Long Distance Dispersal via Catastrophic Events

When I give my lecture in class about animal distributions,  questions often are raised about how did animals get to some remote islands or even islands that are not that far off from the mainland.   Sometimes the answer is that during ice ages the oceans was up to hundreds of meters lower than it is today and so there were land “bridges”  between islands or even between continents.  For example, the Bering land bridge between Alaska and Siberia allowed at least one route of migration for people to North America.  But how did animals get to New Zealand or some Polynesian islands?  How about the Hawaiian Islands. What about several species of elephants that used to live on the Island of Crete in the Mediterranean Sea?  All of these islands are separated by very deep water with no chance of a land bridge in the recent past.

A few days ago we were reminded of the power of  rare yet powerful natural events that can have long term effects far from the location of that event.   The picture to the right is off a dock that washed ashore in Oregon 15 months after it was dislodged by a tsunami from its point of origin in Japan.   Drifting across the entire Pacific Ocean it now find itself on a distant shores and the seaweed and some other small animals and algae that are attacked to the dock are mostly ones that are found only in the western Pacific Ocean.   It is possible now that some of these organisms will make a home on the Oregon coast.   Here a natural process has introduced new species to a place far from their original residence.  Or course, in the past there weren’t large barges but massive tsunamis are recorded on the sides of sea-side mountains in various parts of the world.  Waves 5 times as high as the Japanese tsunami would have carried thousands of tons of trees and other natural debris into the ocean.  For islands withing a few hundred miles of the coast, rodents and other small animals could survive on the vegetation rafts long enough to make it to a new home.   Of course  a breeding pair would  have to make it or at least one pregnant female in order to form a reproducing colony.

Those that study islands have noted a relation ship between the diversity of animals and plants on islands and the proximity of islands to the main land.  This diversity can partially be explained by the relative chances of rare events such as these resulting in animals making it to new locations.     One of the most unique but animal sparse places in the world are the islands of Hawaii.    There are NO amphibians (frogs/salamanders) and NO reptiles (lizards, geckos) that are native to the Hawaiian Islands.  There are frogs and lizards there today but they were most certainly all brought there by people.   There are only two native (before man) mammals on Hawaii.   Those are the Hawaiian Monk Seal and a Hawaiian fruit bat. Both of these are unique (endemic) species to the islands.    While rare events can explain how some animals might get to some islands of the world (even a place like New Zealand) Hawaii is 2000 miles from the nearest continent.   Even material from this tsunami is not predicted to begin to wash up in Hawaii for a few more months.  Even the most durable mammal or reptile is going to have difficult time living on a log in the ocean for 20 months.

The Hawaiian Islands are volcanic by origin.  When the first Hawaiian Island rose up out of the ocean by volcanic processes it is not difficult to imagine that abundant plant and animal life would not have characterized the first island.    Where then did the plants and animals of the islands come from?  There is universal (at least I’ve never heard any other explanation even from young earth creationists) that the plants and animals that are there now migrated in some way to the islands.   The lack of animals other than a bat that is able to fly and a seal which can swim is not surprising.   Even the presence of birds, butterflys, freshwater fish, fungi, and 10s of thousands of species of plants is really amazing considering the 2000 miles of ocean that must be crossed.    There presence there isn’t just the result of their making it there but they must also have been able to find the right habitat to grow and survive and be able to reproduce.   For animals, even birds this is especially difficult.  One bird might be blown out to sea by a storm and get disoriented and fly 2000 miles just barely making it to Hawaii but without a mate it would be doomed to go extinct.  Even two birds would still need to find the appropriate food source when it arrived and their reproduction would have to be very successful in order to establish a new population.

Of course, the plants and birds are also different species than anywhere else in the world so if they migrated from the continent to where they are today why are they not the same at continental species?  But, that is a story for another day.  Today, we are just reminded that rare, extraordinary, events can explain unusual phenomena such as horses or elephants on islands.