Significant ideas:

• A species interacts with its abiotic and biotic environments, and its niche is described by these interactions.

• Populations change and respond to interactions with the environment.

• Any system has a carrying capacity for a given species.

An ecosystem is:

• A community of interdependent organisms and the physical environment they interact with.
• Made up of biotic and abiotic components.

The interdependent part means that you can study this section starting absolutely anywhere – the organisms (biotic), the physical environment (abiotic) or the interactions between them. This section starts with biotic but you can jump to the abiotic part if that makes more sense to you. However, beware - everything is interlinked throughout the topic so there is a lot of cross-referencing to follow.​

Biotic components

The biotic element of the ecosystem is anything that is living and any interactions between the living components. That includes all the organisms (plants and animals), anything that they consume or that consumes them and human influences. The biotic components includes:

• Producers – the plants that convert energy into matter.
• Consumers – animals that eat plants or other animals.
• Decomposers – organisms that breakdown waste into component parts for reuse.
• Interactions that happen between the living components – predation, herbivory, parasitism,mutualism, disease and competition.

Figure 1. Biotic components of an ecosystem.

Species

If you look at species from a purely biological point of view there is a certain amount of debate as to what constitutes a species. In ESS the definition is simple.

Definition

Species is a group of organisms with common characteristics that can interbreed to produce fertile offspring. Panthera leo or lion is an example of a species.

The important point here is the fertile offspring, similar organisms can interbreed but the offspring they produce are infertile. For example the liger is a cross between a male lion (Panthera leo) and a female tiger (Panthera tigris). Parents are of the same genus (Panthera) but different species (leo and tigris). The liger is not considered a species because they are genetically or physically sterile and unable to produce offspring of their own.

Populations

Species do not exist on their own; they live together in groups called populations.

Definition

A population is a group of individuals of the same species living in the same area at the same time.

Populations may be close together or a long way from each other e.g. the Eurasian red squirrel (Sciurus vulgaris). There are 23 subspecies spread across Europe and Siberia. Adjacent populations maybe separated by a road or river; whilst they are technically close enough to meet to interbreed it is unlikely, hence they are considered separate populations. The further apart the populations the less likely they are to interbreed and this has led to the development of the numerous subspecies.

Population size is determined by:

• Births and immigration – which increase population size.
• Deaths and emigration – these decrease population size.

Births + immigration > deaths and emigration (population growth)

Births + immigration < deaths and emigration (population decline)

Births + immigration = deaths and emigration (population is in dynamic equilibrium)

Abiotic components

The biotic elements of the ecosystem interact with the abiotic elements.

Abiotic factors include, temperature, sunlight, pH, salinity and precipitation. These non-living factors strongly influence the living elements of the ecosystem and can operate as limiting factors.

Temperature is an important abiotic factor in all ecosystems. It varies seasonally (though the year) and diurnally (though the day). All organisms have a temperature range within which they can live and if the temperature deviates too much then they will be stressed and may die. Humans for instance have a normal body temperature of 37°C, below 35 or above 42 and we are in trouble. Humans, like other mammals regulate their body temperature and can therefore tolerate a very wide range of environmental temperatures.

Ectotherms such as reptiles rely on the environment to regulate their body temperature so their distribution is strongly controlled by temperature. Water temperature influences the amount of oxygen the water contains and so is vital to aquatic life. The seasonal and diurnal patterns of temperature will effect plant life cycles.

Sunlight is the base of the vast majority of food chains on earth. Solar energy makes photosynthesis possible and enables plants to transform light energy into chemical energy. As with temperature sunlight has diurnal and seasonal fluctuations and this affect the life cycles of both plants and animals. They determine the length of the growing season, mating cycles, when flowers bloom and much more.

In aquatic ecosystems sunlight has an even greater impact because water absorbs light. This means that the deeper you go in a column of water the less light is available. There comes a point where no sunlight can penetrate, by 200 meters there is so little light that photosynthesis is not possible. From 1,000 meters down there is absolutely no light and it is known as the midnight zone. This causes zonation in aquatic systems.

Water is essential to all life on earth and can arrive in an ecosystem as precipitation, groundwater flow or overland flow. It is a basic ingredient of photosynthesis and the medium in which life’s processes take place. In the absence of water plants, wither and die. Animals become weak and confused and will die if access to water is limited for too long. Different organisms have different levels of tolerance to lack of water and desert ecosystems are full of plants and animals that have evolved to tolerate low water levels.

The impact of pH as an abiotic factor differs between aquatic and terrestrial ecosystems but the general principles are the same. All organisms have a tolerance range for pH within which they thrive. In freshwater systems the tolerance range is between pH 6 and 8, above or below that and the ecosystem will start to breakdown. In terrestrial ecosystems the impact of pH is generally seen in soil and the tolerance range of soil microbes is typically pH 6 to 7.

As with all other abiotic factors all organisms have a range of tolerance to salinity. In terrestrial ecosystems the impact of salinity is generally seen in the soil and salinization of the soil causes many problems for agriculture. All aquatic ecosystems are sensitive to changes in salinity, though some organisms (estuarine) have a very wide range of tolerance. Open ocean ecosystems have a salinity of 35 ppm (parts per million, which means 35 grams to every kilogram) whilst freshwater ecosystems have an average salinity on 0.5 ppm. The Dead Sea has a salinity of 250,000 ppm – that is why there is nothing in it!

Habitat

Definition: A habitat is the environment in which a species usually lives.

The habitat of an organism is the natural environment around it. It has the physical and biological resources that an organisms needs – it is the place where it can find food, water shelter (for itself and its offspring) and mates. In most habitats the physical environment includes soil, moisture, temperature and sunlight whilst the biological environment is the food, mate and predators that are around.

The habitat of the Belalong Tree Frog (Rhacophorus belalongensis) is in a small number of the confluences of small tributaries of the Sungai Temburong and Sungai Belalong rivers in the Temburong District, Brunei. This habitat provides the insects it eats (details unknown) and other tree frogs to mate with. It also provides the water for drinking and for its tadpoles to live in.

The example of a habitat given here is an actual location – the rainforests of Brunei. However, habitat is a very broad term and it may not be a geographic location. If the organism is a parasite its habitat is the hosts body. For example, the head louse (Pediculus humanus capitis) is a wingless insect, whose habitat is the human head. Giardia is a gut parasite that spends part of its life cycle in the small intestine of vertebrates (including humans).

Niche

The niche is a diverse concept; it is very broad and encompasses many aspects.

Definition: The niche is the role an organism plays and the position it holds in the environment. It includes all the interactions the organism has with the abiotic and biotic environment.

The niche is the smallest unit of the habitat and it refers to the way an organism fits into the ecosystem, where it lives and what it does. It describes how the organism survives and reproduces.

Population growth curves

The changes in a population in response to changing abiotic and biotic factors can be shown as generalised graphical representations know as J-shaped and S-shaped curves.

J curves show a rapid increase and a boom pattern. At first population grows exponentially and then it suddenly collapses. Collapse is called dieback. Often in this type of curve population exceeds the carrying capacity before it collapses. This is called overshoot. Both of these curves below are idealized. In practice they won’t have ideal shapes.

S curves are associated with exponential growth but above certain size the growth rate slows down gradually, finally transforming into a population of constant size. Numbers stabilize at the carrying capacity of the environment which is the number or load of individuals that an environment can carry or support. S curve’s alternative name is environmental resistance.

In reality we are more likely to see a combination of the two graphs.

The S-shaped curve is likely to be accurate until the population approaches the carrying capacity. Population is unlikely to slow down immediately as there is a time lag between when resources decrease and when that shows in the population growth.

This will cause the overshoot seen in the J-shaped curve. At this point the decrease in resources will show in the population growth rate and it will slow down and then decrease until the population drops back below carrying capacity.

The population will continue to fluctuate around the carrying capacity.

Theory of Knowledge

Scientific models may be accurate with regards to non-human organisms but can they be used to predict human behaviour?

Interactions

Everything discussed in the previous sections comes together in this and the next section on species interactions. Organisms may interact with each other or with the abiotic environment. The S-and J-shaped curves summaries what happens when species interact with the abiotic environment. Now we will look at what happens to populations and species when they interact with each other.

Interactions between species may benefit both individuals (mutualism) or just one (predation) and include predation, herbivory, parasitism, mutualism, disease and competition. Interactions among organisms regulate population size and impact the balance of the food web.

Predation

This is probably one of the best know types of species interaction. Most of us have watched a lion chase a gazelle on television or watched a cat in the garden while it stalks a bird.

Definition

Predation is where one organism (the predator) hunts and kills another (the prey) in order to provide it with the energy for survival and reproduction.

Predation can be by an individual, a group or by a plant.

Figure 1. A lone predator (leopard) catches its prey (Thomson's Gazelle).

Figure 2. A pride of lions (predators) hunting a Cape buffalo.

Figure 3. Predatory plant - the Venus flytrap.

Predation is a good example of evolution in action. Over time the predators that have the best traits (sharp teeth and claws, speed, venom) for predation will be more successful at hunting. They will survive long enough to pass those traits onto their offspring and those characteristics will become more common in the predator population. Also the prey organisms which evolve better avoidance tactics such as speed, camouflage or toxicity will survive long enough to reproduce.

In the short term predation is obviously only beneficial to the predator. However the interaction creates a negative feedback loop in which the predator and prey populations are kept in balance (Figure 4a).

Figure 4a. Negative feedback loop of a predator prey relationship.

The predator prey relationship produces a very typical graph (Figure 4b). The graph shows a number of things:

1. Prey numbers are, on average higher than predator numbers. This is to do with the laws of thermodynamics and loss of energy.
2. The populations peak are out of sync - this is due to the time lag between the numbers of prey decreasing and the numbers of predators decreasing.

Figure 4b. Predator prey population curves.

Herbivory

Definition

Herbivory is the consumption of plant material by an animal (Herbivores).

Herbivores are numerous and there are plenty of examples, Figure 5 shows just one.

At first this appears as a passive act on the part of the plant, but just as predator and prey adapt for survival so do the plants and the herbivores. Plants evolve defense mechanisms to cut down herbivory, these are either structural/mechanical or chemical. The herbivores evolve coping mechanisms to continue eating.

The commonest structural defense is thorns or prickles. These make eating and digestion difficult and painful, herbivores will browse for a while but then move on to something less difficult to eat. An example is the African Acacia tree that has long spines. The giraffe has evolved a long thick skinned tongue that can strip the acacia of its leaves without feeling the impact of the thorns (Figure 5).

Figure 5. The giraffe browsing on the African acacia tree.

Some plants produce chemicals that make them unpalatable (usually bitter) or toxic to the herbivore. The response to this is varied; some animals simply have a varied diet that avoids consumption of too much of the same species thus avoiding toxin build up. Some will vomit and remove the toxins from their system, whilst others build up a tolerance to the toxin. The Amazonian Macaw simply eats clay from the river banks to neutralise the toxins in their diet.

As can be see in Figure 6 the herbivore and plant population growth curves show a similar pattern to the predators and prey graph.

Figure 6. Herbivore plant population curves.

Parasitism

Definition

Parasitism is when an organism (the parasite) takes nutrients from another organism (the host).

Parasites may live on the outside of their host, for example tick and fleas (ectoparasites). The parasites that live on the inside of the host are called endoparasites for example tapeworms spend part of their lives in the gut of their host.

It is obviously not beneficial for a parasite to kill the host as that means it has lost its habitat, however there are times when the population of parasites increases so much that the host dies. In many cases the life cycle of the parasite incudes more than one host. A classic example is the parasite that causes malaria - plasmodium (Figure 7).

Figure 7. Life cycle of the parasite plasmodium.

The parasite host population curve is similar to the predator prey and herbivore plant population curves. There is a lag between peaks in population numbers and one of the species (parasite) outnumbers the other (host). In this case it is because the parasite is significantly smaller than the host and is thus present in pretty large numbers.

Theory of Knowledge

Parasites and predators do very similar jobs but they seem to be viewed differently. What role are the ways of knowing playing in this?

Interactions

Mutualism

Definition

Mutualism is where two organisms of different species exist in a mutually beneficial relationship.

The interactions studied so far cause a drop in the population of one or other species – this is not the case with mutualism where both species benefit from the interaction so an increase in one will cause an increase in the other. Mutualism is vital to ecosystem functioning and it is believed that nearly 50% of all terrestrial plants rely on the fungi around their roots to enable them to absorb inorganic compounds from the soil.

There are numerous examples of mutualism:

• Bacteria in the intestines of cows and other bovines facilitate the digestion of cellulose in their diets. The cow gets better-digested food and the bacteria get a safe stable environment in which to live.
• Corals and an algae live in mutualistic relationship, within each coal polyp is a zooxanthellae algae. The algae photosynthesise and give off oxygen and other nutrients that the coral polyp needs to live. The algae gets CO2 and protection form predators.
• Probably one of the most famous examples of mutualism is that of the oxpecker. The oxpecker is endemic to the African savanna and it can be seen perching on all the large mammals of the savanna (zebra, impala, hippo’s, rhino’s and buffalo). The oxpecker eats the ticks and other parasites that feed on the herds of animals. The benefits both species – the oxpecker gets a meal and the mammals get the parasites removed. In addition the oxpecker acts as an early warning devise against predators and receives an relatively safe place to live.

Figure 1. Female impala being groomed by oxpeckers.

Disease

We all know what disease is, it a departure from the normal state of functioning of any living organism – plant, animal or human. It can affect the whole body or just part of it. It is accompanied by signs and symptoms and may be the result of environmental agents, infective agents, genetic defects or a combination of them. Whatever the cause disease can decimate a population of organisms.

Examples of diseases include:

• Ebola kills large numbers of endangered gorillas and chimpanzees. This has a major effect on the already deleted populations of these species.
• Anthrax is known to have killed 90% of the herbivores in Zimbabwe’s Malilangwe Wildlife Reserve.
• Canine distemper has killed numerous wild animals including black-footed ferrets in Wyoming, African wild dogs and lions.

The list is endless, disease in a normal population of animals can be devastating but in a species that is already endangered the impact can be catastrophic.

International-mindedness

Disease is no respecter of international borders. An outbreak of canine distemper may start in one country but spreads to neighbouring countries very easily.

Competition

Competition is where organisms compete for a resource that is in limited supply (water, food, territory, mates, habitat etc.) The resource must be limited for competition to occur, if it is plentiful there is no need for competition and the population will grow exponentially giving the J-shaped curve. Unlimited resources are rare so competition become important and population growth will slow resulting in the S-shaped curve.

There are two main types of competition. Intraspecific completion occurs when members of the same species compete for a limited resource – water, space, mates, food etc. Most species will demonstrate this type of competition, depending on the environment they are in.

Figure 2. Pheasant males fight for females during mating season.

Interspecific competition is where members of different species compete for a resource that they both need. The resources fought over will be the same as in intraspecific except for mates and maybe habitat. Members of different species do not compete for mates because they can not mate. Also, in some cases different species have different habitats.

If there is not enough of the resource then one or maybe both species will experience lower growth rates and survival of both species may be impossible. An example of this type of competition is between cheetahs and lions. Both are predators on the African savanna and they feed on similar prey and in many cases lions steal a kill from the smaller, lighter cheetahs.

Interspecific competition has more impact on species survival and population dynamics than intraspecific competition. If members of the same species are fighting for the same resource the end result is likely to be a slight drop in population numbers as the weaker members of the species are out competed. But if one species out-competes the other than extinction may be the result.