4.5 - Abiotic & Biotic Factors

Abiotic and biotic factors

• The organisms in a community are shaped by two kinds of environmental factor.
• Abiotic factors are the non-living parts of the environment - the physical and chemical conditions.
• Biotic factors are the living parts of the environment - the effects that organisms have on one another.

Abiotic factors

• Light intensity
• Temperature
• Moisture (water) level
• pH and mineral content of the soil
• Wind intensity and direction
• Carbon dioxide level (matters for plants)
• Dissolved oxygen level (matters for aquatic animals)
• Pollutants, such as toxic chemicals

Biotic factors

• Availability of food
• New predators arriving
• New pathogens (disease) arriving
• Competition - One species outcompetes another for a shared resource such as food or space

Effects on distribution and abundance

• Both abiotic and biotic factors affect the distribution of organisms (where they're found in a habitat) and their abundance (the size of a population).
• A change in a single factor can change a population's size. Because organisms in a community depend on one another, this can have knock-on effects on other populations.
• An organism is usually more common where the factors suit it - e.g. daisies are more common in the open parts of a field, where there's more light than under the shade of trees.

Abiotic changes

• A fall in light intensity, temperature or carbon dioxide level lowers the rate of photosynthesis, so plants grow less and the plant population can fall.
• A drop in soil minerals such as nitrates has the same effect through poorer plant growth.
• The animals that feed on those plants are then affected in turn.

Biotic changes

• New predator - Can cut a prey population.
• New pathogen - Can cause a sharp drop in the species it infects.
• Stronger competitor - Can drive a weaker species' numbers down, e.g. grey squirrels outcompeting red squirrels for the same food.

Measuring abiotic factors

• Abiotic factors are measured directly in the field, giving a number for the condition an organism actually experiences.
• These numbers can then be compared between places or over time.

Equipment for each factor

• Light intensity - Measured with a light meter
• Soil moisture - Measured with a moisture meter
• pH - Measured with a pH meter or indicator
• Temperature - Measured with a thermometer

Reducing errors in readings

• Readings can carry errors - for example accidentally shading a light sensor, or taking a single reading that doesn't represent the whole area.
• To reduce these errors:
  • Take measurements carefully and the same way each time - e.g. holding the sensor identically for every reading.
  • Repeat readings and take several across the area, then average them so the values give a fair picture of the conditions.

Estimating population size by capture-recapture

• Capture-recapture (also called mark-release-recapture) estimates how many animals of one species live in an area.
• It's used for animals that move about - such as woodlice, snails, beetles or butterflies - which can't simply be counted in a quadrat.

The five steps

1. Capture a first sample of the animals (e.g. with a net, a pooter or a pitfall trap) and count them.
2. Mark each one harmlessly - for example a small spot of non-toxic paint on the underside, where predators can't see it.
3. Release the marked animals back where they were caught.
4. Leave time (often a day or two) for them to mix back in with the rest of the population.
5. Capture a second sample from the same area in the same way. Count both the total number caught and how many of them are marked.

---

Formula for population size

The idea behind it is that the fraction of the second sample that's marked is the same as the fraction of the whole population that was marked in the first sample.

---

Worked example - Population of woodlice

40 woodlice are caught, marked and released. The next day, 60 woodlice are caught and 12 of them are marked. Estimate the population size.

---

Assumptions for a reliable estimate

• No population change - No births, deaths or migration in or out between the two samples, so the population stays the same size.
• Marks stay on - The marks aren't lost, for example washed off by rain.
• Marks don't affect survival - A mark that makes an animal easier for predators to spot leaves fewer marked animals to recapture, so the number marked in the second sample is too small and the estimate comes out too high.
• Marked animals mix evenly - They've had time to mix back in with the rest of the population before the second sample is taken.