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

Fig. 54-1 Figure 54.1 How many interactions between species are occurring in this scene?

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)

Competition Interspecific competition (–/– interaction) occurs when two or more species compete for a resource in short supply

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 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 but they cannot share the same niche

Resource partitioning splits a niche into smaller ecological niches, enabling similar species to coexist in a community

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

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 Cryptic coloration, or camouflage, makes prey difficult to spot Video: Seahorse Camouflage

(a) Cryptic coloration Canyon tree frog Fig. 54-5a Figure 54.5a Examples of defensive coloration in animals

Animals with effective chemical defense often exhibit bright warning coloration. Predators are particularly cautious in dealing with prey that display such coloration

Fig. 54-5b Poison dart frog Figure 54.5b Examples of defensive coloration in animals

In some cases, a prey species may gain significant protection by mimicking the appearance of another species In Batesian mimicry, an edible or harmless species mimics a bad tasting or harmful species

(c) Batesian mimicry: A harmless species mimics a harmful one. Fig. 54-5c (c) Batesian mimicry: A harmless species mimics a harmful one. Hawkmoth larva Green parrot snake Figure 54.5c Examples of defensive coloration in animals

In Müllerian mimicry, two or more poisonous or harmful species resemble each other

Müllerian mimicry: Two unpalatable species mimic each other. Fig. 54-5d (d) Müllerian mimicry: Two unpalatable species mimic each other. Cuckoo bee Yellow jacket Figure 54.5d Examples of defensive coloration in animals

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

Fig. 54-6 Figure 54.6 A West Indies manatee (Trichechus manatus) in Florida

Symbiosis Symbiosis is a relationship where two or more species live in direct and intimate contact with one another

Parasitism In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process

Many parasites have a complex life cycle involving a number of hosts Some parasites change the behavior of the host to increase their own fitness

Video: Clownfish and Anemone Mutualism Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species Video: Clownfish and Anemone

(a) Acacia tree and ants (genus Pseudomyrmex) Fig. 54-7a Figure 54.7 Mutualism between acacia trees and ants (a) Acacia tree and ants (genus Pseudomyrmex)

(b) Area cleared by ants at the base of an acacia tree Fig. 54-7b Figure 54.7 Mutualism between acacia trees and ants (b) Area cleared by ants at the base of an acacia tree

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

Fig. 54-8 Figure 54.8 A possible example of commensalism between cattle egrets and water buffalo

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

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

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? A: 25% B: 25% C: 25% D: 25% A: 80% B: 5% C: 5% D: 10%

Determining the number and abundance of species in a community is difficult, especially for small organisms Molecular tools can be used to help determine microbial diversity

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 Food chains link trophic levels from producers to top carnivores Video: Shark Eating a Seal

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

Food Webs A food web is a branching food chain with complex trophic interactions

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

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

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 Fish larvae Fish eggs Zooplankton

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

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 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

Field studies of sea stars exhibit their role as a keystone species in intertidal communities

Fig. 54-15a EXPERIMENT Figure 54.15 Is Pisaster ochraceus a keystone predator?

Number of species present Fig. 54-15b RESULTS 20 15 With Pisaster (control) Number of species present 10 Without Pisaster (experimental) 5 Figure 54.15 Is Pisaster ochraceus a keystone predator? 1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 Year

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”

Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community

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 (a) Salt marsh with Juncus (foreground) With Juncus Without Juncus (b)

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

(a) Soon after fire Fig. 54-21a Figure 54.21 Recovery following a large-scale disturbance (a) Soon after fire

(b) One year after fire Fig. 54-21b Figure 54.21 Recovery following a large-scale disturbance (b) One year after fire

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

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

1941 1907 Dryas stage Pioneer stage, with fireweed dominant 1860 Fig. 54-22-2 1941 1907 2 Dryas stage 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

1941 1907 Dryas stage Pioneer stage, with fireweed dominant 1860 Fig. 54-22-3 1941 1907 2 Dryas stage 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

1941 1907 Dryas stage Pioneer stage, with fireweed dominant 1860 Fig. 54-22-4 1941 1907 2 Dryas stage 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 3 Alder stage

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

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

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

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

Fig. 54-29 Figure 54.29 White-band disease visible on a coral

Human activities are transporting pathogens around the world at unprecedented rates Community ecology is needed to help study and combat them

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