Populations and Ecosystems
The study of inter-relationships between organisms and their environment.
Abiotic - non living components
Biotic - living components
The supporting layers of land, air and water that surrounds the earth is called the biosphere.
Ecosystems - all of the interacting abiotic and biotic features of a specific area
Populations - a group of individuals from the same species that interbreed in a habitat
- Can be difficult to define boundaries of populations
Community - populations of different species that live and interact within the same habitat
Ecological niche - how an organism fits into its environment, where it lives and what it does. This includes all biotic and abiotic requirements for an organism to live. No two species will occupy the exact same niche
Due to time and the amount of damage it would cause, only small area within a habitat are studied; these samples represent the population as a whole.
The larger the number of samples, the more representative of the community the results will be.
Random sampling - quadrats are used
Quadrats - size of quadrat is based on size of species being measured
- use a random number generator to find coordinates to place quadrat to avoid bias
Systematic sampling - transects are used
Line transect (species along line are counted) - used to illustrate a transition along which communites of plants/ animals change
Belt transect (species between two lines are counted) - provides information on the density of a species
(no. individuals caught in first sample x no. caught in second sample) / no. recaptured
= Estimate of population
- Proportion of marked to unmarked individuals in the second sample is the for the whole population
- Individuals in the first sample distribute themselves evenly
- The population has a definite boundary
- No immigration or emigration
- Birth rate and death rate are low
- Marking non-toxic and conspicuous
- Marking is not lost
Variation in Population Size
Population Growth Curves
Growth curves of populations usually have three main phases:
1) A period of short growth due to the fact there is only a limited number of interbreeding individuals; this is known as the lag phase
2) A period of rapid growth, caused by the increase in organisms that are able to reproduce; this is known as the exponential phase
3) Population size begins to level off as there are limiting factors on the population growth such as the availablity of resources; this is known as the stationary phase
Variation in Population Size
No population growth will continue indefinitely. This is because in time there will eventually be limiting factors that will limit the population size. The factors are either biotic or abiotic.
Abiotic factors include:
Temperature - each species has an optimum temperature at which it survives best. The closer a group of organisms is to this temperature the faster the growth rate.
Light - light is the ultimate source of energy for an ecosystem. For plants, the greater the light intensity the more energy they have to produce spores and seed for reproduction.
pH - affects the function of enzymes. Enzymes work best at different pH levels.
Water and humidity - humidty affects transpiration rates in plants and the rate of evaporation of water from animals
Biotic factors include:
Intraspecific or interspecific competition
Competition and Predation
Interspecific cometition - between members of different species
When two species are competiting for limited resources the one that uses these resources the most effectively will ultimately eliminate the other.
Intraspecific competition - between members of the same species
Populations that undergo intraspecific competition are often limited by the number of resources available.
Predation - occurs when one organism consumes another
Effect of predators on population size:
Predators eat their prey thereby reducing the population of prey. With fewer prey available the preadtors compete with one another for the prey that is left. Predator population decreases due to some predators not being able to catch enough prey. With fewer predators around, fewer prey are consumed and population of prey increases again. More prey available, predator population also increases.
Human Population Size and Growth Rate
There are two major factors that have caused an increase in the size of the human population:
- The development of agriculture
- The development of manfacturing (Industrial Revolution)
Factors that affect this:
Death Rate and Birth Rate
Immigration (joining a population from outside) and Emigration (leaving a population)
Population growth = (Births + Immigration) - (Deaths + Emigration)
% growth rate = (Population change during a period / Population at start of period) x 100
Factors Affecting Birth Rates
Economic conditions - less developed countries tend to have higher birth rates
Cultural/religious backgrounds - some countries/religions encourage larger families
Social pressures - in some countries, a larger family improves social standing
Birth control - the extent to which contraception/abortion is available affects birth rate
Political factors - governments can influence birth rates through education and taxation
Birth rate = (Number of births per year x 1000) / Total population in the same year
Factors Affecting Death Rates
Age profile - the greater the proportion of elderly people the higher the death rate
Life expectancy at birth - residents of more developed countries live longer
Food supply - poor nutrition will cause an increase in death rate
Safe drinking water - poor quality of drinking water will cause an increase in water born diseases thus increasing death rate
Medical care - access to medical care will reduce death rate
Natural disasters - the more prone a region is to drought/famine, the higher the death rate
War - will cause an increase in death rate
Death rate = (Number of deaths per year x 1000) / Total population the same year
Energy and ATP
ATP is an immediate energy source. Both animals and plants breakdown organic molecules to make ATP.
What is energy?
- Energy is ability to do work
- It can take a variety of forms, including light, thermal, electrical, kinetic, etc
- It can change from one form to another
- It cannot be created or destroyed
- It is measured in joules (j)
Why do organisms need energy?
- Metabolism - chemical processes
- Active transport
- Production of enzymes
- Maintaining body temperature
Water is used to convert ATP into ADP, in a hydrolysis reaction:
(ATP + H20 --> ADP + Pi + Energy)
Photosynthesis - Light Dependent Reaction
Three main stages of photosynthesis:
1) Capturing of light energy
2) LDR - splitting of water, products are reduced NADP, ATP and O2
3) LIR - CO2 is reduced to produce sugars and other organic molecules
Leaves are adapted to bring together the 3 raw materials of photosynthesis.
Adaptations - air spaces, waxy cuticle, xylem, stomata, thin upper epidermis, palisade layer
The making of ATP (Light Dependent Reaction)
Chlorophyll absorbs light energy
2 electrons in the chlorophyll molecule are boosted to higher energy levels and passed on to electron carriers
Electrons transferred along an electron transfer chain
Electrons lose energy at each stage, which is used to make ATP from ADP + Pi
Photosynthesis - Light Dependent Reaction
The electrons that are lost from the chlorophyll are replaced by electrons released during the photolysis of water.
2H2O --> 4H+ + O2 + 4e-
Light is needed for the reaction to take place
The H+ ions create reduced NADP
O2 is used in respiration
The electrons return to chlorophyll
Light Independent Reaction
The products from the light dependent reaction are ATP and reduced NADP are used in the LDR
The Calvin Cycle
1) CO2 from the atomsphere diffuses into the leaf through the stomata and makes it way to the stroma
2) CO2 combines with ribulose biphosphate (RuBP) using an enzyme and produces two molecules of glycerate-3-phosphate (GP)
3) ATP and reduced NADP from the LDR are used to convert the GP into triose phosphate (TP)
4) The NADP is reformed
5) Some TP is converted into useful organic substances, but most is regenerated into RuBP using ATP from the LDR
Stroma contains enzymes for reaction to take place
Stroma fluid surrounds grana, so products can readily diffuse into stroma
Conversion of glucose to ATP
- Aerobic respiration - requires O2 and produces CO2, H2O and lots of ATP
- Anaerobic respiration - O2 absent and produces ethanol in plants and lactate in animals along with little ATP
1) Glucose is phosphorylated twice to make phosphorylated glucose (6 carbon compound). 2 ATPs are used to supply the Pi groups
2) The phosphorylated glucose breaks down to form triose phosphate (which is a 3 carbon compound)
3) Hydrogen is then removed from these molecules and reduces NAD. 2 ATPs are made from the conversion of TP into pyruvate, using ADP and Pi.
2 ATP molecules are yielded
Link Reaction and Kreb's Cycle
The Link Reaction
Pyruvate + NAD + Coenzyme A --> Acetyl coenzyme A + Reduced NAD + CO2
The Krebs Cycle
1) Acetyl coenzyme A reacts with a 4 carbon compound to form a 6 carbon compound
2) The 6 carbon compound loses CO2 and hydrogen twice to produce one ATP molecule and two reduced NAD molecules
3) FAD and NAD are reduced regenerating a 4 carbon compound
Electron Transport Chain
Takes place in the inner membrane of the mitochondria
Reduced NAD and reduced FAD from the Krebs cycle are used
Stages of the electron transport chain
1) Reduced NAD and reduced FAD are oxidised, releasing a proton and an electron
2) The protons are actively transported into the intermembranous space
3) The electron is taken up by an electron carrier, the carrier becoming reduced
4) The electron is passed on, so old electron carrier is oxidised and new one is reduced
5) By passing it down a chain of electron carriers the electron loses energy. It is this energy that is used to combine ADP + Pi to form ATP
6) Protons accumulate and diffuse back into the cell through special protein channels and combine with the electrons and water to from water
7) Oxygen is therefore the final acceptor of electrons in the electron transport chain
Since oxygen is the final receptor of electrons in the electron transport chain, when it is not present, ATP cannot be produced in this way. Instead ATP is produced anaerobically.
Once produced in glycloysis, products such as pyruvate and hydrogen must be constantly removed. Furthermore, the hydrogen from NAD must be released so it can be used again. In order to do this the pyruvate will react with reduced NAD.
In plants pyruvate is converted into ethanol and water:
Pyruvate + Reduced NAD --> Ethanol + Carbon dioxide + NAD
In animals pyruvate is converted into lactate:
Pyruvate + Reduced NAD --> Lactate + NAD
Food Chains and Food Webs
The ultimate source of energy in an ecosystem comes from sunlight
The energy is converted into an organic form using photosynthesis which is then passed between organisms
Producers - photosynthetic organisms that obtain energy through the photosynthesis of sunlight
Consumers - organisms that feed off of other organisms. They do not produce their own food by photosynthesis. Consumers can be primary, secondary, etc depending on which stage in the food chain they are
Decomposers - when producers/consumers die, the energy they contain can be accessed by decomposers that will break down the larger more complex molecules that they are made up of into smaller simple components again
- Describes the relationships between organisms
- Each stage of the chain is known as a "trophic level"
- In reality most animals do not rely upon a single food source
- Within a single habitat there may be many food chains linked together
Energy Transfer Between Trophic Levels
Energy losses in food chains
Only 1-3% of the energy available to plants is converted into organic matter; this is because:
- Over 90% of the suns energy is reflected back into space by the atomsphere
- Not all wavelengths of light can be absorbed by plants in photosynthesis
- Light may not actually fall on the chlorophyll molecule
- Limiting factors may slow down photosynthesis
The rate at which energy is stored is called "net production"
Net production = Gross production - Respiratory losses
Only about 10% of the energy stored in plants is passed on to primary consumers. Secondary and tertitary consumers are moe efficient at transferring energy. But energy is still lost because of:
- Some of the organism not being eaten
- Some parts can be eaten but not digested
- Some parts lost in excretion
- Some of the energy lost in respiration
Energy Transfer Between Trophic Levels
Calculating the efficiency of energy transfers
Energy transfer = (Energy available after the transfer / Energy available before) x 100
Pyramids of Number:
As you go up the trophic level the number of organisms gets fewer e.g Grass -> Rabbit -> Fox
No account is taken for size and the number individuals can be so great it can be almost impossible to count them all though, so this method has significant drawbacks.
Pyramids of Biomass:
This method does take size into account
Biomass is the total mass of plants/animals of species in a given place
Biomass can be unreliable as the amount of water stored in an organism can vary
Dry mass is therefore measured instead, but this means organisms must be killed
Pyramids of Energy:
Most accurate representation of energy flow in a food chain but collecting data is complex
What is an agricultural ecosystem?
Made up of plants and animals used to make food for humans. Agriculture tries to ensure that as much of the energy available from the sun is transferred to humans as possible.
What is productivity?
The rate at which something is produced. The rate at which plants for example assimilated energy from the sun into chemical energy is called gross productivity. Some of this is used for respiration, the remainder is called net productivity.
Net productivity = Gross productivity - Respiratory losses
Comparisions of Natural and Agricultural Ecosystems
To maintain an agricultural ecosystem it is important to prevent a climax community from forming by excluding other species in that community. Pests, diseases and weeds have to be controlled. This involves energy:
- Food - farmers use energy to do work on the farm
- Fossil fuels - farms have become mechanised and so many different machines are used
- In natural ecosystems this is low
- Energy input in agricultural ecosystems removes limiting factors to improve productivity
- Other species removed to reduce competition
- Fertiliser is added to the soil
Chemical and Biological Control of Pests
What are pests and pesticides?
A pest is an organism that competes with humans for food/space. Pesticides are poisonous chemicals that kill pests. Herbicides kill plants, insectides kill insects etc. An effective pesticide should:
- Be specific
- Be cost effective
- Not accumulate
Uses other organisms and does not eradicate pest but simply controls it.
- Acts more slowly, interval of time between introducing the control and seeing effect
- The control organism could become a pest
- Pests do not become resistant
- Very specific and cost effective
Chemical and Biological Control of Pests
Integrated Pest Control System
This uses all forms of pest control with the aim to determine an accepted level of the pest rather than trying to eradicate it
- Choosing animal/plant varieties that are as pest resilient as possible
- Managing the environment and ensuring there are nearby habitats for predators
- Regulating crops
- Removing pests by mechanically
- Biological agents
- Pesticides as last resort
Intensive Rearing of Domestic Livestock
Intensive Rearing and Energy Conversion
As you move down a food chain, energy is gradually lost to respiratory losses. This is because in mammals, the rate of respiration is high since the organism needs to maintain a high body temperature. This leaves a little amount of energy to be converted into biomass. Respiratory losses in farming are decreased by:
- Movement being restricted so little energy is lost in muscle contraction
- The environment is warm so less energy needed to maintain body temperature
- Nutrition is carefully controlled to ensure organisms recieve the optimum amount and type of food so that there is maximum growth and little wastage
- Predators are excluded and so there is no loss to other organisms
- Selectively breeding animals that are more efficient in converting the food they eat into biomass
- Using hormones to increase growth rate
The Carbon Cycle
Nutrients are recycled
1) Nutrients taken up by producer as simple inorganic molecules
2) The molecule is incorporated into more complex molecules with producer
3) When the producer is eaten, the nutrient passes to the consumer
4) Passes through food chain and when organism dies the complex molecules are broken down by saprobiotic organisms
Variations in the rates of respiration and temperature give rise to brief fluctuations of oxygen and carbon dioxide in the air. CO2 has dramatically increased in recent years due to:
- The combustion of fossil fuels, which releases previously locked up carbon
- Deforestation - reduces large amounts of photosynthesising biomass that can remove CO2 from the air
The sea allows for large amounts of CO2 from the air to dissolve thus lowering the concentration.
The Greenhouse Effect and Global Warming
When solar radiation reaches the earth, some has been reflected back into space and some has been absorbed by the atomsphere. The radiation that reaches the earth is absorbed and then reemitted back into space. However, some of this radiation is absorbed by clouds and greenhouse gases that reflect the radiation back to earth. This causes a heating effect known as the greenhouse effect.
CO2 - increase due to humans
Methane - Produced by the breakdown of organic molecules of which other organisms are made of
The mean average temperature has increased, cannot say for certain how much we have to do with this.
- Affects niches available in a community, leading to alterations in species distribution
- Melting of polar ice caps
- High temperatures might mean crop failure
The Nitrogen Cycle
Plants take up nitrates via active transport since they are moving against a concentration gradient. There are four main stages of the nitrogen cycle:
1) Ammonification - production of ammonia from organic compounds containing ammonium, saprobiotic bacteria feed on the materals releasing ammonia which converts to ammonium into the soil.
2) Nitrification - carried out by saprophytic bacteria in the soil, converting ammonium into nitrite ions and then into nitrate ions. Oxygen is required for this.
3) Nitrogen Fixation - nitrogen gas is converted into nitrogen containing compounds.
- Free living bacteria reduce gaseous nitrogen into ammonia, which they then use for amino acids.
- Mutualistic nitrogen-fixing bacteria provide plants with amino acids in return for carbohydrates
4) Denitrification - denitrifying bacteria convert soil nitrates into gaseous nitrogen when little oxygen is present.
Use of Fertilisers
The need for them:
- All plants need mineral ions, especially nitrogen, from the soil
- Specific areas of land are often used to grow crops
- Plants use up all the nitrogen containing compounds in the soil
- Normally they would be returned when the plant died and was broken down by saprobiotic bacteria, however in farming they are harvested so the nitrogen is not replaced
- The amount of nitrates in the soil decreases
Fertlisers are designed to replace what is lost:
Natural - consists of decaying/dead organisms as well as animal waste
Artifical - minerals obtained from rocks and other stuff
Environmental Consequences of Using Fertilisers
The effects of nitrogen fertilisers:
Nitrogen containing fertilisers can have detrimental affects such as:
Reduced species diversity - nitrogen favours certain species, so other could be outcompeted
Leaching - leads to pollution of watercourses
Eutrophication - caused by leaching
- Rain water can dissolve soluble nitrates and carry them deeper into the soil beyond the reach of plants
- The nitrates may then find there way to water courses and into water used for human consumption
- High levels of nitrates in water can cause inefficient transport of oxygen to the brain
Eutrophication - Because of leaching plants grow exponentially and cover upper layers of water. This means sunlight can't reach the bottom of the lake or pond. The plants on the bottom die. Saprophytic bacteria grow exponentially feeding on the decaying plant matter and using up oxygen and producing nitrates. Organisms that need oxygen, like fish die.
Populations and Ecosystems
Succession - changes that take place within an ecosystem
Barren land such as bare rock can be formed by the eruption of a volcano or a glacier retreating.
The first stage of succession is the colonisation of a pioneer species.
Pioneer species tend to have:
- a tolerance to extreme conditions
- the ability to fix nitrogen from the air
- the ability to photosynthesise light
- the ability to disperse seeds across vast distances
- seeds that repidly germinate
Pioneer species change the abiotic environment by dying and releasing nutrients such as nitrates for the production of amino acids and proteins for the organisms that follow. The growth of mosses provide habitats for insects and other animals.
At each stage of succession a certain type of species will change the environment to make it less hostile.
Climax community - consists of animals and plants which have established equilibrium
Populations and Ecosystems
During succession there are a number of common features such as:
- Environment becomes less hostile - soil forms, nutrients are more plentiful, plants provide shelter from wind
- Greater number of habitats
- Biodiversity increases - habitats become occupied by species
- More complex food webs due to high species diversity and therefore increased biomass
Genotype and phenotype
The genotype sets the limits to which characteristics can vary. Any change to the genotype is called a mutation and it will be passed on the next generation if it is present in gametes.
Phenotype is an observable characteristic of an organism. It will vary depending on the genotype and the environmental conditions.
Genes and alleles
Genes determine the proteins and compounds produced.
An allele is one of the different forms of a gene. If both alleles are the same in a homologous pair of chromosomes it is known as homozygous and if they are different it is known as heterozygous. When there are two different alleles and one expresses itself over the other it is known as the dominant allele.