Distributions of Organisms Three fundamental processes in biogeography: 1. evolution 2. extinction 3. dispersal The relative importance of these processes varies from one group to another. Dispersal: migration by the taxon across a barrier from A to B Vicariance: erection of a barrier between A and B, both of which were already occupied by the taxon The type of phylogenetic pattern observed for dispersal vs vicariance differs. Dispersal The role of successful dispersal in biogeography is an analog of beneficial mutations of the genome - rare, but significant events that are hard to track from step to step. 1. long-distance dispersal my be infrequent and stochastic 2. can t ignore it just because it s hard to study For a species to expand it s range it must be able to: 1. travel to a new area 2. survive the passage 3. establish a viable population at the endpoint 5-1
Long-distance dispersal (aka jump dispersal) three important consequences: 1. mechanism can explain distribution of organisms 2. accounts for similarities of distant biotas 3. emphasizes the impact of humans as transporters of organisms Diffusion Typically has three stages: 1. invasion and range expansion; very slow and may need several introductions 2. after establishment, range expansion is exponential 3. range expansion slows and stabilizes when ecological barriers encountered Secular migration occurs very slowly: hundreds of generations -- long enough for evolution Mechanisms: Active vs Passive Dispersal Active Movement under own power = active: e.g., animals. Passive most organisms disperse by passive means rather than active means Plants, animals, fungi and microbes can disperse passively 5-2
Barriers to dispersal Physiological barriers Ecological and Behavioral barriers Dispersal Routes and Biotic Exchange Corridor: dispersal route that permits the movement of many or most taxa from one region to another Filter: dispersal route that is more restrictive than a corridor. Sweepstakes Route: coined by G. G. Simpson to describe rare, chance dispersal across a major barrier. Dispersal with continental plate: rafting via continental drift Colonization Dispersal is not sufficient -- must be able to survive and reproduce to establish a new population. Successful colonists must be able to survive physical stresses and biological hazards that differ from their adaptive regimes. 5-3
Endemism, Disjunction and Biogeographic Regions Geographic distributions have limits - most species are confined to restricted regions Endemics tend to be concentrated in certain regions: Australia, southern Africa, Madagascar, New Zealand, New Caledonia, islands Europe, North America and Asia tend to share species with each other Provincialism: different groups of plants and animals showing similar patterns of endemism and having the same types of habitats Disjunction: two or more closely related taxa occur in widely separated regions, but absent from intervening areas. Endemism Endemic -- occurring nowhere else originated in place and never dispersed; or survived in only part of more extensive range (relictual endemic) lowest taxonomic categories that are endemic tend to have more narrowly distributed than families and orders Classification of endemics: by place of origin -- in place (autochthonous); dispersed from other place (allochthonous) - relictual is a type of allochthonous endemic by taxonomy or geography - taxonomic relicts are the sole survivors of once diverse groups; biogeographic relicts are the narrowly endemic survivors of a formerly widespread taxon by age - paleoendemic (old); neoendemic (recent) 5-4
Cosmopolitanism taxa that are widely distributed throughout the world (e.g., humans); Biogeographic regions Endemics tend to be clumped in particular regions, and biota within a region is more homogeneous than between adjacent regions. 1. Most closely related species tend to have overlapping ranges within restricted parts of continents or oceans 2. unrelated groups of taxa show similar patterns of distributions 3. disjunctions may be common Zoogeographic provinces (equivalent to phytogeographic regions): Holarctic -- North America, Eurasia and northern Africa Nearctic vs Palearctic Neotropical (Central and South America, West Indies) Ethiopian (subsaharan Africa, and Madagascar) Orienta (Southeast Asia and adjacent continental islands) Phytogeographic regions - endemics defined by climatic and physical barriers There are ca. 37 floristic regions of the world organized under six floristic kingdoms (Holarctic, Paleotropic, Neotropic, Holantarctic, Cape, Australian) 5-5
Holarctic -- same as zoogeographic province Paleotropic -- subsaharan Africa excluding the Cape of South Africa, Madagascar, India, Indonesia, Polynesia Neotropical - central America, South America north of Patagonia, West Indies Holantarctic - Juan Fernadez Islands, Patagonia, Subantarctic Islands, New Zealand Cape -- Southern Cape of Africa -- about 90% of plants are endemic Australian - the entire continent Comparisons of Faunal and Floristic regions Faunal and Floral regions are very similar to one another. A comparison of faunal and floral similarities between different regions shows that there is more similarity among southern hemisphere plants than among mammals Reasons: 1. families of flowering plants evolved and dispersed earlier than mammalian families Cretaceous: opposums are the only mammal family (1% of 94 families) known, whereas 31 (22% of 141) angiosperm families had already appeared by that time. Palaeocene: three families of mammals (3%) - hedgehogs, armadillos, horses, compared with 18 (13%) of the angiosperm families Eocene: 15 (16%) of the mammals vs 33 (23%) angiosperms 5-6
2. many mammal lineages have gone extinct 3. Dispersal capacity of angiosperms much higher than mammals Biogeographic Lines Limits of regions and provinces noticed because of a rapid turnover of taxa at the boundaries between regions -- e.g., sharp boundary of Wallace s Line Wallace s line corresponds to the Sunda Shelf Lydekker s line west of New Guinea responds tot he edge of the Sahul Shelf More difficult to show biogeographic lines in new world Islands -- refer to Table 10.2 Continental islands -- formed as part of continent and subsequently separated Islands surrounded by shallow waters were probably connected to mainland as recently as the Pleistocene - flora and fauna related to mainland Islands that have broken away from continent may be found in open ocean - instead of volcanic rocks, have continental rocks (Kerguelen Island [(I) upper Cretaceous], Madagascar [(I) 100 mybp], New Zealand [(P) 80-90 mybp], New Caledonia [(P) 50 mybp], Greater Antilles [(A) 80 mybp], Seychelles [(I) 65 mybp], South Georgia [(A) 45 mybp]) 5-7
Oceanic islands - usually of recent origin and have emerged from the ocean floor without prior connection to continent have biotas with species more closely related to other species in same archipelago than to species on continent; limited taxonomic richness Hotspot island chains include: Carolines (P), Hawaiian Archipelago (P), Kodiac Bowie Island Chain (P), Marquesas (P) Fracture zone island chains: Clipperton Island (P), Galápagos (P) Island arcs formed in association with trenches: Aleutians (P), Lesser Antilles (A), Lesser Sunda Islands (I), Marianas (P), New Hebrides, Solomons (P), Tonga (P), Kermadec (P) Islands formed at midoceanic ridges: Ascension Island (A), Azores (A), Faeroes, Gough Island (A), Tristan da Cunha (A) Continental islands tend to have biotas that are taxonomically richer than oceanic islands Marine Regions and Provinces Much less difference among marine biotas in different oceans than among terrestrial biotas on different continents. 5-8
Disjunction Distributions in which closely related organisms live in widely separated areas. Southern Hemisphere has some classic patterns associated with the break-up of Gondwanaland Australia, Africa, South America, New Zealand, Madagascar Southern beeches (Nothofagus), Restoniaceae, Acacia, mayflies, lungfishes, galaxoid fishes, clawed frogs of Pipidae, ratite birds (emus, cassowaries, ostriches, rheas, tinamous, kiwis), marsupials. North/South America disjunctions: Larrea tridentata, mesquite (Prosopis), paloverde (Cercidium) -- deserts of both continents; Westcoast amphitropical: Microseris Laurasian disjunctions: Rhododendron, Magnolia Eastern Asia/Eastern North America -- Liquidamber (styracflua/orientalis), Liriodendron (yellow poplar), Illicium (star anise), Desmodium (Fabaceae) -- numerous examples. Good systematic studies are essential for understanding disjunction and mechanisms responsible for disjunctions. 5-9