Chapter 45 Community Ecology. 45.1 Fighting Foreign Fire Ants Native to Brazil, imported fire ants...

Chapter 45Community Ecology

45.1 Fighting Foreign Fire Ants

• Native to Brazil, imported fire ants (Solenopsis invicta) nest in the ground and have painful stings

• Global trade and shipping brought fire ants to the US and to other countries around the world

• Fire ants have a negative impact on native species of plants, insects, birds, and other animals

Red Imported Fire Ants

Communities

• Species interactions such as competition or predation are one focus of community ecology

• A community is all the species that live in a region

• Species interactions and disturbances can shift community structure (types of species and their relative abundances) in small and large ways

45.2 Which Factors Shape Community Structure?

• Community structure refers to the number and relative abundances of species in a habitat

• Habitat• The type of place where a species normally lives

• Community• All species living in a habitat

Species Diversity

• Communities vary in their species diversity

• Two components of species diversity:• Species richness: the number of species• Species evenness: the relative abundance of each species

Community Structure

• Many factors influence community structure• Abiotic factors such as climate• Gradients of topography• Species interactions (direct and indirect)

• Symbiosis refers to direct, long-term interactions:• Commensalism: One species benefits and the other is

neither benefited nor harmed• Mutualism: Both benefit• Parasitism: Parasite benefits, host is harmed

Table 45-1 p810

Commensalism

Take-Home Message: What factors affect species in a community?

• The types and abundances of species in a community are affected by physical factors such as climate and by species interactions.

• A species can be benefited, harmed, or unaffected by its interaction with another species.

45.3 Mutualism

• Mutualism is a species interaction in which each species benefits by associating with the other• Flowering plants and animal pollinators• Birds that disperse seeds• Lichens, mycorrhizae, and nitrogen-fixing bacteria that

help plants obtain nutrients• Animals share nutrients with mutualistic microorganisms in

their gut• Two species may protect one another

Obligate Mutualism: Yucca and Moth

Mutual Protection

Take-Home Message: What are the effects of participating in a mutualism?

• A mutualism benefits both participants.

• In some cases, two species form an exclusive partnership. In others, a species provides benefits to, and receives benefits from, multiple species.

• Participating in a mutualism has both benefits and costs. Selection favors individuals who maximize their benefits while minimizing their costs.

45.4 Competitive Interactions

• Resources are limited; individuals of different species often compete for access to them

• Interspecific competition hurts both species

• Competition among individuals of the same species is more intense than interspecific competition

The Niche

• Each species requires specific resources and environmental conditions that we refer to as its ecological niche

• Both physical (abiotic) and biological (biotic) factors define the niche

• The more similar the niches of two species are, the more intensely the species will compete

Interspecific Competition

• Interference competition• One species actively prevents another from accessing a

resource

• Exploitative competition• Species reduce the amount of a resource available to the

other by using that resource

Interference Competition

Effects of Competition

• Competitive exclusion• When two species require the same limited resource to

survive or reproduce, the better competitor will drive the less competitive species to extinction in that habitat

• Competitors can coexist when their resource needs are not exactly the same• Competition suppresses growth of both species

Competitive Exclusion in Paramecium

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ANIMATED FIGURE: Competitive exclusion

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Competing for Pollinators

Mimulus Lobelia

Resource Partitioning

• Resource partitioning is an evolutionary process by which species become adapted to use a shared limiting resource in a way that minimizes competition (directional selection)

• Example: Eight species of woodpecker in Oregon feed on insects and nest in hollow trees, but the details of their foraging behavior and nesting preferences vary

Character Displacement

• Over generations, directional selection leads to character displacement – the range of variation for one or more traits is shifted in a direction that lessens the intensity of competition for a limiting resource

• Example: Where two species of salamanders coexist, differences in body length becomes more pronounced

Character Displacement in Salamanders

ANIMATED FIGURE: Hairston's experiment

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Take-Home Message: What happens when species compete for resources?

• In some interactions, one species actively blocks another’s access to a resource. In other interactions, one species is simply better than another at exploiting a shared resource.

• When two species compete, selection favors individuals whose needs are least like those of the competing species.

45.5 Predator–Prey Interactions

• Predation is an interspecific interaction in which one species (predator) captures, kills, and eats another species (prey)

• Relative abundances of predators and prey shift over time in response to species interactions and changing environmental conditions

Predator Responses to Changes in Prey Density

• Type I response (passive predators)• Number of prey killed depends on prey density

• Type II response• Number of prey killed depends on the predator’s capacity

to capture, eat and digest prey

• Type III response• Number of kills increases only when prey density reaches

a certain level

Responses of Predators to Prey Density

A Type 2 Response

Figure 45-9b2 p814

ANIMATED FIGURE: Predator-prey interactions

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Cyclic Changes in Abundance

• Time lag in predator response to prey density can lead to cyclic changes in abundance

• When prey density is low, predators decline, prey are safer, prey numbers increase

• When prey density is high, predator numbers increase, prey numbers decline

Canadian Lynx and Snowshoe Hare

Predator and Prey

Take-Home Message: How do predator and prey populations change over time?

• Predator populations show three general patterns of response to changes in prey density.

• Population levels of prey may show recurring oscillations.

• The numbers in predator and prey populations vary in complex ways that reflect the multiple levels of interaction in a community.

45.6 An Evolutionary Arms Race

• Predators select for better prey defenses, and prey select for more efficient predators

• Prey defenses include exoskeletons, unpleasant taste, toxic chemicals or stings, and physical adaptations such as camouflage

Coevolution of Predators and Prey

• Predator and prey populations exert selective pressures on one another

• Genetic traits that help prey escape will increase in frequency

• Defensive improvements select for a countering improvement in predators

• Example: Spraying beetles and grasshopper mice

Defense and Counter Defense

Some Physical Adaptations of Prey

• Warning coloration• Many toxic or unpalatable species have bright colors and

patterns that predators learn to avoid

• Mimicry• A harmless animal looks like a dangerous one

• Camouflage• Body shape, color pattern and behavior that make an

individual blend in with its surroundings

Warning Coloration and Mimicry

Camouflage in Prey and Predators

Coevolution of Herbivores and Plants

• With herbivory, an animal feeds on plants

• Two defenses have evolved in response to herbivory:• Some plants withstand and recover quickly from the loss

of their parts• Some plants have physical deterrents (spines, thorns,

tough leaves); or chemical deterrents (secondary metabolites that taste bad or sicken herbivores)

Take-Home Message: How do predation and herbivory influence community structure?

• In any community, predators and prey coevolve, as do plants and the herbivores that feed on them.

• Defensive adaptations in plants and prey can limit the ability of predators or herbivores to exploit some species in their community.

45.7 Parasites and Parasitoids

• With parasitism, one species (parasite) benefits by feeding on another (host), without immediately killing it

• Endoparasites such as parasitic roundworms live and feed inside their host

• An ectoparasite such as a tick feeds while attached to a host’s external surface

Endoparasites

Ectoparasites

Parasite Diversity

• Parasitism has evolved in members of a diverse variety of groups

• Bacterial, fungal, protistan, and invertebrate parasites feed on vertebrates

• Lampreys attach to and feed on other fish

• Parasitic plants that withdraw nutrients from other plants

Dodder: A Parasitic Plant

Strangers in the Nest

• With brood parasitism, one egg-laying species benefits by having another raise its offspring• Examples: European cuckoo, cowbird

• One cowbird can parasitize 30 nests per season, decreasing the reproductive rate of the host species

Cowbird with Foster Parent

Parasitoids

• Parasitoids are insects that lay eggs in other insects

• Their larvae develop in the host’s body, feed on its tissues, and eventually kill it

• As many as 15 percent of all insects may be parasitoids

• Example: parasitoid wasps

Biological Pest Controls

• Some parasites and parasitoids are raised commercially for use as biological pest control agents

• Example: Parasitoid wasps lay eggs in aphids

• Introducing a species into a community as a biological control has both advantages and risks

Biological Pest Control Agent

Take-Home Message: Effects of parasites, brood parasites, and parasitoids

• Parasites reduce the reproductive rate of host individuals by withdrawing nutrients from them.

• Brood parasites reduce the reproductive rate of hosts by tricking them into caring for young that are not their own.

• Parasitoids reduce the number of host organisms by preventing reproduction and eventually killing the host.

45.8 Ecological Succession

• Ecological succession is a process in which one array of species replaces another over time

• It can occur in a barren habitat such as new volcanic land (primary succession) or a disturbed region in which a community previously existed (secondary succession)

Pioneer Species

• Primary succession begins when pioneer species such as lichens and mosses colonize a barren habitat with no soil

• Pioneer species are opportunistic colonizers of new or newly vacated habitats

• Pioneers help build and improve soil for later successional species

Primary Succession: Alaska’s Glacier Bay

ANIMATED FIGURE: Succession

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