Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

44 D The use of genetic and species diversity

vertical evolution or isolation of subpopulations) (cf

for example, Barbault and Sastrapradja, 1995).

D 2.2

The role of species within ecosystems

Every organism is linked to its environment via a system

of interactive relationships. First of all, there are

links to the abiotic environment: eg organisms are

dependent on light, water, nutrients or structural features

of their environment. The activities of the

organism influence the environment by changing the

site conditions, using resources or being available for

other organisms. Furthermore, each species is situated

within a web of interactions with the biotic environment,

ie with other individuals of the same or

other species. The participating organisms can be

divided into the following roles depending on the

nature of these relations (Begon et al, 1996): competitors

(within a species and between species),

predators (predator-prey relationship), decomposers

(decomposition of dead organic matter), mutualists

and symbionts (mutually supportive relationship),

parasites and pathogens. Organisms can generally

assume various roles either simultaneously or over

the course of their life cycle.

These interactions between organisms are a prerequisite

for evolution and thus contribute over the

long term to the formation of species. This is because

they promote processes such as avoiding interspecies

competition by niche differentiation or the

mutual ‘evolutionary escalation’ in the case of the coevolution

of species. For example, the length of the

calyx and the proboscis of pollinating butterflies

evolved in parallel. In the short term it is however

possible for biotic interactions in one area to lead to

a reduction of the number of species, for example

when two species are competing for the same

resource, one of the two can be displaced.

Furthermore, a number of indirect interactions

link the organisms of a given ecosystem together.

Thus a complex web emerges in which each organism

or species influences the other in very specific ways.

Complex systems, or ecosystems, are made up of

these diverse webs of abiotic and biotic interaction.

These are characterized by non-linearity and have

self-organizing capabilities.

D 2.3

Ecosystem processes

By ‘ecosystem’ we mean the complex interactions of

a biotic community and its abiotic environment. In

the first formulation of the term, by Tansley (1935), it

also constituted a discrete, that is clearly delineated,

unit with a recognizable character. Thus, the definition

has a clear spatial association. The basic components

are the producers (generally green plants, but

also microorganisms), consumers (animals and

microorganisms that feed off living organisms) and

decomposers (microorganisms and animals that

break down organic matter). They are linked to one

another through food chains and webs by which they

pass on the energy they have absorbed from the producers.

This energy flux is not distributed equally.

Depending on environmental conditions, different

paths will predominate at different times (Zwölfer,

1994), just like the road system of a city that is used

differently depending on the time of day and day of

the week. Another essential feature of ecosystems is

the conversion of nutrient elements and other substances

in biogeochemical cycles.

When environmental conditions change, the network

of interactions within the ecosystem reacts:

resources are overused or go unused, and in response

species regroup and food webs are reordered.

Ecosystems are adaptable, but can also be degraded

by human influence, ie their structure and the

processes in the system can gradually change both in

quality and quantity in such a way that the biological

diversity and resources are lost or that the system

becomes more vulnerable.

The explanation of nutrient cycles gives us our

first access point to understanding the role of the various

organisms within ecosystem processes. Fig.

D 2.3-1 shows the nitrogen (N) cycle in a terrestrial

ecosystem. Organisms need nitrogen to build up

amino and ribonucleic acids; it is therefore an essential

nutrient.

A large number of organisms are involved at each

stage of the cycle. Ecologically, it is important that the

turnover happens at very different rates. So, in the

soil ammonium-N and nitrate-N can run through a

cycle within a day that goes via the amino-N of

microorganisms and their excretions. The turnover

via plants has a constant of around one to ten years.

Turnover takes over 1,000 years if denitrification to

N 2

gas takes place. It is also important to know that

the cycle can take a series of shortcuts. If the mycorrhizal

fungi break down proteins directly then no

other microorganisms are involved. Similarly, plants

can compete with microorganisms for the free

ammonium or nitrate – with varying physiological

consequences.

This idealized representation of the nitrogen cycle

does not in itself clarify the role of biological diversity

because, in principle, a cycle of that sort could be

maintained with very few participants, such as happens

in sewage treatment plants where the nutrient

supply is kept permanently high and the abiotic con-

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