Photosynthesis is fundamental to energy flow through an ecosystem. It is achieved by the autotrophic organisms that convert the energy of sunlight into chemical energy. Energy fixation is achieved by photosynthesis .
All organisms require energy to carry out cellular activity, growth and reproduction. They obtain that energy from the food they eat.
PRIMARY PRODUCTIVITY : The amount of light energy converted into chemical energy by autotrophs in an ecosystem during a given period of time. Measured by the rate of accumulation of BIOMASS (dry weight of vegetation) in the ecosystem [usually expressed as per unit area in a given time e.g. g/m 2 /yr]
GROSS PRIMARY PRODUCTIVITY (GPP): the total primary productivity
HOWEVER: Remember that plants not only photosynthesise, they also respire . Therefore not all the material produced is stored (and available as food for the primary consumers). Some of it is used for cellular respiration and other metabolic activities by the plant itself.
The energy available to the next level in a food chain or food web is the gross primary productivity (GPP) minus the energy used by the plant during respiration (R) and is called the NET PRIMARY PRODUCTIVITY (NPP)
This can be written as an equation: NPP = GPP - R
Net primary productivity (NPP) is of interest because it represents the chemical energy available to the primary consumers (herbivores) and is the beginning of the flow of energy through an ecosystem.
Energy fixation and productivity are the basis of ecosystem productivity. In this section the aim is to discuss how that energy flows through an ecosystem, and to consider the efficiency of that energy flow.
Numbers of organisms in a given area in a given time are counted and then grouped into trophic levels.
Pyramids of numbers typically show a broad base of producers and a successive decrease in number of animals at each level
There are several situations that show inverted pyramids of numbers. For example, a tree, as the primary producer, supports large numbers of insects, which in turn are the food source of large numbers of birds or other predators
Each block = total dry mass of organisms at that trophic level.
At each level the biomass decreases . Normally a pyramid of biomass would have a broad base, getting narrower at each succeeding level.
Occasionally, however, inverted pyramids of biomass can be found where the primary consumers outweigh the primary producers. E.g. In aquatic ecosystems the primary producers are algae. They are very productive and have a high turnover rate . This means that they grow in numbers rapidly but are also eaten in large numbers by zooplankton and small fish. Thus at any given time the biomass of the producers will be less than that of the primary consumers.
Despite an inexhaustible influx of energy in the form of sunlight, continuation of life depends on recycling of essential chemical elements. These elements are continually cycled between the environment and living organisms as nutrients are absorbed and wastes released.
The cycling of nutrients from the decomposition of dead or decaying matter , provides essential elements required for metabolic processes, such as photosynthesis, and constructing fundamental organic molecules, such as amino acids and nucleic acids.
The area where plant roots and soil come into contact.
There are large numbers of micro-organisms here that differ in species composition.
The major micro-organisms found are the bacteria whose growth is stimulated by various nutrients released by the plant roots. In exchange, the by-products of microbial metabolism that are released into the soil stimulate plant growth.
These organisms feed on waste or dead organic matter such as dead leaves, dead bodies, or waste products, decomposing it by producing enzymes to break it down. Action of detritivores increases the activity of decomposers :-
Detritus eating soil invertebrates
e.g. Earthworms, woodlice, spiders and nematodes .
Physically reduce detritus particle sizes to produce humus [ FRAGMENTATION = Larger surface area for decomposers to work on!]
Enhance fertility of the soil by incorporating leaf surface litter into the soil
Fungi & Bacteria
Use waste materials as energy, carbon & nutrient sources
1) FIXATION : the reduction of atmospheric nitrogen to ammonia by CYANOBACTERIA . Rhizobium fix nitrogen in the root nodules of legumes. Catalysed by enzyme complex NITROGENASE . LEGHAEMOGLOBIN is a molecule made by both the plant and the bacteria which limits the amount of O 2 reaching the bacteria. This is important as nitrogen-fixing is an anaerobic process
2) NITRIFICATION : the conversion of ammonium to nitrite to nitrates by NITROSOMONAS and NITROBACTER. Nitrates and ammonium are assimilated by plants into proteins and amino acids. They are lost by leaching and denitrifying bacteria. Aerobic process.
3) DENITRIFICATION : returns nitrogen to the atmosphere
4) AMMONIFICATION : the decomposition of organic nitrogen to ammonia
Water saturation of the soil affects the cycling of nitrogen. i.e. H 2 0 = O 2 anaerobic/aerobic affect different stages of cycle
1) Stable coexistence : where both populations remain stable
2) Cyclical variations : regular increases and decreases occur in the populations
3) Erratic swings : large scale “blooms” can take place at an irregular time, due to unstable populations of prey or predator, where a small change in the environment can have a major effect on the animal.
Involves a palatable, unprotected species (the mimic) that closely resembles a dangerous, poisonous or protected species (the model) and therefore is similarly avoided by predators
The scarlet king snake on the left is the mimic, and the coral snake on the right is the poisonous one SCARLET KING SNAKE CORAL SNAKE "Red on yellow, kill a fellow. Red on black, won't hurt Jack."
A ‘GRAZER’ = any species that moves from one ‘victim’ to another feeding on part of it without actually killing it outright e.g. grasshoppers that jump from plant to plant, chewing a portion of the leaves as they go
Grazers, like predators, can both INCREASE or DECREASE species diversity depending on the intensity of the feeding of the grazers and on the type of plant being grazed
In this case on organism will often show AGGRESSION to prevent another organism sharing a resource, e.g. territorial behaviour of the robin. The territory contains just enough resources for the breeding pair
In plants, this can be seen in the ability for some to GROW QUICKLY and block the sunlight out for others e.g. by growing in a lateral manner
Species that share the same habitat and have similar needs frequently use resources in somewhat different ways - so that they do not come into direct competition for at least part of the limiting resource
This is called RESOURCE PARTITIONING
The Competitive Exclusion Principle Early in the twentieth century, two mathematical biologists, A.J.Lotka and V. Volterra developed a model of population growth to predict the outcome of competition Their models suggest that two species cannot compete for the same limiting resource for long. Even a minute reproductive advantage leads to the replacement of one species by the other
These invasive, non-native species are a major threat to the environment because they ...
can change an entire habitat, placing ecosystems at risk
crowd out or replace native species that are beneficial to a habitat
damage human enterprise, such as fisheries, costing the economy millions of dollars
The zebra mussel, accidentally brought to the United States from southern Russia, transforms aquatic habitats by filtering prodigious amounts of water (thereby lowering densities of planktonic organisms) and settling in dense masses over vast areas. At least thirty freshwater mussel species are threatened with extinction by the zebra mussel
[HANDOUT / RESEARCH]
The Importance of Survival for Weak Competitors
Species diversity is important to all ecosystems
The diversity provides flexibility when the environment changes
Therefore competitors change when the environment changes
A less competitive species survives as they can adapt its niche slightly, and therefore maintain a presence
They are a valuable reserve for an alternative ecosystem. Without an alternative, if the environment were to change then the stability of the environment would be in jeopardy
Symbiotic Relationships - Parasitism - Commensalism - Mutualism SYMBIOSIS refers to relationships between organisms of DIFFERENT species that show an intimate association with each other Symbiotic relationships provide at least ONE of the participating species with a nutritional advantage 3 types of symbiosis have been recognised depending on the nature of the relationship:
Humans are the INTERMEDIATE HOST and RESERVOIR of the parasite, and the mosquito is the DEFINITIVE HOST and VECTOR .
Female anopheline mosquitoes become infected only if they take a blood meal from a person whose blood contains mature male and female stages of the parasite.
A cycle of development and multiplication then begins with union of the male and female gametocytes in the stomach of the mosquito and ends with parasites, called sporozoites, in its salivary glands, which are infective to humans.
The time required for the complete maturation of the parasite in the mosquito varies and depends on the Plasmodium species and external temperature.
Lifecycle of Malaria Parasite 1 2 3
The gametocytes are ingested by the female mosquito in a bloodmeal from an infected human. The gametocytes fuse to produce a zygote.The zygote secrete a cyst containing sporozoites formed from meiotic divisions
Sporozites enters the liver cell and during the next two weeks the intracellular parasite reproduces by mitosis within a liver cell to form as many as 200,000 merozoites! On maturation, the merozoites rupture the liver cells and are are released into the blood where they invade human red blood cells
In the red blood cells, the parasite matures asexually to produce another 10-20 merozoites which in turn can rupture the red blood cell and invade more liver cells or red blood cells
Vertebrate hosts infected with microparasites mount an immunological response
Vertebrate hosts infected with ectoparasites have other behavioural strategies e.g.
Preening or grooming each other to remove ectoparasites e.g. chimpanzees.
Move away from the infected area e.g. caribou move to higher altitudes during the summer months when the mosquito population is particularly dense to avoid attacks
Plants respond to parasitic infection in several ways:
e.g. in tobacco plants, if just one leaf is infected with the tobacco mosaic virus, there is an increase in the defensive chemicals throughout the plant-protects the plant from a variety of parasites and from the effects of grazing by herbivores. In addition, the plant will often kill the cells in the area that has been infected by the parasite, causing localised cell death. This deprives the parasite of its source of food and prevents parasitic spread to other cells.
There may be several different organisms growing in an infected sample, although most will have appeared after the initial disease has weakened the host.
Koch's postulates need to be satisfied in order to identify the organism that is causing a disease
Koch was one of the original researchers into tuberculosis, in the 19th century. In an attempt to define what an infectious disease actually is, he formulated his famous postulates, which now bears his name. Basically if,
1. An organism can be isolated from a host suffering from the disease AND
2. The organism can be cultured in the laboratory AND
3. The organism causes the same disease when introduced into another host AND
4. The organism can be re-isolated from that host THEN
The organism is the cause of the disease and the disease is an infectious disease
Most plants have to search through the soil with their roots to find nitrogen which is a critical nutrient required for growth
Legumes, however, form symbiotic relationships with Rhizobium bacteria
The Rhizobium live in little nodules in the roots of the legumes and fix atmospheric nitrogen into ammonium or nitrate, forms of nitrogen that can be used by the plant i.e. Rhizobium turn air into fertiliser!
The plant benefits because it gains nitrogen.
The bacteria benefit because they get sugars and nutrients to survive
The Costs, Benefits and Consequences of Interactions - Interaction between species - Interactions with the environment We have studied various types of biotic interaction that exist between species in an ecosystem. Now we are going to look at these interactions again, but this time we are going to concentrate on the COSTS , BENEFITS and CONSEQUENCES that these interactions have to the different species
Effects of Host Health and Environmental Factors
In most symbiotic relationships, a STABLE relationship exists between the two species involved
This is perhaps most important in parasitic relationships where it is necessary that the host, although affected in a negative way by the relationship, nevertheless remains healthy enough to be able to tolerate the parasite without being affected too seriously. If it is, it may die, which would be detrimental both to the host and to the parasite
People who are HIV positive and whose immune system is therefore compromised, tend to be more at risk from opportunistic infections like pneumonia and tuberculosis than individuals whose immune system is healthy
Seedlings which are grown in overcrowded conditions tend to grow spindly and weak and are more at risk of infections (which can pass more quickly from one individual to another in overcrowded conditions) than ones given more space. This is why gardeners 'thin' their crops of seedling plants, so that those which remain will have a better chance of growing into healthy adult plants, producing more, larger blooms etc .
Soft fruits such as raspberries are prone to a parasitic fungal infection called Botrytis . However, how badly the fruit is affected by the parasite is dependent in part by how humid the environment is
An example of the use of drugs to alter the balance in the hosts favour is the use of anti-fungal ointment and powder in the treatment of athlete's foot - a common fungal infection in humans
Also many farmers regularly include antibiotics in their animals' feed to prevent infection and so speed up the rate of growth of the animals. This activity is controversial, however, as it may be partly responsible for the evolution of antibiotic resistant bacteria
Information on a physical response to an environmental change can be turned into a RESPONSE CURVE
This allows you to identify the optimum conditions and the range in which the organism will survive
Response curves vary with SPECIES , STAGE OF LIFE and HEALTH of organism when exposed to the stress
It also varies with the TYPE and INTENSITY of the stressful situation
Response curve Although organisms can tolerate a range of external environmental changes, they function most efficiently at certain optimum environmental conditions. An organism's responses to a changing environmental factor can be studied in the laboratory and a tolerance, or performance, curve can be produced
Process of maintaining constant internal environment
Maintains INTRA cellular and EXTRA cellular fluids at a relatively constant ionic and osmotic compositions despite fluctuations in external conditions
Abiotic factors e.g. water, light, temp., soil nutrients etc largely determine what organisms live there, homeostasis has enabled organisms to inhabit a diverse range of environments and to exist within narrow physiochemical ranges
When this external environment CHANGES there are 2 basic patterns of response:
CONFORMATION : change in internal environment with the external environment
REGULATION : maintenance of internal environment regardless of changes in the external environment
Where internal variables fluctuate DIRECTLY with the external environment
Survival depends on the cellular resistance to damage
Osmoconformers : Marine invertebrates, such as crabs, shrimp and jellyfish .They are isosmotic and their body fluids are isotonic with their environment (conc. of solutes in the extracellular fluids is equal to that of the surrounding seawater). Therefore, no osmotic gradient exists, so no water enters or leaves the body of the organism. Osmoconformers do not alter their internal solute concentration
Poikilotherms : Animals whose body temperature varies with the surrounding environment. These are usually ectotherms (cold-blooded) that absorb heat from the surrounding environment e.g. snakes, lizards and marine fish
Where the internal variables are maintained at levels DIFFERENT from their environment. This requires significant ENERGY COST
Osmoregulators : e.g. freshwater organisms, terrestrial animals, body fluids are not isotonic with the environment and so need to use energy to regulate their internal osmolarity by excreting excess water or taking in additional water.They use a variety of osmoregulatory mechanisms to do this.
In hypotonic environments : they GAIN water by osmosis
In hypertonic environments : they LOSE water by osmosis
Homeotherms : are animals that maintain a constant body temperature. These are usually endotherms (warm-blooded) that derive heat from metabolism e .g. mammals, insects, birds
Habitat occupation of conformers & regulators
CONFORMERS can only survive in habitats which provide their particular environmental conditions, although they conserve energy by not regulating
REGULATORS use a lot of energy to carry out their homeostatic activities. However, the huge advantage they have is that they can colonise a range of different habitats since they can maintain their internal environment - thus can exploit habitats which conformers cannot!
RESTING SPORES : found in a wide diversity of forms. Temperature and draught resistant stages exist in bacteria, fungi, plants and lower animals
HIBERNATION – a period of inactivity in mammals associated with animals physiological changes resulting in a lowering of metabolic rate to conserve energy during periods of environmental extremes e.g. polar bear, dormice
AESTIVATION – a period of inactivity associated with hot, dry periods [usually summer] during which the organism remains in a state of torpor with reduced metabolic rate e.g. lung fish
DIAPAUSE – a form of dormancy typically found at a specific stage in an insect life history and involving complete cessation of growth and development together with suspended metabolism. This is controlled by hormones
African and South American lungfish are capable of surviving seasonal desiccation of habitats by burrowing into mud and aestivating throughout the dry season
Changes in physiology allow the lungfish to slow its metabolism to greater than 1/60th of the normal metabolic rate, and protein waste is converted from ammonia, to less-toxic urea, (normally, lungfish excrete nitrogenous waste as ammonia directly into the water)
(c) HUMAN IMPACT ON THE ENVIRONMENT Changes to Ecosystems (Changes in complexity, Effects of Intensive Food Production, Effects of Increased Energy Production, Pollution)
ALLOGENIC SUCCESSION: species composition is disturbed by environmental factors unrelated to the organisms present e.g. Hurricanes, forest fires, flooding, climate changes
AUTOGENIC SUCCESSION : the changes in environmental conditions which leads to changes in species composition in an ecosystem are caused by the biological processes of the organisms themselves e.g. trees shading and killing plants underneath that require high sunlight
Occurs where an existing community has been cleared by some disturbance. TOP SOIL PRESENT
Disturbance can be either natural e.g. forest fire, hurricane or man-made e.g. deforestation, agriculture
Faster than primary succession
Pioneer communities tend to be annual plants
In 1850, Connecticut was almost entirely open land cleared for farming or timber. Today, Connecticut has been mostly reforested through the process of secondary succession as farming has left the state since the 1800's This area has not been cleared in over fifty years. These trees represent the CLIMAX COMMUNITY for the rainfall, temperature and soil of this area This area has not been has not been mowed in about ten years. Shrubs and evergreen trees have moved in. These are the INTERMEDIATE species This area has been mowed within the last year. The plants are all annuals or herbaceous perennials. These are the PIONEER species
When an animal dies, bacteria immediately start breaking down the organic materials. This produces a smell which attracts insects such as flies who lay their eggs on the body. Within a few hours the flies' eggs have hatched and the larvae (or maggots) begin to feed on the animal's soft tissue. Several types of beetle also feed on the dead remains, lay eggs and, when hatched, these larvae will feed on the dead remains as well. Now spiders begin to approach, not to feed on the dead animal, but to feed on the insects which are on the animal's body. The fact that degradative succession always occurs in the same sequence is used by forensic entomologists. These scientists can tell approximately when a victim died because of which insects inhabit the body when it is found
As succession takes place, the ecosystem tends to become more COMPLEX and more STABLE
Human activities, as well as natural disasters, can reduce the complexity in ecosystems. This reduction in complexity is shown by, for example, a reduction in the number of species present, a decrease in the number and variety of habitats and niches and a decrease in the complexity of food webs
CHANGES IN ECOSYSTEM COMPLEXITY Increase in complexity shown by: Number of species Population size Biological Productivity Habitat/Niche Variety Complexity of Food Webs Loss of complexity caused by: Monoculture Eutrophication Toxic Pollution Oxygen depletion AUTOGENIC SUCCESSION ALLOGENIC SUCCESSION DEGRADATIVE SUCCESSION Geophysical Forces (e.g. Climatic Extremes) Associated with Decomposition Primary Secondary Barren Land Colonisation by Pioneer Species e.g. moss, microbes Disturbance of Existing Community
It is estimated that by 2050 this figure will increase to 9.4 billion
Sustaining this huge and ever increasing population would not be possible without agriculture. According to the World Health Organisation, over 3.5 million tonnes of food are required every day to provide the minimum calorific intake for today's population and this needs to increase by 83,000 tonnes daily to accommodate the increasing population
Since only 11% of land surface is suitable for agriculture, the growing demand for food can only be achieved by increasing productivity
Effects of intensive food production : MONOCULTURE
Agriculture or forestry in which a single species is cultivated over a large area for ECONOMIC EFFICIENCY
With increased mechanisation and additional use of FERTILISERS and PESTICIDES , farmers can manage larger areas of land
Crops are selected for their PRODUCTIVITY (speed of growth/yield) or DISEASE RESISTANCE
This is the process by which toxins e.g. mercury, poisons, become more and more concentrated with each successive link in a food chain
Biomagnification results from biomass at each trophic level being produced from a much larger biomass ingested from the level below. The top-level carnivores are usually most severely affected by toxic compounds released into the environment
The pesticide DDT was used to control mosquitoes and agricultural pests
DDT persists in the environment and is transported by water to areas away from the point of application
Because it is soluble in lipids and collects in fatty tissues of animals, the concentration is magnified at each trophic level and reached such high concentrations (10 X 10 6 increase) in top-level carnivorous birds that calcium deposition in eggshells was disrupted
Reproductive rates declined dramatically since the weight of nesting birds broke the weakened shells
Oxygen depletion caused by aerobic decomposition seriously affects the freshwater ecosystem
The extent of this pollution can be assessed using the BIOCHEMICAL OXYGEN DEMAND (BOD) test that measures the amount of dissolved oxygen in water
The BOD test is a mandatory water quality test used to estimate the amount of biodegradable organic material there is present in water
A HIGH BOD indicates a HIGH LEVEL of organic pollution in the water. As there is a significant amount of organic matter present in the water a lot of oxygen is required by the micro-organisms to degrade it. A LOW BOD indicates a LOW LEVEL of organic pollution in the water. Less oxygen is required by the micro-organisms because less organic matter is present in the water
The mass of dissolved oxygen, in grams per cubic metre or milligrams per cubic decimetre, taken out of solution by a water sample incubated in darkness at 20°C for five days
Because of the huge increase in the production of greenhouse gases in the last 150 years or so, the greenhouse effect is increasing and this is thought to be contributing to GLOBAL WARMING . Why is this important?
Globally, the ten hottest years on record have all occurred since the beginning of the 1990s
Current climate models predict that global temperatures could warm from between 1.4 o c to 5.8 o c over the next 100 years, depending on the amounts of greenhouse gases emitted and the sensitivity of the climate system
An example of the effect of increasing temperature on organisms is exemplified by the phenomenon known as coral bleaching
The zooxanthellae provide the coral polyps with nutrients produced by photosynthesis which, along with the nutrients the polyps gain by preying on tiny planktonic organisms, enables the coral to grow and reproduce quickly enough to produce reefs. The coral in turn provides the algae with a protected environment and a steady supply of carbon dioxide for photosynthesis. The tissues of the corals themselves are transparent - their colours come from the zooxanthellae living inside them. Under stress e.g. rise in sea temp, corals expel their zooxantheallae , which leads to a lighter or completely white appearance, hence the term "bleached" The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called ZOOXANTHELLAE that live within their tissues