Chapter 54 Community Ecology

Overview: A Sense of Community A biological community is an assemblage of populations of various species living close enough for potential interaction Figure 54.1 How many interactions between species are occurring in this scene? Hornworm caterpillar (herbivore) and its preferred food, a tomato plant (producer). Parasitic wasp – lays eggs inside caterpillar, and the larvae that emerge from the eggs feed on the caterpillar’s tissues. The larvae then develop into adult wasps inside the white cocoons on their host’s back. This interaction will eventually kill the caterpillar.

Community Interactions Interactions between and within populations influence patterns of species distribution and abundance: Competition, parasitism, predation and commensalism can affect population dynamics. Relationships among interacting populations can be characterized by positive and negative effects. Many complex symbiotic relationships exist in an ecosystem, and feedback control systems play a role in the functioning of those ecosystems.

Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved. Ecologists call relationships between species in a community interspecific interactions Examples are competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism) Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0) Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply This is competition between two different species.

Competitive Exclusion Strong competition can lead to competitive exclusion, local elimination of a competing species The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Ecological Niches The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche An ecological niche can also be thought of as an organism’s ecological role Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

A. distichus perches on fence posts and other sunny surfaces. Fig. 54-2 A. distichus perches on fence posts and other sunny surfaces. A. insolitus usually perches on shady branches. A. ricordii Figure 54.2 Resource partitioning among Dominican Republic lizards Cooperative behavior within or between populations contributes to the survival of the populations: Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei

EXPERIMENT High tide Chthamalus Chthamalus Balanus realized niche Fig. 54-3 EXPERIMENT High tide Chthamalus Chthamalus realized niche Balanus Balanus realized niche Ocean Low tide RESULTS High tide Figure 54.3 Can a species’ niche be influenced by interspecific competition? As a result of competition, a species’ fundamental niche may differ from its realized niche Chthamalus fundamental niche Ocean Low tide

Percentages of individuals in each size class Fig. 54-4 G. fuliginosa G. fortis Beak depth 60 Los Hermanos 40 G. fuliginosa, allopatric 20 60 Daphne Percentages of individuals in each size class 40 G. fortis, allopatric 20 Figure 54.4 Character displacement: indirect evidence of past competition Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species…this should make sense…why? An example is variation in beak size between populations of two species of Galápagos finches 60 Santa María, San Cristóbal Sympatric populations 40 20 8 10 12 14 16 Beak depth (mm)

Predation Predation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison Prey display various defensive adaptations Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls Animals also have morphological and physiological defense adaptations Cryptic coloration, or camouflage, makes prey difficult to spot

(c) Batesian mimicry: A harmless species mimics a harmful one. Fig. 54-5 (a) Cryptic coloration Canyon tree frog (b) Aposematic coloration Poison dart frog (c) Batesian mimicry: A harmless species mimics a harmful one. Hawkmoth larva Figure 54.5 Examples of defensive coloration in animals Cryptic coloration: blend into the environment / camouflage makes prey difficult to spot. Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration. Predators are particularly cautious in dealing with prey that display such coloration In some cases, a prey species may gain significant protection by mimicking the appearance of another species. In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model. In Müllerian mimicry, two or more unpalatable species resemble each other. (d) Müllerian mimicry: Two unpalatable species mimic each other. Cuckoo bee Green parrot snake Yellow jacket

Herbivory Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

Symbiosis Symbiosis is a relationship where two or more species live in direct and intimate contact with one another Parasitism Mutualism Commensalism In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process. Parasites that live within the body of their host are called endoparasites; parasites that live on the external surface of a host are ectoparasites Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species. A mutualism can be Obligate, where one species cannot survive without the other Facultative, where both species can survive alone In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected. Commensal interactions are hard to document in nature because any close association likely affects both species

Fig. 54-UN2 All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy. Cooperative behavior within or between populations contributes to the survival of the populations: Mutualism: lichens, bacteria in digestive tracts, mycorrhizae.

Concept 54.2: Dominant and keystone species exert strong controls on community structure. In general, a few species in a community exert strong control on that community’s structure Two fundamental features of community structure are species diversity and feeding relationships

It has two components: species richness and relative abundance Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: species richness and relative abundance Species richness is the total number of different species in the community Relative abundance is the proportion each species represents of the total individuals in the community The stability of populations, communities and ecosystems is affected by interactions with biotic and abiotic factors. The structure of a community is measured in terms of species richness/diversity.

A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25% Fig. 54-9 A B C D Community 1 Community 2 Figure 54.9 Which forest is more diverse? Two communities can have the same species richness but a different relative abundance A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10%

Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor in community dynamics Food chains link trophic levels from producers to top carnivores A food web is a branching food chain with complex trophic interactions

A terrestrial food chain A marine food chain Fig. 54-11 Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Secondary consumers Carnivore Carnivore Figure 54.11 Examples of terrestrial and marine food chains Primary consumers Herbivore Zooplankton Primary producers Plant Phytoplankton A terrestrial food chain A marine food chain

Fig. 54-12 Humans Smaller toothed whales Baleen whales Sperm whales Crab-eater seals Leopard seals Elephant seals Birds Fishes Squids Figure 54.12 An antarctic marine food web Carnivorous plankton Euphausids (krill) Copepods Phyto- plankton

Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton Fig. 54-13 Sea nettle Juvenile striped bass Figure 54.13 Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast Species may play a role at more than one trophic level Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community Fish larvae Fish eggs Zooplankton

Limits on Food Chain Length Each food chain in a food web is usually only a few links long Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis The energetic hypothesis suggests that length is limited by inefficient energy transfer The dynamic stability hypothesis proposes that long food chains are less stable than short ones Most data support the energetic hypothesis

Number of trophic links Fig. 54-14 5 4 3 Number of trophic links 2 1 High (control): natural rate of litter fall Medium: 1/10 natural rate Low: 1/100 natural rate Figure 54.14 Test of the energetic hypothesis for the restriction of food chain length. Researchers manipulated the productivity of tree-hole communities by providing leaf litter input at three levels. Reducing energy input reduced food chain length, a result consistent with the energetic hypothesis. Productivity

Species with a Large Impact Certain species have a very large impact on community structure Such species are highly abundant or play a pivotal role in community dynamics Field studies of sea stars exhibit their role as a keystone species in intertidal communities

Dominant Species Dominant species are those that are most abundant or have the highest biomass Biomass is the total mass of all individuals in a population Dominant species exert powerful control over the occurrence and distribution of other species One hypothesis suggests that dominant species are most competitive in exploiting resources Another hypothesis is that they are most successful at avoiding predators Invasive species, typically introduced to a new environment by humans, often lack predators or disease

Keystone Species A population of organisms has properties that are different from those of the individuals that make up the population. The cooperation and competition between individuals contributes to these different properties. Species-specific and environmental catastrophes affect species distribution and abundance, and sometimes these can be severe. Keystone species exert strong control on a community by their ecological roles, or niches In contrast to dominant species, they are not necessarily abundant in a community

EXPERIMENT RESULTS Fig. 54-15 Figure 54.15 Is Pisaster ochraceus a keystone predator? Conclusions: acts as a keystone species, exerting an influence on the community that is not reflected in its abundance. 20 15 With Pisaster (control) Number of species present 10 5 Without Pisaster (experimental) 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

Otter number (% max. count) Grams per 0.25 m2 Number per 0.25 m2 Fig. 54-16 100 80 Otter number (% max. count) 60 40 20 (a) Sea otter abundance 400 300 Grams per 0.25 m2 200 100 (b) Sea urchin biomass Figure 54.16 Sea otters as keystone predators in the North Pacific Observation of sea otter populations and their predation shows how otters affect ocean communities Sea otters feed on sea urchins, and sea urchins feed mainly on kelp. In areas where sea otters are abundant, sea urchins are rare and kelp forests are well developed. Where sea otters are rare, sea urchins are common and kelp is almost absent. Over the last 20 years, orcas have been preying on otters as the whales’ usual prey has declined. As a result, sea otter populations have declined and the loss of this keystone species has allowed sea urchin populations to increase, resulting in a loss of kelp forests. 10 8 Number per 0.25 m2 6 4 2 1972 1985 1989 1993 1997 Year (c) Total kelp density Food chain

Foundation Species (Ecosystem “Engineers”) Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale

Fig. 54-17 Figure 54.17 Beavers as ecosystem “engineers”

Number of plant species Fig. 54-18 8 6 Number of plant species 4 2 Figure 54.18 Facilitation by black rush (Juncus gerardi) in New England salt marshes Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community (a) Salt marsh with Juncus (foreground) With Juncus Without Juncus (b)

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels In this case, presence or absence of mineral nutrients determines community structure, including abundance of primary producers The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above In this case, predators control herbivores, which in turn control primary producers

(number of individuals Fig. 54-19 RESULTS Control plots 300 Warmed plots (number of individuals Nematode density per kg soil) 200 100 Figure 54.19 Are soil nematode communities in Antarctica controlled by bottom-up or top-down factors? Predator: E. antarcticus Prey: S. lindsayae Conclusion: the prey species’ increase in density as the predator density declined suggests that this soil nematode community is controlled by top-down factors. Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control E. antarcticus S. lindsayae

Polluted State Restored State Fish Abundant Rare Zooplankton Rare Fig. 54-UN1 Polluted State Restored State Fish Abundant Rare Zooplankton Rare Abundant Pollution can affect community dynamics Biomanipulation can help restore polluted communities Algae Abundant Rare

Concept 54.3: Disturbance influences species diversity and composition. A disturbance is an event that changes a community, removes organisms from it, and alters resource availability Fire is a significant disturbance in most terrestrial ecosystems It is often a necessity in some communities

Log intensity of disturbance Fig. 54-20 35 30 25 Number of taxa 20 15 Figure 54.20 Testing the intermediate disturbance hypothesis The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance High levels of disturbance exclude many slow-growing species Low levels of disturbance allow dominant species to exclude less competitive species Conclusions: number of invertebrate taxa peaks when the intensity of flooding was at intermediate levels. 10 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Log intensity of disturbance

(a) Soon after fire (b) One year after fire Fig. 54-21 Figure 54.21 Recovery following a large-scale disturbance The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance (a) Soon after fire (b) One year after fire

Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance Primary succession occurs where no soil exists when succession begins Secondary succession begins in an area where soil remains after a disturbance

Primary Succession

Secondary Succession

Ecological Succession Early-arriving species and later-arriving species may be linked in one of three processes: Early arrivals may facilitate appearance of later species by making the environment favorable They may inhibit establishment of later species They may tolerate later species but have no impact on their establishment

Human Disturbance Humans have the greatest impact on biological communities worldwide Human disturbance to communities usually reduces species diversity Humans also prevent some naturally occurring disturbances, which can be important to community structure

Fig. 54-24 Figure 54.24 Disturbance of the ocean floor by trawling

Island Equilibrium Model Species richness on islands depends on island size, distance from the mainland, immigration, and extinction The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point

Concept 54.5: Community ecology is useful for understanding pathogen life cycles and controlling human disease. Ecological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions Pathogens can alter community structure quickly and extensively

Fig. 54-29 Figure 54.29 White-band disease visible on a coral Pathogens can have dramatic effects on communities For example, coral reef communities are being decimated by white-band disease Human activities are transporting pathogens around the world at unprecedented rates Community ecology is needed to help study and combat them

Community Ecology and Zoonotic Diseases Zoonotic pathogens have been transferred from other animals to humans The transfer of pathogens can be direct or through an intermediate species called a vector Many of today’s emerging human diseases are zoonotic