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

BIOTOPE.

ECOSYSTEM = BIOTOPE + BIOCENOSE

Ecosystems are made of MATTER and ENERGY:

MATTER:

- in the habitat: inorganic matter in rocks and minerals, soil, water, etc

- in the living beings: organic matter

- limited

- 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.

ENERGY:

- 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:

trophic chains

- 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:

FOOD WEBS

TROPHIC PYRAMIDS:

- of NUMBER of individuals

- of BIOMASS

- of ENERGY

Some times pyramids of numbers and biomass can be “inverted”.

Energy pyramids can NEVER be inverted.

WHY?

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.

PHOSPHORUS CYCLE

CARBON CYCLE

Key terms:

- RESPIRATION

- PHOTOSYNTHESIS

- COMBUSTION

NITROGEN CYCLE

Key Terms:

- FIXATION

- ASSIMILATION

- NITRIFICATION

- DENITRIFICATION

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

- immigration

- emigration

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

population size:

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

biotic relationships)...

- 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

R-strategy species:

- reproduce rapidly (high birth rates)

- abundant offspring

- little investment in care of the

offspring: much of the offspring dies

- live in changing, unstable

environments

- bacteria, insects, fish... (small-

sized species)

K-strategy species:

- slow reproduction (low birth

rates)

- 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

PRIMARY SUCCESSION.

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.

FERNSHERBS, BUSHES

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

replace r-strategists,

as they are better

competitors in stable

environments.

Animal species follow the changes in the plant species as the succession

progresses.

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.

Carballeira

Aciñeira

Bosque de faias

Tropical forest

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.