**8. Deglaciation**

244 Biodiversity Conservation and Utilization in a Diverse World

**7. The Late Wisconsin Glaciation** 

climatic zones.

Guyana and northeast Brazil are the same as in Costa Rica, Honduras, sub-humid portions

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

This took place between about 25,000 years and 10,000 years ago in North America, although the timing of the glacial maximum is highly diachronous. Harris (2010) describes the evidence for the spectacular southward shift of the climatic belts along the western Cordillera during the main event (Figure 1). Permafrost extended down to Arizona and New Mexico, and the arid areas of Mexico moved south into Venezuela and the Guianas. The intertropical convergence zone moved at least 6 degrees to the south, so that sand dunes developed on the slopes just inland of the present-day coast of the Guianas (Harris, 2010a)

The ice sheets covered most of Canada (Figure 2), bull-dozing the landscape and destroying the vegetation in their paths. The cold events also allowed the migration of the biota adapted to the drier conditions occurring today in the area from the Mid-West down to Mexico to move south into South America across the Isthmus of Panama, since the climatic zones had moved south (Harris, 2010a). The rest of the biota had to adapt very quickly, or for those species living south of the ice sheets, migrate down the mountain sides and find a suitable refugium. Those that could not adapt perished. The Butterfly species, dependent on specific plants for survival, would have had to follow the changes in distribution of those plants. The species that migrate with seasonal changes would have needed to modify their migration patterns, probably shortening the distance of travel to adapt to the changing

Refugia around the ice sheets included Eastern Beringia (Harris, 2004), various islands along the northwest Pacific coast, the chief of which were the Queen Charlotte Islands (Calder & Taylor, 1968), unglaciated areas in southwest Alberta around Plateau Mountain (Harris, 2007a, 2008), the "Driftless Area" of southwest Wisconsin (Nekola & Coles, 2001, 2010), the Grand Banks east of New England, postulated areas in the far north of the Arctic Islands, and the area south of the ice sheets in the United States (Rogers et al., 1991). This was a time of rapid speciation of plants in the northern refugia (Harris, 2007a, 2008), the number of new species increasing with severity of the climatic change in the refugium. At least two species

of Mexico and the south coast of the United States of America (FNA, 1993 - ?).

America have managed to cross the Land Bridge (Stewart & Stark, 2002, p.16-17).

and in northern Venezuela (Rabassa et al., 2005, Rabassa, 2008).

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 about 4ka on Baffin Island.

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

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

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 247

**Figure 3.** Present-day wintering sites and summer breeding range of the Monarch Butterfly (Partly after Urquhart, 1960). In winter, they migrate south, the western population forming giant clusters on groups of trees in California (Brower & Malcolm, 1991), e.g., in a gulley above the Pacific Ocean at Goleta, with the eastern population overwintering at several locations along the Caribbean coast of Mexico and near Mexico City (Wassenaar & Hobson, 1998). The Floridian group is believed to overwinter on the Yucatan

In the Western Cordillera of the United States, the basins dried up and the biota moved up the mountains, forming isolated ecosystems on the various mountain ranges. This accounts for the considerable endemism found in this region in the plants, reptiles and birds. In the case of the Kangaroo rats, the sharp boundaries between species distribution may be partly caused by competition (Brown & Lomolino, 1998). However in general, many species of birds and 80% of the butterflies rely on one or two families or even species of plants for food, so their distribution is limited by the distribution of the food plant. Additional tropical and subtropical species moved north, adding to the species richness of the region. Cacti,

creosote bush and the associated biota moved into the semi-deserts from the south.

In the case of the trans-continental snake *Diadophis punctatus* (Figure 4)*,* phylogeographic studies of the present–day populations in the United States indicate that there are 13 populations with distinctive DNA that are apparently adapted to specific environmental conditions (Fontanella et al., 2008). The nine southern populations have stable distribution boundaries and appear to represent a pre-glacial distribution that was relatively unaffected by the Late Wisconsin cold event (Figure 4). In the case of the 4 northern populations, they

Peninsula.

**Figure 2.** Position of the ice sheets in North America at 10,000 and 7,000 years before present (after Denton and Hughes, 1981). Also shown is the distribution of the proglacial drainage and the proglacial lakes that permitted the fishes and other aquatic biota to migrate northwards across present-day drainage divides. They were followed by many terrestrial snails that require moist microenvironments.

The flora and associated fauna of the more humid regions moved north through Mexico, divided into two coastal biotas by the semi-desert of the interior. The western group moved north into California and the Southern Rocky Mountain States, while the eastern biota moved east along the Gulf coast of the United States before moving further north. The results of this can be seen in the present distribution of the Monarch Butterfly (Urquhart, 1960). It feeds on various species of Milkweed that are found in southeast and southwest Canada, where it spends its summers (Figure 3). In winter, the two populations migrate south, the western population over-wintering on trees in the sheltered coastal canyons near Goleta, while the eastern populations over-winter in Florida or along the eastern coast of Mexico.

The Role that Diastrophism and Climatic Change Have Played in Determining Biodiversity in Continental North America 247

246 Biodiversity Conservation and Utilization in a Diverse World

Mexico.

**Figure 2.** Position of the ice sheets in North America at 10,000 and 7,000 years before present (after Denton and Hughes, 1981). Also shown is the distribution of the proglacial drainage and the proglacial lakes that permitted the fishes and other aquatic biota to migrate northwards across present-day drainage divides. They were followed by many terrestrial snails that require moist microenvironments.

The flora and associated fauna of the more humid regions moved north through Mexico, divided into two coastal biotas by the semi-desert of the interior. The western group moved north into California and the Southern Rocky Mountain States, while the eastern biota moved east along the Gulf coast of the United States before moving further north. The results of this can be seen in the present distribution of the Monarch Butterfly (Urquhart, 1960). It feeds on various species of Milkweed that are found in southeast and southwest Canada, where it spends its summers (Figure 3). In winter, the two populations migrate south, the western population over-wintering on trees in the sheltered coastal canyons near Goleta, while the eastern populations over-winter in Florida or along the eastern coast of

**Figure 3.** Present-day wintering sites and summer breeding range of the Monarch Butterfly (Partly after Urquhart, 1960). In winter, they migrate south, the western population forming giant clusters on groups of trees in California (Brower & Malcolm, 1991), e.g., in a gulley above the Pacific Ocean at Goleta, with the eastern population overwintering at several locations along the Caribbean coast of Mexico and near Mexico City (Wassenaar & Hobson, 1998). The Floridian group is believed to overwinter on the Yucatan Peninsula.

In the Western Cordillera of the United States, the basins dried up and the biota moved up the mountains, forming isolated ecosystems on the various mountain ranges. This accounts for the considerable endemism found in this region in the plants, reptiles and birds. In the case of the Kangaroo rats, the sharp boundaries between species distribution may be partly caused by competition (Brown & Lomolino, 1998). However in general, many species of birds and 80% of the butterflies rely on one or two families or even species of plants for food, so their distribution is limited by the distribution of the food plant. Additional tropical and subtropical species moved north, adding to the species richness of the region. Cacti, creosote bush and the associated biota moved into the semi-deserts from the south.

In the case of the trans-continental snake *Diadophis punctatus* (Figure 4)*,* phylogeographic studies of the present–day populations in the United States indicate that there are 13 populations with distinctive DNA that are apparently adapted to specific environmental conditions (Fontanella et al., 2008). The nine southern populations have stable distribution boundaries and appear to represent a pre-glacial distribution that was relatively unaffected by the Late Wisconsin cold event (Figure 4). In the case of the 4 northern populations, they

are currently migrating northwards into the areas with suitable climates following deglaciation. The preglacial populations in these four areas would have been extirpated in the northern part of these ranges during the cold event. Since the snake has limited ability to migrate over long distances, the adjustment of its northern boundaries of distribution is still taking place as it expands northwards into suitable environments.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 249

movement by the ice in the mountain valleys and the dry area on the Prairies (MacDonald & Cwynar, 1985). Western Red Cedar only penetrated the Coast Ranges into the Interior

**Figure 5.** The influence of the residual ice masses on the migration of some of the common tree species

In the case of the vascular plants of the present-day alpine tundra and boreal forest, revegetation in western Canada came primarily from Eastern Beringia (Figure 6), with only limited northward movement from the southern refugium (Harris, 2007a, 2008). The species in the SW Alberta refugium spread west across the limestone mountains, while those that survived the glaciation around the Queen Charlotte Islands moved north and south along the west side of the Coast Ranges. In the east, the vegetation gradually moved north

following the retreating ice front in the period between 10,000 and 7,000 years ago.

Plateau of British Columbia about 3,000 years ago.

into the deglaciated areas about 10,000 years ago.

**Figure 4.** The distribution of the present-day distinct haplotypes (populations that have distinctive DNA) of the snake, *Diadophis punctatus* (after Fontanella et al., 2008). The distributions of the southern populations are stable, and may have lived in the same area since before the Late Wisconsin glaciations because of an acceptable microclimate. Also shown are the populations that are slowly expanding northwards to occupy areas that are now suitable for its existence, but were too cold for the snake during the Late Wisconsin Glaciation.

The formerly glaciated areas are another matter. The migration of species into the areas vacated by the glaciers was highly dependent on glacial history, distribution of refugia, the climate which limited the possible migrations, and the waterways in the case of aquatic and associated biota.

The downwasting of glaciers in western Canada and North Dakota slowed the recolonization process of trees (Figure 5), even though deglaciation commenced earlier at about 15,000 years ago. The Jack Pine, which survived the glaciations in the northeast United States, was able to move west along the Laurentide ice front, whereas the Lodgepole Pine that used the west coast as a refugium was largely blocked off from eastward movement by the ice in the mountain valleys and the dry area on the Prairies (MacDonald & Cwynar, 1985). Western Red Cedar only penetrated the Coast Ranges into the Interior Plateau of British Columbia about 3,000 years ago.

248 Biodiversity Conservation and Utilization in a Diverse World

during the Late Wisconsin Glaciation.

associated biota.

taking place as it expands northwards into suitable environments.

are currently migrating northwards into the areas with suitable climates following deglaciation. The preglacial populations in these four areas would have been extirpated in the northern part of these ranges during the cold event. Since the snake has limited ability to migrate over long distances, the adjustment of its northern boundaries of distribution is still

**Figure 4.** The distribution of the present-day distinct haplotypes (populations that have distinctive DNA) of the snake, *Diadophis punctatus* (after Fontanella et al., 2008). The distributions of the southern populations are stable, and may have lived in the same area since before the Late Wisconsin glaciations because of an acceptable microclimate. Also shown are the populations that are slowly expanding northwards to occupy areas that are now suitable for its existence, but were too cold for the snake

The formerly glaciated areas are another matter. The migration of species into the areas vacated by the glaciers was highly dependent on glacial history, distribution of refugia, the climate which limited the possible migrations, and the waterways in the case of aquatic and

The downwasting of glaciers in western Canada and North Dakota slowed the recolonization process of trees (Figure 5), even though deglaciation commenced earlier at about 15,000 years ago. The Jack Pine, which survived the glaciations in the northeast United States, was able to move west along the Laurentide ice front, whereas the Lodgepole Pine that used the west coast as a refugium was largely blocked off from eastward

**Figure 5.** The influence of the residual ice masses on the migration of some of the common tree species into the deglaciated areas about 10,000 years ago.

In the case of the vascular plants of the present-day alpine tundra and boreal forest, revegetation in western Canada came primarily from Eastern Beringia (Figure 6), with only limited northward movement from the southern refugium (Harris, 2007a, 2008). The species in the SW Alberta refugium spread west across the limestone mountains, while those that survived the glaciation around the Queen Charlotte Islands moved north and south along the west side of the Coast Ranges. In the east, the vegetation gradually moved north following the retreating ice front in the period between 10,000 and 7,000 years ago.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 251

**Figure 7.** Migration routes of the various species of Succinedae that survived the Late Wisconsin glaciations south of the ice sheets, and moved north about 10,000 years ago following the proglacial drainage shown in Figure 2 (partly after Harris and Hubricht, 1975). Most of these species must live within one or two meters of water, and so they subsequently followed the water draining north from

The biota that live all or part of their life in fresh water and could tolerate cold climates, e.g., fish and some insects (Lehmkuhl, 1980, Morgan & Morgan, 1980), moved along the main drainage-ways and proglacial lakes, e.g., Glacial Lake Agassiz, crossing into other watersheds using the temporary spillways. Many came from the unglaciated areas to the

Lake Agassiz into the south end of Hudson Bay about 8700 years ago.

south, but others came from Beringia.

**Figure 6.** Migration of selected alpine tundra species from the Beringian refugium along the western Cordillera of North America contrasted with the limited northward migration from south of the maximum extent of the Cordilleran ice sheet (modified from Harris, 2007a). This migration had to occur in the short period when it was cold enough for their movement after the down-wasting of the ice sheet and before the mountain valleys became warm enough for invasion by the Montane Boreal Forest.

Various species of Succinidae (Mollusca) survived the glaciations in refugia (Figure 7), either in Beringia (e.g., *Succinea strigata*) or south of the ice sheets in the Rocky Mountains (*Oxyloma nuttalliana*), on the Central Plains (e*.g., S. indiana*), south of the Great Lakes (*Catinella avara, O. retusum,* and *S. ovalis*), on the exposed Grand Banks (now inundated by the sea) or in the unglaciated parts of New England (e.g., *O. groenlandicum* and *O. verrilli*). A new species (*O. missoula*) evolved in the Lake Bonneville drainage, but it did not migrate elsewhere during the Holocene. Most of the species of the Succineids need to be close to water (c. 1-2 m) and the bodies of terrestrial snails consist of about 50% water. They must lay down a thin layer of mucus as they move, so they are particularly dependent on moisture for survival. As a result, they had difficulty moving on to the rocky and sandy substrates of the Laurentian Shield following deglaciation, except where the waters of Glacial Lake Agassiz drained northwards into Hudson Bay.

Climate certainly limited many northward migrations, e.g., many vascular plants and the freshwater bivalves. The latter only entered the previously glaciated Cordilleran areas in northern Washington, northern Montana and in Southern British Columbia. A few species were able to move further north in the Prairie Provinces, but many only just entered Southern Ontario and the Red River Valley in Manitoba. They only occur along the margins of the Shield on the calcareous rocks and sediments in southern Québec, though the late deglaciation and colder climate there undoubtedly inhibited the northward migration.

Agassiz drained northwards into Hudson Bay.

**Figure 6.** Migration of selected alpine tundra species from the Beringian refugium along the western Cordillera of North America contrasted with the limited northward migration from south of the maximum extent of the Cordilleran ice sheet (modified from Harris, 2007a). This migration had to occur in the short period when it was cold enough for their movement after the down-wasting of the ice sheet and before the mountain valleys became warm enough for invasion by the Montane Boreal Forest.

Various species of Succinidae (Mollusca) survived the glaciations in refugia (Figure 7), either in Beringia (e.g., *Succinea strigata*) or south of the ice sheets in the Rocky Mountains (*Oxyloma nuttalliana*), on the Central Plains (e*.g., S. indiana*), south of the Great Lakes (*Catinella avara, O. retusum,* and *S. ovalis*), on the exposed Grand Banks (now inundated by the sea) or in the unglaciated parts of New England (e.g., *O. groenlandicum* and *O. verrilli*). A new species (*O. missoula*) evolved in the Lake Bonneville drainage, but it did not migrate elsewhere during the Holocene. Most of the species of the Succineids need to be close to water (c. 1-2 m) and the bodies of terrestrial snails consist of about 50% water. They must lay down a thin layer of mucus as they move, so they are particularly dependent on moisture for survival. As a result, they had difficulty moving on to the rocky and sandy substrates of the Laurentian Shield following deglaciation, except where the waters of Glacial Lake

Climate certainly limited many northward migrations, e.g., many vascular plants and the freshwater bivalves. The latter only entered the previously glaciated Cordilleran areas in northern Washington, northern Montana and in Southern British Columbia. A few species were able to move further north in the Prairie Provinces, but many only just entered Southern Ontario and the Red River Valley in Manitoba. They only occur along the margins of the Shield on the calcareous rocks and sediments in southern Québec, though the late deglaciation and colder climate there undoubtedly inhibited the northward migration.

**Figure 7.** Migration routes of the various species of Succinedae that survived the Late Wisconsin glaciations south of the ice sheets, and moved north about 10,000 years ago following the proglacial drainage shown in Figure 2 (partly after Harris and Hubricht, 1975). Most of these species must live within one or two meters of water, and so they subsequently followed the water draining north from Lake Agassiz into the south end of Hudson Bay about 8700 years ago.

The biota that live all or part of their life in fresh water and could tolerate cold climates, e.g., fish and some insects (Lehmkuhl, 1980, Morgan & Morgan, 1980), moved along the main drainage-ways and proglacial lakes, e.g., Glacial Lake Agassiz, crossing into other watersheds using the temporary spillways. Many came from the unglaciated areas to the south, but others came from Beringia.

## **9. Altithermal/Hypsithermal event**

The post-glacial warming culminated in a period of time when the mean annual air temperature was about 2 degrees warmer than at present in the Eastern Cordillera of Southern Alberta (Harris, 2002b). It reached its peak about 9,000 years ago in the southern United States, but the peak only affected the Prairie Provinces of Canada from about 6,500- 4,500 years before today. It was the result of a weakening of the cP/cA air masses, resulting in the northward movement of the Arctic front (Figure 1). The vegetation zones migrated up the mountains, while those species that were at the mountain tops were extirpated. The Boreal Forest approached the Arctic Ocean about 5,000 years ago (Anderson et al., 1989, Ritchie and Hare, 1971) with White Spruce having been reported on the Tuktoyaktuk Peninsula at about the same time. It was at this time when the last remnants of the mammalian Megafauna disappeared in the isolated parts of Alaska and Siberia.

The Role that Diastrophism and Climatic Change

Have Played in Determining Biodiversity in Continental North America 253

Prairie by 4,000 years before today, while the Aspen Parkland moved about 1 degree south. These cooler events were separated by warmer intervals at least as warm as today, with that just before the last Neoglacial being even warmer. These changes favoured some species, but caused others to migrate latitudinally or altitudinally, or become extirpated. Similar

**Figure 8.** Distribution of 4 species of the genus *Gastropoda* that exhibit evidence of having extended their ranges westwards and northwards during the Altithermal/Hypsithermal warm event from their main area of present-day distribution. Today, remnants of the extended populations are found in favourable microenvironments along the river floodplains that they followed during the migration process. They are known to have been extirpated in the vicinity of Lake Louise, Alberta, since the late

changes took place throughout the continent.

1800s.

In the southern Canadian Cordillera, it began after 6,830 years ago. The westerly rainbearing mP air mass as well as the cold cA/cP air mass weakened relative to the cT air mass, which was therefore able to move north into the southern Yukon Territory before turning east. This resulted in drought conditions across the Prairies with drying up of the ponds and lakes. The Prairie vegetation moved north of latitude 60° (Strong & Hills, 2003), and these authors concluded that it may have migrated westwards into the interior plateau of the Cordillera along the Liard River valley. Remnants of prairie-type vegetation may be found today around Carmacks and Kluane, Yukon Territory, e.g., *Krascheninnikovia lanata* (Douglas et al., 2001).

Harris & Pip (1973) demonstrated that there was considerable northward migration of land snails along the main river valleys. Tiny land snails that are currently limited to the eastcentral United States of America migrated along the major river flood plains to the Cordillera along the North Saskatchewan and Missouri rivers. Today remnants of these populations can be found surviving at sheltered places, but are slowly being extirpated (Figure 9). Thus *Gastrocopta armifera, G. similis* and *G. pentodon* were present in the vicinity of Lake Louise in 1880 but were extirpated when the CPR railroad was built. There are still some isolated occurrences of these species remaining at sheltered isolated sites along the river valleys.

#### **10. Neoglacial events**

A series of three cold Neoglacial events began about 4,500 years ago during which the MAAT was significantly cooler, and periodic localized increases in precipitation caused the glaciers to advance a short distance down-valley. The balance of the air masses north of about 48 degrees latitude changed (Figure 1), but this did not affect the area further south. There was also no change in the position of the zone of inter-tropical convergence. It was during the first event (3500 years ago) that the western red cedar (*Thuja plicata)* finally migrated up the Skeena River valley from the coast and colonized the wetter portions of the interior of southern British Columbia (MacDonald & Cwynar, 1975). The Prairies exhibited a large area of sand dunes during the last Neoglacial event (Wolfe et al., 2001; Wolfe & Hugenholtz, 2009). That is why the fur traders chose to paddle up the North Saskatchewan River rather than cross the sand dunes to reach the mountains. Similar sand seas existed in Nebraska. In the north, the Boreal Forest retreated 5 degrees to the south on the adjacent Prairie by 4,000 years before today, while the Aspen Parkland moved about 1 degree south. These cooler events were separated by warmer intervals at least as warm as today, with that just before the last Neoglacial being even warmer. These changes favoured some species, but caused others to migrate latitudinally or altitudinally, or become extirpated. Similar changes took place throughout the continent.

252 Biodiversity Conservation and Utilization in a Diverse World

**9. Altithermal/Hypsithermal event** 

**10. Neoglacial events** 

The post-glacial warming culminated in a period of time when the mean annual air temperature was about 2 degrees warmer than at present in the Eastern Cordillera of Southern Alberta (Harris, 2002b). It reached its peak about 9,000 years ago in the southern United States, but the peak only affected the Prairie Provinces of Canada from about 6,500- 4,500 years before today. It was the result of a weakening of the cP/cA air masses, resulting in the northward movement of the Arctic front (Figure 1). The vegetation zones migrated up the mountains, while those species that were at the mountain tops were extirpated. The Boreal Forest approached the Arctic Ocean about 5,000 years ago (Anderson et al., 1989, Ritchie and Hare, 1971) with White Spruce having been reported on the Tuktoyaktuk Peninsula at about the same time. It was at this time when the last remnants of the

mammalian Megafauna disappeared in the isolated parts of Alaska and Siberia.

In the southern Canadian Cordillera, it began after 6,830 years ago. The westerly rainbearing mP air mass as well as the cold cA/cP air mass weakened relative to the cT air mass, which was therefore able to move north into the southern Yukon Territory before turning east. This resulted in drought conditions across the Prairies with drying up of the ponds and lakes. The Prairie vegetation moved north of latitude 60° (Strong & Hills, 2003), and these authors concluded that it may have migrated westwards into the interior plateau of the Cordillera along the Liard River valley. Remnants of prairie-type vegetation may be found today around Carmacks and Kluane, Yukon Territory, e.g., *Krascheninnikovia lanata* (Douglas et al., 2001).

Harris & Pip (1973) demonstrated that there was considerable northward migration of land snails along the main river valleys. Tiny land snails that are currently limited to the eastcentral United States of America migrated along the major river flood plains to the Cordillera along the North Saskatchewan and Missouri rivers. Today remnants of these populations can be found surviving at sheltered places, but are slowly being extirpated (Figure 9). Thus *Gastrocopta armifera, G. similis* and *G. pentodon* were present in the vicinity of Lake Louise in 1880 but were extirpated when the CPR railroad was built. There are still some isolated

occurrences of these species remaining at sheltered isolated sites along the river valleys.

A series of three cold Neoglacial events began about 4,500 years ago during which the MAAT was significantly cooler, and periodic localized increases in precipitation caused the glaciers to advance a short distance down-valley. The balance of the air masses north of about 48 degrees latitude changed (Figure 1), but this did not affect the area further south. There was also no change in the position of the zone of inter-tropical convergence. It was during the first event (3500 years ago) that the western red cedar (*Thuja plicata)* finally migrated up the Skeena River valley from the coast and colonized the wetter portions of the interior of southern British Columbia (MacDonald & Cwynar, 1975). The Prairies exhibited a large area of sand dunes during the last Neoglacial event (Wolfe et al., 2001; Wolfe & Hugenholtz, 2009). That is why the fur traders chose to paddle up the North Saskatchewan River rather than cross the sand dunes to reach the mountains. Similar sand seas existed in Nebraska. In the north, the Boreal Forest retreated 5 degrees to the south on the adjacent

**Figure 8.** Distribution of 4 species of the genus *Gastropoda* that exhibit evidence of having extended their ranges westwards and northwards during the Altithermal/Hypsithermal warm event from their main area of present-day distribution. Today, remnants of the extended populations are found in favourable microenvironments along the river floodplains that they followed during the migration process. They are known to have been extirpated in the vicinity of Lake Louise, Alberta, since the late 1800s.

#### **11. Post-Neoglacial changes**

The last Neoglacial event ended about 110 years ago with the development of the presentday distribution of air masses (Figure 1), resulting in a minor change in temperature, but important changes in precipitation. There was a marked reduction in precipitation in the Eastern Cordillera, causing the vegetation that required more moisture to become limited to the high precipitation areas around Lake Louise (Harris, 2012). After 1943, the precipitation on the Canadian Prairies increased, and the sand dune field has largely become stabilized, though it is now used for irrigation farming. Grassland species such as the Prairie Swift Fox, the Sage Grouse and Black-Footed Ferret have almost been extirpated on the Canadian Prairies. In the last 500 years, the European settlers have gradually modified the landscape, starting in the east and south as well as at isolated coastal regions in the west. This has had an enormous effect on the biota, especially by destruction of habitat. An additional factor is the importation of species from other countries, especially Europe. Forty percent of the flora of Nova Scotia is from elsewhere (Zinck, 1998). Ships discharging water into North American waterways that is used as ballast are introducing fresh water fish and mollusks, e.g., the Zebra Mussel into the rivers and lakes. These then devastate the indigenous species.

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Have Played in Determining Biodiversity in Continental North America 255

Professor G. M. MacDonald (UCLA) kindly commented on an earlier version of part of the

Alley, N. F., 1976. The palynology and paleoclimatic significance of a dated core of Holocene peat, Okanagan Valley, British Columbia. *Canadian Journal of Earth Sciences,* 13: 1133-

Anderson, T. W., Mathewes, R. W. & Schweger, C. E., 1989. Holocene climatic trends in Canada with special reference to the Hypsithermal interval. In Fulton, R. J., Ed.. *Quaternary geology of Canada and Greenland, the Geology of North America,* K-1: 520-528.

Birks, H. H., 2008. The Late-Quaternary history of arctic and alpine plants. *Plant Ecology and* 

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#### **12. Conclusion**

Until the advent of European settlement, climatic changes and diastrophism essentially determined the biodiversity of the biota of North America. The species found today evolved in the last 6 million years in response to the marked cooling of the continent. There had been limited immigration of present-day species from Asia and South America, and little exchange with Europe. The alternating major warm and cold events caused repeated massive migrations latitudinally and altitudinally, unless a given species was fortunate enough to survive in a refugium. Species that could not adapt or migrate quickly enough were extirpated. The climatic changes also resulted in speciation in the vascular flora, though not in many of the insect groups. Speciation in most of the latter takes more time than the duration of most climatic major warm or cold events. This has resulted in a primarily endemic biota that is able to disperse into new environments rapidly. The exceptions are mainly found in the southwest United States on isolated mountain ranges currently surrounded by deserts. Of particular note is the split in the biota of the more humid regions at lower latitudes into eastern and western groups separated by the central semi-arid plains. This split is the result of the early glacial history of the continent. Clearly, the biota of North America has had a unique history that is significantly different to that of the other continents.

#### **Author details**

Stuart A. Harris *Department of Geography, University of Calgary, Canada* 

## **Acknowledgement**

Professor G. M. MacDonald (UCLA) kindly commented on an earlier version of part of the subject matter and Robin Poitras drew the Figures.
