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Relationships and the Environment
Human Impact on the Environment
is the study of inter-relationships between organisms and their environment. Its
aim it to explain why organisms live where they do. To do this ecologists study ecosystems,
areas that can vary in size from a pond to the whole planet.
physical or abiotic part of an ecosystem, i.e. a defined area
with specific characteristics where the organisms live, e.g. oak forest,
deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc.
living or biotic part of an ecosystem, i.e. all the organisms of
all the different species living in one habitat.
living or biological factor.
non-living or physical factor.
members of the same species living in one habitat.
group of organisms that can successfully interbreed
To study the dynamics of a population, or how the distribution of the members of a population is influenced by a biotic or an abiotic factor, it is necessary to estimate the population size. In other words, it will be necessary to count the number of individuals in a population. Such counting is usually carried out by taking samples.
of the most fundamental problems faced by community and population ecologists is
that of measuring population sizes and distributions. The data is important for
comparing differences between communities and species.
It is necessary for impact assessments (measuring effects of disturbance)
and restoration ecology (restoring ecological systems).
It is also used to set harvest limits on
commercial and game species (e.g. fish, deer, etc.).
most cases it is either difficult or simply not possible to census all of the
individuals in the target area. The
only way around this problem is to estimate population size using some form of sampling technique. There
are numerous types of sampling techniques.
Some are designed for specific types of organisms (e.g. plants vs. mobile
animals). As well there are
numerous ways of arriving at estimates from each sampling technique.
All of these procedures have advantages and disadvantages.
In general, the accuracy of an estimate depends on 1) the number of
samples taken, 2) the method of collecting the samples, 3) the proportion of the
total population sampled.
is viewed by statistical ecologists as a science in its own right.
In most cases, the object is to collect as many randomly selected samples
as possible (so as to increase the proportion of the total population sampled).
The accuracy of an estimate increases with the number of samples taken.
This is because the number of individuals found in any given sample will
vary from the number found in other samples.
By collecting numerous samples, the effect of these variations can be
averaged out. The
purpose for collecting the samples randomly is to avoid biasing the data.
Data can become biased when individuals of some species are
sampled more frequently, or less frequently, than expected at random.
Such biases can cause the population size to be either over estimated or
under estimated, and can lead to erroneous estimates of population size.
size generally refers to the number of individuals present in the population,
and is self-explanatory. Density
refers to the number of individuals in a given area. For ecologists density is usually a more useful measure.
This is because density is standardized per unit area, and therefore, can
be correlated with environmental factors or used to compare different
The spatial distribution of a population is a much more complicated matter. Basically, there are three possible types of spatial distributions (dispersions) (see diagrams below). In a random dispersion, the locations of all individuals are independent of each other. In a uniform dispersion, the occurrence of one individual reduces the likelihood of finding another individual nearby. In this case the individuals tend to be spread out as far from each other as possible. In a clumped dispersion, the occurrence of one individual increases the likelihood of finding another individual nearby. In this case, individuals tend to form groups (or clumps).
are often interested in the spatial distribution of populations because
it provides information about the social behaviour and/or ecological
requirements of the species. For
example, some plants occur in clumped distributions because they propagate by
rhizomes (underground shoots) or because seed dispersal is limited.
Clumped distributions in plants may also occur because of slight
variations in soil chemistry or moisture content. Many animals exhibit rather
uniform distributions because they are territorial (especially birds), expelling
all intruders from their territories. Random
distributions are also common, but their precise cause is more difficult to
it is often difficult to visually assess the precise spatial distribution of a
population. Furthermore, it is
often useful to obtain some number (quantitative measure) that describes spatial
distribution in order to compare different populations.
For this reason, there are a variety of statistical procedures that are
used to describe spatial distributions.
are assemblages of many species living in a common environment.
Interactions between species can have profound influences of their
distributions and abundances. Comprehensive
understanding of how species interact can contribute to understanding how the
community is organized. One way to
look at species interactions is to evaluate the level of association between
them. Two species are said to be positively associated if
they are found together more often than expected by chance.
Positive associations can be expected if the species share similar
microhabitat needs or if the association provides some benefit to one (commensualisms)
or both (mutualism) of the species involved.
Two species are negatively associated if they are found together
less frequently than expected by chance. Such
a situation can arise if the species have very different microhabitat
requirements, or if one species, in some way, inhibits the other.
For example, some plants practice allelopathy, the production and release
of chemicals that inhibit the growth of other plant species.
Allelopathy results in a negative association between the allelopathic
species and those species whose growth is inhibited.
quadrat method is used primarily in studies of plant populations, or where
animals are immobile. The principal
assumptions of this technique are that the quadrats are chosen randomly, the
organisms do not move from one quadrat to another during the census period, and
that the samples taken are representative of the population as a whole.
It is often conducted by dividing the census area into a grid.
Each square within the grid is known as a quadrat and represents the
sample unit. Quadrats are chosen at random by using a random number generator or
a random number table to select coordinates.
The number of individuals of the target species is then counted in each
of the chosen quadrats.
use units to measure organisms within the quadrats.
Frequency (f) is an indication of the presence of an organism in a
quadrat area. This gives no
measure of numbers, however the usual unit is that of density – the
numbers of the organisms per unit area. Sometimes
percentage cover is used, an indication of how much the quadrat area is
are used to describe the distribution of species in a straight line across a
habitat. Transects are particularly
useful for identifying and describing where there is a change in habitat.
A simple line transect records all of the species which actually touch
the rope or tape stretched across the habitat.
A belt transect records all the species present between two lines, and an
interrupted belt transect records all those species present in a number of
quadrats places at fixed points along a line stretched across the habitat.
This method of sampling is most useful when dealing with an animal population that moves around. Ecologists must always ensure minimum disturbance of the organism if results are to be truly representative and that the population will behave as normal. In this method individual organisms are captured, unharmed, using a quantitative technique. They are counted and then discretely marked or tagged in some way, and then released back into the environment. After leaving time for dispersal, the population is then recaptured, and another count is made. This gives the number of marked animals and the number unmarked. This can allow ecologists to estimate of the entire population in a given habitat.
following equation is used to
estimate the population:
S = total number of
individuals in the total population
S = S1
S = 8 X 10
population = 40 individuals
depends on the number of species (species richness of a community) in an
ecosystem and the abundance of each species – the number of individuals of
each species. The populations of an
ecosystem can support demands on abiotic and biotic factors.
The growth of populations depends on limiting factors:
physiological adaptations of organisms only allow them to live in a certain range of pH, light, temp etc – it is part of what defines their niche.
Biotic factors (interactions between organisms)
Intraspecific competition occurs between individuals of the same species eg for a patch of soil to grow on, or a nesting site or food.
Interspecific competition occurs between different species needing the same resource – at the same trophic level.
Plant species compete for light, herbivore species compete for plants or carnivore species compete for prey
Predation – a predator is a limiting factor on growth on the population of its prey and the prey is a limiting factor on the predator population
An index of diversity is used as a measure of the range and numbers of species in an area. It usually takes into account the number of species present and the number of individuals of each species. It can be calculated by the following formulae:
N = total number of organisms of all species in the area
d = index of diversity
n = total number of organisms of each species in the area
Great pond snail
Dragon fly larva
d = 2.6
Comparing both indices, 6.05
is an indicator of greater diversity.
The higher number indicates greater diversity
Great pond snail
extreme environments the diversity of organisms is usually low (has
a low index number). This may
result in an unstable ecosystem in which populations are usually dominated by abiotic
factors. The abiotic factor(s)
are extreme and few species have adaptations allowing them to survive. Therefore food webs are relatively simple, with few food
chains, or connections between them – because few producers survive.
This can produce an unstable ecosystem because a change in the population
of one species can cause big changes in populations of other species.
less hostile environments the diversity of organisms is usually high
(high index number). This may
result in a stable ecosystem in which populations are usually dominated by biotic
factors, and abiotic factors are not extreme. Many species have adaptations that allow them to survive,
including many plants/producers. Therefore
food webs are complex, with many inter-connected food chains.
This results in a stable ecosystem because if the population of one
species changes, there are alternative food sources for populations of other
Ecology is concerned with the question: why is a population the size it is? This
means understanding the various factors that affect the population.
a species is introduced into a new environment its population grows in a
characteristic way. This growth curve is often seen experimentally, for
example bees in a hive, sheep in Tasmania, bacteria in culture. The curve is
called a logistic or sigmoid growth curve.
growth curve has three phases, with different factors being responsible for the
shape of each phase. The actual factors depend on the ecosystem, and this can be
illustrated by considering two contrasting examples: yeast in a flask
(reproducing asexually), and rabbits in a field (reproducing sexually).
in a flask
in a field
growth while yeast starts transcribing genes and synthesising
appropriate enzymes for new conditions.
growth due to small population. Individuals may rarely meet, so few
matings. Long gestation so few births.
Rapid Growth Phase
exponential growth. No limiting factors since relatively low density.
growth, though not exponential. Few limiting factors since relatively
growth due to accumulation of toxic waste products (e.g. ethanol) or
lack of sugar.
growth due to intraspecific competition for food/territory, predation,
the end of phase 3 the population is stable. This population is called the carrying
capacity of the environment (K), and is the maximum population supported by
a particular ecosystem.
different factors interact to determine population size, and it can be very
difficult to determine which factors are the most important. Factors can be
split into two broad group: abiotic factors and biotic factors. We’ll look at
7 different factors.
population is obviously affected by the abiotic environment such as:
temperature; water/humidity; pH; light/shade; soil (edaphic factors); mineral
supply; current (wind/water); topography (altitude, slope, aspect); catastrophes
(floods/fire/frost); pollution. Successful species are generally well adapted to
their abiotic environment.
harsh environments (very cold, very hot, very dry, very acid, etc.) only a few
species will have successfully adapted to the conditions so they will not have
much competition from other species, but in mild environments lots of different
species could live there, so there will be competition. In other words in harsh
environments abiotic factors govern who survives, while in mild environments
biotic factors (such as competition) govern who survives.
abiotic factors vary with the seasons, and this can cause a periodic oscillation
in the population size.
This is only seen in species with a
short life cycle compared to the seasons, such as insects. Species with long
life cycles (longer than a year) do not change with the seasons like this.
population obviously depends on the population of its food supply: if
there is plenty of food the population increases and vice versa. For
example red deer introduced to an Alaskan island at first showed a
population increase, but this large population grazed the vegetation too
quickly for the slow growth to recover, so the food supply dwindled and
the deer population crashed.
competition is competition
for resources (such as food, space, water, light, etc.) between members of different
species, and in general one species will out-compete another one. This can
be demonstrated by growing two different species of the protozoan Paramecium in flasks in a lab. They both grow well in lab flasks
when grown separately, but when grown together P.aurelia out-competes P.caudatum
for food, so the population of P.caudatum
falls due to interspecific competition:
competition is competition
for resources between members of the same species. This is more
significant than interspecific competition, since member of the same species
have the same niche and so compete for exactly the same resources.
competition tends to have a stabilising influence on population size. If the
population gets too big, intraspecific population increases, so the population
falls again. If the population gets too small, intraspecific population
decreases, so the population increases again:
competition is also the driving force behind natural selection, since the
individuals with the “best” genes are more likely to win the competition and
pass on their genes. Some species use aggressive behaviour to minimise real
competition. Ritual fights, displays, threat postures are used to allow some
individuals (the “best”) to reproduce and exclude others (the
“weakest”). This avoids real fights or shortages, and results in an optimum
size for a population.
populations of predators and their prey depend on each other, so they tend to
show cyclical changes. This has been famously measured for populations of lynx
(predator) and hare (prey) in Canada, and can also be demonstrated in a lab
experiment using two species of mite: Eotetranchus
(a herbivore) and Typhlodromus (a
predator). If the population of the prey increases, the predator will have more
food, so its population will start to increase. This means that more prey will
be eaten, so its population will decrease, so causing a cycle in both
Parasitism and Disease
and their hosts have a close symbiotic relationship, so their populations also
oscillate. This is demonstrated by winter moth caterpillars (the host species)
and wasp larvae (parasites on the caterpillars). If the population of parasite
increases, they kill their hosts, so their population decreases. This means
there are fewer hosts for the parasite, so their population decreases. This
allows the host population to recover, so the parasite population also recovers:
A similar pattern is seen for
pathogens and their hosts.
population’s niche refers to its role in its ecosystem. This usually
means its feeding role in the food chain, so a particular population’s niche
could be a producer, a predator, a parasite, a leaf-eater, etc. A more detailed
description of a niche should really include many different aspects such as its
food, its habitat, its reproduction method etc, so gerbils are desert
seed-eating mammals; seaweed is an inter-tidal autotroph; fungi are asexual
soil-living saprophytes. Identifying the different niches in an ecosystem helps
us to understand the interactions between populations. Members of the same
population always have the same niche, and will be well-adapted to that niche,
e.g. nectar feeding birds have long thin beaks.
with narrow niches are called specialists (e.g. anteater). Many different
specialists can coexist in the same habitat because they are not competing, so
this can lead to high diversity, for example warblers in a coniferous forest
feed on insects found at different heights. Specialists rely on a constant
supply of their food, so are generally found in abundant, stable habitats such
as the tropics.
with broad niches are called generalists (e.g. common crow). Generalists
in the same habitat will compete, so there can only be a few, so this can lead
to low diversity. Generalists can cope with a changing food supply (such as
seasonal changes) since they can switch from one food to another or even one
habitat to another (for example by migrating).
niche concept was investigated in some classic experiments in the 1930s by Gause.
He used flasks of different species of the protozoan Paramecium,
which eats bacteria.
These two species of Paramecium share
the same niche, so they compete. P.
aurelia is faster-growing, so it out-competes P.
These two species of Paramecium have
slightly different niches, so they don't compete and can coexist.
is important to understand the distribution in experiment 2. P.
caudatum lives in the upper part of the flask because only it is adapted to
that niche and it has no competition. In the lower part of the flask both
species could survive, but only P.
bursaria is found because it out-competes P.
caudatum. If P. caudatum was
faster-growing it would be found throughout the flask.
niche concept is summarised in the competitive exclusion principle: Two
species cannot coexist in the same habitat if they have the same niche.
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?
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
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
such as dandelion, goosegrass (“weeds”) have small wind-dispersed seeds and
rapid growth, so they become established before larger plants.
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.
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
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.
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.
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.
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.
studying ecosystems in any further detail, it is important to appreciate the
difference between energy and matter. Energy and matter are quite different
things and cannot be inter-converted.
living organisms need energy and matter from their environment. Matter is needed
to make new cells (growth) and to create now organisms (reproduction), while
energy is needed to drive all the chemical and physical processes of life, such
as biosynthesis, active transport and movement.
many relationships between the members of a community in an ecosystem can be
described by food chains and webs. Each stage in a food chain is
called a trophic level, and the arrows represent the flow of energy and
matter through the food chain. Food chains always start with photosynthetic producers
(plants, algae, plankton and photosynthetic bacteria) because, uniquely,
producers are able to extract both energy and matter from the abiotic
environment (energy from the sun, and 98% of their matter from carbon dioxide in
the air, with the remaining 2% from water and minerals in soil). All other
living organisms get both their energy and matter by eating other organisms.
this represents a “typical” food chain, with producers being eaten by animal
consumers, different organisms use a large range of feeding strategies (other
than consuming), leading to a range of different types of food chain. Some of
these strategies are defined below, together with other terms associated with
animal that eats other organisms
consumer that eats plants (= primary consumer).
consumer that eats other animals (= secondary consumer).
consumer at the top of a food chain with no predators.
consumer that eats plants or animals.
human that chooses not to eat animals (humans are omnivores)
organism that manufactures its own food (= producer)
An organism that obtains its energy and mass from other organisms
plankton” i.e. microscopic marine producers.
plankton” i.e. microscopic marine consumers.
animal that hunts and kills animals for food.
animal that is hunted and killed for food.
animal that eats dead animals, but doesn't kill them
and waste matter that is not eaten by consumers
Alternative word for detritus
organism that consumes detritus (= detrivores + saprophytes)
animal that eats detritus.
microbe (bacterium or fungus) that lives on detritus.
living together in a close relationship (= parasitism, mutualism,
organisms living together for mutual benefit.
in which only one organism benefits
organism that feeds on a larger living host organism, harming it
microbe that causes a disease.
food chains need not end with a consumer, and need not even start with a
In general as you go up a food chain
the size of the individuals increases and the number of individuals decreases.
These sorts of observations can be displayed in ecological pyramids, which are
used to quantify food chains. There are three kinds:
These show the numbers of organisms at
each trophic level in a food chain. The width of the bars represent the numbers
using a linear or logarithmic scale, or the bars may be purely qualitative. The
numbers should be normalised for a given area for a terrestrial habitat (usually
m²), or volume for a marine habitat (m³). Pyramids of numbers are most often
triangular (or pyramid) shaped, but can be almost any shape. In the pyramids
below, A shows a typical pyramid of numbers for carnivores; B shows the effect
of a single large producer such as a tree; and C shows a typical parasite food
convey more information, since they consider the total mass of living organisms
(i.e. the biomass) at each trophic level. The biomass should be dry mass
(since water stores no energy) and is measured in kg m-2. The biomass
may be found by drying and weighing the organisms at each trophic level, or by
counting them and multiplying by an average individual mass. Pyramids of biomass
are always pyramid shaped, since if a trophic level gains all its mass
from the level below, then it cannot have more mass than that level (you cannot
weigh more than you eat). The "missing" mass, which is not eaten by
consumers, becomes detritus and is decomposed.
chains represent flows of matter and energy, so two different pyramids
are needed to quantify each flow. Pyramids of energy show how much energy flows
into each trophic level in a given time, so the units are usually something like
kJ m-2 y-1. Pyramids of energy are always pyramidal
(energy cannot be created), and always very shallow, since the transfer of
energy from one trophic level to the next is very inefficient The “missing”
energy, which is not passed on to the next level, is lost eventually as heat.
things can happen to the energy taken in by the organisms in a trophic level:
These three fates are shown in this
energy flow diagram:
all the energy that enters the ecosystem will be converted to heat, which
is lost to space.
cycles between the biotic environment and in the abiotic environment. Simple
inorganic molecules (such as CO2, N2 and H2O)
are assimilated (or fixed) from the abiotic environment by
producers and microbes, and built into complex organic molecules (such as
carbohydrates, proteins and lipids). These organic molecules are passed through
food chains and eventually returned to the abiotic environment again as simple
inorganic molecules by decomposers. Without either producers or decomposers
there would be no nutrient cycling and no life.
simple inorganic molecules are often referred to as nutrients. Nutrients
can be grouped as: major nutrients (molecules containing the elements C,
H and O, comprising >99% of biomass); macronutrients (molecules
containing elements such as N, S, P, K, Ca and Mg, comprising 0.5% of biomass);
and micronutrients or trace elements (0.1% of biomass).
Macronutrients and micronutrients are collectively called minerals. While
the major nutrients are obviously needed in the largest amounts, the growth of
producers is usually limited by the availability of minerals such as nitrate and
are two groups of decomposers:
are animals that eat detritus (such as earthworms and woodlice). They digest
much of the material, but like all animals are unable to digest the
cellulose and lignin in plant cell walls. They break such plant tissue into
much smaller pieces with a larger surface area making it more accessible to
the saprophytes. They also assist saprophytes by excreting useful minerals
such as urea, and by aerating the soil.
(or decomposers) are microbes (fungi and bacteria) that live on detritus.
They digest it by extracellular digestion, and then absorb the soluble
nutrients. Given time, they can completely break down any organic matter
(including cellulose and lignin) to inorganic matter such as carbon dioxide,
water and mineral ions.
material cycles can be constructed for elements such as carbon, nitrogen, oxygen
or sulphur, or for compounds such as water, but they all have the same basic
pattern as the diagram above. We shall only study the carbon and nitrogen cycles
[back to top]
this diagram shows, there are really many carbon cycles here with time scales
ranging from minutes to millions of years. Microbes play the major role at all
more carbon is fixed by microscopic marine producers (algae and
phytoplankton) from CO2 dissolved in the oceans than by
terrestrial plants from CO2 in the air.
the Earth's early history (3000 MY ago) photosynthetic bacteria called cyanobacteria
changed the composition of the Earth's atmosphere by fixing most of the CO2
and replacing it with oxygen. This allowed the first heterotrophic cells to
use oxygen in respiration.
large amount of the fixed carbon is used by marine zooplankton to make
calcium carbonate shells. These are not eaten by consumers and cannot easily
be decomposed, so turn into carboniferous rocks (chalk, limestone, coral,
etc). 99% of the Earth's carbon is in this form.
decomposers are almost all microbes such as fungi and bacteria. Most of the
detritus is in the form of cellulose and other plant fibres, which
eukaryotes cannot digest. Only a few bacteria posses the cellulase
enzymes required to break down plant fibres. Herbivorous animals such as
cows and termites depend on these bacteria in their guts.
of the CO2 that was fixed by ferns during the carboniferous era
(300 MY ago) was sedimented and turned into fossil fuels. The recent mining
and burning of fossil fuels has significantly altered the carbon cycle by
releasing the carbon again, causing a 15% increase in CO2 in just
are involved at most stages of the nitrogen cycle:
Fixation. 78% of the
atmosphere is nitrogen gas (N2), but this is inert and can’t be
used by plants or animals. Nitrogen fixing bacteria reduce nitrogen gas
to ammonia (N2 + 6H g 2NH3),
which dissolves to form ammonium ions (NH4+
). This process uses the enzyme nitrogenase and ATP as a source of
energy. The nitrogen-fixing bacteria may be free-living in soil or water, or
they may live in colonies inside the cells of root nodules of leguminous plants
such as clover or peas. This is an example of mutualism as the plants
gain a source of useful nitrogen from the bacteria, while the bacteria gain
carbohydrates and protection from the plants. Nitrogen gas can also be fixed to
ammonia by humans using the Haber process, and a small amount of nitrogen is
fixed to nitrate by lightning.
can oxidise ammonia to nitrate in two stages: first forming nitrite ions NH4+gNO-2
then forming nitrate ions NO-2gNO-3. These are chemosynthetic bacteria, which means they use the energy released by
nitrification to live, instead of using respiration. Plants can only take up
nitrogen in the form of nitrate.
anaerobic denitrifying bacteria convert nitrate to N2 and NOx,
which is then lost to the air. This represents a constant loss of “useful”
nitrogen from soil, and explains why nitrogen fixation by the nitrifying
bacteria and fertilisers are so important.
saprophytes break down proteins in detritus to form ammonia in two stages: first
they digest proteins to amino acids using extracellular protease enzymes,
then they remove the amino groups from amino acids using deaminase
Impact on the Environment
[back to top]
of the main reasons for studying ecology is to understand the impact humans are
having on the planet. The huge increases in human population over the last few
hundred years has been possible due to the development of intensive farming,
including monoculture, selective breeding, huge farms, mechanisation and the use
of chemical fertilisers and pesticides. However, it is apparent that this
intensive farming is damaging the environment and is becoming increasingly
difficult to sustain. Some farmers are now turning to environmentally-friendly organic
farming. We’ll examine 5 of the main issues and their possible solutions.
the middle of the 20th century, farms were usually small and mixed
(i.e. they grew a variety of crops and kept animals). About a third of the
population worked on farms. The British countryside was described by one
observer in 1943 as “an attractive
patchwork with an infinite variety of small odd-shaped fields bounded by
twisting hedges, narrow winding lanes and small woodlands”. Today the
picture is quite different, with large uninterrupted areas of one colour due to
specialisation in one crop - monoculture. Monoculture increases the
productivity of farmland by growing only the best variety of crop; allowing more
than one crop per year; simplifying sowing and harvesting of the crop; and
reducing labour costs.
monoculture has a major impact on the environment:
farmers are now returning to traditional crop rotations, where different crops
are grown in a field each year. This breaks the life cycles of pests (since
their host is changing); improves soil texture (since different crops have
different root structures and methods of cultivation); and can increase soil
nitrogen (by planting nitrogen-fixing legumes).
Hedges have been planted since Anglo-Saxon times to mark field boundaries and to contain livestock. As they have matured they have diversified to contain a large number of different plant and animal species, some found nowhere else in the UK. Since the Second World War much of the hedgerow has been removed because:
it has now become clear that hedgerows served an important place in the ecology
importance of hedgerows is now being recognised, and farmers can now receive
grants to plant hedgerows. However it takes hundreds of years for new hedgerows
to mature and develop the same diversity as the old ones.
the rate of plant growth in usually limited by the availability of
mineral ions in the soil, then adding more of these ions as fertiliser is
a simple way to improve yields, and this is a keystone of intensive farming. The
most commonly used fertilisers are the soluble inorganic fertilisers
containing nitrate, phosphate and potassium ions (NPK). Inorganic fertilisers
are very effective but also have undesirable effects on the environment. Since
nitrate and ammonium ions are very soluble, they do not remain in the soil for
long and are quickly leached out, ending up in local rivers and lakes and
causing eutrophication. They are also expensive.
alternative solution, which does less harm to the environment, is the use of organic
fertilisers, such as animal manure (farmyard manure or FYM), composted
vegetable matter, crop residues, and sewage sludge. These contain the main
elements found in inorganic fertilisers (NPK), but in organic compounds such as
urea, cellulose, lipids and organic acids. Of course plants cannot make use of
these organic materials in the soil: their roots can only take up inorganic
mineral ions such as nitrate, phosphate and potassium. But the organic compounds
can be digested by soil organisms such as animals, fungi and bacteria, who then
release inorganic ions that the plants can use (refer to the nitrogen cycle).
Some advantages of organic fertilisers are:
disadvantages are that they are bulky and less concentrated in minerals than
inorganic fertilisers, so more needs to be spread on a filed to have a similar
effect. They may contain unwanted substances such as weed seeds, fungal spores,
heavy metals. They are also very smelly!
farmers, a pest is any organism (animal, plant or microbe) that damages
their crops. Some form of pest control has always been needed, whether it is chemical
(e.g. pesticides), biological (e.g. predators) or cultural (e.g.
weeding or a scarecrow). Chemicals pesticide include:
have to be effective against the pest, but have no effect on the crop. They may
kill the pests, or just reduce their population by slowing growth or preventing
reproduction. Intensive farming depends completely on the use of pesticides, and
some wheat crops are treated with 18 different chemicals to combat a variety of
weeds, fungi and insects. In addition, by controlling pests that carry human
disease, they have saved millions of human lives. However, with their widespread
use and success there are problems, the mains ones being persistence and bioaccumulation.
of these are illustrated by DDT (DichloroDiphenylTrichloroethane), an
insecticide used against the malaria mosquito in the 1950s and 60s very
successfully, eradicating malaria from southern Europe. However the population
of certain birds fell dramatically while it was being used, and high
concentrations of DDT were found in their bodies, affecting calcium metabolism
and causing their egg shells to be too thin and fragile. DDT was banned in
developed countries in 1970, and the bird populations have fully recovered.
Alternative pesticides are now used instead, but they are not as effective, and
continued use of DDT may have eradicated malaria in many more places.
This refers to how long a pesticide remains active in the environment. Some
chemicals are broken down by decomposers in the soil (they’re biodegradable)
and so are not persistent, while others cannot be broken down by microbes
(they’re non biodegradable) and so continue to act for many years, and
are classed as persistent pesticides. The early pesticides (such DTT)
were persistent and did a great deal of damage to the environment, and these
have now largely been replaced with biodegradable
insecticides such as carbamates and pyrethroids.
(or Biomagnification). This
refers to the built-up of a chemical through a food chain. DDT is not soluble in
water and is not excreted easily, so it remains in the fat tissue of animals. As
each consumer eats a large mass of the trophic level below it, DTT accumulates
in the fat tissue of animals at the top of the food chain. This food chain shows
typical concentrations of DDT found in a food chain (in parts per million, ppm):
high concentration of DDT in birds explains why the toxic effects of DDT were
first noticed in birds.
refers to the effects of nutrients on aquatic ecosystems. These naturally
progress from being oligotrophic (clean water with few nutrients and
algae) to eutrophic (murky water with many nutrients and plants) and
sometimes to hypertrophic (a swamp with a mass of plants and detritus).
This is in fact a common example of succession. In the context of pollution
“eutrophication” has come to mean a sudden and dramatic increase in
nutrients due to human activity, which disturbs and eventually destroys the food
chain. The main causes are fertilisers leaching off farm fields into the
surrounding water course, and sewage (liquid waste from houses and factories).
These both contain dissolved minerals, such as nitrates and phosphates, which
enrich the water.
producer growth is generally limited by availability of minerals, a sudden
increase in these causes a sudden increase in producer growth. Algae grow
faster than larger plants, so they show a more obvious “bloom”, giving
rise to spectacular phenomena such as red tides. Algae produce oxygen, so
at this point the ecosystem is well oxygenated and fish will thrive.
the fast-growing algae will out-compete larger plants for light, causing
the plants to die. The algae also grow faster than their consumers, so
many will die without being consumed, which is not normal. These both lead
to a sudden increase in detritus. Sewage may also contain organic matter,
which adds to the detritus.
microbes can multiply quickly in response to this, and being aerobic they
use up oxygen faster than it can be replaced by photosynthesis or
diffusion from the air. The decreased oxygen concentration kills larger
aerobic animals and encourages the growth of anaerobic bacteria, who
release toxic waste products.
Oxygen Demand (BOD). This
measures the rate of oxygen consumption by a sample of water, and therefore
gives a good indication of eutrophication. A high BOD means lots of organic
material and aerobic microbes, i.e. eutrophication. The method is simple: a
sample of water is taken and its O2 concentration is measured using
an oxygen meter. The sample is then left in the dark for 5 days at 20°C, and
the O2 is measured again. The BOD is then calculated from: original O2
concentration – final O2 concentration. The more oxygen used up
over the 5 days (in mg.dm-3) the higher the BOD, and the higher the
BOD the more polluted the water is. This table shows some typical BOD values.
ecosystems can slowly recover from a high BOD as oxygen dissolves from the air,
but long-term solutions depend on reducing the amount of minerals leaching into
the water. This can be achieved by applying inorganic fertilisers more
carefully, by using organic fertilisers, by using low-phosphate detergents, and
by removing soluble minerals by precipitation in modern sewage plants. As a last
resort eutrophic lakes can be dredged to remove mineral-rich sediment, but this
is expensive and it takes a long time for the ecosystem to recover. This has
been done in the Norfolk Broads.
activities often affect whole ecosystems. There
are potential conflicts between the need/wish to produce things useful to humans
in the short term and the conservation of ecosystems in the long term.
are the natural climax communities.
They have high diversity, with complex food webs.
Humans have been clearing areas of forest for thousands of years –
leading to deforestation over large areas of Europe, Asia and North
America. Recent and present
deforestation affects manly tropical rain forests.
Tropical rain forests have been estimated to contain 50% of the world’s
standing timber. They represent a
huge store of carbon and sink for carbon dioxide and their destruction may
increase atmospheric concentrations of carbon dioxide by 50%.
They are important in conserving soil nutrients and preventing
large-scale erosion in regions of high rainfall.
They contain a large gene pool of plant resources.
Growth in the human population is increasing demand for land for farming.
causes local extinction of species of trees.
This particularly affects hardwoods, which are in demand for timber,
and softwoods for making paper.
Loss of trees:
Removes the bases of many food webs
Removes the habitats of many other species
Causes local extinction of other populations, or reduction in their size
Reduced the number of species present and numbers of individuals present
Leads to a lower biomass and productivity per hectare.
the diversity produces a less stable and more extreme environment, where abiotic
factors also become more extreme.
Means less photosynthesis
Usually involves burning unwanted trees, and expanding human populations burn more wood for fuel.
With the trees gone (mainly in tropical rain forests) there has been large scale erosion due to the high rainfall in these area washing away the soil.
Less carbon dioxide is removed from the atmosphere and more is added. This adds to the problem of global warming
In forests, most of the nitrate ions (and other mineral ions) absorbed by plants come from decomposition of organic remains – the ions are recycled
Many of the decomposing fungi live in association with the roots of trees
The soil is often a poor source of mineral ions.
Deforestation results in:
Reduced input to the nitrogen cycle
Slower and less recycling of nitrates (and other ions)
Increased loss of nitrates by leeching
soil loses fertility can support lower numbers and fewer species of plants à
involves managing the Earth’s resources so as to restore and maintain a
balance between the requirements of humans and those of other species.
Many attempts are being made to encourage sustainable use of forests.
This involves measuring and comparing yields and profits from
deforestation with alternative uses.
A study in 1989 (obviously the figures will be much different now) of an Amazon rainforest in Peru showed that each hectare of the forest produces fruit and latex (rubber) with an annual market value of $700. If, however, the trees are cut down, the total value of their wood is $1000. Now, trees can only be felled once, but fruit and latex can be harvest every year. This study into sustainable management can allow governments to be persuaded that more money can be made from rainforests by exploiting them on a sustainable basis than by destroying them, therefore it may be worth their while to preserve them.
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Last updated 27/06/2004