ECOSYSTEMS

What is an Ecosystem?

The biological community together with its associated non-living environment

Ecosystems consist of biotic and abiotic components

Biotic Components of an Ecosystem

Living things

Divided into two groups based on their source of food:

Autotrophs

Heterotrophs

Autotrophs

Require only inorganic nutrients and an outside energy source to produce organic nutrients

Also called producers as they produce food

Two types: Photoautotrophs – produce organic nutrients via

photosynthesis

Chemoautotrophs – produce organic nutrients using energy obtained from the oxidization of inorganic compounds

Examples of Autotrophs

Algae

PlantsCyanobacteria Photosynthetic protists

Chemosynthetic bacteria

Heterotrophs

Require a source of preformed organic nutrients

Also called consumers as they consume food

All animals are heterotrophs, as are fungi, many bacteria and parasitic plants

Parasitic herb (Hyobanche sanguinea), Namaqualand

Types of Heterotrophs

Herbivores – Feed on plants or algae

Carnivores – Feed on other animals

Omnivores – Feed on plants and animals

Detrivores – Feed on decomposing organic matter

Decomposers – Break down organic material

Examples of Heterotrophs

Herbivore Herbivore Carnivore

Carnivore Decomposer Decomposer

Energy Flow & Chemical Cycling

Every ecosystem is characterised by two fundamental processes:

1. Energy flow One-way flow of energy through biotic community

Begins when producers absorb solar energy

2. Chemical cycling Cyclical flow of materials from the abiotic

environment, through the biotic community and back again to the abiotic environment

Energy Flow & Chemical Cycling

Energy flows through an ecosystem

Chemicals cycle through an ecosystem

Energy Flow

With few exceptions, all ecosystems depend on solar energy as a primary energy source

Primary productivity is the rate at which producers convert solar energy into biomass

Primary productivity is dependent upon sunlight, nutrient and water availability

Productivity of Ecosystems

Tropical rain forests have high productivity because of abundant rainfall and sunlight

Estuaries and marshes have high productivity because of high nutrient influx from rivers and streams

Productivity of Ecosystems

Energy Flow

Some of the energy captured from the sun is lost to the environment as heat when producers use energy for cellular respiration

Some energy also goes to detritus feeders through waste production and death

Only a portion (~10%) of the energy captured by producers is passed on to consumers

Energy Flow

Consumers also lose energy due to respiration, excretion, and death

At each trophic level, only ~10% of the energy is passed up to the next trophic level

All the energy taken in by producers is ultimately lost as heat through respiration

Energy Flow

Only 10% of the energy taken in by a herbivore goes to carnivores

The rest goes to detritus feeders, or is lost to the environment as heat

Energy Flow

Energy flow through an ecosystem is one way; it is not recycled

Autotrophs must continue to capture the sun’s energy for ecosystems to persist

Trophic Levels

Ecosystems have a trophic structure that represents the different feeding relationships that determine the route of energy flow and the pattern of chemical cycling

Species in an ecosystem are divided into different trophic levels based on their main source of nutrition

Trophic Levels

Trophic levels include:

1. Primary producers

2. Primary consumers

3. Secondary consumers

4. Tertiary consumers

5. Detritivores and decomposers

Primary Producers

Autotrophs support all other trophic levels either directly or indirectly by synthesising sugars and other organic molecules using light energy

EXCEPTION: In hot water deep-sea vents, chemosynthetic bacteria form the basis of the food chain. These bacteria obtain energy from chemical sources such as hydrogen sulfide (H2S) instead of from sunlight

Primary Consumers

Primary consumers are herbivores that consume the primary producers

For example, herbivorous insects, grazing mammals, seed-eating birds, aquatic zooplankton and some fish

Secondary Consumers

Secondary consumers are carnivores that eat herbivores

E.g. spiders, frogs, insect-eating birds, lions, many fish, sea-stars

Tertiary Consumers

Tertiary consumers are carnivores that eat other carnivores

Detritivores & Decomposers

These are consumers that derive energy from organic wastes and dead organisms

E.g. bacteria, fungi, and scavengers such as cockroaches and vultures

This level is an important part of the recycling process as it returns chemicals to the primary producers

Food Chains

A food chain depicts the transfer of energy from one trophic level to the next

Food Webs

Feeding relationships in nature are usually much more complex than a simple chain

E.g. several primary consumers may feed on the same plant species and one primary consumer may eat several types of plant

These complex feeding relationships are depicted in a diagram called a food web

Antarctic Food Web

Ecological Pyramids

Food chains lose energy between trophic levels

Only about 10% of the energy of one trophic level is available to the next trophic level

This explains why few top carnivores can be supported in an ecosystem

The flow of energy between successive trophic levels can be depicted as an ecological pyramid

Productivity Pyramids

A productivity or energy pyramid depicts the amount of energy contained in each trophic level from the lowest, the producers, to the highest, the top carnivores

Trophic levels are stacked in blocks proportional in size to the energy acquired from the level below

Productivity Pyramid

Biomass Pyramid

In a biomass pyramid, each tier symbolises the total dry weight of all organisms in that level

QUESTION

Why could more people be supported on Earth if people were vegetarians than if they ate meat?