Range expansion is one of the expected responses of species to climate change. As formerly unsuitable habitat becomes suitable, species are likely to shift their ranges in order to track the changes in habitat and climate.
However, few landscapes provide a homogeneous path to expansion. High-quality patches are often surrounded by a low-quality matrix, and active management is needed to maintain connectivity between the high-quality areas. A good question is how to manage heterogeneous landscapes so that they best support range expansion in the face of climate change.
Recent research uses theoretical models to predict how species will expand their ranges through heterogeneous landscapes. Two key components inform the models: species movement patterns and population growth rates. A common assumption is that species move the fastest through high-quality habitat, and that population growth is highest in high-quality habitat. If this were the case, then range expansion would be fastest in landscapes composed of 100% high-quality habitat.
However, the empirical evidence doesn’t support this. In a review of 70 studies looking at 78 species, animals moved faster through low-quality habitat in almost ¾ of the studies. This suggests there is a trade-off between faster movement in low-quality habitat and higher population growth in high-quality habitat. The best management practice in heterogeneous landscapes is not necessarily to create as much high-quality habitat as possible.
This empirical evidence is applied theoretically with the example of the Baltimore checkerspot butterfly, a meadow specialist in the eastern U.S. that has been declining at the southern edge of its range and increasing in northern locations. To test how the Baltimore checkerspot might expand its range in the future, an integrodifference equation model incorporating movement and demographic processes was run across landscapes varying in heterogeneity. Model results showed that the optimal landscape for range expansion for Baltimore checkerspots has about 15% high-quality habitat. Adding more high-quality habitat beyond that actually reduced the rate of range expansion.
The trade-off between movement and population growth means that heterogeneity in the landscape can be beneficial in the long run. This heterogeneity doesn’t necessarily have to favor high-quality habitat in order to support successful range expansion. Even small percentages of high-quality habitat can help make a landscape more permeable to range expansion by increasing connectivity. This allows species to better track climate change by moving quickly to new patches that will support population growth.