Several factors affect the likelihood of new species establishing themselves on an island. Plant and animal species must find a way to traverse open water or skies to reach an island. Of great importance is the distance of water separating the island from the mainland or another island, which would permit ‘island hopping’. The greater the distance, the fewer the number of mainland species will successfully make the journey. Animals must traverse seven to 20 mile stretches of open water to reach San Juan Island. Water temperature is also a factor. Some animals can swim or float, but as water temperature drops, so does the number of animal species that can tolerate the increased cold. The waters surrounding the San Juan Islands are relatively cold, about 50 degrees Fahrenheit. Finally the varying combination of open water distance and temperature affect a species’ ability to reach an island. Many bird, bat and insect species can fly across open water to reach an island, but some species can cross greater expanses of open water then others. Some species may reach islands by ‘hitchhiking’. They may drift on a log or ‘ride’ another animal host as in the case of fleas or ticks. Plants may reach the island as seeds drifting on the wind, or incased in bird or animal droppings.

Species confined to an island are more vulnerable then species on the mainland. For example, bears and wolves once lived on San Juan Island, but with the arrival of Europeans and Americans and their livestock in the 1850s, they were extirpated in a few short years. These animals existed as small populations and could not easily replenish their numbers with young or migrate to a safer place. Because of island biogeography, a species that is threatened or under pressure cannot easily move to a more hospitable place. Because of the geography of islands, an island species may have no defense against another species that will protect it from being eaten or crowded out of its habitat. For example, many of the native prairie plants of San Juan Island were vulnerable to grazing, having no natural protection such as thistles or bad taste. When Euro-Americans brought their grazing animals to the island, these tender plants were gobbled up. They were replaced by both native and non-native plants that were less appealing to livestock.

Number of Species of Mammals
Washington mainland 100
British Columbia 97
Vancouver Island 32
San Juan Island 19
American Camp 10