9.1 Ecosystem Structure and Function

Ecosystems and their components

Ecosystems are networks of living organisms interacting with each other and their physical environment within a defined area. Understanding how ecosystems are structured and how they function gives you the foundation for nearly everything else in ecology.

This section covers the building blocks of ecosystems (biotic and abiotic factors), how energy moves through them, how matter gets recycled, and how feeding relationships create structure from the ground up.

Ecosystem definition and characteristics

An ecosystem includes all the living organisms in an area plus the non-living environment they interact with. The key idea is that organisms and their surroundings function as a connected system, not as separate parts.

• Ecosystems range hugely in size. A small pond counts as an ecosystem, and so does the entire Amazon rainforest.
• Ecosystem boundaries are often fluid and can overlap. The transition zone between two ecosystems is called an ecotone (think of where a forest gradually becomes a grassland).
• Ecosystem resilience is the capacity of an ecosystem to absorb disturbances and recover while maintaining its basic structure and function. Resilience depends on factors like biodiversity, functional redundancy (having multiple species that perform similar roles), and adaptive capacity.

Key ecosystem components and services

Every ecosystem has two categories of components:

• Biotic factors are the living parts: producers (plants, algae), consumers (animals), and decomposers (bacteria, fungi).
• Abiotic factors are the non-living parts: temperature, precipitation, soil composition, light availability, and atmospheric gases.

These components interact constantly. For example, soil composition (abiotic) determines which plants (biotic) can grow, which in turn determines which herbivores can survive there.

Ecosystems also provide ecosystem services, which are the benefits humans get from functioning ecosystems:

• Provisioning services supply tangible resources like food, clean water, and timber
• Regulating services maintain environmental stability, such as climate regulation and water purification
• Supporting services underpin everything else, including nutrient cycling and soil formation
• Cultural services provide non-material benefits like recreation, aesthetic value, and spiritual enrichment

Abiotic and biotic factors in ecosystems

Abiotic factors and their influence

Abiotic factors are the physical and chemical conditions that shape where organisms can live and how well they can grow.

• Temperature affects metabolic rates and determines which organisms can survive in a given area
• Precipitation controls water availability and strongly influences vegetation type (desert vs. rainforest, for instance)
• Soil composition impacts nutrient availability and plant growth
• Light availability drives photosynthesis rates and influences animal behavior
• Atmospheric gases like $O_2$ and $CO_2$ are essential for respiration and photosynthesis

Abiotic factors often act as limiting factors, meaning they constrain how much a population can grow. Two principles describe this:

• Liebig's Law of the Minimum states that growth is controlled by the scarcest essential resource, not by the total amount of all resources. If nitrogen is scarce but everything else is abundant, nitrogen limits growth.
• Shelford's Law of Tolerance describes the range of environmental conditions an organism can survive in. Every species has an optimal range, and conditions outside that range cause stress or death.

Together, abiotic factors determine an ecosystem's carrying capacity, which is the maximum population size the environment can sustain over time.

Biotic factors and ecological interactions

Biotic factors include all the living organisms in an ecosystem and the ways they interact with each other. These interactions shape community structure in powerful ways.

• Competition occurs when organisms compete for limited resources like food, space, or mates
• Predation involves one organism consuming another for energy
• Mutualism is a relationship where both species benefit (bees pollinating flowers while collecting nectar is a classic example)
• Ecosystem engineering happens when organisms physically modify their environment. Beavers building dams create entire wetland habitats, and corals construct reef structures that support thousands of species.

The concept of niche describes an organism's role in its ecosystem, including the resources it uses and the conditions it needs:

• The fundamental niche is the full range of conditions and resources a species could use if there were no competition or predation.
• The realized niche is the narrower range a species actually uses, because competition and predation push it out of parts of its fundamental niche.

Keystone species have an outsized effect on ecosystem structure relative to their abundance. Sea otters, for example, keep sea urchin populations in check. Without otters, urchins overgraze kelp, and entire kelp forest ecosystems collapse. Wolves in Yellowstone play a similar role by regulating elk behavior and populations.

Energy flow and matter cycling

Energy flow through ecosystems

Energy moves through ecosystems in one direction: it enters (usually as sunlight), passes through organisms, and eventually dissipates as heat. Two laws of thermodynamics govern this process:

• The first law says energy can't be created or destroyed, only transformed. When a plant captures sunlight, it converts light energy into chemical energy stored in glucose.
• The second law says no energy transformation is 100% efficient. Some energy is always lost as heat at every step.

Solar radiation is the primary energy source for most ecosystems. Producers capture it through photosynthesis and convert it to chemical energy. In some ecosystems (like deep-sea hydrothermal vents), chemosynthesis replaces photosynthesis, with organisms using chemical reactions instead of sunlight to produce organic compounds.

Energy transfer between trophic levels follows the 10% rule: only about 10% of the energy at one trophic level gets passed to the next. The rest is lost as heat or used by organisms for their own metabolism. This is why top predators are rare and ecosystems can't support many trophic levels.

Ecological pyramids visualize this pattern:

• Energy pyramids show decreasing energy at higher trophic levels (these are always upright)
• Biomass pyramids show total mass of organisms at each level (usually upright, but can be inverted in some aquatic systems where producers reproduce rapidly)
• Numbers pyramids show organism abundance at each level (can vary in shape depending on the ecosystem)

Matter cycling in ecosystems

Unlike energy, matter is recycled through ecosystems. The same atoms get used over and over, moving between living organisms and the physical environment through biogeochemical cycles.

The major cycles to know:

• Carbon cycle: Carbon moves between the atmosphere (as $CO_2$), living organisms, oceans, and rocks. Photosynthesis pulls carbon out of the atmosphere; respiration, decomposition, and combustion release it back.
• Nitrogen cycle: Atmospheric $N_2$ is converted to usable forms through nitrogen fixation (by certain bacteria), then cycled through nitrification and eventually returned to the atmosphere via denitrification.
• Phosphorus cycle: Phosphorus cycles between rocks, soil, water, and organisms. Unlike carbon and nitrogen, it has no significant atmospheric component, so it cycles more slowly.
• Water cycle (hydrologic cycle): Water moves through evaporation, precipitation, runoff, and transpiration by plants.

Organisms need both macronutrients (nitrogen, phosphorus, potassium, required in large amounts) and micronutrients (iron, zinc, manganese, needed in trace amounts but still essential).

Decomposers are critical to all nutrient cycles. Bacteria and fungi break down dead organic matter and release nutrients back into the soil or water, making those elements available for producers to use again. Without decomposers, nutrients would stay locked in dead tissue and ecosystems would grind to a halt.

Trophic levels and interactions

Trophic structure and energy transfer

Trophic levels represent the feeding positions of organisms in a food chain. Each level is one step removed from the energy source:

1. Producers (autotrophs) form the base. Photoautotrophs (plants, algae) use sunlight; chemoautotrophs (certain bacteria) use chemical energy.
2. Primary consumers (herbivores) feed directly on producers.
3. Secondary consumers (carnivores) eat primary consumers.
4. Tertiary consumers (top predators) feed on other carnivores.
5. Omnivores eat both plants and animals, so they can occupy multiple trophic levels depending on what they're eating.

A food chain shows a single linear path of energy transfer (grass → rabbit → fox). A food web is more realistic, showing the interconnected feeding relationships across an entire community. Most organisms are part of multiple food chains within a web.

Ecological efficiency is the percentage of energy transferred from one trophic level to the next. It typically ranges from 5-20%, depending on metabolic rates, foraging behavior, and how efficiently organisms digest their food.

Trophic interactions and ecosystem dynamics

Different types of consumers play distinct roles:

• Herbivores (rabbits, deer) feed on plants
• Carnivores (lions, eagles) consume other animals
• Omnivores (bears, humans) eat both
• Detritivores (earthworms, millipedes) feed on dead organic matter (detritus)
• Decomposers (bacteria, fungi) break down organic matter at the molecular level

Trophic cascades occur when a change at one trophic level ripples through the entire food web. There are two types of control:

• Top-down control: Predators regulate prey populations, which in turn affects the trophic levels below. When wolves were reintroduced to Yellowstone, they reduced elk numbers and changed elk grazing behavior. This allowed streamside vegetation to recover, which stabilized riverbanks and even altered river dynamics.
• Bottom-up control: Resource availability at the base of the food web limits populations at every level above. If nutrient-poor soil limits plant growth, there's less food for herbivores, which means fewer carnivores too.

Both types of control operate simultaneously in most ecosystems, but one often dominates depending on the system. Recognizing whether an ecosystem is primarily top-down or bottom-up controlled helps predict how it will respond to disturbances.