Populations and Ecosystems

Ecology

The study of living things and their interactions with each other and their environment

Ecosystems

A characteristic community of interdependent species interacting with the abiotic components of their habitats

Populations

All the members of one species in an area that can breed with each other

Community

All the members of all species in an area

Habitat

The place in an ecosystem where an organism lives

Niche

The role of an organism in an ecosystem, generally a feeding role

Biotic

All the living and organic components of an ecosystem

Abiotic

All the non-living parts of an ecosystem

Population size equation

Reproduction + Immigration = Death + Emigration

Bacteria Growth Curve

Lag phase - Synthesis of enzymes and DNA replications], numbers of individuals rises slowly

Log phase - High food availability, exponential growth, cells divide rapidly and reproduction exceeds death rate and the population doubles for each unit of time

Stationary phase - Nutrients start to run out, reproduction and deaths int he population stabilise

Death phase - Toxins in the broth build up and nutrients run out to such an extent that deaths exceed reproduction and populations decreases

Animal Growth Curve

Lag phase - An animal first arrives in an area and the populations increases very slowly as there are not enough individuals to breed and time is needed for them to reach sexual maturity

Log phase - With plenty of food, the population increases exponentially, competition for food, habitats and territory is low so population increases exponentially

Carrying capacity - Population maximum. This is where the competition for food between the animals is very high and there is not enough food for all individuals. There can also be environmental resistance, which causes the population to fall until it is under the carrying capacity. This causes fluctuations

Density Dependent Factors

Competition, Predation, and Disease.

The effect of the factor on the population is greater when it is larger.

They tend to be biotic, and limit the size of the population therefore determining carrying capacity.

Interspecific competition

Between different species

Intraspecific Competition

Between members of the same species

Density Independent Factors

Affects large or small populations just the same. They tend to be abiotic factors and are sudden changes to the environment due to natural disasters.

Food chains

Energy is transferred through ecosystems by food chains, The source of all energy in food chains is sunlight. This energy is used to fix carbon dioxide molecules in photosynthesis by Photoautotrophs.

Food chain levels

Energy is transferred through feeding primary consumers (herbivores) then to secondary consumers (carnivores) and on to higher levels of they can be sustained.

Food web

A food web represents the energy flow through an ecosystem and seeks to represent the more complex feeding relationships, where some organisms gain energy at more than on trophic level

First energy transfer

The first energy transfer is from sunlight to plants. The efficiency is about 0.2% as not all the light hitting the leaf can be used in photosynthesis. Some energy is reflected by the waxy cuticle, some is the wrong wavelength to be absorbed, and some is transmitted through the leaf as it doesn’t hit a chloroplast

Gross Primary Productivity (GPP)

The fate of production of organism molecules by photosynthesis in a given area per unit time. The units are kJ m^-2 yr^-1. The o ant itself uses some of these organic molecules to respire - some energy is lost as heat and carbon dioxide returned to the atmosphere

Net Primary Productivity (NPP)

The organic molecules assimilated represent this. These are the organic molecules that are available to the next trophic level, the primary consumers

Equation for Energy Flow

GPP - R = NPP

R = Respiration

NPP transfers to the next level

Not all of the NPP is transferred to the primary consumer as they don’t tend to eat the whole plant. Of the energy is consumes, some of it won’t be digestible (cellulose) , this will pass straight through the gut and be egressed as faeces (F). The rest will be assimilated (taken into the cells). Any excess animal acids will be converted to nitrogenous waste and will be excreted as uric acid (U). Some of it will be used in restoration to provide ATP for movement and will be lost as heat (R). What is left of the food is consumed (C) and is the productivity of the caterpillar (P). The productivity is then passes on to a secondary consumer.

Equation for productivity

P = C - (F + U + R)

Energy efficiency loss

As energy is lost at each trophic level, less and less is available to be passes on at each level. Eventually, there will not be enough energy to sustain a higher trophic level. This limits how many trophic levels there are in a food chain.

Energy loss through respiration

Much of the energy loss is from respiration, so when comparing efficiencies, one thing to consider is how much respiration is going on and why.

Aquatic food chains tend to be longer than terrestrial as aquatic animals don’t use much energy to support themselves due to buoyancy from water. Also there are not many endothermic animals in aquatic systems so less respiratory losses. Invertebrates are ectothermic and therefore lose less energy from respiration as they don’t use the heat from respiration to keep warm. Therefore ectotherms are more efficient

Smaller organisms have a higher SA:V ratio, so a small mammal or bird will lose more heat and will have to respire at a higher rate to keep warm. Larger organisms are more efficient.

Energy losses from excretion

Other losses are those from faeces, as herbivores consume a higher proportion of cellulose which is less digestible so they lose a lot of their food as faeces. This makes them less efficient than carnivores

Increasing productivity

In agriculture productivity can be increases by cutting down respiratory losses by keeping the animals warm and restricting movement. Another way would be to feed herbivores more digestible food

Succession

Succession is a change in species composition and communities over time

Primary Succession

Takes place on land where there has been no existing life e.g bare rock or sand dunes

Stages of Succession

The stages are called seres, each sere is characterised by a particular dominant species

The first organisms that colonise bare rock in a succession are called the pioneer species, These are Lichens which arrive on the rock as spired blown in the wind or carried by animals. They erode the rock and as they decompose a little soil builds up

The next organisms are mosses, again blown in by spores, which furniture erode the rock and again decompose.

The soil formed will eventually be deep enough for grasses to take root from seeds being blown in or carried by animals.

The grass ecosystem is eventually replaces by herbaceous plants and then shrubs

The final stage of succession is called the climax community and is woodlands in the UK

Climax Community

Stable end point where there are no further successional stages - although this does not mean there are no more changes

Seral stages

Pioneer species —> Mosses —> Grasses —> Herbaceous plants —> Shrubs —> Climax community (Trees)

Land changes in succession

Soil gets deeper, the ground becomes more nutrient rich as decomposition increases. Water holding capacity and biodiversity of the soil increases. This is beached each stage of succession makes conditions more favourable for the next community. As more plants of different types become established and off differing growth heights and forms, there are more habitats, food sources and niches. As biodiversity increases, so does competition. The climax community may have slightly lower biodiversity because of competition for light and nutrients under the tree canopy

Speed of succession

Depends on climate or proximity to places where there are already established species

Secondary Succession

Starts where there are seeds or spores available in the soil. As soil is already established, and seeds can germinate, rooted plants to grow much faster than primary succession. This type of succession happens on cleared land - perhaps by construction digging, tree felling or following a fire

Deflected Succession

Where a sub climax community is maintained usually by human interference. This can be by grazing animals so that shrubs and trees cannot get established or mowing lawns and school fields. In some cases, conservation of particular habitats involved deflecting succession like wildflowers meadows or heather moorland. Moorland is maintained as a subclimax by controlled burning, to provide ideal conditions for grouse. The heather roots are unharmed by the fire and regenerate supplying fresh green shoots for grouse to feed on , The nature heather provides ideal nesting sites

The Carbon Cycle

Describes the way in which carbon atoms are circulated around ecosystems.

Organisms involved in the decay of dead organic material

Detrivores - Animals with a tube gut that feed on dead, organic materials. These are animals like wood lice and earthworms. In the process of consuming their food they help to speed up decomposition by increasing the surface area of the dead organic material for the action of saprophytes

Saprophytes - Fungi and bacteria that carry out extracellular digestion of the organic material to obtain nutrients, They decompose the dead material

The Carbon Cycle Processes

Photosynthesis - Photoautotrophs remove carbon dioxide from the atmospher and ‘fix’ it into organic molecules.

Respiration - All organisms respire and return carbon dioxide to the atmosphere

Fossil Fuels - Formed millions of years ago and are derived from the partially decayed remains of plants. Combustion of plant material and fossil fuels releases carbon dioxide into the atmosphere.

Human Impact on the Carbon Cycle

Much of the carbon in the Earth is ‘locked away’ in peat bogs, tundra, fossil fuels, and trees, This carbon is locked away for long periods of time. One of the ways in which humans are impacting the carbon cycle is by releasing this locked up carbon by combustion to provide energy for homes and transport. This increases combustion of carbon based fuels is a contributing factor to the global rise of atmospheric carbon dioxide. Deforestation and pollution of oceans is decreasing the rate of photosynthesis that removes carbon dioxide from the atmosphere

Effects of increases Carbon Dioxide levels

Increased carbon dioxide leads to the greenhouse effect. Heat from the sun gets reflected to the planet by carbon dioxide and other greenhouse gases leading to temperature increases. Up to a certain level the greenhouse effect makes the planet more habitable, however one of the consequences of an increased greenhouse effect is climate change, Much of our climate depends on oceanic and stratospheric currents and warming of the atmospher and oceans disrupts the currents and impact on the climate.

Global warming causes polar ice to melt and pushed the organisms adapted to live there to the brink of extinction. The range of organisms are increasing as a result of global warming. Organisms living in warmer climates near the equator are thriving, pushing the usual organisms, furniture north and south - and there are limits to how far they can go, so global warming is affecting species distribution on the planet and leading to extinction

Carbon Footprint

Defined as the total amount of carbon dioxide attributable to an individual, product or service over a year. Activities to redirect e carbon footprint may involve; utilising ‘green energy’ sources, reducing food miles, reducing meat in the diet, suing public transport, walking or cycling

The Nitrogen Cycle

All living organisms need a source of nitrogen in order to synthesise molecules such as DNA, chlorophyll and proteins. At first glance, it may seem very easy for plants to obtain their nitrogen as the atmosphere contains 78% by volume of nitrogen. However, atmospheric nitrogen is inert and unavailable to plants or animals for use. Only some specialised micro-organisms have the enzymes to utilise nitrogen gas. Plants squally abstain the nitrogen they need by absorbing nitrate ions or ammonium ions through their roots by active transport. Some plants can obtain much of their nitrogen by forming a mutualistic relationship with bacteria that can utilise nitrogen gas.

Nitrogen Cycle Summary

Nitrates and ammonium ions that are taken up by plants are assimilated into nitrogen containing biological molecules. Herbivores then obtain their source of nitrogen by eating the herbivores or each other. Both animals return nitrogen to the soil via excretory products and when they die are decomposed. Detrivores feed on the egested material and dead organisms and break down the material into smaller particles increasing the SA for decomposers, They egest and excrete compounds containing nitrogen for decomposers to use

Decomposition

Decomposers feed on dead organic material saprophytically. They secrete enzymes into the food and absorb the small soluble products. Proteins and other nitrogen containing compounds are broken down into smaller particles increasing soluble molecules which are absorbed for use by the organism. Any excess nitrogenous compounds are converted into ammonium and excreted by the micro organisms. Nitrogenous compounds are converted into ammonium in the soil

Nitrification

Process by which ammonium is oxides into nitrite and then nitrate by bacteria. The nitrogenous compounds ammonium and nitrite are being used as a source of energy by the bacteria which therefore have a chemoautotrophic mode of nutrition.

Ammonium —> Nitrite —> Nitrate

NH4+ —> NO2- —> NO3-

The bacteria that carry out these reactions are called nitrifying bacteria. Nitrosamines converts ammonium to nitrite and Nitrobacter converts nitrite to nitrate

Nitrogen Fixation

Conversion of nitrogen gas into ammonium. Carried out by nitrogen fixing bacteria. Some nitrogen fixing bacteria are free living in the soil like Azotobacter. Some live in a mutually beneficial relationship with a plant. Rhizobium lives freely in most soils where it can carry out nitrogen fixation, however most nitrogen fixation by Rhizomium occurs inside the roots of legumes. The bacteria invade the roots of the legumes and the plant responds by forming a nodule in which the bacteria lives. Nitrogen gas diffuses into the nodule from the soil and is utilised by the bacteria to make ammonium and then amino acids. Excess ammonium and amino acids are exported to the plant for protein synthesis. Nitrogenous compounds fixation requires much ATP and the Rhizobium gains the sugars for respiration from the plants photosynthetic products.

Nitrogenous compounds fixation takes place in anaerobic conditions and the plant provides this by producing a pigment called leghemoglobin which takes up oxygen in the root nodules, removing it from the bacterial environment

Denitrification

Nitrogen releases back into the atmosphere. This occurs as a result of the actions of denitrifying bacteria such as Pseudomonas. These bacteria use nitrate to provide oxygen for respiration, the nitrogen atoms remaining are converted into nitrogen gas.

Nitrate —> Nitrogen Gas

NO3- —> N2

Occurs in anaerobic conditions, The most common cause of anaerobic conditions in nature is water logging, where air spaces in soil are filled with water. Water-logged soils are therefore often deficient in nitrates, although in paddy fields photosynthetic nitrogen fixing prokaryotes provide the rice plants with nitrogen compounds. Carnivorous plants often live in water logged soils, gaining carbohydrates from photosynthesis and their nitrogenous compounds from digestion of invertebrates

The Nitrogen Cycle and Agriculture

Crop plants require nitrates to manufacture proteins, nucleotides and chlorophyll to produce the yield of the crop. Agricultural practices involve manipulation of the nitrogen cycle to maximise the availability of nitrates to maximise yields.

Artificial fertilisers can be applied to farm land, these can be inorganic nitrogen compounds or can be organic (animal manure). Inorganic fertilisers are highly soluble and can leach into water courses, this is a cause of eutrophication. A further issue with fertiliser is that many of our native wildflowers are adapted to live on nutrient poor soil and are easily outcompeted in nutrient rich soil by fast growing grasses

Eutrophication

Excess nutrients get into water bodies. The algae are able to exploit them, and the population increase rapidly creating algal bloom. Algae can block sunlight from reaching bottom rooted plants which can’t photosynthesise and die. When the algae die, they and the other dead plants are decayed by aerobic bacteria. The population increase of aerobic bacteria means that the oxygen in the water gets used up and the water becomes too low in oxygens to support aquatic invertebrates and fish

Land draining

Soil does not get water logged, so denitrification gets slowed down and with more oxygen nitrification speeds up, adding more nitrate to the soil. More decomposition can take place too.

Land draining is not always desirable for wildlife, wetland ecosystems being drained leads to a loss of habitats and therefore biodiversity. Peat rich habitats are more subject to destruction wild fires which release locked in carbon

Ploughing land

Aerates the soil, more nitrification can occur as conditions are aerobic