Shifting habitat causes niche offsets and range asymmetries in experimental populations
How species ranges are impacted by climate change requires understanding how range dynamics are altered when the habitat is moving. Theory suggests that properties of a species' range including extent, abundance, and symmetry can change dramatically under moving habitat and that stochasticity plays an important role. We used Tribolium castaneum in laboratory microcosms to investigate the effect of moving habitat on range dynamics in experimental landscapes. The experiment used a 2 x 2 x 2 x 2 factorial design including two habitat transitions between suitable and unsuitable habitat (abrupt or gradient), speed of movement of the suitable habitat (1 or 2 patches per generation), dispersal range (long or short, by varying the length of the dispersal period), and size of the suitable habitat (2 or 6 patches). We followed habitat movement for 12-15 generations in 10 replicates of each treatment. To track the spatial population response to habitat movement over time we censused beetles in each patch of every landscape in each generation.
We found that shifting habitat alters the geometry of a population's range in two important ways. First, the population is not centered on the good habitat leading to a niche offset; the mode may even be outside the suitable habitat. The niche offset depends on habitat speed, habitat geometry, and dispersal. This result is potentially problematic for species distribution models that rely on a correlation between a species' abiotic niche and its current distribution. Second, the distribution of abundances across the range is skewed depending on habitat speed, habitat geometry, and dispersal. In fast moving large habitat with a sharp boundary and long range dispersal the range was negatively skewed with a long tail at the trailing edge, whereas the range was positively skewed with a long leading edge for a similar moving habitat gradient. Asymmetry of the range appears to be associated with the population being close to a critical speed below which it cannot keep pace with the shifting habitat, suggesting that range asymmetry could be a useful indicator of populations at risk. Finally, intrinsic stochasticity caused variability in the range and time to extinction such that populations often went extinct suddenly from an apparently healthy state.