ESS SL 2.1 Individuals, Populations, Communities & Ecosystems Notes

Introduction to Ecological Systems

  • Biosphere:

    • The life-supporting zone around Earth where air (atmosphere), water (hydrosphere), and land (lithosphere) meet.

    • Contains all living organisms (plants, animals, fungi, microorganisms).

    • Functions as a large, complex system of living communities interacting with each other and non-living components.

    • Measures approximately 20 km from top to bottom, with most life existing in a narrow range of 6.5 km (500 m below sea level to 6 km above sea level).

  • Species:

    • A group of organisms sharing common characteristics that interbreed to produce fertile offspring (biological species concept).

    • Members of the same species share a gene pool, breeding and producing offspring with similar traits.

  • Populations:

    • A group of organisms of the same species living in the same area at the same time which interbreed.

    • Considered an interbreeding unit of a species.

    • A species may consist of multiple populations.

    • Example: Eastern Grey Kangaroo in Australia has populations in Queensland, New South Wales, Victoria, etc.

    • Geographic isolation can lead to unique characteristics and adaptations within each population due to local factors like habitat, climate, and food availability.

  • Community:

    • Includes all different populations (of different species) living in the same area at the same time.

    • A collection of interacting populations within an ecosystem.

    • Species depend on each other for food, shelter, pollination, seed dispersal, etc.

  • Habitat:

    • The local environment in which an organism, species, population, or community normally lives.

    • E.g., badgers, deer, oak trees, and ants in a woodland habitat.

    • Description includes geographical and physical locations, and the type of ecosystem needed for survival.

  • Ecosystems:

    • A community of living organisms interacting with their physical environment in a specific area.

    • Includes biotic (living) components like plants, animals, fungi, and microorganisms.

    • Includes abiotic (non-living) components like soil, water, air, sunlight, temperature, humidity, and minerals.

    • Abiotic components provide essential resources and conditions for biotic survival.

    • Interactions between biotic and abiotic components shape ecosystem structure and dynamics.

    • Influences biodiversity, nutrient cycling, and ecosystem services.

    • Vary in size from small ponds to vast forests.

    • Unique characteristics shaped by geography, climate, and species present.

    • Open systems in which both energy and matter can enter and exit.

    • Energy:

      • Enters through photosynthetic organisms capturing sunlight.

      • Exits through heat released during cellular respiration and trophic transfers.

    • Matter:

      • Enters/exits as water, nutrients, gases, or waste products.

Classification & Taxonomic Tools

  • Classification:

    • Organizing and categorizing species based on similarities and differences to manage the enormous diversity of life on Earth.

    • Species grouped into a hierarchy of categories based on shared biological characteristics.

    • Allows for quick identification and prediction of characteristics.

    • Hierarchical system: higher ranks contain more organisms with less similarity, lower ranks contain fewer organisms with more similarity.

    • First step is to place a species into a genus.

    • A genus is a category including one or more closely related species sharing common characteristics.

    • Each species gets a two-part name: genus name followed by species name.

    • Genus name is capitalized and written first; the species name is not capitalized.

    • Both genus and species are italicized or underlined.

    • Example: Homo sapiens (humans), where Homo is the genus and sapiens is the species.

    • Genus usually contains more than one species, but Homo sapiens is the only existing species in its genus today.

    • Other species in the Homo genus include Homo neanderthalensis (Neanderthals) and Homo erectus.

    • Example: The genus Canis includes:

      • Canis lupus (Grey wolf)

      • Canis familiaris (Domestic dog)

      • Canis latrans (Coyote)

      • Canis aureus (Golden jackal)

      • Canis simensis (Ethiopian wolf)

      • Canis mesomelas (Black-backed jackal)

      • Canis anthus (African golden wolf)

      • Canis adustus (Side-striped jackal)

      • Canis lupaster (African wolf)

    • These Canis species share similar body structures, behaviors, and genetic traits, indicating a close evolutionary relationship.

  • Taxonomic Tools:

    • Tools used to identify an organism and classify it.

    • Identification means determining which species an individual organism belongs to.

    • Comparison of specimens with reference collections:

      • Taxonomists compare unknown specimens with well-documented reference collections.

      • Reference collections contain a large number of similar organisms that have already been identified and classified.

      • Method involves physically comparing the specimen to known samples.

      • Relies on the taxonomist's expertise and the quality of the reference collection.

      • Example: Botanist identifying a plant specimen by comparing it to a collection at a botanical garden.

      • Apps can identify species by comparing a photo to an online database.

    • DNA surveys:

      • Analyzing an organism's DNA to determine its species.

      • Compares the DNA sequence of the specimen with known sequences.

      • Provides precise and reliable identification, especially for closely related species.

      • Example: Wildlife conservation researchers using DNA surveys to distinguish between similar butterfly species.

    • Dichotomous keys:

      • Tools used to identify organisms based on their characteristics.

      • Consist of paired statements or questions with two possible answers.

      • Choices lead to further pairs of statements, eventually identifying the organism.

      • Example: Dichotomous key for identifying species in the Serengeti ecosystem.

      • Limitations:

        • Limited scope: Designed for a limited number of species.

        • Inaccuracies: Dependent on the accuracy of the information provided.

        • Variability: Organisms' physical characteristics can vary.

        • Time-consuming: Especially for beginners.

        • Expertise required: Familiarity with the organisms is needed.

        • Limited to physical characteristics: Doesn't account for behavior or habitat.

Factors Affecting Populations

  • Biotic & Abiotic Factors:

    • Factors determining population distribution can be abiotic or biotic.

    • Biotic: Living components of an ecosystem.

    • Abiotic: Non-living, physical factors influencing organisms.

  • Biotic Factors:

    • Living, biological factors influencing ecosystems and communities.

    • Interactions between organisms within a population or community.

    • Include: Predation, Herbivory, Parasitism, Mutualism, Disease, Competition.

    • Examples:

      • Availability of food:

        • More food increases survival and reproduction chances.

        • Rainforests have a rich food supply, supporting many species.

        • Deserts have a poor food supply, supporting fewer species.

      • New predators:

        • Unbalanced ecosystems if a new predator is introduced.

        • Red foxes in Australia caused the decline of native species.

      • New pathogens:

        • Populations have no immunity, may decline or be wiped out.

        • Avian flu caused deaths of over 6,000 bar-headed geese in Qinghai Lake, China.

      • Competition:

        • Better-adapted species outcompete others for resources.

        • North American grey squirrels outcompeted native red squirrels in the UK.

  • Abiotic Factors:

    • Non-living, physical factors influencing ecosystems and communities.

    • Include: Temperature, Sunlight, pH (soil and water), Salinity, Dissolved oxygen, Soil texture, Moisture and precipitation levels, Minerals and nutrients, Wind intensity, Carbon dioxide levels (for plants).

    • Changes can affect survival, reproduction, and ecosystem functioning.

    • Can be quantified (measured) to determine species distribution.

    • Examples:

      • Temperature: Affects photosynthesis, metabolism, growth, and reproduction.

      • Sunlight: Required for photosynthesis, increases plant growth rates.

      • pH: Affects nutrient availability, influencing plant growth and aquatic organisms.

      • Salinity: Affects the health and survival of aquatic organisms.

      • Dissolved oxygen: Essential for aquatic organisms; low levels can lead to hypoxia.

      • Soil texture: Influences water retention, nutrient availability, and root penetration.

      • Moisture and precipitation: Determine water availability for organisms.

      • Minerals and nutrients: Different plants are adapted to different soil mineral content.

      • Wind intensity: Affects transpiration rate and disperses seeds and pollen.

      • Carbon dioxide: Required for photosynthesis, affects plant growth.

  • Ecological Niches:

    • Describes the abiotic and biotic conditions and resources an organism or population responds to and depends on.

    • Each species has its own distinct niche.

    • If two species try to occupy the same niche, they compete, and one will outcompete the other.

    • Example: Three North American warbler species occupy the same habitat but feed at different heights within trees, avoiding competition.

  • Population Interactions:

    • Populations are characterized through Size, Density, Distribution, Age structure, Growth rate, Interaction with each other.

    • Populations interact through: Herbivory, Predation, Parasitism, Mutualism, Disease, Competition.

    • Resulting in ecological, behavioral and evolutionary consequences.

    • Herbivory:

      • When an organism feeds on a plant.

      • Carrying capacity of herbivores is affected by plant numbers.

      • High herbivory rates can decrease plant populations.

    • Predation:

      • When one animal eats another.

      • Lowers carrying capacity of prey species.

      • Can lower carrying capacity of predator species due to decreased prey numbers.

      • Predator-prey cycles: Numbers of predators and prey rise and fall in cycles.

    • Parasitism:

      • Parasites live closely with a host species, benefiting from the host.

      • Parasites harm the host, lowering the host's carrying capacity.

      • Example: Fleas on mammals, malaria parasite infecting red blood cells in humans.

    • Mutualism:

      • Both species benefit, increasing carrying capacity of both.

      • Example: Bees and flowering plants, bees gaining nectar and flowers gaining pollination.

    • Disease:

      • Pathogens (bacteria, viruses, fungi, and protozoa) cause diseases.

      • Diseases lower the carrying capacity of infected species.

      • Changes in disease incidence cause populations to fluctuate.

    • Competition:

      • Intraspecific competition: Between members of the same species.

      • Interspecific competition: Between members of different species.

      • Intraspecific competition can lower carrying capacity due to decreased food availability.

      • Interspecific competition occurs between species with similar niches, decreasing carrying capacity of one or both species.

Population Growth

  • Carrying Capacity:

    • The maximum stable population size an ecosystem can support, determined by competition for limited resources.

    • Abiotic and biotic factors prevent every individual from reproducing.

    • Ensures species population size is limited, reaching carrying capacity.

    • The graph plateaus when the environmental (abiotic and biotic) factors that stop individuals from surviving and reproducing result in the population no longer being able to grow in size.

  • Density-dependent Factors & Negative Feedback Mechanisms:

    • Population size is regulated by density-dependent factors and negative feedback mechanisms.

    • Density-independent factors (e.g., climate, temperature, rainfall, soil fertility) may influence population size.

    • Density-dependent factors' impact varies with population density.

    • Examples:

      • Competition for resources: Increased competition for food, water, and shelter as population density increases.

      • Increased risk of predation: Higher population density increases predator encounters.

      • Pathogen transmission: Dense populations facilitate the spread of diseases and parasites.

    • Negative feedback mechanisms:

      • Density-dependent factors drive negative feedback, returning a population to equilibrium.

      • Resource scarcity, predation, and disease outbreaks reduce population growth rates as density rises.

  • Population Growth Curves:

    • Growth can be exponential or limited by carrying capacity.

    • If there are no limiting factors, population growth follows a J-curve (exponential growth).

    • When density-dependent limiting factors operate, the curve becomes S-shaped.

    • J-curves:

      • Growth pattern in an environment with unlimited resources.

      • Three phases:

        • Lag phase: Initial slow growth when the population is small.

        • Exponential growth phase: Population growth accelerates rapidly as the number of individuals increases.

        • Crash phase: Significant overshoot, may lead to a sudden decrease in population.

    • S-curves:

      • Growth pattern in a resource-limited environment.

      • Four phases:

        • Lag phase: The initial growth is slow when the population is small.

        • Exponential growth phase: With low or reduced limiting factors, the population expands exponentially into the habitat.

        • Transitional phase: Increased competition between individuals for the same limiting factors or resources, which competition result in a lower rate of population increase.

        • Plateau phase: The population reaches its carrying capacity determined by the limiting factors. Changes in limiting factors impact carrying capacity. The population reaches its carrying capacity and fluctuates around a set point determined by the limiting factors

Human Populations

  • Human Population Growth:

    • Limiting factors on human population growth

      • Human societies are increasingly able to overcome the limiting factors that delayed human population growth in the past.

      • This has had many negative consequences for the sustainability of ecosystems

      • Reasons:

        • Elimination of natural predators: Removing predators leads to unchecked growth. Reintroduction of wolves in Yellowstone helped control elk population.

        • Technological advances: Agriculture and medicine advancements reduce mortality, increase food production.

          • The Green Revolution in the mid-20th century, with the introduction of high-yield crop varieties and modern agricultural techniques , significantly increased food production globally

        • Degradation of the environment: Extracting resources provides living space and land for food production, increasing human population growth rates.

          • Environmental degradation continues to facilitate the extraction of energy sources , such as fossil fuels , which are vital for sustaining growing populations

  • Assessing carrying capacity for human populations:

    • Scientists use various methods to estimate the carrying capacity of an environment for a given species: These methods include field observations , population surveys , mathematical modelling and data analysis

    • Estimating carrying capacity is challenging due to:

      • The broad and changing ecological niche of humans:

        • Populations in ecosystems tend to reach equilibrium when the availability of resources matches the population's needs. However , humans have a broad and dynamic ecological niche , constantly adapting through technological innovations and changes in consumption patterns

      • Mobility of resources: Humans can move and exploit resources beyond their immediate habitat.

        • For example , global trade allows societies to access resources like food and materials from around the world, solving the problem of local resource limitations

      • Technological advancements: Humans modify the environment and overcome limitations.

        • For example , the development of agriculture and irrigation techniques has allowed humans to increase food production and support larger populations beyond what the natural environment could sustain

      • Cultural and social factors: Influence fertility rates and migration patterns.

        • For example , these can affect fertility rates and migration patterns , making it difficult to accurately predict or estimate carrying capacity for human populations

      • Changing lifestyles and consumption patterns: Varying lifestyles impact resource demands.

        • For example , urbanised societies with high levels of consumption may strain the carrying capacity of their surrounding areas due to increased resource demands and waste generation

      • Adaptive capacity: Humans adapt and innovate in response to changing conditions.

        • More so than any other species , humans have the ability to adapt and innovate in response to changing environmental conditions. This adaptability can affect carrying capacity by influencing resource use efficiency and the development of technological solutions

      • Disputed estimates of carrying capacity: Estimates are often disputed due to uncertainties.

Studying Populations

  • Sampling Strategies:

    • Sampling methods extract a smaller sample from a larger population to make inferences about the population.

    • Population: The whole set of things you are interested in.

      • E.g., all year 11 pupils at a school.

      • Can also refer to makes and models of mobile phones.

    • Sample: A selected part (subset) of the population that data is collected from.

      • E.g., a certain number of pupils from year 11.

    • Random sample: Every item in the population has an equal chance of being selected.

      • E.g., every pupil in year 11 has the same chance of being selected.

    • Biased sample: The sample is not random.

      • E.g., the teacher asks pupils from just one class

    • Advantages of using a population:

      • Accurate results.

      • All options/opinions/responses included.

    • Disadvantages of using a population:

      • Time-consuming.

      • Expensive.

      • Large amounts of data to organize and analyze.

    • Advantages of using a sample:

      • Quicker to collect data.

      • Cheaper.

      • Less data to organize and analyze.

    • Disadvantages of using a sample:

      • Small sample size can lead to unreliable results.

      • Sampling methods can usually be improved by taking a larger sample size.

      • A small sample size can lead to unreliable results.A sample can introduce bias.

      • Particularly if the sample is not random.

      • A sample might not be representative of the population.

      • Only a selection of options/opinions/responses might be accounted for.

      • The members/items used in the sample may all have similar responses.

  • Random and systematic sampling strategies:

    • Random sampling: Positions of sampling points are completely random.

      • For example , sampling points can be selected using a random number generator to create a set of random coordinates

      • Beneficial because it means there will be no bias by the person carrying out the sampling.

    • Systematic sampling: Positions chosen by the person carrying out the sampling and a regular pattern is used to select sample points.

      • There is a possibility that the person choosing could show bias towards or against certain areas.

  • Transect sampling:

    • Systematic sampling allows researchers to investigate the effect of the presence of certain environmental features on species distribution.

    • Transect: A line along which samples are taken.

    • Used when there is a clear change in the physical conditions across the area being studied

    • Line transect: Record the identity of the organisms that touch the line.

    • Belt transect: Place quadrats at regular intervals along the tape and record the abundance or percentage cover of each species within each quadrat

  • Quadrat Sampling:

    • Quadrats are square frames made of wood or wire.

      • They can be a variety of sizes e.g. 0.25 m or 1 m.

    • Placed on the ground and the organisms within them are recorded.

    • Used to estimate population size by recording The number of an individual species and Percentage cover.

    • Estimating population size:

      • Quadrats must be laid randomly in the area to avoid sampling bias.

      • This random sampling can be done by converting the sampling area into a grid format and labelling each square on the grid with a number

    • Estimating percentage cover and percentage frequency:

      • Percentage cover:

        • Estimate of the area within a given quadrat covered by the plant or animal being sampled

      • Percentage frequency:

        • The number of squares in which the species occurs divided by the number of possible occurrences

        • % frequency=(number of quadrat squares in which species presenttotal number of quadrat squares)×100\% \text{ frequency} = (\frac{\text{number of quadrat squares in which species present}}{\text{total number of quadrat squares}}) \times 100

  • Capture–mark–release–recapture & the Lincoln Index:

    • Different methods are required for estimating the number of individuals in a population of motile animals (i.e. animals that are mobile

    • The capture-mark-release-recapture method is commonly used alongside the Lincoln index (a statistical measure used to estimate population size)

    • The Lincoln index can be used to estimate the abundance or population size of a species in a given area

    • For a single species:

      • The first large sample is taken—as many individuals as possible are caught, counted and marked in a way that won’t affect their survival

      • The marked individuals are returned to their habitat and allowed to randomly mix with the rest of the population

      • When a sufficient amount of time has passed another large sample is captured

      • The number of marked and unmarked individuals within the sample are counted

      • The proportion of marked to unmarked individuals is used to calculate an estimate of the population size (the Lincoln index)

      • The formula for calculating the Lincoln index is:

        • Population size estimate=M×NR\text{Population size estimate} = \frac{M \times N}{R}

        • Where:

          • MM = number of individuals caught in the first sample (i.e. number of marked individuals released)

          • NN = number of marked and unmarked individuals caught in the second sample (i.e. total number of individuals recaptured)

          • RR = number of marked individuals in the second sample (i.e. number of marked individuals recaptured)

    • Limitations when using the mark-release-capture method:

      • The marked individuals must be given sufficient time to disperse and mix back in fully with the main population

      • The marking doesn't affect the survival rates of the marked individuals

      • The marking remains visible throughout the sampling and doesn't rub off

      • The population stays the same size during the study period (i.e. there are no significant changes in population size due to births and deaths and there are no migrations into or out of the main population)

Ecosystem Functioning & Sustainability

  • Sustainability of ecosystems:

    • Sustainability is a fundamental property of ecosystems.

    • It refers to the ecosystem's ability to maintain balance and productivity over time.

    • Ecosystems naturally regulate themselves to sustain life within them.

  • Balanced inputs and outputs:

    • In a steady-state ecosystem, inputs and outputs are balanced

      • Inputs include energy, nutrients and water entering the ecosystem

      • Outputs include energy, nutrients and waste leaving the ecosystem

    • This balance ensures the ecosystem's long-term stability and resilience

    • These inputs and outputs can be illustrated with ecosystem flow diagrams

    • Flow diagrams demonstrate the movement of energy and nutrients within ecosystems

    • These diagrams highlight the interconnectedness of biotic and abiotic factors within an ecosystem

  • Evidence of long-term sustainability:

    • Some ecosystems have persisted for millions of years, indicating their long-term resilience and sustainability

      • Tropical rainforests are a prime example of long-term sustainable ecosystems

      • Despite changes in climate and other external factors, these ecosystems have endured

      • Its great biodiversity and complex interactions contribute to its resilience

  • Human impacts on ecosystem stability:

    • Human activity can disrupt the stability of ecosystems, leading to tipping points

    • Tipping points are critical thresholds where small changes can trigger significant shifts in the ecosystem

    • These shifts can lead to the collapse of the original ecosystem and the establishment of a new equilibrium

    • The example used is Deforestation in the Amazon Rainforest

      • Impact on climate: Reduced transpiration leads to a decrease in the amount of water vapour in the local atmosphere

      • Feedback loop: Deforestation can create a positive feedback loop where reduced precipitation leads to further forest loss

      • New equilibrium: If deforestation continues at its current rate, it may not be long until the Amazon Rainforest reaches a new equilibrium state

  • Understanding the role of keystone species:

    • Keystone species are organisms within an ecosystem that have a disproportionately large impact on the structure and function of the ecosystem relative to their abundance

    • The presence of keystone species can help regulate the population sizes of other species and maintain higher levels of biodiversity

    • Examples mentioned are:

      • Purple sea stars

      • African elephants

  • Human impacts on biosphere integrity:

    • The planetary boundaries model identifies nine key Earth system processes essential for maintaining a stable planet

      • These boundaries represent safe operating limits for human activity to prevent irreversible environmental changes

    • Changes beyond these boundaries can lead to detrimental effects on Earth's systems and human well-being

    • Biosphere integrity (one of the nine critical processes) refers to the overall health and diversity of life on Earth

    • Human activity has significantly impacted biosphere integrity, pushing it beyond critical thresholds

    • Disturbances to ecosystems have led to severe loss of biodiversity, disrupting ecological balance and resilience

    • Ecosystems and species diversity are highly interlinked, with each depending on the other:

    • Evidence from extinction rates provide tangible evidence that the planetary boundary for biosphere integrity has been crossed

    • Human-induced factors such as habitat destruction, pollution and climate change have driven extinction rates to unprecedented levels
      Addressing ecosystem damage and species loss is essential to avoiding reaching these critical tipping points

    • Avoiding critical tipping points:

      • Reversing the loss or "erosion" of biosphere integrity is crucial to preventing catastrophic shifts in Earth's ecosystems

      • Addressing ecosystem damage and species loss is essential to avoiding reaching these critical tipping points

      • Ecosystem conservation efforts aim to preserve the structure, function and diversity of ecosystems

      • Preserving species is a key factor in maintaining ecosystem integrity

      • Various conservation strategies can help to protect ecosystems and preserve species diversity, including:

        • Habitat conservation

        • Species conservation

        • Sustainable resource management