Evolution and geographic distribution of bears have been influenced by continental drift, sea level fluctuations, climatic changes in each region of bear habitat, and geographic shifts in climatic zones, as well as consequent separation, mixing or extirpation of biotas, and evolution of species and bio-communities. This section reviews the general history of these events to set the stage for understanding their effects on bears.
* Continental drift
* Climatic change
o Temperature and moisture
o Seasonality: Pleistocene vs. Holocene
* Sea level fluctuation
* Travel barriers and routes
o Water
o Deserts and grasslands
o Ice and mountains
* Biota
o Mammoth steppe and other grasslands
o Vegetative diversity and distribution
o Habitat carrying capacity
o Animals of possible significance to bears
* Megafaunal extinctions
o Types of species that became extinct
o Causes of extinction
* Current status of the world’s bears.
* Summary
CONTINENTAL DRIFT
Worldwide distributions of plants, animals, and other biota have been strongly influenced by continental drift. This is of most significance to our understanding of bears in terms of the which continents are connected by land bridges, and when these bridges occurred.
Drift began at least as early as the Triassic or Jurassic (Table 2:1), when Pangaea separated into Laurasia and Gondwana, which in turn eventually fragmented into modern continents. Later, some of these continents rejoined in new configurations. Siberia and Alaska touched, forming Berengia in the late Cretaceous (70 MYA--million years ago). During the Eocene (45 MYA), the Indian subcontinent collided with Asia, buckling the Earth's crust at their juncture, creating the Himalayas and adjacent mountains of southeast Asia. After being separated in the Paleocene (60 MYA), North and South America rejoined via the Isthmus of Panama during the mid-Pliocene (6 MYA). (Pileou 1979; Neill 1979). Only the rejoining of North and South America occurred recently enough to directly influence bears.
CLIMATE CHANGE
Changes have occurred in mean annual levels of temperature and moisture, and in seasonal variations of their timing and intensity. These phenomena are important for understanding bears because of the ways in which climate has affected food supply, cover, and other habitat features for bears, and the impacts of this on bear population dynamics. Temporal and spatial variation in climate and vegetation has fostered evolution by bears into a diversity of niches on three continents.
Temperature and Moisture
At the end of the Paleozoic, Earth's climate chilled. Ice covered Gondwana in thick sheets for millions of years before the Earth warmed again. For the next 248 MY during the Mesozoic and early Tertiary, Earth's climate was typically subtropical or tropical. Dominant animals during the Jurassic and Cretaceous included dinosaurs; mammals were insignificant. It was during the Cretaceous that birds and flowering plants proliferated; some of the species were similar to modern ones. Weather during the Cretaceous was mild even as far north as Beringia, which was sometimes lifted above sea level. By contrast, shallow seas encroached on the continents, including portions of what are now North America and Europe. These epicontinental seas left marine sediments on what later became dry land. Among these sediments were limestone formations which were later eroded by streams to form caves. Some of these caves eventually became important to bears. During the Mesozoic and Tertiary, ice was rare even at the poles and mountain peaks. (Pileou 1979; Neill 1979).
The Paleocene was warm and the Eocene even more so, with little polar ice. Mammals began to proliferate. Especially along the coasts, tropical biota extended into what are now temperate latitudes, and subtropical taxa into what is now the subarctic, including Alaska. Nut-producing trees flourished, including oak (Quercus), beech (Fagus), and hickory (Carya), which would eventually become major foods for bears. Temperate forest ranged through Canada, Alaska, southern Greenland, and northern Eurasia. Higher latitudes hosted trees and shrubs such as birch (Betula), willow (Salix), spruce (Picea), pine (Pinus), and hazel (Corylus). (Neill 1979).
In the Oligocene, climate began to cool. Tropical biomes retreated south into Mexico and subtropical forest into the mid-latitudes of North America, although coral still lived as far north as Labrador. Much of the Earth's land was covered by a nearly unbroken canopy of forest (Guthrie 1984). Northern hemisphere deciduous forests of the Tertiary were probably comparable to those still surviving in the American Appalachians and some parts of eastern Asia (Fig. 2:1) (Whittaker 1975). The Rocky Mountains finally rose high enough to produce a significant rain shadow, increasing aridity east of the Rockies; this favored replacement of temperate forests with open woodland and grassland. Epicontinental seas disappeared except along the southeastern coastal plain. Similar changes occurred in Eurasia; epicontinental seas drained and mountain building continued, including elevation of the Himalayas and formation of the Alps. Beringia was submerged. (Neill 1979).
During the Miocene, climate warmed. The European climate was moist and subtropical, with heavy forest (Kurte'n 1976). In North America, tropical vegetation again flourished as far north as Virginia along the east coast, which received abundant rainfall. But in western and central North America the climate continued to become increasingly arid, further reducing wetland and aquatic habitats, and replacing of forests by temperate grasslands. The fluctuation between forest and temperate grassland was repeated numerous times after climatic cooling resumed at the end of the Miocene and during the Pliocene, as well as during the Pleistocene and Holocene. (Neill 1979).
Pliocene elevation of the Sierra and Cascade mountains created a vast arid basin between them and the Rockies. The eastern United States remained wet and forested. The coastal plain drained, joining Florida to the mainland. Fauna belonged mainly to the same families as modern species, although their geographic distributions were quite different. (Neill 1979).
Cooling of the Earth continued gradually through the first 1.5 MY of the Pleistocene, leading to the Great Ice Age. During intermittent periods of the next 0.5 MY, vast amounts of ice accumulated, forming first at the poles and in mountains, then spreading to lower latitudes and altitudes of both hemispheres. Ice covered most of Canada and the northern United States, except for a refugium in Alaska and the Yukon (an area where precipitation was too sparce for much snow to fall, allowing a wide variety of animals to survive there even during glacials) (Fig. 2:2). Individual glaciers formed on high mountains in the Cascades, in the Sierras as far south as southern California, and in the Rockies as far south as Arizona; as well as in Mexico and Central America. Glaciation was also extensive in Europe, but not in Asia. The ice sheet covered most of the British Isles and Scandinavia. South of there, only high mountains were glaciated--for instance the Alps and Pyrenees, and individual peaks from Portugal to Italy, Greece, and Romania, as well as the Atlas mountains of North Africa (Fig. 2:3). The major ice sheet extended from Europe through Asia northeast paralleling the North Atlantic and Arctic coasts to the Taimyr Peninsula. Individual glaciers occurred on mountain ranges as far south as the Himalayas and Hindu Kush, and on individual peaks as far east as Korea and Japan (Fig. 2:4). (Neill 1979).
In the southern Hemisphere, Antarctica and parts of south America were covered by ice sheets. Ice extended from Terra del Fuego through southern Chilie and southwestern Argentina, then along the Andes, including individual high peaks near the equator. (Neill 1979).
The Pleistocene ice made at least four major advances (Table. 2:2; Fig. 2:1), the most extensive of which was the Riss/Illinoian. These advances were intersperced by interglacials when the climate was sometimes warmer than at present, shrinking ice caps below their current size, but probably never eliminating them. Lesser fluctuations in extent of the ice sheets also occurred during each major glacial and interglacial periods Peak advances ("stadials") were punctuated by moderate retreats ("interstadials"). For example, the Wu"rm/ Wisconsin glaciation closed with the Dryas stadial, followed 12,000 YBP by the Allero"d interstadial, and finally 11,000 YBP by the Upper Dryas stadial which lasted until 10,000 YBP--ending the Pleistocene and starting the Holocene. During glaciations, accumulation of vast amounts of water as ice both lowered sea levels and increased aridity on portions of the continents. (Pielou 1979). Much of North America south of the ice sheet was forested during the early Wisconsin glaciation (Wright 1970; Kiltie 1984); but by the late Wisconsin and continuing during the Holocene, forest was progressively replaced by grasslands east of the Rockies.
During cool arid periods of the Pleistocene, the Amazonian rain forest shrank to a few remnant patches (Fig. 2:5), being largely replaced by a paramo-like biome, whereas the reverse occurred during warm moist interglacials. However, parts of Peru that are now desert were well-watered savannah during the late Pleistocene (Campbell 1973).
Postglacially, deserts developed to the south and west of the Rockies. Axlerod (1979) considers them larger than has been typical in the past. The weather patterns that create them may be somewhat new. Moist winds rise from the tropics, move north to temperature latitudes where they drop rain, then turn back toward the equator, absorbing moisture as they approach the tropics. (Neill 1979). Perhaps during Pleistocene glaciations, this arid latitudinal weather band was compressed or eliminated.
Holocene temperatures are not as warm and ice accumulations not as small as in some previous interglacials. So the Holocene may be another interglacial, rather than post-glacial. Holocene temperature peaked about 7000 YBP, followed by cooling that was most severe during roughly 1550 to 1850 AD. (Pielou 1979).
Seasonality: Pleistocene vs. Holocene
The North American climate now tends to be more continental than during the Pleistocene and perhaps earlier glacial periods (Webb 1977). Previous interglacials had cooler summers and usually frost-free winters, whereas glacial winters tended to be colder than modern ones. Seasonal variation in climate increased during the Wisconsin glacial and Holocene (Taylor 1965). Similar changes occurred in other parts of the world, outside of the wet tropics; summers became hotter, winters colder, and precipitation more seasonal (Axelrod 1967; Slaughter 1967). Hibbard (1960) concluded that "present day climates with their seasonal extremes of temperature and aridity are geologically atypical, even in the Pleistocene." (See Kiltie 1984 for a review of contrary opinions).
Increasing continentality during the Holocene seems to have been accompanied by shortening of the growth seasons for plants and animals (Axelrod 1967; Slaughter 1967). Annual variation in timing of growth season onset may have also increased. (Guthrie 1984). "Growth season" for plants refers to the period of increasing size of the plant and/or proliferation of leaves or fruit. Guthrie (1984:269) defines the "growth season" for ungulates as "the average period of the year in which the animals are near their maximum potential for daily somatic growth." Somatic growth refers to increase in stature (muscle, bone, and other non-adipose tissue). By contrast, elsewhere in this book, I include under "growth season" even any period when only fatness mainly adipose tissue increases. Growth seasons for animals, even herbivores, correlate only roughly with those of plants. For phenology varies among species, as does seasonal availability of their nutrient- or calorie-rich tissues. An animal may switch from plant species to species according to when each is most palatable, extending the animal's own growth peak beyond the nutrient peak for any of its forage species.
Glacial, interglacial, and Holocene variations in climate and vegetation may thus account for observed variation in body size of large mammals (Guthrie 1984), including some bears.
SEA LEVEL FLUCTUATION
Sea level changes may have affected bears in at least two major ways, first by increasing or decreasing availability of certain habitats, and second by opening or closing travel routes between continents (North America to Asia, Europe to North Africa), as well as between continents and islands in North America, Asia, and Europe.
Throughout the Mesozoic and Cenozoic eras, sea levels have fluctuated markedly with respect to the continents. For over 150 MY, large portions of the continents were covered by seas, especially along their modern coasts, but also in areas that are now deep inland such as central North America, the Europe-Asia boundary, and much of southern Asia. Draining of those areas was due at least partially to up-warping of the continents by plate tectonics. But glaciation, even prior to the Ice Age lowered sea level by nearly 70 m to levels comparable to our own. (Pielou 1979; Neill 1979).
The most recent of the tectonic sea level changes may have had some effect on bears. But the most influential sea level fluctuations were caused by Pleisticene glaciations. The northern coasts of North America and Europe were depressed by the weight of glacial ice. Accumulation of water in ice lowered worldwide sea level 100-160 m below what we have now. During interglacials, melting of ice to much less than current amounts elevated world sea level over 50 m above the current level; the total global variation in sea level was nearly 230 m. (Pielou 1979; Neill 1979).
TRAVEL BARRIERS & ROUTES
Barriers to travel--including water (mainly oceanic), deserts, grasslands, icesheets, glaciers, and mountain ranges--have had a large impact on previous and modern distributions of bears throughout the world and on individual continents and islands.
Water
Since the formation of Beringia, about 65 MYA, it has been above sea level numerous times, beginning during the Cretaceous, and then again during the Eocene. It was flooded during the Oligocene, drained during the Miocene, again flooded during much of the Pliocene, and intermittently drained during the Pleistocene. Alternate draining and flooding of Beringia were due primarily to buckling of the continental plates during the Mesozoic and Tertiary, but to glacial-caused changes in sea level during the Pleistocene. When Berengia was drained, it provided a land bridge between North America and Asia over which many species passed. Travel was probably easiest during the Mesozoic when the high latitude climates were mild. During the late Tertiary and especially the Pleistocene, only more cold tolerant species could survive the trip. (Neill 1979) It was then that bears immigrated from Asia to North America (Kurte'n & Anderson 1980).
South America separated from Africa during the early Cretaceous, about 100 MYA. Up to about that time, South and North America were connected by the Proto-Antilles (Pielou 1979). This chain of mountains may have once provided a complete land bridge, judging by the great similarity of reptilian fauna on the two continents (Simpson 1965), but which sank except for their peaks. The Proto-Antilles eventually moved east relative to the Americas, forming the Greater Antilles islands (Pielou 1979). For many millions of years, South America was largely isolated. Then, during the late Tertiary, new islands formed between the continents. Finally, about 6 million years ago, this area rose above sea level, creating a land bridge, the Isthmus of Panama. (Pielou 1979). Thus, South American biota evolved in 3 phases: pre-isolation, isolation, and post-isolation (Simpson 1965). Bears are post-isolation colonists.
Africa separated from Eurasia in the early Tertiary, then rejoined in the later Tertiary, about 17 MYA, forming land bridges at Suez which is now above sea level, and at Gibralter which is now flooded. However, during some Pleistocene glaciations, lower sea levels allowed bears and other species to cross the Strait of Gibralter and colonize North Africa.
Movement of biota from Asia to offshore islands and vice versa depended heavily on declines in sea level exposing land bridges. Areas of the continental shelf joining the Asian mainland to the islands of Japan, Taiwan, Borneo, Java, Sumatra, and Sri Lanka were shallow enough to have been drained during glaciations. This permitted migration of terrestrial species, including bears. But these animals could not reach islands such as the Celebes that were separated by deep oceanic trenches. (Pielou 1979; Neill 1979).
Lowering of sea level during glaciations also permitted bears to colonize offshore islands in Alaska (Kodiak, Admirality, etc.) and British Columbia. Given that some of these islands were glaciated during the Pleistocene (Pileau 1979:Fig. 4:1), it is not obvious what habitat existed on them for bears during periods when sea level would have been low enough for colonization.
Deserts and Grasslands
Deserts in North America, Asia, the mid-East and North Africa (Fig. 2:1) may have also been barriers to biotic dispersal. But the degree of impairment could have depended on degree of aridity. There is reason to question whether those areas were as arid and impassable during glaciations as currently. At least intermittently, North American deserts were smaller and milder during the Pleistocene, than post-glacially (Axlerod 1979; Guthrie 1984). The same could have been true in Eurasia and Africa. It is noteworthy that during the Holstenian and Eemian interglacials, Europe supported numerous mammals--hippo (____), rhino (Dicerorhinus spp.), hyaena (Crocuta crocuta), leopard (Panthera pardus), and lion (P. leo)--that are now typically African (Kurte'n 1976). At one or more periods, these species must have travelled between Africa and Eurasia. This would have been facilitated if the sub-tropical latitudes were moister than currently.
In grasslands, aridity may have been less of a limiting factor to some species. But for various animals including bears, that depend on trees for specific foods, hiding cover, or arboreal refuges, wide belts of grassland could have been impassable.
2:IV.C. Ice and Mountains
During Pleistocene glaciations, the aridity of eastern Asia, Beringia, Alaska, and the Yukon kept those areas largely ice free. So animals and plants that could tolerate the arid, cold climate could migrate from Asia across Beringia to the Alaska-Yukon refugium, or vice versa. But the path to rest of North America was blocked by ice (Fig. 2:_) until an interglacial. No ice barrier existed to separate major continental habitats in Asia or Europe. But travel was blocked on a local scale by glaciated mountain ranges such as the ... Himalayas, and Hindu Kush in Asia. In North America, travel was likewise blocked by glaciated portions of the Cascades, Sierras, and Rockies. The Andes probably provided a comparable barrier in South America. For a smaller variety of organisms, mountains are also barriers even during non-glacial periods.
BIOTA
The evolution of bears has probably been shaped by changes over time in plant cover; plant food types, quality and abundance; animal prey/carrion; predators on the bears themselves; competitors; and other enemies. Insight on these phenomena will be derived later based on the following information about characteristics of Pleistocene and Holocene flora and fauna.
Mammoth Steppe And Other Grasslands
During the Mesozoic, relatively moist climates fostered forestation of most land areas. This continued during the early Tertiary. But by the Miocene, increasing aridity favored reduction of forests and formation of other biomes, including extensive shrublands and grasslands. Temperate grasslands were periodically replaced by forest, but in general temperate latitude grasslands were much more common during the Cenozoic than previously. (Neill 1979; Guthrie 1984).
Ungulates had originally evolved mostly as forest dwellers. But the spread of unforested habitats, especially grasslands fostered a wave of evolution that produced a phenomenal diversity of ungulates in the northern hemisphere. (Guthrie 1984).
During Pleistocene glaciations, the grasslands that occurred at high latitudes, from western Canada and Alaska through Asia to England, comprised a distinctive biome that Guthrie (1984) calls the "mammoth steppe." Although it encompassed the same plants as modern tundra, it also included many species now found only in the temperate zone. The plant diversity of mammoth steppe was more comparable to that of the modern African savannah than to any other extant grassland--enabling the mammoth steppe to support a comparably great diversity of ungulates. In more specific terms, however, the mammoth steppe has no post-glacial counterpart. (How these biomes changed during the brief interglacials, which account for about 10% of the Pleistocene, isn't known.) (Guthrie 1984). He states (p.259) that:
The most curious aspect of this mammoth steppe is its peculiar mixture of biota, without counterpart among modern biomes. This is why the species associations we find there appear so disharmonius. Many forms appeared in Alaska which are now associated with southern grasslands: badgers (Taxidea), ferrets (Mustela), bison (Bison), and grama grass (Bouteloua). Eurasiatic forms like saiga antelope (Saiga), horses (Equus), and lion (Panthera) were mixed with the North American short-faced bears (Arctodus), and camels (Camelops). Other species stretch the imagination: sabertooth cats (Homotherium), yaks (Bos), [mastodons (Mammut), wooly mammoths (Mammuthus),] and bonnet-horned musk oxen (Symbos). All these appeared in a heterogenous mix with large mammals present in Alaska today: moose (Alces), caribou (Rangifer), muskoxen (Ovibus), sheep (Ovis), and others ....
Vegetative Diversity and Distribution
Vegetative diversity in Pleistocene biomes was generally higher than in extant biomes. It was once believed that Pleistocene biomes were much like modern ones. The occurence of northern species at low latitudes and southern species at high latitudes were attributed to biomes being shifted south and compressed during glacials, then shifted north and expanded during interglacials. However, more recent evidence on plants and animals indicates that many species had a much wider latitudinal range during the Pleistocene simultaneously. Northern species occured in the south concurrently with southern species and vice versa. Furthermore, Pleistocene flora contained a few species which have since become extinct. Strong latitudinal (and probably altitudinal) zonation of biomes, such as we find now, is a Holocene phenomenon. Pleistocene biomes were more patchy--a distinction comparable to stripes vs. plaids. (Guthrie 1984). Vegetative changes during and after the Pleistocene exerted strong selection pressures on ungulates and other herbivores, probably including bears.
Guthrie attributes the plaid vs. stripe difference in geographic patterning of biomes to differences in climate. Although the Pleistocene was colder during glaciations, but sometimes warmer during interglacials than is the Holocene, seasonal variation in temperature and humidity was apparently less and growing seasons for plants and animals longer during the Pleistocene. Also, the climate may have varied less with latitude (and perhaps altitude) then than now.
Habitat Carrying Capacity
It is likely that Pleistocene habitats tended to have higher carrying capacity than modern ones. There are at least four reasons for thinking this: (1) The soils may have been richer (Guthrie 1984), due in part to current or recent glaciation providing abundant silt for the formation of new or deeper soils. The richness of soils derived from glacial silt is well known to agriculturalists (Geist 1978, pers. comm.). As those soils aged into the Holocene, their depth may have been reduced by erosion, and some nutrients depleted. (2) A longer growing season for plants, when they are rich in nutrients while low in fiber and toxins, would have produced more food for herbivores, including bears and some of the species they prey upon or scavenge. (3) A wider diversity of plants would have increased the likelihood that bears and other herbivores would have been able to find some nutritious food within their home ranges at any time of their own growth season and perhaps during their "off-season" (which is when some bear species hibernate). High plant species diversity facilitates an herbivore moving from species to species, foraging on each plant during its peak of palatability; diversity also reduces the likelihood that all food sources will be impaired by bad weather or some other factor during a given season or year. In other words, vegetative simplicity promotes a boom-bust food supply, whereas diversity provides a more dependable, steady high-quality supply. (4) Levels of anti-herbivory toxins in many plants probably increased from the Pleistocene to the Holocene (Guthrie 1984).
Animals of Possible Significance to Bears
Most plants living during the Pleistocene still survive, and no major new taxa have evolved during the Holocene. So changes in plant food supply for bears during the Pleistocene and Holocene would have involved mainly changes in species associations, geographic distribution, and anti-herbivory defenses. By contrast, there have been major changes in fauna during the same period. Many species of large herbivores and predators, including some predatory/scavenging birds, have become extinct or suffered drastic range reductions. In toto, these faunal changes could have had substantial impact on the evolution and geographic distribution of bears. The impacts of specific fauna will be addressed later in the book. For now let us concentrate on the general topic of Pleistocene-Holocene extinctions, mainly of megafauna.
MEGAFAUNAL EXTINCTIONS
During the Pleistocene, bears underwent considerable evolution, including the appearance of new species, modification of old ones, and geographic displacement or complete extinction of others. Extinctions were particularly common at the end of the Pleistocene when so many other large mammals and birds were extirpated or suffered severe range reductions. To the extent that the general causes of megafaunal extinction can be understood, they may help us understand why some species of bears disappeared while others remain--for now.
Megafaunal extinction was a worldwide phenomena, but concentrated on the continents occupied by bears--North America, South America and Europe--as well as Australia, with lesser extinctions in Asia and especially Africa. It encompassed a variety of "large" mammals, that is species variously characterized as being above the size of lagomorphs (Guthrie 1984) or above 44 kg adult size (Martin 1984). It also included numerous birds, mainly large species, that were either commensals with large mammals or scavenged large mammal carrion (Steadman & Martin 1984). Other taxa suffered fewer extinctions, some of which may have also been consequences of megafaunal culling (Guthrie 1984).
Two major causes of extinction have been proposed: climatic deterioration or overkill by humans. Biologists differ widely in their estimate of the relative importance of these two factors, but few doubt that both were involved (see Martin & Klein 1984).
Types of Species That Became Extinct
That large species--among mammals and birds--suffered the highest rates of extinction is now obvious. But what other traits distinguished those which perished from those which survived? Size is certainly not the only factor, since some species that survived are as large or larger than some sympatric species that perished. Clearly, too, fate was not merely a matter of differences between species themselves; for some species extirpated on one continent or region survived elsewhere. For example, horses and tapirs were extirpated from North America, but survived in Asia and South America, respectively. Another major variable identified by Guthrie (1984) is the type of digestive system. Although Guthrie's analysis focuses on Holarctic ungulates, it is the most comprehensive theory I have encountered on ecological changes over the Pleistocene-Holocene transition, changes which may be highly relevant to bears.
Photosynthetic or Structural Plant Tissue Feeders
Holarctic ungulates that feed on photosynthetic ("green") tissue or structural plant tissues fall into two categories according to their digestive systems: ruminants or monogastric caecalids. Both groups eat monocot and dicot vegetation, including leaves, stems, and sometimes twigs or bark. Growing leaves and seeds may contain fairly high concentrations of protein or/and lipid. The leaves and sometimes stems of rapidly growing forbs and grasses, or the leaves of shrubs and trees, may contain up to 20-30% protein (dry-weight), whereas at maturity these same tissues contain as little as 3-4% protein. Lipids (mainly galactolipids) in these tissues are also most abundant during early growth. Availability of lipids declines with both maturity of the tissue and later drying. (Robbins 1983:236).
Green vegetation provides energy to herbivores mainly in the form of carbohydrates. Non-structural carbohydrates such as soluable sugars and starches are readily digestible even by monogastrics, including humans and bears. Hemicellulose, a structural molecule, can be partly digested in the gastric stomach. But complete digestion of hemicellulose and other structural carbohydrates such as pectin and cellulose requires the aid of fermentation by microflora. Such microflora are housed in the rumen-reticulum of ruminants and in the caecum of caecalids. In sequence along the digestive tract, a rumen-reticulum occurs before the gastric stomach, whereas a caecum occurs between the stomach and the colon. Pectin is fermented more readily than cellulose. Although cellulose is abundant in both monocots and dicots, pectins are most prevalent in dicots. (Robbins 1983).
Other plant structural compounds include lignin, cutin, suberin, and silica. Lignin is undigestible even with the aid of fermentation; cutin-suberin is only slightly digestible. But both groups of compounds--occurring in woody tissue and herbs, but scarce in grasses--impair digestion of cell wall polysaccharides. Silica content also increases proportionately as a plant matures, lowering digestability. Silica is less abundant in dicots than in grasses, sedges, and many lower plants. Important non-structural compounds include anti-herbivore toxins whose impacts range from repelling the herbivore to poisoning it or/and its microflora. (Robbins 1983). In general dicots tend to be more toxic than monocots, which depend more on silica for protection from herbivores (Guthrie 1984). Toxicity tends to increase in green tissue as it matures or after nearby tissue has been stressed or bitten by herbivores (Robbins 1983; Guthrie 1984).
Whether an ungulate has a rumen or only a caecum determines how much nutrition it can derive from different foods and how susceptible it is to plant toxins. A rumen permits degradation of some plant toxins; thorough digestion of structural carbohydrates; and production of a balanced diet of amino acids, fatty acids, vitamins, etc. before these foods reach the stomach and intestine. This assures minimal poisoning by toxins and maximum absorption of nutrients. However, to obtain these benefits, the ruminant must maintain a fairly high quality (low fiber) diet. Ruminants lack an efficient mechanism for separating indigestible highly fibrous tissue (e.g., grass stems) from more digestible and nutritious tissue. So fiber will accumulate while higher quality tissue is fermenting, until fiber eventually blocks the digestive tract. (Guthrie 1984).
By contrast, a caecalid passes the vegetative matter through its stomach removing readily-digestible compounds before passing food through the intestine, where fermentable plant tissue is passed into the caecum and indigestible roughage shunted to the colon for excretion. This is less efficient for protecting animals from plant toxins and for absorbing nutritious byproducts of microfloral fermentation. But it allows caecalids to subsist on coarser forage (e.g., grass stems) than ruminants. Coarse forage tends to be energy-rich but nutrient-poor (protein, vitamin, lipid, etc.), and low in toxins. However, caecalids must supplement their bulk intake of coarse graminoids with a varied diet of more nutritious but more toxic forbs and shrub/tree leaves. To obtain adequate nutrition and avoid poisoning, caecalids need more forage variety than ruminants. Caecalids eat only a little of each toxic plant, such that interactions among these compounds in the stomach can detoxify some of them, and that the amounts absorbed into the bloodstream do not excede detoxifying capacity of the liver. It can handle a little of each of several toxins better than a comparable total amount of one toxin. (Guthrie 1984).
Guthrie points out that although some ruminants (especially large ones) were extirpated from the Holarctic, monogastric caecalids such as mammoths, mastodonts, and equids were hardest hit--all of them being eliminated from the higher latitudes of North America where they once flourished.
So too among sloths, large monogastrics may have been more vulnerable than smaller ruminant-like species. Families of (extinct) ground sloths include Megalonychidae, Megatheriidae, and Mylodonitidae. Guthrie (1984) describes the giant Sphaeralcea as monogastric. Analysis of its fossil dung from over a 30,000 yr period show that the toxic plant mormon tea (Ephedra) gradually replaced other supplements to its dietary staple globemallow (Sphaeraicea), perhaps because those other plants progressively disappeared from ground sloth range. By contrast, Eisenberg (1981) describes the arboreal extant sloths (Choloepus and Bradypus of family Bradypodidae) as having a multi-chambered stomach--suggesting that it is analogous to an ungulate rumen, where leaves or grasses (graminoids) are fermented and toxins degraded before entering the pyloric stomach. These sloths also have very long gestation--11.5 months in Choloepus, followed by a 2-4 month lactational anestrus; interbirth interval is 14-15 months (Eisenberg 1981). Reproduction was presumably even slower in the extinct ground sloths.
Lagomorphs and some other taxa of medium-sized mammals have a caecum for fermentation.
Eaters of Other Plant Tissue
Monogastric mammals (including bears) that lack a rumen or caecum specialized for fermentation of fiber are less able to subsist mainly on green plant tissue. It is eaten primarily during early growth when it is least fibrous, silaceous and toxic. For species whose teeth can withstand the wear of grinding silica-rich tissue, foraging may focus more on the green tissue of low-toxin monocots than on that of high-toxin dicots. These herbivores may also depend heavily on fruits, nuts, roots, bulbs, corms, or tubers.
Plant leaves and stems tend to be fairly uniform in their energy content (3.9-5.1 kcal/g dry weight), mainly in the form of carbohydrates. Seeds tend to be richer in energy and to vary more between taxa. Seed energy content averages lowest for grass (4.5 kcal/g), then progressively higher in deciduous shrubs and trees (5.0 kcal/g), then coniferous trees (6.1 kcal/g). (Robbins 1983).
The seeds in fruits and nuts are propagules designed to move a plant's offspring out of range of competition with its parent, and secondarily to colonize new habitat (Neill 1979). Seeds are transported by animals that eat them. Seeds dependent on animal transport for dispersal are "designed" to be palatable to animals, yet for at least some of the seeds to survive ingestion and digestion such that they emerge with the feces in viable condition, perhaps able to exploit nutrients in the feces for their later growth. Thus, seeds commonly contain toxins (for instance tanin in acorns [Quercus spp.]) that protect them from digestion during their passage through a herbivore's gut. Furthermore, fleshy fruits may be high in toxins before they ripen (hence their strongly bitter or sour taste) to prevent fruit consumption before the seeds mature; ripening includes detoxification (Robbins 1983). Some non-poisonous plants produce substances that mimic the bitter or sour flavors of toxins to deter herbivores.
Underground plant organs, including roots, bulbs, corms and tubers, sometimes store plant nutrients, for instance as starch--as in familiar crops such as potato, carrot and beet. Reproductive organs may also be buried to protect them from excessive heat or cold; this is particularly common in deserts and the arctic tundra (Colinvaux 1973). Underground storage of nutrients and buds also protects them from many herbivores; hence some species of tuber are relatively unprotected by toxins poisonous to mammals, making them attractive to the few species able to reach them.
Certain highly digestible fungi such as truffles (Tuberales) grow underground. No truffle is known to be poisonous to animals or humans. Dispersal of their spoors is aided when they are dug up and consumed by a mammal; importance of this mode of dispersal isn't known. Some above-ground fungi are also readily digestible and non-toxic for most mammals. (Lincoff 1981).
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The evolutionary "arms race" between plant "weapons" (structural and chemical) and herbivore defenses may have played a significant role in megafaunal extinctions and generally in the evolution of bears.
CAUSES OF EXTINCTION
Guthrie (1984) attributes the greater extinction of monograstric caecalid ungulates over ruminants to declines in forage induced by climatic change and by degradation of soils (presumably through erosion, leaching and perhaps exhaustion of some nutrients). A Holocene shortening of the plant growth season and an increase in seasonal weather extremes have reduced plant diversity throughout much of the Holarctic and probably other continents. Simplification of plant communities and their intensified latitudinal zonation increased selection pressures for antiherbivory defenses. The Pleistocene-Holocene transition was thus characterized by reduction in plant diversity, higher plant toxicity, shorter growth seasons for plants and animals, and perhaps less predictable growth seasons.
Monogastric caecalid ungulates were more vulnerable to these changes than ruminants. Declining diversity of plant communities impairs a caecalid's opportunity to find enough forage diversity to meet its nutrient needs while avoiding poisoning by the increasingly toxic plants. Caecalids were also more vulnerable to shortening of the growth season because they tend to have longer gestation lengths than comparably-sized ruminants, a difference that was compounded by extremely large size of some caecalids such as mammoths and mastodonts; gestation length is directly related to size. (Guthrie 1984; Kiltie 1984).
One might expect similar vulnerabilities by other kinds of herbivorous mammals that eat toxic plant tissues. This is illustrated by sloths. If the extinct giant ground sloths were indeed monogastric (Guthrie 1984) and extant species indeed ferment vegetation in a rumen-like organ (Eisenberg 1981), extinction of the slow-gestating monograstric taxa while the quicker-gestating ruminant-like species survived, would be comparable to the pattern Guthrie identifies for ungulates.
Changes in vegetation and herbivores would have in turn affected other vertebrate and invertebrate taxa, including dependent predators, scavengers, and commensals.
Guthrie's hypothesis of a decline in vegetative carrying capacity and thus in nutritional status for ungulates is supported by the fact that among both the species that became extinct and those that remain, there was reduction in body size and in "low priority growth tissues" such as horns and antlers (Guthrie 1984). I have found no mention of whether size of the brain, which includes other low priority tissues (Geist 1978), also decreased.
There are reasons to be skeptical of any of the hypotheses linking extinctions to climate. For example, Kiltie (1984) argues that increased seasonality and reduced growth seasons are unlikely to have eliminated large ungulates, despite the long gestation lengths of these species.
The major alternative theory for Pleistocene extinctions is overkill of the fauna by humans. There is some parallel between the spread of humans and the timing of megafaunal extinctions; and humans are known to have hunted some of these species (Martin 1984). Although the megafauna may have been particularly able to defend themselves against humans, their low rates of reproduction would have impaired compensation for the losses that did occur. (Martin 1984; Kiltie 1984). Nevertheless, many of the extinct species show no evidence of having been hunted by humans or dependence on hunted species; and many coexisted with humans for thousands of years without extinction (Kurte'n 1976; Anderson 1984; Guthrie 1984). Furthermore, modern observations on difficulties of humans killing large ungulates and predators with primitive weapons raise serious doubt that even more primitive cultures would have fared better against the larger and sometimes more numerous Plistocene megafauna--particularly species such as the saber- or scimitar-toothed felids and the cave bear (Geist 1989b). Simulation models have been devised to test whether human predation could have accounted for so much extinction, but the results are inconclusive (Whittington & Dyke 1984)--which is not surprising, considering our limited understanding of and ability to model even ungulate population dynamics, to say nothing of a complex system of herbivore-herbivore-predator-habitat relationships.
The relative impacts by climatic change and human hunting remain highly controversial. But at least as far as bears are concerned, climatic change seems a more feasible mechanism, as will be discussed later.
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Having set the stage by identifying characteristics of the evolutionary environment of bears, let us now address their phylogeny, ecological radiation, and geographic dispersal.
CAUSES OF THE SHIFT
FROM PLAID TO STRIPED BIOME ZONATION
In support of the hypothesis that the altered climate could have reduced local diversity and increased latitudinal zonation of biota, Guthrie (1984) offers the following statements:
1) I argue that this Pleistocene species diversity is primarily the character of the growing season combined with the kinds, degrees, and variations of seasonal stress. Given a long and internally varied growth season, plant competition can result in temporal or phenological species displacement rather than spatial elimination. Thus a long and varied growth season on a productive substrate increases sympatric tolerance. This diversity normally occurs in a moderate to fine-grain mosaic. The complex vegetative mosaic is then reflected in the animals: insect species emerge and reproduce in a sequential pattern, tracking flowering and seed production. The diversity is seen among birds and mammals which also experience more heterogeneous associations. (p.266).
2) The Holocene's less fertile soil and a less varied and shortened growth season increased plant competition. The resulting competitive displacement sorted species more geographically--a shift from plaids to stripes (fig. 13.1). The shortened growth season heightened the competitive edge of some species. In a local area some species should exclude others that could potentially grow in the same circumstances but could not survive the new competitive pressure. (p. 267).
3) Axlerod (1967) argued] that one finds the highest species diversity in mild climates, without marked seasonal extremes. Such equable climates allow species which cannot tolerate cold winters to live beside those species which cannot tolerate hot summers. As the climate shifts to a more continental regime marked by hot summers and cold winters, species diversity declines. (pp. 287-288.).
Transition to the Holocene with a more variable climate, more extensive grasslands and deserts, and shorter growing seasons accompanied increasing seasonality. In a physical environment with more stable physical conditions, natural selection shifted emphasis from meeting the challenges of physical stressors to the challenges of interspecific competition. Individual plant species became more specialized for the narrower ranges of climatic seasonality; specialists out-competed generalists. Particular species became confined to fewer, less diverse biomes; and these biomes commonly occurred as east-west (latitudinal) and altitudinal strips. And plants increased certain classes of toxins. Simplification of the flora, and its latitudinal and altitudinal zonation, along with increasing toxicity of many plant species, favored ruminant over caecalid ungulates.
One "finds the highest species diversity in mild climates, without marked seasonal extremes. Such equable climates allow species which cannot tolerate cold winters to live beside those species which cannot tolerate hot summers. As the climate shifts to a more continental regime marked by hot summers and cold winters, species diversity declines" (Guthrie 1984, paraphrasing findings by Axelrod 1967).
Nevertheless, as is well known from the rocky intertidal zones and estuaries of sea coasts, steep environmental gradients can promote high species diversity within small areas. Not only are these areas ecotones (sea-land and sea-river, respectively), but there is an "altitudinal" gradient in how much of the time each horizon of the intertidal zone is exposed to marine vs. terrestrial conditions, and how long each horizon of the estuary is exposed to marine vs. aquatic/brackish conditions. Great interspecies variation in tolerances produces "altitudinal" zonation of the biota. (REF REF). Analogously, increasing latitudinal and altitudinal gradients in weather severity and in growing season length might have accentuated corresponding zonation in biota.
Chapter 7.
GEOGRAPHY, CLIMATE & BIOTA
Part II:
EVOLUTIONARY ENVIRONMENT