Table of Contents
What Is an Ecosystem?
An ecosystem is the functional unit formed by all organisms in a given area (the biotic community or biocenosis) together with their physical environment (the biotope) and all the interactions between them.
In other words, an ecosystem includes:
- The living components (plants, animals, fungi, microorganisms)
- The nonliving components (light, water, soil, temperature, minerals, etc.)
- The flows of energy and cycles of matter that connect them
Unlike the broader “biosphere” (the sum of all ecosystems on Earth), the ecosystem concept is used at many scales: a rotting log, a pond, a forest, a coral reef, or even a city park can all be considered ecosystems, as long as we look at them as functioning wholes.
Components of Ecosystems
Biotic Components
Biotic components are the living parts of an ecosystem. They are often grouped by their role in energy flow and nutrient cycles:
- Producers (autotrophs)
Primarily green plants, algae, and some bacteria that build organic matter from inorganic substances using an external energy source (mostly sunlight). They form the base of most food webs. - Consumers (heterotrophs)
Organisms that obtain energy by eating other organisms or their products. - Primary consumers: herbivores that eat producers
- Secondary and higher-order consumers: carnivores and omnivores that eat other consumers
- Special cases: parasites, predators, and scavengers, which differ in how they exploit other organisms
- Decomposers and detritivores
Bacteria, fungi, many invertebrates, and some larger animals that break down dead organic matter and waste. They release inorganic nutrients back into the environment, making them available to producers again.
These groups are linked in food chains and food webs and form trophic levels (feeding levels).
Abiotic Components
Abiotic components are the nonliving environmental factors that shape which organisms can live in an ecosystem and how they interact:
- Climate factors: light, temperature, precipitation, wind
- Chemical conditions: pH, salinity, nutrient concentrations, oxygen and carbon dioxide in water and air
- Physical structure: soil type, rock, water depth, flow velocity, topography
- Disturbance regime: frequency and intensity of fires, floods, storms, landslides
These factors are not just a backdrop; they actively structure communities. For example, water depth and light penetration determine which algae can grow in a lake, while soil characteristics influence which plants dominate a forest.
Structure of Ecosystems
Spatial Structure
Ecosystems are not homogeneous; they have spatial patterns:
- Horizontal structure (mosaics and patches)
Ecosystems are often mosaics of microhabitats: sunny clearings vs. shaded understory in a forest; shallow vs. deep zones in a lake; reef crest vs. lagoon in a coral ecosystem. - Vertical structure (stratification)
Many ecosystems are vertically layered: - Forests: canopy, understory, shrub layer, herb layer, root zone
- Lakes: surface (epilimnion), deeper (hypolimnion), and transition zones
- Oceans: light-rich surface layer vs. dark deep sea
These structures influence light, temperature, humidity, and therefore species distributions and interactions.
Trophic Structure
The trophic structure shows how energy and matter move through feeding relationships:
- Trophic levels:
- 1st level: producers
- 2nd level: primary consumers
- 3rd level and above: higher consumers
- Decomposers act throughout, feeding on all levels’ wastes and remains
- Food chains vs. food webs:
- A food chain is a simplified, linear sequence (e.g., grass → rabbit → fox).
- A food web is the complex network of overlapping food chains that more realistically describes feeding relationships.
Trophic structure affects the number and abundance of organisms at each level and is closely tied to energy flow and nutrient cycling.
Functional Groups
Beyond trophic levels, organisms can be grouped by function:
- Pollinators, seed dispersers
- Nitrogen‑fixing bacteria, mycorrhizal fungi
- Engineers (e.g., beavers constructing dams, earthworms altering soil)
Different species can perform similar functions (functional redundancy), contributing to resilience if one species is lost.
Types of Ecosystems
Ecosystems are often categorized by their dominant environmental conditions and life forms. A few broad types:
Terrestrial Ecosystems
- Forests: dominated by trees, often with strong vertical stratification; can be tropical, temperate, or boreal.
- Grasslands and savannas: dominated by grasses and herbs, with scattered shrubs or trees; often shaped by grazing and fire.
- Deserts: low precipitation, sparse vegetation, strong temperature extremes; organisms show pronounced adaptations to drought.
- Tundra: cold, short growing season, permafrost; low shrubs, mosses, lichens dominate.
Aquatic Ecosystems
- Freshwater ecosystems: rivers, streams, lakes, ponds, wetlands.
- Flowing waters (lotic systems) vs. standing waters (lentic systems).
- Marine ecosystems: oceans, coral reefs, estuaries, coastal wetlands.
- Strong vertical (light, pressure) and horizontal (coasts vs. open ocean) gradients.
Special and Human‑Influenced Ecosystems
- Wetlands: transitional zones between land and water; high productivity and importance for nutrient retention and biodiversity.
- Urban ecosystems: cities and suburbs; heavily shaped by humans, but still containing complex communities and flows of energy and matter.
- Agroecosystems: fields, orchards, plantations, pastures; managed by humans with altered species compositions and nutrient cycles.
Each type has characteristic structures, species compositions, and dominant processes, yet all share the same basic principles of energy flow and nutrient cycling.
Ecosystem Functioning
Productivity
Ecosystem productivity describes how much organic matter is produced:
- Primary production:
- Gross primary production (GPP): total energy fixed by producers (e.g., by photosynthesis).
- Net primary production (NPP): energy left over after producers’ own respiration, available to consumers and decomposers.
$$ \text{NPP} = \text{GPP} - \text{Respiration of producers} $$ - Secondary production:
Production of new biomass by consumers (e.g., growth of herbivores and carnivores).
Productivity varies strongly between ecosystems (e.g., tropical rainforests and coral reefs are highly productive; deserts and deep oceans much less so).
Energy Flow and Trophic Efficiency
Energy flows one‑way through ecosystems:
- Enters (mainly) as sunlight
- Is transformed into chemical energy by producers
- Passes through consumers and decomposers
- Is lost as heat due to metabolic processes at each step
Only a fraction of energy at one trophic level becomes biomass at the next (often roughly 10–20%: trophic efficiency). This leads to:
- Fewer individuals and less biomass at higher trophic levels
- Limits on how many levels a food chain can sustain
Nutrient Cycling
Elements such as carbon, nitrogen, and phosphorus are recycled within ecosystems through biotic and abiotic processes:
- Taken up by producers from soil, water, or air
- Integrated into organic matter
- Passed on through food webs
- Released by respiration, excretion, and decomposition
Each ecosystem expresses global nutrient cycles (such as those studied in the separate nutrient‑cycle chapters) in its own specific way, depending on climate, soils, and species present.
Stability, Resilience, and Resistance
Ecosystem functioning is often discussed in terms of:
- Resistance: ability to withstand disturbance without major change
- Resilience: ability to recover after disturbance
- Stability: overall constancy of structure and function through time
Diversity, functional redundancy, and network complexity can influence these properties. For example, if multiple species fulfill similar roles, the loss of one species may be less disruptive.
Succession and Ecosystem Development
Ecosystems are not static; they change over time. Ecological succession describes the more or less directional change in species composition and ecosystem structure after a disturbance or on newly available surfaces.
Primary vs. Secondary Succession
- Primary succession:
Begins on previously lifeless substrates without soil (e.g., fresh lava, exposed rock after glacier retreat).
Early stages are dominated by pioneer species (e.g., lichens, mosses), which contribute to soil formation. - Secondary succession:
Begins where a previous community existed but has been disturbed (e.g., after fire, logging, storms), with soil already present and often seeds or rootstocks remaining.
Over time, both forms of succession may lead to more complex communities with:
- More biomass and organic matter in soils
- More developed vertical structure
- More interactions among species
Traditional concepts describe a relatively stable “climax community” under given climatic conditions, but in many real landscapes, repeated disturbances create a shifting mosaic of successional stages.
Disturbance and Dynamic Equilibria
Disturbances (fires, floods, storms, insect outbreaks, human activities) are integral parts of many ecosystems:
- Some species and processes depend on periodic disturbance (e.g., fire‑adapted plants).
- Ecosystems often exist in dynamic equilibria, fluctuating around typical states rather than achieving an unchanging end point.
Ecosystem Boundaries and Scales
Ecotones and Gradients
Ecosystem boundaries are often gradual, not sharp lines:
- Ecotones are transition zones between different ecosystems (e.g., forest–grassland boundary, shoreline between land and water).
- These areas often have:
- Steep environmental gradients
- Species from both neighboring ecosystems
- Sometimes increased diversity and unique species
Nested Systems
Ecosystems are nested within each other and vary in scale:
- A puddle, a pond, a lake, and an entire river basin can each be treated as ecosystems.
- Local ecosystems are embedded in larger landscape and regional contexts, and ultimately in the biosphere.
Processes at small scales (e.g., nutrient cycling in soil microhabitats) can influence larger‑scale patterns (e.g., regional productivity and water quality), and vice versa.
Human Influence on Ecosystems
Humans are now major drivers of ecosystem change:
- Land‑use change: deforestation, agriculture, urbanization alter ecosystem type, structure, and function.
- Pollution and nutrient input: excess nutrients (eutrophication), toxic substances, and plastics disrupt communities and processes.
- Climate change: alters temperature, precipitation, and disturbance regimes, shifting ecosystem boundaries and composition.
- Biological invasions: introduced species can outcompete native species or alter key processes.
These influences can:
- Simplify food webs
- Reduce biodiversity
- Change productivity and nutrient cycling
- Impair ecosystem services (e.g., clean water, soil fertility, climate regulation)
At the same time, humans manage and restore ecosystems through conservation, protected areas, ecological restoration, and sustainable use, aiming to maintain or recover their functioning and diversity.
Ecosystems and Ecosystem Services
Ecosystems provide services that support human life and well‑being:
- Provisioning services: food, freshwater, wood, fibers, medicinal resources
- Regulating services: climate regulation, flood control, pollination, water purification, erosion control
- Supporting services: soil formation, primary production, nutrient cycling
- Cultural services: recreation, aesthetic value, spiritual significance, education and research
Understanding ecosystems as functional units helps in recognizing their value, predicting the effects of human actions, and guiding conservation and sustainable management.