Interdependence

Interdependence - SQA Key definitions: 

• Assimilation - A measure of the efficiency with which an organism extracts what it needs from the food it consumes, converting it to biomass or useful energy.

• Autotroph - An organism that produces its own food, usually through photosynthesis.

• Biomass - The total mass of organic material within a particular trophic level. 

• Carnivore - An animal that only eats other animal material.

• Carrying capacity - The maximum population that can be supported with the available resources in an area.

• Climatic factor - Any long-term atmospheric or weather condition that influences the distribution, abundance, and adaptations of organisms in an ecosystem.

• Climax community - The final, relatively stable, and mature stage of an ecosystem that results from the process of ecological succession.

• Community -  All the varied organisms that live together in an ecosystem. 

• Competition - The interaction between organisms, or species, where the success of one is at the expense of the other, e.g. competing for food, mates or nest sites. 

• Density-dependent - Factors which reduce a population when numbers are high and allow a population to increase when numbers are low, for example, the abundance of food or water. 

• Density-independent - Factors which affect population number regardless of the population’s size, for example, wildfire or flooding. 

• Ecological efficiency - The efficiency, expressed as a percentage, with which energy and biomass are transferred between trophic levels in a food chain. 

• Ectotherm - An organism that relies on the external environment for temperature control e.g. reptiles.

• Edaphic factor - Any physical, chemical, or biological characteristic of the soil that influences the organisms living within or on it.

• Endotherm -  An organism that uses internally-generated heat to maintain body temperature, regardless of external temperature e.g. mammals. 

• Exponential population growth model - J-shaped growth model, in which the population increases over time regardless of limits on resources.

• Grazing - The process by which an organism feeds on parts of a plant (or other multicellular producer like algae) without typically killing the entire organism outright.

• Gross primary productivity - A measure of the energy which is captured and converted into organic matter by autotrophs. It represents the maximum amount of energy that enters the food chain from an external source. 

• Herbivore - An animal that only eats plant material.

• Heterotroph - An organism that obtains energy by consuming other organisms. 

• Interdependence - The mutual reliance of organisms on one another and on the abiotic components of their environment for their survival, well-being, and life processes.

• Inter-specific competition - Competition between different species for resources, for example, Red Squirrels and Grey Squirrels competing for food and nesting sites. 

• Intra-specific competition - Competition within a species for resources, for example, male Red Deer competing for territory and mates. 

• Logistic population growth model - S-Shaped growth model, in which the population grows until it reaches the carrying capacity of the ecosystem.

• Net primary productivity - A measure of the overall energy available to the primary heterotrophs.

• Niche - The 'role' that a species plays within the wider biological community. It describes how an organism interacts with its environment.

• Omnivore - An animal that eats both plant and animal material.

• Parasitism - A symbiotic relationship by which one organism, the parasite, benefits at the expense of another organism, the host.

• Population - The number of organisms of one species in an ecosystem.

• Population crash - The rapid drop in size of a population, caused by a population overshoot. 

• Population dynamics - The study of how and why the size, density, and structure of a population change over time and location.

• Population oscillation - The cycle of population growth and decline around the carrying capacity level in an ecosystem. 

• Population overshoot - A population where growth has exceeded the carrying capacity of an ecosystem. 

• Predator-prey cycle - The balanced state of the populations of predators and prey in an ecosystem. An increase in the number of either species would result in an increase in the other, counteracting the change.

• Primary succession - Occurs in an area where no life or soil existed before. This process begins from a bare rock, meaning organisms must start from scratch. This can take a very long time, from several hundred to thousands of years.

• Respiration - Glucose and oxygen are converted into carbon dioxide and water, releasing energy that the organism can use. 

• Secondary productivity - A measure of the production of new biomass in heterotrophs following the consumption of autotrophs or other heterotrophs. 

• Secondary succession - Occurs in an area where a previously existing community has been disturbed or destroyed, but the soil remains intact. This allows for a much more rapid succession process, from decades to hundreds of years. 

• Seral stages - The intermediate stages of an ecological community that occur during the process of ecological succession.

• Succession - The sequence of changes by which the mix of species in an ecological community develops over time. 

• Trophic - To consume to gain energy. 

• Trophic level - The position of an organism in a food chain, indicating the number of organisms through which energy has passed. 

Succession

Succession is the sequence of changes by which the mix of species in an ecological community develops over time. It involves a series of communities, where one set of species creates conditions that favour the establishment of new, different species, eventually leading to a more stable and mature community. 

There are two key types of Succession : 

The Stages of Primary Succession

A site undergoing primary succession will pass through a series of expected 'Seral Stages'. Seral stages are the intermediate stages of an ecological community that occur during the process of ecological succession.

Lichens are the pioneer species that initiate primary succession because they don't require soil and can colonise bare rock. They secrete acids that chemically weather the rock, which releases the mineral matter necessary for soil formation. Furthermore, lichens capture moisture and dust particles, contributing organic matter and promoting the creation of the first, or virgin soil.

As this thin soil layer forms around the lichens, mosses or grasses begin to colonise the rock. These new species quickly outcompete the lichens for space and light, causing the lichen population to decline. The developing layer of mosses and grasses, in turn, provides ideal germination conditions for seeds carried in by wind or animals.

These seeds develop into shrubs and small trees, which continue to grow, adding substantial organic matter to the deepening soil. Eventually, as the soil becomes richer and more extensive, larger tree species can colonise the area, ultimately creating a stable, self-perpetuating forest community known as the climax community.

The Stages of Secondary Succession

Secondary succession occurs when an existing ecosystem is disrupted, such as by a wildfire, logging, or abandonment of a farm field, but the soil and seed bank remain intact. Since soil and seeds are already present, this process is much faster than primary succession. The seral stages again follow the same progression as primary succession, just at a much more rapid pace, starting with pioneer species, moving through herbaceous plants, into shrubs and small trees, to finally mature forest. 

Climax Communities

Climax communities are the final, relatively stable, and mature stage of an ecosystem that results from the process of ecological succession. There are two factors which determine the type of climax community : 

• Climatic factors

• Edaphic factors

Climax Communities : Climatic Factors

Climatic factors are any long-term atmospheric or weather condition that influences the distribution, abundance, and adaptations of organisms in an ecosystem. These include factors such as temperature, light level, precipitation, wind and humidity. 

The three videos below show the processes within the atomsphere which cause the climate zones above : 

Climax Communites : Edaphic Factors

Soil is a very complicated material that is formed by the mixing together of the products of weathering (rocks, gravel, sediments etc.) with decaying organic matter (dead leaves and other plant material etc.). 

Edaphic factors are any physical, chemical, or biological characteristic of the soil that influences the organisms living within or on it. 

Climax Communities : Earth's Biomes

A biome is a large geographical region of the Earth characterised by its dominant plant life, which gives a good indication of the climax communities for each region : 

Though there will be variations within these biomes as to which climax community is present, caused by topography (the shape and features) of a region : 

Interdependence : Food Webs

An ecological Niche is the 'role' that a species plays within the wider biological community. It describes how an organism interacts with its environment and is made up of the following:- 

• The adaptations that the organism has to suit its habitat 

• The food/ nutrients it consumes 

• What preys on the organism

The fact-file below shows the niche of a Great Crested Newt, the largest Newt found in Scotland:-

Food Chains

A food chain shows how plants and animals get their energy. At each link in a food chain, 90% of the energy is lost, so the organism next in the chain only gains 10%. This is why there are relatively few top predators (such as lions) compared to their prey (such as wildebeest). 

Energy can be lost from a food web in different ways:-

• Wasted as undigested food (faeces)

• Used by the organism for heat and movement

An example of a food chain is shown below, the arrows showing the direction of energy flow:-

Food Chains : Autotrophs

A food chain always starts with an Autotroph. An autotroph is an organism that produces its own food. Most autotrophs are green plants, algae or bacteria which make their food by Photosynthesis using energy from sunlight. 

In Photosynthesis, water and carbon dioxide are converted into glucose (a type of sugar) and oxygen. This oxygen is either used in respiration or released from the organism as a waste gas. 

The diagram below shows the word equation for Photosynthesis:-

There is a small number of autotrophs that do not use Photosynthesis to produce their food, but use other chemical processes instead. An example of this can be found around Hydrothermal Vents on the ocean floor. 

Hydrothermal Vents form when seawater seeps down through the sea bed in a region of volcanic activity. As the water passes down through the rock, minerals become dissolved in the water. The water is then heated by underground magma and rises back up to the ocean floor. The hydrothermal vent forms where this mineral-rich water escapes.

The autotrophs around a hydrothermal vent are a type of bacteria. Instead of Photosynthesis, these bacteria use the chemicals released into the water by the hydrothermal vents to make their food, a process known as 'Chemosynthesis'. 

Food Chains : Heterotrophs

Heterotrophs are organisms that obtain energy by consuming other organisms. Heterotrophs can be split into five groups, based on what they consume (eat) : 

Both Autotrophs (producers) and Heterotrophs (consumers) release the energy from their food through a process known as 'Respiration', occurring in all living cells. 

In respiration, glucose and oxygen are converted into carbon dioxide and water, releasing energy that the organism can use. 

The diagram below shows the word equation for respiration:-

Food Webs

The food chains described above show a simple explanation of the way energy moves from organism to organism in an area, but in reality it is more complex than this, as organisms usually consume more than one food source. When two or more food chains are linked together, they form a 'Food Web'. 

Food Chains & Webs : Trophic Levels

Trophic levels refer to the position of an organism in a food chain, indicating the number of organisms through which energy has passed. Trophic levels can be represented by numbers, beginning with autotrophs at the first trophic level, and progressing to heterotrophs at the second and higher trophic levels. 

The highest trophic level of a food chain is usually trophic level 4 or 5, as energy is lost at each stage, allowing fewer organisms to be supported at higher levels. 

There are several ways to measure the energy changes across the tropic levels. 

Gross Primary Productivity - This is a measure of the energy which is captured and converted into organic matter by autotrophs. It represents the maximum amount of energy that enters the food chain from an external source. 

Net Primary Productivity - This is a measure of the overall energy available to the primary heterotrophs. It is equal to the gross primary productivity minus the energy required for respiration in the autotrophs. 

Secondary Productivity - This is a measure of the production of new biomass in heterotrophs following the consumption of autotrophs or other heterotrophs. 

Food Chains & Webs : Ecological Efficiency 

Another way to represent the energy moving through the trophic levels of a food chain is 'Ecological Efficiency'. This is the efficiency, expressed as a percentage, with which energy and biomass are transferred between trophic levels in a food chain. 

Organisms use energy for respiration and movement, and energy is lost from food webs through heat or undigested material. As a result, ecological efficiency is usually around 10% between each trophic level.

The ecological efficiency is affected by how efficiently an organism is at extracting useful nutrients from its food and converting this to biomass or usable energy; this process is known as 'Assimilation'. This varies between species, but as a general rule, the following can be used as a guide : 

Food Chains & Webs : Endotherms & Ectotherms

Organisms fall into two groups when considering 'Body Temperature' : 

Population Dynamics

Food Webs show the complex interdependence between all of the organisms within an ecosystem, with a change in any part of the web affecting the whole ecosystem.

Due to this, it is very important that any changes we try to make to an ecosystem are very carefully managed. When a species is introduced to a new area, either by accident or intentionally, the species has the potential to disrupt the entire food web. 

The study of how and why the size, density, and structure of a population change over time and location is known as 'Population Dynamics'. 

Population Growth Models

There are two key population growth models; modelling how a population can change over time : 

There are two types of factors which affect population dynamics : 

• Density-dependent factors : Factors which reduce a population when numbers are high and allow a population to increase when numbers are low, for example, the abundance of food or water. 

• Density-independent factors : Factors which affect population number regardless of the population’s size, for example, wildfire or flooding. 

Density-Dependent Factors : Carrying Capacity

The 'Carrying Capacity' of an area is the maximum population that can be supported with the available resources in the area. 

When a population enters a new area, the organisms take advantage of the abundant resources, and the population grows rapidly. 

After some time, however, the population reaches and exceeds the carrying capacity of the area, which is known as a 'Population Overshoot'. Through density-dependent factors such as predation, disease or lack of food, the population begins to drop rapidly, which is known as a 'Population Crash'.  

Once the population falls below the carrying capacity, the density-dependent factors seen previously no longer limit growth, and the population increases again. 

This cycle repeats, keeping the population size approximately stable. This movement around the carrying capacity is known as a 'Population Oscillation'. 

Density-Dependent Factors : Predator-Prey Cycles

In an ecosystem, the numbers of predators and prey will usually be found in a balanced state. An increase in the number of either species would result in an increase in the other, counteracting the change. This is known as a negative feedback loop. 

A classic example of this is the Fox/Rabbit population model. If the rabbit population in an ecosystem increases, then there will be more food available for foxes, so the fox population will increase. But an increased fox population would eat more rabbits, reducing the rabbit population. Fewer rabbits mean less food for foxes, so the fox population would also decrease, and the cycle begins again.

If the numbers of either predator or prey changed dramatically over a short period of time, for example, due to a disease,  then the ecosystem could be thrown out of balance and wide variations in populations could be seen. There is even the risk of a population failing to survive, with consequences for the entire ecosystem. 

The embedded website below runs a simulation of the Fox/Rabbit population model. Starting variables (number of foxes to rabbits etc.) can be changed and the effects of these can be modelled:-

Density-Dependent Factors : Grazing

Grazing process in which an organism feeds on parts of a plant (or other multicellular producer like algae) without typically killing the entire organism outright. examples of this include : 

With low grazing intensities, the biodiversity of grassland is low because a few species of plants, such as grasses, are able to outcompete the others and dominate the ecosystem, leading to low biodiversity.  

As the grazing intensity increases, the biodiversity increases as the dominant plant species are kept in check by grazers, and the weaker competitors are therefore also able to grow.

At very high grazing intensities, the biodiversity decreases because only plants with adaptations to tolerate the effects of grazing can survive, reducing the overall number of species that can survive in that area. 

Density-Dependent Factors : Competition

Intraspecific competition : Competition within a species for resources, for example, male Red Deer competing for territory and mates. 

Interspecific competition : Competition between different species for resources, for example, Red Squirrels and Grey Squirrels competing for food and nesting sites. 

Interspecific Competition Case Study : Squirrels

The introduction of grey squirrel from America between 1876 and 1929 as an ornamental species in country estates has led to a huge impact on the native Red Squirrel population. 

Grey Squirrels are larger than the Red Squirrels and live in denser groups, so when introduced into a new area, the Grey Squirrels will quickly outcompete the Red Squirrels for food and resources.  

The Grey Squirrel is also a carrier (a carrier of a disease can spread it without showing symptoms themselves) for the disease 'Squirrel Pox', which is almost always fatal to Red Squirrels. 

The maps below show how the range of the Red Squirrel has reduced over the last 75 years:-

In order to protect native Red Squirrel populations and reduce the damage to the wider ecosystem, Defra (The UK Government Department for Environment, Food & Rural Affairs) has begun to control the population of Grey Squirrels though the use of contraceptive-laced food:-

Intraspecific Competition Case Study : Red Deer 

Red Deer have been studied for over  50 years in terms of intraspecific competition; in this case, over mates. During the rut (mating season : mid-September to early November), mature stags compete directly and aggressively for the right to control a group of hinds (female deer). This involves roaring contests and parallel walking to display size and fitness. If this is not enough to show dominance, the stags will engage in fighting, where stags lock antlers, attempting to overpower their opponents. This can, and regularly does, result in serious injury and death.

The Isle of Rùm Red Deer Project is one of the world's longest-running and most complete studies of a wild mammal population. Since 1972, researchers have continuously monitored virtually every individual Red Deer in the North Block of the Isle of Rùm, a National Nature Reserve in the Scottish Inner Hebrides. This has allowed researchers to understand the ecological and evolutionary forces that shape wild animal populations by monitoring every individual in a population, something that is usually impossible to do. 

Density-Dependent Factors : Parasitism 

Parasitism is a symbiotic relationship by which one organism, the parasite, benefits at the expense of another organism, the host. The parasite gains resources, shelter, or nourishment, whereas the host is harmed by the loss of these. 

Unlike predation, a true parasite rarely kills its host quickly. The parasite depends on the host remaining alive, often for extended periods, to complete its life cycle. However, when host populations are dense, parasites and diseases can spread rapidly, leading to increased host mortality or reduced reproductive rates, thereby regulating the host population size.

There are two types of parasitism :  

Density-Independent Factors 

Density-independent factors are those that affect population numbers regardless of the population’s size, for example, wildfire or flooding :