Life processes in the biosphere
Adaptations to the environment
All organisms must be adapted to the range of abiotic and biotic conditions that exist in their habitat if they are to survive.
Species become better adapted through processes that involve the production of new characteristics by random mutations, followed by natural selection, which gives an increased chance of survival to better adapted individuals. This is the process of evolution.
Organisms that are poorly adapted may die if the environmental factors are unsuitable, or if other species are better adapted.
For any specific condition or factor most organisms survive within a relatively narrow range,
A population with a large gene pool is more likely to survive environmental changes more effectively as some individuals will be adapted to new conditions.
Although the non-adapted individuals would die, the survivors would be able to breed and rebuild the population because their offspring will be adapted to surviving the new conditions.
Factors that control survival
Abiotic
Light
pH
Water
Mineral Nutrients
Biotic
Food supply
Pollination
Seed dispersal
Disease
Ecological succession
All species have a stage in their life cycle when movement to colonise new habitats can take place. This is essential to avoid extinction as changing conditions make the current location unsuitable for survival.
When a species colonises an area, it will change the habitat, which may make it suitable for other species to colonise that could not have survived there before.
The full range of processes and stages in these changes can best be seen in a newly created habitat which currently has no life e.g. a new pond, or bare rock exposed by a retreating ice sheet a landslide, or created by a volcanic eruption.
The stages in the sequence of events that follows are called seres. Each sere has a prefix named after the starting conditions:
Water- Hydrosere
Bare rock- Lithosere
Sand- Psammosere
The process of changes in a sere is called ecological succession and includes the following stages:
The first living organisms to colonise must be adapted to abiotic conditions which are more extreme than later on when the habitat will support a higher biomass e.g. less shelter from strong winds, brighter sunlight, or more extreme temperatures.
The first organisms to colonise are called 'pioneer species'.
As time passes and populations increase, the pioneer species change the habitat and make it suitable for species that were not able to survive before. The new colonisers may out-compete the pioneer species which become less dominant and may eventually die out.
These new colonisers also change the conditions, making the habitat suitable for colonisation by more new species.
As the changing abiotic conditions become less extreme the adaptations that are needed for survival are increasingly based on biotic factors and inter-species relationships.
The sequence of new species colonising, thriving, then dying out, continues until a final community of species develops which remains dominant as long as the climate does not change. This is called the climax community. The species that make up the community are controlled by the climate, so it is often called the climatic climax community.
Rainfall and temperature are critical determinants of a biome.
E.g. areas which have water all year typically develop into woodland. The type of woodland that develops is controlled by the temperature, e.g. tropical rainforest at higher temps, temperate deciduous at moderate temps, and boreal conifer forest in cooler environments.
Areas where rainfall varies from season to season usually become grassland e.g. tropical savannah and temperate grassland
Seres
A sere is the sequence of stages in ecological succession during which an uncolonised habitat develops into the climax community.
Seres are often named after the conditions at the beginning of succession.
Hydrosphere
When an area of freshwater is created it is usually quickly colonised by single-celled algae from the soil (pioneer species). Birds, such as herons and ducks, and flying insects, such as water beetles and pond skaters, can bring in spores and seeds of algae, diatoms, and plants. The lake edges start to be colonised by rooting plants such as reeds, lilies, and reed mace. However, the open water may be too deep for such plants so only floating plants live there. The more the area is colonised by plants the more food and shelter there is so more animals that arrive can survive.
As plants grow and die, the lake gradually fills in with dead organic matter as well as soil and sediments carried in from the surrounding area. As the water becomes shallower, conditions continually change until rooted plants can survive. Emergent plants, that have their leaves above the water, shade the submerged plants which gradually die out.
As sediments fill the lake and open water recedes, aquatic species progressively disappear. As soil accumulates and develops to the extent that it can support the weight of much bigger plants, trees that can survive in waterlogged soil, such as willow and alder, start to colonise the area.
As transpiration by trees removes water and more sediments accumulate the soil becomes drier. Trees such as oak will be able to colonise. These larger trees create a denser canopy, providing shade which inhibits the growth of smaller plants.
Eventually, the community is dominated by the largest trees, with other plants and animals living in, on, or under them. Few of the original species involved in the early stages of the ecosystem's development are still present.
Lithosphere
The development of a community of species on bare rock, created by a cliff fall, the retreat of a glacier, or a volcanic eruption is called a lithosere. The initial abiotic conditions are very harsh and unsuitable for most organisms. Temperatures are extreme, water availability is severely limited and there is no soil. Simple autotrophs such as lichens and algae are the first to colonise the area (pioneer species).
Conditions improve as DOM and rock fragments gradually accumulate. Mosses colonise and a thin layer of soil starts to build up. Grasses and ferns then colonise. As the layers of soil form and plants get larger, the abiotic factors become less extreme. The conditions are never as hot or cold / wet or dry, as they were at the beginning of the community's development and the development of the soil makes plant nutrients more available.
Seedlings of less hardy plants can survive under the shade of the larger plants. Once pollinating insects become established flowering plants colonise and survive as pollination takes place.
Once the soil is deep enough and the edaphic factors (soil factors) are suitable, trees can colonise. The first tree species to establish usually have wind-blown seeds, eg. birch trees, while the seeds of later species are dispersed by animals, e.g. beech and oak trees. In the early stages of development, a hydrosere and a lithosere are very different from each other.
This is due to their different original conditions. However, as succession occurs these differences are reduced and the final communities are very similar to each other because they are controlled by the climate of the region which is the same for both communities.
Psammosere
A psammosere involves ecological succession that starts on sand.
Plant nutrients are not readily available, drainage is rapid so water supplies are poor and moving sand makes it difficult for plants to establish a good root hold without being covered and killed.
As plants succeed in colonising, the sand is stabilised, nutrient supplies increase, organic matter builds up, and water becomes more available.
Eventually, the sequence of changes in the community of species will establish the same climatic community that would have been produced by a hydrosere or lithosere.
Conservation of plagioclimax communities
Human activity that deflects the climax community occurs regularly, so the natural climax community does not have time to reform and a new community of species develops. This is called a plagioclimax.
A long-term balance between succession and human activities that maintain many plagioclimax communities produce habitats that don't appear to change e.g. grassland, lowland heathland, and moorland.
Conservation of climax communities such as natural rainforest, coral reefs, mangroves and Antarctica typically takes the approach of minimal human impact.
However, in the UK, conservation management of plagioclimax communities is usually through maintenance of the traditional activities that created them.
Methods of how to maintain plagioclimax communities
Burning
Encourage nutrient cycling
Removal of vegetation that may not be useful for grazing
To stimulate dormant seeds e.g. healthier
Grazing
Prevents development of climax species
Maintain habitats
Important for germination
Introduction of animal dung to increase nutrient levels
Coppicing
Encourages competition + diversity
Small-scale impact on habitats
Pollarding
Similar to coppicing but allows re-growing branches to avoid being eaten by local deer or cattle
Mowing
Removal of shrubs + thorns that may not be eaten by grazers
Population control
Increasing or decreasing populations to support species with lower breeding rates
Removal of unwanted species
Dedicated exclusion sites
Aim to reduce human activity
Controlling access to areas to allow the repopulation
Secondary succession
Human activities that disturb climax communities e.g. tree felling, ploughing, and burning, recreate the conditions that were suitable for the species that colonised the area earlier in the sequence of ecological by pioneer new species climax succession, so these species recolonise the area.
If the habitat is left alone, succession will continue and the climax community is recreated (this is secondary succession).
This occurs faster than primary succession because the soil doesn’t have to develop and many seeds are already present so there is less delay for colonisation.
- The reintroduction of organisms into an area of bare habitat previously occupied by plants and animals.
Original vegetation may be removed by:
Forest fires
Floods
Storms
Deforestation, grazing, mowing
Soil is already present, and contains dormant seeds, so succession is quicker.
Plants and animals can move in from surrounding areas.
Species diversity + ecological stability
In extreme environments that are dominated by abiotic factors, populations are likely to fluctuate dramatically, so species diversity is often low.
Where few food species are available, a change in the availability of one food species will have a big impact on the number of predators.
This will then impact the availability of the food species, creating cycles of population rise and fall.
Less abiotically extreme environments usually have higher biodiversity. These become more stable ecosystems in which populations are dominated by biotic factors e.g. tropical rainforests and coral reefs.
The level of diversity can be quantified using several different calculations. one common way to measure biodiversity is Simpson’s Diversity Index
Estimating species numbers
We haven’t discovered all species in existence, so it is not easy to estimate how many there are.
We can use the past ‘rate of discovery’ for estimating the number.
The gradual reduction in the rate of finding new species can be used to estimate the total number of species that have not yet been discovered and therefore the total number of species that exist
New species are currently being discovered at a rate of about 20,000/year.
Current estimates for the total number of species vary from 5 to 100 million.
Only 2 million species have so far been named.
Some ecosystems are so inaccessible e.g. the deep-sea floor and the canopy of tropical rainforests, that they have yet to be fully researched
Population dynamics + population regulation
Population dynamics involves the processes that can cause populations to change in size and structure.
Population increase: reproduction rate is high and the death rate is low.
Population decrease: reproduction rate is low and the death rate is high.
Successful wildlife conservation maximises the factors that increase and minimise the factors that reduce.
The environmental factors that affect the death rate can be controlled by good environmental management.
Birth rate
The maximum birth rate (natality rate) is determined by the natural ability of the species to reproduce.
Evolution has produced birth rates for each species that are appropriate for the death rate.
Species with lower chances of survival have higher birth rates.
Mice- gestation period 3 weeks, producing give birth to 5-6 babies.
Between 5-10 litters per year.
Can mate immediately after giving birth, meaning mice can birth a second litter in as little as 25 days after the first.
Death rate
The death rate is controlled by environmental factors such as disease, drought, predation + shortage or food
r-selected species
These species that can respond rapidly to low survival; rates
They reach sexual maturity quickly, produce many young + can disperse widely. r-selected species include mice, locusts + greenfly
K-selected species
These are species that recover slowly from a decline in population.
They usually reach sexual maturity at an older age, produce few young, but often live for a long time.
An increase in the death rate caused by a change in the habitat or by human exploitation may cause a population crash.
The low reproduction rate of k-selected species may make it impossible to replace the losses.
E.g. whales, elephants, rhinos
Maximum Sustainable Yield (MSY)
Is the maximum number of individuals in a species that can be sustainably harvested.
There must be enough adults left to breed and produce more young.
Current population
Numbers of births and deaths;
Number of individuals immigrating and emigrating
Population = starting population + births + immigrants - deaths - emigrants
Factors affecting mortality rates
Density independent factors
Factors where the population density has no effect on the chances of survival of an individual, e.g. drought, flood, volcanic eruption.
Density dependent factors
Factors where the chances of an individual surviving depend on the population density of the species. Survival chances are usually higher when the population density is low and lower when the population density is high. E.g.:
Food supply:
Intra-species competition for food is greatest when the population density is high.
Disease:
Closer individuals= faster disease spread.
As the population density increases the density dependent factors become more important until the combined mortality rate (caused by density dependent factors plus density independent factors) forms a long-term balance with the birth rate.
Carrying capacity
The greatest population that an area can support indefinitely without damaging or over-exploiting the environment. The mortality rate in a population changes if the population size is above or below the carrying capacity, so that the population size changes back to the carrying capacity
Predator-prey population relationship
More prey = more predators as more food available for them.
The high predator population causes the prey population to decline.
Low prey = food shortage for the predators so their population declines.
The low predator population then allows the prey population to rise again.
Artificial population control
We can artificially control populations through culling to enable the species or habitats to survive where natural control mechanisms no longer regulate the population. This is needed if:
The breeding rate of an endangered species is low, so a captive breeding and release programme is needed to maintain or increase the population
A non-indigenous species is introduced which reduces populations of indigenous species because it is a predator, competitor or pathogen.
An indigenous predator has been removed so its prey species becomes over-populated and needs to be culled to avoid the ecological damage it may cause by its impact on other species. e.g.
As wolves in Scotland have been exterminated their prey species, Red Deer, must be culled to prevent their population rising too high which would lead to over-grazing.