Species distribution models are a group of methods often used to estimate
consequences of global change, to assess ecological status and for other ecological
applications. The main idea behind species distribution models is that the
geographical distributions of species can, to a large part, be explained by
environmental factors and that species distributions therefore can be predicted in
time or space. For robust and reliable applications, models need to be based on
sound ecological principles, predictions need to be as accurate as possible, and
model uncertainties need to be understood.
Two approaches are available for modelling entire species communities: (1) each
species can be modelled individually and independently of other species or (2)
community information can be incorporated into the models. The first study in this
thesis compares these two modelling approaches for predicting phytoplankton
assemblages in lakes. The results showed that predictive accuracy was higher when
species were modelled individually. The results also showed that phytoplankton can
be used for model-based assessment of ecological status. This finding is important
because phytoplankton is required for assessing the ecological status of European
water bodies according to the European Water Framework Directive.
Dispersal barriers in the landscape or limited dispersal ability of species might be a
reason for species being absent from suitable habitats, and these factors might
therefore affect model accuracy. The second study in this thesis examines the
influence of dispersal and the spatial configuration of ecosystems on prediction
accuracy of benthic invertebrate and phytoplankton distribution and assemblage
composition. The results showed only a minor influence of spatial configuration and
no effect of flight ability of invertebrates on model accuracy. However, the models
used may partly account for dispersal constraints, since dispersal-related factors, such
as lake surface area, are included as predictor variables. The result also showed that
composition of littoral invertebrate assemblages was easier to predict at sites located
in well-connected lake systems, possibly because the relatively unstable littoral zone
necessitates a need for species to re-colonize disturbed habitats from source
populations
