Ecology
see also Ecology
History of Ecology
Fundamental principles of ecology
Biosphere and biodiversity
Main articles:
Biosphere,
Biodiversity,
Unified neutral theory of biodiversity
For modern ecologists, ecology can be studied at several levels:
population level (individuals of the same species),
biocenose
level (or community of species),
ecosystem
level, and
biosphere level.
The Earth, from an ecological point of view, consists of several
compartments, the
hydrosphere (or sphere of water), the
lithosphere (or sphere of soils and rocks) and the
atmosphere (or sphere of the air). The
biosphere,
sometimes described as fourth envelope, is the part of the planet on which the
life developed. It is a very thin surface layer, which goes down to 11000
meters of depth to rising up to 15000 meters of altitude, although the
majority of life live in the zone located between -100 meters and +100 meters.
Life first developed in the hydrosphere, at low depth, in the
photic
zone. Multicellular organisms then appeared and colonized
benthic zones.
Terrestrial life developed later, after the
ozone
layer protecting living beings from
UV rays formed.
Diversification of terrestrial species is thought to be increased by the
continents
drifting apart, or alternately, colliding. Biosphere and biodiversity are
inseparable characteristics of the Earth. Biosphere is defined as being the
sphere of life, whereas
biodiversity is its diversity. The sphere is the container, whereas
diversity is the contents. This diversity is expressed at the same time at the
ecological level (ecosystem), population level (intraspecific diversity) and
species level (specific diversity).
The biosphere contains great quantities of elements such as
carbon,
nitrogen
and oxygen.
Other elements, such as
phosphorus,
calcium,
potassium
are also essential to
life. At the ecosystem and biosphere level, there is a permanent recycling
of all these elements, which alternate between the mineral state and the
organic state.
While there is a slight input of geothermal energy, the bulk of the
functioning of the ecosystem is primarily based on the input of
solar
energy. Plants convert
light into into
chemical energy by the process of
photosynthesis, which creates
glucose (a
simple sugar) and releases free
oxygen.
Glucose thus becomes the secondary energy source which drives the ecosystem.
Some of this glucose is used directly by other organisms for energy. Other
sugar molecules can be converted to other molecules such as
amino
acids. Plants use some of this sugar, concentrated in
nectar to
entice pollinators to aid them in reproduction. (Honeybees concentrate the
sugar still further as honey, which can be said to be "stored summer sunshine").
Cellular respiration is the process by which organisms (like
mammals)
breakdown the glucose back to its constituents,
water and
carbon dioxide, gaining back the stored energy the sun originally gave to
the plants. The proportion of photosynthetic activity of plants to the
respiration of other organisms determines the specific composition of the
Earth's atmosphere, particularly its oxygen level.
Global air currents mix the atmosphere and maintain nearly the same
balance in areas of intense biological activity and areas of slight biological
activity.
Water is also exchanged between the hydrosphere, the lithosphere, the
atmosphere and the biosphere in regular
cycles. The
oceans are large tanks, which store water, ensure a thermal and climatic
stability, as well as the transport of the chemical elements thanks to large
oceanic currents.
For better understanding of how the biosphere works, and the dysfunctions
related to human activity, American scientists carried out, under
greenhouses, a small-scale model of the biosphere, called
Biosphere II.
The concept of an ecosystem
Main article:
Ecosystem
The first principle of ecology is that each living organism has an ongoing
and continual relationship with every other element that makes up its
environment. An
ecosystem
can be defined as any situation where there is interaction between organisms
and their environment.
The ecosystem is composed of two entities, the entirety of life (called the
biocenose)
and the medium that life exists in (the
biotope).
Within the ecosystem, species are connected and dependent upon one another in
the food
chain, and exchange
energy and
matter
between themselves and with their environment.
The concept of an ecosystem can apply to units of variable size, a
pond, a field, or
a piece of deadwood. A unit of smaller size is called a
microecosystem. For example, an ecosystem can be a stone and all the
life under it. A mesoecosystem could be a
forest, and a
macroecosystem a whole
ecoregion,
with its
watershed.
The main questions when studying an ecosystems are:
- how could be carried out the colonization of an arid area?
- What are the ecosystems dynamics and changes
- how does an ecosystem interact at local, regional and global scale
- is the current state stable?
- what is the value of an ecosystem? How does the interaction of
ecological systems provide benefit to humans, especially in the provision of
healthy water?
Ecosystems are often classified by reference to the biotopes concerned. The
following ecosystems may be defined :
Another classification can be done by reference to its communities (for
example a
human ecosystem).
Dynamics and stability
Main articles:
ecological factor,
geobiochemical cycle,
Homeostasis,
Population dynamics
The biotope is a region environmentally uniform, characterized by a whole
set of geological, geographical and climatological parameters, which are
called abiotic
ecological factors:
- water, is
at the same time, an essential element to life, as well as a
milieu
- air, which
provides oxygen and carbon dioxide to living species, and allows the
dissemination of
pollen and
spores
- soil, at
the same time source of nutriment and support of development
-
temperature, which should not exceed certain extremes, even if tolerance
to heat is significant for some species
- light,
allowing
photosynthesis.
Biocenose, or community, is a group of populations of plants, animals,
micro-organisms. Each
population is the result of procreations between individuals of same
species and
cohabiting in a given place and at a given time. When a population consists of
an insufficient number of individuals, the species is threatened with
extinction, either by underpopulation, or by because of
consanguinity. A population can be reduced for several reasons, for
example, disappearance of its habitat (destruction of a forest) or by
excessive predation (such as the hunting of a given species).
Biocenose is characterized by
biotic ecological
factors of two types: intraspecific and interspecific relations.
Intraspecific relations are those which are established between
individuals of the same species, forming a population. They are relations of
co-operation or
competition, with division of the territory, and sometimes organization in
hierarchical societies.
Interspecific relations, i.e. those existing between different species,
are numerous, and usually described according to their beneficial, detrimental
or neutral effect (for example,
symbiosis
(relation ++) or
competition (relation --)). The most significant relation is the relation
of predation
(to eat or to be eaten), which leads to the essential concepts in ecology of
food
chains (for example, the grass is consumed by the herbivore, itself
consumed by a carnivore, itself consumed by a carnivore of larger size).
Ecological niche is the area shared by two species when they live at the
same place with the same type of
diet.
The existing interactions between the various living beings go along with a
permanent mixing of mineral and organic substances, absorbed by organisms for
their growth, their maintenance and their reproduction, to be finally rejected
as waste. These permanent recyclings of the elements (in particular
carbon,
oxygen and
nitrogen)
as well as the
water are called
biogeochemical cycles. They guarantee a durable stability of the biosphere
(at least when human influence and
extreme weather phenomena are left aside). This self-regulation, supported
by negative
feedback controls, ensures the perenniality of the ecosystems. It is
showed by the very stable concentrations of most elements of each compartment.
This is referred to as
homeostasis. The ecosystem also tends to evolve to a state of ideal
balance, reached after a
succession of events, the
climax (for example a pond can become a
peat bog).
Spatial relationships and subdivisions of land
Main articles:
Biome,
ecozone
Ecosystems are not isolated from each other, but are interrelated. For
example, water
may circulate between ecosystems by the means of a
river or
ocean current. Water itself, as a liquid medium, even defines ecosystems.
Some species, such as
salmon or
freshwater eels
move between marine systems and fresh-water systems. These relationships
between the ecosystems lead to the concept of a biome.
A biome is a
homogeneous ecological formation that exists over a vast region, such as
tundra or
steppes. The
biosphere
comprises all of the Earth's biomes -- the entirety of places where life is
possible -- from the highest mountains to the depths of the oceans.
Biomes correspond rather well to subdivisions distributed along the
latitudes, from the
equator
towards the poles,
with differences based on to the physical environment (for example, oceans or
mountain ranges) and to the
climate.
Their variation is generally related to the distribution of species according
to their ability to tolerate temperature and/or dryness. For example, one may
find
photosynthetic
algae only in the photic part of the ocean (where light
penetrates), while
conifers
are mostly found in mountains.
Though this is a simplification of more complicated scheme,
latitude
and altitude
approximate a good representation of the distribution of
biodiversity within the biosphere. Very generally, the richness of
biodiversity (as well for animal than plant species) is decreasing most
rapidly near the
equator (as
in Brazil)
and less rapidly as one approaches the
poles.
The biosphere may also be divided into
ecozone,
which are very well defined today and primarily follow the continental
borders. The ecozones are themselves divided into
ecoregions, though there is not agreement on their limits.
Ecosystem productivity
In an ecosystem, the connections between species are generally related to
food and their
role in the
food
chain. There are three categories of organisms:
- producers -- plants which are capable of
photosynthesis
- consumers -- animals, which can be primary consumers (herbivorous),
or secondary or tertiary consumers (carnivorous).
- decomposers --
bacteria,
mushrooms
which degrade organic matter of all categories, and restore minerals to the
environment.
These relations form sequences, in which each individual consumes the
preceding one and is consumed by the one following, in what are called
food
chains or
food network. In a food network, there will be fewer organisms at each
level as one follows the links of the network up the chain.
These concepts lead to the idea of
biomass
(the total living matter in a given place), of
primary productivity (the increase in the mass of plants during a given
time) and of
secondary productivity (the living matter produced by consumers and the
decomposers in a given time).
These two last ideas are key, since they make it possible to evaluate the
load capacity -- the number of organisms which can be supported by a given
ecosystem. In any food network, the energy contained in the level of the
producers is not completely transferred to the consumers. Thus, from an energy
point of view, it is more efficient for humans to be primary consumers (to get
nourishment from grains and vegetables) than as secondary consumers (from
herbivores such as beef and veal), and more still than as a tertiary consumer
(from eating carnivores).
The productivity of ecosystems is sometimes estimated by comparing three
types of land-based ecosystems and the total of aquatic ecosystems:
- the forests (1/3 of the Earth's land area) contain dense biomasses and
are very productive. The total production of the world's forests corresponds
to half of the primary production.
- savannas, meadows, and marshes (1/3 of the Earth's land area) contain
less dense biomasses, but are productive. These ecosystems represent the
major part of what humans depend on for food.
- extreme ecosystems in the areas with more extreme climates -- deserts
and semi-deserts, tundra, alpine meadows, and steppes -- (1/3 of the Earth's
surface) have very sparse biomasses and low productivity
- finally, the marine and fresh water ecosystems (3/4 of Earth's surface)
contain very sparse biomasses (apart from the coastal zones).
Humanity's actions over the last few centuries have seriously reduced the
amount of the Earth covered by forests (deforestation),
and have increased agro-ecosystems (agriculture).
In recent decades, an increase in the areas occupied by extreme ecosystems has
occurred (desertification).
Ecological crisis
Generally, an
ecological crisis is what occurs when the
environment of life of a species or a population evolves in an
unfavourable way to its survival.
It may be that the environment quality degrades compared to the species
needs, after a change of abiotic
ecological factor (for example, an increase of temperature, less
significant rainfalls).
It may be that the environment becomes unfavourable for the survival of a
species (or a population) due to an increase pressure of
predation
(for example overfishing).
Lastly, it may be that the situation becomes unfavourable to the quality of
life of the species (or the population) due to raise in the number of
individuals (overpopulation).
Ecological crises may be more or less brutal (occurring between a few
months to a few million years). They can also be of natural or anthropic
origin. They may relate to one unique species or on the contrary, to a high
number of species (see the article on
Extinction event).
Lastly, an ecological crisis may be local (as an
oil spill)
or global (a rise in the sea level related to
global warming).
According to its degree of endemism, a local crisis will have more or less
significant consequences, from the death of many individuals to the total
extinction of a species. Whatever its origin, disappearance of one or several
species often will involve a rupture in the
food
chain, further impacting the survival of other species.
In the case of a global crisis, the consequences can be much more
significant; some extinction events showed the disappearance of more than 90%
of existing species at that time. However, it should be noted that the
disappearance of certain species, such as the dinosaurs, by freeing an
ecological niche, allowed the development and the diversification of the
mammals. An ecological crisis thus paradoxically favored biodiversity.
Sometimes, an ecological crisis can be a specific and reversible phenomenon
at the ecosystem scale. But more generally, the crises impact will last.
Indeed, it rather is a connected series of events, that occur till a final
point. From this stage, no return to the previous stable state is possible,
and a new stable state will be set up gradually (see
homeorhesy).
Lastly, if an ecological crisis can cause extinction, it can also more
simply reduce the quality of life of the remaining individuals. Thus, even if
the diversity of the human population is sometimes considered threatened (see
in particular
indigenous people), few people envision human disappearance at short span.
However,
epidemic diseases,
famines,
impact on health of reduction of
air quality,
food crises, reduction of living space, accumulation of toxic or non
degradable wastes, threats on
keystone species (great apes, panda, whales) are also factors influencing
the
well-being of people.
During the past decades, this increasing responsibility of humanity in some
ecological crises has been clearly observed. Due to the increases in
technology and a rapidly increasing population, humans have more influence on
their own environment than any other
ecosystem engineer.
Some usually quoted examples as ecological crises are
