7.4 Populations in ecosystems

What is the definition of a community?

• All populations of different species.

• Living in the same habitat.

• At the same time.

What is the definition of an ecosystem?

• A community of organisms.

• Plus the non-living (abiotic) components of their environment.

What are two key characteristics of ecosystems?

• They can range from very small to very large.

• They are dynamic systems (populations rise and fall over time).

What is the definition of an ecological niche?

• The specific role of a species within its habitat.

• Includes: What it eats, where and when it feeds.

• Governed by adaptations to abiotic and biotic conditions.

What are the two advantages of species occupying different niches?

• Less competition for resources (food, space).

• Prevents competitive exclusion: If two species occupy the same niche, one will outcompete the other.

What is the definition of carrying capacity?

• The maximum stable population size.

• That an ecosystem can support.

What are the two main categories of factors that influence carrying capacity?

1. Abiotic factors

2. Interactions between organisms (biotic)

List four examples of abiotic factors.

• Light intensity

• Temperature

• Soil pH & mineral content

• Humidity

Name the three types of biotic interactions.

1. Interspecific competition (between different species)

2. Intraspecific competition (within the same species)

3. Predation (predators kill and eat prey)

Describe the general effect of favourable abiotic conditions on a population.

• Organisms are more likely to survive and reproduce.

• This increases the carrying capacity.

Explain the specific effect of increasing light intensity on an ecosystem, step-by-step.

• Step 1: Increases the rate of photosynthesis in plants.

• Step 2: This increases the carrying capacity for plant species.

• Step 3: This increases the number and variety of habitats, niches, and food sources.

• Step 4: This increases the carrying capacity for animal species.

How does interspecific competition affect the population size of the competing species?

• Reduces the availability of a named resource (e.g., food, space) for both species.

• This limits their chances of survival and reproduction.

• Result: Reduces the population size of both species.

What is the potential outcome if one species is better adapted in an interspecific competition?

• The better-adapted species will outcompete the other.

• Result: The population size of the less well-adapted species declines.

• This can lead to its local extinction.

Explain the first phase of how intraspecific competition regulates population size.

• As population size increases, resource availability per individual decreases.

• This causes competition to increase.

• Result: Chances of survival and reproduction decrease, so the population size decreases.

Explain the second phase of how intraspecific competition regulates population size.

• As population size decreases, resource availability per individual increases.

• This causes competition to decrease.

• Result: Chances of survival and reproduction increase, so the population size increases

What happens when the prey population increases?

• Predators have more food.

• Result: More predators survive and reproduce, so the predator population increases.

What is the consequence of an increasing predator population?

• More prey are killed and eaten.

• Result: Fewer prey survive and reproduce, so the prey population decreases.

What happens when the prey population decreases?

• Predators have less food.

• Result: Fewer predators survive and reproduce, so the predator population decreases.

What is the consequence of a decreasing predator population?

• Fewer prey are killed and eaten.

• Result: More prey survive and reproduce, so the prey population increases (and the cycle repeats).

What is the key pattern of predator and prey population sizes over time?

• Their populations fluctuate in cycles.

• The predator population peak occurs after the prey population peak (there is a lag time).

How do you begin estimating the population of a slow-moving organism using quadrats?

• Divide the area into a grid (e.g., using two tape measures at right angles).

How do you select where to place the quadrat?

• Generate random coordinates using a random number generator.

What do you do at each selected coordinate?

• Place a quadrat.

• Count the number or frequency of the named species inside it.

How do you ensure a reliable estimate?

• Repeat the process a large number of times (e.g., 10 or more).

• Calculate the mean number per quadrat.

What is the final calculation for total population size?

• Population size = (Total habitat area / Quadrat area) x Mean per quadrat

Outline the four key steps of the mark-release-recapture method.

1. Capture, mark, and release a sample of the population.

2. Ensure marking is not harmful and doesn't affect survival and predation.

3. Allow sufficient time for marked individuals to redistribute evenly and randomly within the whole population.

4. Capture a second sample and count how many are marked.

What is the formula for estimating the total population size (N)?

• N = (Number in first sample x Number in second sample) / Number marked in second sample

What is an important modern alternative to physical marking?

• Recording an organism's DNA base sequence.

• A recaptured organism with an identical sequence is identified as previously "marked."

What is the core assumption used to derive the mark-release-recapture equation?

• The proportion of marked individuals in the second sample.

• Is equal to the proportion of the first (marked) sample in the total population.

What is the proportional equation based on this assumption?

• (Number marked in sample 1) / (Total population) = (Number marked in sample 2) / (Total in sample 2)

Rearrange this proportion to get the standard population size (N) formula.

• N = (Number in sample 1 x Number in sample 2) / (Number marked in sample 2)

If 17 lizards are marked and released, and a later sample of 20 contains 10 marked lizards, what is the estimated population?

• Calculation: (17 x 20) / 10 = 34 lizards.

What must happen regarding the distribution of marked individuals?

• There must be sufficient time for marked individuals to redistribute evenly and randomly within the whole population.

What must be true about the marking process?

• The mark is not removed.

• The mark is not harmful does not affect the organism's chance of survival or predation.

What must be true about movement into and out of the population?

• There must be limited or no immigration or emigration.

What must be true about the population size between samples?

• There must be no births or deaths.

• OR the birth rate and death rate must be equal (population size is constant).

Why is the mark-release-recapture method often unreliable in very large areas?

• It is unlikely that organisms will distribute randomly/evenly.

• Result: There is a much lower chance of recapturing the marked individuals.

What is the first event in primary succession?

• Colonisation by pioneer species (the first species to colonise a barren area).

What is the key role of pioneer species (and species at each stage)?

• They change the abiotic conditions.

• Example: They die and decompose, forming soil (humus/organic matter) that retains water.

What are the two consequences of these changed abiotic conditions?

• The environment becomes less hostile / more suitable for other species with different adaptations.

• It becomes less suitable for the previous species, so better-adapted species outcompete them.

What is the general trend for biodiversity during succession?

• Biodiversity increases as succession progresses.

What is the end point of succession called?

• The climax community – the final, stable community where no further succession occurs.

What is true about the species present in a climax community?

• The same species are present over a long period.

• Result: It is a stable community.

What is true about the abiotic factors?

• Abiotic factors remain fairly constant over time.

What is true about the population sizes?

• Populations are fairly stable, fluctuating around the carrying capacity of the environment.

What is conservation?

• The protection and management of species and habitats in order to maintain biodiversity.

How is the conservation of habitats linked to the management of succession?

• It involves preventing further succession to stop a climax community from forming.

What is one method for preventing later stages of succession?

• Removing or preventing the growth of species associated with later stages of succession.

• Example: Allowing grazing by animals.

What is the term for an ecosystem preserved at a non-climax stage?

• A plagioclimax.

What is the primary goal of managing succession in this way?

• To preserve the ecosystem in its current stage of succession.

What are the two key benefits of preventing later succession?

• Early species are not outcompeted by later, more competitive species.

• Result: Specific habitats and niches are not lost.

What is the core conflict between human needs and conservation?

• Human demand for natural resources (e.g., timber, land, water).

• Result: This leads to habitat destruction and biodiversity loss.

Why is conservation necessary despite this conflict?

• To protect habitats, niches, species, and biodiversity.

What is the importance of managing this conflict effectively?

• It maintains the sustainability of natural resources.

What is the definition of sustainability in this context?

• Meeting current needs.

• Without compromising the ability of future generations to meet their needs.