Notes on The Living World: Ecosystems (Modules 1–3)
1-1 How do we define ecosystem boundaries?
Ecosystems defined by a combination of living (biotic) and nonliving (abiotic) components. Abiotic examples: temperature range, humidity, rainfall, nutrients. Biotic examples: plants, animals, fungi, bacteria.
Boundaries can be well-defined (e.g., a cave) or subjective/administrative (e.g., management boundaries of a national park).
Climate is a key determinant of ecosystem characteristics; extreme contrasts: Death Valley (hot, 50°C / 120°F) vs Antarctica (cold, down to -85°C / -120°F).
Water availability can define an ecosystem (deserts vs lakes/ocean bodies).
Boundaries help identify biotic/abiotic components and trace energy and matter cycling.
The biosphere is the global system of all ecosystems, a ~20 km (12-mile) thick layer encircling Earth where life resides.
Symbiosis: two species living in a close and long-term association within an ecosystem.
Example: caves often have clear boundaries from stream entry to stream exit; flowering riverine interactions across boundaries can connect ecosystems.
Figure references (Fig. 1.1, Fig. 1.2a–b) illustrate well-defined (cave) vs broader, border-transcending ecosystems (Greater Yellowstone Ecosystem).
Not all ecosystems have fixed boundaries; energy and matter cross boundaries (e.g., bats moving between cave interior and exterior; carbon and nitrogen cycling across systems).
Important terms:
Biosphere: global sum of all ecosystems.
Ecosystem boundary: delimitation of a given ecosystem’s extent, often influenced by climate, geology, and human criteria.
1-2 How do competing species respond to limited resources?
Competition for limited resources is a core interaction among species within ecosystems.
Competitive exclusion principle: two species competing for the same limiting resource cannot coexist under the same environmental conditions.
Gause’s Paramecium experiments demonstrated this: when P. caudatum and P. aurelia were grown separately, both thrived; when grown together, the better competitor outcompeted the other, driving it to extinction.
Concept of realized niche vs fundamental niche: when two species share a resource, competition can constrain realized niches, leading to exclusion or niche differentiation.
Examples of competition in plants:
Goldenrods outcompete other wildflowers in old-field New England by taller growth and more light capture.
Wild oat (Arena fatua) outcompetes crops on the Great Plains by earlier seed maturation.
Resource partitioning: evolution of reduced overlap in resource use to minimize competition; can be temporal, spatial, or morphological.
Example (temporal): wolves and coyotes active at different times of day to reduce overlap in hunting similar prey.
Example (spatial): desert plants with different root depths to access different soil moisture zones.
Example (morphological): Darwin’s finches with beaks adapted to different seed sizes; over generations, overlap declines and beaks specialize on large vs small seeds.
Result: competition can drive diversification and niche partitioning, reducing direct competition over time.
1-3 Which interactions involve one species consuming another?
Predation: one organism kills and consumes another (the prey).
Parasitoids: a specialized predator that lays eggs inside a host; larvae consume the host from the inside, often leading to host death (e.g., some wasps and flies).
Parasitism: one organism lives on or in another (the host), typically benefiting the parasite and harming the host.
Pathogens: diseases-causing parasites (viruses, bacteria, fungi, protists, helminths).
Herbivory: animals consuming plants or algae.
Predators can regulate prey populations (e.g., red foxes, wolves, big cats).
Predator-prey dynamics can cause cascading ecological effects (e.g., Isle Royale moose–wolf dynamics; fox–hare–grouse dynamics).
Examples of prey defenses against predation:
Behavioral: hiding, reduced movement, vigilance.
Morphological: camouflage (e.g., satanic leaf-tailed gecko); spines (porcupines, stingrays, pufferfish).
Chemical: toxins or distasteful compounds (poison dart frog skin toxins).
Mimicry: non-toxic species mimicking toxic prey to deter predators.
1-4 Which species interactions cause neutral or positive effects on both species?
Mutualism: both species benefit and experience increased survival or reproduction; can involve behavioral and ecological interdependencies.
Example: Acacia trees and certain ant species (Pseudomyrmex) where ants receive nectar/nectaries while trees gain protection from herbivores and competitors; ants patrol and defend the tree, while nectarants feed on specialized nectaries.
Coral reefs: coral provides habitat and substances, algae provide sugars via photosynthesis; mutualistic partnership critical to reef ecosystems (algae receive CO2 and nutrients; coral obtains sugars).
Lichens: symbiosis between fungi and algae/cyanobacteria; fungi provide nutrients; algae provide carbohydrates via photosynthesis.
Commensalism: one species benefits; the other is unaffected.
Example: tree branches serve as perch sites for birds; birds gain food search and nesting sites, trees are not harmed or helped by birds.
Coral reefs as shelter for fish species; fish benefit from shelter, coral is not significantly affected.
Invasive species can disrupt these interactions by introducing novel interactions with no shared evolutionary history, often with negative outcomes for natives.
1-5 How do invasive species represent novel interactions?
Native vs exotic (alien) species definitions:
Native species: living within historical range, often co-evolved with locals.
Exotic (alien) species: introduced outside historical range.
Invasive species: exotic species that spread rapidly and cause harm to native biota (often due to lack of natural enemies in the new region).
Mechanisms: invasive species may lack coevolved predators/pathogens, allowing populations to explode and disrupt local communities.
Examples in North America: rats; zebra mussels (Dreissena polymorpha); kudzu vine (Pueraria lobata).
Pathways: intentional introductions (e.g., honeybees Apis mellifera introduced for honey; red foxes introduced for hunting) and accidental introductions (rats on ships).
Consequences: altered competitive dynamics, predation pressure, disease dynamics, habitat alteration.
Summary: invasive species illustrate how novel interactions can destabilize ecosystems due to missing historical ecology.
Module 2: Terrestrial Biomes
2-1 How do we define terrestrial biomes?
Terrestrial biomes are defined by the dominant plant growth forms in a region and the climate (annual patterns of temperature and precipitation).
Climate is summarized as average annual temperature and precipitation (not weather).
Growth forms reflect adaptations to climate; e.g., cacti-like plants in deserts, evergreen vs deciduous trees in various biomes.
Nine terrestrial biomes are organized around latitudinal and altitudinal patterns, each with characteristic climate, plants, and animals.
Distinctions:
Habitat: a specific area where a species lives within a biome; habitat is a subset of a biome.
Biome: large geographic region with a characteristic climate and growth forms.
The boundary between biomes can shift with climate change; historical shifts occurred with ice ages; pollen cores from lake sediments reveal past biome distributions.
Example: Greater Yellowstone Ecosystem extends beyond Yellowstone National Park boundaries to include ~20 million ha of land outside the park due to species’ ranges.
Note: Some biomes are particularly large (e.g., Greater Yellowstone) while others are small (e.g., rain-filled tree holes).
2-2 How can climate diagrams describe biome information?
Climate diagrams plot monthly average temperature and monthly precipitation to visualize patterns across a year.
Growing season: months when temperature is above 0°C.
Temperature axis and precipitation axis provide a visual sense of constraints on plant growth.
Rule of thumb from the diagram: for every 10°C increase in temperature, plants require ~20 mm of precipitation to grow, illustrating whether growth is constrained by temperature or precipitation. This can be expressed as:
ext{P}_{ ext{required}}( ext{Δ}T) \,=\ 2 \times \Delta T \,\text{mm}.Climate diagrams help explain how humans use biomes (e.g., warm, wet regions for crops; warm and drier regions for grains and grazing; colder regions for forests).
Important definitions:
Climate: long-term average weather patterns in a region (decades).
Weather: short-term atmospheric conditions.
2-3 What are the nine terrestrial biomes?
Tundra
Very cold, treeless; low-growing vegetation; permafrost; waterlogged, shallow soils; slow decomposition; nutrients limited; short growing season (~4 months).
Common mammals: muskox, Arctic fox, polar bear.
Alpine tundra exists at high elevations elsewhere.
Main threat: warming climate increasing permafrost thaw and greenhouse gas release.
Taiga (Boreal forest)
Dominated by coniferous evergreen trees (pine, spruce, fir).
Found roughly 50°–60° N (Europe, Russia, North America).
Cold, long winters; short cool summers; slow decomposition; nutrient-poor soils with thick organic layer.
Common animals: beaver, brown bear, wolverine.
Industries: logging; mining; oil/gas extraction.
Temperate Rainforest
Coastal biome with moderate temperatures and high precipitation.
Locations: west coast of North America (CA to AK), southern Chile, parts of Australia, New Zealand.
Very large trees (e.g., Sequoia sempervirens); can be hundreds to thousands of years old.
Soils leached, nutrients low; epiphytes and lichens common; large coniferous trees.
Common animals: black-tailed deer, Pacific giant salamander, Pacific treefrog.
Human impact: extensive logging; conversion to single-species plantations.
Temperate Seasonal Forest (Temperate Deciduous Forest)
Warm summers, cold winters; >1 m (39 inches) precipitation/year.
Dominated by broadleaf deciduous trees (beech, maple, oak, hickory); some conifers.
High soil fertility and longer growing season; high productivity.
Common animals: white-tailed deer, red fox, gray squirrel.
Historical conversion to agriculture; some regrowth since.
Shrubland (Woodland/Chaparral)
Hot, dry summers; mild, rainy winters; 12-month growing season.
Fire-adapted flora; plants resprout after fires; seeds may require fire to germinate (serotinous seeds).
Typical plants: drought-tolerant shrubs (yucca, sagebrush, scrub oak).
Common animals: California quail, black-tailed jackrabbits, coyotes, rattlesnakes.
Temperate Grassland (Cold Desert / Prairies)
Cold winters, hot, dry summers; grasses predominate; fire and grazing shape communities.
Regions: Great Plains (NA), pampas (South America), steppes (Eurasia).
Tallgrass prairies have higher rainfall and deep soils; shortgrass prairies are drier.
High productivity in temperate grasslands due to a long growing season and nutrients; most tallgrass prairie converted to agriculture; remaining shortgrass prairies used for grazing and grains.
Common animals: bison, praire chickens, prairie dogs, snakes.
Hot Desert (Subtropical Desert)
Located near 30° N and 30° S; hot temperatures, extremely dry conditions, sparse vegetation.
Plant adaptations: small or modified leaves (spines), thick outer layers to reduce water loss; CAM photosynthesis common in succulents.
Common animals: tortoises, camels, roadrunners.
Savanna (Tropical Seasonal Forest)
Warm temperatures; distinct wet and dry seasons.
Trees drop leaves during dry season; grasses dominate open landscapes.
Regions: Central America, Atlantic coast of South America, southern Africa, Australia, sub-Saharan Africa.
Grazing and fire help maintain openness; commonly acacia and baobab in many savannas.
Fauna: large grazers (gazelles, zebras); large predators (lions, cheetahs).
Tropical Rainforest
Warm, wet, with little seasonal variation; high productivity and biodiversity.
Found between 20° N and 20° S of the equator; located in Central/South America, Africa, SE Asia, NE Australia, and numerous tropical islands.
Vegetation layers: emergent trees, canopy, subcanopy (understory); epiphytes; lianas.
Common animals: jaguars, orangutans, green-eyed tree frogs (examples vary by region).
Soils are often nutrient-poor due to rapid decomposition and leaching; high biodiversity per hectare; deforestation is a major threat.
2-4 What are the causes of changing boundaries of terrestrial biomes?
Biome boundaries shift with climate changes (historical and ongoing).
Ice ages caused substantial temperature declines and biome shifts; pollen cores from lake sediments reveal past biome distributions.
After the last glacial maximum (~10,000 years ago), taiga and temperate seasonal forests expanded northward as glaciers retreated.
Modern climate change and human activity are shifting biome boundaries today (e.g., warming, altered precipitation patterns).
The Greater Yellowstone Ecosystem example shows how boundaries can extend beyond administrative borders when species use larger areas for survival.
3-1 What are the major freshwater biomes?
Freshwater biomes have low salinity and include:
Streams and rivers: flowing freshwater; energy primarily from terrestrial detritus; high-oxygen environments in rapids; energy transitions toward the mouth where flow slows and rooted vegetation can establish.
Lakes and ponds: standing bodies of water with distinct zones; depth determines plant zones; zonation includes littoral, limnetic, profundal, and benthic zones.
Freshwater wetlands: characterized by saturation or shallow standing water; highly productive; include swamps (emergent trees), marshes (non-woody vegetation), and bogs (acidic, sphagnum moss).
Global importance: biodiversity, flood control, water purification, habitat for migratory birds; status: wetlands have been heavily drained for agriculture and development, reducing ecosystem services.
3-2 What are the major marine biomes?
Marine biomes include high-salinity environments and are categorized as:
Estuaries / salt marshes: zones where freshwater mixes with seawater; highly productive and filter contaminants; nurseries for many fish and shellfish.
Mangrove swamps: salt-tolerant trees along tropical/subtropical coasts; stabilize coastlines and provide habitat for marine life; roots trap sediment and organic material.
Intertidal zones: shore areas between high and low tides; harsh desiccation and temperature swings; organisms include barnacles, sponges, mussels, crabs, sea stars.
Coral reefs: among Earth’s most diverse marine biomes; corals host photosynthetic algae (zooxanthellae) and build limestone skeletons; require shallow, sunlit water; high biodiversity but threatened by pollution, sedimentation, and coral bleaching.
Open ocean: deep-ocean waters beyond continental shelves; atmosphere-surface interactions; zonation includes photic, aphotic zones, and the benthic zone at the ocean floor.
Coral reefs face challenges including pollutants, sediments, and bleaching; global bleaching events link to warmer water and ocean acidification.
In the open ocean, the photic zone supports photosynthesis by phytoplankton; the aphotic zone relies on chemosynthesis by bacteria in some deep-sea communities (e.g., tube worms).
Great Barrier Reef example: vast reef system with high species richness (e.g., >400 coral species, ~1,500 fish species, ~200 bird species).
3-3 The Open Ocean and Zones
Open ocean: deep-ocean waters beyond shore; sunlight penetration declines with depth and turbidity; the photic zone is the upper light-penetrating layer where photosynthesis occurs; the aphotic zone lacks sufficient light for photosynthesis.
Benthic zone: ocean floor; hosts diverse communities; detritus-based energy flows support many organisms.
Chemosynthesis: some deep-sea bacteria derive energy from chemical reactions (e.g., methane, hydrogen sulfide) rather than sunlight; forms base of deep-ocean food webs (e.g., tube worms).
3-1 to 3-3: Summary of aquatic biomes and their drivers
Major drivers distinguishing aquatic biomes: salinity, depth, water flow, nutrient levels, and light availability.
Freshwater biomes: streams/rivers (flow), lakes/ponds (standing water), freshwater wetlands (saturated soils).
Marine biomes: estuaries/salt marshes, mangrove swamps, intertidal zones, coral reefs, open ocean.
Ecological services: drinking water sources, habitat for diverse species, nutrient cycling, flood mitigation, sediment stabilization, and climate regulation.
TABLE 3.1: Summary of the aquatic biomes
Freshwater: Streams & rivers (flowing), Ponds & lakes (standing, shallow to deep), Freshwater wetlands (swamps, marshes, bogs).
Marine: Estuaries / salt marshes, Mangrove swamps, Intertidal zones, Coral reefs, Open ocean.
Key distinguishing features: salinity, depth, and water flow (flowing vs standing).
3-4 Photosynthesis, nutrient cycling, and zones in water bodies
Streams and rivers: energy sources transition from terrestrial detritus to aquatic algae as flow slows; high-oxygen environments in rapids; downstream support for fish like trout if conditions are well-oxygenated.
Lakes and ponds: zonation (littoral, limnetic, profundal, benthic) defines where photosynthesis and detritus-based energy support communities.
Wetlands: productive, flood buffering, pollutant filtration, biodiversity hotspots.
Coral reefs and mangroves: nutrient-poor waters sustain energy through symbioses and detrital inputs; reefs provide structure and habitat; mangroves stabilize coastlines and support fisheries.
4- General notes on human impacts and conservation
Biome boundaries shift with climate change; past shifts linked to ice ages; current shifts linked to emissions and land-use change.
Invasive species are a major concern across biomes due to novel interactions and lack of natural enemies.
Wetlands and coral reefs are among the most threatened aquatic biomes due to pollution, sedimentation, habitat destruction, and climate change (warming, acidification).
ext{P}_{ ext{required}}( ext{Δ}T) \approx 2 \,\text{mm per }^\circ\text{C} \times \Delta T.
Biosphere thickness: areas: Yellowstone boundaries expanded to include roughly20,000,000 ha≈50,000,000 acres.20,000,000 ha≈50,000,000 acres.
20{,}000{,}000\ ha = 10{,}000 m²; 1{,}500\ \Descriptive depth/zone depths: Open-ocean light penetration often up to ∼200 m∼200 m
\sim 200\ \text{m}; photic zone contains photosynthetic organisms; aphotic zone lacks sufficient light; benthic zone is the ocean floor.
Representative biomes and climate ranges:
Tundra: very cold; short growing season; permafrost; low nutrient soils.
Taiga: cold winters; conifers; nutrient-poor soils; slow decomposition.
Temperate rainforest: high precipitation; large trees; leaching lowers soil nutrients.
Temperate seasonal forest: moderate climate; high productivity; deciduous trees.
Shrubland: hot dry summers; fire-adapted flora.
Temperate grassland: fire and grazing shape communities; grasses dominate; agriculture history.
Hot desert: sparse vegetation; CAM/photosynthesis adaptations.
Savanna: warm; distinct wet/dry seasons; grasses with scattered trees.
Tropical rainforest: high biodiversity; layered forest structure; nutrient-poor soils.
Invasive species examples (exotic species):
Rats, zebra mussels, kudzu; native–exotic interactions lack shared evolutionary history and can cause rapid ecosystem disruption.
Key definitions recap:
Ecosystem: living and nonliving components interacting in a system.
Abiotic: nonliving environmental components (temperature, humidity, rainfall, nutrients).
Biotic: living organisms (plants, animals, fungi, bacteria).
Biosphere: all ecosystems on Earth; life-supporting layer ~20 km thick.
Habitat vs biome distinction: habitat is a niche within a biome; biome is a large geographic region defined by climate and growth forms.
Mutualism, commensalism, parasitism, predation, herbivory, competition: key interaction types with distinct effects on participant species.