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Ecosystems
Basic Concepts
Life is sustained by the interactions of many organisms
functioning together, interacting through their physical
and chemical environments
This is referred to as an ecosystem
Basic characteristics of ecosystems:
Ecosystem structure
Ecosystem processes
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecosystem Structure
Ecosystem structure
An ecosystem has two major parts
Living
Non-living
Called the ecological community is the set of species interacting within
the ecosystem
Consists of the physical-chemical environment including the local
atmosphere, water and mineral soil (on land) or other substrate (in water)
Ecosystem processes
An ecosystem has two major processes
Cycling of elements
Flow of energy
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Communities
Defined in two ways
A set of interacting species found in the same place and
functioning together, thus enabling life to persist
The community exists of all the species found in an area,
whether or not they are known to interact
(Practical/operational definition)
One form of interaction found within communities is the
transfer of energy, chemical elements and compounds
between different species (known as a food chain)
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Communities (terminology)
Food Chain
Food Web
The linkage of who feeds on whom
A network of who feeds on whom, consisting of complex
linkages
Trophic Level
All the organisms in a food web that are the same number of
feeding levels away from the original energy source
In most ecosystems this original source is the sun
In others it is the energy in certain inorganic compounds
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Communities (terminology)
Photosynthesis
The process whereby green plants and algae create sugars
from CO2, water and sunlight:
6‫ܱܥ‬ଶ + 6‫ܪ‬ଶ ܱ + ݁݊݁‫ܪ ଺ܥ → ݕ݃ݎ‬ଵଶ ܱ଺ + 6ܱଶ
Autotrophs
Those organisms who photosynthesise (1st Trophic Level)
“organisms that produce their own food”
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Communities (terminology)
Heterotrophs
Organisms that feed on other organisms
Herbivores:
Carnivores:
Feed on plants, algae or photosynthetic bacteria (2nd Trophic Level)
Feed directly on the herbivores (3rd Trophic Level)
Feed on third level carnivores (4th Trophic Level)
Decomposers (detritivores)
Organisms that feed on dead organic material (occupies the
highest trophic level in a given ecosystem)
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
4th Trophic level
3rd Trophic level
2nd Trophic level
FLOW OF ENERGY
5th Trophic level
1st Trophic level
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Complexity (A simple food web)
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Complexity (A more complex food web)
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecosystems as Systems
Ecosystems are open systems having both a flow of
matter and energy
Ecosystems vary greatly in the their structural complexity
as well as the clarity of their boundaries
What all ecosystems therefore have in common is NOT
size or shape, but processes which give them the ability
to sustain life
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Cycling of Elements
21 chemical elements required by at least some form of life
Elements must be available at the right time, in the right
amounts, and in the right ratio compared to other elements
They must also be recycled (food to waste back to food)
For complete recycling several species need to interact
A simple ecosystem (theoretical)
Via photosynthesis green plants/algae produce sugars
These sugars are used to build organic compounds (e.g. wood y
tissue)
HOWEVER, green plants cannot break this material down into its
original organic compounds. The presence of other species are
required
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Flow of Energy
All life requires energy
Energy is the ability to do work
Ecosystem Energy Flow: the movement of energy through
an ecosystem from the external environment through a
series of organisms and back to the external environment
Energy enters an ecosystem in one of two ways:
Energy fixed by organisms which moves through the food chain
Heat energy transferred by air/water currents or through
convection through sediments/soils to warm living things
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Laws of Thermodynamics
First law of thermodynamics
Aka law of the conservation of energy
In any physical or chemical change, energy is neither created
nor destroyed but merely changed from one form to another
Second law of thermodynamics
Whenever some work is done, some energy is inevitably
converted to heat (which from our organisms perspective gets
“lost” to the surrounding environment)
More specifically this deals with a change in the state of the
energy from organised to disorganised (which we refer to as
entropy)
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
An impossible ecosystem
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Energy Flow
The net flow of energy through an ecosystem is always a
one-way flow
For example:
Green plants/algae make sugars during photosynthesis using
energy derived from the sun
Herbivores use the sugars manufactured by plants as their
main source of energy (process of respiration)
Therefore, ecosystems must exist between a source of
usable energy, and a sink for degraded energy
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Biological Production and Biomass
Biomass: the total amount of organic matter in any
ecosystem
Production: The change in biomass over a given period
Biological production: The capture of usable energy from
the environment to produce organic matter or organic
compounds (also referred to as energy fixation)
Gross production: the increase in stored energy before
any is used
Net production: the amount of newly acquired energy
stored after some energy has been used
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Biological Production and Biomass
We “burn” a fuel in order to obtain energy (respiration)
Respiration: The complex series of chemical reactions in
organisms that make energy available for use. Output is
CO2, H2O and water
‫ܪ ଺ܥ‬ଵଶ ܱ଺ + 6ܱଶ → 6‫ܱܥ‬ଶ + 6‫ܪ‬ଶ ܱ + ‫ݕ݃ݎ݁݊ܧ‬
Primary production: The production carried out by
autotrophs
Secondary production: The production carried out by all
heterotrophs
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Energy Efficiency
How efficiently organisms use energy is important in
terms of the conservation of biological resources
No system can be 100% efficient (2nd Law of
Thermodynamics) therefore energy is degraded as it
flows through the food web
Energy efficiency: The ration of output to input as well as
the amount of useful work obtained from some amount
of available energy
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Energy Efficiency
Can be viewed in tow ways (depending on your perspective)
Converts a lot of energy to stored reserves while using little for
biological functions (Herbivores)
Uses as much energy as possible and stores very little (Carnivores)
Trophic-level efficiency: The ration of production of one
trophic level with that of the next lower trophic level
Never very high
More than 90% of energy transferred between trophic levels is lost
as heat
Less than 10% (typically 1% in natural systems) is fixed as tissue
Example: Cattle require ±7.2kg of vegetable matter to produce ±0.45
kg of meat
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Stability and Succession
Ecosystems are dynamic and constantly changing due to
external as well as internal processes
When disturbed ecosystems can recover through
environmental succession if the damage is not too great.
Succession: The process of development and establishment of
an ecosystem
Primary succession: Establishment and development of an
ecosystem where one did not exist previously
e.g. vegetation growing on recent lava flows
Secondary succession: Reestablishment of an ecosystem after
disturbances. There are remnants of a previous biological
community including organic matter and seeds
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Ecological Stability and Succession
Early-successional species: also known as pioneers, they
have evolved to the environmental conditions in early
stages of succession (grow rapidly and spread easily)
Late-successional species:These tend to be slower
growing, longer lived and can persist under intense
competition with other species
Middle-successional species: Characteristics in between
the other two types
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
How Species Facilitate Succession
Facilitation
Interference
An earlier successional species changes the local environment in ways
that make it suitable for other species characteristic of a later
successional stage
Situations where an earlier successional species changes the local
environment so it is unsuitable for a species in a later successional stage
Life History Differences
Not true succession
Occurs because differences in time it takes for species to establish
themselves give the appearance of succession
Chronic Patchiness: A situation where ecological succession does not
occur. One species may replace another, or an individual of the first
species may replace it, but no overall general temporal pattern is
established
Characteristic of highly disturbed environments
ENV 101: Introduction to Environmental
Science - Lecture 4
2014
Nutrient Cycling and Succession
One of the most important effects of succession is a change in
the storage of chemical elements necessary for life
In terrestrial environments the storage of these chemical
elements increase:
Increased storage
Increased rate of uptake
Organic matter stores chemical elements. If it it accumulates then the
amount of elements accumulate
Presence of root nodules that can assimilate atmospheric nitrogen
(nitrogen fixation) increasing the uptake by the plant
Decreased rate of loss
The presence of live and dead organic material decreases rates of erosion
which would remove these chemical elements from the ecosystem
ENV 101: Introduction to Environmental
Science - Lecture 4
2014