Abiotic conditions affect survival
Definition
Abiotic factor
An abiotic factor is a non-living physical or chemical feature of the environment that affects organisms.
- The organisms found in a habitat depend on whether its physical and chemical conditions allow them to survive, grow and reproduce.
- Different species have different tolerances, so a condition favourable to one species may restrict another.
- Temperature affects enzyme-controlled reactions, including photosynthesis, respiration and growth.
- At low temperatures these reactions are often slower, while temperatures above an organism’s tolerance can disrupt enzyme activity and survival.
- Temperature can also change the availability of liquid water and the amount of oxygen dissolved in water.
- Light intensity affects the rate of photosynthesis and therefore the amount of material plants can make for growth.
- Where light is limited, fewer plants may grow or shade-tolerant species may become more common.
- Many woodland plants flower before the trees develop a full leaf canopy, allowing them to use the stronger light that reaches the ground in early spring.
- Moisture supplies water for photosynthesis, transport and maintaining cell turgor.
- A shortage of water can cause wilting and stomatal closure, reducing carbon dioxide uptake and photosynthesis.
- Waterlogged soil can restrict oxygen around roots and therefore limit aerobic respiration.
- Soil acidity, measured using pH\mathrm{pH}pH, affects the availability of mineral ions and the activity of roots and soil microorganisms.
- Some species grow better in acidic soils and others in more alkaline soils, so differences in soil pH\mathrm{pH}pH can change the plant community.
- Mineral ion availability affects growth because plants need substances such as nitrates to make amino acids and proteins.
- Dissolved oxygen affects aquatic organisms that need oxygen for aerobic respiration, so a fall in oxygen concentration can reduce their survival.
Exam technique
- For an environmental change, connect the named factor to a biological process, then explain its effect on survival, growth or reproduction.
- For example, reduced light can lower photosynthesis, leaving less material for plant growth and potentially less food for herbivores.
Biotic factors involve other organisms
Definition
Biotic factor
A biotic factor is an influence of living organisms on other organisms in a community.
- Food availability affects how much energy and raw material consumers can obtain.
- With less food, some organisms grow more slowly, produce fewer offspring or die, so the population may decrease.
- An increase in food may allow more individuals to survive and reproduce, provided another resource does not become limiting.
- Predators reduce prey numbers by killing and eating individuals.
- Introducing a predator can reduce a prey population, whereas removing a predator may allow that population to increase.
- Pathogens can cause disease that reduces survival or reproduction, changing the abundance of affected species.
- Competitors reduce the share of a limited resource available to each organism.
- A newly arriving species may use the same food or growing space as an existing species and reduce its abundance.
- The effect of a biotic change depends on the other organisms present, so the response should be explained using the relationships given.
Common Mistake
A biotic factor is an influence involving organisms, such as predation or food availability; temperature and light intensity are abiotic factors.
Factors change abundance and distribution
- Abundance describes how many organisms of a species occur in an area, whereas distribution describes where they occur.
- A plant may be abundant in damp ground but absent from a nearby dry patch because the patches offer different conditions.
- Several factors act together, so a site with suitable light may still lack a species if its soil is too dry or the species has not reached it.
- A limiting factor restricts population growth when a required resource or suitable condition is insufficient.
- As a population grows, food or space may become harder to obtain, so its growth can slow and its size may fluctuate around the level the habitat can support.
- The levelling-off shown in these diagrams represents restricted net population growth, not the complete stopping of births and deaths.
- A change in resources or conditions can change the population size that the habitat can support.
- Across larger regions, temperature and rainfall help determine which broad communities of plants and animals can occur.
Practical
Investigating a habitat gradient
- Aim: investigate whether the abundance of one plant species changes with light intensity from open grassland towards a shaded area.
- Apparatus: use a tape measure, a quadrat of fixed area, a light meter, a plant identification guide, a recording sheet and instruments for checking soil moisture and temperature.
- Method, field sampling:
- Identify the plant reliably and choose a transect that crosses the change in shade without crossing a major change in soil type or management.
- Lay the tape in a straight line from the open area into shade and mark fixed sampling distances, for example every 2 m2\,\mathrm{m}2m.
- Place the quadrat on the same side of the tape at each distance and count the rooted individuals of the chosen species.
- Use the same quadrat area and the same boundary rule, such as including plants rooted on the top and left edges but excluding those on the bottom and right edges.
- For a plant that is difficult to separate into individuals, estimate percentage cover using a gridded quadrat and use that measure consistently throughout.
- Record light intensity at the same height and orientation in each quadrat, keeping your body from shading the sensor.
- Record soil moisture and temperature at the sampling points so that alternative explanations can be considered.
- Repeat the transect along several parallel lines across the same habitat transition, using the same sampling intervals.
- Take measurements within a short period under similar weather conditions, because changing cloud cover can alter light readings.
- Results and processing:
- Retain the paired light and abundance measurements and plot a scatter graph with light intensity on the horizontal axis and abundance on the vertical axis.
- Describe whether abundance tends to rise, fall or show no consistent change with light intensity, and support the statement with values from the results.
- Sampling design:
- A transect is useful for finding changes along an environmental gradient because its sampling positions follow that gradient.
- To compare the overall abundance of a species in two areas, use random quadrat positions within each defined area to reduce selection bias.
- Choose positions from random coordinates rather than placing quadrats where the chosen plant is easiest to see.
- Use the same quadrat area, identification method and counting rules in both habitats, and take enough samples to represent variation within each area.
- Evaluation:
- A field correlation does not establish that light is the only cause, because moisture, soil conditions and other factors may change along the transect as well.
- An association between a factor and abundance supports an explanation, but a controlled investigation is needed to isolate the effect of that factor more securely.
- Safety and care: check for uneven ground and harmful plants, minimise trampling, avoid uprooting plants and wash hands after fieldwork.
Self review
- How can low light intensity reduce plant growth?
- Why can waterlogging reduce the survival of roots?
- How can an increase in food change a consumer population?
- How does distribution differ from abundance?
- Why does a correlation between light and abundance not prove that light is the only cause?
