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Krakatoa: A case study of species dispersal

May 26, 2014

Islands are intensively studied by ecologists because they are hothouses for evolution.  Physical isolation results in the evolution of new species that are related to their mainland ancestors and the result is many endemic species of plants and animals which exist only on that island. 

Some islands originated when continents broke up into smaller pieces as a result of continental drift.  Madagascar and New Zealand are examples of islands that were originally attached to a continent.  These islands brought some of the inhabitants of the continent with them.  But many islands arose from the ocean as a result of volcanic activity and were therefore born bare as a newborn babe without vegetation or inhabitants.  All subsequent life on these volcanic islands arrived by dispersal from elsewhere via ocean currents, winds, or carried by traveling animals, most recently by humans.

Krakatoa map

Krakatoa is such a volcanic island in the Indonesian archipelago.  It has a long record of volcanic eruptions which both destroyed much of the island and created new islands.  Many of these eruptions occurred during prehistoric periods, but many have been recorded by human history.  These recent eruptions have created an evolutionary laboratory that enables us to answer the perplexing question of how quickly the dispersal of species occurs. 

The cataclysm of 1883

Krakatoa eruption, lithograph 1888

Krakatoa eruption, lithograph 1888

In August 1883 a series of volcanic eruptions on Krakatoa produced one of the most cataclysmic events of recorded human history.  The force of the blast was the equivalent of 13,000 times the nuclear bomb that devastated Hiroshima in 1945.  The blast could be heard as far as 3,000 miles away.  Shock waves from the blast reverberated around the planet 7 times.  The blast sent an ash cloud 50 miles into the atmosphere. The weather of the entire planet was altered by the ash cloud.  The temperature dropped by 1.2 Degrees Celsius in the year after the eruption and the climate did not return to normal until 1888.  The blast and subsequent tsunami killed over 36,000 people and destroyed two-thirds of the island.

 Scientists believe that nothing living survived the blast:  “no plant, no animal, no seed, no spore.” (1)  The first scientists visited Krakatoa nine months after the blast.  They reported finding nothing alive except a single spider.  Spiders are notoriously successful dispersers because the webs they weave can become sails on the wind.

Krakatoa is quickly repopulated

In 1886—three years after the eruption—the first botanical expedition arrived on Krakatoa.  They found mosses, algae, flowering plants and eleven species of fern.  They speculated that the arrival of algae enabled the spores of ferns to become established on the otherwise bare ground.  Amongst the plants there were two species of grasses.  Scientists assume that most of these plants arrived via wind, but some species could have arrived as seeds carried by the surf.

Further colonization of the barren island then began to accelerate.  By 1887, young trees were found as well as dense grassland and many ferns.  Butterflies, beetles, flies and a single monitor lizard were found in 1889.   The species of monitor lizard found in 1889 is known to be a good swimmer and is a “versatile opportunist” on land, which means it’s not a fussy eater and it can eat less often than other lizards.

By 1906, there were a hundred species of vascular plants, covering the summit in green and a grove of trees along the shore, including tamarisk and coconut palm.  The coconut is found on virtually any sunny beach in the Pacific because its seeds float in their large protective shell wherever the current carries them.

Fifty years after the eruptions of 1883, the island was home to 171 species of plants.  One botanist estimated that 40% of the plants came on the wind, 30% floated on the sea and most of the remainder were brought by animals.   The eruption of 1883 produced huge quantities of pumice–a lightweight, sponge-like volcanic glass—that floated on the ocean creating rafts that were observed for years after the eruption:  “…a ship’s captain…who encountered pumice on the Indian Ocean, lowered a boat for a closer look, ‘It was curious and interesting to note how it had been utilized by animals and low types of life as habitations and breeding places.’”  (1)

These early arrivals were effective dispersers, but they also had to be capable of surviving inhospitable conditions on arrival.  The order of arrival is therefore an important factor in determining successful establishment.  For example, animals won’t survive if they arrive before needed food resources.  The plants most likely to survive are capable of self-pollinating, that is they don’t require a partner to reproduce.

San Francisco is not an island

How does this experience on Krakatoa compare to our experience in the San Francisco Bay Area?  We’re so glad you asked!!

The many projects all over the Bay Area that destroy non-native vegetation are not isolated islands.  They are surrounded by more non-native vegetation which quickly re-populates the bare ground created by these projects.  Dispersal into small plots of land within San Francisco is much easier than onto isolated Krakatoa.  The majority of these projects do not have the resources to replant the areas in which non-native vegetation is eradicated.  The fiction is that native plants will magically reappear when non-natives are destroyed.  But we can see that the result is the return of the hardiest non-native weeds such as hemlock, star thistle, oxalis, and broom.  These hardy creatures don’t need to be planted.  Their seeds are carried by the wind or remain dormant in the ground to germinate when someone foolishly destroys the trees that provide shade and suppress germination of weeds.

California Academy of Sciences, April 2011

California Academy of Sciences, April 2011

Even when natives are planted, they are quickly out-competed by non-natives.  The best local example of that hard, cold fact is the living roof on the California Academic of Sciences.    When the California Academy of Sciences reopened in San Francisco in August 2008, its “living roof” was considered its most unique feature.  Thirty species of native plants were candidates for planting on the roof.  They were planted in test plots with conditions similar to the planned roof and monitored closely.  Only nine species of native plants were selected for planting on the roof because they were the only plants that were capable of self-sowing from one season to another, implying that they were “sustainable.”  A living demonstration of “sustainability” was said to be the purpose of the living roof.

Two of six of the predominant species on the roof after 2-1/2 years were native.  Four of six of the predominant species were mosses that are “cosmopolitan,” which means they are found everywhere.  They weren’t planted on the roof and were therefore “volunteers.”

The monitoring project divided the roof into four quadrants.  In February 2011, non-natives outnumbered natives in two of the quadrants.  Although natives outnumbered non-natives in the other two quadrants, non-natives were also growing in these quadrants.

The consultant who advised the Academy about what to plant on the roof would not be surprised by this monitoring report.  He advised the Academy to walk the streets of San Francisco and identify the plants growing from the cracks in the sidewalks.  These are the plants he advised the academy to plant because these are the plants that are adapted to current conditions in the city.  The Academy rejected this advice because they were committed to planting exclusively natives on the roof.

The many projects that are destroying non-native vegetation are not sustainable.  They are surrounded by non-native vegetation which is better adapted to current climate, soil, and atmospheric conditions.  Non-native vegetation will out-compete the natives that are not adapted to current conditions.  If these projects were merely futile, perhaps we could shrug and move on.  But we can’t turn a blind eye because these projects are harmful to the environment.  They use huge quantities of toxic herbicide and they are destroying healthy trees that are performing many valuable ecological functions.  These are not harmless experiments.


(1)    David Quammen, Song of the Dodo, Scribner, 1996.

(2) Some information for this post is from Wikipedia

7 Comments leave one →
  1. May 26, 2014 6:25 am

    Very, very good analysis and post. Though long, I read every word. It is not too difficult for learned people to see the errors of this movement. How many examples are needed before the “native only” folks get off the high horse and realize that wildlife, especially insect life need places to feed and reproduce. How many bees do we lose? Pesticides stay in the soils and bees ingest it through the nectar they drink and pollen they collect. And pesticides are used in clearing non-natives most efficiently.

  2. May 26, 2014 10:03 am

    i love it, have already reposted.

    *R* o b i n *S* h e r r e r c: 4 1 5 . 5 3 3 . 6 0 8 3 *r o b i n s h e r r e r . . .*

  3. Don E permalink
    May 26, 2014 2:07 pm

    I watched a program on PBS about the aftermath of the Mt. St. Helens eruption. The first life that appeared? Gophers! It may be futile but I continue to fight gophers in some areas.

    As applied by the native plant advocates, the concepts of sustainability, natural, and biodiversity are so much BS. However, humans have preferred one species over another and have manipulated the environment to favor the preferred species for thousands of years. In that sense I have no problem if people prefer a native garden. All urban gardens, native or non-native, require maintenance. The issue is the cost and priorities. The judicious use of chemicals to manage gardens does not cause major harm, which is exaggerated. Without them we would need an army of gardeners or volunteers to maintain accessibility and an attractive appearance of native or non-native gardens.

    I don’t know about grasses and shrubs, but I have found that trees native to the region are easier to grow and maintain and look better. Street trees may be another matter because of many other factors. I see mostly non-natives in my neighborhood in the City. Outside the City I have found that volunteer natives do the best. But “volunteer” non-natives do okay also. It is argued that natives support greater “biodiversity” in general even though some non-native trees support more biodiversity than some native trees. At least that is the conventional wisdom among entomologists. I like many non-natives but have had difficulty with them. For example, trying to grow Japanese maple in the Bay Area’s dry windy environment is a challenge and they just don’t look as good as they do in Japan or the East Coast.

    • May 26, 2014 7:07 pm

      We agree that everyone has a right to their horticultural preferences. By all means plant exclusively natives in your garden if that is what you prefer. We ask only that native plant advocates quit destroying everything in our public lands that they do not prefer. Our public lands are owned by everyone. Therefore, they have no right to impose their personal preferences on everyone. Even then, we have no objection to them planting what they prefer. We only object to them killing everything else. Our request is very modest, yet it is completely rejected by the extremism of the native plant movement. They continue to claim more land and expand their projects despite our objection to their destructive agenda.

    • May 30, 2014 7:04 pm

      Not that I agree with all of what he speaks, but he cites that non-natives often are beneficial and not always “I have found that volunteer natives do the best,” as quote above.
      Here is the link to his video.

  4. June 8, 2014 5:06 pm

    Thank you so much for this excellent post yet again. I finally went back to Cesar Chavez park in Berkeley, where theoretically we won in stopping the planned poisoning of the native California Ground Squirrels and Botta’s Pocket Gopher by the city of Berkeley. Yet I saw almost no burrows and only one squirrel, which makes me wonder if someone else is going ahead with the murdering and what this means for the Burrowing Owls for next winter.

    Meanwhile, though, there are the multi-million dollars of non-native landscaping there, and yet there is also is the usual campaign to kill the broom. No attempt to kill the hillsides of non-native grasses, but just the beautiful sweet-scented broom. Some has been cut and piled up dry and dead (fire hazard, just like the several years of cut and piled dead brush still in the EBRPs in the Oakland hills.)

    And remember the carrying on about how the squirrels “MIGHT” cause toxic leakage into the bay there? Well, there are stands of broom not yet cut and very dead (even more fire hazard), which could only be possible if they had poisoned them. What about protecting the bay water from such toxins? Hypocrisy….

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