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Transcript
Chapter 24: History and Biogeography Robert E. Ricklefs The Economy of Nature, Fifth Edition (c) 2001 by W. H. Freeman and Company History and Biogeography The origin and maintenance of the earth’s biodiversity is one of ecology’s central issues: has earth’s biodiversity been maintained at a steady state or has it varied through time? the answer to this question can help us choose between equilibrium and nonequilibrium viewpoints We can look to the fossil record for evidence of past changes in biological diversity: the record over the past 600 million years yields useful information about the history of biodiversity (c) 2001 by W. H. Freeman and Company Diversity has generally increased over geologic time Biodiversity in the Fossil Record Findings vary for different groups: diversity has remained constant in some groups, while increasing in others: increasing in flowering plants, fishes, birds, and mammals diversity has also decreased dramatically in many groups at various times: regional species pools have declined because of catastrophic events and grown because of biological diversification we must question whether ecological systems ever truly achieve equilibrium (c) 2001 by W. H. Freeman and Company The Tropical Zone Can we find explanations for tropical biodiversity at large temporal and spatial scales? tropical conditions appeared on earth much earlier than colder conditions tropical and subtropical zones now cover much more area than temperate and polar regions, even more so in times past (c) 2001 by W. H. Freeman and Company A Record of Dramatic Changes Both high and low latitudes experienced drastic fluctuations in climate and extent during the Ice Age of the past 2 million years: during periods of glacial expansion: the tropics experienced low rainfall and reduced temperatures rain forest habitats were restricted and fragmented (c) 2001 by W. H. Freeman and Company Biological History The history of life reveals itself to us in: the geochemical record of past environments fossil traces left by long-extinct taxa geographic distributions and evolutionary relationships of living species the most obvious consequences of this history is the nonuniform distribution of plant and animal forms over the earth’s surface: every part of the earth has a distinctive fauna and flora (c) 2001 by W. H. Freeman and Company Phylogenetic Effects Morphology, physiology, and behavior of organisms reflect: conditions and resources of their present environments constraints imposed by the characteristics of their ancestors: marsupials are presently most diverse in Australia this pattern is not the result of unique ecological properties of the continent of Australia, but rather of evolutionary inertia characteristics shared by a lineage irrespective of environmental (c) factors effects 2001 byare W. H.phylogenetic Freeman and Company Australia’s unusual terrestrial organisms: (a) Eucalyptus; (b) Banksia inflorescence; (C ) red kangaroo Phylogenetic effects influence ecology. Phylogenetic effects affect the structure and functioning of ecosystems: would Australian ecosystems function in the same manner if eucalyptus were replaced by some other kinds of plants? would forests of different kinds of trees be less susceptible to fires? if so, what consequences would this have for ecosystem function? (c) 2001 by W. H. Freeman and Company The history of life can be gauged by the geological time scale. Ecologists recognize key features of the geological record: earth formed 4.5 billion years ago life arose within the first billion years life remained primitive for most of earth’s history ancient physical environments were quite different from those of the present: the early atmosphere had little oxygen and early microbes used anaerobic metabolism increased oxygen led to diversification of complex life forms (c) 2001 by W. H. Freeman and Company The Geologic Record About 590 Mya, most of the modern phyla of invertebrates appeared in the fossil record: these early animals began to protect themselves with hard shells, which make excellent fossils the Paleozoic era is thus the first of three major divisions of geologic time reflecting diversification of animals: Paleozoic: 590 Mya to 248 Mya Mesozoic: 248 Mya to 65 Mya Cenozoic: 65 Mya to present (c) 2001 by W. H. Freeman and Company Hardened outer shells in the Cambrian seas Continental Drift The continents are islands of low-density rock floating on the denser material of the earth’s interior and carried along by convection currents: the movements of the continents over time are called continental drift These movements have two important ecological consequences: positions of continents, ocean basins influence climate continental drift creates and breaks barriers to dispersal (c) 2001 by W. H. Freeman and Company Continental Drift: Mesozoic to Present In the early Mesozoic era, 200 Mya, continents formed a single giant landmass called Pangaea By 144 Mya (beginning of the Cretaceous period) the northern continents (Laurasia) had separated from the southern continents (Gondwana) at this time Gondwana itself was also breaking apart By the end of the Mesozoic era (65 Mya), South America and Africa were widely separated, and many other patterns were emerging. (c) 2001 by W. H. Freeman and Company Positions of the continents have changed over geologic time Mya: 200 million years before) Continental drift changed routes of dispersal (units in millions of years – when dispersal routes were broken or created) Consequences of Continental Drift Details of continental drift have yet to be resolved, but implications for evolution of animals and plants are clear; for example: the distributions of the flightless ratite birds (such as ostriches) are the results of connection between the southern continents that made up Gondwana: these birds are descended from a common Gondwanan ancestor splitting of a widely distributed ancestral population by continental drift is called vicariance (c) 2001 by W. H. Freeman and Company Lineages of ratite birds separated by fragmentation of Gondwana Biogeographic Regions The modern distributions of animals led Alfred Wallace to recognize six major biogeographic regions: these correspond to landmasses isolated millions of years ago by continental drift over the course of this isolation, the animals and plants of these regions evolved independently and developed distinctive characteristics (c) 2001 by W. H. Freeman and Company Major zoogeographic regions of the earth – based on distribution of animals Biogeographic Regions 1 Nearctic - North America maintained connections to Palearctic for 100 My Palearctic - Eurasia shares many groups of plants and animals with Nearctic Ethiopian - Africa has a long history of isolation from the rest of the world (c) 2001 by W. H. Freeman and Company Biogeographic Regions 2 Australian - Australia has a long history of isolation from the rest of the world Oriental - Southeast Asia isolated from rest of tropical world, but has some affinities to Palearctic, where a high percentage of trees are derived from tropical forests Neotropical - South America isolated from Nearctic until about 3 Mya (formation of isthmus of Panama) (c) 2001 by W. H. Freeman and Company Changes in Climate 1 The distribution of heat over the surface of the earth depends largely on circulation of the oceans: 50 to 30 Mya, polar regions were covered by oceans that extended to tropical regions, resulting in much warmer polar climates after this time, drifting continents curtailed this circulation, resulting in a cooling and drying trend at high latitudes result was greater stratification, during later Tertiary, of temperate and tropical biotas with distinctive adaptations (c) 2001 by W. H. Freeman and Company Changes in Climate 2 During the past 2 million years, gradual cooling of the earth gave way to violent oscillations in climate, the Ice Age or Pleistocene epoch: glacial advances drove temperate species southward and may have restricted tropical species to isolated refuges with moist conditions migrations of forest trees in eastern North America have been well documented: after the last glacial retreat beginning 18,000 years ago, a general pattern of reforestation ensued (c) 2001 by W. H. Freeman and Company Changes in Climate 3 Migrations of trees in eastern North America from 18,000 years ago to present are known from pollen grains deposited in bogs and lakes: the compositions of communities shifted as species migrated across the landscape in particular, the composition of forests during the past 18,000 years has: included combinations of species that do not occur today lacked combinations of species that do occur at present (c) 2001 by W. H. Freeman and Company Catastrophes 1 The Mesozoic era (Cretaceous period) ended with a catastrophic disturbance 65 Mya: evidence points to collision of an asteroid with Earth that struck in shallow seas off the Yucatan Peninsula of Mexico: much of earth’s biomass was destroyed by massive tidal waves, fires, and ensuing darkness and cold temperatures among the groups falling victim to this mass extinction were the dinosaurs other groups (birds, mammals) survived and may have taken advantage of empty niches vacated by extinct groups (c) 2001 by W. H. Freeman and Company Catastrophes 2 Major catastrophes have occurred at intervals of 10 to 100 millions of years: such events have disrupted ecosystems and changed the course of community development: thousands of years may be required for environmental conditions to return to normal such events may also: eliminate species and thus reduce diversity foster rapid evolutionary responses to new conditions create opportunities for development of new biological associations (c) 2001 by W. H. Freeman and Company Convergence Convergence is the process whereby unrelated species living under similar ecological conditions come to resemble one another more than their ancestors did: there are numerous examples of convergence: woodpecker-like birds that fill the woodpecker niche in many systems lacking woodpeckers similarities of plants and animals of North and South American deserts similar body forms of dolphins and penguins, which both resemble tuna, whose swimming lifestyle they share (c) 2001 by W. H. Freeman and Company convergence Convergence is often incomplete. Detailed study often turns up remarkable differences between plants and animals occupying superficially similar habitats: the ancient Monte Desert of South America lacks the bipedal, seed-eating, water-independent rodents of North America (kangaroo rats) and Asia (gerbils) superficially similar lizards of Australia and North America differ in diet, optimal activity temperature, burrowing behavior, and annual cycle (c) 2001 by W. H. Freeman and Company Isolation has unique evolutionary consequences. Many of the unique attributes of the reptile fauna of Australia may be related to poor soils: most Australian soils are old, deeply weathered, and have few nutrients plants have low nutrient content and high levels of toxic substances these plants support few insects birds, which depend on insects, are not common released from bird predation, lizards have proliferated in ways not possible elsewhere (c) 2001 by W. H. Freeman and Company Local Community Diversity To what extent are community attributes, such as diversity, convergent? do local processes determine numbers of coexisting species? are numbers of species in communities occupying similar habitats independent of the regional species pool? to what extent do regional differences in diversity also contribute to local diversity? (c) 2001 by W. H. Freeman and Company A Test for Local Control of Diversity If regional processes influence local diversity, then local communities should sample regional species pools in the same proportion: this would result in a linear relationship between local and regional diversity local control would result in saturation, beyond which increasing regional diversity would add nothing to local diversity Available data support the idea that communities are open to invasion at any level of diversity when more species are present in the regional species pool seen in data for fish communities in Africa and South America (c) 2001 by W. H. Freeman and Company Processes on many scales regulate biodiversity. History and geographic position may influence diversity of an entire region and its local inhabitants: interactions of species within local habitats make up only half of the diversity equation! for example, mangroves in the Indo-West Pacific region are far more diverse than mangroves in the Caribbean: both regions have roughly equal areas of a similar variety of mangrove habitats (c) 2001 by W. H. Freeman and Company Indo-West Pacific versus Caribbean Mangroves Differences in diversity in mangroves of these regions appear related to several factors: plant taxa have invaded the mangrove habitat more frequently in the Indo-West Pacific region fewer lineages in the Indo-West Pacific region appear to have suffered extinctions: wet conditions may have prevailed in Southeast Asia through much of the Tertiary, while adjacent terrestrial habitats in the Caribbean many have been dry during the latter Tertiary fragmentation of the Indo-West Pacific habitats may have isolated populations and fostered (c) 2001 by W. H. Freeman and Company speciation Summary 1 Life first appeared several billion years ago, but modern forms appeared about 590 Mya, the point marking the beginning of the Paleozoic era. More recent eras include the Mesozoic (beginning 248 Mya) and Cenozoic (beginning 65 Mya). Continental drift has altered climates and pathways of dispersal among the continents. (c) 2001 by W. H. Freeman and Company Summary 2 Animals and plants have evolved to some extent independently in each of six major biogeographic regions. The climate of the earth cooled during the Cenozoic, leading to greater distinctions between tropical and temperate biotas. Glacial advances during the Ice Age resulted in shifts in distribution and extinctions of many species of plants and animals. (c) 2001 by W. H. Freeman and Company Summary 3 Catastrophes have punctuated the development of life on earth, resulting in mass extinctions and new opportunities for surviving lineages. Convergence is often observed in biota of similar but geographically isolated regions. Nonconvergence in diversities of biotas from similar habitats indicates the role played by regional species pools in determining local diversity. (c) 2001 by W. H. Freeman and Company