IB Biology: Ecosystems

B4.1 C4.1 D4.2

Species

A group of organisms that can potentially interbreed to produce viable, fertile offspring.

Population

A group of organisms of the same species living together in the same area.

Community

A group of populations living and interacting with each other within a given area.

Habitat

Environment in which a species normally lives. Alternatively, the location of an organism.

Ecosystem

A community and its abiotic environment (habitat).

Ecology

Study of the relationships between living organisms/between organisms and their habitats.

Limiting factor

Component of an ecosystem which limits the distribution or numbers of a population.

What does a limiting factor do?

Defines optimal survival conditions according to its effect on a species when the factor is in deficiency or excess.

Biotic factors

Interations (intra- or inter-specific) between organisms

Abiotic factors

Environmental factors like sunlight, rainfall, soil pH, etc.

How does temperature limit plants?

• Higher temps increase rate of water loss by evaporation and may denature enzymes

• Low temps can cause plant sap to freeze, leading to expansion of frozen water in xylem, which can cause split trunks

  • Some species produce antifreeze proteins to prevent crystallization of cells

Xerophytes

Plant species adapted to dry, arid environments (e.g. cacti, succulents)

Hydrophytes

Plant species adapted to survive in frequently waterlogged soil (e.g. rice)

How does light availability affect plants?

• Light is essential for photosynthesis. If plants cannot access it, they will die.

• Low-growing plants typically have dark green leaves (more chlorophyll)

• Some seaweeds have pigments adapted to absorbing blue light, as red doesn’t penetrate the water

How does salinity limit plants?

High salinity may be toxic and makes osmosis difficult for plants, which may lead to cell death.

Halophytes

Plants adapted to high salinity levels (e.g. mangroves)

Glycophytes

Plants that are easily damaged from high salinity

Poikilotherms

Animals that cannot maintain thermal homeostasis and must occupy environments according to temperature needs (cold-blooded animals)

Homeotherms

Animals that can regulate their own internal body temperatures and thus occupy a wider range of habitats (warm-blooded animals)

How is territory a limiting factor for animals?

• Determines an animal’s capacity to attract mates, rear young, forage for food, and avoid predators

• May be temporary (breeding sites, migration stops) or permanent

• In some species, juveniles have different environmental requirements than adults (e.g. tadpoles vs. frogs)

  • May lead to intra- or inter-species competition

How is food availability a limiting factor for animals?

• Animals may need a specific source of food (e.g. cows need grass)

• Seasonal/geographic variations may directly affect food availability (e.g. seasonal migrations)

Law of Tolerance

Populations have optimal survival conditions within minimum and maximum thresholds. Populations exposed to the extremes of a limiting factor begin to die off.

Who proposed the Law of Tolerance?

Victor Ernest Shelford (1911)

Zones classified by the Law of Tolerance

• Zones of intolerance: extremes of limiting factor; unlivable conditions

• Zones of stress: survivable but result in reduced reproduction/survival rates

• Optimal zone: conditions favouring max reproductive success and survivability

Quadrats

Rectangular frames of known dimensions used to establish population densities

Transects

Straight lines along abiotic gradient from which population data can be recorded to determine a pattern

Limiting factors for coral reefs

• Can only grow in shallow, clear water to allow for photosynthesis (< 25 m)

• Zooxanthellae (photosynthetic algae) cannot survive < 18 C

• In higher ocean temperatures (> 35 C) zooxanthellae leave coral tissue, resulting in coral bleaching

• Ocean acidification leads to lower calcification of coral and destruction of existing reefs

Optimal growth range of reef-building corals

Shallow, 20-30 degree C water (tropical and sub-tropical regions)

Biomes

Geographical areas with particular climates that sustain specific communities of plants and animals

Factors affecting biome distribution

Temperature and rainfall, which vary according to latitude, longitude, altitude, and ocean proximity

Tropical rainforest characteristics

• High annual rainfall

• High average temperatures

• Nutrient-poor soil

• High biodiversity

Temperate forest characteristics

• Moderate temperatures

• Clear changes in season

• High precipitation levels

Taiga/boreal forest characteristics

• Cold and icy climate (0-15 C)

• Low precipitation

• Densely packed

• Low biodiversity

Savannah characteristics

• 20-30 C

• Moderate precipitation

  • Seasonal droughts are common

• Widely spaced trees

Temperate grassland characteristics

• Moderate temperatures

• Moderate precipitation

• Grasses are the dominant vegetation, with trees and shrubs being largely absent

Mediterranean/chaparrel characteristics

• Moderate temperatures

• Rainy winters and dry summers

• Dry, woody, fast-growing shrubs make up the majority of vegetation

Desert characteristics

• Extreme temperatures (< 0 to > 30 degrees)

• Low precipitation (< 30 cm/year)

• Dominant plant species are xerophytes (adapted for water conservation)

Tundra characteristics

• Freezing temperatures

• Very low precipitation

• Low-growing vegetation

• Perennial plants may grow during the summer

Mountain characteristics

• High altitudes (> 10,000 feet)

• Temperatures are typically low due to high altitudes

• Weather conditions subject to rapid change

Adaptations for desert survival

• Expire: parents die but leave behind tough eggs/seeds

• Evade: change activity from day to night or above ground to below to avoid extreme temperatures

• Endure: adaptations like storing water in leaves enable survival in harsh conditions

What four factors affect population size?

1. Natality (birth rate)

2. Mortality (death rate)

3. Immigration (new members arriving)

4. Emigration (members leaving)

Equation to determine population size

Population size = (Immigration + Natality) - (Mortality + Emigration)

Non-motile/sessile sampling method

Sessile species can be sampled with quadrats

Motile species sampling method

Capture-mark-release-recapture

• Estimates are based on Lincoln index

Capture-mark-release-recapture method

• Area defined and marked off

• Selection of individuals captured, marked, counted, and released (n1)

  • Marking must not be easily removable or adversely affect animal’s survival prospects

• After sufficient time is allowed for marked individuals to re-integrate, a second capture (n2) is made

• Unmarked and marked individuals (marked on second capture = n3) counted

• Lincoln index used to calculate population size

Lincoln index

Estimated population = (n1 * n2) / n3

Requirements of Lincoln index

• Sampling is random

• n1 is randomly distributed post-release

• Marking individuals will not affect mortality or natality rates

Exponential growth

• Occurs in an ideal environment with unlimited resources

• Population numbers and birth rates rise exponentially

• Visible in very small populations or those that have newly colonised a region

Biotic potential

Maximal growth rate of a population

Logistic growth

• Occurs when populations approach a finite carrying capacity

• Environmental resistance occurs and slows growth rate

• Results in a sigmoidal (s-shaped) curve plateauing at k (carrying capacity)

• Will be seen in any stable population occupying a fixed geographic area

Carrying capacity

Maximum number of a species that can be sustainably supported by the environment

Population clocks

• Provide current projections of estimated populations based on previous data

  • Typically gathered from regional and national census data

• May be used to assume rates of change

• Questionably accurate but can be used to identify changing demographics

Density-dependent factors

• Influenced by relative size of a population

• May be predators, available resources, nutrient supply, disease, accumulation of waste, etc.

Desity-independent factors

• Not influenced by population size

• Natural phenomena (earthquakes, tsunamis, etc), abiotic factors (temperature, soil pH, etc), weather

Exponential growth phase

• Lag period (low growth) initially

• As numbers of individuals rise, natality exceeds mortality and leads to a rapid growth in numbers

• Low mortality due to abundant resources and minimal environmental resistance

Transitional phase

• Resources become limited, leading to competition for survival

• Natality rates drop and mortality rates rise

Plateau phase

• Mortality is equal to natality

• Population has reached carrying capacity

  • It will oscillate around CC as the environment/limiting factors rise and fall in response to the population size

Negative feedback

Return of a system to its original starting state

Top down control

Pressures applied by a higher trophic level to control population dynamics of an ecosystem

Bottom up control

Availability/abundance of lower trophic levels control nutrient availability to higher trophic levels

Human activity can often limit resource availability and thus exert bottom up pressure on an ecosystem

Intraspecific competition

Organisms belonging to the same species compete for resources to survive

Herbivory

• May be harmful or beneficial to plant species

  • Beetle feeding on all of a crop plant’s leaves leads to crop failure, while fruit-eating animals spread the plant’s seeds as they travel

Predation

Predator and prey populations are intertwined—if prey populations decrease, so do predator populations

Pathogenicity

Microbe causing disease in a host

• May decrease carrying capacity of host population

• Changes in the incidence of pathogen can cause populations to oscillate around CC

Symbiosis

Long-term close interaction between species

• May be obligate (necessary for survival) or facultative (beneficial but unneeded)

  • Mutualism: both species benefit

  • Commensalism: one species benefits, the other is unaffected

  • Parasitism: one species benefits to the detriment of the other

Competition

The fitness of one species is lowered by the other’s presence

• Limited resource supply typically triggers one of two responses

  • Competetive exclusion: one species uses resources better and drives the other to local extinction

  • Resource partitioning: both species alter their use of the environment to divide the resources between them

Allelopathy

Chemical inhibition of one organism by another due to the release of allelochemicals that act as germination or growth inhibitors

Endemic species

Native to a given geographic region

Alien species

Transferred from their natural habitat to new environment

Invasive species

An alien species that has a detrimental effect on pre-existing food chains and threatens biodiversity by stealing resources from native species

Competitive exclusion principle

Two species cannot inhabit identical niches within a community. One will have a competitive advantage and survive at the expense of the other.

Invasive species advantages

• Large fundamental niche

• Fast reproduction rates

• Lack natural predators

• May possess features suited to new environment

  Case studies: Wild rabbits and cane toads in Australia

Positive association

Species in a given environment that are typically found together—indicates a symbiotic, predator-prey, or other necessary relationship

Negative association

Species that are not often found together within the same community—indicates competition

Chi-squared tests

Applied to data from quadrat sampling to determine statistical significance

Ecosystem stability

Ecosystems are largely self-contained and can be self-sustaining over long time periods (rainforests of Southeast Asia are estimated to be ~70 million years old)

Components required for ecosystem stability

• Supply of energy (typically light)

• Recycling of nutrients

  • Sapotrophic decomposers recycling inorganic nutrients and bacteria detoxifying harmful waste products

• Genetic diversity—higher diversity leads to more resilience

Deforestation

Permanent destruction of a forest via the removal of its trees

Effects of deforestation

• Fewer trees leads to less moisture in the air

• Less natural litter leads to reduced humus production and lower soil nutrition

• A rapid loss of nutrients from leaching damages rocks with chemical weathering

• Topsoil layer is easily thinned/eroded

• Infertile soil prevents vegetative growth, lowering biodiversity and damaging the nutrient cycle

• Logging alters distribution of plant species

• Removal of the canopy leads to a loss of soil nutrients due to water runoff

Mesocosm

Enclosed environment allowing a small part of a natural environment to be observed under controlled conditions

Keystone species

A species with a disproportionately large impact on the environment relative to its percentage of the population

Sustainability

Capacity for a biological system to remain diverse and productive indefinitely

Sustainable yield

Amount of a natural resource that can be taken from an ecosystem without reducing the base stock

Common agricultural impacts

• Soil erosion: impact of water and wind leads to deterioration of soil

• Leaching: loss of water-soluble nutrients from the soil

• Pollution due to chemicals like pesticides and fertilizers

• Crop transportation leads to high carbon footprint

Eutrophication

Enrichment of a (typically aquatic) ecosystem with chemical nutrients. Common around agricultural lands where artificial fertilizers are prevalent

Effects of eutrophication

• Algal blooms as a result of increased nutrients

• Saphotrophic microbes increase when algae dies

• Higher decomp rate leads to higher biochemical oxygen demand by sapotrophic bacteria

• Sapotrophs consume available dissolved oxygen, leading to deoxygenation of water supply

• Higher turbidity of water leads to lower oxygen production by seaweeds

• Stresses marine organisms and may lead to lower biodiversity

Biomagnification

Process in which chem substances are present in a higher concentration in each trophic level

Bioaccumulation

Buildup of chemical substance in a single organisms tissues

Why does biomagnification occur?

Organisms at higher trophic levels must consume more biomass to meet nutrition requirements. Energy transformations are only ~10% efficient, meaning higher-order consumers must eat more to reach energy demands, leading to higher concentration of chemical substances as the trophic level increases.

Macroplastic vs. microplastic

Macroplastic: > 1mm

Microplastic: < 1mm

Gyres

Oceanic convergence zones where currents deposit plastic debris

Persistent Organic Pollutants

Toxic contaminates absorbed by plastic molecules. Microplastics absorb more of these due to their higher surface area. When marine animals ingest micro- and macroplastics, bioaccumulation and biomagnification of POPs occurs, which damages their stomachs and may cause starvation due to blocking the digestive tract.

Rewilding

Reintroducing lost animal species to their natural environments, including apex predators and keystone species.

Goal of rewilding

Aims to restore ecosystems and increase natural biodiversity by allowing wildlife and natural processes to reclaim areas no longer under human management.

Ecological succession

Process by which a sequence of increasingly complex communities develop over time

Primary succession

Communities developing on entirely new land without established soil

Process of primary succession

• Pioneer species: first colonisers. Typically lichen or moss, whose death and decomposition creates organic soil for plant growth

• Plants add humus to soil, which leads to increased soil depth and changes soil pH

  • Soil mineral content increases and rocks are broken down by the actions of roots

  • Soil aerated and water retention increases

  • Changes allow for growth of larger plants, which reduce erosion through root binding action

• Larger plants outcompete smaller shade-intolerant plants

Primary succession changes

• Net primary productivity is high in pioneer communities and reduces as it nears the climax community

• Gross primary productivity increases

• Plant size increases

• Complexity of food webs increases

• Nutrient availability and importance of nutrient cycling within ecosystem increase

• Species diversity increases

Secondary succession

Occurs when succession starts on existing soil following the upheaval of a preexisting ecosystem. It may be a cycle of communities instead of a single climax community and may occur due to seasons, natural disasters, or significant changes in animal activity.

Process of secondary succession

• Upheaval leads to removal of existing biota

  • Results in development of a new ecosystem

• Grasses and herbacious plants grow

• Fast growing trees develop to their fullest; shade tolerant trees develop in understory

  • Fast trees may be overtaken by larger, slower-growing trees as the ecosystem reverts to its prior state