Having just taught a section on Evolution in my
General Biology class at Hawaii Pacific
University, your questions are timely.
First off, land plants appeared on Earth about 470 millions of years ago, and animals moved from the ocean to the land approximately 360 millions of years ago, so it is entirely possible for all of the plants and animals on the Channel Islands to have been living there before they were in fact islands, and before they are where they are now.
That said, before man started moving around in canoes and ships and airplanes, all of the plants and animals present on island chains like Hawaii,
which formed in the middle of the Pacific, had to
have arrived here by "wind, waves and wing". Same goes for the Galapagos, although those islands are much, much closer to a continent that the Hawaiian Islands.
There are now thousands of native plants
on Hawaii, and about half as many on the Galapagos
islands. So plants blew to Hawaii on the wind,
floated from another island or from the mainland,
or hitched a ride on the outside or the inside of
birds (many seeds survive and germinate after
being eaten). Likewise, animals flew here, or were blown off course, or floated on plant debris, or swam here. It makes sense that in Hawaii we have no native reptiles or amphibians (these species don't swim well) and the only native mammals either fly (one species of bat) or swim (several species of marine mammals). So it is also possible that the plants and animals migrated or dispersed out to the Channel Islands from the "mainland" (the coast), and continue to do so today. In addition, new species can evolve from the original species that first colonized the islands.
In Hawaii, we have over 50 species of Hawaiian
Honeycreepers, forest birds that are thought to
have evolved from one original species that landed
here millions of years ago. Hawaiian Honeycreepers are found only in Hawaii and nowhere
Lastly, man is famous for introducing
species into environments that they would have
never found on their own. The early settlers of
Hawaii probably came from Tahiti, and brought with
them over 30 species of plants, and a couple
species of animals (dogs, pigs, and probably
rats). All in sailing canoes, and without the aid of a metal compass! These plants provided food in case the new islands did not have enough to
survive on, raw materials for building new canoes
and "houses", and medicine.
Today, humans introduce thousands of new species either on purpose or by accident. This has threatened the survival of many native species, especially on islands, since the introduced species are not introduced with their predators and diseases. Soon our native forests and forest birds may be gone because of these and other introduced species.
Most of the 50 or so species of Hawaiian
Honeycreepers are endangered, and many have
already gone extinct since Captain Cook first
visited the islands in the 1700's.
I know for a fact that the Santa Catalina Island fox is endemic to the Channel Islands, which means that it evolved into a new species once on the island, and this species is found nowhere else. The original fox species (probably a Gray fox) could have swum to the island from the mainland, or lived in the region before the islands became separated from the mainland.
According to some information I have, it's early ancestors arrived at the islands about 15,000 years ago, which isn't
too long ago, so the early foxes probably swam or
rafted out to the islands, and weren't transported
there. It is now endangered.
As for the Torrey Pines, since they occur naturally in only two spots (San Diego County and Santa Rosa Island), this implies to me that Santa Rosa Island must have been in contact with or very near the region of the coast where the San Diego Torrey Pines are presently located. If the pines are able to disperse long distances, you would expect to see them on other islands, especially Catalina, which is farther south. It is possible that the pines were more widespread on the mainland and made it out to more than one island and what we are seeing today is just two remnant forests.
I'd have to know more about the biology of the tree and its fossil record. Pollen tends to preserve well in the fossil record (or better than most other plant parts), so that's one way scientists would be able to determine the past distributions of Torrey Pines.
As for other plants and animals, since these groups have been around on Earth for >300 million years, it is hard to say whether certain species traveled up on the islands from their original location, or migrated out to the islands once the islands began to move. One way biologists can tell this is to look for fossils on the islands.
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