populations in ecosystems YEAR 2 Flashcards
ecology
- study of inter-relationships between organisms and their environment. Environment includes both non-living (abiotic) factors and living (biotic factors)
ecosystems
- dynamic systems made up of a community and all the non-living factors of its environment.
Need to consider two major processes within in ecosystem: - flow of energy through the system
- cycling of elements within the system
Within each ecosystem there are a number of species. Each species is made up of a group of individuals that make up a population
populations
- a group of individuals of one species that occupy the same habitat at the same time and are potentially able to interbreed. An ecosystem supports a certain size of a population of a species called the carrying capacity
The size of a population can vary as a result of: - the effect of abiotic factors
- interactions between organisms eg intraspecific and interspecific competition and predation
community
-all the populations of different species living and interacting in a particular place at the same time
habitat
- where an organism normally lives
- characterised by physical conditions and the other types of organisms present
- within an ecosystem there are many habitats
- within each habitat there are smaller units each with their own microclimates. These are called microhabitats eg crevice on the bark of an oak tree may be the microhabitat for a lichen
ecological niche
- niche describes how an organism fits into the environment. Refers to where an organism lives and what it does there. Includes all the biotic and abiotic conditions to which an organism is adapted in order so survive, reproduce and maintain a viable population
- some species may appear very similar, but their nesting habits or other aspects of their behaviour will be different, or may show different levels of tolerance to environmental factors eg pollutants or a shortage of oxygen or nitrates
- no two species occupy exactly the same niche. This is known as the competitive exclusion principle
population growth
- when a population grows in size slowly over a period of time, possible to plot a graph of numbers in a population against time.
- If the population grows rapidly over a short period of time eg in bacteria, plot a graph of the logarithmic number of bacteria against time, which then makes the graph linear instead of exponential
over time things change and a factor may become limiting, slowing down the bacterial growth. For example: - mineral ions consumed as population becomes larger
- population so large that bacteria at the surface prevent light reaching those at deeper levels
- other species are introduced into the pond, carried by animals or wind, and some of these species may use the bacteria as food or compete for light or minerals
- winter brings much lower temperatures and lower light intensity
No population will continue to grow indefinitely as certain factors limit growth: food availability, light , water, oxygen, shelter, accumulation of toxic waste, disease, predators - the carrying capacity will be determined by these limiting factors
abiotic factors
- temperature. All species have different optimum temperatures. In plants and cold-blooded animals as temps fall below the optimum, enzymes work more slowly and so their metabolic rate is reduced. Populations therefore have a smaller carrying capacity. As temps are above optimum, enzymes begin to denature, so carrying capacity is reduced. In warm blooded animals although they can maintain a relatively constant body temperature, the further the temperature of the external environment gets from their optimum, the more energy they spend trying to maintain their body temperature. This leaves less energy for growth and reproduction so carrying capacity of population reduced
- light. At high light intensity the carrying capacity of both plants and animals that feed on them increase as rate of photosynthesis increases
- pH. Population of organisms larger where the appropriate pH exists
- water and humidity. When water is scarce will consist of only species well adapted to dry conditions. In dry air conditions populations that are adapted to tolerate low humidity will be larger than those that arent
When any abiotic factor is below the optimum, fewer individuals are able to survive because their adaptations aren’t suited to the conditions. If no individuals have adaptations to allow survival, populations will become extinct
intraspecific competition
- when individuals of the SAME species compete with each other for resources such as food, water, breeding sites etc. Availability of these resources determines the size of a population. The greater the availability, the larger the population. Availability of resources also effects the degree of competition between individuals
- eg limpets competing for algae (their main food). As more algae becomes available, the larger the limpet population becomes
interspecific competition
- when individuals of DIFFERENT species compete for resources such as food, light, water etc
- when populations of two species are in competition one will normally have a competitive advantage over the other. The population of these species will gradually increase in size while the population of the other will diminish. If the conditions remain the same, this will lead to the complete removal of one species. This is known as the competitive exclusion principle
- the principle states that where two species are competing for limited resources, the one that uses these resources most effectively will ultimately eliminate the other. No two species can occupy the same niche indefinitely when resources are limiting. This means there must be slight differences in the niches for different species to co-exist together
- to show how a factor influences the size of a population necessary to link it to birth and death rates of individuals, as for example an increase in food supply may just lead to larger individuals
predation
- predator is an organism that feeds on another organism, prey
- as predators have evolved they have become better adapted for capturing their prey- faster movement, more effective camouflage, better means of detecting prey.
- when a population of a predator and a population of its prey are brought together in a laboratory, the prey is usually exterminated by the predator. This is largely because the range and variety of the habitat provided is normally limited to the confines of the laboratory. In nature the situation is different. The area over which the population can travel is far greater and the variety of environment is much more diverse. Means prey can escape predation as harder to find and catch. Although prey population falls low, rarely becomes extinct.
- difficult to obtain reliable data on natural populations. Size can be estimated from sampling and surveys
effect of predator-prey relationship on population size
- predators eat their prey, reducing prey population
- with fewer prey available the predators are in greater competition with each other for the prey that are left
- the predator population is reduced as some individuals are unable to obtain enough prey for their survival or to reproduce
- with fewer predators left, fewer prey are eaten so more survive and reproduce
- prey population therefore increases
- with more prey now available as food, the predator population in turn increases
In natural ecosystems- organisms eat a range of foods and therefore the fluctuations in population size are often less sever
- disease and climatic factors are significant for cyclic functions in populations
- population changes are important in evolution as there is a selection pressure which means those individuals who are able to escape predators, withstand disease or an adverse climate, are more likely to survive to reproduce. The population therfore evolves to be better adapted to the prevailing conditions
investigating populations
- to study a habitat, often necessary to count the number of individuals of a species in a given space. This is known as abundance. Impossible to identify and count every organism as to do so would be time consuming and would cause damage to the habitat being studied. For this reason only small samples of a habitat are studied in detail. As long as these samples are representative of the habitat as a whole, any conclusion drawn from these findings will be reliable.
quadrats
- Point quadrat consists of a horizontal bar supported by two legs. Set intervals along the horizontal bar are ten holes, through each of which a long pin may be dropped. Each species that touches the pin is then recorded
- A frame quadrate consists of a square frame divided by string or wire into equally sized subdivisions. Quadrat is placed in different locations within the area being studied. The abundance of each species within the quadrat is then recorded
When using quadrats need to consider:
- size of quadrat to use. Depend on size of plants or animals being counted and how they are distributed. Where a population isn’t evenly distributed throughout the area, a large number of small quadrats will give more representative results than a small number of large ones
- number of quadrats to record within the study area. The larger the number of quadrats the more reliable the results will be. Balance between time available and reliability of results. Greater the number of different species in the area, the greater the number of quadrats required
- the position of each quadrat needs to be made by random sampling to produce statistically significant results
sampling at random
Avoiding bias ensures data is reliable. To randomly sample:
-lay out two long tape measures at right angles along two sides of the study area
- then obtain a series of coordinates by using random numbers taken from a table or generated by a computer
- place a quadrat at the intersection of each pair of coordinates and record the species within it
systematic sampling along belt transcets
- sometimes MORE informative to measure the abundance and distribution of a species in a systematic rather than a random manner. eg when measuring the distribution along a line of succession, from sand dunes up into a woodland
- belt transect can be made by stretching a string or tape across the ground in a straight line. A frame quadrat is laid down along the line and the species is recorded. It is then moved its own length along the line and the process repeated. Gives a record of species in a continuous belt
measuring abundance
- random sampling with quadrats and counting along transects are used to obtain measures of abundance. Abundance is the number of individuals of a species within a given area. For species that don’t move around it can be measures using:
- frequency- likelihood of species occurring in a quadrat
- percentage cover. An estimate of the area within a quadrat that a particular plant species covers. Can measure this by counting the number of squares within the quadrat that are dominated by the species you are measuring. Useful where a species is particularly abundant or difficult to count. Advantages are that allows for rapid data collection and don’t need to count for individual plants, however less useful if several overlapping layers
Need to ensure sample is large, so many quadrats are used and the mean of the quadrats are obtained to ensure results are reliable. The larger the number of samples, the more representative of the community as a whole the results will be
mark-release-recapture techniques
- used to estimate abundance of motile/ fast moving animals
- a known number of animals will be caught and marked in some way. They will then be released back into the community
- then a given number of individuals will be collected randomly and the number of marked individuals is recorded
estimated population size = (total number of individuals in the first sample x total number of individuals in the seconds sample) / number of marked individuals recaptured
assumptions for mark-release-recapture
- proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole
- marked individuals released from the first sample distribute themselves evenly amongst the remainder of the population and have sufficient time to do so
- the population has a definite boundary so there is no immigration into or emigration out of the population
- few if any deaths and births within the population
- method of marking is not toxic to individual nor does it make the individual more liable to predation
- mark is not lost of rubbed off during the investigation
succession
- succession is the changes over time in a species that occupies a particular area
- succession takes place in a series of stages. At each stage new species colonise the area and these may alter the environment in a way that makes it:
- less suitable for the existing species. As a result the new species may out-compete the existing one and so take over a given area
- more suitable for other species with different adaptations. As a result this species may be outcompeted by the better adapted new species
In this way there is a series of successional changes which alter the abiotic environment. These alterations can result in a less hostile environment that makes it easier for other species to survive. As a consequence new communities are formed and biodiversity may be changed and/ or increased
The first stage of this type of succession is the colonisation of an inhospitable environment by organisms called pioneer species. They make up a pioneer species and have features the suit them to colonisation
features of pioneer species
- asexual reproduction so that a single organism can rapidly build up a population
- the production of vast quantities of wind-dispersed seeds or spores so can easily reach isolated situations
- rapid germination of seeds so dont require period of dormancy
- ability too photosynthesis. Not dependant on animal species
- ability to fix nitrogen from the atmosphere as often soil has few or no nutrients
- tolerance to extreme conditions
eg on an area of bare rock the pioneer species is lichens. As lichens die and decompose they release sufficient nutrients to support a community of small plants. They then change the abiotic environment and allow for it to becomes less hostile, so more species can survive
in a stable community eg a deciduous oak woodland, often a balanced equilibrium. Climax community is reached and remains mostly stable over a long period of time
common features of a succession
- non-living (abiotic) environment becomes less hostile, eg soil forms and nutrients are more plentiful
- leads to a greater number and variety of habitats and niches
- this increases the biodiversity as different species occupy these habitats. Especially evident in the early stages, decreasing when climax community is reaches as dominant species out-compete pioneer species
- the increased biodiversity leads to more complex food webs
- this leads to increased biomass, especially during mid-succession
Climax communities are in a stable equilibrium with the prevailing climate.
succession when land that has already sustained life is suddenly altered
- eg due to land clearance or a forest fire
- process by which the ecosystem returns to its climax community is the same as described above but normally occurs more rapidly as soil already exists in which spores and seeds often remain alive in soil, and there is an influx of plants and animals through dispersal and migration of surrounding areas
- this is called SECONDARY SUCCESSION
- because the land has been altered in some may, some of the species in the climax community will be different
what is conservation
- management of the earths natural resources by humans in such a way that maximum use of them can be made in the future. Involves active intervention by humans to maintain ecosystems and biodiversity. Main reasons for conservation:
- personal
- ethical. Other species have occupied earth far longer than we have so should be allowed to coexist with us
- economic. Living organisms contain gigantic pool of genes with capacity to make millions of substances, many of which ma be valuable in the future
- cultural and aesthetic
conserving habitats by managing succession
- many of the pioneer species are no longer present as part of the climax community. This is because their habitats have disappeared as a result of succession, or species have out-competed other species or have been taken over by human activity. One way of conserving these habitats and hence the species they contain is by managing succession in a way that prevents change to the next stage
- if the factor preventing further succession is removed then the ecosystem can develop naturally into its climatic climax
