4.4 Ecotsystems Flashcards
4.4 Ecosystem: (a) Populations
In this section you are learning:
• How populations grow
• The distinction between r- and K-selected species
• The ways in which population interact
• The general principles of biological control of pest species
• The dynamics of populations
Understanding how Populations Grow
What is a population?
A group of organisms of the same species located in a particular area.
*Population numbers can remain stable or grow/decline over time. *
Factors which influence population growth include:
•Death Rate
•Birth Rate
•Immigration
•Emmigration
Note: the ability for a population to increase its size can be studied in closed conditions
The Phases of Population Growth
1. Lag Phase: Very slow increase in number (number may even decrease for a time. A stage when nutrient assimilation takes place - may involve bacteria activating genes and producing appropriate enzymes to metabolise a particular food substrate.
2. Exponential Phase: Bacteria divide exponentially. No restriction to growth (there is abundant resources present) and bacteria can divide at the maximum rate. They can divide to produce two new bacteria as often as every 20 minutes, therefore, the increase in number can be exponential.
3. Stationary Phase: Food supply may become limited, therefore, the number of new individuals produced decreases. Waste product and toxins may accumulate to a level which restricts growth. The birth rate and death rate approach equilibrium.
4. Decline Phase: The death rate exceeds the birth rate, therefore, the population declines, sometimes very rapidly in a population ‘crash’. In bacterial populations, this can be due to a build up of toxic waste and/or the nutrient supply declining.
Key Terms associated with Population Growth
• Biotic Factors - The effects of other living organisms whether the same species or other species, for example, supply or predation.
• Abiotic Factors - Factors in the physical or chemical environment (non-living), for example water, nutrient, light and oxygen availability.
• Biotic Potential - The maximum rate of growth in a population as seen in the exponential phase. The reproduction potential of a population under optimum conditions with unlimited resources .
• Carrying Capacity - The maximum number of the population that the ecosystem can support. This is detained by the amount of resources available.
• Environmental Resistance - The restriction by the environment on the population reaching its maximum growth rate and biotic potential. This may be due to many factors including nutrient shortage, accumulated of waste, climate or biotic factors.
• Renewable Source - These resources are replaced on a regular basis and allow a population to remain stationary or in the stable phase.
• Non-renewable Source - These resources are not replaced. These tend to cause the flattening out of the stationary phase and the rapid fall of decline phase
Other Types of Growth Curves
J-shaped Curve
The J-shaped growth curve is typical for protoctistans.
The characteristics of a protoctist
These are eukaryotic and contains both plat and animal characteristics. It contains organisms that don’t fit into other kingdoms.
Competition Between Organisms
Intraspecific competition is between individuals of the same species and interspecific is where there is competition between members of a different species. This is most intense when two different species attempt to occupy the same niche and can lead to competitive exclusion.
Paramecium
•When grown separately, with controlled amounts of food, P. caudatum & P. aurelia both showed S-shaped curves.
• When grown together, the two species could coexist for a limited period of time.
• Eventually P. aurelia tended to out-compete P. caudatum as it was smaller and faster growing. They could not occupy the same niche.
Predator-Prey Relationships
It is obvious that predators affect the size of prey populations, but equally prey can affect the predator population.
A decrease in prey causes a decrease in the food supply for predators and therefore leads to a decrease in predators.
If there are large numbers of prey there will be more food available for predators so their numbers will increase. This increase will cause the numbers of prey to decrease which will cause the predator numbers to decrease and so on.
Predator-prey cycles are naturally self-regulating and tend to have a number of features in common:
• Predatory peaks and troughs lag behind prey - time delay depends on features including rate and time in which predators can produce offspring.
• Although lagging, the length of the predator and prey cycle are usually similar.
• The number of predators is normally significantly lower than the number of prey at equivalent points on the cycle.
Population Dynamics
As the growth curves show, the number of individuals that make up a population fluctuates over time.
The change of any population size is determined by:
•Birth Rate
•Death Rate
•Immigration
•Emigration
Population Growth Equation
Population growth= (BR-DR) + (I-E)
This equation can be applied to any population. Seasons can have a very obvious effect on populations.
Populations can also change from year to year. This can be for many reasons such as changes to food supply and abiotic factors e.g. temperature.
r-selected Species and K-selected Species
Characteristics of the species itself can influence population dynamics. Most species fit into either r- or K- selected species.
r-selected species
- Grow quickly
- Rapid reproduction
- Oppurtunistic
- Colonise new habitats
- Decline rapidly
- Boom and bust
- Population increases rapidly
E.g. Bacteria and Fungi
K-selected species
- Larger organisms
- Dominant
- Stable
- Survive in current habitat ~ not good at adapting to new habitats
- Population stays close to carrying capacity
Population Interactions
Competition is a -/- interaction: Negative for both species/ both species suffer. It can lead to elimination of other species.
Eg. Red and Grey squirrels
Mutualism is a +/+ interaction: Benefits both. Complex relationship. Both evolve to point where they can’t survive without eachother.
Predation, Parasitism and Grazing are +/- interaction: One species gains from the relationship while the other suffers.
Unlike predators, parasites generally do not kill their hosts and may exist on or within their hosts. The parasitic nematode (roundworm) Trichinella spiralis is an example of a parasite that affects Man.
What is a Parasite?
An organism that lives on/in another organism benefiting from them and causing harm over an extended period of time.
Predation refers to interactions between different species where one organism (the predator) obtains its needed food resources by killing and eating another organism (the prey).
This includes eating of plants by herbivores although this is commonly referred to as grazing.
The differences between a predator-prey relationship and a parasite-host relationship are:
1. Parasites live on/in the host
2. The parasite causes harm to the host over an extended period of time
3. Parasite is usually smaller than the host
Pests and Pest control
Biological control is a method of controlling pest species populations by deliberately introducing predator species that target the pest. This can be a predator, a competitor, a parasite or a pathogenic organism.
What is a pest?
A species that damages a valuable/ commercial crop species, causing economic damage.
As with many living things, the population of a pest is regulated by its natural predators and
parasites. Many alien species can become pests since they are no longer controlled by a natural predator. They can cause a lot of damage to crops etc, so it is worth spending the money to controlthe pest.
Biological control > chemical pesticides
Biological control benefits the environment by reducing the need for chemical pesticides, especially broad spectrum pesticides. These kill beneficial organisms such as natural predators of the pests.
Chemical Control
The graph above shows that the pest can experience pest resurgence in that its numbers increase rapidly due to the elimination of a natural predator. In pest resurgence after the use of insecticide, the number of pests can rise to above what it was before the insecticide was applied.
Biological Control
The graph below shows how effective biological control can reduce pest numbers below the threshold of economic damage.
An effective biological control integrates naturally into the ecosystem and does not need to be continually re-introduced. Effective biological control possesses the following advantages:
• No chemical damage to the environment or bio-accumulation
• Targets only the pest species
• Needs little additional action and saves money on the continued use of pesticides
• The development of resilience by the pests is unlikely
• Pest resurgence unlikely
Biological Control has its limitations and is not always successful. These include:
• The pest in unlikely to be totally eliminated. It’s density will be reduced below the threshold for economic damage
• Biological control will only work well if the species. An adapt and thrive in the ecosystem into which it is introduced
• It is important that the introduced control species does t outcompete other native species or damage non pest species
Features of a successful predator biological control programme:
-Increased rate of reproduction
-Good searching abilities
-Must be disease free
4.4 Ecosystems: (b) Communities
In this section you will be learning:
•The concept of an ecological community
•The concept of an ecosystem
• The process of community development
Definitions
Ecological Community: A biotic component of ecosystems and involves interactions between autotrophic (self-feeding) and heterotrophic populations.
Ecosystem: Community of different species that are interdependent and interact with each other and their abiotic environment. Involving energy flows and nutrient and gas exchange.
The Process of Community Development (Succession)
Ecosystems are dynamic units that are constantly changing as energy and nutrients flow from the abiotic environment to the biotic community and as organisms interact with one another. In newly-formed habitats and in areas subjected to a disturbance such as a fire or flood, changes in the biotic community follow in a process called succession.
Ecological Succession is: The change over time in ecosystems. Involving changes to both the community (species present) and the abiotic environment due to the ongoing interaction between these two components.
Note: Each stage in succession is called a sere
Primary Succession
This occurs on newly formed, barren substrates that have not been previously colonised e.g. lava field, disused quarry. The exposed land provides a very harsh and hostile environment for life. There will be no soil present to support plants.
1st stage:
•Pioneer species colonises area eg lichen
•Lichen can grow on bare rock and tolerates desiccation
•Over time lichen degrades the rock and helps weathering to form an embryonic soil
•Lichen will die and decompose and the ‘soil’ will develop to a stage where is can support mosses as the decomposing nutrients enter the soil
Next stage:
• Soil depth and fertility increases with time due to the decomposition of mosses
• Number of plant species increases
• Plant biomass increases
Final stage:
•After a number of seres a climax community develops
Climax Community: This is the stable end stage of a succession which is in equilibrium with the environment
In favourable climatic conditions, succession continues and the enriched soil may eventually support the growth of shrubs and trees; biotic factors become increasingly important in determining which species colonise the habitat as succession proceeds.
Note:
If the composition of the climax community is determine by the climate it is called a climatic climax. If it is determined by biotic factors, such as grazing, it is called a biotic climax.
All primary successions have the following common features:
• They are predictable. Pioneer species will always be the initial colonisers and similar climax communities develop in similar conditions
• Abiotic environment becomes less hostile as soils form and plant growth provides shelter
• The height and biomass of vegetation increases
• Communities become increasingly complex, with more complex food chains and more niches
• Increased biodiversity (until mid-late succession)
•Communities at later seres become more stable
Succession Stages Diagram
Secondary Succession
Primary succession can be interrupted or the climax community can be damaged/destroyed by events such as fire, wind damage or human interference. This type of succession is called secondary succession.
Secondary succession is generally completed in much less time that primary succession; soil is already present and surviving seeds, together with seeds from neighbouring communities, quickly colonise the area.
Secondary succession on an abandoned agricultural field takes about 120 years to develop form the pioneer annual plants to the climax woodland community.
Pyramids of Energy (Productivity)
This represents the flow of energy through each tropic level and is constructed using energy values determined from a given areas over a specified period of time (eg 5 years) for each trophic level.
It is usually measured in (kJ m-2 yr-1).
Pyramids of energy give the most accurate representation of the energy at a particular level but the values are more difficult to be obtained over a time period to compare the before and after.They are useful in comparing ecosystems.
Stable ecosystems will always represent energy flow as a pyramid.
The Efficiency of Energy Flow through Ecosystems
Photosynthesis is the principal route by which energy from the Sun is made available to the communities within an ecosystem. [Only a small percentage of energy reaching the Earth’s atmosphere is used by producers to make organic compounds].
Energy Losses between the Sun and Plants:
• Energy is reflected back into space by dust or clouds
• Energy is absorbed by water vapour or dust and reradiated as heat energy
• Will miss the leaves of a plant
Between 0.5% and 1% of the light reaching the leaf surface will be converted into chemical energy. Photosynthetic reactions are inefficient with much of the energy being lost as heat.
Productivity of an Ecosystem
Gross primary production (GPP): The energy in organic compounds produced by plants in photosynthesis
Net primary production (NPP): The energy available for other tropics levels once energy used in respiration has been subtracted from GPP.
(NPP = GGP- respiration)
The transfer of energy between producers and primary consumers and among consumers is much more efficient than the conversion of solar energy into organic compounds, however it is still relatively inefficient.
Only 5-10% of NPP is transferred to herbivores. Why?
• Much plant material can’t be accessed, eg roots
• Plant material is difficult to digest due to cellulose and lignin
• Excretory losses (metabolic waste) is energy that is not available to be transferred to the next trophic level
• Much every is used in respiration to generate ATP. This is lost as heat - a by prosucblf respiration
• Respiration losses are high in mammals and birds to maintain their body temperature
• Some plants or plant parts enter the decomposed food chain and are not available to primary consumers
At 10-20%, energy transfer to carnivores is more efficient. Why?
More of an animal can be eaten and digested. Losses due to excretion, uneaten structures (eg fur), respiration or through death and entry into the decomposed food chain.
Energy Transfer
Calculating the Efficiency of Energy Transfer through Trophic Levels
Energy Efficiency and the Human Diet
The production of animal products is much less efficient than using crops. This inefficiency means that more energy is available to humans through eating plant products rather than animal products.
Animal products require more land so a high consumption of meat is only possible in countries with a low population density or through importing meat from other countries.
The energy used in the production of new tissue in animals is referred to as secondary productivity eg cows. The efficiency of secondary productivity can be worked out using energy budgets.
In intensive farming, maximising P (by using high energy foods) and reducing any of R, U and/or F (most easily done with R by confinement) can lead to increases in growth and profit.
Carbon Cycle
• In photosynthesis, producers are able to fix inorganic carbon into organic products
• Respiration breaks down organic compounds are broken down to produce ATP, with carbon dioxide released as a waste product
• Saprobiotic microorganisms (decomposers) break down organic molecules in dead organisms during decay and decomposition and release carbon dioxide via respiration
• Fossilisation is when dead organisms are unable to decay. When fossils are burnt the carbon dioxide is released via combustion
Note:
Saprobiotic - certain bacteria contain enzymes that carry out extracellular digestion and absorb the products
Nitrogen Cycle
• Nitrogen cycle is nessecary for living organisms to make Nitrogen containing compounds essential for life. These include proteins, DNA, RNA, etc.
• Nitrogen normally enters the plants as nitrate ions absorbed from the soil via active transport
• The N containing compounds in plants enter the consumer pathway by being eaten by animals. Eventually N containing compounds are excreted (as urea), egested in feces or end up in non living organic matter following death
• Decay and decomposition by decomposers
Extra
Ecological Succesion BioFact Sheet
Populations BioFact Sheet
