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Community Ecology
Section 1 – Species
Symbioses à close interactions between species Predation, parasitism, competition, mutualism,
commensalism are all types of symbioses.
Predation
• One individual (predator) captures, kills and eats another individual (prey)
• Influences where and how species live by determining relationships in food web
Predators, prey, and natural
• Natural selection favors adaptations that improve efficiency of predator at finding, capturing and eating prey
• Ex. Rattlesnakes à sharp sense of smell, specialized heat-‐sensitive pits below nostrils
• Pits enable snake to strike at warm-‐blooded prey
Prey’s survival
• Depends on ability to avoid being caught
• Natural selection favors traits that improve avoiding, escaping or discourage predators
• Fast speed, hiding, poison
Camouflaged animals
Mimicry• Trickery is important in antipredator defenses
• Mimicry à a harmless species looks like a poisonous species
• Sometimes also two or more dangerous species look similar (bees, wasps)
The coral snake, a very poisonous species.
The scarlet king snake, a harmless species.
This moth caterpillar defends itself by mimicking a snake.
This is the Monarch bu>erfly. It feeds on poisonous milkweed, and tastes very bad to predators.
This is the Viceroy bu>erfly, and tastes very good to predators, but is mistaken for the Monarch bu>erfly.
Plant-‐herbivore interactions
• Ecologists classify relationship between plants and herbivores as predatory
Plant adaptations
• Through natural selection, plants have evolved adaptations that protect them from being eaten
• Physical defenses
• Chemical defenses
Physical defenses• Thorns, sticky hairs, tough leaves
• Difficult to eat
Chemical defenses• Plants make chemicals from products of their
metabolism (secondary compounds) that are poisonous, irritating or bad-‐tasting
• Ex. Strychnine from leaves of plants in genus Strychnos
• Strychnine kills people
Other examples
• Nicotine – toxic to insects, in tobacco leaves
• Poison oak/poison ivy – causes irritating itching rash on most people
Medicinal secondary • Morphine – pain-‐killer
• Atropine – used to treat glaucoma (eye disease)
• Codeine – used for respiratory infections
• Taxol – used in cancer treatments
• Quinine – used to treat malaria
Parasitism
• Parasitism à one individual is harmed while the other benefits
• Parasite feeds on the host
• Parasitism does not usually kill host right away
• Lives on host for long time
Kinds of parastites• Ectoparasites à external parasites
Endoparasites• Endoparasite à internal parasite
Evolution of parasites and
• Parasites can have strong negative effect on host
• Parasitism stimulates defensive evolution in hosts
• Skin – important defense that prevents most parasites from entering body
• Other parts of body vulnerable – eyes, mouth, nose – defended chemically by tears, saliva, mucus
• Parasites that still get in are attacked by immune system
Adaptations of parasites
• Parasites usually specialized for their host
• Ex. Tapeworms – do not have digestive system • Live in small intestine and absorb nutrients
directly through skin
Competition
• Competition à results from fundamental niche overlap
• Two or more species use the same limited resource
• Some plants release toxins into soil to prevent other plant species from living there
Research studies on
• Russian ecologist G. F. Gause was one of first to study competition in lab
• Used test tubes stocked with food supply of bacteria
• Raised species of paramecium separately and in different combinations
• Paramecium caudatum and Paramecium aurelia grew well when separated
• When two species combined, P. caudatum always died out
• P. aurelia was better predator of bacteria
• (see graph)
• Competitive exclusion à describes situations where one species is eliminated from a community because of competition for same limited resource
• One species uses resource more efficiently
• Has reproductive advantage that eventually eliminates competition
Joseph Connell’s study of
• Connell studied 2 species of barnacles
• Semibalanus balanoides and Chthamalus stellatus
• Live in intertidal zone – part of shore that is exposed during low tide
• Each species formed a separate band in intertidal zone
• Chthamalus lived higher on rocks than Semibalanus
• Demonstrated difference partly because of competition
• When rock covered with Chthamalus was transferred to lower zone, it was able to tolerate conditions there
• Eventually Semibalanus settled on rock and crowded out Chthamalus
• Connell concluded that competition limited the range of Chthamalus
• Even though Chthamalus could survive in lower area, competition prevented it
• Semibalanus could not tolerate long periods of drying when tide was out
• Restricted to lower zone
Competition and community
• Competition has important influence on a community
• Composition of a community can change through competitive exclusion
• Competitors can also evolve niche differences or structural differences that lower competition
• The differences evolved are greatest where ranges of competitors overlap
• This is called character displacement
Darwin’s finches
• Example of character displacement
• Finches eat seeds
• Birds with larger beaks can crack open and eat larger seeds
• Beak size shows diet
• Two species of finch are very similar
• On islands where only one of them is found, birds of two species have same size beak
• If they are both on one island their beak size is different
• Evolved character displacement that lets them feed on different sized seeds
• Reduces competition
• Competition is likely to be most intense between 2 closely related species that require the same resources
• When similar species live in same area they use only part of resources
• This is called resource partitioning
• Ex. 3 species of warblers that live in fir trees and feed on insects
• Warblers hunt for insects in different parts of trees
• Competition is reduced
Mutualism
• Mutualism à a cooperative relationship in which both species gain some benefit
• Some are so close that neither species can live without the other
• Ex. Ants and bulls horn acacia bush
• Ants nest inside thorns of bush and get food from plant
• Ants protect acacia from predators and cut any plants that shade the bush from the sun
Pollination
• One of the most important mutualistic relationships
• Animals pollinate flowering plants
• These are pollinators
• Animal feeds on flower, then takes pollen to another flower
Commensalism
• Interaction where one species benefits and the other is NOT affected
• Some cases may be mutualism but we don’t know what the other benefit is yet
• Ex. Cattle egrets and Cape buffalo in Tanzania
• Birds feed on small animals (insects or lizards scared out of bush by buffalo)
Properties of communities
Section 2
Species richness and species
• Species richness à number of species it contains
• Species diversity à relates number of species in community to relative large quantity of each species
• Two measures provide different information
• Species richness is just a count of the species in the community
• Each species contributes 1 count to total, whether there is 1 or 1 million individuals
• Species diversity suggests importance of species because it accounts for how common each species is in community
• To calculate, must measure/estimate population size of all species in community
Patterns of species richness
• Species richness changes with latitude (distance from equator)
• The closer a community is to the equator, the more species it will contain
• Species richness greater in tropical rain forests
• Cover 3% of planet but contain 50% of world’s species
• Ex. E.O. Wilson and Terry Erwin
• Identified nearly as many species of ants in 1 tree in Peru as in the entire British Isles
• Why are there more species in the tropics that there are in other places?
Hypothesis 1
• Temperate habitats are younger
• Formed from since last ice age
• Tropical habitats not affected by ice ages
• Climate is more stable in tropics
• Allowed species to specialize to greater extent than in temperate regions where climate changes more
Hypothesis 2
• Because plants can do photosynthesis year round in tropics there is more energy to support more organisms
• Probably the species richness is a combination of several things
The species-‐area effect
• Another pattern of species richness is that larger areas usually have more species than smaller areas
• This is the species-‐area effect
• Most often applied to islands
• Area clearly limited by geography
Ex. Caribbean
• In Caribbean, more species of reptiles and amphibians live on large islands (ex. Cuba) than on small islands (ex. Redonda)
• Because all of them are close together it cannot be because of latitude
• Larger areas usually contain more habitats so can support more species
• Species-‐area effect has 1 important consequence
• Reducing the size of a habitat reduces the number of species it can support
• Now natural habitats are reducing quickly under pressure from human population
• About 2% rainforest destroyed each year
• Result -‐ extinction
Species interactions and
• Interaction among species sometimes promotes species richness
• Several studies shown predators can prevent competitive exclusion from happening with their prey
1960 Robert Paine• Showed importance of
sea star, Pisaster, in keeping species of communities in Washington coast
• He removed all sea stars from one site and for years prevented more from settling there
• Dramatic shift in community
• Mussel Mytilus which coexisted with other species before now spread over habitat and crowded out other species
• Species richness of community fell from 15 to 8 during the study
• Mytilus was superior competitor for space on rocks
• Pisaster held population of Mytilus in check
Community stability
• One of the most important characteristics of a community is how it responds to disturbance
• Stability indicates its resistance to change
• Stability directly related to species richness
• The more species in a community, the more links between species
• The links would spread out effects of disturbance and prevent disruption of community
• Ex. Farm crops – one species of crop
• If outbreak of insect pests – whole crop can be ruined (community is disrupted)
David Tilman and John
• Investigated how the response to drought in small plot of grasses was affected by species richness
• Grew varying numbers of species in many small plots
• Each subjected to drought
• Plots with more species present lost smaller percentage of plant mass than plots with fewer species
• Plots with more species also took less time to recover
• Conclusion – species richness improves stability
successionSection 3
• Disturbances like fires, landslides, hurricanes, floods trigger a sequence of changes in the composition of a community
• Certain species do well immediately after disturbance
• Then replaced by other species
• Over time, composition of community changes
Successional changes in
• Succession à the gradual, sequential regrowth of species in an area
• You can see early stages of it in abandoned fields, empty lots, along roads, sidewalks
• Where weeds are pushing up through cracks in concrete
• Ecologists recognize 2 types of succession
• Primary succession à development of a community where there was no life before
• Ex. Bare rock, sand dune, volcanic eruption
• Secondary succession à sequential replacement of species that follows disruption of an existing community
• Ex. Natural disaster, human activity
• Any new habitat is an invitation to many species that are adapted to be good pioneers
• Pioneer species à species that dominate in early succession
• Usually small, fast-‐growing and fast-‐reproducing
• Well adapted for invading and occupying disturbed habitat
• Pioneer species good at spreading out their seeds
Primary succession• Often goes very slowly
because minerals for plant growth are unavailable
• Ex. When glaciers retreated from eastern Canada about 12,000 years ago
• Left large area of deserted rock
• All soil scraped away
• This place is called the Canadian Shield
• Place where plants and most animals couldn’t live
• Freezing and thawing over and over broke rocks up
• Lichens – mutualistic relationship between fungi and algae – colonized barren rock
• Acids in lichens and mild acid rain washed nutrients from rock
• Eventually dead organic matter from lichens with minerals began to form soil
• A few grasslike plants could grow
Canadian shield today
Secondary succession
• Happens where an existing community was cleared by a disturbance
• Soil left undamaged
• Usually takes about 100 years for original ecosystem to return
• Starts with annual grasses then perennial grasses and shrubs
The complexity of
• Traditional description is that the community goes through predictable series of stages until it reaches stable end point
• Climax community
• Organisms in each stage change environment in ways that make it less favorable for their survival but better for organisms that will succeed them
• Each stage paves the road for the next
• Climax community is constant for long period of time
• Not always that simple
• Some “climax communities” are not stable and continue to change
• Actual path of succession may depend on several things
• Identities of species there
• Order that they arrive
• Climate
• Etc.
