Niches, models, and climate change: Assessing the assumptions and uncertainties

As the rate and magnitude of climate change accelerate, understanding
the consequences becomes increasingly important. Species
distribution models (SDMs) based on current ecological niche
constraints are used to project future species distributions. These
models contain assumptions that add to the uncertainty in model
projections stemming from the structure of the models, the algorithms
used to translate niche associations into distributional
probabilities, the quality and quantity of data, and mismatches
between the scales of modeling and data. We illustrate the application
of SDMs using two climate models and two distributional
algorithms, together with information on distributional shifts in
vegetation types, to project fine-scale future distributions of 60
California landbird species. Most species are projected to decrease
in distribution by 2070. Changes in total species richness vary over
the state, with large losses of species in some ‘‘hotspots’’ of
vulnerability. Differences in distributional shifts among species will
change species co-occurrences, creating spatial variation in similarities
between current and future assemblages. We use these
analyses to consider how assumptions can be addressed and
uncertainties reduced. SDMs can provide a useful way to incorporate
future conditions into conservation and management practices
and decisions, but the uncertainties of model projections must
be balanced with the risks of taking the wrong actions or the costs
of inaction. Doing this will require that the sources and magnitudes
of uncertainty are documented, and that conservationists and
resource managers be willing to act despite the uncertainties. The
alternative, of ignoring the future, is not an option.