Chapter 54 Community Ecology

Overview: A Sense of Community A biological community is an group of populations of various species living close enough for potential interaction

Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved Relationships between species in a community are called interspecific interactions Examples: competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism) Interspecific interactions affect the survival and reproduction of each species the effects can be (+), (–), or no effect (0)

Competition Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply In Alaska, the lynx and the fox must compete with each other for prey such as the snowshoe hare.

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 One species will be more efficient and thus reproduce more rapidly than the other. This will eventually lead to the local elimination of the inferior competitor.

Ecological Niches Ecological niche --The total of a species’ use of biotic and abiotic resources In other words…a niche is an organism’s ecological role (its “job”) 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 Ex: Seven species of Anolis Lizards live close together in the Dominican Republic. They all eat the same food. Competition, however, is reduced because each lizard species has a preferred perch, thus occupying a distinct niche.

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 A. insolitus A. aliniger A. christophei A. distichus A. cybotes A. etheridgei

As a result of competition, a species’ fundamental niche may differ from its realized niche Fundamental niche = niche potentially occupied Realized niche = part of the niche actually occupied

Two barnacle species have a stratified distribution on coastal Fig. 54-3 EXPERIMENT High tide Chthamalus Two barnacle species have a stratified distribution on coastal rocks. Is this the result of interspecific competition between the two? To find out, researchers removed the Balanus from the rocks. Balanus Chthamalus realized niche Balanus realized niche Ocean Low tide Results were that the Chthamalus moved into the area formerly occupied by Balanus. Thus, interspecific competition makes the realized niche of Chthamalus much smaller than its fundamental niche RESULTS High tide Figure 54.3 Can a species’ niche be influenced by interspecific competition? Chthamalus fundamental niche Knowing that Balanus cannot survive high on the rocks because it dries out during low tides, how does its realized niche compare with its fundamental niche? Ocean Low tide

Character Displacement Character displacement is a tendency for characteristics to be more divergent in populations that geographically overlapping rather than populations that are geographically separate An example is variation in beak size between populations of two species of Galápagos finches Where these two species live separately, their beak sizes are about the same---indicating they eat the same seeds. Where they live together (on Santa Maria and San Cristobal) they must compete for food and overtime have evolved differences that allow for different beak sizes thus, eating different seeds.

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 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

Video: Seahorse Camouflage 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 such as: Cryptic coloration, or camouflage, makes prey difficult to spot Canyon Tree Frog Video: Seahorse Camouflage

Aposematic coloration bright warning coloration indicating a chemical defense (poison) Batesian Mimicry —a palatable or harmless species mimics an unpalatable or harmful species Poison Dart Frog Hawkmoth larva Green parrot snake

Müllerian mimicry-- two or more unpalatable species resemble each other. Each species gains an advantage because the more unpalatable prey there are, the more quickly predators adapt to avoid prey with that appearance. (This is also why warning colors seem to be universal: yellow, black, and red) Cuckoo bee 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 Symbiotic relationships can be harmful, helpful or neutral

Parasitism In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process Endoparasites live within the body of their host (tapeworm); Ectoparasites live on the external surface of a host (leech) Parasites may account for 1/3 of all the species on Earth Many parasites have a complex life cycle involving a number of hosts (blood fluke—infects snails and humans)

Video: Clownfish and Anemone Mutualism Mutualism (+/+ interaction), is an interspecific interaction that benefits both species A mutualism can be Obligate, where one species cannot survive without the other (termites and digestive microbes) Facultative, where both species can survive alone (acacia trees and ants) Ants get food in the form of nectar Trees get protection as ants keep the area at the base of the tree free from fungus, herbivores, debris, etc. Video: Clownfish and Anemone

Commensalism 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 Cattle egrets and water buffalo

Concept 54.2: Dominant and keystone species exert strong controls on community structure Two fundamental features of community structure are species diversity and feeding relationships

Species Diversity Species diversity of a community is the variety of organisms that make up the community It has two components: 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

Which forest is more diverse? Fig. 54-9 Which forest is more diverse? Both of these forest communities have the same species richness because they each have four species of trees. A B C D Community 1 Community 2 Figure 54.9 Which forest is more diverse? A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10% Their relative abundance, however is very different. Most people would therefore describe Community 1 as being more diverse.

Video: Shark Eating a Seal Trophic Structure Trophic structure is the feeding relationships between organisms in a community It is a key factor in community dynamics Video: Shark Eating a Seal

Food chains link trophic levels from producers to top carnivores Fig. 54-11 Quaternary consumers Food chains link trophic levels from producers to top carnivores 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

A food web is a branching food chain with complex trophic interactions Fig. 54-12 Humans A food web is a branching food chain with complex trophic interactions 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

Species may play a role at more than one trophic level This is true for many omnivores. Sea nettle Juvenile striped bass Fish eggs Zooplankton

Limits on Food Chain Length Each food chain in a food web is usually only a few links long (about 5) 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 Only about 10% of the energy stored in one level of a food chain is passed on to the next The dynamic stability hypothesis proposes that long food chains are less stable than short ones The longer the food chain, the more slowly top predators can recover from environmental shocks. Most data support the energetic hypothesis

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

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 Ex: Maple trees in Northeastern American Forests impact abiotic factors such as shading and soil.

How does a species become the dominant 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 lend support to the second hypothesis. They are typically introduced to a new environment by humans and often lack predators or disease, therefore attaining a high biomass.

The impact of a dominant species on a community can be seen when it is accidentally lost Ex: American Chestnut: formally the dominant tree of Northeastern American forests. All were killed by a fungus between 1910 and 1950. Few mammals and birds were harmed by the loss, but 7 species of moths and butterflies went extinct.

Keystone Species 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 Example: Pacific Sea Otter Observation of sea otter populations and their predation shows how otters affect ocean communities

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 When sea otters are abundant, kelp forests thrive As the otters are hunted by orcas, their numbers decline. Sea urchin populations rise, causing the loss of kelp forests. (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 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

Bottom-Up and Top-Down Controls The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels N V H P In this case, presence or absence of mineral nutrients (N) determines the amount of vegetation (V) which controls the number of herbivores (H) and the number of predators (P)

The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above N V H P In this case, predators control herbivores, which in turn control the amount of vegetation and the uptake of nutrients The effects of any manipulation thus move down the trophic structure as alternating +/- effects

Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control

Concept 54.3: Disturbance influences species diversity and composition The nonequilibrium model of a community structure describe describes communities as constantly changing following disturbances 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

The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance. They do this by opening up habitats for occupation by less competitive species High levels exclude many slow-growing species Low levels allow dominant species to exclude less competitive species

The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance The dominant species in Yellowstone is the Lodgepole pine. It releases seeds only after exposure to extreme heat. The fire is actually healthy for a forest like this.

(a) Soon after fire (b) One year after fire Fig. 54-21 Figure 54.21 Recovery following a large-scale 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. May take hundreds or thousands of years Secondary succession begins in an area where soil remains after a disturbance

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

Retreating glaciers provide a valuable field-research opportunity for observing succession Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

Current Iceberg Front Glacier Bay

1941 1907 Pioneer stage, with fireweed dominant 1860 Glacier Bay Fig. 54-22-1 1941 1907 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska Alaska 1760

Dryas stage—after 30 years Pioneer stage, with fireweed dominant 1860 Fig. 54-22-2 1941 1907 2 Dryas stage—after 30 years 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska Alaska 1760

Alder stage—after another few decades Fig. 54-22-3 1941 1907 2 Dryas stage—after 30 yrs. 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska Alaska 1760 3 Alder stage—after another few decades

Spruce stage—after two centuries Alder stage—after another few decades Fig. 54-22-4 1941 1907 2 Dryas stage—after 30 yrs. 1 Pioneer stage, with fireweed dominant 5 10 15 1860 Kilometers Glacier Bay Figure 54.22 Glacial retreat and primary succession at Glacier Bay, Alaska Alaska 1760 4 Spruce stage—after two centuries 3 Alder stage—after another few decades

Succession is the result of changes induced by the vegetation itself On the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen content As this occurs, more and more plant species can develop over time

60 50 40 Soil nitrogen (g/m2) 30 20 10 Pioneer Dryas Alder Spruce Fig. 54-23 60 50 40 Soil nitrogen (g/m2) 30 20 Figure 54.23 Changes in soil nitrogen content during succession at Glacier Bay 10 Pioneer Dryas Alder Spruce Successional stage

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 These photos show disturbances on the ocean floor before (top) and after (bottom) deep-sea trawlers have passed.

Concept 54.4: Biogeographic factors affect community biodiversity Latitude and area are two key factors that affect a community’s species diversity

Latitudinal Gradients Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate The greater age of tropical environments may account for the greater species richness Climate is likely the primary cause of the latitudinal gradient in biodiversity Two main climatic factors correlated with biodiversity are solar energy and water availability They can be considered together by measuring a community’s rate of evapotranspiration Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

Area Effects The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species A species-area curve of North American breeding birds supports this idea

Fig. 54-26 1,000 100 Number of species 10 Figure 54.26 Species-area curve for North American breeding birds 1 0.1 1 10 100 103 104 105 106 107 108 109 1010 Area (hectares)

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

Pathogens and Community Structure Pathogens can have dramatic effects on communities For example, coral reef communities are being decimated by white-band disease As the staghorn coral dies, algal species take its place, and the fish community is dominated by herbivores feeding on the algae.

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

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

Fig. 54-UN2

You should now be able to: Distinguish between the following sets of terms: competition, predation, herbivory, symbiosis; fundamental and realized niche; cryptic and aposematic coloration; Batesian mimicry and Müllerian mimicry; parasitism, mutualism, and commensalism; endoparasites and ectoparasites; species richness and relative abundance; food chain and food web; primary and secondary succession

Define an ecological niche and explain the competitive exclusion principle in terms of the niche concept Explain how dominant and keystone species exert strong control on community structure Distinguish between bottom-up and top-down community organization Describe and explain the intermediate disturbance hypothesis

Explain why species richness declines along an equatorial-polar gradient Define zoonotic pathogens and explain, with an example, how they may be controlled