The Globalization of Ecology by Humans

Humans have had a profound impact on our planet wherever they have lived.  Since the industrial age man’s impact on the environment has even extended beyond where he lives as atmospheric changes impact remote places such as the polar regions. Humans are a particularly restless species of animal, forever fleeing whatever conditions threaten existence or seeking a better life ahead.  Because we are an adaptable species with the capacity to alter conditions, we have a wider range of movement than other animals. 

Wherever humans have gone they have taken with them the seeds of plants, at first unwittingly and then purposefully after the advent of agriculture.  Edward Salisbury, a 19th Century British horticulturalist, reported raising 300 plants of 80 different species from the debris in his trouser cuffs despite wearing spats.(1)

The impact of Native Americans on the landscape

Native Americans arrived in North America about 13,000 years ago and slowly migrated to South America.   The megafauna that Native Americans found when they arrived disappeared soon after they arrived.  Although not all scientists agree, there is strong evidence that the megafauna, such as enormous buffalo and mastodons were hunted to extinction by the first human inhabitants of the New World.  The loss of these huge herbivores was a factor in the development of grasslands that were not adapted to heavy grazing.(2)

Native Americans setting grass fire, painting by Frederic Remington, 1908

The most profound alteration of the ecology of the New World by Native Americans was their use of fire to assist their hunting and gathering activities.  Burning grassland to the ground encourages the growth of new sprouts that improve grazing and attract the animals the Native Americans hunted.  And burning grassland funneled the animals into the hunt.  These frequent burns prevented the succession of grassland to shrubs and forest, maintaining prairies that were essentially manmade. 

The arrival of Europeans in the New World

This process of transplanting plants around the globe was greatly accelerated by the age of exploration which began in earnest in the 15th century.  When Columbus arrived in the New World in 1492, he found a land that had already been radically altered by Native Americans, though he had no way of knowing that. 

The Europeans that eventually settled the New World were ignorant of the impact of Native Americans on their new home until the advent of archaeology in the 20th century.  Their ignorance was based on their mistaken perception that the Native American population was small.(3) 

In fact, the population of Native Americans was decimated by the time the Europeans settled the New World nearly 200 years after the early explorers arrived at the end of the 15th century.  The early explorers introduced European diseases to which they were immune, but the Native Americans were not.  From just a few localized contacts with Europeans these diseases–such as measles, smallpox, and syphilis–spread quickly through Native American communities, sometimes reducing the population by as much as 90%.  These deadly epidemics were largely unwitnessed by the tiny population of early explorers.

Alfred Crosby speculates in Ecological Imperialism that Native Americans were particularly vulnerable to these diseases because they had virtually no domesticated animals.  He gives us a fascinating explanation of how Europeans developed their immunities by living in close proximity with animals for centuries.

The history of Native Americans was initially recorded by the European settlers who arrived in numbers nearly 200 years after initial contact in the 15th century.  Their perception that they found a pristine wilderness, largely untouched by human hands, has persisted to this day in the public’s mind.   It is that perception that has led native plant advocates to the conclusion that the pre-European landscape should be the goal of restorations.

The impact of Europeans on the landscape

As much impact as Native Americans had on our landscape it pales in comparison to that of Europeans.  The importation of domesticated animals—such as cattle, pigs, horses, and sheep—changed the ecology of the New World quickly.  Because the population of early settlers was small and the grazing resources vast, these animals quickly became huge herds of feral animals, roaming places such as the pampas of Argentina and the prairies of the mid-west of North America.  The native grasses were quickly replaced by the non-native grasses spread by these wandering herds. (4)

Herd of sheep, Mono County, CA. Creative Commons

Likewise, the agricultural practices of European settlers have introduced virtually everything we eat today in North America.  Nearly all of our fruits, vegetables, and cereals are not native to North America.  If non-native food were to be banished from California, we would be reduced to a diet of game, acorn mush and a few species of nuts and berries.   The diet of Europeans would be equally impoverished if they lost the potatoes, corn, tomatoes, etc. that came to them from the New World.

USDA photo

The Anthropocene:  The geologic age of humans

The Earth’s ecology is in a constant state of change.  Humans are not the source of much of that change.  The continents shift.  The climate oscillates.  Plants are moved by the wind and tide.  Animals move without the assistance of humans, often taking plants with them.  But change has been accelerated by the restless movements and activities of humans. 

Scientists have recently begun to advocate for the naming of a new geologic era, the Anthropocene.   The Anthropocene would officially acknowledge that the environment has been altered by humans.  Although the beginning of this era is still being debated by scientists, Erle Ellis (University of Maryland, Baltimore) believes that an appropriate beginning would be about 6,000 years ago, when humans domesticated animals. 

Why are we suddenly afraid of change?

Changes in the ecology of North America resulting from the movement of humans have been going on for thousands of years and have greatly accelerated in the last 500 years.  Until very recently, these changes were broadly perceived as improvements.  We enjoy a more varied diet than our ancestors and we are shielded from famine by our access to food from anywhere in the world.  We find the products of the New World useful in other ways, such as the rubber that is essential to transportation.

Suddenly, for little apparent reason, we are afraid of everything new in our environment.  The media is full of panic-stricken reports of alien “invasions.”  Most of these panic attacks eventually prove to be baseless.  The newcomers are eventually absorbed into the landscape or they disappear without a trace.  Or scientists eventually explain that the newcomers are better adapted than their predecessors to the climate, water, and soil conditions created by the activities of man. 

We asked a scientist who advises us occasionally this question:  Since new species of plants and animals have been introduced to our landscape for hundreds, if not thousands of years, why the sudden panic?  His reply was:  “It’s trendy. It’s ‘cutting edge.’ It’s ‘hot.’ It’s…fundable.”   

In the service of a scientific fad that has spread into the popular culture, introduced species of plants and animals are being needlessly destroyed.  Many of these new species are better adapted to present conditions than their predecessors.  Many are essential to our way of life and the animals with which we share the planet. 

(1) Richard Mabey, Weeds:  In Defense of Nature’s Most Unloved Plants, Harper-Collins, 2011

(2) Alfred Crosby, Ecological Imperialism. Cambridge University Press, 2004

(3) Charles Mann, 1491, Random House, 2005

(4) Crosby

Pangaea: The first but not the last globalization of ecology

The continents have been sliding about on the Earth since it was “created”(1) approximately 4.5 billion years ago.  Although geologists tell us that the continents came together and broke apart several times prior to the formation of the supercontinent geologists call Pangaea, this is the geologic period of most interest to us because life forms were sufficiently complex by that period that we can recognize their modern counterparts.

The supercontinent Pangaea

Pangaea is said to have been assembled about 237 million years ago, during the Early Triassic Period, shortly after the great Permian extinction, the period of the most extensive extinctions of plant and animal species in the history of the Earth.   Pangaea began to break apart about 50 million years later, but the African and South American continents remained fused–into a continent dubbed Gondwana–until about 100 million years ago. (1)

During that period of nearly 160 million years, many new life forms emerged and others died out.  Cone-bearing plants replaced some spore-bearing plants before Pangaea formed and dominated the Earth during much of Pangaea’s existence.  The first true mammals, flowering plants, birds, lizards, and salamanders appeared before the break up of Pangaea was complete.

What are the implications of the development of new species of life on Earth at a time when there was a single, unified continent?  That is the question we are considering today.  Obviously, the transport of plant and animal species into new territories is facilitated by their proximity.  Seeds are more easily transported by wind and animals if they need not cross barriers such as oceans, as they must today.   As a result there was greater homogeneity of species during the geologic periods of Pangaea.  And species diversified rapidly when Pangaea broke up into the 7 continents of today. (2)  These diversified species have common ancestors. 

Even after Pangaea began to break up into separate continents, there were land bridges between some of the continents during periods of glaciations when water was locked into ice, draining the oceans.  Animals could travel over these land bridges from one continent to another, often bringing plant species with them, usually unwittingly.  That’s how the first humans in North America and ultimately South America traveled from Asia about 13,000 years ago at the time of the last ice age.

The common ancestry of many plants and animals is one of many reasons why the concept of “native” is ambiguous and is often debated.  We will consider a few examples in which the designation of a particular plant as native or non-native seems debatable.

Is the Dawn Redwood native to California?

Dawn redwood (Metasequoia glyptostroboides) is closely related to our redwood trees, Coast Redwood and Giant Sequoia.  Dawn redwood is unique in being a conifer that is also deciduous (loses its foliage in winter), unlike our redwood trees which are evergreen.  Dawn redwoods were until recently considered native to remote regions of China where they are considered “critically endangered.”

Dawn redwood in spring. Wikimedia Commons

However, scientists at the Museum of Paleontology at UC Berkeley tell us that there is fossil evidence that dawn redwoods grew in California about 40 million years ago.  Dawn redwoods now grow successfully in the Bay Area.  There is a famous specimen in front of McLaren Lodge in Golden Gate Park, headquarters of San Francisco’s Recreation and Park Department.  Every autumn, when the tree turns red, park staff receives calls from the public expressing their concern that the beautiful tree is dying.

Dawn redwoods died out in California during the last ice age because the climate was cooler than dawn redwoods could tolerate.  So, now that the climate has warmed again, and dawn redwoods are back, why not welcome them as a “return of the natives?”  That’s the kind of flexibility that makes sense to us, particularly in a time of rapidly changing climate.

Dawn redwood in autumn. Wikimedia Commons

Unfortunately, we don’t find such flexibility in the native plant ideology.  Dawn redwoods are rare both in California and in China from which it was reintroduced, and it is therefore not one of the trees that native plant advocates demand be eradicated.  Monterey pine and Monterey cypress are not so fortunate.  These are also trees for which fossil evidence suggests that they lived in San Francisco in the distant past and their native range is less than 150 miles down the coast in Monterey.  Both tree species are also considered threatened in their native range.  Yet, native plant advocates demand their eradication in San Francisco.

This is an example of the rigidity of the native plant ideology that has earned them the reputation of fanatics.

Does Rhododendron ponticum “belong” in Britain?

We told our readers in a recent post that Rhododendron ponticum is one of only about a dozen plants in Britain that are considered “invasive.”  It is a stunningly beautiful plant which is being aggressively eradicated in Britain.  Richard Mabey in Weeds:  In Defense of Nature’s Most Unloved Plants offers this explanation for why this particular plant is “invasive” in Britain:

“The next most serious weed is probably rhododendron which, unusually, has the ability to invade existing ancient woodland, especially in the west of Britain.  This may be because, if one employs a very long time scale, it is not strictly an alien.  The species that forms impenetrable thickets in western Britain is Rhododendron ponticum, whose pollen remains have been found in deposits in Ireland dating back to the last interglacial.  The species was plainly accustomed to growing in Atlantic woodland and may have retained a genetic “memory” of how to cope with this habitat and its competing species.  But it didn’t grow spontaneously in Britain for the next 30,000 years, and all the current feral colonies are regarded as originating from garden escapes.”(3)

Rhododendron ponticum. Wikimedia Commons

Once again, we wonder if “welcome home” isn’t a more appropriate response to this beautiful plant.  We find the definition of “native” as arbitrary as the definition of “invasive.”  Both seem to be terms used by people who abhor change.  And in a rapidly changing world, does such resistance to change make any sense?  We don’t think so. 

(1) The use of the word “created” implies no particular origin of the earth, merely its beginning.

(2) Crosby,Alfred, Ecological Imperialism, 2nd Edition, Cambridge, University Press, 2004

(3) Mabey, Richard, Weeds:  In Defense of Nature’s Most Unloved Plants, Harper-Collins, 2010