human impacts on the environment

Factors controlling population size

There are factors in an ecosystem that affect all the organisms living in it, these factors constantly change so ecosystems are dynamic

Reason 1

Intensity of energy flowing through the ecosystem varies

Reason 2

Biological cycles such as the nitrogen cycle vary the mineral availability

Reason 3

Habitats change over time as succession occurs

Reason 4

New species arrive and some species are no longer present

As a result

The number of individuals in a population doesn’t remain the same , size of a population at a particular time is determined by 4 factors

the 4 factors which determine the size of a population

Birth rate

Death rate or mortality

Immigration

Emigration

How does this work

Birth and immigration increases 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

Extra on birth rate

It also refers to hatching, reproduction by binary fission, all other ways that living organisms increase their numbers

Strategies

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

Type of species 1

Fugitive species- species that are poor at competition so they rely on large capacity for reproduction and dispersal to increase their numbers , invade a new environment rapidly

Example of fugitive species

Algae colonising bare rock

rose bay willow herb colonising soil cleared by fire

Equilibrium species

Control their population by competition within stable habitat , their usual pattern of growth is a sigmoid (S) shaped curve called one-step growth curve

When is this seen

When bacteria are put into fresh nutrient solution or when rabbits are newly introduced to an island

Phase 1 of the one- step growth curve

Lag phase: initially, the population doesn’t increase but then there is a period of slow growth

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Period of adaptation or preparation for growth with intense metabolic activity especially enzyme synthesis

Example in organisms

In bacteria it may last a few minutes to several days

In sexually reproducing organisms , lag phase represents time for individuals to reach sexual maturity ,to find a mate and gestate their young

• Phase 2 of the one-step growth curve

Exponential phase: as numbers increase as long as there is no factor limiting growth, more individuals become available for reproduction

Bacterial cells for example

They divide at a constant rate and the population doubled per unit time , the cell number increases logarithmically and so the exponential phase is also called log phase

However the rate of increase

Cannot be maintained indefinitely because environment resistance sets in, the population still increases but more slowly so the gradient of the graph decreases

• less food available

• concentration of waste products becomes increasingly toxic

• not enough space or nesting sites

Environmental resistance

The environmental factors that slow down population growth

For bacteria these factors include

Available food

Overcrowding

Competition

Accumulation of toxic waste

Another situation

In a less artificial situation, other factors play a part e.g. for rabbit on an island these factors apply but there are additional biotic factors

• predation

• parasitism and disease because increased pop density allows infection to spread more rapidly

• competition from other species for nesting sites and food

Additionally

Abiotic factors such as temp and light intensity may also play a role role

Phase 3 of the one-step growth curve

Stationary phase which 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 environment

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The actual number depends on the resources available e.g. more food increasing carrying capacity

the population

not absolutely constant it fluctuates around carrying capacity in response to environmental change such as the number of predators

Death phase

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

Predators and prey

Predators are normally larger than their prey, tend to kill before they eat

Numbers

The abundance of prey limits the number of predators and the number of predators controls the number of prey

Missed out a part

Calculating population increase from a graph

When a population increase is very large the range of numbers is too great to plot on a linear scale so a log 10 scale is used instead

What is a log 10 scale

In which each mark on the population scale is ten times the previous mark

Give an example of when a log 10 scale is used

for a population of bacteria in a test tube

The actual numbers

Is the antilog of the number on the scale e.g. antilog (10) of 5 is 10^5

Density dependant factors

Have more effect if the population in a given area is larger, affect greater proportion of the population if the population is denser, they are biotic factors

Example 1

Parasitism, if the population is denser parasites are transmitted more efficiently and greater proportion of individuals are affected

Example 2

If the prey density is higher, predator encounters prey more readily and eats a greater proportion of them

Density independent factors

Effect of abiotic factors doesn’t depend on the population density , the effect is the same regardless of the size of the population, usually due to a sudden change in an abiotic factor e.g. flood or fire

Example

Daphnia, the water flea undergoes population crashes when the temp suddenly falls

Honeybee populations are killed by neonicotinoid insecticides

Temp fluctuations

In general the birth rate and death rate regulates the size of a population , however populations fluctuate they don’t 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

Regulation

Population size is regulated by negative feedback

Part 1 of negative feedback

If the population rises above the set point, a density dependant factor increases mortality or reduces breeding to such an extent that the population declines

Part 2 of negative feedback

If that the population falls below the set point environmental resistance is temporarily relived so that the population rises again

Biogeography

The study of species abundance and distribution

New habitat

When a new habitat is assessed, physical features are described first because they determine the number and types of plants that live there

Examples of physical features

Soil type, rainfall , temperature

Specific examples that I haven’t mentioned

Animals in a habitat

Depend on the plants, so in a new habitat plants are described before animals

Abundance of a species

Measure of how many individuals exist in a habitat

The number of individuals in a species in a given area or volume

How can animal abundance be assessed

1)Calpture-Mark-Recapture experiment using the Lincoln index calculation

2)kick sampling in a stream and counting aquatic invertebrates

How can the abundance of plant species be assessed

1)using a quadrant to calculate the mean number of individuals in several quadrants 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

Distribution of a species

The area or volume in which organisms of a species are 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

Results

A small area indicates that a species is under threat of extinction

When is this used 1

Botanists use this technique to assess the distribution of threatened plant species

When is this used

Mining companies and road building authorities can then be lobbied to protect specific sites and hence mitigate species loss

If the habitat is not uniform

A transect is a useful technique for displaying the variation in organisms and its correlation with a changing abiotic factor

What is a transect

A line along which abundance is assessed , shows the organisms that lie on a line at measured intervals

Suitability of a transect

suitable for assessing plants or sessile animals but it’s not suitable for motile animals because they move, instead the distribution of animals is assessed by direct observation of individuals or their nests, faecal deposits or markings on vegetation

Belt transect

Shows the abundance data for a given area at measured distances along the transect, a quadrat is placed at each coordinate along the transect and readings are taken

What can readings be taken for

The density of chosen species

The percentage frequency of chosen species

The percentage area cover for all species

Description of an ecosystem

A community comprises many species living and interacting together, interaction of orgs with each other and with the non living factors in their environment such as aid, soil, water comprises an ecosystem

Definition of an ecosystem

Characteristic community of interdependent species interacting with the abiotic components of their habitat

Ecosystems can be (1)

Small: human large intestine and community of microorganisms

Large: seas cover about 70%of the earths surface , pacific basin is the largest marine ecosystem, whole planet may be considered to be one large ecosystem

Ecosystems can be (2)

Temporary: puddle left after rain

last millions of years: lake Baikal in Siberia existed for 25 million years

Energy and ecosystems

Biotic and abiotic components of an ecosystem are linked by energy flow and by the cycling of nutrients

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Energy is the ability to do work, no change happens unless energy changes occur , the function 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 (laws of thermodynamics)

Energy source 1

Energy derived from unequal distribution of protons allowed non living system in cavities of alkaline hydrothermal vents to make the transition into livings sytem

Energy source 2

Early organisms used the energy released by chemical reactions to make carbohydrates by chemosynthesis , the electrons they need to reduce CO2 or methane to sugar are derived from oxidation of inorganic molecules such as H2 or hydrogen sulphide

Additionally

Some archea and bacteria still do but they tend to inhabit marginal ecosystems

Energy source 3

The most significant energy source for ecosystems now is the light energy radiating from thr sun because light is the energy source for photosynthesis

Much more info on each definition that I haven’t included

Habitat

Place in which an organism lives, microhabitat is a very small area that differs from its surroundings and has the features that make it suitable for a particular species

Community

Interacting populations of two or more species in the same habitat at the time

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 organism through a food chain

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Energy available to a trophic level contributes to its biomass , food chains can therefore be thought of as a means of transferring biomass

Trophic level

Feeding level, the number of times that energy has been transferred between the sun and successive organisms along a food chain

Biomass

Mass of biological material in living or recently living organisms

Start of the food chain

Green plants, Cyanobacteria and some protists are producers because they incorporate the suns energy into carbohydrates which are the food and therefore the source of energy for successive organisms in the food chain

How do they do that

They trap solar energy and synthesise sugars from inorganic compounds by photosynthesis,

However

Only a small proportion of the total energy that reaches the plant as light is incorporated into the plants tissues

Consumers

Herbivores are primary consumers, they are animals that feed on plants

Carnivores are secondary, tertiary and higher consumers they are animal that feed on other animal

Each of these groups

Operates at a feeding or trophic level with energy passing to a higher trophic level as material is eaten and energy in the food consumed is incorporated into molecules of the consumer

Energy loss

As energy is passed along the food chain , there is a loss from the food chain at each level, as energy is lost at each tropic level the energy flowing through the ecosystem reduces and ultimately the energy leaves the system as heat

Therefore

The number of links in a chain is limited to 4 or 5 because after 4 or 5 trophic levels there is not enough energy to support another one, although the actual length of a food chain may depend upon several interacting factors

Factor 1 affecting length of a food chain

The more energy that enters a food chain in the first tropic level. I.e. the more energy fixed in photosynthesis , the longer the food chain can be

Example

A tropical food chain which have high light all year, tend to be longer than arctic food chains which have much less light all year

Factor 2 affecting length of a food chain

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

Factor 3 affecting length of a food chain

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

Factor 4 affecting length of a food chain

Larger ecosystems can support longer food chain

Factor 5 affecting length of a food chain

3-dimensional environments such as aquatic systems and forest canopies have longer food chains than 2-dimensional habitats such as grasslands

What does a food web show

Shows how organisms in a community interact with each other through food they eat m food chain is a linear sequence of organisms in a food web

Grazing food chain with 3 consumers summary: producer (1st trophic level) —> primary consumer e.g. herbivore (2nd trophic level) —> secondary consumer (3rd trophic level) —> tertiary consumer (4th trophic level)

When producers and consumers die

Energy remains in the organic compounds of which they are made , detritivores and decomposes feed as saprobionts so they derive their energy from dead and decaying organisms and they contribute to the recycling of nutrients

Saprobionts

A microorganism that obtains its food from the dead or decaying remains of other organism

Detritivores

Feed on small fragments of organic debris (detritus) , the remains of dead organisms and fallen leaves

Examples of detritivores.

Earthworms, wood lice and milipedes

Decomposers

Obtain their nutrients from dead organisms and animal waste , they complete the process of decomposition started by detritivores

Examples of decomposers

Microbes such as fungi and bacteria