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