Table of Contents:
A typical phenomenon along environmental, i.e.
abiotic, gradients is an abrupt change in species composition
(zonation). At the Aral Sea, where plants are colonising the
vast exposed seabeds, the abiotic conditions along a gradient
that follows the receding sea water are characterised by an
increase in salt and inundation stress. Motivated by this
situation, the general spatio-temporal vegetation dynamics and
species diversity patterns along environmental gradients have
been investigated with a grid-based simulation model. The main
attributes characterizing a plant type in the model are the
maximum seed dispersal distance, stress tolerance, seed
production and longevity. The study mainly focused on two
questions: 1) which conditions favour coexistence of the
different plant types, and 2) do the different plant types
overlap in their spatial distribution or do they segregate
along the stress gradient. The simulation experiments revealed
that not only the interplay of dispersal and stress tolerance
attributes of the plant types, together with the abiotic stress
gradients, determine the successional dynamics. In addition,
intra- and interspecific competition decisively govern the
spatio-temporal vegetation dynamics. Furthermore the
simulations showed that successions depend on stochastic
establishment events and do not end in a distinct final state.
This underlines the importance of the consideration of
stochastic processes in the modelling of successions.
13 main results are summarized:
1. The spatial pattern of
species richness at the desiccated sea floor of the Aral Sea
comprises two main zones. The first zone on the older areas has
a higher species diversity than the second zone, the areas
desiccated from the mid eighties onwards. This phenomenon
motivated the theoretical investigations of spatio-temporal
patterns along stress gradients.
2. A detailed literature
review on community models revealed that spatially explicit,
grid and rule based models are excellent tools to investigate
spatio-temporal species diversity patterns.
3. With a Partial
Differential Equation model after DeAngelis and Post (1991),
the conditions for coexistence and zonation were investigated
for a two species system along an environmental gradient. In
this deterministic model, the two species only separated into
zones along the gradient under strong interspecific
4. A spatially explicit, rule based simulation
model for up to 8 different plant types was developed. The
central process of vegetation dynamics is the lottery
competition of seedlings for space.
5. Simulations identified
stress tolerance as the most important plant attribute for the
successful establishment of an additional test type.
number of coexisting plant types increases with the strength of
intraspecific competition, whereas spatial segregation of
different types only occurred with weak intraspecific
7. The use of a global seed pool instead of
spatially explicit seed dispersal reduced the number of
coexisting types for all levels of intraspecific competition.
The number of persisting species is increased by spatial
aggregation or segregation of species along the gradient.
Compared to a linear stress gradient, both a random spatial
distribution of stress intensity as well as the homogenous
distribution of stress intensity reduced the species diversity.
9. A stepwise stress gradient leads to a zonation of
different types. For strong intraspecific competition, the
model results correspond to the empirical pattern substantially
(spatial zonation in a species-rich and a species-poor area).
10. A trade-off between stress tolerance and competition
strength increased diversity and stabilized the spatial pattern
of zonation against an increase of intraspecific competition.
Without such a trade-off, an increase in intraspecific
competition resulted in an overlapping spatial distribution of
the plant types.
11. When introducing additional types and
investigating their potential to establish in this community,
seed production for an annual test type and longevity for an
perennial test type have been identified as the most important
plant attributes for the successful establishment in the
12. An additional trade-off between fecundity
and longevity, where the costs of reproduction limit the
longevity of a plant type, increased the diversity only in
combination with the trade-off between stress tolerance and
competition strength. Otherwise the most stress tolerant annual
type has the potential to become a dominant super species.
For replicates with the same parameterisation and initial
conditions, different final plant type compositions are
possible for the majority of all simulation experiments, i.e.
successions do not inevitable end in a distinct final climax