ECOSYSTEMS – LIFE SCIENCE

Energy in Ecosystems

Energy moves continuously through ecosystems, powering life at every level. In most ecosystems, energy flows through four main feeding levels, each representing a step in the transfer of energy.

Producers: Producers, usually plants and some algae, convert sunlight into stored chemical energy through photosynthesis. This forms the basis of most food chains.

Primary Consumers: These are herbivores that obtain energy by eating producers. Examples include deer, rabbits, and many insects.

Secondary Consumers: These carnivores feed on primary consumers. Examples include wolves, snakes, and some birds.

Decomposers: Organisms such as bacteria, fungi, and some insects that break down dead plants and animals. Decomposers return nutrients to the environment and obtain energy from all three other levels.

Flow of Energy: We can visualize energy movement in ecosystems using models such as energy pyramids.

Energy Pyramid Diagram

The base of the pyramid—the producers—has the most energy available. At each higher level, there is less available energy. The size of each level on the pyramid is proportional to the amount of energy available to organisms at that level.

Conservation of Energy

When energy moves through an ecosystem, only a small fraction is passed from one feeding level to the next. Most of the energy is lost along the way—often as heat.

Solar energy entering a leaf, photosynthesis, respiration, heat loss, and energy available to herbivores.

Not all sunlight that reaches a plant is absorbed. Of the energy that is absorbed: Some is used in photosynthesis to make food. Some is lost through respiration. Some is lost as heat. Only what remains becomes available to the next level when the plant is eaten. Because of these losses, energy flow through an ecosystem is one-way—it cannot be recycled like matter.

Matter in Ecosystems

Food Chains: A food chain is a simple model that shows how energy and matter move from one organism to another in a straight line. Arrows in a food chain indicate the direction of energy flow—from the eaten to the eater.

Food Webs : While food chains show one path of energy flow, food webs are more realistic because they illustrate all the possible feeding relationships within a community.

Multiple interconnected food chains with arrows representing feeding relationships.

In food webs: Consumers may feed at multiple levels. Omnivores, like humans, eat both producers and other consumers. For example, a duck may eat snails (primary consumers) and also eat fish (higher-level consumers). Food webs also help us understand the interdependence of organisms. For example, if pollution destroys aquatic plants, herbivores that depend on those plants will decline. This loss affects predators that feed on those herbivores. Thus, changes at one level can impact the entire web.

Capacity for Change

Populations and Carrying Capacity: A population is a group of organisms of the same species living in a specific area. Every environment has a carrying capacity, which is the maximum number of individuals of a species it can support over time without degrading the environment. When a population is below its carrying capacity:

• Birth rates usually exceed death rates.
• The population will grow until the carrying capacity is reached.
• When the population nears its carrying capacity: Resources become limited. Growth slows and eventually levels off.

This graph should show three phases:

• Lag phase: slow initial growth as organisms mature and adapt.
• Exponential growth phase: rapid increase when resources are plentiful.
• Stationary phase: growth levels off as the carrying capacity is reached.

Understanding these patterns helps scientists predict how populations will respond to environmental changes, resource availability, and human impact. By studying energy flow, matter cycling, food webs, and population dynamics, we can better understand how ecosystems function and how all living things are connected.

Predator–Prey Relationships

Predator–prey relationships occur when one organism (the predator) feeds on another (the prey). These interactions are essential for maintaining the balance and health of ecosystems. Energy flows through an ecosystem via food chains and food webs, and predation is one of the primary mechanisms moving energy from one trophic level to another. Predation also acts as a limiting factor on population sizes, preventing overpopulation and resource depletion.

The cyclical pattern of prey and predator populations over time. Typically, the prey population rises first, supporting an increase in the predator population. As predators consume more prey, the prey population drops, causing a subsequent decline in the predator population. Then, with fewer predators, the prey population rebounds.

Key features of predator–prey cycles:

• When prey numbers decline sharply, predator populations often decline afterward due to reduced food availability.
• As predator numbers fall, prey have a chance to recover and increase in number.
• This cyclical pattern helps keep both populations in balance.

Predation also contributes to the overall health of prey populations. Predators typically target the young, old, sick, or injured individuals. This natural selection pressure helps maintain a healthier, more resilient prey population by removing weaker individuals and reducing the spread of disease.

Symbiosis

Symbiosis is another important type of relationship in ecosystems. Symbiosis occurs when members of different species live in close, long-term association with one another. These relationships can take several forms, each with distinct impacts on the organisms involved.

Types of Symbiosis

• Mutualism: Both species benefit from the relationship. Example: Bees pollinate flowers while feeding on their nectar.
• Commensalism: One species benefits, while the other is neither helped nor harmed. Example: Barnacles attach to whales for transportation without affecting the whale.
• Parasitism: One species (the parasite) benefits at the expense of the other (the host). Example: Ticks feeding on the blood of mammals.

Importance of Symbiosis

• Symbiotic relationships are critical to the survival of many species. They can influence:
• Nutrient cycling (e.g., nitrogen-fixing bacteria in plant roots)
• Species distribution (e.g., coral and algae partnerships that form coral reefs)
• Population dynamics (e.g., parasite-host relationships limiting host population growth)

Symbiosis demonstrates the deep interconnectedness of life in ecosystems. By examining these relationships, scientists can better understand how species depend on one another and how ecosystems function as integrated, balanced systems.

 Disruption of Ecosystems

Changing Conditions in Ecosystems

Ecosystems are dynamic systems that are constantly changing. These changes can occur: Rapidly and dramatically, as with floods, wildfires, hurricanes, or volcanic eruptions. Slowly and gradually, such as when new species are introduced or environmental conditions change over time.

Factors that influence the balance (equilibrium) of an ecosystem include: Availability of food and water; Changes in temperature; Competition between native and introduced species

When these conditions change, the entire ecosystem may be affected. Species may need to adapt to new conditions, move to new habitats, or risk population decline.

Common Types of Ecosystem Disturbances

Ecosystems can experience many types of disturbances that lead to changes in their structure and composition.

• Wildfires: Burn large areas of vegetation but can also clear space for new growth.
• Floods: Change water levels rapidly, displacing animals and reshaping habitats.
• Storms/Hurricanes: Uproot trees and alter coastlines.
• Invasive Species: Non-native species outcompete native organisms and disrupt food webs.

These disturbances may initially seem harmful but can also be part of natural cycles that renew and reshape ecosystems.

Ecological Succession

When an ecosystem is disturbed, it often goes through a process called ecological succession. Succession is the gradual change in species composition in an area over time following a disturbance.

Bare soil left after fire. Pioneer species such as grasses and small plants colonize the area. Shrubs and young trees grow next. Mature forest eventually develops.

Through succession, even after major disturbances like forest fires, ecosystems can recover as new plants grow and animal species return.

Extinction

Sometimes, the changes in an ecosystem are so great that certain species cannot survive. Extinction is the complete disappearance of a species when the last of its members dies. Extinction can happen due to natural processes, such as: Climate change, Competition with other species, Natural disasters. However, human activities are now the leading cause of extinction worldwide. These activities include:

Habitat destruction, such as clearing forests for farmland or urban development. Overhunting and overfishing. Pollution of air, water, and soil. Introduction of invasive species that disrupt native communities.

When a species goes extinct, it can affect the entire ecosystem. Other species that relied on it for food or other ecological roles may also decline. Protecting habitats and reducing harmful human impacts are critical for preventing extinction and preserving biodiversity.