(c) 2001 by W. H. Freeman and Company 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 zThe origin and maintenance of the earths biodiversity is one of ecologys central issues: yhas earths biodiversity been maintained at a steady state or has it varied through time? xthe answer to this question can help us choose between equilibrium and nonequilibrium viewpoints zWe can look to the fossil record for evidence of past changes in biological diversity: ythe record over the past 600 million years yields useful information about the history of biodiversity Diversity has generally increased over geologic time (c) 2001 by W. H. Freeman and Company Biodiversity in the Fossil Record zFindings vary for different groups: ydiversity has remained constant in some groups, while increasing in others: xincreasing in flowering plants, fishes, birds, and mammals ydiversity has also decreased dramatically in many groups at various times: xregional species pools have declined because of catastrophic events and grown because of biological diversification ywe must question whether ecological systems ever truly achieve equilibrium (c) 2001 by W. H. Freeman and Company The Tropical Zone zCan we find explanations for tropical biodiversity at large temporal and spatial scales? ytropical conditions appeared on earth much earlier than colder conditions ytropical 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 zBoth high and low latitudes experienced drastic fluctuations in climate and extent during the Ice Age of the past 2 million years: yduring periods of glacial expansion: xthe tropics experienced low rainfall and reduced temperatures xrain forest habitats were restricted and fragmented (c) 2001 by W. H. Freeman and Company Biological History zThe history of life reveals itself to us in: ythe geochemical record of past environments yfossil traces left by long-extinct taxa ygeographic distributions and evolutionary relationships of living species zthe most obvious consequences of this history is the nonuniform distribution of plant and animal forms over the earths surface: yevery part of the earth has a distinctive fauna and flora (c) 2001 by W. H. Freeman and Company Phylogenetic Effects zMorphology, physiology, and behavior of organisms reflect: yconditions and resources of their present environments yconstraints imposed by the characteristics of their ancestors: xmarsupials are presently most diverse in Australia xthis pattern is not the result of unique ecological properties of the continent of Australia, but rather of evolutionary inertia xcharacteristics shared by a lineage irrespective of environmental factors are phylogenetic effects Australias unusual terrestrial organisms: (a) Eucalyptus; (b) Banksia inflorescence; (C ) red kangaroo (c) 2001 by W. H. Freeman and Company Phylogenetic effects influence ecology. zPhylogenetic effects affect the structure and functioning of ecosystems: ywould Australian ecosystems function in the same manner if eucalyptus were replaced by some other kinds of plants? xwould forests of different kinds of trees be less susceptible to fires? xif 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. zEcologists recognize key features of the geological record: yearth formed 4.5 billion years ago ylife arose within the first billion years ylife remained primitive for most of earths history yancient physical environments were quite different from those of the present: xthe early atmosphere had little oxygen and early microbes used anaerobic metabolism xincreased oxygen led to diversification of complex life forms (c) 2001 by W. H. Freeman and Company The Geologic Record zAbout 590 Mya, most of the modern phyla of invertebrates appeared in the fossil record: ythese early animals began to protect themselves with hard shells, which make excellent fossils ythe Paleozoic era is thus the first of three major divisions of geologic time reflecting diversification of animals: xPaleozoic: 590 Mya to 248 Mya xMesozoic: 248 Mya to 65 Mya xCenozoic: 65 Mya to present Hardened outer shells in the Cambrian seas (c) 2001 by W. H. Freeman and Company Continental Drift zThe continents are islands of low-density rock floating on the denser material of the earths interior and carried along by convection currents: ythe movements of the continents over time are called continental drift zThese movements have two important ecological consequences: ypositions of continents, ocean basins influence climate ycontinental drift creates and breaks barriers to dispersal (c) 2001 by W. H. Freeman and Company Continental Drift: Mesozoic to Present zIn the early Mesozoic era, 200 Mya, continents formed a single giant landmass called Pangaea zBy 144 Mya (beginning of the Cretaceous period) the northern continents (Laurasia) had separated from the southern continents (Gondwana) yat this time Gondwana itself was also breaking apart zBy the end of the Mesozoic era (65 Mya), South America and Africa were widely separated, and many other patterns were emerging. 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) (c) 2001 by W. H. Freeman and Company Consequences of Continental Drift zDetails of continental drift have yet to be resolved, but implications for evolution of animals and plants are clear; for example: ythe distributions of the flightless ratite birds (such as ostriches) are the results of connection between the southern continents that made up Gondwana: xthese birds are descended from a common Gondwanan ancestor xsplitting of a widely distributed ancestral population by continental drift is called vicariance Lineages of ratite birds separated by fragmentation of Gondwana (c) 2001 by W. H. Freeman and Company Biogeographic Regions zThe modern distributions of animals led Alfred Wallace to recognize six major biogeographic regions: ythese correspond to landmasses isolated millions of years ago by continental drift yover the course of this isolation, the animals and plants of these regions evolved independently and developed distinctive characteristics Major zoogeographic regions of the earth based on distribution of animals (c) 2001 by W. H. Freeman and Company Biogeographic Regions 1 zNearctic - North America ymaintained connections to Palearctic for 100 My zPalearctic - Eurasia yshares many groups of plants and animals with Nearctic zEthiopian - Africa yhas a long history of isolation from the rest of the world (c) 2001 by W. H. Freeman and Company Biogeographic Regions 2 zAustralian - Australia yhas a long history of isolation from the rest of the world zOriental - Southeast Asia yisolated from rest of tropical world, but has some affinities to Palearctic, where a high percentage of trees are derived from tropical forests zNeotropical - South America yisolated from Nearctic until about 3 Mya (formation of isthmus of Panama) (c) 2001 by W. H. Freeman and Company Changes in Climate 1 zThe distribution of heat over the surface of the earth depends largely on circulation of the oceans: y50 to 30 Mya, polar regions were covered by oceans that extended to tropical regions, resulting in much warmer polar climates yafter this time, drifting continents curtailed this circulation, resulting in a cooling and drying trend at high latitudes xresult 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 zDuring the past 2 million years, gradual cooling of the earth gave way to violent oscillations in climate, the Ice Age or Pleistocene epoch: yglacial advances drove temperate species southward and may have restricted tropical species to isolated refuges with moist conditions ymigrations of forest trees in eastern North America have been well documented: xafter 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 zMigrations of trees in eastern North America from 18,000 years ago to present are known from pollen grains deposited in bogs and lakes: ythe compositions of communities shifted as species migrated across the landscape yin particular, the composition of forests during the past 18,000 years has: xincluded combinations of species that do not occur today xlacked combinations of species that do occur at present (c) 2001 by W. H. Freeman and Company Catastrophes 1 zThe Mesozoic era (Cretaceous period) ended with a catastrophic disturbance 65 Mya: yevidence points to collision of an asteroid with Earth that struck in shallow seas off the Yucatan Peninsula of Mexico: xmuch of earths biomass was destroyed by massive tidal waves, fires, and ensuing darkness and cold temperatures xamong the groups falling victim to this mass extinction were the dinosaurs xother 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 zMajor catastrophes have occurred at intervals of 10 to 100 millions of years: ysuch events have disrupted ecosystems and changed the course of community development: xthousands of years may be required for environmental conditions to return to normal ysuch events may also: xeliminate species and thus reduce diversity xfoster rapid evolutionary responses to new conditions xcreate opportunities for development of new biological associations (c) 2001 by W. H. Freeman and Company Convergence zConvergence is the process whereby unrelated species living under similar ecological conditions come to resemble one another more than their ancestors did: ythere are numerous examples of convergence: xwoodpecker-like birds that fill the woodpecker niche in many systems lacking woodpeckers xsimilarities of plants and animals of North and South American deserts xsimilar body forms of dolphins and penguins, which both resemble tuna, whose swimming lifestyle they share convergence (c) 2001 by W. H. Freeman and Company Convergence is often incomplete. zDetailed study often turns up remarkable differences between plants and animals occupying superficially similar habitats: ythe ancient Monte Desert of South America lacks the bipedal, seed-eating, water-independent rodents of North America (kangaroo rats) and Asia (gerbils) ysuperficially 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. zMany of the unique attributes of the reptile fauna of Australia may be related to poor soils: ymost Australian soils are old, deeply weathered, and have few nutrients yplants have low nutrient content and high levels of toxic substances ythese plants support few insects ybirds, which depend on insects, are not common yreleased from bird predation, lizards have proliferated in ways not possible elsewhere (c) 2001 by W. H. Freeman and Company Local Community Diversity zTo what extent are community attributes, such as diversity, convergent? ydo local processes determine numbers of coexisting species? xare numbers of species in communities occupying similar habitats independent of the regional species pool? yto 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 zIf regional processes influence local diversity, then local communities should sample regional species pools in the same proportion: ythis would result in a linear relationship between local and regional diversity ylocal control would result in saturation, beyond which increasing regional diversity would add nothing to local diversity zAvailable 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 yseen in data for fish communities in Africa and South America (c) 2001 by W. H. Freeman and Company Processes on many scales regulate biodiversity. zHistory and geographic position may influence diversity of an entire region and its local inhabitants: yinteractions of species within local habitats make up only half of the diversity equation! yfor example, mangroves in the Indo-West Pacific region are far more diverse than mangroves in the Caribbean: xboth 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 zDifferences in diversity in mangroves of these regions appear related to several factors: yplant taxa have invaded the mangrove habitat more frequently in the Indo-West Pacific region yfewer lineages in the Indo-West Pacific region appear to have suffered extinctions: xwet 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 yfragmentation of the Indo-West Pacific habitats may have isolated populations and fostered speciation (c) 2001 by W. H. Freeman and Company Summary 1 zLife first appeared several billion years ago, but modern forms appeared about 590 Mya, the point marking the beginning of the Paleozoic era. zMore recent eras include the Mesozoic (beginning 248 Mya) and Cenozoic (beginning 65 Mya). zContinental drift has altered climates and pathways of dispersal among the continents. (c) 2001 by W. H. Freeman and Company Summary 2 zAnimals and plants have evolved to some extent independently in each of six major biogeographic regions. zThe climate of the earth cooled during the Cenozoic, leading to greater distinctions between tropical and temperate biotas. zGlacial 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 zCatastrophes have punctuated the development of life on earth, resulting in mass extinctions and new opportunities for surviving lineages. zConvergence is often observed in biota of similar but geographically isolated regions. zNonconvergence in diversities of biotas from similar habitats indicates the role played by regional species pools in determining local diversity.