02. Ecosystems, Energy Flow, and Biodiversity

Energy enters ecosystems through producers

An ecosystem is an interconnected system in which organisms interact with one another and with abiotic factors such as sunlight, temperature, water, soil, and nutrients. Two processes organize ecosystem function:

• Energy flow: Energy enters most ecosystems as sunlight, moves through organisms, and is eventually released as heat.

• Matter cycling: Elements such as carbon, nitrogen, phosphorus, and water move between organisms and the physical environment.

The most important starting point is Primary productivity. Plants, algae, phytoplankton, and some bacteria act as primary producers, or autotrophs. They convert inorganic materials into organic matter. Most producers use photosynthesis:

Some bacteria, including organisms near deep-sea hydrothermal vents, use chemosynthesis instead. They obtain energy by oxidizing inorganic substances such as hydrogen sulfide.

Gross primary productivity (GPP) is the total rate at which producers capture and store energy. Producers use some of this energy for cellular respiration. Net primary productivity (NPP) is the remaining energy:

NPP is the new producer biomass available to herbivores, other consumers, and decomposers. For example, if GPP is $20{,}000$ $\text{kcal}/\text{m}^2/\text{/year}$ and respiration uses $8{,}000$ $\text{kcal}/\text{m}^2/\text{/year}$, then:

Productivity depends on light, water, temperature, carbon dioxide, and nutrients. Aquatic systems are often limited by nitrogen, phosphorus, or iron, while terrestrial systems may be limited by water, soil fertility, temperature, or growing-season length. Tropical rain forests, estuaries, swamps, and shallow coastal waters are often highly productive. Deserts, polar regions, and nutrient-poor open oceans are often less productive. High productivity does not necessarily mean high biodiversity; an agricultural field may produce substantial biomass while containing relatively few species.

Takeaway: Energy entering through producers sets the amount of biomass that can support the rest of the ecosystem.

Feeding relationships form trophic networks

A trophic level is a feeding position in an ecosystem. The principal levels are:

1. Primary producers: Organisms that create organic matter from inorganic substances.

2. Primary consumers: Herbivores that feed directly on producers.

3. Secondary consumers: Organisms that eat primary consumers.

4. Tertiary consumers: Consumers that eat secondary consumers.

5. Apex consumers: Predators at the highest position in a particular food web.

Decomposers, especially fungi and bacteria, break down dead organisms and wastes from all trophic levels. They return nutrients to the environment. Detritivores such as earthworms and millipedes physically consume and fragment dead organic matter, making decomposition easier.

A food chain is a linear feeding sequence. For example, grass is eaten by a grasshopper, the grasshopper is eaten by a frog, the frog is eaten by a snake, and the snake is eaten by a hawk. The direction of an energy arrow runs from the organism being eaten to the organism that consumes it.

In nature, most organisms have several food sources and several predators. A food web combines many interconnected food chains and therefore gives a more realistic picture of ecosystem structure. In a grassland, for example, a hawk may eat snakes, mice, and small birds; mice may eat seeds and insects; and snakes may eat mice, frogs, and insects. If drought reduces grass production, insects and mice may decline, affecting frogs, snakes, and hawks in turn.

An omnivore can occupy more than one trophic level. When it eats plants, it acts as a primary consumer; when it eats herbivores, it acts as a secondary consumer.

Takeaway: Food webs distribute producer-derived energy through many linked pathways, so a change in one population can spread through the ecosystem.

Energy decreases across trophic transfers

Energy flows through an ecosystem in one direction. It enters as light or chemical energy, becomes biomass in producers, passes to consumers, and is eventually dissipated as heat through cellular respiration and other metabolic processes. Unlike matter, energy is not recycled within an ecosystem.

At every transfer, some energy is unavailable to the next trophic level because organisms use energy for movement, growth, maintenance, and reproduction; release heat during respiration; fail to consume all available biomass; or cannot digest and assimilate everything they consume.

Trophic-level transfer efficiency measures the percentage of production that moves from one trophic level to the next:

The commonly used $10\%$ rule estimates that about $10\%$ of the energy available at one trophic level reaches the next. This is an approximation rather than a universal constant. If producers store $10{,}000$ energy units, primary consumers might receive about $1{,}000$ units, secondary consumers about $100$ units, and tertiary consumers about $10$ units.

The declining energy supply helps explain why food chains usually contain only a few transfers. It also helps explain why persistent pollutants can be especially harmful to top predators. Biomagnification occurs when a pollutant becomes more concentrated at successively higher trophic levels. A chemical present at low concentration in water may accumulate in plankton, become more concentrated in fish, and reach its highest concentration in fish-eating birds or mammals.

Takeaway: Energy decreases as it moves upward, while some persistent pollutants can become more concentrated upward.

Ecological pyramids compare ecosystem structure

Ecologists compare trophic levels using three kinds of ecological pyramids.

• A pyramid of energy shows the amount of energy available at each trophic level, usually per unit area per unit time. It is always upright because energy is lost at every transfer.

• A pyramid of biomass shows the total dry mass of organisms at each trophic level at a particular time. It is often upright on land but may be inverted in aquatic systems.

• A pyramid of numbers shows the number of individual organisms at each trophic level. It may be upright, inverted, or irregular.

An aquatic biomass pyramid may be inverted because phytoplankton have a small standing biomass but reproduce rapidly. They can be consumed soon after reproducing and support a larger standing biomass of zooplankton. This does not mean that consumers contain more energy than producers; it reflects rapid producer turnover.

A single large tree can support thousands of insects, producing an inverted pyramid of numbers. Thus, the three pyramids answer different questions:

• Energy: How much energy is available?

• Biomass: How much living material is present at one time?

• Numbers: How many individual organisms are present?

Takeaway: Pyramid shape depends on what is measured, so energy, biomass, and numbers pyramids should not be interpreted as interchangeable.

Ecosystems support human well-being

Ecosystem services are benefits people obtain from functioning ecosystems. The same ecosystem can provide several kinds of services at once.

• Provisioning services provide tangible products such as food, fresh water, timber, fibers, fuel, medicines, and genetic resources.

• Regulating services help control environmental conditions through climate regulation, carbon storage and sequestration, flood reduction, erosion control, water purification, pollination, and pest or disease control.

• Cultural services provide nonmaterial benefits such as recreation, tourism, education, aesthetic enjoyment, spiritual meaning, and cultural identity.

• Supporting services maintain the conditions required for other services, including soil formation, nutrient cycling, primary production, decomposition, and habitat provision.

A coastal wetland can illustrate the connections. It may provide habitat for fish and birds, store carbon, filter pollutants, reduce storm damage, and support recreation. Damaging one component of the wetland can therefore affect several service categories at the same time.

Takeaway: Human well-being depends on ecological processes that produce both material benefits and less visible regulating, cultural, and supporting benefits.

Biodiversity creates ecological options

Biodiversity is the variety of life at multiple levels:

• Genetic diversity: Variation in genes within a species.

• Species diversity: The number and relative abundance of species in a community.

• Ecosystem diversity: The variety of ecosystems and habitats in a region.

Species diversity includes species richness, the number of species present, and species evenness, how evenly individuals are distributed among those species. A community containing ten species in similar numbers has greater evenness than a community in which one species dominates and the other nine are rare.

Biodiversity can support ecosystem functioning in several ways:

1. Different species may use different resources, reducing competition.

2. Species may perform complementary ecological roles.

3. Functional redundancy may allow several species to perform similar functions.

4. Genetic variation may help populations adapt to environmental change.

5. Complex food webs may reduce dependence on a single species or interaction.

Greater diversity often increases the range of ecological responses available when conditions change, but it does not guarantee that an ecosystem will remain unchanged. A disturbance that affects all species, such as extreme warming or a widespread pathogen, can threaten a diverse ecosystem.

Takeaway: Biodiversity is more than a species count: it includes variation within species, among species, and among ecosystems, and it can create multiple ways for ecological functions to continue.

Biodiversity and resilience shape ecosystem change

Ecosystem resilience is the ability of an ecosystem to absorb disturbance and retain or recover its structure and function. Two related concepts clarify how ecosystems respond:

• Resistance is the ability to withstand disturbance with little change.

• Resilience is the ability to recover after disturbance.

Disturbances may be natural, such as wildfire, drought, flooding, and hurricanes, or human-caused, such as pollution, habitat destruction, overharvesting, and invasive species introduction. A prairie adapted to periodic fire may resist some fire-related changes and recover rapidly because native grasses regrow afterward.

Repeated or severe disturbance can push an ecosystem across a threshold into an alternative stable state. For example, a clear lake may become persistently algae-dominated after excessive nutrient input. Recovery may then require more than simply removing the original disturbance.

Biodiversity can support resilience by providing alternative species with similar functions, genetic variation for adaptation, multiple feeding pathways, habitat complexity, and nearby undisturbed areas that can supply recolonizing organisms. Resilience also depends on connectivity, reproductive rates, soil and water conditions, disturbance frequency, and the severity of human impacts.

A forest food web demonstrates these connections. Oak trees capture solar energy and produce biomass. Insects, mice, and deer consume plant material; songbirds and frogs consume insects; snakes consume mice and frogs; and foxes and hawks consume snakes, mice, and birds. Fungi and bacteria decompose dead organisms and return nutrients to the soil. If a disease greatly reduces oak trees, less energy enters the web, and populations of dependent consumers and predators may decline. If other plant species can replace some oak functions, the forest may be more resilient; if oak provides unique habitat or is the only major producer, the effects may be greater.

Strategies that can protect resilience include conserving habitat and diversity, protecting keystone and foundation species, maintaining wildlife corridors, restoring wetlands and forests, reducing pollution and unsustainable harvesting, preventing invasive species, preserving genetic diversity, allowing appropriate natural disturbance regimes, and monitoring ecosystems before thresholds are crossed.

Takeaway: Primary productivity determines how much energy enters an ecosystem, food webs distribute that energy, biodiversity supplies alternative functions and pathways, and resilience describes how the whole system responds to change.