Module 4.1

Populations are in dynamic equilibrium and are affected by a number of factors.

Ecosystems are dynamic moving from colonisation to climax communities via succession.

Know a population is all the organisms of on species in a habitat.Understand that in a habitat a population occupies a niche as a result of adapting to abiotic and biotic factors. (learn definitions)`Appreciate how abiotic factors, interspecific/intraspecific competition and predation affect population size.Be able to investigate populations using random quadrats, transects. Use % cover and frequency as measures of abundance. Use mark release recapture for mobile species.Be aware of risk, ethical issues in taking samples when doing investigations.Be aware that the data gathered will need to be analysed statistically.Analyse and interpret data relating to distribution of organisms, recognising correlations and causal relationships.Know human population size and structure, population growth rate, age pyramids, survival rates and life expectancy.Calculate population growth rates from birth and death rate data.Relate human population size and structure to demographic transition.Know pioneer species colonise a habitat, change it so more, new different species can survive.As succession occurs the habitat changes from harsh to less harsh abiotic conditions allowing more different species to survive i.e. for diversity to increase. (refer to module 2.1)Conservation involves management of succession i.e. change. It is not about stopping natural change.Evaluate conflicting data relating to the conservation of a habitat.

Key Words:

Biotic: an ecological factor that makes up part of the living environment of an organism. Examples include food availability, competition and predation.

Abiotic: an ecological factor that makes up part of the non-biological environment of an organism, e.g. temperature, pH, rainfall and humidity.

Habitat: the place where an organism normally lives, which is characterised by physical conditions and the species of other organisms present.

Niche: all conditions and resources required for an organism to survive, reproduce and maintain a viable population.

Population: a group of individuals of the same species that occupy the same habitat at the same time.

Community: the organisms of all species that live in the same area.

Ecosystem: more or less self-contained functional unit in ecology made up of all the interacting biotic and abiotic factors in a specific area.

Interspecific: competition between organisms of different species.

Intraspecific: competition between organisms of the same species.

Investigating populations:

Random sampling using frame quadrats or point quadrats Systematic sampling along transects.

Quadrats

The size of the quadrato Depends on what size plant/animals being counted and how they are distributed

within the area.o Larger the species the larger the quadrat.

The number of sample quadrats to record within the study areao Larger the number of sample quadrats the more reliable the results will be.

The position of each quadrat within the study area.o To produce statistically significant results a technique known as random sampling

must be used.

Random sampling

Avoid bias Ensures valid results Lay out two long tape measures at right angles, along two sides of the study area Obtain a series of coordinates by using a random number generator. Place a quadrat at the intersection of each pair of coordinates and record the species within

it.

Systematic sampling along transects

Sometimes more informative

Measuring abundance

The number of individuals of a species within a given space. Frequency

o The likelihood of a particular species occurring in a quadrato A species occurs in 15 out of 30 quadrats – the frequency of its occurrence is 50%o Does not provide information on the density and detailed distribution of a species.

Percentage covero An estimate of the area within a quadrat that a particular plant species covers.o It is useful where a species is particularly abundant or is difficult to count.o Data can be collected rapidly and individual plants do not need to be counted.o Less useful where organisms occur in several overlapping layers.

To obtain reliable results it is necessary to ensure that the sample size is large, mean of all samples are obtained. The larger the number of samples the more representative of the community as a while will be the results.

Mark release recapture techniques

To measure abundance of animals/insects

estimated population

¿ totalnumber of individuals∈the first sample xtotalnumbe r of individuals∈the second samplenumber of marked individuals recaptured

Relies on a number of assumptionso 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.o That the marked individuals released from the first sample are distributed evenly

amongst the rest.o The population has a definite boundary so that these is no immigration into or

emigration out of the populationo There are few deaths and births within the populationo Method must not be toxic to individual or affect its liability to predators.o The marked label is not rubbed off in the investigation.

Variation in population size

Population growth curves

1. Periods of slow growth as the initially small number of individuals reproduce to slowly build up their numbers.

2. Period of rapid growth.3. Population growth declines until its size

remains more or less stable.

Population size

Limiting factors arise over time e.g. algae Mineral ions are used up as the population becomes larger Population becomes so large that algae 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 algae as food or compete for light or minerals.

Winter brings much lower temperatures and lower light intensity of shorter duration.

Various limiting factors that affect the size of a population are of two basic types:

Abiotic factors – are concerned with the non-living part of the environmento Temperature – each species had a different optimum temperature.o Light – ultimate source of energy – photosynthesis etco pH – actions of enzymeso Water and humidity – when water is scarce – populations are small and consist only

of species that are well adapted to living in dry conditions. Humidity effects transpiration rates.

Biotic factors – are concerned with the activities of living organisms and include, for example, competition and predation.

Competition

Intraspecific

Same species compete with one another for resources. Examples

o Limpets competing for algaeo Oak trees competing for minerals, light etc.o Robins

Interspecific

Individuals from different species compete for resources. Where populations of two species initially occupy the same niche, one will normally have a

competitive advantage over the other. The population with the advantage will increase whereas the other will diminish.

o Competitive exclusion principle.

Predation

When one organism is consumed by another Effect of predator-prey relationship on population size

o Predators eat their prey, thereby reducing the population of prey.o With fewer prey available the predators are in greater competition with each other

for the prey that are left.o The predator population is reduced as some individuals are unable to obtain enough

prey for their survivalo With fewer predators left, fewer prey are eateno The prey population therefore increases.o With more prey now available as food, the predator population in turn increases.

Could be other reasons such as disease, climate

Important in evolution as they create selection pressures enabling survival.

Human populations

Explosion in human populationo The development of agricultureo The development of manufacturing and trade that created the industrial revolution

Factors affecting the growth and size of human populationso Immigration – where individuals join a population from outsideo Emigration – where individuals leave a population

Population growth = (births + immigration) – (death + emigration)

% population growth rate= populationchange duringthe periodpopulationat the start of the period

x 100

Factors affecting birth rateso Economic conditionso Cultural and religious backgroundso Social pressures and conditionso Birth controlo Political factors

Factors affecting death rateo Age profileo Life expectancy at birtho Food supplyo Safe drinking water and effective sanitationo Medical careo Natural disasterso War

Population structure

Demographic transition

stage 1 – small and stable – birth rate and death rate is high.

Stage 2 – early expansion – high birth rate but decreasing death rate.

Stage 3 – late expansion – decreasing birth rate and a low death rate.

Stage 4 – large and stable – low birth rate and death rate.

Succession 

Ecosystems are not fixed, but constantly change with time. This change is

called succession. Imagine a lifeless area of bare rock. What will happen to it as time

passes?

1.       Very few species can live on bare rock since it stores little water and has few

available nutrients. The first colonisers are usually lichens, which have a

mutualistic relationship between an alga and a fungus. The alga

photosynthesises and makes organic compounds, while the fungus absorbs

water and minerals and clings to the rock. Lichens are such good colonisers that

almost all “bare rock” is actually covered in a thin layer of lichen. Mosses can

grow on top of the lichens. Between them, these colonisers start to erode the

rock and so form a thin soil. Colonisers are slow growing and tolerant of extreme

conditions.

2.       Pioneer species such as grasses and ferns grow in the thin soil and their roots

accelerate soil formation. They have a larger photosynthetic area, so they grow

faster, so they make more detritus, so they form better soil, which holds more

water.

3.       Herbaceous Plants such as dandelion, goosegrass (“weeds”) have small wind-

dispersed seeds and rapid growth, so they become established before larger

plants.

4.       Larger plants (shrubs) such as bramble, gorse, hawthorn, broom and

rhododendron can now grow in the good soil. These grow faster and so out-

compete the slower-growing pioneers.

5.       Trees grow slowly, but eventually shade and out-compete the shrubs, which are

replaced by shade-tolerant forest-floor species. A complex food web is now

established with many trophic levels and interactions. This is called the climax

community.

These stages are called seral stages, or seral communities, and the whole

succession is called a sere. Each organism modifies the environment, so creating

opportunities for other species. As the succession proceeds the community becomes

more diverse, with more complex food webs being supported. The final seral stage is

stable (assuming the environment doesn’t change), so succession stops at the

climax stage. In England the natural climax community is oak or beech woodland

(depending on the underlying rock), and in the highlands of Scotland it is pine

forests. In Roman times the country was covered in oak and beech woodlands with

herbivores such as deer, omnivores such as bear and carnivores such as wolves

and lynxes. It was said that a squirrel could travel from coast to coast without

touching ground.

Humans interfere with succession, and have done so since Neolithic times, so in the

UK there are few examples of a natural climax left (except perhaps small areas of

the Caledonian pine forest in the Scottish Highlands). Common landscapes today

like farmland, grassland, moorland and gardens are all maintained at pre-climax

stages by constant human interventions, including ploughing, weeding, herbicides,

burning, crop planting and grazing animals. These are examples of an artificial

climax, or plagioclimax.

Primary succession  starts with bare rock or sand, such as behind a retreating glacier, after

a  volcanic eruption, following the silting of a shallow lake or seashore, on a new sand dune,

or on rock scree from erosion and weathering of a mountain.

Secondary succession  starts with soil, but no (or only a few) species, such as in a forest

clearing, following a forest fire, or when soil is deposited by a meandering river.

Conservation

Human intervention to maintain ecosystems and biodiversity Main reasons for conservation

o Ethical – respect for living thingso Economic o Cultural and aesthetic

Conserving habitats by managing successiono Climax communities make habitats disappearo Conservation stops succession from moving onto the next stage and enables the

species to live.