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

Invasion or Natural Succession?

In a recent post we considered the changes in our landscape that have occurred as a result of climate change.  In this post we examine more historical sources of change in the landscape.

Native plant advocates in the Bay Area choose to replicate the “pre-settlement” landscape that existed in the late 18th century.  The arbitrary selection of this date does not take into account that Native Americans had lived in the Bay Area for approximately 10,000 years.  Throughout that period Native Americans altered the landscape by setting fires to promote food production as well as to provide materials for cultural activities such as basket weaving.  Fires were used to improve forage for the animals they hunted and visibility during the hunt, and to funnel animals into their hunts.  Fires also promoted the growth of their food sources such as acorn production.  (1)

Unlike some parts of California, fire ignition in the San Francisco Bay Area is rarely caused by lightening, making this anthropogenic (caused by man) source of fire the predominant cause of fire historically.  (2)

After the arrival of the Spanish in the late eighteenth century, cattle and sheep grazing was the predominant economic activity in California and continued to be an important activity into the early 20th century.  These early ranchers also introduced non-native grasses which had greater nutritional value for their herds.  The non-native annual grasses out-competed the native bunch grasses, resulting in California grassland that is 99% non-native today (3).

The fires set by Native Americans and the cattle grazing of the early Californians were both instrumental in preventing the natural succession of grassland to chaparral and scrub and subsequently to woodlands.  Modern land use and management policies have suppressed fire and reduced grazing in the Bay Area.  Consequently grasslands are succeeding to chaparral and scrub.  Although managers of public lands often describe these changes in the landscape as “invasions,” Jon Keeley (Ph.D. biologist, USGS) considers them a natural succession:

“These changes are commonly referred to as shrub invasion or brush encroachment of grasslands.  Alternatively, this is perhaps best viewed as a natural recolonization of grasslands that have been maintained by millennia of human disturbance.”  (4)

Serpentine Prairie restoration. East Bay Regional Park District

So, if the succession of grassland to shrubland is natural, why do managers of public lands believe it is necessary to prevent—or even reverse– this succession?  

Serpentine Prairie. 500 trees were destroyed, including many oaks.

For example, the “Wildfire Plan” of the East Bay Regional Park District is even more ambitious than halting natural progression of the landscape.  In many instances it proposes to return the landscape to an earlier version of the native landscape.  Here are a few examples of management actions in the “Wildfire Plan” that are intended to roll back biological time to sustain native landscapes from an earlier period:

  •  “[Native] Grasslands and Herbaceous Vegetation…these widely-spaced trees will not cause an active crown fire because of the discontinuity of tree crowns.  They could, however, provide a seed source for invasion of grassland habitats by woodland species and should be considered for removal to maintain desirable and declining grassland habitat.” (page 131)
  • “[Native] Maritime Chaparral…Favor chaparral community by removing oak, bay, madrone buckeye, and other trees under 8 inches diameter at breast height that are encroaching upon the maritime chaparral.”  (page 136)
  • “[Native] North Coastal Scrub…Shift species composition towards native scrub species or consider conversion to grasslands, where appropriate on historic grassland sites…” (page 140)
  • “[Native] Coyote Brush Scrub…In most treatment areas, encourage conversion to grasslands by reseeding with native grasses…after brush removal.”  (page 149)
Serpentine Prairie being weeded by hand. Mowing will be required during the restoration. Prescribed burns will be required to maintain it as prairie.

The return of the existing landscape to earlier, historical versions requires the removal of native trees and shrubs, as well as dangerous, polluting prescribed burns.  In so doing, a permanent commitment to periodic prescribed burns is made to maintain the landscape as grassland.  And what will this accomplish? If this strategy is successful the landscape would be returned to a version of the landscape in the late 18th century, even though that landscape was actually created by the Native Americans and maintained by subsequent grazing by early European settlers.

As we often do on Million Trees, we ask the managers of our public lands to explain their strategy for artificially maintaining our landscape at an arbitrarily selected point in time.  Should we run the risks of prescribed burns for the sole purpose of replicating an 18th century landscape that was created by Native Americans?  Since California grassland is now almost entirely non-native, what is the point of preventing its succession by destroying native plants?  We don’t understand what would be accomplished by such artificial manipulation of the landscape. 

(1) “The Use of Fire by Native Americans in California,” M. Kat Anderson in Sugihara, Fire in California’s Ecosystems, 2006.  

(2) “Central Coast Bioregion,” Frank Davis & Mark Borchert in Sugihara, Fire in California’s Ecosystems, 2006.

(3) Natural History of California, Schoenherr, UC Press, 1992

(4) “Fire history of the San Francisco East Bay region and implications for landscape patterns,” Jon E. Keeley, International Journal of Fire, 2005.