Spring 2004
Lecture 13: March 25
Biogeography: Spatial patterns of biodiversity; distributions of organisms, both past and present; patterns of variation in the numbers and kinds of living things over the earth.
Plant realms as seen today:
Realms are the largest subdivisions; one level lower are biomes.
Biomes:
If one looks at one group of organisms (e.g., bivalves, snails) then the word 'province' is usually used to designate one large region and its characteristic species; usually, there are several provinces (of one type of organism) within a biome.
In my opinion the most probable theory is: at higher latitudes there is more chance to become extinct, and fewer options to diversify.
Explanation: on islands, species come in from mainland; species also go extinct. After some time, equilibrium is reached, at the point where the island has as many species as it can hold.
Species-area theory has been much used, e.g., in making rain forest wild life preserves one large area is much better than 2 small ones. Note; this model breaks down once the area becomes as large as a continent (2 continents can hold more species than 1): at what size is that boundary?.
Recently, the species-area theory has been much criticized. For instance; how do we know the islands are (or were, before human actions) actually in equilibrium? During the last ice age all islands were much larger (lower sea level): did species numbers fluctuate with sea level? If they did, how rapidly did the number of species adapt to larger/smaller size? No evidence.
1. Plate tectonics: in the past, organisms probably lived in such a way that they could be divided into biomes as well. But these regions are moved around; continents split and recombined. For fossils we thus have to reconstruct the plate tectonic configuration before we can make inferences about paleobiogeography. Example: southern continents were organized in one continent (Gondwana land) before the Jurassic. Older fossils clearly document this (e.g., Glossopteris flora). Even modern floras show still similarities between these southern continents, with some plant families present only on the continents that once formed Gondwana.
There are many barriers to dispersal. Without these barriers, every species would be everywhere. Barriers are species-specific: they affect different species differently.
Clearly, different species have different capability of dispersion. Coconut can float for months in salt water and still keep the seed viable; mangroves have floating 'seedlings'. All flying organisms do not only fly but are also possibly blown off course in storms. Elephant and deer are good swimmer, and commonly are part of island faunas.
The opposite of barriers to dispersal are corridors for dispersal.
Vicariance: splitting of a taxon's range
If a large continent splits apart (or if sea level rises and creates various islands), the evolution of the various groups in the newly formed subcontinents can be expected to reflect that splitting up of the continent (with more clearly related groups on pieces of continent that were split off most recently). One could thus draw a 'cladogram' for the continents (which was last connected to which other), and then compare such 'area cladograms' with cladograms for groups of organisms living on the various continents. If a species is split into several (from then on, independently evolving) new species, the new species are called vicar species. Vicariance biogeography distributions of monophyletic groups of taxa over areas are explained by the reconstruction of area cladograms. These area cladograms are hypotheses of historical relationships between areas and are derived from phylogenetic and distributional information of the monophyletic groups concerned.
When formation of barriers or splitting up of areas triggered speciation (i.e. vicariance), all species are endemic to their own area. In such simple cases, derivation of an area cladogram is trivial. Replacement of taxa in the taxon-phylogeny by their areas of distribution results in area cladograms with an own and unique terminal node for each area. However, real data are mostly the result of other processes such as extinction of species in part of their range or dispersal of species over the formed barriers.
South America was a separate continent after it split off from Gondwana land in the Early Cretaceous, although it may have been connected to Antarctica until some time in the late Eocene. South America then became connected to North America in the Pliocene. South America contained its early mammals (which had evolved by the Early Cretaceous), which were thus split off (vicariance) from those in other continents, and evolved on their own. The flora also evolved from the Gondwana flora, and has its typical species (e.g., the southern beech, or Nothofagus). On South America a (to us) very strange fauna evolved: glyptodonts (giant, armored armadillos), armadillos, anteaters, sloths, horse-like litopterns, and a range of marsupial carnivores, giant carnivorous birds (Phorusracids), and strange terrestrial crocodiles (Sebecids). During the Cenozoic (probably in the Oligocene), monkeys and rodents arrived from Africa, how we do not know. They must have floated over on logs, or 'island-hopped' part of the way...The rodents evolved into the typical South American rodents (guinea pig, chinchilla, capybara). The monkeys evolved into the "new world monkeys". When North and South America became jointed, there was the "Great American Interchange". More species migrated from North to South (bears, camels, cats, deer, dogs, elephants, horses, peccaries, rabbits, raccoons, skunks, tapirs, weasels) than the reverse (anteaters, armadillos, capybaras, glyptodonts, monkeys, opossums, porcupines, phorusracids-giant birds-, sloths, teratorns, toxodonts), and most of the South American marsupials became extinct (but the opossum successfully migrated from South to North).