In some cases, a bridge or underpass may be enough to join two fragments. This has the potential to mitigate the problem of isolation but not the loss of interior habitat. Wildlife corridors can help animals to move and occupy new areas when food sources or other natural resources are lacking in their core habitat, and animals can find new mates in neighbouring regions so that genetic diversity can increase. Species that relocate seasonally can do so more safely and effectively when it does not interfere with human development barriers. Due to the continuous expansion of urban landscapes, current research is looking at green roofs being possible vectors of habitat corridors. A recent study has found that green roofs are beneficial in connecting the habitats of arthropods, specifically bees and weevils. Another mitigation measure is the enlargement of small remnants to increase the amount of interior habitat. This may be impractical since developed land is often more expensive and could require significant time and effort to restore. The best solution is generally dependent on the particular species or ecosystem that is being considered. More mobile species, like most birds, do not need connected habitat while some smaller animals, like rodents, may be more exposed to predation in open land. These questions generally fall under the headings of metapopulations island biogeography. Genetic risks As the remaining habitat patches are smaller, they tend to support smaller populations of fewer species. Small populations are at an increased risk of a variety of genetic consequences that influence their long-term survival. Remnant populations often contain only a subset of the genetic diversity found in the previously continuous habitat. In these cases, processes that act upon underlying genetic diversity, such as adaptation, have a smaller pool of fitness-maintaining alleles to survive in the face of environmental change. However, in some scenarios, where subsets of genetic diversity are partitioned among multiple habitat fragments, almost all original genetic diversity can be maintained despite each individual fragment displaying a reduced subset of diversity. Gene Flow and Inbreeding Gene flow occurs when individuals of the same species exchange genetic information through reproduction. Populations can maintain genetic diversity through migration. When a habitat becomes fragmented and reduced in area, gene flow and migration are typically reduced. Fewer individuals will migrate into the remaining fragments, and small disconnected populations that may have once been part of a single large population will become