Changes in Vegetation Patterns Following Megafauna Extinctions

C.N. Johnson of James Cook University in Australia shares my fascination with how extinct megafauna influenced past ecosystems.  Not long ago, he published a paper about my favorite discussion topic entitled “Ecological Consequences  of Late Quaternary Megafauna Extinctions.”  He believes the extinctions of so many large mammal species led to 3 major changes in the environment.

1. The replacement of open woodlands and diverse habitat mosaics created by megafauna foraging with uniform closed canopy forests and zonal vegetation.

2. An increase in flammable plant material and thus an increased frequency of fire intervals because the plant biomass was no longer being eaten.

3. The decline of co-evolved plants such as large-seeded fruits dependent upon distribution by dung transport, and plants that put much of their energy into growing defensive thorns to deter herbivory.

Dr. Johnson relates an interesting hypothesis as to how Pleistocene ecosystems responded to megafauna foraging.  The feeding and trampling behavior of mammoths, bison, horses, and ground sloths suppressed woody regeneration.  Scrubby thickets dominated by thorny plants grew in place of highly shaded forests.  And patches of natural pastures grazed into short grass lawns impeded the fires that could potentially burn thickets.  When the thorny vegetation grew thick enough, megaherbivores avoided these locations, allowing shade intolerant species such as herbs, oaks, and hazelnuts to sprout and grow.  The widely spaced oaks and other deciduous trees would eventually shade out the thorny thickets. The megafauna would return to suppress the regeneration of the woodland as the trees aged and died, and the cycle would begin anew.  I can guess who the players were in this scenario here in southeastern North America.  Thorny greenbrier and blackberry vines along with hawthorne grew in areas temporarily left ungrazed.  The thick growth discouraged most animals larger than a rabbit from penetrating this space.  Sun-loving oaks, hazelnuts, persimmon and plum popped up.  Perhaps not coincidentally, hazelnut has a lesser distribution in North America now than it did during the Pleistocene.  A hazelnut was extracted from a mastodon coprolite found in Florida.  Hazelnuts no longer occur in Florida’s wild.

Thorny scrub habitat in south Texas.  The sharp spines of the yucca  could theoretically deter a species of large mammal from chewing on the tree or trampling it.  When the tree grows larger and older, it will shade out the spiny plants that originally protected it.

American Hazelnut Tree

American hazelnut (Corylus americana).  This species was more widespread and common in the south during the Pleistocene because the megafauna suppressed tall forest regeneration.  Hazelnut trees prefer sunny conditions and can’t grow in shade.

Hazelnut_CorylusLarge.jpg

Photo of the actual hazelnut extracted from a Florida mastodon coprolite dated to ~14,000 calender years BP.  Hazelnut no longer occurs in Florida.

Some pollen studies in Europe dating to the last interglacial, when megafauna was abundant and climate similar to that of today prevailed, contradict the above hypothesis.  Shady closed canopy forests dominated much of northern Europe.  However, other pollen studies from evidence collected in river bottomlands do show evidence of higher populations  of diverse sun-loving plant species, and this evidence is associated with  the abundant remains of dung beetles. This suggests patchy environments influenced by megafauna activities did occur near rivers.  This makes sense: large herds of megafauna likely occurred where water was an easily accessible resource.  If we were to go back in time and search for megafauna in Georgia, our best bet would be to travel along river corridors.  Large mammals were probably scarce or even absent in locations far from water.  The ancient environment of the southeast likely consisted of a mosaic of small and large grasslands, open woodlands, closed canopy forests away from where megafauna congregated, and thorny thickets.

The pollen record of southeastern North America does consistently show a large increase in shade tolerant beech approximately correlated with the timing of megafauna extinction.  This suggests forests became more close canopied when large mammals became extinct.

One study discussed in Dr. Johnson’s paper proposed that the extinct Irish elk (Megaloceros giganteus), the largest deer known to science, converted Ireland’s scrub habitat into grassland.  Megaloceros recolonized Ireland ~16,000 years ago when glaciers retreated and the land became habitable.  Willow scrub dominated the landscape initially, but Megaloceros favored browsing on shrubs for the phosphorus content.  Juniper and crowberry replaced the willow, then Ireland became mostly grassland because Megaloceros suppressed the growth of shrubs.  After Megaloceros became extinct here, willow scrub again became the primary type of environment in Ireland.

Scale drawing comparing the extinct Irish elk with a man.  This animal may have transformed Ireland from willow scrub to grassland about 16,000 years ago.

Reference:

Johnson, C.N.

“Ecological Consequences  of Late Quaternary Megafauna Extinctions.” 

Proceedings of the Royal Society of Biological Sciences 2009

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