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

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