energy and ecosystems Flashcards
(19 cards)
food chains
Organisms can be divided into three groups, according to how they obtain their energy and nutrients:
1. Producers. Photosynthetic organisms that manufacture organic substances using light energy, water, CO2 and mineral ions
2. Consumers. Organisms that obtain their energy by feeding on other organisms, rather than using the energy of sunlight directly. Animals are consumers. Those that directly eat producers are called primary consumers because they are the first chain of consumers. Those animals eating primary consumers are secondary consumers and those eating secondary consumers are tertiary consumers. Secondary and tertiary consumers are usually predators, but may also be scavengers or parasites
3. Saprobionts (decomposers). Group of organisms that break down the complex materials in dead organisms into simple ones. In doing so they release valuable minerals and elements in a form that can be absorbed by plants and so contribute to recycling. The majority of this work is carried out by fungi and bacteria
A food chain describes a feeding relationship in which the producers are eaten by primary consumers. These in turn are eaten by secondary consumers, which are eaten by tertiary consumers. In a long food chain the tertiary consumers may be eaten by quaternary consumers. Each stage in thus chain is referred to as a trophic level. The arrows on food chain diagrams represent energy flow
Food web- in reality, most animals don’t rely on a single food source and within a single habitat many food chains will be linked together to form a food web. The problem with food webs is their complexity. In practice it is likely that all organisms within a habitat, even within an ecosystem, will be linked to others by a food web
biomass
Biomass- the total mass of living material in a specific area at a given time.
Fresh mass is easy to access, but the Prescence of varying amounts of water make it unreliable. Measuring the mass of carbon or dry mass overcomes this problem but because the organisms must be killed, usually only made on a small sample, so may not be representative
Measured using dry mass per given area, in a given time. Measured in grams per square meter, where an area is being sampled, eg grassland or seashore
Where a volume is being sampled for, eg pond or ocean, measured in grams per cubic meter
The chemical energy store in dry mass can be estimated using calorimetry. In bomb calorimetry, a sample of dry material is weighed and is then burnt in pure oxygen within a sealed chamber called a bomb. The bomb is surrounded by a water bath and the heat of combustion causes a small temperature rise in this water. As we know how much heat energy is required to raise the temperature of 1kg of water by 1 degrees, if we know the volume of water and the temperature rise, we can calculate the energy released from the mass of burnt biomass in units such as kJ/kg
inefficiency of photosynthesis
The sun is the source of energy for ecosystems. However as little as 1% of this light energy may be capture by green plants and so made available to organisms in the food chain. These organisms then pass on only a small amount of the energy they receive to each successive chain
Plants normally convert between 1-3% of the suns energy into organic matter. Most of the suns energy isn’t converted to organic matter by photosynthesis because:
- not all wavelengths of light can be absorbed and used for photosynthesis
- light may not fall on a chlorophyll molecule
- a factor such as low CO2 may limit the rate of photosynthesis
- over 90% of the suns energy is reflected back into space by the clouds or dust absorbed by the atmosphere
net primary production
The total quantity of the chemical energy store in plant biomass, in a given area or volume, in a given time is called the gross primary production (GPP). However plants use 20-50% of this energy in respiration. The chemical energy store which is left when these losses to respiration have been taken into account, is called the net primary production (NPP)
NPP = GPP - R
net primary production= gross primary production- respiration
The NPP is available for plant growth and reproduction. It is also available to other trophic levels in the ecosystem, such as consumers and decomposers.
Usually less than 10% of this net primary production in plants can be used by primary producers for growth. Secondary and tertiary consumers are slightly more efficient, transferring up to about 20% of the energy available from their prey to their own bodies
why is there a low percentage of energy transferred at each stage
- some of the organism not consumed
- some parts are consumed but cannot be digested and are therefore lost in faeces
- some of the energy is lost in excretory materials such as urine
- some energy losses occur as heat from respiration and lost to the environment. These losses are high in mammals and birds because of their high body temperature when heat is constantly being lost to the environment
net production of consumers can therefore be calculated as:
N = I - (F+R)
N- net production
I- chemical energy store of ingested food
F- the energy lost in faeces and urine
R- energy lost in respiration
It is the relative inefficiency of energy transfer between trophic levels that explains why:
- most food chains have only 4 or 5 trophic levels because insufficient energy available to support a large enough breeding population at trophic levels higher than these
- the total mass of organisms in a particular place (biomass) is less at higher trophic levels
- the total amount of energy available is less at each level as one moves up a food chain
nutrient cycles
Limited supply of nutrient ions in usable form, so important elements such as carbon, nitrogen and phosphorus are recycled. Flow of nutrients within an ecosystem aren’t linear but are cyclic
Simple nutrient sequence:
- nutrient is taken up by producers, as simple organic molecules
- producer incorporates nutrient into complex organic molecules
- when producer is eaten the nutrient passes into consumer
- then passes along the food chain when these animals are eaten by other consumers
- when the producers and consumers die, their complex molecules are broken down by saprobiontic microorganisms, decomposers, that release the nutrient in its original simple form. The cycle is then complete. The role of these saprobionts in nutrient cycles is huge to ensure that nutrients are released for reuse. Without them nutrients would remain locked up as part of complex molecules that cannot be taken up and used again by plants
the nitrogen cycle
Living organisms require a source of nitrogen from which to manufacture proteins, nucleic acids and other nitrogen containing compounds- like chlorophyll. Although 78% of the atmosphere is nitrogen, there are very few organisms that can use nitrogen gas directly. Plants take up most of the nitrogen they require in the form of nitrate ions, NO3- from the soil. These ions are absorbed by active transport by the roots. This is where nitrogen enters the living component of the ecosystem. Animals obtain nitrogen-containing compounds by eating and digesting plants
Nitrate ions are very soluble and therefore easily leach (wash) through the soil, beyond the reach of plant roots. In natural ecosystems, the nitrate concentrations are restored largely by the recycling of nitrogen containing compounds. In agricultural ecosystems, the concentration of soil nitrate can be further increased with the addition of fertilisers.
When plants and animals die, the process of decomposition begins, in a series of steps by which the microorganisms replenish the nitrate concentrations in the soil. This release of nitrate ions by decomposition is most important because in natural ecosystems there are very few nitrate ions available from other sources
main stages in the nitrogen cycle
- ammonification
- nitrification
- nitrogen fixation
- denitrification
ammonification
This process converts organic nitrogen-containing compounds in dead organisms or waste into substances like ammonia or ammonium ions, NH4+
- death or excretion by living organisms releases nitrogen-rich organic substances like urea, proteins, nucleic acids and vitamins
- saprobionts decompose these into simpler substances like NH4+
- NH4+ can be absorbed and assimilated by plants
assimilation refers to the process where plants convert absorbed nutrients, like carbon dioxide, water, and minerals, into organic molecules like sugars and amino acids. This process primarily occurs through photosynthesis, where plants convert light energy, carbon dioxide, and water into sugars.
Nitrification
NH3 and NH4+ are first oxidised to nitrites, NO2- and then to nitrates NO3-, which plants can absorb through their root hair cells via active transport and assimilate
occurs in two stages:
1. Nitrifying bacteria oxidise NH3 and NH4+ into NO2-
2. Other nitrifying bacteria further oxidise NO2- into NO3-
- some bacteria obtain their energy from chemical reactions involving inorganic ions. eg NH4+ –> NO3-. This is an oxidation reaction and so releases energy. Carried out by free-living soil microorganisms called nitrifying bacteria
- require oxygen to carry out these conversions so require soil that has many air spaces. To raise productivity, it is important for farmers to keep soil structure light and well aerated by ploughing. Good drainage also prevents air spaces from being filled with water and so prevents air being forced out of the soil
nitrogen fixation
This process converts atmospheric nitrogen gas, N2, into nitrogen-containing compounds like ammonia
Atmospheric nitrogen can be fixed by:
1. Mutualistic bacteria in the root nodules of leguminous plants- they fix N2 into NH3, which is converted into amino acids for the plant, while the plant provides carbohydrates to the bacteria
2. Free-living bacteria. They fix N2 into NH3 and then amino acids. When they die and are decomposed by saprobionts, nitrogen-rich compounds are released and converted into a form that can be assimilated by plants. Reduction reaction
3. Abiotic processes- lightning, artificial fertilisers and the harbour process also fix N2
denitrification
When soils become waterlogged and have low oxygen concentration, the type of microorganism present changes. Fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria.
This process converts NO3- back into N2. Excessive denitrification can deplete soil nitrogen, negatively impacting plant growth
1. Carried out by denitrifying bacteria
2. Occurs under anaerobic conditions, such as waterlogged soils
phosphorous cycle
Phosphorus is found in DNA/RNA, ATP and the phospholipid bilayer
The phosphorous cycle is different to the nitrogen cycle, as phosphorus is not found as a gas and therefore isn’t in the atmosphere. Instead it is mainly found as a phosphate ion, in mineral form in sedimentary rocks
PO4 -3 ion, phosphate ion is what it most commonly exists as
- Originally dissolved in oceans, lakes and soils
- Absorbed by plants
- Then fed/digested by animals
- These animals then excrete phosphate ions back into oceans ,lakes and soils
- The phosphate ions from animals then decompose/ are excreted, to form phosphate ions in wastes and remains eg bones, shells, guano- guano refers to the waste, especially the faeces and urine, of sea birds. It is a valuable natural fertilizer, particularly rich in phosphate ions, so plays a large part in the phosphorus cycle
- Some of this waste is eroded so renters the lakes, oceans and soils
- Some of the waste undergoes deposition to form phosphate ions in rocks
- These rocks will undergo erosion/ weathering, and also due to use of fertilisers, will re- enter oceans, lakes and soils
- Dissolved phosphate ions in oceans, lakes and soils will undergo sedimentation to form phosphates in rocks
mycorrhizae
Fungal associations between plant roots and beneficial fungi
- The fungi increase the surface area for water and mineral absorption
- Mycorrhizae acts like a sponge so holds water and minerals around the roots
- Makes plants more drought resistant and able to take up more inorganic ions
Their part in the nutrient cycles is therefore improving the uptake of relatively scarce ions, such as phosphate ions
-mutualistic relationship, as the plant benefits from improved water and inorganic ion uptake while the fungus receives organic compounds such as sugars and amino acids from the plant
the need for fertilisers
All plants need mineral ions, especially nitrates from the soil. Much food production in the developed world is intensive, so specific area of land are used repeatedly to achieve maximum yield from the crops and animals that grow on them. Intensive food production makes large demands on the soil because mineral ions are continually taken up by the crops being grown on it
- In natural ecosystems the minerals that are removed from the soil by plants are returned when the plant is decomposed by microorganisms on its death.
In agricultural systems the crop is harvested and then transported from its point of origin for consumption. The urine, faeces and dead remains of the consumer are rarely returned to the same area of land.
-Under these conditions the concentrations of the mineral ions in agricultural land will fall. It is therefore necessary to replenish these mineral ions because, otherwise, their reduced concentrations will become the main limiting factor to plant growth. Productivity will consequently be reduced. To offset this loss of mineral ions fertilisers need to be added to the soil
types of fertilisers
- Natural (organic) fertilisers. Consist of the dead and decaying remains of plants and animals as well as animal wastes such as manure, slurry and bone meal
- Artificial (inorganic) fertilisers. Mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop. Compounds contain the three elements, nitrogen, phosphorus and potassium are almost always present
How fertilisers increase productivity
Plants require minerals for their growth. eg where nitrate ions are readily available, plants are likely to develop earlier, grow taller and have a greater leaf area. This increases the rate of photosynthesis and improves crop productivity
effects of nitrogen containing fertilisers
- Reduced species diversity. because nitrogen rich soils favour the growth of grasses, nettles and other rapidly growing species. These out-compete many other species, which die as a result. Species-rich hay meadows, only survive when soil nitrogen concentrations are low enough to allow other species to compete with the grasses
- Leaching. Process by which nutrients are removed from the soil. Rainwater will dissolve any soluble nutrients such as nitrate ions, and carry them deep into the soil, eventually beyond the reach of plant roots. The leached nitrate ions find their way into watercourses such as streams and rivers, that in turn may drain into freshwater lakes. May have harmful effects on humans if river or lake is a source of drinking water. Very high nitrate ion concentrations in drinking water can prevent efficient oxygen transport in babies and link to stomach cancer in humans. The leached nitrate ions are also harmful to the environment as they can cause eutrophication
- Eutrophication.
eutrophication
Process by which nutrient concentrations increase in bodies of water. It is a natural process that occurs mostly in freshwater lakes and the lower reaches of rivers. Eutrophication consists of the following sequence of events:
1. In most lakes and rivers there is naturally very low concentration of nitrate and so nitrate ions are a limiting factor in plant and algal growth
2. As the nitrate ion concentration increases as a result of leaching, it ceases to be a limiting factor for the growth of plants and algae whose populations both grow
3. As algae mostly grow at the surface, the upper layers of water become densely populated with algae, this is called ‘algal bloom’
4. This dense surface layer of algae absorbs light and prevents it from penetrating to lower depths
5. Light then becomes the limiting factor for the growth of plants and algae at lower depths and so they eventually die
6. The lack of dead plants and algae are is no longer a limiting factor for the growth of saprobiontic bacteria and so these populations too grow, using dead organisms as food
7. The saprobiontic bacteria require oxygen for their respiration, creating an increased demand for oxygen
8. The concentration of oxygen in the water is reduced and nitrates are released from the decaying organisms
9. Oxygen then becomes the limiting factor for the population of aerobic organisms, such as fish. These organisms ultimately die as the oxygen is used up altogether
10. Without the aerobic organisms, there is less competition for the anaerobic organisms, whose populations now rise
11. The anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes, such as hydrogen sulphide, which male the water putrid
Organic manures, animal slurry, human sewage, ploughing old grassland and natural leaching can all contribute to eutrophication, but the leaching of artifical fertilisers is the main cause
