Ecology Notes

Biosphere, Ecosystems, and Habitats

• The biosphere includes all parts of the earth and its atmosphere inhabited by organisms.
• Within the biosphere are numerous ecosystems; major ecosystems are called biomes.
• Within an ecosystem, an organism lives in a habitat and can occupy a smaller area called a microhabitat.

Common Definitions

• Abundance: The number of a particular organism.
• Distribution: Where an organism lives in a habitat and how it is arranged.
• Ecosystem: An environment where organisms interact with each other and their physical/chemical surroundings.
• Habitat: A place where an organism lives.
• Population: A group of organisms of one species living and interacting in a particular area/habitat.
• Community: A group of organisms of different species living and interacting in a particular area/habitat.
• Niche: The role of an organism in its habitat.

Biotic and Abiotic Factors

• Biotic factors: The living elements in a habitat.
• Abiotic factors: The physical/chemical (non-living) factors in a habitat.
• Predator: An organism that hunts and eats other organisms (prey).
• Prey: An organism hunted and eaten by other organisms (predator).

Ecosystem Evolution (Succession)

• Succession: A series of changes in a community of organisms, where species are replacing and succeeding each other over a period of time.
• During succession, both biotic (e.g., plant communities) and abiotic conditions (e.g., edaphic/soil factors) change over time.
• Three types of succession:
  1. Primary succession
  2. Secondary succession
  3. Deflected succession

Primary Succession

• Occurs in land where there is no soil (e.g., barerock or sand dune).
  1. Colonization by pioneer species:
     • Lichens and mosses colonize the area as pioneer species (opportunists).
     • These species can grow in little or no soil.
     • They break up barerock or sand by secreting acids or penetrating using hyphae, forming thin soil.
     • Thin soil allows small plants to grow.
  2. Continuation:
     • Pollination brings in seeds of small plants, which grow in thin soil.
     • When they die, microbes decompose them to form humus, adding minerals to the soil.
     • The soil thickens and retains water.
     • Other dominant plant species move in and outcompete previous species.
     • This process repeats, and humus increases in soil, eventually supporting larger trees with root systems.
  3. Climax community:
     • A stable community is reached, with plants and animals.
     • High biodiversity and a balanced equilibrium of species are achieved.
     • Dominant and co-dominant species give the community its name (e.g., Oak forest).
     • The climax community is self-sustaining and stable under environmental changes.
• Examples of dominant species during primary succession:
  • Sand sedge, marram grass
  • Gorse, heather, sea buckthorn
  • Holly, birch, oak
• Role of birds in primary succession:
  • Birds act as pollinators.
  • Birds distribute seeds in their feces.
  • Bird feces improve soil quality and add mineral ions.
  • Birds eat parasitic insects.
• Factors affecting the time taken for primary succession:
  • Soil fertility
  • Climate
  • Soil pH
  • Soil color
  • Rainfall
  • Humus (%)
  • Temperature

• Climatic climax community: The climax community determined by the climate in that area.
• Characteristics of pioneer species:
  • Can withstand harsh environments.
  • Can grow in little or no soil.
  • Have low mineral requirements.
  • Have fast life-cycles, produce lots of seeds, and have wide seed dispersal mechanisms.

Secondary Succession

• Development of an ecosystem from existing soil that is clear of vegetation.
• Occurs after floods, fires, or human intervention.
• Similar to primary succession, but the soil, seeds, and animals are already present.
• Better adapted plants and animals move in and outcompete less adapted ones, until a climax community is reached.
• Differences between Primary and Secondary Succession:
  1. Primary succession occurs from no soil (e.g., barerock/sand dune), while secondary succession occurs on land that has soil but is cleared of vegetation.
  2. Pioneer species in secondary succession are larger plants, whereas in primary succession, they are algae and mosses.
  3. Primary succession takes longer to reach a climax community than secondary succession.

Deflected Succession

• Human activities can prevent succession, stopping a climax community from developing.
• When succession is stopped artificially like this, the climax community is called a plagioclimax. This process is known as deflected succession.
• Example: Sheep grazing in Britain prevents many grasslands from developing into woodlands.

Effect of Abiotic and Biotic Factors

• The abundance and distribution of organisms are determined by biotic and abiotic factors.

Abiotic Factors

• Light:
  • Affects the rate of photosynthesis.
  • Plants growing in low light intensity are adapted with more chlorophyll or different ratios of pigments.
  • Some plants reproduce earlier in the year to avoid shade by larger plants.
  • Light intensity affects plants, which affects herbivore animals and animals that rely on plants for habitat.
• Temperature:
  • Affects the activity of enzymes involved in metabolism.
  • High temperature causes enzyme denaturation. Low temperature (below $0^\circ C$) causes water content of cytoplasm to freeze, stopping enzyme activity.
  • Animals and plants are adapted to deal with harsh temperatures.
• Wind and Water Current:
  • Wind increases cooling and water loss from organisms.
  • Hurricanes damage trees and affect populations.
  • Only strong swimmers survive high water currents.
  • Wind speed may also favor pollination.
• Water Availability:
  • Water affects the germination and growth of plants, hence of animals dependent on plants.
• Oxygen Availability:
  • When water is cold/fast-flowing, enough oxygen will dissolve.
  • When water is hot/still, dissolved oxygen content drops.
  • Oxygen content affects respiration and growth.
• Edaphic (soil structure and mineral content):
  • Sand is dry and easily drained (water passes through it, washing away minerals), so sand is easily leached. Plants with long roots can grow here.
  • Clay doesn't leach easily but gets waterlogged easily.
  • Loam is an ideal soil type, not easily waterlogged or leached.

Biotic Factors

• Predation:

  • Predator-prey population graph oscillates.
    Size

  Time

  • Initially, prey population is high, with more food available for predators.
  • Predator population increases as the reproduction rate is greater than the death rate.
  • When more predators are present, more prey are hunted, so their death rate is greater than their reproduction rate; hence population size decreases.
  • With a low prey population, there is less food and energy for predators, so the death rate becomes greater than the reproduction rate, reducing the predator population.
  • This pattern repeats.

• Finding a Mate:

  • The abundance of sexually reproducing organisms is greatly affected by the availability of mates.

• Territory:

  • Used to ensure a breeding pair has enough resources to raise young.
  • A territory is a place held and defended by animals from other animals of the same or different species.

• Parasitism and Disease:

  • Diseased animals become weakened and cannot reproduce successfully.
  • Sick predators cannot hunt well, and sick prey are easily caught.
  • Diseases spread in feces.
  • Parasites feed on living organisms, consuming their energy, affecting organisms' energy for reproduction/growth.
  • Areas of high biodiversity are less affected because predators have many prey options.

• Density-Dependent and Independent Factors

  • The abiotic and biotic factors affecting abundance and distribution may be density-dependent or density-independent.
  • Density-independent factors have the same effect regardless of population density/size, e.g., mainly abiotic factors such as temperature. Density-independent factors limit distribution.
  • Density-dependent factors depend on the population density, e.g., mainly biotic factors such as parasitism and disease, as these spread quicker if population density is higher. Density-dependent factors limit abundance.

Intraspecific and Interspecific Competition

• Intraspecific competition: Competition between members of the same species, within the same niche, for a limited resource.

• Interspecific competition: Competition between members of different species for the same limited resource. Their niches must overlap to create competition.

• Species with a faster reproduction rate and greater density will outcompete the other species and reduce its abundance.

• Occupying different niches avoids competition. Two species sharing the same niche cannot coexist.

• P. aurelia and P. caudatum Competition:

  • They are of greater numbers when cultured separately.
  • When cultured together, there is competition for bacteria/food.
  • Since they share the same niche (of feeding on bacteria), they cannot coexist, and P.aurelia outcompetes P.caudatum for bacteria.

Distribution

The distribution of an organism in a habitat usually enters into one of three main patterns:

• Uniform distribution: This occurs when resources are thinly but evenly spread or when individuals of a species are hostile to each other. The territories may be very large or very small, for example, nesting penguins, polar bears, or hawks.
• Clumped distribution: This is the most common distribution seen with herds of animals or groups of plants and animals that have specific resource requirements and therefore group together in areas where those resources are found, for example, a caravan of camels, a pod of dolphins, or a stand of palms.
• Random distribution: This is the result of plentiful resources and no hostility. It is seen, for example, in weeds in a grass lawn.

Abundance and distribution may vary through the day and also through the year. It may be important to repeat measurements at different times.

Ecological Techniques

(Measuring abundance and distribution)

• Measuring abundance and distribution requires one to do sampling.
• Samples are small parts of field, taken to be representative of the whole field when scaled up.
• Sampling is done because it is very difficult to do work on the whole field.
• Sampling can be Random or Non-Random
  1. To avoid bias in your results, the sample should be random. For example, if you were looking at plant species in a field, you could pick random sample sites by dividing the field into a grid using measuring tapes and using a random number generator to select coordinates.
  2. However, sometimes it's necessary to take a non-random sample. Non-random samples can be used in habitats where there's a lot of variety in the abiotic features and/or distribution of species in the habitat and you want to make sure all the different areas or species are sampled.
  3. Systematic sampling is a type of non-random sampling. This is when samples are taken at fixed intervals, often along a line. For example, quadrats (see below) could be placed along a line (called a transect, see next page) in a habitat where the abiotic factors change gradually from one end of the sample area to the other. An example could be the coast where abiotic factors, such as soil salinity, change gradually as you get further away from the sea. A habitat that changes like this is said to have an environmental gradient.
• Quadrats are used for measuring abundance, while transects + quadrats are used to measure distribution.

Quadrats (Random Sampling of Immobile Organisms)

• A square framed tool used for sampling.

• Types of quadrats:

  • Frame quadrat
  • Point quadrat

• Frame Quadrat

  1. Placed at random using random sampling technique, at coordinates determined using a number generator.
  2. Samplings are repeated with many quadrats.
  3. Abundance is measured quantitatively by counting no. of organisms (individual counts) or measuring area covered within the quadrat (percentage cover).
  4. Abundance can also be measured qualitatively using the ACFOR scale.

  Limitations of frame quadrats:

  • Limitations to the area you can sample
  • The randomness (or not) of the sampling sites
  • Decisions made about whether to include or exclude organisms partly covered by the quadrat.

  ACFOR scale :

  • A = abundant
  • C = common
  • F = frequent
  • O = occasional
  • R = rare.

  Limitations of the ACFOR scale:

  • The measurements given on the scale are subjective; two people would probably never give exactly the same scores.
  • There are no set definitions of the terms; for example, how common is common?
  • Species can easily be rated according to how obvious they are rather than how abundant they are.

• Point Quadrat

  1. A point quadrat is a horizontal bar on two legs with a series of holes at set intervals along its length.
  2. Point quadrats are placed on the ground at random points within the area you're investigating.
  3. Pins are dropped through the holes in the frame, and every plant that each pin touches is recorded.

  If a pin touches several overlapping plants, all of them are recorded.
  5) The percentage cover of a species can also be measured by calculating the number of pins that touch a given species as a percentage of the total number of pins dropped.

• Permanent quadrats

  • These are left in place all the time. Can be used to reliably take samples from season to season or year to year.
  • Eg. for penguin nests.

• Aerial quadrats

  • Aerial quadrats are a high-tech version of this technique. Aerial photographs of large areas of land are divided into measured areas and surveyed for numbers of species. The technique is used, for example, when surveying large areas of desert or savannah.

Transects (Non-Random, Systemic Sampling)

• Systematic sampling is a type of non-random sampling. This is when samples are taken at fixed intervals, often along a line.
  You can use lines called transects to help find out how plants are distributed across an area, e.g., how species change from a hedge towards the middle of a field.
• Three types of transect:
  1. Line transects - a tape measure is placed along the transect and the species that touch the tape measure are recorded.
  2. Belt transects data is collected along the transect line using frame quadrats placed next to each other.
  3. Interrupted transects - instead of investigating the whole transect of either a line or a belt, you can take measurements at intervals. E.g. by placing point quadrats at right angles to the direction of the transect at set intervals along its length, such as every $2 m$.

Transects can be used in any ecosystem, not just fields. For example, along a beach.

Measuring Abiotic Factors

• Data loggers combine ways of measuring several different aspects of a habitat or ecosystem. They are relatively cheap and portable and are commonly used.
• Topography the shape and features of the Earth's surface. Includes slope of incline of hill.