An ecosystem is a complex web of interactions between living organisms and their physical environment, functioning as a unified whole. Ecosystems can vary widely in size and complexity, ranging from small ponds or a single tree to vast forests, oceans, and deserts. What unites all ecosystems is the flow of energy and the cycling of nutrients that sustain life within them. This intricate balance of interactions, driven by both biotic (living) and abiotic (non-living) components, is what enables ecosystems to function and support a diversity of life forms.

At the heart of any ecosystem is the interaction between its biotic elements—plants, animals, fungi, and microorganisms. These organisms can be classified into various categories based on their roles. Producers, primarily plants and algae, are organisms that create their own food through photosynthesis, using sunlight to convert carbon dioxide and water into glucose. They are the foundation of the food chain, providing energy for other organisms. Consumers, such as herbivores, carnivores, and omnivores, depend on producers or other consumers for food, while decomposers, including bacteria and fungi, break down dead organic matter, recycling nutrients back into the ecosystem. Each species within an ecosystem occupies a niche, or a specific role, and the interactions between species—such as predation, competition, and symbiosis—help maintain the balance and flow of energy through the system.

Abiotic factors, including sunlight, temperature, water, soil, and air, play a crucial role in shaping ecosystems. These non-living elements determine the types of organisms that can survive and thrive in a particular environment. For example, the availability of water dictates whether an ecosystem will support desert-dwelling plants like cacti or lush rainforests with diverse plant species. Similarly, temperature influences the metabolic rates of organisms and the geographical range of species. The nutrient composition of the soil affects plant growth, while factors like wind and weather patterns influence the distribution of seeds and the migration of animals. These abiotic factors are not static but are constantly interacting with living organisms, creating a dynamic and ever-changing environment.

Energy flow is a fundamental concept in ecosystems. The primary source of energy for most ecosystems is sunlight, which plants and other producers capture through photosynthesis. This energy is then passed along the food chain, from producers to consumers and decomposers. At each level of the food chain, energy is transferred but not in a perfectly efficient manner. Only a portion of the energy consumed by an organism is used for growth and reproduction, while the rest is lost as heat. This loss of energy at each trophic level results in a pyramid structure, where energy diminishes as it moves up the food chain. Consequently, ecosystems tend to have fewer top predators than primary producers.

Nutrient cycling is another essential process that ensures the long-term sustainability of ecosystems. Elements such as carbon, nitrogen, and phosphorus circulate between the living and non-living components of ecosystems in various chemical forms. For instance, carbon is absorbed by plants from the atmosphere in the form of carbon dioxide during photosynthesis and is returned to the atmosphere when organisms respire or decompose. Similarly, nitrogen, which is essential for building proteins, is fixed from the atmosphere by certain bacteria, then taken up by plants, and eventually returned to the soil through decay. These cycles are vital for maintaining ecosystem health, as they replenish the essential nutrients that organisms need to grow and survive.

Ecosystems are not static; they are dynamic systems that are constantly evolving in response to changes in both biotic and abiotic factors. Natural events such as storms, wildfires, and volcanic eruptions can dramatically alter ecosystems by destroying vegetation, reshaping landscapes, and affecting the availability of resources. Additionally, ecosystems undergo long-term changes through processes such as ecological succession, where a disturbed or newly formed habitat is gradually colonised by a series of different species. Over time, these species alter the environment in ways that make it more suitable for other species, leading to the establishment of a stable and diverse community.

Human activities have had a profound impact on ecosystems worldwide. Urbanisation, deforestation, pollution, and climate change are some of the most significant threats to ecosystem stability. The destruction of habitats, coupled with the overexploitation of resources, has led to the decline of biodiversity, the extinction of species, and the disruption of natural processes like nutrient cycling and energy flow. Pollution, including plastic waste, pesticides, and chemical runoff, has contaminated ecosystems, harming wildlife and degrading habitats. Climate change, driven by human-induced greenhouse gas emissions, is altering ecosystems by changing temperature and precipitation patterns, leading to shifts in species distributions and the disruption of established ecological relationships.

Despite the challenges posed by human activity, ecosystems are resilient and possess the capacity to recover from disturbances, given enough time and the right conditions. Conservation efforts around the world aim to protect ecosystems and restore degraded habitats. Reforestation, the creation of protected areas, and the promotion of sustainable agricultural and industrial practices are examples of how humanity is working to preserve ecosystems. Protecting ecosystems is crucial not only for maintaining biodiversity but also for supporting the ecosystem services that human societies depend on. These services include the provision of food, clean water, and air, as well as the regulation of climate and the pollination of crops.

Examples of ecosystems

• Agro-ecosystems - The term agro-ecology can be used in multiple ways, as a science, as a movement and as a practice. Broadly stated, it is the study of the role of agriculture in the world. Agro-ecology provides an interdisciplinary framework with which to study the activity of agriculture.

• Agroecosystem - is the basic unit of study for an agro-ecologist, and is somewhat arbitrarily defined as a spatially and functionally coherent unit of agricultural activity, and includes the living and nonliving components involved in that unit as well as their interactions.

• Aquatic ecosystem - An aquatic ecosystem is an ecosystem located in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems.

• Chaparral - is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico. It is shaped by a Mediterranean climate and wildfire. Similar plant communities are found in the four other Mediterranean climate regions around the world, including the Mediterranean Basin, central Chile, South African Cape Region, and in Western and Southern Australia.

• Coral reef - are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters containing few nutrients. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups.

• Desert - is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Deserts are defined as areas with an average annual precipitation of less than 250 millimeters per year or as areas where more water is lost by evapotranspiration than falls as precipitation.

• Forest - also called a wood, woodland, weald, wellard or holt is an area with a high density of trees. There are many definitions of a forest, based on the various criteria. These plant communities cover approximately 9.4% of the Earth's surface (or 30% of total land area), though they once covered much more (about 50% of total land area), in many different regions and function as habitats for organisms, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the Earth's biosphere.

• Human ecosystem - are complex cybernetic systems that are increasingly being used by ecological anthropologists and other scholars to examine the ecological aspects of human communities in a way that integrates multiple factors as economics, socio-political organisation, psychological factors, and physical factors related to the environment.

• Large marine ecosystem - are regions of the world's oceans, encompassing coastal areas from river basins and estuaries to the seaward boundaries of continental shelves and the outer margins of the major ocean current systems. They are relatively large regions on the order of 200,000 km² or greater, characterised by distinct bathymetry, hydrography, productivity, and trophically dependent populations.

• Littoral zone - The littoral zone refers to that part of a sea, lake or river that is close to the shore. In coastal environments the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged. It always includes this intertidal zone and is often used to mean the same as the intertidal zone. However, the meaning of "littoral zone" can extend well beyond the intertidal zone.

• Lotic - is the ecosystem of a river, stream or spring. Included in the environment are the biotic interactions as well as the abiotic interactions.

• Marine ecosystem - are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content.

• Pond Ecosystem - is an ecosystem located in a body of water.

• Prairie - are considered part of the temperate grasslands, savannas and shrubland biome by ecologists, based on similar temperate climates, moderate rainfall, and grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type. Temperate grassland regions include the Pampas of Argentina, and the steppes of Eurasia.

• Rainforest - are forests characterised by high rainfall, with definitions setting minimum normal annual rainfall between 1750–2000 mm. The monsoon trough, alternately known as the intertropical convergence zone, plays a significant role in creating Earth's tropical rain forests.

• Riparian zone - A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the fifteen terrestrial biomes of the earth. Plant habitats and communities along the river margins and banks are called riparian vegetation, characterised by hydrophilic plants.

• Savanna - is a grassland ecosystem characterised by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses.

• Steppe - in physical geography refers to a biome region characterised by grassland plain without trees apart from those near rivers and lakes. The prairie (especially the short grass and mixed prairie) is an example of a steppe, though it is not usually called such.

• Subsurface Litho-autotrophic Microbial Ecosystem – described as a unique assemblages of bacteria and fungi that occupy pores in the interlocking mineral grains of igneous rock beneath Earth's surface.

• Taiga - also known as the boreal forest is a biome characterized by coniferous forests.

• Tundra -In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes from Russian for uplands, treeless mountain tract.

• Urban ecosystem -are the cities, towns, and urban strips constructed by humans.

Climate Change and What to Expect

Glaciers

Glaciers are some of the most sensitive indicators of climate change, reacting visibly to shifts in temperature. When the climate cools, glaciers advance, and when it warms, they retreat. This natural process of growth and shrinkage contributes to both natural climate variability and amplifies external changes in the environment. Since the 1970s, a world glacier inventory has been maintained, initially using aerial photographs and maps, but now relying heavily on satellite imagery. The inventory monitors over 100,000 glaciers, covering an area of approximately 240,000 square kilometres. Current estimates suggest the remaining ice cover globally is around 445,000 square kilometres. The World Glacier Monitoring Service tracks glacier retreat and mass balance annually. Their findings reveal that glaciers worldwide have been shrinking significantly, with notable retreats during the 1940s, followed by periods of stability or growth in the 1920s and 1970s, and another phase of retreat from the mid-1980s to the present day.

The most significant climate patterns over the last few million years have been the glacial and interglacial cycles, with the current interglacial period lasting about 11,700 years. These cycles are shaped by variations in Earth's orbit, which, in turn, have driven the advance and retreat of continental ice sheets and caused significant sea-level changes. However, other events, such as Heinrich and Dansgaard-Oeschger events and the Younger Dryas, show how glacial variations can influence the climate independently of orbital changes.

Glaciers leave behind moraines, which contain organic material, quartz, and potassium that can be dated to determine when glaciers advanced or retreated. Additionally, the absence of glacier cover can be identified through tephrochronology, using volcanic tephra layers or soil deposits to provide a timeline of glacial movements.

Vegetation

Changes in climate inevitably lead to shifts in the type, distribution, and coverage of vegetation. A mild climate shift might increase precipitation and temperature, encouraging plant growth and the subsequent absorption of atmospheric carbon dioxide. However, more extreme or rapid climate changes can cause vegetation stress, resulting in widespread plant die-offs and desertification in some regions. The impact of climate change on ecosystems can vary significantly, but the threat to global vegetation remains one of the most visible consequences of a changing climate.

Ice cores

Ice core analysis, which involves drilling deep into ice sheets like those in Antarctica, has been crucial in understanding the link between temperature and global sea level variations. The air bubbles trapped in the ice contain atmospheric gases from ancient times, allowing scientists to track historical CO2 levels long before human influence on the environment. These studies have been key in identifying how CO2 levels have fluctuated over thousands of years and how they compare to current levels, providing valuable insights into the effects of greenhouse gas emissions.

Dendroclimatology

Dendroclimatology is the study of tree rings to infer past climate conditions. The width of tree rings can indicate the climate at the time of growth. Wider, thicker rings suggest optimal conditions with plenty of water and warmth, while thinner rings indicate periods of drought or cooler temperatures. By studying these patterns, scientists can reconstruct climate variations going back hundreds or even thousands of years, offering a window into how climate has changed over time.

Pollen analysis

Palynology, the study of pollen, is another tool for understanding past climate conditions. Pollen is extremely resilient and can be preserved in various sediment layers found in lakes, bogs, or river deltas. By examining the different types of pollen found in these layers, scientists can infer how plant communities—and by extension, the climate—have changed over time. Different plants thrive under different conditions, so changes in pollen types provide clues about shifts in temperature, precipitation, and other climatic factors.

Atmospheric chemistry

The composition of Earth’s atmosphere plays a crucial role in regulating the planet's climate. Air is primarily composed of nitrogen, oxygen, and argon, but trace gases, including greenhouse gases like carbon dioxide, methane, and water vapour, have a significant impact on climate regulation. Greenhouse gases trap heat in the atmosphere, and human activities, such as burning fossil fuels, have increased their concentrations, contributing to global warming. The atmosphere also contains trace amounts of natural substances like dust, pollen, sea spray, volcanic ash, and industrial pollutants, such as sulphur dioxide and chlorine compounds. These additional components can also affect climate and air quality, further complicating the environmental impact of human activities.

The effects of climate change are being observed across various systems, from shrinking glaciers to shifts in vegetation and rising greenhouse gas levels in the atmosphere. The study of ice cores, tree rings, pollen, and atmospheric chemistry provides a detailed picture of how climate has changed in the past and how it may continue to evolve in the future. The evidence gathered from these natural records highlights the urgency of addressing climate change, as the impacts are already significant and are expected to accelerate without meaningful intervention.