Biology: Population Size & Ecosystems

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Population Size & Ecosystems

Population growth is controlled by biological and environmental factors. Succession is the change of structure and species composition of communities over time. Energy passes along a food chain as biomass, but its loss limits their length. All organisms require carbon and nitrogen and microorganisms play a key role in recycling them through the ecosystem. Agriculture, deforestation and pollution have drastically altered ecosystems and so agricultural practices must be carefully evaluated and altered to support the continuing increase in human population.

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Population Growth

Factors Controlling Population Size

There are factors in an ecosystem that affect all the organisms living in it and these factors constantly change. So ecosystems are dynamic

1. The intensity of energy flowing through the ecosystem varies.

2. Biological cycles, such as the nitrogen cycle, vary the mineral availability.

3. Habitats change over time as succession occurs.

4. New species arrive and some species are no longer present.

As a result, the number of individuals in a population does not remain the same. The size of a population at a particular time is determined by:

1. Birth rate, or natality, which also refers to hatching, reproduction by binary fission and all other ways that living organisms increase their numbers.

2. Death rate, or mortality.

3. Immigration.

4. Emigration.

Birth and immigration increase population size, while death and emigration decrease it. When the combined effects of birth and immigration exceed those of death and emigration, the population size increases.

Population Size

Different strategies for population growth are used by different species, depending on their characteristics:

1. Fugitive species are species that are poor at competition: instead, they rely on a large capacity for reproduction and dispersal to increase their numbers. They invade a new environment rapidly, e.g. algae colonising bare rock; rose bay willow herb colonising soil cleared by fire.

2. Equilibrium species control their population by competition within a stable habitat. Their usual pattern of growth is a sigmoid (S-shaped) curve called the one-step growth curve. It is seen, for example, when bacteria are put into fresh nutrient solution or when rabbits are newly introduced to an island.

The One-Step Growth Curve

The one-step growth has four phases:

1. The lag phase. Initially, the population does not increase but then there is a period of slow growth. In bacteria, it may last from a few minutes to several days. It is a period of adaptation or preparation for growth, with intense metabolic activity, especially enzyme synthesis. In sexually reproducing organisms, such as rabbits, the lag phase represents the time for individuals to reach sexual maturity, to find a mate and gestate their young.

2. The exponential phase. As numbers increase, as long as there is no factor limiting growth, more individuals become available for reproduction. Bacterial cells divide at a constant rate and the population doubles per unit time. The cell number increases logarithmically and so the exponential phase is also called the log phase.

This rate of increase cannot be maintained indefinitely because environmental resistance sets in:

- There is less food available.

- The concentration of waste products becomes increasingly toxic

- There is not enough space or nesting sites.

The population still increases, but more slowly, so the gradient of the graph decreases.

Environmental resistance includes all the factors that may limit the growth of a population. For bacteria in a flask these factors include:

- Available food

- Overcrowding

- Competition

- The accumulation of toxic waste.

However, in a less artificial situation other factors ma play a part. For the rabbits on an island, these same factors apply but there are additional biotic factors including:

- Predation

- Parasitism and disease because increase population density allows infection to spread more rapidly

- Competition from other species for nesting sites and food.

Abiotic factors such as temperature and light intensity may also play a role in affecting population size.

3. The stationary phase occurs when the birth rate is equal to the death rate. The population has reached its maximum size, which is the carrying capacity for that particular environment, i.e. the maximum number of individuals that an area can support. The actual number depends on the resources available, e.g. more good increases the carrying capacity. The population is not absolutely constant, and it fluctuates around the carrying capacity in response to environmental changes, e.g, the number of predators.

4. The death phase. The factors that slow population growth at the end of the lag phase become more significant and population size decreases until the death rate is greater than the birth rate and the graph has a negative gradient.

Environmental Resistance and the Carrying Capacity

Predators are normally larger than their prey and tend to kill before they eat. The abundance of prey limits the numbers of predators and the number of predators controls the number of prey. A predator-prey relationship causes both populations to oscillate and these oscillations are regulated by negative feedback. In a classic experiment using data collected since 1920, the numbers of lynx and snow-shoe hares in parts of Canada have been estimated. Their numbers fluctuated around the carrying capacity of each because:

1. A large number of lynx predate hares, so the hare population decreases.

2. Then there is not enough food for the lynx, so lynx numbers go down.

3. There is therefore less predation on the hares, so hare numbers increase again.

4. There is more prey for lynx, so lynx numbers go up.

This cycle repeats at approximately 10-year intervals and illustrates how population numbers at the carrying capacity would depend on the numbers in other species. An equivalent explanation would apply to other predator-prey relationships, e.g. owl and mouse numbers in a woodland, and herbivore-plant relationships, e.g. koalas and eucalyptus.

65 million years ago, non-avian dinosaurs became extinct. It is likely that they exceeded their carrying capacity because they ran out of food. Evidence shows an asteroid impact and excessive volcanic activity. They put so much dust into the atmosphere, that the consequent lack of sunlight killed the plants. Herbivorous dinosaurs, e.g. Triceratops horridus, therefore had no food and died. This meant that the carnivorous dinosaurs, e.g. Tyrannosaurus rex, had no food either, so they died.

Calculating Population Increase from a Graph

When a population increase is very large, for example in a population of bacteria in a test tube, the range of numbers is too great to plot a graph on a linear scale. A log10 scale is therefore used, in which each mark on the population scale is ten times the previous mark.

The rate of growth in the exponential phase =

number of bacteria at day 5 - number of bacteria at day 2 per day

9 - 4

As a log scale is plotted, we cannot read the actual numbers at day 9 and at day 4. The actual number is the antilog of the number on the scale, i.e. the number for which the numbers on the axis are the logs.

Therefore, the rate of growth in the exponential phase = antilog 5 - antilog 2 per day

9-4

In the example shown here, the log is a whole number:

Antilog 5 = 10^5 = 100000, and antilog 2 = 10^2 = 100

Therefore the rate of growth in the exponential phase =

99900 = 19980 per day

5

If the log is not a whole number, you can use a calculator to find the antilog.

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Factors That Regulate Population Increase

Some environmental factors have more effect if the population in a given area is larger. These factors affect a greater proportion of the population if the population is denser and so they are called density-dependent factors. They are biotic factors and include disease, parasitism and depletion of food supply, e.g. if a population is denser, parasites are transmitted more efficiently and a greater proportion of individuals are affected. Similarly, if the prey density is higher, the predator encounters prey more readily and eats a greater proportion of them.

The effect of abiotic factors in the environment does not depend on the population density. These are density-independent factors. The effect is the same regardless of the size of a population and is usually due to a sudden change in an abiotic factor, e.g. flood or fire. Daphnia, the water flea, undergoes population crashes when the temperature suddenly falls; honeybee populations are killed by neonicotinoid insecticides.

Populations Fluctuate in Numbers

In general, the balance between the birth rate and the death rate regulates the size of a population. However, populations fluctuate; they do not remain constant in size, although in equilibrium species these fluctuations are not usually large or erratic. The numbers in their populations fluctuate around a set point, which is the carrying capacity. The population size is regulated by negative feedback:

1. If the population rises above a certain point, a density-dependent factor increase mortality or reduces breeding to such as extent that the population declines.

2. If the population falls below the set point, environmental resistance is temporarily relieved so that the population rises again.

A population explosion occasionally occurs, e.g. greenfly. There may be a decrease in the number of their predators or a temporary favourable change in weather. Such fluctuations self-regulate by negative feedback.

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The Abundance and Distribution of Organisms in a Habitat

The study of species abundance and distribution is called biogeography. Alfred R Wallace was the first person to model biogeographic regions, and he defined six around the world. In his study of birds and vertebrates, he saw that rivers and mountain ranges marked the boundaries of many species' ranges. He saw different animals in similar habitats. This did not correspond with the prevailing explanation that organisms were all created to suit a particular environment, and it contributed to his understanding of natural selection.

A New Habitat

When a new habitat is assessed, physical features, e.g. soil type and temperature, are described first because they determine the number and types of plants that live there:

1. A soil derived from granite bedrock, e.g. the Lizard, Cornwall, is more acid than derived from chalk, e.g. the South Downs. Consequently, sundew grows on the Lizard and half of all the native orchid species of the UK grow on the South Downs.

2. If rainfall is very high throughout the year, e.g. parts of south-east Australia, tree ferns may be found, but in some very dry regions, e.g. south-western USA, cactuses are common.

3. A very cold habitat, e.g. parts of Finland, may support only lichens but a warmer habitat, e.g. Guinea, may support a vast number of plant types.

The animals in a habitat depend on the plants, e.g. pandas in the wild are only found where bamboo grows. So in a new habitat, plants are described before animals.

Measuring Abundance

The abundance of a species is a measure of how many individuals exist in a habitat.

Animal abundance can be assessed by:

1. Capture-mark-recapture experiments, using the Lincoln index calculation.

2. Kick sampling in a stream and counting aquatic invertebrates.

The abundance of plant species can be measured by:

1. Using a quadrat to calculate the mean number of individuals in several quadrats of known area, to find the density, i.e. number/metre^2.

2. Estimating percentage cover of a plant in which individuals are hard to recognise.

3. Estimating percentage frequency.

Measuring Distribution

The distribution of a species describes the area or volume in which it is found.

If a habitat is uniform, the positions of the outermost plants can be marked on a map and the area they surround can be measured. A small area may indicate that a species is under threat of extinction. Botanists use this technique to assess the distribution of threatened plant species. Mining companies and road building authorities can then be lobbied to protect specific sites and hence mitigate species loss.

If a habitat is not uniform, a transect is a useful technique for displaying the variation in organisms and its correlation with a changing abiotic factor. A transect is a line along which abundance is assessed.

1. A line transect shows the organisms that lie on a line, measured at intervals.

2. A belt transect shows abundance data for a given area at measured distances along the transect. A quadrat is placed at each co-ordinate along the transect and readings may be taken for:

- The density of chosen species

- The percentage frequency of chosen species

- The percentage area cover for all species.

A kite diagram shows the percentage area cover of a species across a belt transect.

A footpath is most trampled in the middle and less so at the edges. A transect shows which species are most resistant to trampling, because they grow in the middle of the path. Both the line transect and the belt transect for the footpath show that plantain and meadow grass are the least sensitive to trampling as they are found in the middle of the path. Red fescue and London god are only found at the edges: they are more sensitive to trampling. The belt transect, however, being two-dimensional, provides more information about species distribution that the line transect, which is one-dimensional.

A transect is suitable for assessing plants or sessile animals but it is not suitable for motile animals, because they move. The distribution of animals is assessed by direct observation of individuals or their nests, faecal deposits or markings on vegetation.

Kite diagrams are useful because they show clearly the abundance and distribution of species along a transect. But numbers are rounded when they are constructed and so accuracy is lost, in comparison with a table of data.

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Ecosystems

A community comprises many species living and interacting together. The interaction o organisms with each other and with the non-living factors in their environment, such as the air, soil and water, comprises an ecosystem. The biotic and abiotic components of the ecosystem are linked by energy flow and by the cycling of nutrients.

1. Ecosystems can be small, e.g. the human large intestine and its community of microorganisms. Ecosystems can be very large. Seas, for example, cover about 70% of the Earth's surface and the Pacific Basin is the largest marine ecosystem. There are those who consider the whole planet to be one large ecosystem.

2. Ecosystems can be temporary such as a puddle left after the rain or may last many millions of years, e.g. Lake Baikal in Siberia has existed for 25 million years.

Energy and Ecosystems

Energy is the ability to do work. No change happens unless energy changes occur. So the functioning of an ecosystem can be thought of as a sequence of energy changes, in which energy flows through the components of the ecosystem, subject to certain rules, described as the Laws of Thermodynamics.

There are many possible energy sources on earth, e.g. geothermal, electrical and chemical.

1. It is likely that the energy derived from an unequal distribution of protons allowed the non-living systems in the cavities of alkaline hydrothermal vents to make the transition into living systems.

2. Early organisms used the energy released by chemical reactions to make carbohydrates by chemiosmosis. The electrons they need to reduce carbon dioxide or methane to sugar are derived from the oxidation of inorganic molecules such as hydrogen or hydrogen sulphide. Some Archaea and bacteria still do, but they tend to inhabit marginal ecosystems.

3. The most significant energy source for ecosystems now is the light energy radiating from the Sun because light is the energy source of photosynthesis.

The energy to support ecosystems comes mainly from the Sun, incorporated by photosynthesis, but also comes from chemical reactions, incorporated in chemosynthesis.

It is suggested that chemosynthesis may support life below the surface of Mars, on Jupiter's moon, Europa, or Saturn's moons, Enceladus and Titan, and perhaps elsewhere.

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Biotic Components on an Ecosystem

Habitats

A habitat is an ecological or environmental area such as a freshwater stream or a temperate grassland. It provides the means of survival such as food, water, soil, appropriate temperature and pH. A habitat is not necessarily a geographical area, as part of one organism may be the habitat for another, e.g. the human duodenum can be the habitat for a tapeworm. A microhabitat is a very small area that differs from its surroundings and has the features that make it suitable for a particular species, e.g. the cabbage looper, which is a moth larva, feeds on the lower, but not the upper surface of leaves of cabbages and related crops.

Communities

The members of a species, living and interacting together in a habitat from a population. Populations interact to form a community. Community ecology studies the interactions of the species, related to their distribution and abundance, and their genotypic and phenotypic differences. It considers food web structure and predator-prey relationships.

Biomass Transfer

The ultimate source of energy for most ecosystems is sunlight. Photosynthetic organisms convert sunlight energy into chemical energy which passes from organism to organisms through a food chain. The study of the flow of energy through the ecosystem is ecological energetics. The energy available to a trophic level contributes to its biomass. Food chains can, therefore, be thought of as a means of transferring biomass.

Habitat: the place in which an organism lives.

Community: interacting populations of two or more species in the same habitat at the time.

Trophic level: feeding level; the number of times that energy has been transferred between the Sun and successive organisms along a food chain.

Biomass: the mass of biological materials in living, or recently living, organisms.

Ecosystem: a characteristic community of interdependent species interacting with the abiotic components of their habitat.

Food Chains

1. Green plants, cyanobacteria and some Protoctista are called producers because they incorporate the Sun's energy into carbohydrates, which are the food, and therefore the energy source, for successive organisms in the food chain. They trap solar energy and synthesise sugars from inorganic compounds by photosynthesis.

2. Only small proportion of the total energy that reaches the plant as light is incorporated into the plant's tissues.

3. Herbivores are primary consumers, i.e. they are animals that feed on plants. Carnivores are secondary, tertiary and higher consumers, i.e. animals that feed on other animals.

4. Each of these groups operates at a feeding or trophic level with energy passing to a high trophic level as material is eaten.

5. Energy in the food consumed is incorporated into the molecules of the consumer.

6. As energy is passed along the food chain there is a loss from the food chain at each level.

7. As energy is lost at each trophic level, the energy flowing through the ecosystem reduces and ultimately the energy leaves the systems as heat.

Plant parasites such as dodder and mistletoe are primary consumers even though they are plants, because they get their energy from their host, which is also a plant.

Decomposition

When producers and consumers die, energy remains in the organic compounds of which they are made. Detritivores and decomposers feed as saprobionts, i.e. they derive their energy from dead and decaying organisms, and they contribute to the recycling of nutrients.

1. Detritivores are organisms, such as earthworms, woodlice and millipedes, which feed on small fragments of organic debris. This is detritus, the remains of dead organisms and fallen leaves.

2. Decomposers are microbes such as bacteria and fungi that obtain nutrients from dead organisms and animal waste. They complete the process of decomposition started by detritivores.

Saprobiont: a microorganism that obtains its food from the dead or decaying remains of other organisms.

A detritus food chain can be summarised as: detritus ---> detritivore ---> decomposer.

Most of the energy entering a food chain is transferred through these detritus food chains rather than being transferred between consumers.

Food Chains and Their Length

A food web shows how the organisms in a community interact with each other through the food that they eat. A food chain is a linear sequence of organisms in a food web. A "grazing food chain" with three consumers can be summarised as:

producer ---> primary consumer (herbivore) ---> secondary consumer ---> tertiary consumer

Energy is lost at each link along the food chain. After four or five trophic levels, there is not enough energy to support another one. The number of links in the chain is, therefore, normally limited to four or five, although their actual length may depend upon several interacting factors:

1. The more energy that enters a food chain in the first trophic level, i.e. the more energy fixed in photosynthesis, the longer the food chain can be. So trophic food chains, which have high light all year, tend to be longer than Arctic food chains, which have much less light.

2. If energy is transferred more efficiently between trophic levels, the food chain is longer.

3. Predators and prey populations fluctuate and their relative abundance can affect the food chain length.

4. Larger ecosystems can support longer food chains.

5. Three-dimensional environments such as aquatic systems and forest canopies have longer food chains than two-dimensional habitats such as grasslands.

At a given light intensity, a leaf absorbs more energy per unit area if it is thicker, has a thinner cuticle, more chloroplasts, more chlorophyll or more grana.

6. The energy flowing from one organism to another in the food chain originates as sunlight.

7. About 60% of the light energy that falls on a plant may not be absorbed by photosynthetic pigments because it could be:

- The wrong wavelength

- Reflected

- Transmitted straight through the leaf.

Photosynthetic efficiency (PE) is a measure of the ability of a plant to trap light energy:

PE = quantity of light energy incorporated into product x 100

quantity of energy falling on the plant

In wild plants PE may be as low as 1%, but it is higher in crop plants, as they have been selectively bred for high productivity, e.g. the sugarcane, PE = 7-8%. The efficiency depends on a plant's genotype, and environmental factors such as light intensity and temperature.

Primary Productivity

Gross primary productivity (GPP) is the rate of production of chemical energy in organic molecules by photosynthesis in a given area, in a given time, so its units are kJm-2y-1. A substantial proportion of gross production is released by the respiration of the plant to fuel, for example, protein synthesis. The remains is net (or nett) primary productivity (NPP) and this represents the energy in the plant's biomass. This is the food available to primary consumers, or, in crop plants this represents the yield which may be harvested.

GPP - respiration = NPP

Both GPP and NPP are higher if plants have a high photosynthetic efficiency. Unlike crop plants, most plants are not highly selected for productivity and are not grown under ideal conditions, so, in contrast to the figures for crops, a common figure for GPP is around 1% of incident radiation and for NPP, 0.5%.

Energy Flow through Food Chains

Primary productivity is the rate at which producers convert energy into biomass. Secondary productivity is the rate at which consumers accumulate energy from assimilated food in their cells or tissues. Secondary production occurs in heterotrophs, including animals, fungi, some bacteria and some Protoctista.

A primary consumer converts about 10% of ingested energy into biomass. A secondary or tertiary consumer converts about 20%.

As energy is passed along the food chain, there is a loss from the food chain at each level because:

1. There is energy in molecules that are egested. Cows feed on plant material that contains cellulose, which is digested by their mutualistic microorganisms. Its remains pass out of the body in faeces, which contain a high proportion of undigested material. This energy, however, is not loss from the ecosystem as it is available to decomposers. A carnivore's protein-rich diet is more readily and efficiently digested. Consequently, only about 20% of its energy intake is lost in waste products, compared with about 60% in herbivores.

2. Energy lost as heat following processes fuelled by the energy generated in respiration, including muscle contraction.

3. Energy remains in molecules in parts of an animal that may not be eaten, for example horns, fur and bones.

Herbivores, the primary consumers, have a conversion efficiency of about 10%, that is, for every 100g of plant material ingested, only about 10g is incorporated into their biomass. This means that only part of the NPP of the whole ecosystem is transferred to the primary consumers: herbivores do not eat all the vegetation available to them; for example, cattle grazing a field eat grasses and many other small herbaceous plants but not roots and the woody parts of plants. Carnivores are more efficient at energy conversion tha herbivores, because their food is more quickly and easily digested, so they have a conversion efficiency of about 20%.

Calculating the Efficiency of Energy Transfer

The efficiency of energy transfer between trophic levels is calculated using the following equation:

efficiency of energy transfer = energy incorporated into biomass after transfer x 100

energy available in biomass before transfer

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Ecological Pyramids

An ecological pyramid is a diagram that shows a particular feature of each trophic level in an ecosystem. Producers are drawn at the bottom and successive trophic levels are drawn above. The number of organisms, the energy or the biomass contained in each trophic level, is represented by a bar for each level. The area of each bar of the pyramid is proportional to the total number, energy or biomass at each trophic level. These are known as pyramids of numbers, energy and biomass respectively.

Although ecological pyramids are useful in describing ecosystems, they do not take account of the fact that some organisms operate at more than one trophic level at the same time. A human, for example, is an omnivore. When eating an egg sandwich, the human is:

1. A primary consumer when eating the bread, which is made from wheat, a producer.

2. A secondary consumer when eating the egg, which is produced by a chicken, a primary consumer.

The diagram below shows two different pyramids for the same ecosystems.

The Pyramid of Numbers

The pyramid of numbers is relatively easy to construct but:

1. It does not take into account size of organisms

2. It does not recognise the difference between juvenile and adult forms.

3. The range of numbers may be so large that it may be difficult to draw to scale.

4. The pyramid, or part of it, may be inverted if one trophic level has more organisms than the previous trophic level, e.g. a single oak tree may have many thousands of caterpillars eating its leaves.

If you measure the sizes of all the animals in a community, most are small. As their size increase, their numbers decrease, i.e. their sizes and abundance can be drawn as a pyrmaid. This was first described by Elton (1927) and he went on to develop the concept of the pyramid of numbers.

The Pyramid of Energy

The most accurate way of representing feeding relationships is to use a pyramid of energy, which shows the energy transferred from one trophic level to the next, per unit area or volume, per unit time. As material passes up through the food chain, energy is lost from the ecosystem so the area of the bars decreases accordingly. Since only some of the energy is passed on from one level to the next, energy pyramids are never inverted. Pyramids of energy make it easy to compare the efficiency of energy transfer between trophic levels in different communities.

In 1942, Lindeman stated that 10% of energy was lost between trophic levels. This was the first description of a pyramid of energy.

The Pyramid of Biomass

Energy is incorporated into the macromolecules that make up the biomass of an organism. If the available energy is greater, more biomass can be supported, so a pyramid of biomass is closely related to the energy passing through the ecosystem. However:

1. Pyramids of biomass are difficult to measure accurately as, for example, all the plants roots must must be included.

2. They do not indicate productivity or the amount of energy flowing through the ecosystem.

3. Pyramids of biomass may be inverted.

4. In a pyramid of biomass, a trophic level may seem to contribute more to the next trophic level than it actually does. This is because many organisms contain structures with mass that will not transfer to the next trophic level, e.g. bones, beaks.

5. Species with similar biomass have different life spans. A direct comparison of the total biomasses is therefore misleading.

Inverted Pyramid of Biomass

In an aquatic ecosystem, phytoplankton are the major producers. A lot of energy flows through the first trophic level and the phytoplankton reproduce very quickly. Some are eaten immediately, leaving just enough to maintain the population. So their standing crop, i.e. the mass of individuals present at a given time is lower than the biomass of zooplankton, which eat them.

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Community and Succession

Ecosystems are dynamic and subject to change.