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Chapter 4: Species Interactions and Community Ecology

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1 Chapter 4: Species Interactions and Community Ecology

2 Central Case Study: Black and White and Spread All Over
In 1988, discharged ship ballast water accidentally released zebra mussels into Lake St. Clair By 2010, they had spread to 30 states No natural predators, competitors, or parasites They cause millions of dollars of property damage each year 2

3 Species interactions Species interactions are the backbone of communities Effects of species interactions on the participants: Type of interaction Effect on Species 1 Effect on Species 2 Competition Predation, parasitism, herbivory + Mutualism “+”: positive effect “–”: negative effect

4 Competition occurs with limited resources
Competition: multiple organisms seek the same limited resource Food, water, space, shelter, mates, sunlight, etc. Intraspecific competition: between members of the same species High population density: increased competition Interspecific competition: between members of different species Strongly affects community composition Leads to competitive exclusion or species coexistence

5 Results of interspecific competition
Competition is usually subtle and indirect One species may exclude another from using the resource Zebra mussels displaced native mussels in the Great Lakes Quagga mussels are now displacing zebra mussels Or, competing species may be able to coexist Natural selection favors individuals that use different resources or shared resources in different ways

6 Resource partitioning
Resource partitioning: competing species coexist by specializing By using different resources (small vs. large seeds) Or using shared resources differently (active during day vs. night)

7 An exploitative interaction: predation
Exploitation: one member benefits while the other is harmed (+/- interactions) Predation, parasitism, herbivory Predation: process by which individuals of one species (predators) capture, kill, and consume individuals of another species (prey)

8 Predation affects the community
Interactions between predators and prey structure food webs The number of predators and prey influences community composition Predators can, themselves, become prey Zebra mussels eat smaller types of zooplankton Zebra mussels are prey for North American predators (fish, ducks, muskrats, crayfish)

9 Predation can drive population dynamics
Increased prey populations increase food for predators Predators survive and reproduce Increased predator populations decrease prey Predators starve and their populations decrease Decreased predator populations increase prey populations Insert Fig. 4.4

10 Predation has evolutionary ramifications
Natural selection leads to evolution of adaptations that make predators better hunters Individuals who are better at catching prey: Live longer, healthier lives Take better care of offspring Prey face strong selection pressures—they are at risk of immediate death Prey develop elaborate defenses against being eaten

11 Prey develop defenses against being eaten

12 An exploitative interaction: parasitism
Parasitism: a relationship in which one organism (parasite) depends on another (host) For nourishment or some other benefit The parasite harms, but doesn’t kill, the host Some parasites contact hosts infrequently Cuckoos, cowbirds Some live within the host Disease, tapeworms Some live on the hosts’ exterior Ticks, sea lampreys 12 12

13 Parasite – host relationships
Parasitoids: insects that parasitize other insects Kill the host Example: wasp larvae burrow into, and kill, caterpillars Coevolution: hosts and parasites become locked in a duel of escalating adaptations Has been called an evolutionary arms race Each evolves new responses to the other It may not be beneficial to the parasite to kill its host 13 13

14 An exploitative interaction: herbivory
Herbivory: animals feed on the tissues of plants Widely seen in insects May not kill the plant But affects its growth and reproduction Defenses against herbivory include: Chemicals: toxic or distasteful Thorns, spines, or irritating hairs Herbivores may overcome these defenses 14 14

15 Mutualists help one another
Two or more species benefit from their interactions Each partner provides a service the other needs (food, protection, housing, etc.) Symbiosis: a relationship in which the organisms live in close physical contact (mutualism and parasitism) Microbes within digestive tracts Mycorrhizae: plant roots and fungi Coral and algae (zooxanthellae) Pollination: bees, bats, birds, and others transfer pollen from one flower to another, fertilizing its eggs 15 15

16 Pollination In exchange for the plant nectar, the animals pollinate plants, which allows them to reproduce 16 16

17 Ecological communities
Community: an assemblage of populations of organisms living in the same area at the same time Members interact with each other Interactions determine the structure, function, and species composition of the community Community ecologists are interested in how: Species coexist and interact with one another Communities change, and why these patterns exist 17 17

18 Energy passes among trophic levels
One of the most important species interactions Who eats whom? Matter and energy move through the community Trophic levels: rank in the feeding hierarchy Producers (autotrophs) Consumers Detritivores and decomposers 18 18

19 Producers: the first trophic level
Producers, or autotrophs (“self-feeders”): organisms capture solar energy for photosynthesis to produce sugars Green plants Cyanobacteria Algae They capture solar energy and use photosynthesis to produce sugars 19 19

20 Consumers: consume producers
Primary consumers: second trophic level Organisms that consume producers Herbivorous grazing animals Deer, grasshoppers Secondary consumers: third trophic level Organisms that prey on primary consumers Wolves, rodents, birds Tertiary consumers: fourth trophic level Predators Hawks, owls 20 20

21 Detritivores and decomposers
Organisms that consume nonliving organic matter Detritivores: scavenge waste products or dead bodies Millipedes, soil insects Decomposers: break down leaf litter and other nonliving material Fungi, bacteria Enhance topsoil and recycle nutrients 21 21

22 Energy, biomass, and numbers
Most energy that organisms use in cellular respiration is lost as waste heat Less and less energy is available in each successive trophic level Each trophic level contains only 10% of the energy of the trophic level below it There are also far fewer organisms and less biomass (mass of living matter) at the higher trophic levels A human vegetarian uses less energy and has a smaller ecological footprint than a meat eater 22 22

23 Pyramids of energy, biomass, and numbers

24 Food webs show relationships and energy flow
Food chain: a series of feeding relationships Food web: a visual map of feeding relationships and energy flow among organisms Food webs are greatly simplified and leave out most species 24 24

25 Some organisms play big roles
Keystone species: has a strong or wide-reaching impact Far out of proportion to its abundance Removing a keystone species has substantial ripple effects Alters the food web Large-bodied secondary or tertiary consumers

26 Species can change communities
Trophic cascade: predators at high trophic levels indirectly promote populations at low trophic levels By keeping species at intermediate trophic levels in check Extermination of wolves led to increased deer populations … Which overgrazed vegetation … Which changed forest structure Ecosystem engineers: physically modify the environment Beaver dams, prairie dogs, ants

27 Communities respond to disturbances
Communities experience many types of disturbance Removal of keystone species, natural disturbances (fires, floods, etc.) Human impacts cause major community changes Resistance: a community resists change and remains stable despite the disturbance Resilience: a community changes in response to a disturbance, but later returns to its original state Or, a disturbed community may never return to its original state

28 Primary succession Succession: the predictable series of changes in a community After a severe disturbance Primary succession: disturbance removes all vegetation and/or soil life Glaciers, drying lakes, volcanic lava covering the land Pioneer species: the first species to arrive in a primary succession area Lichens: fungi + algae

29 Secondary succession Secondary succession: a disturbance has removed much, but not all, of the biotic community Fires, hurricanes, logging, farming Aquatic systems can also undergo succession Ponds eventually fill in to become terrestrial systems Climax community: remains in place with few changes Until another disturbance restarts succession

30 Communities may undergo shifts
Community changes are more variable and less predictable than early models of succession suggested Conditions at one stage may promote another stage Competition may inhibit progression to another stage Chance factors also affect changes Phase (regime) shift: the overall character of the community fundamentally changes Some crucial threshold is passed, a keystone species is lost, or an exotic species invades Example: overfishing and depletion of fish and turtles has allowed algae to dominate coral reef communities

31 Invasive species threaten stability
Alien (exotic) species: non-native species from somewhere else enters a new community Invasive species: non-native species that spreads widely and become dominant in a community Introduced deliberately or accidentally Growth-limiting factors (predators, disease, competitors, etc.) are absent Major ecological effects Pigs, goats, and rats have destroyed island species But some invasive species (e.g., honeybees) help people

32 Invasive mussels modify communities



35 Controlling invasive species
Techniques to control invasive species include: Removing them manually Applying toxic chemicals Drying them out, depriving them of oxygen Introducing predators or diseases Stressing them with heat, sound, electricity, carbon dioxide, or ultraviolet light Control and eradication are hard and expensive Prevention, rather than control, is the best policy

36 Altered communities can be restored
Humans have dramatically changed ecological systems Severely degraded systems cease to function Restoration ecology: the science of restoring an area to an earlier (presettlement) condition Tries to restore the system’s functionality (e.g., filtering of water by a wetland) Ecological restoration: actual efforts to restore an area Difficult, time-consuming, and expensive It is best to protect natural systems from degradation in the first place

37 Examples of restoration efforts
Prairie restoration: replanting native species, controlling invasive species, controlled fire to mimic natural fires The world’s largest project: Florida Everglades Flood control and irrigation removed its water Populations of wading birds dropped 90–95% It will take 30 years and billions of dollars to restore natural water flow

38 Widely separated regions share similarities
Biome: major regional complex of similar communities recognized by: Plant type Vegetation structure There are about 10 terrestrial biomes


40 Abiotic factors influence biome locations
The type of biome depends on temperature, precipitation Also air and ocean circulation, soil type Climatographs: a climate diagram showing an area’s mean monthly temperature and precipitation Similar biomes occupy similar latitudes


42 Aquatic systems have biome-like patterns
Various aquatic systems comprise distinct communities Coastlines, continental shelves, open ocean, deep sea Coral reefs, kelp forests Some coastal systems (estuaries, marshes, etc.) have both aquatic and terrestrial components Aquatic systems are shaped by Water temperature, salinity, dissolved nutrients Wave action, currents, depth, light levels Substrate type Animals, not plants, delineate marine communities

43 Temperate deciduous forest
Deciduous trees lose their broad leaves each fall They remain dormant during winter Midlatitude forests in Europe, east China, eastern North America Even, year-round precipitation Fertile soils Forests: oak, beech, maple

44 Temperate grasslands More temperature difference
Between winter and summer Less precipitation supports grasses, not trees Also called steppe or prairie Once widespread, but has been converted to agriculture Bison, prairie dogs, ground-nesting birds, pronghorn

45 Temperate rainforest U.S. coastal Pacific Northwest Heavy rainfall
Coniferous trees: cedar, spruce, hemlock, fir Moisture-loving animals Banana slug (UCSC mascot) Erosion and landslides affect the fertile soil Most old-growth is gone as a result of logging

46 Tropical rainforest Southeast Asia, west Africa Central and South America Year-round rain and warm temperatures Dark and damp Lush vegetation Diverse species But in low densities Very poor, acidic soils Nutrients are in the plants

47 Tropical dry forest Also called tropical deciduous forest
Plants drop leaves during the dry season India, Africa, South America, north Australia Wet and dry seasons Warm, but less rainfall Converted to agriculture Severe soil erosion

48 Savanna Tropical grassland interspersed with trees
Africa, South America, Australia, India Precipitation occurs only during the rainy season Animals gather near water holes Zebras, gazelles, giraffes, lions, hyenas

49 Desert Minimal precipitation Sahara: bare, with sand dunes
Sonoran: heavily vegetated Temperatures vary widely Day vs. night, seasonally Soils (lithosols): high mineral content, low organic matter Animals: nocturnal, nomadic Plants: thick skins, spines

50 Tundra Russia, Canada, Scandinavia Minimal rain, very cold winters
Permafrost: permanently frozen soil Residents: polar bears, musk oxen Migratory birds, caribou Lichens, low vegetation, no trees Alpine tundra: on mountaintops

51 Boreal forest (taiga) Canada, Alaska, Russia, Scandinavia
A few evergreen tree species Cool and dry climate Long, cold winters Short, cool summers Nutrient poor, acidic soil Moose, wolves, bears, lynx, migratory birds

52 Chaparral Occurs in small patches around the globe
Mediterranean Sea, Chile, California, south Australia Densely thicketed, evergreen shrubs Highly seasonal biome Mild, wet winters Warm, dry summers Fire-resistant plants

53 Conclusion Species interactions affect communities
Competition, predation, parasitism, competition, mutualism Causing weak and strong, direct and indirect effects Feeding relationships are represented by trophic levels and food webs Humans have altered many communities Partly by introducing non-native species Ecological restoration attempts to undo the negative changes that we have caused 53

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