3.7.4 populations in ecosystems

What is ecology?

The study of inter-relationships between organisms and their environment. The environment includes abiotic factors (non-living) and biotic factors (living) components.

What is the biosphere?

The life supporting layer of land, air and water that surrounds the Earth.

What is an ecosystem?

They are dynamic systems made up of all the interacting abiotic and biotic factors in a specific area. There are two major processes: the flow of energy through the system and the cycling of nutrients within the system.

What is a community?

The sum of all the organisms living in an ecosystem.

What is a population?

A group of interbreeding individuals of the same species in the same space and time that share the same gene pool.

What is the carrying capacity?

A certain size of population of a species that an ecosystem supports.

What is a habitat?

Place where a community of organisms live, this can include microhabitats with microclimates. Characterised by physical conditions and other types of organisms present. There are many habitats in an ecosystem.

What is an ecological niche?

It describes how an organism fits into its environment, and where the organism lives and what it does there, and includes all biotic and abiotic conditions to which an organism is adapted to survive.

Why can population size vary?

As a result of abiotic factors, or from interactions between organisms, such as interspecific and intraspecific competition and predation.

What is population density?

Number of individuals per unit area of a chosen habitat.

What is population density affected by?

Birth - number of new individuals born into a population

Immigration - number of new individuals joining a population

Death - individuals in a population that die

Emigration - number of individuals leaving a population

When must a logarithmic scale be used to plot a growth curve?

Where the population growth rapidly over a short period of time, for example with microorganisms, because there is a large range of data with very small and very large numbers so it is difficult to work out a graphical scale. Each number is converted to a logarithm before being plotted so it easily fits onto the same graph.

How is a log scale identified on a graph?

There are not equal intervals between the numbers on the y axis of the graph.

How to calculate log and reverse log?

Log: press log10 button, then type in the number

Reverse log: press shift then log to get ln, then type in number

What happens to population growth in an ideal environment?

It grows exponentially, and the growth curve is smooth and continues to increase over time. This occurs when there are no limiting factors

What is a logistic growth curve and how does it occur?

An s-shaped curve of population growth. It reflects the fact that in all populations, exponential growth is curtailed by limiting factors, such as disease, competition or food limitations. As competition increases and resources become scarce, populations reach their carrying capacity (K) that causes the growth rate to slow to 0. This capacity can be sustained over a relatively long period which is determined by these limiting factors.

Describe the 4 growth phases in a population:

1. Lag phase - there is no increase in number of organisms

2. Log phase - there is exponential increase in number of living organisms

3. Stationary phase - plateau in number of organisms, rate of birth and death is roughly equal

4. Death phase - exponential decrease in number of living organisms

Explain the population growth stages:

• Lag phase - population growth begins slowly from a few individuals

• Log phase - ideal conditions are reached and maximum growth rate occurs. At s phase, Growth begins to slow as limiting factors such as food, water and space emerge.

• Stationary phase - carrying capacity for a population is reached and population number is stable

• Death phase - a sudden change in environment means it can no longer support the population, such as a drought, and population will crash and cause the process to start again.

What abiotic conditions affect population size?

• Temperature - each species has a different optimum temperature at which it is best to survive, further away from the optimum fewer species are able to survive

• Light - rate of photosynthesis increases as light intensity increases, causing faster plant growth and greater carrying capacity, so the carrying capacity of animals feeding on plants increases

• pH - affects enzyme action, and at optimum pH enzyme activity is greater

• Water/humidity - humidity affects transpiration rates and evaporation of water from animals, water scarcity leads to a few well adapted populations.

Outline a method for investigating the effect of an environmental factor into species distribution (RP 12):

1. Choose a 5×5m area to take samples from, and use a random number generator to make 10 sets of random coordinates

2. Use 2 tape measures to create a set of axes off which coordinates can be read

3. Place the quadrat at each of the coordinates, placing the bottom left corner on the coordinate each time

4. Record percentage cover of the species (count a square if more than half of it is covered).

5. At each coordinate a measure of the independent variable should be taken, eg using a photometer to measure light intensity

What is intraspecific competition?

Occurs when individuals of the same species compete with one another for resources such as food, water, breeding sites etc. The availability of these resources determines population sizes, and greater availability leads to larger population.

What are two examples of intraspecific competition?

• Limpets compete for algae which is their primary food source

• Oak trees competing for resources such as water, mineral ions and light

What is interspecific competition?

Occurs when individuals of different species compete for resources such as food, light and water. One species will normally have a competitive advantage over the other so the numbers of this will gradually increase in size while the population of the other will diminish. This may lead to complete removal of one species, known as the competitive exclusion principle.

What is the competitive exclusion principle?

States that two species competing for the same limited resources cannot coexist at constant population sizes; one will outcompete the other, leading to its extinction. No two species can occupy the same niche indefinitely when resources are limiting.

What is a predator?

An organism that feeds on another organism, known as prey. As they have evolved they have become better adapted to capture prey, and prey become more adept at avoiding predators. They have evolved alongside each other, and if either of them had not matched the adaptations of the other it would probably have become extinct.

Why are predators not able to catch all prey in a natural environment as opposed to a lab setting?

The area over which the population can travel is far greater and the environment is more diverse and there are more refuges. Some prey can escape predation because if there are few they are harder to find and catch, so the population rarely becomes extinct.

What are the disadvantages of using labs and natural fieldwork to study predator/prey populations?

Labs do not necessarily reflect what happens in the wild - predators may be more able to catch the prey.

Natural - it can be hard to obtain reliable data because not all the individuals in a natural population can be counted, and size can only be estimated from sampling and surveys.

Describe the stages of the predator prey relationship:

• Predators eat the prey, reducing the population of prey

• With fewer prey available predators are in greater competition with each other for the prey that are left

• Predator competition is reduced as some individuals are unable to obtain enough prey for their survival to to reproduce

• Fewer prey are therefore eaten so more survive and reproduce, increasing the population

• Predator population increases as more prey are available

Why might fluctuations of predator and prey numbers be less severe in natural ecosystems?

Organisms eat a range of foods so the effects may be less large

What other factors also influence cyclic fluctuations in populations?

Disease and climatic factors.

Why are periodic population crashes important?

Important in evolution, as there is a selection pressure meaning that individuals that are able to escape predators, or withstand disease are more likely to survive to reproduce, so the population evolves to be better adapted to prevailing conditions.

What is abundance?

The number of individuals of a species in a given space.

What are the two main techniques used for studying habitats?

Random sampling using frame or point quadrats

Systematic sampling using a belt transect.

What is a point quadrat?

Consists of a horizontal bar supported by two legs, and at set intervals along the bar there are ten holes where a long pin is dropped and the species that touch it are counted.

What is a frame quadrat?

A square frame divided into equal subdivisions, and designed so it can be folded. The abundance of species is then recorded.

What are the three factors to consider when using quadrats?

• Size of quadrat to use - depending on the size of animals/plants being sampled and how they are distributed. Larger species require larger quadrats. If species is uneven a large number of small quadrats is needed

• Number of sample quadrats to record in the study area - larger samples lead to more reliable results but this must be balanced with the time available to count the organisms. More species means more samples are needed to produce valid results

• Position of each quadrat within the study area - to produce statistically significant result random sampling must be used.

What is the importance of sampling at random?

It avoids bias in collecting data

When is systematic sampling used along belt transects?

This is used when there is a gradual change in communities of plants/animals, for example going into a woodland. The belt transect can be made by stretching a string or a tape across the ground in a straight line.

What is percentage frequency and how is it calculated?

Likelihood of a particular species occurring in a quadrat, and using frame quadrats is counted by seeing how many squares have the species in. It does not give an idea of the density and detailed species distribution, but gives a quick idea of general distribution.

What is percentage cover and how is it calculated?

It is an estimate of the area within a quadrat that a particular plant species covers. The total squares are counted which have from than 50% coverage. This is beneficial where the species is more abundant.

What are the advantages/disadvantages of using % cover and % frequency?

Pros: data can be collected rapidly and individual plants do not need to be counted

Cons: not useful when organisms occur in several overlapping layers

How can you tell if a species is thinly distributed or has a cluttered distribution?

A high percentage frequency value but low % cover shows a thinly distributed population, but similar values for both measurements show a cluttered distribution.

How does mark-release-recapture work?

A known number of organisms are caught with an appropriate trap, marked in some way with UV paint. They are given sufficient time to intermingle with the rest of the population, and then a given number of marked individuals is collected randomly and the number of marked individuals is recorded.

How is population size calculated from mark-release-recapture?

estimated population size = total number of individuals in first sample x total number of individuals in the second sample/

number of marked individuals recaptured

What assumptions does mark-release-recapture rely on?

• Proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the whole population

• Marked individuals released from the first sample distribute themselves evenly amongst the remainder of the population and have enough time

• Population has a definite boundary so no immigration or emigration

• No births or deaths

• Marking is not toxic to the individual and does not make it more vulnerable to predation

• Mark/label is not lost

Give some examples of traps used for animals:

Longworth trap for small rodents

Pitfall traps for small invertebrates

Kick sampling for small aquatic invertebrates

Plankton net - plankton

Tullgren funnel - soil animals

Mist net - bird ringing

What is conservation?

The management of the Earth’s natural resources by humans in a sustainable way (so the maximum use of them can be made in the future).

How is conservation a dynamic process?

It entails careful management of existing resources and reclamation of those already damaged by human activities.

What are the main reasons for conservation?

• Personal - maintain the planet and life support system

• Ethical - other species have occupied the Earth far longer than humans and should be allowed to co-exist, and respect for living things is preferable to disregard for them

• Economic - living organisms have a huge pool of genes with capacity to make many substances which may be valuable in the future, and long term productivity is greater if ecosystems are maintained in their natural state

  • Cultural/aesthetic - organisms enrich our lives, and adds interest, inspiring writers and media to entertain us.

Sustainable meaning?

Enough resources are taken to meet the needs of people today without compromising the needs of people in the future

How can habitats be conserved by managing succession?

Many species existing in the earlier stages of succession before the current climax community are no longer present because habitats have disappeared as a result of succession, or been out-competed by other species/ human activities. Succession can be managed in a way that prevents a change to the next stage. If a factor preventing further succession is removed then the ecosystem can naturally develop into its climatic climax.

List some examples of conservation techniques:

• Seedbanks

• Fishing quotas

• Protected areas like nature reserves

• Captive breeding

• Rewilding

What is succession?

Changes in species that occupy an area over time.

Why might bare rock be exposed?

From volcanic eruption, sand dunes shifting, an earthquake, or a glacier retreating.

What is the first stage of primary succession?

The pioneer species colonises the inhospitable environment, and they make up a pioneer community.

What is lichen made of?

Symbiotic relationship between a fungus and an alga.

What features do pioneer species have that may make them suited to colonisation of an inhospitable environment?

• Asexual reproduction so a single organism can rapidly multiply

• Production of many seeds/ spores so isolated situations can be reached

• Rapid seed germination on arrival

• Photosynthetic abilities as light is normally available if food isn’t

• Able to get nitrogen from the atmosphere

• Tolerant to extreme conditions

Describe the process of succession starting from lichen on bare rock:

Lichens are slow growing and resistant to hostile conditions. As the lichens die and decompose they release sufficient nutrients to support a community, and more resistant tissues form a soil and nutrients for the organisms to follow. This supports a community of moss plants as the environment gets less hostile and more suitable for other species. Ferns then may outcompete the moss, and this continues to erode the rock and increase the organic matter, thickening the soil layer. This holds water and makes it easier for other plants to grow. Flowering plants then shrubs may then follow until the food chains and communities become stable and a climax community is reached which stays stable over long periods.

What are climax communities characterised by?

Rich biodiversity, state of equilibrium, and types of species remain quite unchanged.

What is the difference between primary and secondary succession?

Primary succession begins from an environment completely devoid of life and with no soil, like bare rock, whereas secondary succession results where a biological community used to be, but was disrupted by an event like a fire or agricultural land, but soil is still present.