3.5 Populations and ecosystems

Define population

An interbreeding group of organisms of the same species occupying a particular habitat.

Define birth rate

The reproductive capacity of a population: the number of new individuals derived from reproduction per unit time

Define immigration

The movement of individuals into a population of the same species/

Define equilibrium species

Species that control their population by competition rather than reproduction and dispersal.

What are the phases of the one step growth curve?

1. The lag phase

2. The exponential phase

3. The stationary phase

4. The death phase

Describe the lag phase

Initially the population does not increase but then there is a period of slow growth.

• In bacteria, it is a period of preparation for growth, with rehydration and intense metabolic activity such as enzyme synthesis.

• In sexually reproducing organisms, it represents the time for individuals to reach sexual maturity, find a mate and gestate their young.

Describe the exponential phase

As numbers increase, as long as there is no factor limiting growth, more individuals become available for reproduction. The rate of increase cannot be maintained indefinitely because environmental resistance sets in.

Environmental resistance includes all the factors that may limit the growth of a population.

• For bacteria in a flask, this includes available food, overcrowding, competition, accumulation of toxic waste.

• In a natural environment additional factors, abiotic and biotic, may apply such as predation, parasitism and disease, and competition from other species for nesting sites and food.

Describe the stationary phase

Birth rate = death rate. The population has reached its carrying capacity. The actual number depends on the resources available. The population is not absolutely constant and it fluctuates around the carrying capacity in response to environmental changes.

Describe the death phase

The factors that slow population growth become more significant and population size decreases until the death rate is greater than birth rate.

Define environmental resistance

Environmental factors that slow population growth.

Define biotic

A part of the environment of an organism that is living e.g. pathogens, predators.

Define abiotic

A part of the environment of an organism that is non-living e.g. air temperature, oxygen availability.

Define carrying capacity

The maximum number around which a population fluctuates in a given environment.

What are density dependent factors?

• Environmental resistance factors that may affect a greater proportion of a population if that population is denser.

• They are biotic factors and include disease, parasitism, and depletion of food supply.

What are density independent factors?

• The effect of abiotic factors on the environment does not depend on population density.

• The effect is the same regardless of the size of the population and is usually due to a sudden change in an abiotic factor e.g. flood, fire.

Populations fluctuate around the carrying capacity, regulated by negative feedback:

• If the population reaches above a set point, a density dependent factor increases mortality or reduces breeding that the population declines.

• If the population falls below the set point, environmental resistance is temporarily relieved so that the population rises again.

Define abundance

The number of individuals in a species in a given area or volume.

Animal abundance is measured by:

• Capture-mark-recapture experiments using the Lincoln Index.

• Kick sampling in a stream and counting aquatic invertebrates.

Plant abundance is measured by:

• Using a quadrat to calculate the mean number of individuals in several quadrats of a known area, to find the density.

• Estimating percentage cover of a plant in which individuals are hard to recognise.

• Estimating percentage frequency.

Define distribution

The area or volume in which the organisms of a species are found.

Define ecosystem

A characteristic community of interdependent species interacting with the abiotic components of their habitat.

Define habitat

The place in which an organism lives.

Define community

Interacting populations of two or more species in the same habitat at the time.

Define trophic level

Feeding level: the number of times that energy has been transferred between the sun and successive organisms along a food chain.

Define biomass

The mass of biological material in living or recently living organisms.

Food chains

• Producers incorporate the sun’s energy into carbohydrates, for successive organisms in the food chain. They trap solar energy and synthesise sugars from inorganic compounds by photosynthesis.

• Only a small proportion of the total energy that reaches the plant as light is incorporated into plant tissues.

• Carnivores are secondary, tertiary, quaternary, etc. Each of these groups operates at a trophic level with energy passing to a higher trophic level as material is eaten.

• Energy in the food chain is incorporated into the molecules of the consumer.

• As energy is passed along the food chain there is a loss from the food chain at least level.

• As energy is lost at each trophic level, the energy flowing through the ecosystem reduces and eventually leaves the system as heat.

Decomposition

When producers and consumers die, energy remains in the organic compounds of which they are made. Detritivores and decomposers feed as saprobionts and they contribute to the recycling of nutrients.

What are detritivores?

Detritivores are organisms such as earthworms which feed on small fragments of organic debris (detritus, the remains of dead organisms and fallen leaves)/

What are decomposers?

Decomposers are microbes such as bacteria and fungi that obtain nutrients from dead organisms and animal waste. They complete the process of decomposition started by detritivores.

Define saprobionts

Organisms that derive energy and raw materials for growth from extracellular digestion of dead or decaying material.

The length of a food chain may depend on several interacting factors:

• The more energy that enters a food chain is the first trophic level so the more energy fixed in photosynthesis, the longer the food chain can be. Therefore tropical food chains tend to be longer than arctic food chains.

• If energy is transferred more efficiently between trophic levels. the food chain is longer.

• Predators and prey populations fluctuate and their relative abundance can affect food chain length.

• Larger ecosystems can support longer food chains.

• Three dimensional environments such as aquatic systems and forest canopies have longer food chains than two dimensional habitats such as grasslands.

Photosynthetic efficiency

• The energy flowing from one organism to another in the food chain originates as sunlight.

• About 60% of the light energy that falls on a plant may not be absorbed by photosynthetic pigments because it could be:

• the wrong wavelength

• reflected

• transmitted straight through the leaf.

Define photosynthetic efficiency

The measure of the ability of a plant to trap light energy.

Define gross primary productivity

The rate of production of chemical energy in organic molecules by photosynthesis in a given area in a given time. Measured in Kj m^-2 y^-1.

Define net primary productivity

Energy in the plants biomass which is available to primary consumers. Measured in Kj m^-2 y^-1.

GPP equation

GPP - respiration = NPP

Define primary productivity

The rate at which energy is converted by producers into biomass.

Define secondary productivity

The rate at which consumers convert the chemical energy of their food into biomass. Occurs in heterotrophs.

As energy is passed along the food chain, there is a loss from the food chain at each level because:

• There is energy in molecules that are egested.

• Energy is lost as heat following processes fuelled by the energy generated in respiration including muscle contraction.

• Energy remains in molecules in parts of an animal that may not have been eaten e.g. fur, bones and horns.

Disadvantage of ecological pyramids

An ecological pyramid is a diagram that shows a particular feature of each trophic level in an ecosystem. Producers are drawn at the bottom and successive trophic levels are drawn above. The number of organisms, the energy or biomass contained in each trophic level is represented by a bar for each level. The area of each bar of the pyramid is proportional to the total number, energy or biomass at each trophic level. 

• Although ecological pyramids are useful in describing ecosystems, they do not take into account that some organisms operate at more than one trophic level at the same time.

Pyramids of numbers

• Does not take into account the size of organisms

• Does not recognise the difference between juvenile and adult forms

• Range of numbers may be too large and difficult to draw to scale

• The pyramid may be inverted if one trophic level has more organisms than the previous trophic level.

Pyramids of energy

• Most accurate way of representing feeding relationships as it shows the energy transferred from one trophic level to the next, per unit area or volume, per unit time. 

• Since only some of the energy is passed on from one level to the next, energy pyramids are never inverted.

• Make it easy to compare the efficiency of energy transfer between trophic levels in different communities.

Pyramids of biomass

If the available energy is greater, more biomass can be supported, so a pyramid of biomass is closely related to energy transfer. However

• Difficult to measure accurately as e.g. all the plants roots must be included

• Do not indicate productivity or the amount of energy transfer

• May be inverted

• A trophic level may seem to contribute more to the next level than it actually does because many organisms contain structures with mass that will not transfer to the next trophic level e.g. beak, bones.

• Species with similar biomass may have different lifespans. A direct comparison of their total biomass is therefore misleading.