Download The species-pool hypothesis

The species-pool hypothesis’s role in community assembly.
Bas Roels.
Department of Plant Ecology and Evolutionary Biology, Section Plant Ecology, University of Utrecht.
Introduction:
In the ecological discussion about biodiversity,
many different topics are important and thus
discussed. One such a topic concerns the
explanation of biodiversity; why do species
coexist within a limited space? We try to find
an explanation for the coexistence of species
and mainly an explanation for the big
differences in species diversity between
different systems. The answer on such a
question is hard to give, which partly is caused
by the fact that this is a discussion at the
ecosystem level. Here many processes that act
at once at different levels/scales and at different
time-scales become important to distinguish
between
the
different
mechanisms.
Implementation of all theories that mostly
focus on only one small aspect of the total
system dynamics is hard. However,
overcoming such scaling problems is important
for a good understanding of the mechanisms
acting. It is also possible because most theories
are complementary instead of exclusive
towards each other.
The species-pool hypothesis is concerned with
the question why we find particular numbers of
species within a particular unit of space. In
answering this question, this hypothesis mainly
focuses on evolutionary and historical factors
like speciation and the immigration and
dispersal of species.
Comparison of different ecosystems and
communities shows that the number of species
per unit space differs widely. Even between
communities, which are close to each other or
have many environmental characters in
common big differences in species diversity
exists.
Utrecht, november 1999.
In this paper, I will explain the species-pool
hypothesis and give a short review about its
position towards other theories.
The species-pool hypothesis:
Taylor et al. (1990) first used the term speciespool but the idea behind this hypothesis already
existed earlier. The species-pool hypothesis
gives an explanation for species diversity and
its maintenance from an evolutionary and
historical point of view rather than from an
ecological point of view. This is in contrast
with the more classical theories, which explain
diversity from the ecological point of view. The
classical theory’s say species try to avoid the
negative effects of competition, which would
always be present between coexisting species.
In order to avoid competition, the species
differentiate on a spatial and temporal scale
from each other in terms of resource use,
recruitment etc. After this niche differentiation
competition would be enough reduced to
mediate
coexistence
of
the
species.
Competitive exclusion resulting in a few
dominant species thus would be the expected
outcome and a coexistence of many species
would be extraordinary. In this later case it is
necessary that all species have their own niche
which theoretically is possible but as several
authors already pointed out (see Zobel 1992)
practically this is probably not the case. In
nature, thus more mechanisms besides the
niche differentiation concept are responsible for
mediating high species diversity.
The species-pool hypothesis does not reject this
competition model but states that other
mechanisms also have a big or even bigger
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species-pool the more species will coexist in
the target community and the smaller the local
pool the smaller the actual pool will be. The
species richness on a certain scale thus is
determined by the species richness on the next
larger scale. A species-pool in this sense thus is
the set of species potentially capable of
coexisting in a certain community. According
to this definition besides ecological characters
of the species also dispersal properties and
historical biogeographical processes become
important in determining
the
actual
species
richness.
These
biogeographical processes
are
important
in
determining
the
distribution of species on
a big scale (continental)
and thus determine the
regional or bigger scale
species pool.
From this point it
becomes clear that the
size/area and geological
age of a certain target
community
also
is
important in determining
the
species
richness
according to the speciespool hypothesis. The
bigger the area and the
Figure 1: The species-pool hypothesis; the role of speciation, migration and filtering and
older the age of a
the differentiation in different scale species-pools (after Zobel, 1992).
community or habitat the
more species potentially will be able to coexist
actual species-pool, to distinguish between
in that community. A bigger area mediates
different scales. The actual species-pool is the
more surrounding species thus a bigger species
set of species, which coexists in the community
pool, amount of species that are capable of
under consideration. The local species-pool is
living in that habitat. An older geological age
the set of species in the direct surroundings of
on the other hand gives more opportunity for
the target community. These species are
speciation of species, which also enlarges the
capable to live in the target community and can
species-pool.
immigrate rapidly into it. Finally, there is the
The species-pool hypothesis thus says that
regional species-pool, which consists of all the
variation in species diversity along different
species capable of living in the target
habitats is partly explained by variation in the
community and capable to immigrate into it,
species-pool associated with these habitats. In
even if it would take very long.
determining the species-pool size ecological
The species-pool hypothesis now states that the
conditions of a certain habitat act as a filter
local species pool determines the actual
(figure 1). All species initially present in the
species-pool (figure 1). The bigger the local
impact on nowadays community assemblage.
The species diversity would be the product of
speciation, dispersal and extinction of species
over evolutionary time. These processes are the
mechanisms determining the nowadays species
diversity in a certain place. As is shown in
figure 1 the species-pool hypothesis thus
predicts that local species diversity reflects the
size of the species diversity in the surroundings
of a community. Päertel et al. (1996)
introduced the term’s regional-, local- and
Utrecht, november 1999.
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species-pool but not capable to live under the
conditions in the target community are
excluded from the species-pool. Thus in
determining the species-pool, also the ecology
of the target community is important besides
the
big-scale
dispersal
patterns
and
evolutionary speciation.
Hypothesis testing and modeling:
The first step in testing the species-pool
hypothesis is to define the target community
and the local and regional species-pools. For
defining the target community two methods are
proposed; one defines communities on basis of
environmental characteristics (Eriksson, 1993)
without looking at the inhabitant species. The
other method (Pärtel et al., 1996) defines
communities on a phytosociological manner. In
this latter method the selection items thus are
the inhabitant species. Both methods probably
are good but when comparisons between
communities are made one should be aware
when the second definition is used in defining
communities.
The second step is to define the size of the
bigger scale species-pools, to start with the
local species pool. The first problem is defining
the area which belongs to the local pool. Which
habitats and landscape units are local and
which regional? When the area is defined the
species-pool must be defined. In principle all
species of a local flora could belong to the
species-pool with a certain degree of
membership. In this set of species, one must
make a selection on basis of the ecological
properties of a species, which determines its
capability to survive in the target community.
This step is one of the most difficult in testing
the species-pool hypothesis because the number
of species in local flora’s often are high and
one only knows the ecological conditions under
which the species mainly occurs. Nothing or
less is known about the ecological tolerances of
the species, so an underestimation of the local
species-pool is likely to be made. Thus both
area and ecological factors should taken into
account when defining the local species-pool.
Utrecht, november 1999.
When looking at this definition and manner of
determining the local-species pool the way of
defining the target community becomes more
important then stated before. Of the two
methods described above the first now becomes
prevailing above the second. This first method
defines a target community on basis of
ecological conditions which now is necessary
when the local species-pool is determined
partly on basis of ecological properties of the
species.
When these definitions are stated one expects
to find a relationship between the size of the
actual species-pool and the local species-pool.
One consideration which must be taken into
account here is the fact that this proposed
correlation is biased. Because of the definitions
of local and actual species-pools there is a
certain degree of autocorrelation between the
two. The small-scale richness (actual speciespool) can never be bigger than the larger scale
richness (local and regional species-pools). One
can overcome this problem by formulating the
null hypothesis in a different way and by
adjusting the statistical methods (see Zobel
1997).
With these definitions in mind, it is now
possible to formulate a simple model that
describes the community species-diversity
dynamics. Erikson (1993) formulated the
following model;
dN/dt = i(Ns-N) - eN
with an equilibrium at
N* = Ns(i/i+e).
In this model N is the number of species in a
community (Ns in the species pool, and N* at
equilibrium) i is the immigration constant and e
the extinction constant.
Several important assumptions are incorporated
in this model; firstly, there is no upper limit to
the amount of species in a community.
Secondly the change to invade into or to be
excluded from a community is equal for all
species and thirdly there is a factor for other
possible mechanisms determining species
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diversity (like competition). The extinction and
immigration constants wholly or partly refer to
processes like competition and dispersal
abilities.
Unless these assumptions/simplifications one
can derive some interesting predictions from
this model. When the extinction rate in a
community is very low the model says the
number of species will equal the number of
species in the species-pool. When the process
of species accumulation (under low extinction
rates) is a slow process the model also predicts
that the community will be in a state of nonequilibrium. Because of global extinction and
ongoing speciation in the species-pool N* will
constantly change.
The species-pool hypothesis and other
theories:
The above described model says that the
number of species in a community is depended
on two factors; the size of the local speciespool and of the immigration and extinction
constants, which refer to other mechanisms
regulating species diversity. If one thinks about
these constants not as constants but as variable
processes, the integration of the species-pool
hypothesis with another hypothesis thus is
easily made. The model holds place for other
additional mechanisms.
Zobel (1992) gives a good review about all
different hypotheses explaining species
diversity. He starts his overview with the
formulation of a so-called null community.
Which is an ideal community, undisturbed (no
succession and climatic change) with a
homogenous environment, which contains all
possible species. Competition, disturbances and
other factors are thought to be absent in this
model.
In the null community the set of
species which occurs in it and thus the species
richness is only determined by speciation of
species and their traits which enables them to
grow in that specific community. From the
observation that in the real world not all
communities are so rich in species as would be
expected from this model we must conclude
Utrecht, november 1999.
that there are mechanisms working which
reduce species richness in communities. These
mechanisms can be divided into three groups;
first there is the simple lack of traits which
disables species to live in a community.
Secondly, there can be limitations on dispersal
and immigration of species so they can not
reach the community under consideration.
Finally, there can be interactions between
species that disables a coexistence.
When a species is present in the local speciespool (it has the traits to grow in a community
and the dispersal properties to reach it) but not
in the actual pool the only reason for its
absence can be interaction between species,
competition.
The null community forms a good basis for
examining the factors, which determine
variation in species diversity between and
within habitats. This model is based largely on
the species-pool hypothesis, especially in its
historical and evolutionary point of view. It’s is
demonstrated that theories about species
diversity like the competition model and others
(see Zobel, 1992) are easily integrated into this
model. The species-pool hypothesis thus is
mainly complementary towards other theories.
This vision is also supported from data that
came forth out of experiments testing the
species-pool hypothesis. These experiments
(see Pärtel (1996) for example) searched for an
explanation for variation in species richness.
According to the species-pool hypothesis, this
variation was thought to be caused by variation
in the size of local species-pools. The results
showed significant correlations but no absolute
correlations, thus the species-pool hypothesis
only explains part of the variation and must be
combined with other theories.
Concluding remarks:
One general but important remarks is left to be
made. The lack of experimental data
concerning the species-pool hypothesis is often
stressed in literature. To validate proposed
models and mechanisms it really is necessary to
do experimental experiments, as opposed to
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model and observational experiments. Also to
determine the relative importance of the
different operating mechanisms (species-pool
hypothesis,
competition
model
etc.),
experiments will be necessary.
Literature:
Erikson, O. (1993).
The species-pool hypothesis and plant
community diversity.
Oikos 68, 371-474.
Morton, R.D. (1997).
Regional species pools and the assembly of
local ecological communities.
J. Theor. Biol. 187, 321-331.
Päertel, M., M. Zobel, K. Zobel and E. van
der Maarel (1996).
The species pool and its relation to species
richness: evidence from Estonian plant
communities.
Oikos 75, 111-117.
Taylor, D.R., L.W. Aarsen and C. Loehle
(1990).
On the relationship between r/K selection and
environmental carrying capacity: a new habitat
templet for plant life history strategies.
Oikos 58, 239-250.
Zobel, M. (1992).
Plant species coexistence – the role of
historical, evolutionary and ecological factors.
Oikos 65, 314-320.
Zobel, M. (1997).
The relative role of species pools in
determining plant species richness: an
alternative explanation of species coexistence?
Tree 12, 266-269.
Utrecht, november 1999.
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