Ecosystems

An ecosystem is a dynamic, relatively self-sustaining system that includes multiple communities (biotic factors) and the environment (abiotic factors) they inhabit. The communities are made up of populations of different species that live and interact with one another. Different species will interact not only with each other and with other species but also with their non-living environment. In all ecosystems, the concepts of genetics, populations, and evolution relate to one another. Let’s see how each of these contributes to the diversity in ecosystems.

Types of ecosystems

There are two main types of ecosystems: aquatic and terrestrial.

Aquatic ecosystems

Aquatic ecosystems refer to all ecosystems contained in a body of water. There are two types of aquatic ecosystems: freshwater and marine. Their main energy sources (the produces; see below) are microalgae and macroalgae, as well as some aquatic plants.

Freshwater ecosystems

The water of freshwater ecosystems has no or only a very low salt content. Examples of freshwater ecosystems include lakes, ponds, streams, and wetlands. There are different ways that ecosystems can be classified, but the main three are:

• Lentic: slow-moving water, as in ponds, which are extremely rich in flora and fauna.
• Lotic: fast-moving water, as in streams.
• Wetlands: areas of land covered by water, which are anoxic (they have little or no oxygen) as the soil is saturated with water. Wetlands are important in nitrogen fixation (the release of free nitrogen,${\mathrm{N}}_2$).

Freshwater ecosystems account for only about 3% of the earth’s water supply. Humans and other living organisms depend on freshwater ecosystems for a supply of fresh water.

Marine ecosystems

Marine ecosystems are water bodies that contain high amounts of salt, such as coral reefs, mangroves, open oceans, and abyssal plains. They are classified according to the depth and other features of the shoreline. Ecosystems, such as coral reefs and mangroves, are responsible for food supply and job provision. Communities from poorer countries often heavily rely on jobs in fisheries.

Similarly to freshwater ecosystems, marine ecosystems suffer from overpopulation and climate change, which cause overfishing, pollution, and other issues.

Terrestrial ecosystems are ecosystems that exist solely on land, as in the following examples.

Deserts

Deserts are usually found in a very warm climate (although there are exceptions, such as the cold deserts in Greenland), with sparse vegetation and less than 25 cm of annual rainfall. Animals and plants in deserts are very well-adapted to the extreme environment. For example, cacti conserve water by storing it in their thick stems and have spines to defend themselves from predators.

Forests

Forests, characterised by their trees, are oxygen-making powerhouses (along with the algae in aquatic environments, which are sadly frequently overlooked). Rainforests are tropical climate forests that have an incredible species diversity. Temperate forests (classified by an abundance of deciduous trees, high humidity, and high precipitation) have lower biodiversity but are equally important. Deforestation, primarily due to human intervention, is one of the main issues affecting the survival of forests. They are exploited for timber, cut down for agricultural land development, and degraded due to climate change.

Grasslands

Grasslands are largely covered by grasses and other herbaceous vegetation, but either lack or have very few trees. They are known by different names throughout the world, such as the steppes in Europe or the savannas in Africa. Grasslands are usually found in areas where forests cannot be supported, often due to a lack of rain.

Fig. 1 - Interactions between marine and terrestrial ecosystems

Food webs in the ecosystems

Food webs in ecosystems are extremely complex. Food chains are often used for simplification purposes, especially when showing the movement of energy through the trophic levels. Food webs consist of producers, consumers (primary, secondary, etc.) and decomposers.

Fig. 2 - Arctic marine food web

Producers and consumers

Producers in the aquatic ecosystems include aquatic plants and algae, while in terrestrial ecosystems, they consist solely of plants. Producers harvest the sun’s energy and absorb inorganic nutrients to convert them into food via photosynthesis. The primary consumers can then access the energy.

Decomposers

Decomposers are crucial to complete the nutrient cycle and return inorganic ions back to the soil. Decomposers are organisms that break down organic matter from plants and animals into inorganic matter that can then again be used by the primary producers. Examples of decomposers include fungi, bacteria, worms, and insects.

Biotic and abiotic interactions in the ecosystems

Living organisms, which interact with biotic and abiotic factors in their ecosystem, develop adaptations to survive in their environment. Let’s take the example of a savanna ecosystem featuring widely spaced trees in a grassland.

• In a savanna, trees (the producers) have deep roots to be able to absorb water that is usually found deep in the soil. The roots also protect the trees from fires, which normally do not damage them, so the trees can regrow.
• Prey animals, such as zebras feeding on the grasses, use their camouflage to hide from predators. Others, such as meerkats, use alarm calls to warn other meerkats when they have detected a predator.
• Predators, too, use camouflage to stalk their prey.
• Migration to find water sources is prominent in both predators and prey.

There are other biotic and abiotic interactions not covered here.

Genetics in the ecosystems

Individuals of the same species are very similar to one another genetically. They have the same number of chromosomes, the same number of genes, and the same types of genes. However, different individuals can possess different combinations of alleles of these genes.

Alleles are versions of the same gene. They are inherited from the individual’s parent or parents, and different genes may have different patterns of inheritance. For instance, some genes are inherited randomly and independently from others. Some are inherited alongside the individual’s sex, and some are linked to other genes.

Alleles can interact with one another to produce different characteristics. Some alleles are dominant and suppress others, while some can be codominant with other alleles and create intermediate characteristics.

The alleles an individual inherits contribute to determining their observable characteristics. Different environmental factors, such as the availability of resources or light, can also help to shape these. An individual’s characteristics determine its fitness or ability to survive and reproduce in its environment. Alleles are responsible for the genetic diversity in species. The more alleles there are in the genome of a species, the greater its genetic diversity is going to be. You can read more about this concept in the article on genetic diversity.

Populations and evolution

Individuals of the same species living together in a habitat form a population. Alleles can have different frequencies in the population, with those that increase the chances of survival usually being more frequent. Natural selection occurs when alleles that increase fitness (‘survival of the fittest’) rise in frequency. In small populations, alleles can also see a random increase in frequency due to genetic drift. The change in allele frequencies over time is called evolution.

Natural selection can occur in various ways. It can stabilise a population by favouring average characteristics, or it can favour one extreme trait over its opposite. When two or more different traits can afford individuals with the same level of fitness, natural selection can also diversify the population.

When different populations of the same species are isolated from one another and no longer interact, genetic variations can accumulate between them. Over time, these differences may lead to an inability to breed and produce fertile offspring with one another. On the other hand, new species can evolve when populations only breed among themselves. All species develop from existing ones through evolution by natural selection, which means that all species go back to a common ancestor. All this is part of the theory of evolution, which is a fundamental concept in biology.

Population size in ecosystems

The size of a population is affected by both living and non-living factors in its environment, which has limited resources and thus can only sustain a certain number of individuals. This causes competition for resources and reproductive opportunities in a population. Competition, which is essential for maintaining the number of populations, also occurs between populations and even within communities, as some species prey on others.

Over time, ecosystems can change in a process known as ecological succession. Understanding the stages of succession has important applications in conservation. The complexity of the conflict between human needs and conservation makes resolution a difficult but not unattainable task.

Human impact on the ecosystems

Humans have many impacts on ecosystems, some of which are listed below:

• Pollution, which is caused, for instance, when untreated waste is released into freshwater ecosystems. This not only affects species in the ecosystem that may be crucial for fisheries but also creates higher risks to human health.

• Climate change, which is due to the accumulation of greenhouse gases (e.g. carbon dioxide) in the atmosphere. Climate change has led to more extreme weather, including floods and droughts. Ecosystems that are run down are less resilient to changes and have lower recovery rates or may not recover at all.

• Mining, which, among other things, can alter soil profiles, cause erosion (which, in turn, causes more nutrient run-off from the land into streams and rivers), and lead to deforestation.

• Deforestation, which leads to the loss of important producers that absorb carbon dioxide and produce oxygen.