symmetricus, and three-spined sticklebacks, Gasterosteus aculeatus. These small fish are eaten by young steelhead trout, Oncorhynchus mykiss. Steelhead and large roach eat predatory invertebrates, and large roach also feed directly upon benthic algae.
comparison of the relationship between Oecophylla and citrus trees to that between ants and acacias
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 southeastern China in orange trees. Consequently, farmers must generally provide shelter and food for the ants during winter. The labor and expense of maintaining these ants through the winter may be reduced by mixed plantings of orchard trees. During winter the ants are mostly in pomelo trees, which are larger and have thicker foliage than orange trees, characteristics that reduce cooling rates on winter nights. In this situation, farmers do not have to add new nests of Oecophylla each spring. The farmers of southeastern China have employed Oecophylla as a keystone species in a complex citrus-based food web for a long time. However, the results would not be the same with just any ant species. The citrus growers required a species that acts in a particular way.
paine's hypothesis for keystone species and diversity
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, a concept that was later refined with further research. 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 reduced 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. 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 Atlantic Ocean over continental shelves includes 81 species, 16% of which are predators. In contrast, the zooplankton community of the Sargasso Sea contains 268 species, 39% of which are predators (Grice and Hart 1962). Paine set out to determine if similar patterns occur in marine intertidal communities.
Paine's second study in new zealand to determine the importance of keystone species
The intertidal community along the west coast of New Zealand is similar to the intertidal community along the Pacific coast of North America. The top predator is a sea star, Stichaster australis, that feeds on a wide variety of invertebrates, including barnacles, chitons, limpets, and a mussel, Perna canaliculus. During 9 months following Paine's removal of sea stars, the number of species in the removal plot decreased from 20 to 14 and the coverage of the area by the mussel increased from 24% to 68%. As in Mukkaw Bay, the removal of a predaceous sea star produced a decrease in species richness and a significant increase in the density of a major prey species. Again, the mechanism underlying disappearance of species from the experimental plot was competitive exclusion due to competition for space. 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. 17.10). This is reassurance to ecologists seeking general ecological principles. Many other studies quickly followed the lead taken by Paine's pioneering work.
John Orrock, Martha Witter, and O. J. Reichman (2008) found that an assumed competitive interaction between an invasive exotic plant (black mustard) and a bunchgrass native to the grasslands of California (N. pluchra) is actually apparent competition.
The invasive exotic plant, black mustard, Brassica nigra, has been long implicated in the competitive displacement of plants native to the California grasslands, including the native perennial bunchgrass, Nassella pulchra. However, Orrock, Witter, and Reichman hypothesized that N. pulchra populations might be limited more by small mammalian herbivores and granivores than by competition with B. nigra. They tested their hypothesis using a field experiment in which they established 14 study sites at varying distances from patches of B. nigra. At each study site, they applied three experimental treatments: (1) a fenced exclosure that excluded all small mammals, mainly mice, ground squirrels, and rabbits; (2) a control consisting of a fenced area that produced the disturbance associated with constructing an exclosure but allowed small mammal entry; and (3) an unfenced control. Orrock, Witter, and Reichman obtained results in support of their hypothesis (fig. 17.7). Small mammal activity was greatest near patches of B. nigra and declined away from patches of the invasive exotic plant. Small mammals apparently shelter in patches of B. nigra but do not feed on it. Instead, they feed on vegetation in the surroundings. As a consequence, the densities of both seedlings and adult N. pulchra were reduced in control plots, where small herbivorous mammals had access, near B. nigra patches. However, where small mammals were excluded, seedling and adult N. pulchra densities were high regardless of their distance from B. nigra patches. Orrock, Witter, and Reichman propose that B. nigra patches provide areas of protective cover for the small mammals in their study area from which they forage across the surrounding landscape. The result is a gradient of decreasing herbivore pressure away from B. nigra patches, producing a gradient of increasing density in the N. pulchra population with distance from B. nigra patches. *Orrock, Witter, and Reichman concluded that while the distribution patterns suggest competition, B. nigra actually suppresses N. pulchra indirectly by sheltering mammalian herbivores that feed on the native grass. In short, this is an example of apparent competition (fig. 17.8).*
what is the advantage of only focusing on the strong interactions in a food web?
The most comprehensible of Winemiller's food webs, though still complex, were those that focused on only the strongest trophic links (fig. 17.3b). In addition to simplifying food web structure, focusing on the strongest feeding relationships identifies and emphasizes the more biologically significant trophic interactions.
It appears that ants that disperse seeds have a significant influence on the structure of plant communities in the species-rich fynbos of South Africa. Caroline Christian (2001) observed that native ants disperse 30% of the seeds in the shrublands of the fynbos. -why are seed-dispersing ants important to the persistence of fynbos plants? -effect of the argentine ant (presence vs absence)
The plants attract the services of these dispersers with food rewards on the seeds called elaiosomes. However, the Argentine ant, Linepithema humile (fig. 17.19), which does not disperse seeds, has invaded these shrublands. Christian documented how the invading Argentine ants have displaced (as they have in other regions) many of the native ant species in the fynbos. In addition, she discovered that the native ant species most impacted by Argentine ant invasion are those species most likely to disperse larger seeds Seed-dispersing ants are important to the persistence of fynbos plants because they bury seeds in sites where they are safe from seed-eating rodents and from fire (seeds are the only life stage of many fynbos plants to survive fires). In a comparison of seedling recruitment following fire, Christian found substantial reductions in seedling recruitment by plants producing large seeds in areas invaded by Argentine ants (fig. 17.20). Meanwhile, small-seeded plants, whose dispersers are less affected by Argentine ants, showed no reduction in recruitment following fire. Christian's results, like Bshary's, reveal the influence of mutualists acting as keystone species within the communities they occupy. Other studies are revealing the importance of other mutualists, such as pollinators and mycorrhizal fungi, as keystone species.
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? -influence of Littorina on algal diversity -how was this studied?
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 sea star Pisaster. Methods: Lubchenco 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. 17.12). Results: As the density of Littorina 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.
One of the most widely distributed cleaner fish in the Indo-Pacific region is the cleaner wrasse, Labroides dimidiatus - a true mutualism What effect might cleaning activity by L. dimidiatus have on the diversity of fish on coral reefs? This is the question addressed with a series of field experiments by Redouan Bshary of the University of Cambridge.
The study area consists of a sandy bottom area approximately 400 m from shore dotted with reef patches in water depths from 2 to 6 m. Bshary chose 46 reef patches separated from other patches by at least 5 m of sandy bottom. He identified and counted the fish species present during dives on these reefs and noted the presence or absence of cleaner wrasses on each reef patch. In addition, he performed experimental removals of cleaner wrasses from reefs and introductions of these cleaners to reef patches where there were none. Bshary followed the responses of the fish community to natural disappearances and experimental removals and natural colonization and experimental introductions. In doing so, he gained insights into the influence of these tiny mutualists on reef fish diversity. results: Bshary observed a median reduction in fish species richness of approximately 24% where cleaner wrasses disappeared or were removed. Where cleaner wrasses were added, either naturally or experimentally, he observed a median increase in fish species richness of 24%. Bshary's results indicate that the cleaner wrasse acts as a keystone species on the coral reefs of the Red Sea. Mutualists that act as keystone species have also been found on land. Thus, L. dimidiatus *is a mutualistic keystone species*.
Overall, what did Paine's FIRST experiment show?
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 Mukkaw Bay with a similar experiment in New Zealand.
What would happen if Littorina preferred to eat competitively inferior species of algae? -what microenvironment could this be observed on? -in this scenario, at what density of Littorina would algal diversity be highest?
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, Lubchenco found that algal diversity was highest when Littorina densities were low
Teja Tscharntke (1992) has worked intensively on a food web associated with the wetland reed Phragmites australis. -foundation species
This reed grows in large stands along the shores of rivers and other wetlands, where it is a dominant, or foundation, species. Foundation species are those that have substantial influences on community structure as a consequence of their high biomass—for example, abundant phragmites, an abundant tree in a forest, or a coral on a reef. Tscharntke represented these trophic interactions with a food web that captures the essential interactions among species in this community (fig. 17.5). Though Tscharntke's web still contains plenty of complexity, his depiction focuses the reader on the most important interactions in the community. 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. Identifying strong interactions allows us to determine which species may have the most significant influences on community structure.
food web
a summary of the feeding interactions within a community. It is one of the most basic and revealing descriptions of community structure. A food web is, essentially, a community portrait based on feeding relationships
If this reduction in species richness following removal of the sea star was due to competitive exclusion, what was the resource over which species competed?
s we saw in chapters 11 and 13, 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 gooseneck 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: mussels and gooseneck barnacles
Gregory Martinsen, Elizabeth Driebe, and Thomas Whitham of Northern Arizona University uncovered an indirect commensalism (fig. 17.6) in which beavers, Castor canadensis, indirectly benefit a herbivorous beetle species, Chrysomela confluens, through their effects on cottonwood trees, Populus spp. (Martinsen, Driebe, and Whitham 1998).
the beetles were concentrated on cottonwood sprouts growing from the stumps of trees that had been felled by beavers. Searching for a mechanism to explain these higher beetle concentrations, Martinsen, Driebe, and Whitham found that, compared to leaves on undamaged cottonwood trees, the leaves on stump sprouts had higher concentrations of the defensive chemicals used by cottonwood trees to repel beavers and other herbivorous mammals. However, C. confluens is not repelled by the tree's defensive chemicals and instead uses them for its own defense. In addition, the leaves on stump sprouts were also higher in nitrogen compared to leaves on uncut trees, making them a better food source for the beetles, which grew 20% larger and developed 10% faster when feeding on them. This is an example of a commensalism—beaver activity creates better conditions for the beetle population, without itself being affected. However, because the positive effect of beavers on the beetles is mediated through a third species, cottonwood trees, it is an indirect commensalism.
Paine described a food web from the intertidal zone at Mukkaw Bay, Washington, which lies in the north temperate zone at 49∘ N and has 13 species. Paine also described a subtropical food web (31∘ N) from the northern Gulf of California, a much richer web that included 45 species.
washington: Paine pointed out that the sea star Pisaster commonly consumes two other prey species in other areas, bringing the total food web diversity to 13 species. Ninety percent of the energy consumed by the middle-level predator, Thais, consists of barnacles. Meanwhile, the top predator, Pisaster, obtains 90% of its energy from a mixture of chitons (41%), mussels (37%), and barnacles (12%). northern gulf of cali: But like the food web at Mukkaw Bay, Washington, the subtropical web was topped by a single predator, the sea star Heliaster kubiniji (see fig. 17.9). However, six predators occupy middle levels in the subtropical web, compared to one middle-level predator at Mukkaw Bay. Because four of the five species in the snail family Columbellidae are also predaceous, the total number of predators in the subtropical web is 11. These predators feed on the 34 species that form the base of the food web. aine 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 in the zooplankton communities that had helped inspire Paine's hypotheses. As Paine went from Mukkaw Bay to the northern Gulf of California, overall web diversity increased from 13 species to 45 species, a 3.5-fold increase. However, at the same time, the number of predators in the two webs increased from 2 to 11, a 5.5-fold increase. According to Paine's predation hypothesis, this higher proportion of predators produces higher predation pressure on prey populations, which in turn promotes the higher diversity in the northern Gulf of California intertidal zone. Encouraged by these patterns, Paine designed a field experiment to evaluate his hypothesis that intertidal predators enhance species diversity.
