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Communities in Motion A biological community is an assemblage of populations of various species living close enough for potential interaction Some interactions are beneficial to both of the species involved For example, the bluestreak cleaner wrasse swims inside the mouth of a moray eel and eats tiny parasites inside its mouth © 2017 Pearson Education, Inc.

Figure 54.1 Which species benefits from this interaction?

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, 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) © 2017 Pearson Education, Inc.

Competition Competition (–/– interaction) occurs when species compete for a resource that limits survival and reproduction Resources must be in short supply for competition to occur Strong competition can lead to competitive exclusion, local elimination of a competing species Russian ecologist G.F. Gause concluded that two species competing for the same limiting resources cannot coexist permanently in the same place © 2017 Pearson Education, Inc.

Ecological Niches and Natural Selection An ecological niche is the sum of an organism’s use of biotic and abiotic resources; it can 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 Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community © 2017 Pearson Education, Inc.

A. insolitus usually perches on shady branches. 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 A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei

Both species are normally nocturnal (active during the night) The common spiny mouse and the golden spiny mouse show temporal partitioning of their niches Both species are normally nocturnal (active during the night) Where they coexist, the golden spiny mouse becomes diurnal (active during the day) © 2017 Pearson Education, Inc.

Character Displacement 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 An example is variation in beak size between populations of two species of Galápagos finches © 2017 Pearson Education, Inc.

Floreana, San Cristóbal Sympatric populations 40 G. fuliginosa G. fortis Beak depth Percentages of individuals in each size class 60 Los Hermanos 40 G. fuliginosa, allopatric 20 60 Daphne 40 G. fortis, allopatric 20 Figure 54.4 Character displacement: indirect evidence of past competition 60 Floreana, San Cristóbal Sympatric populations 40 20 8 10 12 14 16 Beak depth (mm)

Exploitation Exploitation refers to any +/– interaction in which one species benefits by feeding on the other species Exploitative interactions include predation, herbivory, and parasitism © 2017 Pearson Education, Inc.

Predation Predation (+/– interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey Predators have adaptations that enable them to find, identify, catch, and subdue their prey For example, many predators have claws, fangs, or poison © 2017 Pearson Education, Inc.

Predation Prey display various adaptations to avoid being eaten Behavioral defenses include hiding, fleeing, and forming herds or schools Animals also have morphological and physiological defense adaptations For example, mechanical and chemical defenses protect species such as porcupines and skunks © 2017 Pearson Education, Inc.

Cryptic coloration: camouflage Mechanical defense Chemical defense Porcupine Skunk Cryptic coloration: camouflage Aposematic coloration: warning coloration Poison dart frog Canyon tree frog Batesian mimicry: A harmless species mimics a harmful one. (Müllerian mimicry: Two unpalatable species mimic each other. Figure 54.5 Examples of defensive adaptations in animals Venomous green parrot snake Yellow jacket Nonvenomous hawkmoth larva Cuckoo bee

Cryptic coloration, or camouflage, makes prey difficult to spot Animals with effective chemical defenses often exhibit bright warning coloration, called aposematic coloration Predators are particularly cautious in dealing with prey that display such coloration Cryptic coloration, or camouflage, makes prey difficult to spot 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 © 2017 Pearson Education, Inc.

In Müllerian mimicry, two or more unpalatable species resemble each other Mimicry has also evolved in many predators to enable them to approach prey For example, the mimic octopus can take on the appearance and movement of more than a dozen marine animals © 2017 Pearson Education, Inc.

Mimicking a sea snake Mimicking a flounder Mimicking a stingray Figure 54.6 The mimic octopus Mimicking a stingray

Herbivory Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga Large mammals are the most familiar herbivores, but most herbivores are invertebrates Herbivores have many specialized adaptations For example, many herbivores have specialized teeth or digestive systems for processing vegetation Plants may produce toxic or distasteful chemicals or mechanical defenses, such as spines or thorns © 2017 Pearson Education, Inc.

Figure 54.7 An herbivorous marine mammal

Parasitism 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 © 2017 Pearson Education, Inc.

Parasitism Many parasites have a complex life cycle involving multiple hosts Some parasites change the behavior of the host in a way that increases the likelihood that the parasite will be transmitted to the next host Parasites can significantly affect the survival, reproduction, and density of their host population, directly or indirectly © 2017 Pearson Education, Inc.

Positive Interactions Ecological communities are heavily influenced by positive interactions, where at least one species benefits and neither is harmed Mutualism (+/+) and commensalism (+/0) are positive interactions © 2017 Pearson Education, Inc.

Mutualism Mutualism (+/+ interaction) is a common interspecific interaction that benefits both species In a mutualism, both species incur costs, but the benefits to each partner exceed the costs In some mutualisms, each species depends on the other for their survival and reproduction; in others, both species can survive alone © 2017 Pearson Education, Inc.

Hollow thorns that house stinging ants of the genus Pseudomyrmex Figure 54.8 Mutualism between acacia trees and ants Hollow thorns that house stinging ants of the genus Pseudomyrmex Area cleared by ants around an acacia tree

Commensalism Commensalism (+/0 interaction) is another common interaction in which one species benefits and the other is neither harmed nor helped For example, shade-tolerant wildflowers depend on the shade provided by forest trees, but the trees are not affected by the wildflowers © 2017 Pearson Education, Inc.

Commensalism Some interactions that are typically commensal may at times become mutualistic For example, cattle egrets typically benefit from the insects flushed out of the grass by bison and have no effect on the bison At times, they may remove and eat ectoparasites from the bison’s skin, making the interaction mutualistic © 2017 Pearson Education, Inc.

Figure 54.9 Commensalism between cattle egrets and African buffalo

Commensalism Positive interactions can have significant influence on the structure of ecological communities For example, the black rush affects community diversity by making the soil more hospitable for other plant species © 2017 Pearson Education, Inc.

With Juncus Without Juncus Number of plant species 8 6 4 2 Figure 54.10 Facilitation by black rush (Juncus gerardii) in New England salt marshes Salt marsh with Juncus With Juncus Without Juncus

Concept 54.2: Diversity and trophic structure characterize biological communities The 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 number of different species in the community Relative abundance is the proportion each species represents of all individuals in the community © 2017 Pearson Education, Inc.

H = – (pA ln pA + pB ln pB + pC ln pC + …) Shannon Diversity Index Diversity can be compared using a diversity index Shannon diversity index (H) H = – (pA ln pA + pB ln pB + pC ln pC + …) where A, B, C . . . are the species, p is the relative abundance of each species, and ln is the natural logarithm © 2017 Pearson Education, Inc.

Shannon Diversity We can calculate the Shannon diversity index for the two communities discussed in the text For community 1, p = 0.25 for each species, so H = – 4(0.25 ln 0.25) = 1.39 For community 2, H = –[0.8 ln 0.8 + 2(0.05 ln 0.05) + 0.1 ln 0.1] = 0.71 © 2017 Pearson Education, Inc.

A B C D Community 1 A: 25% B: 25% C: 25% D: 25% Community 2 Figure 54.11 Which forest is more diverse? Community 2 A: 80% B: 5% C: 5% D: 10%

Shannon Diversity Communities with higher diversity are more productive; they produce more biomass (the total mass of all organisms) more stable in their productivity better able to withstand and recover from environmental stresses more resistant to invasive species, organisms that become established outside their native range © 2017 Pearson Education, Inc.

Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor affecting community structure and dynamics Food chains link trophic levels from producers to top carnivores The position an organism occupies in a food chain is called its trophic level © 2017 Pearson Education, Inc.

A terrestrial food chain A marine food chain Quaternary consumers Carnivore Carnivore Tertiary consumers Carnivore Carnivore Carnivore Secondary consumers Carnivore Primary consumers Herbivore Figure 54.14 Examples of terrestrial and marine food chains Zooplankton Plant Primary producers Phytoplankton A terrestrial food chain A marine food chain

Food Webs A food web is a group of food chains linked together forming complex trophic interactions A species may play a role at more than one trophic level in a food web Food webs can be simplified by grouping species with similar trophic relationships into broad functional groups isolating a portion of a community that interacts very little with the rest of the community © 2017 Pearson Education, Inc.

Sperm whales Smaller toothed whales Baleen whales Elephant seals Crabeater seals Leopard seals Birds Fishes Squids Figure 54.15 An Antarctic marine food web Carniv- orous plankton Copepods Krill Phyto- plankton

Sea nettle Juvenile striped bass Fish larvae Zoo- plankton Fish eggs Figure 54.16 Partial food web for Chesapeake Bay estuary Zoo- plankton Fish eggs

Limits on Food Chain Each food chain in a food web is usually only a few links long The energetic hypothesis suggests that length is limited by inefficient energy transfer Only about 10% of the energy stored in organic matter at each trophic level is converted to organic matter at the next trophic level For example, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass and 1 kg of carnivore biomass © 2017 Pearson Education, Inc.

Limits on Food Chain © 2017 Pearson Education, Inc.

Limits on Food Chain Dominant species are those that are most abundant or have the highest biomass One hypothesis suggests that dominant species are most competitive in exploiting resources Another hypothesis is that they are most successful at avoiding predators or disease Invasive species, typically introduced to a new environment by humans, may become dominant because they lack natural predators or parasites © 2017 Pearson Education, Inc.

Limits on Food Chain Keystone species exert strong control on a community by their ecological roles, or niches In contrast to dominant species, they are not usually abundant in a community Field studies of sea stars illustrate their role as a keystone species in intertidal communities © 2017 Pearson Education, Inc.

Number of species present Experiment Results Number of species present 20 15 With Pisaster (control) Figure 54.18 Inquiry: Is Pisaster ochraceus a keystone species? 10 Without Pisaster (experimental) 5 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structure For example, beaver dams can transform landscapes on a very large scale © 2017 Pearson Education, Inc.

A disturbance is an event that changes a community, removes organisms from it, and alters resource availability The nonequilibrium model describes communities as constantly changing after disturbance © 2017 Pearson Education, Inc.

Characterizing Disturbance The types of disturbances and their frequency and severity vary among communities Storms and fire are significant sources of disturbance in many ecosystems The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance © 2017 Pearson Education, Inc.

Soon after fire One year after fire Figure 54.22 Recovery following a large-scale disturbance One year after fire

Ecological Succession Ecological succession is the sequence of changes in community composition following a disturbance Primary succession occurs where no soil exists when succession begins Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics Succession is the result of changes induced by the vegetation itself On the glacial moraines, pioneer plant species facilitate later arrivals by increasing soil nitrogen content © 2017 Pearson Education, Inc.

Ecological Succession 1941 1907 1860 1 Pioneer stage 2 Dryas stage Glacier Bay Alaska Figure 54.23_5 Glacial retreat and primary succession at Glacier Bay, Alaska (step 5) 1760 5 10 15 Kilometers 4 Spruce stage 3 Alder stage

Ecological Succession Secondary succession begins in an area where soil remains after a disturbance For example, abandoned agricultural land may return to its original state through secondary succession © 2017 Pearson Education, Inc.

Human Disturbance Humans have the greatest impact on biological communities worldwide Both terrestrial and marine ecosystems are subject to human disturbance Human disturbance to communities usually reduces species diversity © 2017 Pearson Education, Inc.

Before trawling After trawling Figure 54.25 Disturbance of the ocean floor by trawling After trawling

Concept 54.4: Biogeographic factors affect community diversity Latitude and area are two key biogeographic factors that affect the species diversity of biological communities Species richness is especially great in the tropics and generally declines in a gradient toward the poles Two key factors affecting latitudinal gradients of species richness are evolutionary history and climate Tropical environments may have greater species richness because there has been more time for speciation to occur © 2017 Pearson Education, Inc.

Two main climatic factors correlated with biodiversity in terrestrial communities are sunlight and precipitation They can be considered together by measuring a community’s rate of evapotranspiration, the evaporation of water from soil plus transpiration of water from plants Potential evapotranspiration is the measure of potential water loss, assuming water is available © 2017 Pearson Education, Inc.

Vertebrate species richness 200 Vertebrate species richness (log scale) 100 50 Figure 54.26 Energy, water, and species richness 10 500 1,000 1,500 2,000 Potential evapotranspiration (mm/yr)

Concept 54.5: Pathogens alter community structure locally and globally Community structure is universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions Pathogens can be particularly virulent in a new habitat Human activities are transporting pathogens around the world at unprecedented rates For example, H1N1, the virus that causes “swine flu,” spread around the world from Veracruz, Mexico, causing more than 18,000 deaths within two years © 2017 Pearson Education, Inc.

Community Ecology and Zoonotic Diseases Zoonotic pathogens are transferred to humans from other animals The transfer of pathogens can be direct or through an intermediate species called a vector Many of today’s emerging human diseases are zoonotic Avian flu is a highly contagious virus of birds Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds © 2017 Pearson Education, Inc.

Figure 54.UN06 Summary of key concepts: interspecific interactions