Support from Ken Purdy

Troubled about Corona Virus?  Wonder why this disease is so successful? The Corona Virus is certainly one of the most dangerous species to land in the United States and in many other nations since the great Black Death of the 14th century.  Many are wondering, how could this be happening?  Well, being invasive is one major way that species on Earth have always managed to survive—not through one lifetime, but for many generations, for very long times.  Many species are exceptionally good at finding new open homes around the world.  Sometimes the result is wonderful for people and sometimes, like today with the corona virus, terrible.  But at the same time, we have the great abundance of foods that we have caused to invade by planting corn, potatoes, and beautiful flowers from one part of the world to decorate many another place. Yes, many species persist precisely because they are such good invaders and because we are so good at helping them invade.  Ironically, we have often done our best to help them invade our nation.


Biological invasions are natural; they occur without human interference and have done so as long as life has existed on our planet.  Indeed, one of the basic features that has made it possible for life to persist for more than three and a half billion years on Earth is this ability of various species to invade new habitats and reinvade previously occupied ones.

Here is one striking story of what I am writing about:   Zoologist David Challinor, a former Vice-president of the Smithsonian Institution and, during his lifetime one of my closest friends, directly observed an initial plant invasion on  a new island 25 km off Iceland’s south coast ,named Surtsey.  In 1964, while camping on Iceland’s south coast  with a group of scientists, David witnessed from the shore a vertical plume of volcano, rocks and smoke from an open fissure about 100 m below the water surface. That group stayed for weeks watching the eruption.    When ocean water met molten lava at that shallow depth, large pieces of the seabed rose up 300 meters into the air.  Pulsed explosions occurred with each contact of cold sea water with molten lava at the fissure.  The rapid cooling of the lava resulted in fine-grained crystal particles calledtephra.




Image credits: The Surtsey Research Society[i]

About three and a half years later in 1967, the explosions ceased. The volume of tephra that had rained from the sky created this small new island, named Surtsey by these scientific explorers.  It was roughly 2 kilometers in diameter and about 100 meters high.  A few weeks after the eruptions ceased, the lava had cooled and hardened enough so that it was safe to walk on the surface, although molten lava was still visible flowing deep below through occasional surface cracks.  A group of botanists, including my friend David Challinor, immediately found the first flowering plant, a sea rocket, had already sprouted on this still warm rocky island.   That it flowered so soon after the lava stopped flowing illustrated the speed with which biological invasions can occur.  This was a tiny new islet, surrounding by Arctic ice and cold sea water, a long distance from any land glowing with beautiful flowering plants or plants of any kind.  Other botanists later discovered mosses and grasses which continued the biological invasion, and the newly formed Surtsey Research Society stimulated long-term monitoring of the invasion process.


Cattle egret. Adults foraging beside cow. Rural Napier, Hawke’s Bay, November 2015. Image © Adam Clarke by Adam Clarke Zealand Birds Online.

A recent, ongoing example of a biological invasion also unassisted directly by people, but indirectly influenced by human alteration of land, is that of the cattle egret, a ubiquitous white bird familiar to travelers who originally saw it among African wildlife.  These birds feed on insects stirred up by the grazing of large herbivores, originally the Cape buffalo, which the egrets followed closely as they made their grazing rounds.  This bird probably evolved in the flood plains of the African tropics, but adapted to irrigated fields, especially in southern Africa.  But as native people began to introduce domestic cattle, the egrets soon began following them as well, feeding on the insects that were parasites of the cattle as well as the buffalo.  As the number of buffalo and other large wild herbivores declined, in large part because people were killing them for food, the cattle egret moved with them to feed on the insects on these large animals.  My guess is that it was a pleasure, so to speak for both the African buffalo and the domestic cattle to have these beautiful white birds cleaning them of the unpleasant parasites.

More surprising was the amazing capacity for transoceanic migration of these birds, which flew from west Africa to South America.  Eventually, enough egrets arrived to establish a New World breeding population.  This transoceanic invasion was not such a difficult feat for these amazing birds because, if aided by favorable winds, they can fly the 2900 km from West Africa to South America in about 40 hours.

First reported in South America in the 1880s, cattle egrets rapidly expanded their range, especially as coastal tropical forests were cleared for cattle ranching in the twentieth century.  Once established in South America, they migrated north, reaching the United States in 1951.  Just five years later they had spread from Texas to New England — several thousand kilometers!

Islands, Ecological Islands, and the Ability to Migrate Long Distances

As long as life has persisted on Earth, Life has spread around the Earth by invasion.  Even isolated oceanic islands are invaded by species that can travel long distances as adults, eggs, seeds, or other propagules.  Coconut palms are common on tropical islands because their nuts float and remain viable for long periods in saltwater.  Some insects are blown long distances and birds can fly thousands of miles.  Even snakes can travel across oceans on rafts of vegetation or today by hiding in ships’ cargoes or even in the wells of jet plane landing gear.

An isolated island with favorable habitats offers survival opportunities to species that can reach it.  Once on the island and free of former predators, competitors, and parasites, a species adapted to a specific mainland environment may evolve to occupy the island’s vacant niches.  For example, a small group of invading seed-eating birds reaching an island with no resident nectar- or insect-eating birds evolved into new species to exploit unused food sources.  Among these were these were the finches that Charles Darwin studied in the Galapagos Island. These birds helped him discover how species could evolve.  Eventually 16 bird species evolved in the Hawaiian Islands from a single ancestral species and happened, by luck we would say, to find those islands.



It is common for humans to bring other species with them to new habitats.  Today, alien species are inadvertently but regularly introduced throughout the world by extensive jet travel and even refugee resettlement.  Such invasions probably increased In the first decade of the 20th century and undoubtedly will continue, The U.S. Department of Agriculture monitored travelers carrying containers to the United States to see if any foreign plants were being brought in by travelers.  Officials opened containers that seemed likely to hold small plants that people were bringing into the United States.  They found 69,000 exotic plants people were attempting to bring into the nation, sometimes for commercial resale, other times because the plants were just pretty. Travelers thought these would add to the beauty of the American countryside and that of other nations.  Most were brought in by air travelers—42,000 during the years the U.S. Department of Agriculture tracked these invasions.

The problem was that these exotic plants and their soils often carried disease organisms and other kinds of pests that could seriously damage large volumes of native American plants.  For example, one employee of the Department of Agriculture, born in Italy, loved a certain onion that grew in his mother’s garden in Italy.  On a trip home, he dug up one of these onions, with enough soil so he could easily plant it in his American garden.  When staff of the Department of Agriculture found out that this exotic onion had been brought in and planted, they immediately dug it up with the surrounding soil brought from Italy.  They found that the soil contained an onion disease which, had it been left to proliferate in American soil, would have spread widely throughout the nation, possibly destroying large areas of commercially farmed native American onions.  That Italian onion and its soil were destroyed, preventing the potentially very damaging disease it carried from spreading. That was, indeed, a close brush with species disaster.

Not all human-induced invasions are detrimental.  Crops and decorative plants have been successfully introduced around the world.  Maize, potatoes and tomatoes, New World plants, are well-known examples of species that have become staples spreading from Mexico to Arizona and huge areas of the United States, providing food, along with beans and gourds, also abundant sources of human food in the Americas, in Europe and elsewhere.  With the exploration of the New World, many plant species were discovered and transported to Europe to decorate gardens and parks in many European cities.

(Photograph by Daniel B. Botkin)


Ornamental plant invasions, like crop invasions, were done with the best of intentions and generally were successful.  Few become pests. Most are now delightful additions to local environments.  Some intentional invasions have provided great commercial benefits.  The North American Douglas fir, a native of the Pacific Northwest, is today an important timber tree in Great Britain.  The Monterey Pine is the most important timber tree in New Zealand and in other nations of the southern hemisphere where it grows larger and faster than it does in its restricted northern California natural range.

These successes, along with spectacular dangerous runaway species invasions that become problems, raise an important question:  If so many introductions are successful, why are there also such spectacular failures?  The answers can be found in some of the fundamental properties of Earth’s biological species.

Many Species from Around the World are Planted and Grow Well in Santa Barbara, Santa Barbara. California






The process of invasion requires an inoculating event.  But once that event occurs, the features necessary for a successful invader are these:  rapid reproduction; the ability to migrate long distances; and competitive advantages over native plants and animals already existing in the environment they invade.  In addition, habitats and ecosystems required by the invader must be in good condition.

Rapid Growth

The importance of rapid reproduction for an invader was recognized in ancient times by Aristotle, who wrote about a pregnant mouse which was inadvertently enclosed in a jar of millet.  “After a short while” the jar was opened and “120 mice came to light.”  Biological populations can increase exponentially as with a sum of money left in a bank account at compound interest.  This point can be illustrated by the following story about the Elephant Seal.


Once numbering only ten or twenty at the south end of South America, the Elephant Seal has recovered and its large populations now fill the Pacific Coast of North America from the southern tip to San Francisco, CA.

THE NECESSITY OF BIOLOGICAL INVASION:  Biological Invasions and Evolution

In approximately the year 1900, the British Museum of Natural History sent forth an expedition to collect any remaining specimens of elephant seals for the Museum’s collection in London.  Museum officials thought it better for the species to be kept in a collection where they could be studied than to be shot for their blubber in the wilds of California.  That expedition failed to find any elephant seals. But the intended hunting, supposedly undertaken as a noble effort to savethe seals , had covertly been intended to kill off that species.  When the hunt ceased, the animals multiplied to more than 60,000 in 60 years, an average rate of increase of about 9% per year.  Among the reasons this population growth could be sustained is that elephant seals feed on deep-dwelling fish not normally harvested by people, and there was still beach space on southern offshore islands in South America where these big animals could molt and reproduce.


Because the earth’s environment is dynamic, species must adapt to change if they are to survive.  This is familiar to many people who live in areas that experience varieties of natural damage. Storms flood streams and rivers, which in turn scour flood plains.  Fires burn forests and prairies. Volcanoes cover landscapes with ash and lava and hill slopes erode.  Natural events continually alter the earth’s surface. The results may be either exacerbated or dampened by human activity or intervention.  Environmental changes help some species survive and hinder others, which explains the varying extinction rates over the three billion years that life has evolved on Earth.  Persistent species survive by adjusting to environmental change.

Among fish, salmon present one of the most impressive examples of a natural capacity to invade new streams.  Salmon and their relatives were once plentiful in the cold waters throughout the coasts of the northern hemisphere, thriving in fast-flowing, forested streams and rivers.  Salmon lay their eggs in the headwaters of streams where the young remain until they migrate to the ocean.  There, depending on the species, they spend from one to six years, after which they return to fresh water to spawn and create another generation.  Folk knowledge was that salmon always returned to their natal stream.  Any such salmon that did not do so was thought to have made a “mistake.”  Modern salmon studies suggest that such “mistakes” are not uncommon.  They may not be mistakes at all.

Along the Pacific coast of North America, about 15% of salmon return to the “wrong” stream.  In fact, the “mistake” improves the salmon’s ability to occupy new rivers when their natal stream becomes unsuitable for spawning.  Fires and storms can destroy large areas of riverbank forests, causing excessive silt to run into the spawning streams.  The salmon lay eggs in gravel that must have precise qualities for depth and texture.  Over time, gravel washes downstream and must to be renewed by occasional rockslides.  Initially, after such a rockslide, a stream can be so loaded with gravel that it is blocked and not available for salmon reproduction.  However, eventually stream erosion will restore the gravel bed for spawning.  These changes — from good to poor salmon habitat and back — are repeated over decades or even centuries.

SALMON IN A REPRODUCTION POOL (Photograph by Daniel B. Botkin)


If salmon were genetically coded to return always to their natal stream, the species would have disappeared long ago.  That a consistent, small percentage (10-15%) return to a “wrong” stream insures that, under changing environmental conditions, some spawning will always occur.


Biological invasions are a fascinating characteristic of life, one whose rate humans have greatly altered to their advantage and disadvantage.  Humans ignore its properties at their own risk and at the risk of many of Earth’s life forms. If the ability to invade is a natural and necessary feature of life, why should new invasions cause concern?  Because humans are relatively short-lived, and the time scale of their interest is short.  In the short run, as the preceding examples illustrate, results of an invasion can be negative, causing the extinction of some native species and the increase of others.  However, a period of adjustment must follow the initial invasion.

There are several important lessons to be learned about moving species around the earth.  Prior to introducing a species from one part of the world to another, careful tests must be made to ensure that the invader does not become a pest.  For rare and endangered local species, determined efforts are needed to help them reoccupy their former habitats and to ensure that these habitats and ecosystems are kept in adequate condition.

From a human perspective, biological invasions can be either advantageous or detrimental.  Since the Industrial Revolution, more human-augmented invasions seem to have been negative than positive.  Although biological invasion is a natural process, the short-term result frequently disrupts native species, but over millennia, a fragile balance is often reached between invaders and local organisms.  Successful biological invasion of Homo sapiens throughout the world has been largely dependent upon our successful introduction of domestic crops and livestock.  Humans have greatly altered the rate at which invasions occur.  This has enabled the migration of organisms that would not otherwise have taken place.  Animals that can swim or fly only short distances are transported in ships and airplanes; plant seeds that can stick to boot soles, the hooves of domestic stock, and the mud on car tires are carried everywhere.  Contemporary transportation systems increase the likelihood of biological invasions on a scale heretofore unimagined and with concomitant unexpected results.