December 2, 2014

Introduction: What is Biogeography?

Biogeography is the study of the distributions of living organisms. The major goal of biogeography is to explain why different regions of the earth are inhabited by different types of organisms. Biogeographers seek to discover what historical and ecological factors explain why a species lives in one particular area but not in another area.

Biogeography is largely a historical science, rather than an experimental science. Distributions can be observed, but the initial conditions that led to those distributions are unknown. Biogeographers seek to infer initial conditions of distribution by studying the principles of ecology, the fossil record, the present and past relationships of the continents, and the processes of speciation and adaptation. Because of its historical nature, much of the data is unavailable, and considerable speculation may be involved in reconstructions of the past.

The study of biogeography can be conducted at various scales, depending on the purpose of the study. This necessitates that we identify the taxonomic unit being studied and the geographic units comprising its distribution. If a part of the group or a part of its distribution is ignored, we may easily arrive at incorrect conclusions. Alternatively, combining two natural groups into one artificial group is likely to lead to incorrect interpretations. In this blog, we will be concerned with large-scale patterns, focusing on how groups of species are distributed over the surface of the earth. For our purposes we will mostly use the taxonomic family as the taxonomic unit representing a lineage and the continents as the geographic units.

Biogeographical Patterns and Processes

Most distributional patterns fit into one of three categories: widespread, narrowly endemic, or disjunct. Widespread species are those found on several continents. The common egret (Ardea alba) is a widespread species, while the cat family (Felidae) is a widespread family. Narrow endemics are restricted to a single area, often small in extent. The Torrey pine (Pinus torreyana) is endemic to a small area of southwestern California and the nearby island of Santa Rosa. The kangaroo family (Macropodidae) is endemic to Australia. Disjunct distributions are those with a major gap in the distribution, especially at the continental or sub-continental scale. Creosote bushes (genus Larrea) have a disjunct distribution, being found in the deserts of North America and in South America, with a large gap in between. The family of "clawed" frogs (Pipidae) is disjunctly distributed in South America and Africa. Biogeography attempts to explain the origins of these various kinds of distributional patterns.

Three biogeographical processes are thought to be responsible for the observed patterns of species distributions: dispersal, extinction, and speciation. Dispersal is the movement of populations from one region to another. This may be accomplished by a gradual expansion of range, such as the opossum in North America, or by an unusual circumstance, such as when the cattle egret crossed the Atlantic Ocean from Africa to South America in the late 1930s. Dispersal normally occurs continuously until blocked by some condition acting as a dispersal barrier. Excellent dispersal potential may produce a widespread distributional pattern. Failure to disperse may result in a narrowly endemic distribution. One or more accidental dispersals may produce a disjunct distribution.

Extinction and speciation may alter distributional patterns. Extinction in part of a species’ range (extirpation) may leave a single relictual population that is narrowly endemic, or a small number of disjunct populations. Speciation may result in morphological divergence of two isolated populations, producing a new species[1] with a narrowly restricted distribution. Extensive speciation and extinction may make it difficult to interpret the biogeographical history of a group.

Ecological factors are also important in explaining present distributions, especially those of subcontinental scale. For example, parrots are largely restricted to tropical areas because they depend on a year-around supply of fruit, which is typically not available in more temperate areas. However, parrots are found in temperate New Zealand and temperate South America, and the Carolina parakeet once lived in the southeastern United States. Distributions of all living species are influenced by ecological factors.

The Flood and Biogeography

A creationist interpretation of biogeographical distributions must begin with the Flood. Unfortunately, creationists do not have a generally accepted flood model, and the Bible does not tell us whether fossils were all produced by the Flood. Much more study is needed in this area. However, we do not know of any other process that could account for the richness of the fossil record within a short chronology for life on Earth, and so we accept, as a working hypothesis, that most of the fossil record is the result of the catastrophic events of the Flood.

The Biblical Flood is described very briefly in Genesis 6-9. The passage focuses on the experience of Noah and his family, and the effects on the animals are only of secondary interest. The passage emphasizes two important features: the global extent of the flood, and the ark as a refuge from the flood.

A global flood. The Flood described in Genesis was global in extent. However, the brief, general description in Genesis leaves considerable leeway in interpreting the details as they apply to biogeography. For example, it seems reasonable to suppose that the Flood may not have had the same effects in all regions of the earth. Flooding could have been the result of torrential rains, or of rising sea levels, or of gigantic tsunamis due to tectonic movements, or of water thrown from the ocean by asteroidal impacts. These factors may have been of different extent in different regions. Some regions may have been inundated for long periods of time while other regions may occasionally have had some mountaintops rising temporarily above the water surface. The Biblical text may even permit the idea that the flood covered the land surface sequentially, rather than simultaneously. The common coverage of the present continents with marine sediments indicates that all regions of the earth were covered at one time or another during the flood, whether sequentially or simultaneously.

The ark as a refuge for terrestrial vertebrates. It is obvious that a flood would have different effects on marine creatures and on terrestrial creatures. Terrestrial vertebrates in particular would be unlikely to survive a global flood, while many marine invertebrates would be expected to survive. Considerable speculation and discussion has occurred over how the flood might have affected different types of organisms, and which animals were on the ark.

It appears that the ark was designed specifically to save terrestrial vertebrates, including humans. Numerous invertebrates probably came with them, and Noah undoubtedly took some seeds with him to plant after the flood, but many species from these groups must have survived through the flood. Genesis 7:21-23 specifies the kinds of creatures that were to be destroyed in the flood: those that walk on the surface of the ground and breathe through nostrils. This description applies only to terrestrial vertebrates. The ark was designed to preserve representatives of the terrestrial vertebrates, which would otherwise be obliterated by the flood.

Refugia for other groups. The Flood seems to have involved a complex series of catastrophic events that affected the entire surface of the earth. Billions of fossils are found in the rocks, probably the result of these catastrophic events. If the catastrophic processes that killed and preserved these fossils were uniformly applied to all areas of the earth's surface, it is unlikely that any macroscopic organisms could survive. Yet the oceans and rivers have a wide diversity of organisms that would not be expected to find refuge in the ark. These organisms are descended from flood survivors, and seem to imply the existence of local areas where conditions were calm enough to permit some individuals to survive. Such areas could be called “refugia.”

There may have been numerous marine refugia, as well as fresh-water refugia in which the present groups of marine and fresh-water organisms were preserved through the Flood. Mats of floating vegetation may have acted as refugia to preserve a diversity of terrestrial invertebrates through the Flood, including insects, spiders, worms, etc. After the Flood, these refugia, scattered in various parts of the world, would act as dispersal centers which would serve to quickly populate the newly available habitats of the post-Flood world.

Extinction. Not all species were preserved through the Flood. Many thousands of fossil species are extinct, most of them presumably destroyed by the Flood. Even entire groups of species were destroyed, including the trilobites and ammonites in the sea, the dinosaurs and therapsids on the land, and the pterodactyls in the air. The fossil record also shows evidence of mass burials in which many individuals were rapidly killed and covered by sediments. Frequently, large numbers of species disappear from the fossil record at the same stratigraphic level, a phenomenon known as a mass extinction. These observations suggest that survival through the flood may have required exceptional circumstances.

Effects of the Flood on Biogeographic Distributions

We have no information about the distributions of plants and animals at creation. The flood would have destroyed whatever distributional patterns might have been present. Most present species distributions probably do not date back further than the Biblical flood. We will examine how the flood might have influenced the distributions of various types of living organisms.

Starting Points for Dispersal

After the Flood, groups of organisms would begin to reproduce and disperse from the locations where they were preserved during the Flood. Terrestrial vertebrates that were preserved in the ark would disperse from Southwest Asia after the Flood. Marine and freshwater plants and animals would disperse from many different locations around the world, depending on where the survivors were found. Terrestrial invertebrates would disperse from their own refugia, perhaps logs or rafts of vegetation that had become beached as the waters receded.

There are far more species of invertebrates than of terrestrial vertebrates, and far more organisms would have survived the flood in aquatic refugia than in the ark. Predicting the distributional patterns of the surviving invertebrates would require knowledge of the locations of the various aquatic refugia. This information is not available, and creationary biogeographers have not attempted to propose where such refugia might have been. The situation is different for the terrestrial vertebrates, which are thought to have dispersed only from the ark. An important research goal of creation biogeography is to explain the distributions of terrestrial vertebrates as the result of dispersal from the ark, which is believed to have landed in Southwest Asia.

Dispersal Processes

Barriers. Members of a species tend to increase their distribution by dispersing until they encounter a barrier. A barrier is any environmental obstacle that prevents an organism from dispersing further. For marine organisms, oceans provide dispersal corridors while land masses act as barriers. For terrestrial organisms, oceans act as barriers while land masses act as dispersal corridors. Salinity, temperature differences and substrate type may produce impassible barriers for aquatic species, while moisture, temperature differences, availability of food, and topography may produce important barriers for terrestrial species.

Active and passive dispersal. Dispersal may be active or passive. Active dispersal means intentional locomotion by individuals seeking to enter new territory. Active dispersal may be accomplished by walking, flying, or swimming. Passive dispersal means transport by other organisms or by physical forces. Passive dispersal may be accomplished by wind, water currents, or transport by other organisms.

Ocean currents. If the surface of the earth was largely, or entirely, covered by water simultaneously during the Flood, powerful oceanic currents would be expected to develop. These would probably form a circum-global equatorial current along with one or more circular currents called gyres, rotating clockwise in northern hemisphere and counter-clockwise in the southern hemisphere. In the absence of exposed land, these powerful oceanic currents might flow at speeds of hundreds of miles per hour, and might transport organisms over long distances. During Miocene sedimentation, a circum-global equatorial current is believed to have passed between North America and South America and between Eurasia and Africa. This was probably near the end of the Flood, or possibly even after the Flood, and could have provided a means of dispersal that is no longer available.

Clues from islands. Islands provide information on the relative dispersal abilities of different groups. Not all islands are equal in their potential for information about dispersal. Continental islands are separated from the mainland by shallow water, and were connected to the rest of the continent at a time when sea level was lower than at present. Continental islands typically have abundant diversity, including freshwater fish. Examples of continental islands include Java, Trinidad, Britain, and Ceylon. Changes in sea level would also explain former land connections between North America and Asia. These considerations imply that past conditions were different from those of today. Species on continental islands probably arrived by the same dispersal methods used by these same species today.

Volcanic islands probably represent areas that formed during or after the flood, and have never been connected to any continent. Species found on volcanic islands must have dispersed across part of the ocean. Examples include certain types of plants and invertebrates, as well as birds, lizards and some bats. Volcanic islands rarely have land mammals, and almost never have native amphibians or primary freshwater fish. These observations indicate that the best dispersers to islands are those that can travel over the sea itself, such as certain plants, or through the air, such as flying animals and members of the “aerial plankton” such as tiny spiders. Lizards and other kinds of organisms that are able to grasp vegetation or hide inside rotten logs may occasionally be able to raft to islands, and are intermediate in dispersal ability. The least able dispersers to islands are those groups unable to survive in salt water, such as primary freshwater fish and amphibians. Mammals that cannot grasp vegetation are not good dispersers to islands beyond at most a few kilometers from a source area.

Some present insular distributions seem very difficult to explain. For example, iguanid lizards are found from South America to southern North America, and also on Madagascar, Samoa and Fiji. It appears that these lizards somehow dispersed from South America to Fiji. This seems impossible at the present time, but past conditions may have been different. Perhaps the flood resulted in large numbers of floating islands of plant material that might have been carried by ocean currents to various parts of the world, transporting various kinds of plants, invertebrates and lizards with them. This might have been enhanced by the circum-equatorial ocean current thought to have existed before closure of the Panamanian land bridge and the collision of Africa with Europe.

Patterns of Distribution

A global Flood would greatly alter the distributions of species and groups. Some species would be greatly reduced from their former distributions. Others might be transported to new areas where they would increase their distribution. These processes might produce any of the three basic distribution patterns: widespread, narrowly endemic, or disjunct.

Plants, invertebrates, and aquatic and marine organisms that survived through the Flood might be expected to have distributions that appear relictual. That is, their present range may be much more restricted than their fossil distribution. The flood might destroy most individuals of such a species, but leave isolated populations of survivors, perhaps in areas distant from the ark. This process could produce disjunct distributions if the species survived in more than one region. Some possible examples might include groups now restricted to the southern hemisphere, such as Nothofagus trees (present in South America and the Australian region), Araucaria trees (present in South America and the Australian region), characiform fish (present in South America and Africa), pleurodiran turtles (present in South America, Africa, Madagascar and Australia), and pipid frogs (present in South America and Africa). Alternatively, the species might be a restricted endemic if it survived in only one isolated location. Some examples might include the Australian lungfish (Neoceratodus), the New Zealand frogs, (Leiopelma) and the New World snapping turtles (Chelydridae).

The Flood might have produced cosmopolitan distributions also, most likely among pelagic groups such as foraminifera, algae, and some fish such as lantern fish. However, post-flood dispersal could also produce cosmopolitan distributions, and it might not be possible to distinguish between the two historical processes.

Conclusions

Biogeographic distributions have been influenced by both historical factors and current ecological conditions. Much of biogeography can be explained in the context of the biblical account of creation and the flood, keeping in mind that creationist theory includes formation of large numbers of groups of locally adapted species from a created ancestor, and that the animals that depended completely on the ark for protection through the flood are the terrestrial vertebrates – those that breathe through nostrils and walk on the ground.[2] Some biogeographic problems are without answers at present. For example, neither creationists nor evolutionists have provided a satisfactory explanation for this question of how certain South American mammals dispersed to that continent before it was connected with North America. Perhaps future fossil discoveries will help provide tentative answers. In the meantime, biogeography remains a fascinating topic for study.

 

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L. James Gibson, PhD

Geoscience Research Institute

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[1] Note: development of new species is compatible with creation theory, assuming that many original kinds were created, from which numerous species may have descended, with minor changes.

[2] Genesis 7:2