III. Ecology
Key focus of this chapter: population and communities.
This chapter reviews populations, communities, ecosystems, and the biosphere.
A. Hierarchical levels of organization
Hierarchical levels | Main features |
Organism | • Individual living thing |
Species | • Under the biological species concept, organisms that interbreed to produce viable, fertile offspring • Reproductively isolated from other such groups |
Population | • Individuals of the same species living in an area |
Community | • Interacting populations of different species sharing an area |
Ecosystem | • Biotic factors and abiotic factors |
Biosphere | • Sum of all global ecosystems • Earth and atmosphere |
B. Species
1. Historical views of evolution
a. Aristotle (384 – 322 B.C.)
- Scale of nature – life forms ranked on a ladder by complexity from simple to complex.
- He viewed species as fixed.
b. Carolus Linnaeus (1707 – 1778)
- Binomial nomenclature system of taxonomy.
- He initially interpreted similarities as a pattern of creation, rather than common ancestry.
c. Georges Cuvier (1769 – 1832)
- Paleontology – the study of fossils.
- Catastrophism – sudden catastrophic events caused extinctions recorded in fossil layers.
d. James Hutton (1726 – 1797)
- Gradualism – large changes accumulate through slow, continuous processes.
e. Charles Lyell (1797 – 1875)
- Uniformitarianism – geological processes operating today also operated in the past.
f. Jean Lamarck (1744 – 1829)
- Proposed inheritance of acquired characteristics (not the general mechanism of adaptive evolution).
- A part of the body in frequent use becomes larger and stronger, while a part in disuse gradually deteriorates.
g. Charles Darwin (1809 – 1882)
- Voyage of HMS Beagle, including the Galápagos Islands.
– observations helped him develop the theory of evolution by natural selection.
- Origin of species
– descent with modification from common ancestors.
– species evolve through natural selection.
- Natural selection
– heritable traits that increase reproductive success become more common over generations.
- Artificial selection
– divergent results of intentional breeding by human selection.
2. Evolution and structure
- Evolution: a change of the genetic composition in the population over time.
- Divergent and convergent evolution.
3. Geographic separation
- Allopatric speciation
– new species from population by geographic isolation
- Sympatric speciation
– new species from population without geographic isolation
– new species share geographic region
C. Population
1. Sources of genetic variation
a. Mutation – creates new alleles
b. Sexual recombination – creates new combinations of existing alleles
2. Alteration of genetic composition
a. Genetic drift
: random changes in allele frequencies; effects are strongest in small populations.
- Bottleneck effect
– sudden disasters reduce the size of population.
- Founder effect
– a small founding group establishes a population with allele frequencies that may differ from the source population.
b. Gene flow
- Movement of alleles between populations through migrating individuals or gametes
c. Natural selection
: main source of adaptive evolution
- Geographic variation
– a cline is a gradual change in traits or allele frequencies along a geographic gradient; phenotype alone does not prove genetic differences.
- Patterns of selection
– stabilizing selection: shifting the overall population to favored middle values and removing extreme values.
– disruptive selection: shifting the overall population possessing favored values to both ends of the distribution.
– directional selection: shifting the overall population possessing favored values to one extreme end of the distribution.
3. Population dynamic
a. Exponential growth
- Population size increases exponentially.
- Birth rate exceeds death rate when immigration and emigration are ignored.
b. Logistic Growth
- Sigmoid curve or S-curve.
- Growth slows near carrying capacity (K); net growth is zero at K in the ideal model.
- Carrying capacity: Maximum population size that environment can sustain.
- Life histories: r/K selection summarizes traditional tendencies, not rigid categories.
Life histories | Features |
K-selection | • Traits favored by competition near carrying capacity (K) • Stable environments • Low reproductive rate • Slow growth • Small number of offspring • Roughly constant in size • Long life span • Eg/ Human |
r-selection | • Traits favoring a high intrinsic population growth rate (r) • Unstable environments • High reproductive rate • Rapid growth • Numerous offspring • Short life span • Eg/ Bacteria |
D. Communities
1. Niche and habitat
- Niche: relational position or role of population in an ecosystem.
- Habitat: environmental areas where organisms live.
2. Species community
Interaction | Classification | Main characteristics |
- / - | Competition | • Two organisms in the same environment compete to get the same resource • Both species negatively affected |
+/- | Herbivory | • Herbivore is positively affected and plant or alga is negatively affected |
Parasitism | • Parasite is positively affected and host is negatively affected • Eg/ Tapeworm living in the digestive tract and human, fungal agent (athlete’s foot) and human | |
Predation | • Predator is positively affected and prey is negatively affected | |
+ / 0 | Commensalism | • One species is positively affected and another species is not affected • Eg/ Cattle egrets and water buffalo, remora and shark |
+ / + | Mutualism | • Both species are positively affected • Some endosymbioses are mutualistic • Eg/ Tick bird and rhinoceros, acacia tree and ants, lichens (fungus & algae), nitrogen-fixing bacteria living in the root nodules of legumes |
3. Defensive adaptation
: adaptations that reduce predation.
Defensive adaptation | Classification | Features |
Coloration | Aposematic coloration | • Warning coloration to predator (Chemical defenses) • Eg/ Poison arrow frog |
Cryptic coloration | • Camouflage (Morphological defense) | |
Mimicry | Batesian mimicry | • Palatable species mimics an unpalatable species • Harmless species mimics a harmful species |
Mullerian mimicry | • Two or more unpalatable species are similar to each other and have aposematic signals. |
4. Innate and learned behaviors
a. Fixed action pattern (FAP)
- An innate behavioral sequence triggered by a sign stimulus; once initiated, it typically runs to completion.
- Eg/ Species-specific courtship displays.
b. Imprinting
- Learning during a sensitive period.
- Innate predispositions interact with learning.
- Young animals form lasting associations that influence later behavior.
- Eg/ Young geese following their mother, flying imprinted cranes with an ultralight plane.
c. Habituation
- Reduced response to a repeated, harmless stimulus.
- Eg/ Baby is no longer startled by sound of a car horn after repeated exposure.
d. Spatial learning
- Learning locations and spatial relationships in the environment.
e. Associative learning
: learning associations between stimuli or between a behavior and its consequences.
Associative learning | Summary |
Classical conditioning | • Learning by pairing a neutral stimulus with an unconditioned stimulus • After pairing, the conditioned stimulus elicits a conditioned response • Eg/ Salivating mouth of a dog after ringing of a bell |
Operant conditioning | • Learning from voluntary behaviors with reward or punishment • Behavior → consequences → altered probability of behavior • Trial and error learning • Eg/ A rat presses a lever more often when presses yield food pellets |
f. Extinction
- A conditioned response weakens when the conditioned stimulus is repeatedly presented without the unconditioned stimulus; operant responses decline without reinforcement.
5. Ecological succession
: community composition changes over time, often after disturbance
a. Primary succession
- Succession with the lifeless area where soil is not formed
- New surfaces exposed by lava flows or retreating glaciers
- Pioneer species with algae, lichen, mosses, and fungus
- Communities
Communities | Features |
Pioneering community | • Early colonizing organisms; examples depend on the substrate • Eg/ Lichens and mosses on bare rock; herbs on disturbed soil |
Climax community | • Late successional community • Relatively stable under prevailing conditions; disturbance can restart succession • Eg/ Shade trees |
b. Secondary succession
- Succession with the area where soil is already formed.
- Disturbed area formed by forest fire.
E. Ecosystem
1. Biotic and Abiotic factors
- Biotic factors: all living organisms in ecosystem
- Abiotic factors
– nonliving physical and chemical factors.
– eg/ water, temperature, sunlight, wind, nutrients.
2. Energy flow in ecosystem
a. Food chain
- Persistent pollutants that accumulate in tissues can biomagnify at higher trophic levels.
Trophic structures | Features and examples |
Producer | • Plants • Green plants, algae, phytoplankton |
Herbivores | • Primary consumers • Animals that eat producers, including herbivorous zooplankton |
Primary carnivores | • Secondary consumers • Animals that eat herbivores and zooplankton |
Secondary carnivores | • Tertiary consumers • Animals that eat primary carnivores |
Tertiary carnivores | • Quaternary consumers • Animals that eat secondary carnivores |
Decomposers | • Organisms that break down nonliving organic materials • Eg/ Bacteria, fungi |
Detritivores | • Organisms that eat nonliving organic materials • Eg/ worms, arthropods |
3. Nutrient cycles
: pathway of biogeochemical elements through both abiotic and biotic factors.
a. Water cycle
- Essential to all living organisms.
- Evaporation of water by solar energy → condensation of water vapor into clouds → precipitation
b. Phosphorus cycles
- Components of ATP, nucleic acids, and phospholipids in living organisms.
- Plants absorb phosphate from soil → consumers → decomposers return phosphate
c. Carbon cycles
- Essential to all living organisms as organic structures.
- Photosynthesis fixes CO2; respiration and decomposition return CO2 to the environment
d. Nitrogen cycles
- Components of amino acids, nucleic acids in living organisms.
- Nitrogen gas (N2) → ammonia (NH3) → ammonium (NH4+) → nitrites (NO2–) → nitrate (NO3–)
Bacteria for nitrogen cycles | Functions |
Nitrogen fixing bacteria | • Reduction of atmospheric nitrogen • Fixing nitrogen gas (N2) to ammonia (NH3) |
Ammonification | • Conversion of organic nitrogen to ammonia (NH3) or ammonium (NH4+) |
Nitrifying bacteria | • Oxidation of ammonium to nitrite and nitrate • Converting ammonium (NH4+) to nitrites (NO2−) and nitrites (NO2−) to nitrate (NO3−) |
Denitrifying bacteria | • Reduction of nitrate to gaseous nitrogen • Changing nitrate (NO3−) to nitrogen gas (N2) |
- Nitrogen cycles
4. Temperature relation
a. Climate
- Microclimate
– climate of local zone surrounded by community of organisms.
– important to individuals of a species.
- Macroclimate
– global or broad regional climate.
– important to distribution patterns of species.
b. Thermoregulation
- Homeothermic animals
– keeping constant body temperature.
– eg/ Most mammals and birds; endothermy (internal heat production) is a distinct concept
- Poikilothermic animals
– changing body temperature with environmental temperatures.
– eg/ Many reptiles and amphibians; ectothermy refers to reliance on external heat
5. Water and salt regulation
a. Freshwater bony fish
- In hypotonic freshwater, they excrete abundant dilute urine and actively take up salts through the gills.
b. Marine bony fish
- In hypertonic seawater, they drink seawater, excrete excess salts through the gills and kidneys, and produce relatively little urine.
F. Biosphere
1. Water structure
a. Lake zones
Lake zones | Features |
Littoral zone | • Shoreline areas |
Limnetic zone | • Open surface area away from littoral zone |
Photic zone | • Area exposed to sunlight for photosynthesis • Euphotic zone: enough light for net photosynthesis; depth depends on water clarity |
Aphotic zone | • Insufficient light for net photosynthesis |
Benthic zone | • Lake bottom and its sediment |
b. Marine zones
Marine Zones | Features | |
Littoral zone | Intertidal zone | • Exposed air and submerged area • Many different types of organisms • Important area for study of ecology |
Neritic zone | • Water over the continental shelf; its edge is often near 200 m depth • Extends seaward from the low-tide mark to the shelf edge • Often well-lit, nutrient-rich waters; coastal salinity can vary | |
Photic zone | Epipelagic zone | • Surface – 200 m depth • Most photosynthesis takes place |
Aphotic zone | Mesopelagic zone | • 200-1000m depth |
Bathypelagic zone | • 1000-4000m depth | |
Abyssal zone | • 4000-6000m depth | |
Hadal zone | • Deepest area of marine environment • Below 6000m depth | |
2. Water relations
a. Rain shadow
- When moist air passes the front side of the mountains, it becomes cool at high altitudes, and precipitation occurs by decreasing moisture-holding capacity.
- Descending air warms on the leeward side, lowering relative humidity and producing a dry rain shadow.
- Eg/ Death Valley (Sierra Nevada)
b. Seasonal turnover
: spring and fall mixing in many temperate lakes redistributes oxygen and nutrients.
- Breaking down thermal stratification of summer and winter seasons.
- Spring turnover
– surface water warming toward 4 °C becomes denser; reduced density differences allow wind-driven mixing.
- Fall turnover
– surface water cooling toward 4 °C becomes denser; wind helps mix the lake
3. Terrestrial biomes
Terrestrial biomes | Main features |
Tropical rain forest | • Very high terrestrial species diversity • Around region of equator |
Savanna | • Tropical grassland with distinct wet and dry seasons • Transition between tropical forest and desert in the interiors of continents |
Desert | • Dry places with low precipitation • Water conservation for plants and animals • Succulent plants of shrubs and cacti |
Chaparral | • High temperature and dry summer and mild temperatures and rainy winter. • Evergreen plants with spiny shrubs and small trees |
Temperate grassland | • Seasonal climates, often with hot summers, cold winters, and periodic drought • Forb and grass plants • Many farmlands |
Temperate deciduous forest | • Hardwood and broad-leaved deciduous forests • Summer rain and winter snow |
Coniferous forest (Taiga) | • Coniferous tree with needle like leaves • Rain and snow; seasonal precipitation varies by region • Long and cold winter • Many large mammals |
Tundra | • Found in far north with low precipitation • Small shrubs or vegetation • Long and cold winter and short and cool summer • Permafrost |
Polar ice | • North and south pole • Almost no precipitation • Limited life of organisms |