1.5 Population Size and Ecosystems

Animal population growth curve

LAG

• period of adaptation to the environment, few animals are initially present so growth is slow.

EXPONENTIAL

• population doubles per unit time as resources are plentiful and no limiting factors

STATIONARY

• population growth slows due to competition

• reproduction rate = death rate

• carrying capacity is reached

(DEATH)

• death rate > birth rate

• occurs due to a population crash (plants eaten)

• natural disaster or new disease

Bacterial/Yeast population growth curve

LAG

• rate of cell division is slow so population growth is slow

• enzymes are being synthesised

EXPONENTIAL

• population doubles in a unit time

• cells divide at their maximum rate (binary fission/budding)

• no limiting factors

STATIONARY

• cells dying at the same rate as they are being produced

• limiting factors - nutrients running out

DEATH

• cell death > rate of cell production

• build up of toxic waste (ethanol - yeast, lactic acid - bacteria)

What is INTRAspecific competition?

Competition within the SAME species. Limits population size and is important in natural selection

What is INTERspecific competition?

Competition between individuals of DIFFERENT species. Two species cannot occupy the same niche in a specific habitat. The more the niche overlaps, the more competition there is.

What do plants compete for?

• light

• water

• mineral ions

• space

What do animals compete for?

• food and water

• shelter

• space

• reproductive partners

What is the definition of carrying capacity?

The maximum population size that can be sustained over a period of time in a particular habitat.

What is the definition of environmental resistance?

Factors which slow down population growth.

What factors contribute to environmental resistance?

BACTERIAL CULTURE

• nutrients - glucose/C source

• space

• pH

• build up of toxic waste

ANIMAL/PLANTS

biotic factors —>

• predators

• intraspecific competition

• parasitism/disease

abiotic factors —>

• light intensity

• temperature

• water

• minerals

• accumulation of toxic waste

What is the definition of a density dependent factor and what are examples?

Density dependent factor — BIOTIC

A biotic factor is LIVING

• disease/parasitism

• accumulation of toxic waste

• food availability

• predation

• availability of territory/shelter

What is the definition of a density independent factor and what are examples?

Density independent factor — ABIOTIC

An abiotic factor is NON-LIVING

• soil pH

• light availability

• mineral availability

• temperature

• water

What is photosynthetic efficiency (PE)?

Definition, unit, equation and value.

PE is a measure of how well a plant is able to capture light energy.

Unit: %

Value: 1-2%

Equation: GPP ÷ quantity of light energy falling on the plant x 100

What is gross primary productivity (GPP)?

Definition, unit, equation and value.

GPP is the rate of production of chemical energy by photosynthesis in a given area in a specific time period.

Unit: KJ m-2 year-1

Value: 0.2% of energy from sun hitting earth

Equation: NPP + respiration

What is net primary productivity (NPP)?

Definition, unit, equation and value.

NPP represents the potential food available to primary consumers.

Unit: KJ m-2 year-1

Value: 0.1% of energy from sun hitting earth

Equation: GPP - respiration

What is primary productivity?

GPP

What is secondary productivity?

The rate at which consumers accumulate energy in their cells or tissues.

Why is energy transfer between plants and herbivores very inefficient?

Cellulose is difficult to digest.

More energy is left in herbivore faeces than in carnivore.

Lots of energy is used during the digestive process.

Why is energy transfer between herbivores and carnivores more efficient?

Because they are able to digest their high protein diets more easily.

10% of the energy ends up as herbivore biomass

20% of the energy can be transferred from herbivore to carnivore

Photoautotroph definition

Uses light energy to make complex organic molecules.

Chemoautotroph definition

Uses chemical energy to make complex organic molecule

What do the arrows in a food chain show?

The direction of energy flow.

What happens to energy in the food chain?

energy used in respiration, egestion, excretion. not all will pass to the next trophic level

not all tissue is eaten

energy required for:

• active transport

• muscle contraction

• cell division

• maintaining body temperature

What is the definition of succession?

The change in structure and composition of species in a community over time.

What is primary succession?

Succession that occurs in a habitat that has never been colonised before. (bare rock, volcanic soil)

Pioneer species stage of succession.

• first plants to colonise a new habitat, highly adapted to very harsh conditions (extremeophiles)

• LICHENS - slow growing symbiotic relationship between fungi and algae able to tolerate total desiccation and low nutrients

• algae photosynthesise

• fungal hyphae grip rock and trap soil fragments

• MOSS - able to root into the first soil formed by decomposing lichen and total desiccation, could not survive on bare rock alone

Grassland community stage of succession.

• decomposing moss and lichen establish soil sufficient for the roots of herbaceous plants 

• seeds are blown by the wind into the habitat, establishing grassland

Shrub stage of succession

• heather, perennial

• they outcompete the grasses by blocking sunlight from reaching them

Climax community stage of succession

• forest dominated by trees, wild variety of plants and animals

• the final stage of succession where the species composition is relatively stable.

• the community reaches EQUILIBRIUM

Changes noticeable over time during primary succession

1) SOIL

• increase in depth

• increase in minerals (nitrates + phosphates)

• Humus - increase in water content

2) increase in BIODIVERSITY

• increase in no. of different species

• increase in niches

• increase in food and shelter

3) increase in BIOMASS

4) increase in STABILITY

What is secondary succession?

Repopulation of an area that has been previously colonised due to the habitat being disturbed or damage.

Happens in less time than primary succession:

• Soil, seeds/roots, humus ALREADY PRESENT

Why is secondary succession a lot faster than primary succession?

• already an existing seed bank in the soil

• root systems, stumps and other plant parts can rapidly regenerate

• fertility + structure of soil has been modified which makes it more suitable for growth and colonisation

Which factors affect secondary succession? 

MIGRATION

• vital

• arrival of spores, seeds and animals

INTERSPECIFIC COMPETITION

• occurs at all stages

• how they progress

FACILITATION

• invasion of later species depends on conditions created by earlier colonists

• increases competitive ability of species which are then able to displace them.

What are pyramids of NUMBER?

The number of organisms at each trophic level shown as a graph with each level forming a horizontal bar over the previous one.

What are pyramids of BIOMASS?

Created to overcome the problem of inverted pyramids of number.

Biomass is used at each trophic level instead (Kg/m2)

DRY MASS is used (dehydrated in oven until no change in mass)

UNETHICAL —> kills organisms

May also be inverted due to differences in PRODUCTIVITY

How are minerals released from dead organisms to be used again?

Detritivores and decomposers break down the remains and waste products of organisms and the minerals return to the soil for plants to take up again.

Saprotrophs feed by secreting enzymes onto matter to digest it, freeing some enzymes in the process.

What are the 5 key processes in the carbon cycle?

• Photosynthesis (from atmosphere and stores in oceans0

• Respiration (co2 returned to carbon stores in air and oceans)

• Decomposition (co2 released from dead organisms

• Fossilisation (formation of fossil fuels from dead organisms (NOT DECAYED) and formation of limestone from skeletons)

• Combustion

Draw the carbon cycle

What is the process of the greenhouse effect?

UV radiation from the sun reaches Earth, not all of this reaches as some is reflected back to space

Radiation that reaches the surface is absorbed and re-emitted as IR

This radiation is absorbed by greenhouse gases and warms the Earth.

A rise in co2 has caused an increase in this effect

Causes of the greenhouse effect

• burning of fossil fuels

• deforestation

• methane increase (rice crops, cattle, rotting material in landfill sites)

• CFCs more active as a greenhouse gas

Consequences of the greenhouse effect

• melting of polar icecaps and thermal expansion of water, leading to flooding and coastal erosion

• increased frequency of natural disaster climates

• climate change having serious effects on food production

• habitat changes

What is a carbon footprint and where can they be from?

A carbon footprint is the total amount of carbon dioxide attributable to the actions of an individual in the course of one year.

DIRECT emission → home and transport

INDIRECT emission → goods and services consumed

Draw the nitrogen cycle

What is the main principle of the nitrogen cycle?

Shows the flow of organic and inorganic nitrogen within an ecosystem. Nitrogen is converted between nitrogenous compounds and atmospheric nitrogen.

What are the 4 processes in the N cycle?

• Nitrogen fixation

• Ammonification

• Nitrification

• De-nitrification

What is nitrogen fixation?

The process that converts atmospheric nitrogen gas into compounds e.g. ammonium/nitrates by combining the nitrogen with hydrogen or oxygen

ACHIEVED BY:

• lightning

• Haber process

• Nitrogen fixing bacteria

How does nitrogen fixation occur via lightning?

enormous energy of lightning breaks N2 apart and enables the nitrogen atoms to combine with oxygen, forming nitrogen oxides.

These dissolve in rain water, forming nitrate ions

How does nitrogen fixation occur via the Haber process?

Under high pressure and temperature, with the use of a catalyst, atmospheric nitrogen and hydrogen are combined to form ammonia or ammonium ions

How does nitrogen fixation occur via bacteria? (AZOTOBACTER)

Free living bacteria in the soil (azotobacter)

Convert nitrogen gas from soil air spaces into ammonium using nitrogenase

Nitrogen gas is reduced to ammonium ions, bacteria use these to make amino acids, but release excess into the soil.

SOME plants can absorb these

Why is there a problem with azotobacter?

They require aerobic conditions for respiration but N fixing is reduction which oxygen inhibits

however, these bacteria have very fast metabolic rates so use up O2 quickly so nitrogenase isn’t inhibited

How does nitrogen fixation occur via N fixing bacteria (rhizobium)

these have a symbiotic relationship with leguminous plants as they live in root nodules

inside the nodules, these bacteria also reduce nitrogen gas to ammonium ions using the enzyme nitrogenase

How do both organisms benefit from rhizobium?

bacteria makes ammonium ions, which are then combined with keto acids (carbs provided by host plants) to make their own amino acids

any excess ammonium ions made by the bacteria enter the phloem of the plant and are used to make amino acids

Why are root nodules of legumes pink? Why is this needed?

Due to the presence of leg-haemoglobin

the leg-haemoglobin has a high affinity for oxygen, so binds to it and keeps the o2 pp low inside the root nodules

the bacteria carry out aerobic respiration but the oxygen would inhibit nitrogen fixation (reduction)

What is the process of ammonification?

Bacteria and fungi secrete protease which hydrolyse proteins in dead and decaying organic matter to amino acids.

They also secrete deaminase, which deaminate the amino acids to produce ammonia. These are then reduced to ammonium ions.

What is the process of nitrification?

Carried out by nitrifying bacteria.

Ammonium ions are converted to nitrite ions by nitrosomonas

Nitrite ions are converted to nitrate ions by nitrobacter (oxidation)

What is the process of denitrification?

Nitrates are lost from ecosystems and converted back to nitrogen gas by denitrifying bacteria such as pseudomonas (reduction)

bad in farming as they are anaerobic so are a particular problem in waterlogged soil which lack oxygen

Why do plants/animals need nitrogen?

• amino acid synthesis

• nucleotide synthesis (DNA/RNA, ATP)

• Chlorophyll

• Chitin

Human negative impact on the nitrogen cycle

• organic + inorganic fertilisers can lead to eutrophication

• adding fertilisers reduces biodiversity

• Bogs destroyed to provide peat

Human positive impact on nitrogen cycle

Improve nitrogen circulation:

• draining land to reduce anaerobic conditions (installing drainage systems to reduce waterlogging)

• planting fields of legumes to encourage nitrogen fixation

• ploughing fields to improve soil aeration and encourage aerobic conditions

Why is eutrophication bad?

• nitrates in fertilisers are leached out into waterbodies due to rain

• causes algal bloom

• algae growth cuts off light to submerged photosynthesising plants which die

• saprotrophic bacteria decompose the dead plants and respire aerobically

• this gives water a high biological oxygen demand and it becomes ANOXIC

• biodiversity of plants and animals decreases in water

• they die due to lack of o2

How to calculate degrees of freedom?

N1 + N2 - 2