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UNIT 6: ECOSYSTEMS
What Is An Ecosystem?An ECOSYSTEM is formed by a group of living organisms
(community) that interact with each other and with the surrounding
physical environment. All ecosystems have two parts:
- Biotic factors (living things), which form the BIOCENOSE
- Abiotic factors (non-living): water, soil, light, temperature, pH, etc.
The non-living part of an ecosystem is also called HABITAT or
ECOSYSTEM = BIOTOPE + BIOCENOSE
Ecosystems are made of MATTER and ENERGY:
- in the habitat: inorganic matter in rocks and minerals, soil, water, etc
- in the living beings: organic matter
- MATTER IS RECYCLED in the ecosystems:
PRODUCERS: transform inorganic
matter into organic matter that will form all
the living beings of the ecosystem
CONSUMERS: get their matter
from producers or other consumers.
DECOMPOSERS: recycle organic
matter transforming it into inorganic matter.
- The SUN is the main energy source for most ecosystems: unlimited
- AUTOTROPHIC organisms (plants, algae and some bacteria)
transform a small part of the solar energy and use it to transform
inorganic matter into organic matter (rich in energy: chemical energy)
- HETEROTROPHIC organisms (animals,
protozoa, fungi, bacteria) get their energy from
the autotrophs or other heterotrophs:
- Only a small part of the energy passes on to
the next trophic level. Most part is lost (faeces,
heat, non-ingested food...).
- ENERGY FLOWS in the ecosystems.
FOOD (or trophic) CHAINS:Chain that links species that eat and are eaten by others; each level is a TROPHIC LEVEL:
AUTOTROPHS or PRODUCERS: make their
own organic matter.
HETEROTROPHS or CONSUMERS: get their
organic matter from other organisms:
- primary consumers (herbivores)
- secondary consumers (carnivores)
- tertiary consumers (supercarnivores)
DECOMPOSERS: are not represented in food chains. They recycle organic matter by
breaking down organic matter from dead organisms.
FOOD (or trophic) CHAINS are usually more complex:
- of NUMBER of individuals
- of BIOMASS
- of ENERGY
Some times pyramids of numbers and biomass can be “inverted”.
Energy pyramids can NEVER be inverted.
There are big losses of energy as it flows from one trophic level to the next
one. Only 10% (on average) of the energy in one level passes to the next one.
This is known as the 10% rule.
POPULATION: a group of individuals of the same species living in the same place at
the same time and interbreeding.
How much can a population grow?
- birth rate
- death rate
When a species colonises a new area, if the conditions are favourable, it will have an
EXPONENTIAL GROWTH: (J-shaped curve)
But... can this go on indefinitely?
What will happen when resources start to be scarce
(food, space) or the environment is not so
favourable (diseases, adverse climate....)?
Self-regulation of the Ecosystems:
When a population grows, normally limiting factors will appear, that will limit the
Limiting Factors of an Ecosystem:
- Abiotic factors: food, light, temperature, water, soil...
- Biotic factors: relationships with other living beings:
Intraespecific Relationships: COMPETENCE
Interspecific Relaitionships: COMPETENCE, MUTUALISM, SYMBIOSIS,
COMMENSALISM, PARASITISM, PREDATION
We can classify these factors as:
- dependent on the population density: diseases, competence (and most
- independent on the population density: temperature, light, draughts...
ENVIRONMENTAL RESISTANCE includes
all the factors that may limit the growth of a
populations (both biotic and abiotic)
BIOTIC POTENTIAL of a population is the
maximum rate at which it can reproduce,
given all the resources it needs (max. birth
rate, min. death rate)
So, in real-life conditions, a population encounters environmental
resistance, and the typical graph that shows how it grows is the S-shaped
curve (LOGISTIC GROWTH):
CARRYING CAPACITY (K) is the
maximum population size that can be
supported by a particular environment.
Evolution of the human population.
What kind of growth does it follow? Why do
you think it is so?
Once that a population has reached its
carrying capacity, its size remains stable
around it, with small periodic variations
that are called FLUCTUATIONS.
Changes in the environment (plagues, diseases, new predators,
competence.... ) may lead to a new carrying capacity for a population:
Strategies for Survival: r-strategy and k-strategy
- reproduce rapidly (high birth rates)
- abundant offspring
- little investment in care of the
offspring: much of the offspring dies
- live in changing, unstable
- bacteria, insects, fish... (small-
- slow reproduction (low birth
- produce few offspring
- big investment in care of the
offspring, which takes time to
develop and grow adult.
- live in stable environments, to
which they are well adapted.
- big-sized species
Ecosystems change over time: ECOLOGICAL SUCCESSIONS
An ECOLOGICAL SUCCESSION is an ordered sequence of changes in the
composition (species) and structure of a community over time, until it reaches
a stable state called climax community.
- Successions happen because the living beings modify their environment
by making it more favourable for more complex organisms. Environmental
factors such as climate (draught, floods...) fires, volcano eruptions, may also
alter the ecosystems.
Given enough time and
favourable conditions, a
pond may transform
into a forest.
If a succession begins in a new area, where no previous community was
established (e.g. on a sand dune, or on a new volcanic island, it is called a
Pioneer species, with little nutritional needs, small size and resistant to
unstable conditions, are the first to establish on the bare land. They are
r-strategists such as lichens and mosses. The activity of these organisms
produce the meteorization of the rock
(both physical and chemical).
When these first colonizers die, their organic matter helps to create a fertile
soil, where other (a little more complex) organisms will establish: ferns, herbs
and bushes, and finally, trees.
As these new plants die, the humus (organic part of the soil) increases, and
the soil becomes deeper and more structured.
K-strategists start to
as they are better
competitors in stable
Animal species follow the changes in the plant species as the succession
Finally -ideally- all the ecosystems progress in a succession until they reach a
final state called CLIMAX COMMUNITY, where the maximal biodiversity and
stability is achieved.
Different CLIMAX COMMUNITIES in different latitudes.
Bosque de faias
SECONDARY SUCCESSIONS happen on a place with the previous
ecosystem was completely or partially destroyed by natural or artificial causes
(fires, volcano eruptions, floods...). These ecosystems keep their soil, so
secondary successions are normally faster than primary successions.