**5. Onset of major cold events in North America**

238 Biodiversity Conservation and Utilization in a Diverse World

These events appear to have resulted in a dramatic drop in mean annual air temperatures around 35 million years ago (Frakes, 1979). This heralded the end for many of the tropical species of biota inhabiting the northern regions. They were replaced by species that could tolerate the cooler temperatures, and these changes in the biota have continued until today. The loss of the warm ocean currents entering the Arctic Basin would appear to be the most likely cause of the abrupt change in mean annual air temperature around the basin and the

The Cenozoic was when the Mammals became the dominant land animals, aided by the presence of fur and by being warm-blooded. Within 10 million years, they had become greatly diversified and lived in almost all micro-environments. They included herbivores and carnivores, e.g., whales and bats. Those weighing more than about 45 kg are referred to as the Megafauna, and first appeared in Eocene times (55-30 million years ago). These became abundant in the early and middle Cenozoic, but largely died out in the Pliocene and Pleistocene. They included herbivores such as *Coryphodon*, *Uintatherium* and *Arsinoitherium*, together with carnivores such as *Andrewsarchus* and smaller wolf-like predators and sabretoothed cats. About 40 million years ago, the first camels evolved (Harrington, 1978). At first, they were rabbit-sized with four toes, but they became much larger about 24 million years ago. By about 5 million years before present, some were substantially larger than the present-day camels of Africa and Asia (Harrison, 1985). They were common on the dry scrub grasslands of central North America from 600,000 years ago until about 10,000 years before today. A larger species of camel entered Alaska and crossed the Bering land bridge to Asia about 5 million years before present, and evolved into the species still surviving in Africa and Asia. About 20,000 years ago, they died out in North America. Among the other mammals that originated in North America are the horses, mastodons and mammoths. These also migrated across the Bering Strait, but subsequently died out in North America. Our ancestors are believed to have helped kill them off, though some mammoths survived in relatively inaccessible areas of Alaska (Haile et al., 2009) and Northern Siberia (Boeskorov, 2004, 2006, Vartanyan et al., 1993) well into the Holocene. The last wild horse carcass in Siberia dates from as late as 2,150 years ago, but horses were reintroduced into

consequent extinction of so many species and genera that had survived for so long.

North America during the second invasion by the Spanish Conquistadors.

times.

Stewart & Stark (2002, Figure 3.2) also conclude that a considerable number of genera of Plecoptera (stoneflies) moved across the Bering Land Bridge in both Miocene and early Pliocene times, since today part of their distribution extends south of Alaska to California. Before the closure of the North Atlantic Land Bridge, there was an exchange of at least 5 genera of stoneflies with Europe that now exhibit an Amphi-Atlantic pattern of distribution. There was also a tremendous explosion of species in the other terrestrial groups including amphibians, birds, fish, insects and reptiles. Many are endemics, often with a very limited distribution. Weber (1965) discussed the plant geography of the southern Rocky Mountains, and determined that there were also a number of species now living in Colorado and California that are also found in sub-tropical Asia. He concluded that they must have crossed the Bering Land Bridge while the climate was still sub-tropical in late Cenozoic About 3.5 million years ago, the first major cold event (extensive glaciations and development of permafrost) affected the Cordillera of western North America. Cold conditions also extended across northern Canada, and there must have been a substantial southward movement of the climatic zones. Another brief connection with Asia across the Bering Land Bridge occurred when the sea level dropped during this first major cold event. There were to be 5 subsequent occasions when the land bridge was open, the last one being during the late Wisconsin cold event about 15,000-20,000 years ago. Altogether there were about 13 major cold events (Harris, 1994; 2000; 2005) separated by shorter interglacials.

The biota that do not live in the Arctic or Subarctic could not move across the Bering Land Bridge during the few times it was open during the last 3.5 million years. These include the Hispine beetles (Staines, 2006). Only one species of Lasiopogon (*L. hinei)* is found on both sides of the Bering Strait (Cannings, 2002), but that species is exceptional in its wide range throughout Eurasia.

The second major cold event at 3 million years ago produced the most extensive glaciation in Alaska and the Yukon Territory. The moisture came primarily from the open Arctic Ocean, but by the time the third major cold event occurred (2.58 million years before today), the Arctic Ocean had frozen over and permafrost with tundra was present along the Arctic coast. This split the temperate humid vegetation into separate eastern and western

populations. These were forced to move south along the respective coasts as cooling continued, and any components of the biota that could not adjust to the changing environment were extirpated. Since the climate, topography and micro-environments were different on the two sides of North America, different Temperate and Subtropical species that lived in the more humid areas evolved on the two sides of the Continent. The eastern populations have significantly more species than the western populations. Likewise, the animal populations in the two areas show distinctive species adapted to the local environment. Only the Boreal Forest and Tundra biomes are distributed across the northern part of the continent. Further south, the dry central steppe (Prairies) separates the two populations of biota that are adapted to wetter climates. The Tropical conditions moved south into Central America, while Subtropical climates were limited to the extreme southern United States, even during the Interglacial periods.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 241

exposed by the eustatic drop in sea level along the Grand Banks area off Newfoundland, the Gulf coast of the southern United States and along the Beringian land bridge. Undoubtedly in the north, these provided expanded ranges for the arctic mammalian fauna, as well as the limited number of arctic insects such as butterflies (Layberry et al., 1998). Many Arctic mammalian species were destined to become extirpated by a combination of hunting and climate change, but 6 species of butterflies still live in both Alaska and the adjacent part of Siberia. The existence of closely related but different species of butterflies in eastern Beringia (Alaska and the Yukon Territory) and in western Beringia (East Siberia) is apparent

In the southwest United States, the climatic changes were accompanied by widespread tectonic movements and volcanism (Wahrhaftig & Birman, 1965). This orogenic activity started in middle Miocene times and is continuing today. Pluvial lakes developed in the inland drainage basins (Morrison, 1965) and the sediments in these basins contain scattered vertebrate fossils, freshwater mollusks and diatoms, some dating back to 3.4 million years ago. Fossils in the marine terraces provide further evidence of the climatic changes and their effects on the local biota. The isolated volcanic mountains tend to have local endemic biotas that have evolved to cope with the local microenvironments. During pluvial events, animals such as voles are believed to have descended from the mountains and became widespread in the Great Basin and Mohave Deserts (Findley & Jones, 1962, Norris, 1958). Southward movement of sage voles and other animals resulted in their presence as fossils in the Isleta Caves of New Mexico (Harris & Findley, 1964). Around Lake Bonneville, speciation resulted in the appearance of a new species of *Oxyloma* (*O. missoula,* Succinidae) that is limited to that

In the southeast United States, a narrow zone of subtropical and tropical vegetation may have persisted along the coast during the glaciations, but permafrost conditions were present along the higher and more northern parts of the Appalachian Mountains. Once again there is clear evidence of the southward migration of the biota during the glaciations. Remains of mammoths and other cold arctic fauna are well known from along the eastern seaboard of the United States, and have even been found in the sediments on the shallow sea floor that would have been exposed as dry land due to eustatic sea level changes during the glaciations. There was a gradual change in the mammalian fauna throughout the sequence of glaciations, indicating that speciation was fostered by the climatic changes (see Hibbard et al., 1965, for a summary). This contrasts with only slow speciation being reported in the insect world, since speciation in insects seems to have been slower than the speed of the major climatic changes (Matthews, 1980). Other animals, e.g., the Mollusca, tended to evolve in a similar way to the mammals in response to the environmental changes during

The aquatic biota also had to adjust their ranges. The north-south Mississippi River was very important, since it facilitated the migration south of fish and aquatic mollusks from the interior of the continental United States. The biota had to move into the warmer waters close to the Caribbean Sea which also diminished in size as sea levels dropped due to the accumulation of ice on land. During the last few glaciations, the Mississippi River acted as a spillway for the

evidence of Holocene speciation there.

particular drainage area (Harris & Hubricht, 1982).

the last 3.5 million years.

Initially, the later glaciations only affected relatively small areas in the north, but subsequently, the ice caps have become far more extensive. Permafrost was particularly widespread across the northern parts of North America, and facilitated the spreading of the Tundra flora south along the western Cordillera (Harris, 2007a). Only in the last 100ka have the Milankovitch cycles become significantly correlated with the onset of glaciations (Harris, 2012, Imbrie & Imbrie, 1980). During the last cold event and probably during the earlier events of the last million years, permafrost extended down to the southern part of Arizona and New Mexico, so the biota of all the climatic zones had to migrate long distances to survive or else find local refugia. The ice sheets wiped out all the vegetation in their path as they advanced, and when they retreated, the biota had to rapidly migrate north again over distances in excess of 1500 km (Dynesius & Jansson, 2000; Harris, 2010a). The result was that only those species of plants and animals that could migrate, adapt or find a suitable refugium could survive.

Refugia were present throughout these climatic changes, providing suitable habitat for the biota, whether the changes involved mean annual air temperature, precipitation, or both. In the more northerly mountain valleys, a combination of cold air drainage, temperature inversions and steam fog provide a buffering of the mean annual air temperature, so that the effects of cooling events were greatly reduced (Harris, 2007b, 2010b). This undoubtedly helped the biota in the eastern part of Beringia survive during the major cold events of the last 3 million years.

It should be noted that there are multiple kinds of refugia. Until now, most of the literature only discusses the effects of variations in temperature (e.g., Willis and Whittaker, 2000; Stewart et al., 2010). As these authors point out, refugia exist for species during both warmer and colder conditions. However, the vegetation is actually controlled by a number of factors, of which the moisture regime is undoubtedly equally important. Since the vegetation cover is a critical part of the ecosystem, it is also a major factor in providing a suitable microenvironment for animals.

The colder climate of the Late Wisconsin event would have resulted in an expansion of the ranges of species adapted to the cold conditions southwards and also on to lands becoming exposed by the eustatic drop in sea level along the Grand Banks area off Newfoundland, the Gulf coast of the southern United States and along the Beringian land bridge. Undoubtedly in the north, these provided expanded ranges for the arctic mammalian fauna, as well as the limited number of arctic insects such as butterflies (Layberry et al., 1998). Many Arctic mammalian species were destined to become extirpated by a combination of hunting and climate change, but 6 species of butterflies still live in both Alaska and the adjacent part of Siberia. The existence of closely related but different species of butterflies in eastern Beringia (Alaska and the Yukon Territory) and in western Beringia (East Siberia) is apparent evidence of Holocene speciation there.

240 Biodiversity Conservation and Utilization in a Diverse World

United States, even during the Interglacial periods.

refugium could survive.

last 3 million years.

suitable microenvironment for animals.

populations. These were forced to move south along the respective coasts as cooling continued, and any components of the biota that could not adjust to the changing environment were extirpated. Since the climate, topography and micro-environments were different on the two sides of North America, different Temperate and Subtropical species that lived in the more humid areas evolved on the two sides of the Continent. The eastern populations have significantly more species than the western populations. Likewise, the animal populations in the two areas show distinctive species adapted to the local environment. Only the Boreal Forest and Tundra biomes are distributed across the northern part of the continent. Further south, the dry central steppe (Prairies) separates the two populations of biota that are adapted to wetter climates. The Tropical conditions moved south into Central America, while Subtropical climates were limited to the extreme southern

Initially, the later glaciations only affected relatively small areas in the north, but subsequently, the ice caps have become far more extensive. Permafrost was particularly widespread across the northern parts of North America, and facilitated the spreading of the Tundra flora south along the western Cordillera (Harris, 2007a). Only in the last 100ka have the Milankovitch cycles become significantly correlated with the onset of glaciations (Harris, 2012, Imbrie & Imbrie, 1980). During the last cold event and probably during the earlier events of the last million years, permafrost extended down to the southern part of Arizona and New Mexico, so the biota of all the climatic zones had to migrate long distances to survive or else find local refugia. The ice sheets wiped out all the vegetation in their path as they advanced, and when they retreated, the biota had to rapidly migrate north again over distances in excess of 1500 km (Dynesius & Jansson, 2000; Harris, 2010a). The result was that only those species of plants and animals that could migrate, adapt or find a suitable

Refugia were present throughout these climatic changes, providing suitable habitat for the biota, whether the changes involved mean annual air temperature, precipitation, or both. In the more northerly mountain valleys, a combination of cold air drainage, temperature inversions and steam fog provide a buffering of the mean annual air temperature, so that the effects of cooling events were greatly reduced (Harris, 2007b, 2010b). This undoubtedly helped the biota in the eastern part of Beringia survive during the major cold events of the

It should be noted that there are multiple kinds of refugia. Until now, most of the literature only discusses the effects of variations in temperature (e.g., Willis and Whittaker, 2000; Stewart et al., 2010). As these authors point out, refugia exist for species during both warmer and colder conditions. However, the vegetation is actually controlled by a number of factors, of which the moisture regime is undoubtedly equally important. Since the vegetation cover is a critical part of the ecosystem, it is also a major factor in providing a

The colder climate of the Late Wisconsin event would have resulted in an expansion of the ranges of species adapted to the cold conditions southwards and also on to lands becoming In the southwest United States, the climatic changes were accompanied by widespread tectonic movements and volcanism (Wahrhaftig & Birman, 1965). This orogenic activity started in middle Miocene times and is continuing today. Pluvial lakes developed in the inland drainage basins (Morrison, 1965) and the sediments in these basins contain scattered vertebrate fossils, freshwater mollusks and diatoms, some dating back to 3.4 million years ago. Fossils in the marine terraces provide further evidence of the climatic changes and their effects on the local biota. The isolated volcanic mountains tend to have local endemic biotas that have evolved to cope with the local microenvironments. During pluvial events, animals such as voles are believed to have descended from the mountains and became widespread in the Great Basin and Mohave Deserts (Findley & Jones, 1962, Norris, 1958). Southward movement of sage voles and other animals resulted in their presence as fossils in the Isleta Caves of New Mexico (Harris & Findley, 1964). Around Lake Bonneville, speciation resulted in the appearance of a new species of *Oxyloma* (*O. missoula,* Succinidae) that is limited to that particular drainage area (Harris & Hubricht, 1982).

In the southeast United States, a narrow zone of subtropical and tropical vegetation may have persisted along the coast during the glaciations, but permafrost conditions were present along the higher and more northern parts of the Appalachian Mountains. Once again there is clear evidence of the southward migration of the biota during the glaciations. Remains of mammoths and other cold arctic fauna are well known from along the eastern seaboard of the United States, and have even been found in the sediments on the shallow sea floor that would have been exposed as dry land due to eustatic sea level changes during the glaciations. There was a gradual change in the mammalian fauna throughout the sequence of glaciations, indicating that speciation was fostered by the climatic changes (see Hibbard et al., 1965, for a summary). This contrasts with only slow speciation being reported in the insect world, since speciation in insects seems to have been slower than the speed of the major climatic changes (Matthews, 1980). Other animals, e.g., the Mollusca, tended to evolve in a similar way to the mammals in response to the environmental changes during the last 3.5 million years.

The aquatic biota also had to adjust their ranges. The north-south Mississippi River was very important, since it facilitated the migration south of fish and aquatic mollusks from the interior of the continental United States. The biota had to move into the warmer waters close to the Caribbean Sea which also diminished in size as sea levels dropped due to the accumulation of ice on land. During the last few glaciations, the Mississippi River acted as a spillway for the

melting ice to the north, so the biota would have had to find refugia in smaller tributary streams. During deglaciation, they would migrate upstream to reclaim their previous habitats, or find new ones. Migratory birds presumably altered their migration patterns as they do today, to dwell in suitable habitats. When a cold event ended, they would adjust their migrations to make use of the new environments as they became available.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 243

 **Origin Common Name** 

 **Skunks and otters** 

 **Camels and llamas** 

 **Giant ground sloths Three-toed sloth** 

**Table 1.** Some mammal families that took part in the Great American Faunal Interchange (after Webb,

About 3.5 million years ago, the Panamanian Seaway between North and South America began a step-wise closure. The Isthmus of Panama was finally dry land by about 2.4 million years before today. South America had been moving northwestwards and had finally run into the southern margin of the North American plate. The northwards motion of the South America plate is still resulting in volcanism and the formation of the islands in the Caribbean Sea. Both plants and animals have taken part in exchanges between North and South America since then. During Interglacials, Tropical Rain Forest occurred along the Isthmus and permitted the exchange of that biota, but during major cold events, savannah conditions occurred there, allowing an exchange of the biota found in much drier areas (Harris, 2010a). As a result, many of the species of grasses and other vascular plants of

 **North America Rabbits Squirrel Field mice** 

 **Cats** 

 **Foxes Bears Horses** 

 **Deer** 

 **South America Opossums Armadillos** 

 **Anteaters Monkeys Porcupines Guinea pigs** 

**6. Closing of the Panamanian Seaway** 

1997 and MacDonald, 2003).

**Figure 1.** Changes in distribution of the main air masses during the main climatic extremes of the last 15,000 years along a north-south transect from the Arctic Ocean to Brazil along the Cordillera of the Americas (modified from Harris, 2010a). Note firstly that the zone of intertropical convergence moved south into northern Brazil during the Late Wisconsin cold event, and secondly, that the latitudinal range of the cold cA/cT air extended some 20o south of the ice sheets almost to the Mexican border along the Cordillera.


**Table 1.** Some mammal families that took part in the Great American Faunal Interchange (after Webb, 1997 and MacDonald, 2003).

#### **6. Closing of the Panamanian Seaway**

242 Biodiversity Conservation and Utilization in a Diverse World

melting ice to the north, so the biota would have had to find refugia in smaller tributary streams. During deglaciation, they would migrate upstream to reclaim their previous habitats, or find new ones. Migratory birds presumably altered their migration patterns as they do today, to dwell in suitable habitats. When a cold event ended, they would adjust their

**Figure 1.** Changes in distribution of the main air masses during the main climatic extremes of the last 15,000 years along a north-south transect from the Arctic Ocean to Brazil along the Cordillera of the Americas (modified from Harris, 2010a). Note firstly that the zone of intertropical convergence moved south into northern Brazil during the Late Wisconsin cold event, and secondly, that the latitudinal range of the cold cA/cT air extended some 20o south of the ice sheets almost to the Mexican border

along the Cordillera.

migrations to make use of the new environments as they became available.

About 3.5 million years ago, the Panamanian Seaway between North and South America began a step-wise closure. The Isthmus of Panama was finally dry land by about 2.4 million years before today. South America had been moving northwestwards and had finally run into the southern margin of the North American plate. The northwards motion of the South America plate is still resulting in volcanism and the formation of the islands in the Caribbean Sea. Both plants and animals have taken part in exchanges between North and South America since then. During Interglacials, Tropical Rain Forest occurred along the Isthmus and permitted the exchange of that biota, but during major cold events, savannah conditions occurred there, allowing an exchange of the biota found in much drier areas (Harris, 2010a). As a result, many of the species of grasses and other vascular plants of Guyana and northeast Brazil are the same as in Costa Rica, Honduras, sub-humid portions of Mexico and the south coast of the United States of America (FNA, 1993 - ?).

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 245

of land snails (*Gastrocopta rogersensis* and *Vertigo meramecensis*) are regarded as having evolved in the Driftless Area (Nekola and Coles, 2001, 2010), while about 40 species of Arctic-Alpine vascular plants evolved in Eastern Beringia (Harris, 2007a; 2008). This represents far more speciation in a given sized population than during the same period in

About 14,000 years ago, there was a marked change in the relative strengths of the air masses affecting North America (Figure 1). This resulted in the northward movement of the climatic zones and the zone of intratropical convergence. It was not a continuous process; the climate fluctuated with both warmer and colder periods, thus complicating the revegetation process. The exact timing of the fluctuations and their areal extent is still being examined. Localized readvances of glaciers provide evidence of these fluctuations, as do variations in pollen, diatom and finger clam distribution at the base of the oldest post-glacial sediments at the bottom of lakes in the formerly glaciated areas. During the early part of the deglaciation process, the Cordilleran ice sheet had rapid local readvances, but in general, it down-wasted in situ with widespread ice stagnation in the valleys in British Columbia and in the Prairies from south-central Alberta across to North Dakota (Alley, 1976, Harris, 1985, Prest et al., 1968). In the Cordillera, the mountain tops appeared first from under the ice and there were numerous lakes in the valleys, with ice blocking the centre of the valley. The former water levels are marked by gravel terraces and hanging deltas along the valley walls. In the main part of central and eastern Canada, the ice persisted as a single entity centered on Hudson Bay (Figure 2) until about 9,000 years ago (Prest et al., 1968). It then split into two parts, one centered on the highlands of northern Québec and the other on the highlands west of Hudson Bay. Complete deglaciation did not occur until 6-7ka in these centers and

The warming first commenced in the south, and slowly and jerkily moved north (Wright, 1970). Thus the prairie vegetation at 20,000 years before present was limited to a narrow north-south zone extending from central Oklahoma to the east coast of Mexico (Ross, 1970). Lehmkuhl (1980) discusses the movement of insects into the evolving Prairies with their harsh temperature regime and unpredictable precipitation. In the case of the grasshoppers that are found today across the Prairies, Ross reports that only 3 out of 82 species are endemics, 31 species have moved in from the west, 34 from Mexico, 10 from the Caribbean coast, and 7 from the eastern part of the United States. In the case of aquatic Mollusca, Clarke (1973) estimates that only about 21 species out of a potential 103 species now populate the western interior of Canada, probably due to the vast distances, poor dispersal mechanisms, and harsh climate. In

The flora and fauna adapted to the cold permafrost land would have had to contract their ranges with some cold-adapted butterflies and rock crawlers (Gylloblattidae) still surviving in small areas on mountain summits along the Cordillera of western North America. The mountain sheep and goats survive in this way, whereas the larger herbivores such as

contrast, mammals and birds could migrate readily across the region.

Mammoths could not survive in the southern parts of their former ranges.

most environments south of the ice sheets.

**8. Deglaciation** 

about 4ka on Baffin Island.

There was also a spectacular interchange of mammalian faunas between the two continents (Table 1, modified from Webb, 1997) that has been called "the Great American Interchange" (Marshall et al., 1982). The mammals from North America have tended to displace the marsupials of South America, whereas the marsupials could only move a limited distance north into the United States due to the cold winters and competition from the indigenous mammalian species. A small camel species crossed the Panamanian Land Bridge into South America and evolved into the present-day Llamas, Alpacas, Guanacos, and Vicunas. However only two genera (*Anacroneuria* and *Amphinemura*) of the Plecoptera which occur as a distinct Austral-American group in South America and two Nearctic groups in North America have managed to cross the Land Bridge (Stewart & Stark, 2002, p.16-17).
