Biodiversity related to running waters
PD Dr. Carsten Hobohm from University of Lueneburg, Germany
Lecture 4/2002 in Innsbruck, Austria
Summary
Many organisms are adapted to running waters. Some of them live within the water for their
whole lifecycle.
Because of disturbances or unfavorable seasons in running waters dispersal over long distances is
advantageous for plant and animal species living in rivers.
Worldwide only few species are endemics of only a single river.
Some examples of plant and animal species diversity, of endemic and non-endemic species and
their relationship to different habitats related to running waters are discussed.
Biodiversity
Life and its extraordinary diversity is the unique wealth which distinguishes the plant earth from
all other planets in the universe. Mankind is a critical element of this wondrous spectrum, and thus
biodiversity in all its aspects represents the very foundation of human existence. We have only
become aware of the real dimensions of this wealth during the last thirty years, and today we know
some 1.7 million different species probably representing far less than 10 or 20 % of the actual
diversity. At the same time it is increasingly evident that - due to the rapid growth of the world`s
population which is about six billion people and is driving the extreme exploitation of natural
resources - this diversity is undergoing a dramatic change. Fundamental genetic resources, which
evolved over more than three billion years, probably are being lost at a dramatic pace.
Many quantitative approaches to biogeography have been published (see e.g. Barthlott & Kier
2001, Barthlott & al. 1999, Coleman 1981, 1982, Connor & McCoy 1979, Hobohm 2000a, Iwasa
& al. 1994, Malyshev & al. 1994, Malyshev 2000, MacArthur & al. 1966, MacArthur & Wilson
1967, Myers & Giller 1988, Simberloff 1974, Whittaker 1972, Wilson & Shmida 1984).
The numbers of species and endemics of many groups of organisms are well known in many parts
of the world, for example the numbers of vascular plant and mammal species in individual
countries or national parks.
According to Mittermeier & al. (1999) and Hobohm (2002) many biodiversity hotspots are
concentrated in the tropics. E.g. mammals, birds, reptiles, amphibians, insects and vascular plants
are show high concentrations in the tropics.
However, until now neither the function of water, especially of running waters, for biodiversity nor
the relationship between biodiversity and running waters has been analyzed in detail.
It is impossible to explain or discuss the systematics, ecology or foodweb of rivers in a single
lecture. Therefore the following topics will bring only examples which can show the complexity of
biodiversity related to running waters.
Species richness and ecosystem processes - theoretical relationships
Diversity would not be interesting if the level of diversity were the same everywhere. Fortunately,
no matter how diversity is defined, or what types of organisms are being considered, there is a
phenomenal variation in diversity across the entire range of living systems.
No single process or theory can explain a phenomenon as complex as biological diversity. The
intellectual challenge and scientific value of the study of diversity lies in the conceptual synthesis
required to understand a complex phenomenon that is influenced by many different interacting
factors and processes.
Theory and data have, until very recently, focussed - at least very often only - on the relationship
between species richness and ecosystem processes. Taking this state of affairs as our starting point,
within any one site or location, we can imagine ecosystem processes responding in one of three
ways to reductions in species richness.
1. The redundant species hypothesis suggests that there is a minimal diversity necessary for
ecosystem functioning, but beyond this minimum, most species are redundant in their roles; above
this minimum, adding or deleting species has no detectable effect on the process or processes in
question.
2. In marked contrast to (1), the rivet hypothesis postulates that all species are important (by
analogy with the rivets holding an aeroplane together, so that ecosystem processes are
progressively more impaired as species are lost from the system. If some rivets are missing the
aeroplane is able to fly.
3. The idiosyncratic hypothesis again postulates that species are important in ecosystem processes,
but particular species identities matter more than species richness per se. In consequence,
ecosystem processes change erratically and unpredictably as species are lost from the system.
Providing the species pool is reasonably large (thereby avoiding strong stochastic effects), the
most likely theoretical relationship between loss of species and ecosystem processes is pattern,
consistent with the rivet hypothesis. The fundamental underspinning mechanism is that species
differ in their ecologies, that is, no two species have identical niches. But there are some
important, and still poorly understood, subleties underlying this simple statement.
4. The sampling hypothesis. Species differ intrinsically in their potential maximum size, growth
rate, and so on. Hence, mixtures of many species are more likely to contain high-yielding species
than monocultures or low-diversity mixtures. In consequence, random samples of species from a
pool will, on average, show higher biomass, and higher productivity, as species richness increases.
5. The niche complementary hypothesis. Here the niche differences are explicit. Niche-differences
between species ensure that as species richness increases in ecological assemblages, so does the
range of functional spce occupied by the assemblage. More diverse communities are more spacefilling above ground, have a wider range of requirements for ressources, and so on.
6. The posive species interactions hypothesis. Species do not compete with each other in
communities. It is becoming increasingly clear that some plant species for example benefit from
the presence of other species in the community.
These hypotheses are good for understanding ecological conditions and species compositions in an
ecosystems. But ecosystems cannot exist by themselves. For a good understanding we do need the
knowledge of some dimensions more.
In general, three groups of mechanisms are important:
* mechanisms of evolution
* processes which provide dispersal
* ecological processes
time
species
diversity of a
region
processes of evolution
(evolutionary times)
ecological mechanisms
(ecological times)
dispersal
(intermediate times)
area
fig. dimensions of species diversity
Biodiversity and running waters in space I: Biodiversity in running waters
There is a foodweb in every zone of a river which is connected with other foodweb structures in
the semi-terrestrial and terrestrial surroundings and other zones of the river. This complex is part
of different other lectures of the IP. As a result, it will not been discussed here.
Biodiversity and running waters in space II: Biodiversity close to the river (riverside)
Biodiversity related to running waters is not only the biota of running waters. There are a lot of
biotopes directly connected with river systems. Different habitats are typical parts of riversides in
Europe. They can be ordered in different ways:
* from wet (small lakes) to dry (dunes, rocks)
* according to biomasses, pure substrates (riverbanks) to forests (e.g. with Salix spp.)
* according to human activities from untouched to artificial
and so on.
Biodiversity and running waters in space III: Biodiversity indirectly dependent on rivers
Many plant and animal species profit from the existance of rivers, but they are not connected with
or directly dependent on rivers.
For example many storks are breeding near large rivers. They are feeding in many biotopes, e.g.
wet places, but also in cultivated areas outside river-valleys. Many other birds use rivers for
orientation as traffic lines.
Biodiversity and running waters in time
The evolution, also the biology of dispersal, population dynamics in different groups of organisms
and in different limnic and terrestrial habitats, depends on the quality and quantity of running
waters in space and time.
Biodiversity and running waters in time I: ecological times
Biodiversity and running waters in time II: dispersal
Because of disturbances or unfavorable seasons in running waters dispersal over long distances is
advantageous for plant and animal species living in rivers. The result are highly adapted
organisms living in or close to running waters. Comparing the species numbers of limnic and
terrestrial organisms, rivers are poor. This is an effect of area sizes, age and unfavorable
conditions. Lands are larger and older than rivers - thinking in geological times. As a result
unfavorable conditions, e.g. drought or negative temperatures can extinct whole populations in
rivers whereas it is more easy to survive in large terrestrial ecosystems.
On the other hand, e.g. terrestrial vascular plant species in Central Europe seem to be concentrated
where large rivers come together. This fact is not easy to understand, because habitat diversity very
often is contradictory in these landscapes.
Biodiversity and running waters in time III: evolutionary times
Compared with terrestrial ecosystems most rivers are very young. Within running waters
worldwide not many species are endemics of only a single river.
But e.g. some fishes or river-dolphins (Inia geoffrensis in Amazonas, Lipotes vexillifer in the
yang Tse, Plantanista minor in the rivers Ganges, Brahmaputra and Meghna, Platanista
gangetica in river Ganges) are endemics in some large and old river-systems. Some plant species,
e.g. Oenanthe conioides in the Elbe river-valley, are also endemics related to running waters.
Running waters can devide terrestrial lands in different parts. For example many organisms, e.g.
many small mammals, reptiles or amphibians, are not able to cross large rivers. Separation is one
possibility for initiating genetic isolation. Genetic isolation on the other hand can result in adaptive
radiation. So evolution can be stimulated by separation.
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