Compared to an area of the same size on the mainland, the Channel Islands are home to fewer species of land animals. In scientific terms, the "species diversity" of terrestrial animals is relatively low. That low species diversity reflects the challenges of a species arriving on the islands and then adapting to unique island conditions. With fewer distinct species, the ecology of the islands is simpler, though the relationships between species have more consequences, and the plants and animals that persist sometimes evolve to become unique from relatives on the mainland.
The park's five islands only support four native land mammals: the island fox, the island deer mouse, the harvest mouse and the spotted skunk. The island foxes and deer mice have evolved into separate sub-species on each island, resulting in eight unique mammal species found only on the Channel Islands.
The number of native reptile and amphibian species is likewise low: only four lizards, one salamander, one frog, and two non-venomous snakes. None of these species is found on all of the islands, and no island supports all seven species. For example, the island night lizard, a threatened species found nowhere else in the world, occurs on only three islands: one within the park and two owned by the U.S. Navy outside park boundaries.
Because birds can fly, it is easier for them to reach the Channel Islands than it is for other land animals, and their species diversity is relatively high. Not all birds that visit the islands stay all year, though. Migratory birds, infrequent visitors, and rare species that arrive only after being blown off course during spring and summer migrations all add to the count. Like the island fox and deer mouse, the island scrub-jay has evolved into a unique (endemic) island species. The largest landbird native to the islands is the bald eagle. It was only recently reintroduced.
Bats are not seen frequently, but they are common on Santa Cruz and Santa Rosa islands. Bats fill an important role, or "niche," by consuming large numbers of insects and pollinating some plants. Eleven species of bats have been found on Santa Cruz, and the island is home to a rare maternity colony of Townsend's big-eared bats.
Current research focusing on terrestrial animals includes several studies on the island fox, one on the effects of deer mice on the recovery of vegetation communities, and one on the response of lizards to the removal of rats on Anacapa. Ongoing monitoring efforts are measuring changes in amphibian and reptile populations in response to the removal of pigs from Santa Cruz Island, the number of bats occupying maternity colonies on Santa Cruz, and mouse population dynamics as they relate to changes in fox numbers on San Miguel Island.
Each island has a unique complement of land animals, dependent in many ways on the size of the island. Over time some have evolved into new species and are present today. Others, like the pygmy mammoth, the Santa Barbara Island song sparrow, and the giant deer mouse, evolved into unique island species before going extinct. It may not appear so to relatively short-lived humans, but the islands are still changing and evolving, and what lives here in the future may be very different than what lives here today.
