populations in ecosystems

what we learn in this topic:

• how living organisms form communities within ecosystems through which energy is transferred and elements are recycled

• how populations of different species live in communities and how competition for survival arises both within and between these populations

• how populations within a single community are affected by living and non living factors in an ecosystem

what is ecology?

the study of the inter-relationships between organisms and their environment - including both abiotic factors (temp, rainfall) and biotic factors (competition, predation)

ecosystems:

dynamic systems made up of a community and all the non living factors of its environment. they can range in size from small to large

what are the 2 major processes to consider in an ecosystem?

• the flow of energy through the system

• the cycling of elements within the system

example of an ecosystem: freshwater pond/lake:

• has its own community of plants to collect the necessary sunlight energy to supply the organisms within it

• nutrients like nitrate ions and phosphate ions are recycled within the pond/ lake

• there is little/ no loss or gain between it and other ecosystems

what are there within each ecosystem?

a number of species, made up of a group of individuals that make up a population

what is a population?

a group of individuals of one species that occupy the same habitat at the same time and are potentially able to interbreed

carrying capacity:

an ecosystem supporting a certain size of population of a species

what can the size of a population vary as a result of?

• the effect of abiotic factors

• interactions between organisms, eg. intraspecific and interspecific competition and predation

populations in an oak woodland:

• populations of nettles, worms, green woodpeckers, beetales

• the boundaries of a population are often difficult to define

• eg. all the mature green woodpeckers can breed with one another and so form a single population

• populations of different species form a community

community:

all the populations of different species living and interacting in a particular place a the same time

community: in an oak woodland:

a community may include a large range of organisms like oak trees, hazel shrubs, bluebells, nettles, sparrow hawks, blue tits, ladybirds, aphids, woodlice, earthworms, fungi and bacteria

habitat:

the place where an organism normally lives and is characterised by physical conditions and the other types of organisms present

habitat: oak woodland:

• in an ecosystem there are many habitats

• leaf canopy of trees for blue tits

• decaying log for woodlice

• stream flowing for aquatic plants and water beetles

• banks for a water vole

microhabitat:

smaller units in each habitat with their own microclimate eg. the mud at the bottom of the stream for a bloodworm

niche:

how an organism fits into the environment. where an organism lives and what it does there, including all the biotic and abiotic conditions to which an organism is adapted in order to survive, reproduce and maintain a viable population

how to understand/ explain a niche:

some species may appear very similar, but their nesting habits or other aspects of their behaviour will be different, or they may show different levels of tolerance to environmental factors (eg. pollutant or a shortage of oxygen/ nitrates)

competitive exclusion principle:

no 2 species occupy the same niche

population size:

the number of individuals in a population

what are populations like?

dynamic in that they vary in size and composition over time

what possible where a population grows in size slowly over a period of time?

its possible to plot a graph of number in a population against time (harder where a population grows rapidly)

eg. growth of microorganisms:

• table 1 shows increase in population size of bacterium that initially doubles its numbers each hour

• if we try to plot a graph of numbers against time using a time scale that allows us to differentiate each point, the curve runs off the graph after the point plotted at 4 hours

• in these cases it is necessary to use a logarithmic scale to represent the number of bacteria

• the logarithms of bacterial numbers are shown

• when the graph of log bacterial numbers is plotted against time all points can be represented on the graph and we can see that rhetorical rate of growth starts to slow after 8 hours

SITUATION:

• situation where a single photosynthetic bacterial cell, capable of asexual reproduction, is placed in a newly created pond

• its summer and so there is plenty of light and the temp of the water is 12 degrees - mineral nutrients have been added to the water

• in these circumstances the bacterial cell divides rapidly because all the factors needed for the growth of the population are present

• no limiting factors

in situation, what things may change over time?

• mineral ions are consumed as the population becomes larger

• the population becomes so large that the bacteria at the surface prevent light reaching those at deeper levels

• other species are introduced into the pond, carried by animals or the 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 of shorter duration

• the growth of population slows (possibly ceases), and the population size may diminish

• over winter the population is likely to reach a constant size - abiotic and biotic factors affect this

• changes in the factors will influence the population size

what is the carrying capacity?

each population has a certain size that can be sustained over a relatively long period and this is determined by these factors

abiotic factors that influence the size of a population:

• temperature

• light

• pH

• water and humidity

temperature:

each species has a different optimum temperature at which it is best able to survive. the further away from this optimum, the fewer individuals in a population are able to survive and the smaller the population that can be supported

in plants and cold blooded animals…

• enzymes work slower so metabolic rate is reduced

• populations have a smaller carrying capacity

• at temps above optimum, the enzymes work less efficiently as they gradually undergo denaturation - populations carrying capacity is reduced

in birds, mammals, and the warm blooded animals…

• can maintain a relatively constant body temp regardless of external temp

• you may think the carrying capacity would be unaffected by temp, but the further the temp of the external environment gets from their optimum temp, the more energy these organisms expend in trying to maintain their normal body temp

• leaves less energy for individual growth and so they mature slower and reproductive rate slows

• carrying capacity of population is reduced

light:

• ultimate source of energy for most ecosystmes

• basic necessity of life

• rate of photosynthesis increases as light intensity increases

• more photosynthesis = plants grow faster and more spores and seeds produced

• carrying capacity therefore greater

• carrying capacity of animals that feed on plants therefore greater

pH:

• affects action of enzymes

• each enzyme has an optimum pH where it operates most effecitvely

• population of organisms is larger where the appropriate pH exists and smaller, or non existent, where pH is different from the optimum

water and humidity:

• where water is scarce, populations are small and consist only of species that are well adapted to living in dry conditions

• humidity affects the transpiration rates in plants and the evaporation of water from the bodies of animals

• in dry air conditions, the populations of species adapted to tolerate low humidity will be larger than those with no adaptations

what happens when any abiotic factor is below the optimum for a population?

fewer individuals are able to survive as their adaptations aren’t suited to the conditions. if no individuals have adaptations that allow survival, the population becomes extinct

extra info: the influence of abiotic factors on plant populations:

extra info: the growth and size of human populations:

when does competition happen?

where 2+ individuals share any resource (eg. light, food, space, oxygen) that is insufficient to satisfy all their requirements full

intraspecific competition:

where such competition arises between members of the same species

interspecific competition:

where competition arises between members of different species

what determines the size of a population?

the availability of such resources like food, water, breeding sites, etc

the greater the availability…

… the larger the population

what does availability of resources also affect?

the degree of competition between individuals which results in a smaller population

examples of intraspecifc competition:

• limpets competing for algae, which is their main food. the more algae available, the larger the limpet population becomes

• oak trees competing for resources. in a large population of small oak trees some willl grow larger and restrict the availability of light, water, and minerals to the rest, which then die. in time the population will be reduced to relatively few large dominant oaks

• robins competing for breeding territory. female birds are normally only attracted to males who have established territories. each territory provides adequate food for one family of birds. when food is scarce, territories become larger to provide enough food. there are therefore fewer territories in a given area and fewer breeding pairs, leading to a smaller population size

what happens when populations of 2 species are in competition?

one will normally have competitive advantage over the other

therefore what will happen to the species?

one will gradually increase in size and the other will diminish

what happens if conditions remain the same?

it will lead to the complete removal of one species - the competitive exclusion principle

competitive exclusion principle:

where 2 species are competing for limited resources, the one that uses these resources most effectively will ultimately eliminate the other - no 2 species can occupy the same niche indefinitely when resources are limiting

example:

2 species of sea birds (shags and cormorants) appear to occupy the same niche, living and nesting on the same type of cliff face and eating fish from the sea. analysis of their food, however, shows that shags feed largely on sand eels and herring, whereas cormorants eat mostly flat fish, gobies and shrimps. they therefore occupy different niches

what’s it necessary to do to show how a factor influences the size of a population?

link it to the birth rate and death rate of individuals in a population

example:

an increase in food supply doesn’t necessarily mean there will be more individuals - it could just result in bigger individuals

what is it therefore important to show?

how a factor, like a change in food supply, affects number of individuals in a population

example:

a decrease in food supply could lead to individuals dying of starvation and directly reduce the size of a population

what does an increase in food supply mean?

more individuals are likely to survive and so there is an increases probability that they will produce offspring and the population will increase. this effect therefore takes longer to influence population size

extra info: the effects of interspecific competition on population size:

extra info: competing to the death:

extra info: effects of abiotic and biotic factors on population size:

what is a predator?

an organism that feeds on another organism, know as their prey

what has happened as predators have evolved?

they’ve become better adapted for capturing their prey with faster movement, better camouflage and better means of detecting prey

how have prey also become more adept for avoiding predators?

better camouflage, more protective features like spines, and better concealment behaviour

what would have happened if prey or predators hadn’t matched the adaptations of each other?

both would’ve probably have become extinct

when does predation occur?

when one organism is consumed by another

what would happen if prey and predators are brought together in a laboratory?

the prey would be exterminated by the predator as the range and variety of the habitat is limited in a lab

why is extermination less likely in a habitat?

the area over which the population can travel is much greater and the environment is much more diverse - eg. more potential refuges

how can prey therefore escape predation in these circumstances?

they are harder to find and catch, so although the prey population falls to a low level it rarely becomes extinct

why isn’t lab condition findings very reliable?

they don’t necessarily reflect what happens in the wild

why’s it hard to obtain reliable data on natural populations?

its not possible to count all the individuals in a natural population - its size can only be estimated from sampling and surveys, which doesn’t guarantee accuracy

relationship between predators and prey and its effect on population size:

• predators eat their prey, reducing the prey population

• with fewer prey available the predators are in competition with each other for the prey that are left

• the predator population reduces as some individuals are unable to obtain enough prey for survival or to reproduce

• with fewer predators left, fewer prey are eaten so more survive and can reproduce

• the prey population increases

• more prey available so the predator population increases

what is the case in natural ecosystems?

organisms eat a range of foods and so fluctuations in population size are often less severe

diagram to show relationships between prey and predator populations:

what else leads to changes in organism populations?

disease and climatic factors

selection pressure:

those individuals who are able to escape predators, or withstand disease or adverse climate are more likely to survive and reproduce. the population therefore evolves to be better adapted to the prevailing conditions

extra info; the Canadian lynx and the snowshoe hare:

abundance:

the number of individuals of a species in a given space

why are only small samples of a habitat usually studied in detail?

its impossible to identify and count every organism. to do so would be time consuming and cause damage to the habitat being studied

as long as the samples are representative of the habitat as a whole…

…any conclusion drawn from the findings will be reliable

sampling techniques used for the study of habitats:

• random sampling using frame quadrats or point quadrats

• systematic sampling along a belt transect

point quadrat:

• horizontal bar supported by 2 legs

• at set intervals along the horizontal bar are 10 holes, through each of which a long pin may be dropped

• each species that the pin touches is then recorded

frame quadrat:

• a square frame divided by string or wire into equally sized subdivisions

• often designed so that it can be folded to make more compact for storage

• the quadrat is placed in different locations within the area being studied

• the abundance of each species within the quadrat is then recorded

factors to consider when using quadrats:

• size of quadrat to use

• number of sample quadrats to record in the study area

• the position of each quadrat within the study area

size of quadrat to use:

• will depend on the size of the plants or animals being counted and how they are distributed within the area

• larger species require larger quadrats

• where a population of species 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 sample quadrats to record within the study area:

• the larger the number of sample quadrats the more reliable the results will be

• as the recording of species within a quadrat is a time consuming task a balance needs to be struck between the reliability of the results and the time available

• the greater the number of different species present in the area being studied, the greater the number of quadrats required to produce reliable results for a valid conclusion

the position of each quadrat within the study area:

to produce statistically significant results a technique called random sampling must be used

why is random sampling important?

to avoid any bias in collecting data, ensuring reliable data

example: investigating the effect of grazing animals on the species of plants growing in a field:

• choose 2 fields as close together as possible in order to minimise soil, climatic, and other abiotic difference

• one field is regularly grazed by animals like sheep, whereas the other hasn’t been grazed for many years

• then take many samples at many random sites in each field by placing the quadrat on the ground and recording the names and numbers of every species found within the area of the quadrat

brief method of sampling:

1. lay out 2 long tape measures at right angles, along 2 sides of the study area

2. obtain a series of coordinates by using random numbers taken from a table or generated by a computer

3. place a quadrat at the intersection of each pair of coordinates and record the species within it

how is it sometimes more informative to measure?

the abundance and distribution of species in a systematic way rather than a random manner

when is this important?

when some form of gradual change (transition) in the communities of plants and animals takes place

example:

the distribution of organisms along a line of succession, such as, through sand dunes by the edge of the sea and inland up into woodland

how can a belt transect be made?

by stretching a string or tape across the ground in a straight line. a frame quadrat is then laid down alongside the line and the species within it recorded. it is then moved its own length along the line and the process repeated - gives a record of species in a continuous belt

what does random sampling and transects obtain?

measures of abundance

abundance:

the number of individuals of a species within a given area

how can abundance be measured? (for species that don’t move)

• frequency

• percentage cover

frequency:

• the likelihood of a particular species occurring in a quadrat

• eg. a species occurs in 15/30 quadrats, the frequency is 50%

• this method is useful where a species, like grass, is hard to count

• gives a quick idea of the species present and their general distribution within an area

• it doesn’t provide information on the density and detailed distribution of a species

percentage cover:

• an estimate of the area within a quadrat that a particular plant species covers

• useful where a species is particularly abundant or is difficult to count

• the advantages in these situations are that data can be collected rapidly and individual plants don’t need to be counted

• its less useful where organisms occur in several overlapping layers (more probably plants)

what’s necessary to obtain reliable results:

ensure that the sample size is large, that is, many quadrats are used and the mean of all the samples is obtained. the larger the number of samples, the more representative of the community as a whole will be the results

why are these above measures not possible with motile animals?

motile animals move away when approached. they are often hidden and different to find and identify

what is the mark release recapture technique?

a known number of animals are caught, marked and then released by into the community. some time later, a given number of individuals are 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 second sample) / number of marked individuals recaptured

what assumptions does this technique rely on?

• the 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

• the 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 that there is no immigration into or emigration out of the population

• there are few, if any, deaths and births within the population

• the method of marking isn’t toxic to the individual nor does it make the individual more conspicuous and therefore more liable to predation

• the mark or label isn’t lost or rubbed off during the investigation

ecosystem:

made up of all the interacting biotic and abiotic factors in a particular area within which there are a number of communities of organisms

are ecosystems the same forever?

no, they are dynamic