Zoogeography is the study of the distribution of animals on Earth. It is the study of where species live and why they happen to live there. A fundamental axiom of zoogeography is that the geographic range of a species is limited. No species is everywhere; some have broad ranges, while others have quite small ones. Zoogeographers seek to determine patterns in the distributions of species.
In this lecture, we will study how historical geological factors, climatalogical factors, and ecological factors may limit or determine species distributions.
In spite of minor and major temporary declines along the way, there has been an increasing trend in biodiversity over time. This is thought by some ecologists to be a result of changes in the continental land masses. Continental drift is the gradual breaking up (or collision) of the continents that has occurred steadily over the past 200 million years.
In Silurian and Devonian times (> 395 mya), three major continents existed: Gondwanaland (composed of South American, central and south Africa, Australia, Antarctica, and India) was in the southern hemisphere; Europe and North America formed another continent; and Asia lay to the north. This period was characterized by the early evolution of fishes and amphibians, especially in Euramerica. Gondwanaland lasted until the lower Permian, when early reptiles were also represented in Euramerica.
During the Permian (280 mya) period, all of the continents met and formed Pangaea. Extensive faunal similarities among these formerly separate continents is apparent from fossil records from the upper Permian and Triassic. The collision of the continents allowed a major radiation of reptiles from Eurasia to spill new groups into virtually all parts of Pangaea.
During Jurassic times (195 mya), Gondwanaland started to separate from Pangaea (Laurasia) as the Atlantic Ocean opens the gap. Fossil records indicate that land routes allowing migration still existed between Europe an Africa and between Alaska and Siberia.
During the Cretaceous period (136 mya), the widening of the Atlantic finally separated Euramerica from Africa and South America from Africa. Australia and Antarctica remained together but separated from Africa. Wherever the marsupial mammals may have originated, they may have reached Australia from South American across Antarctica. The subsequent separation of these three landmasses may have prevented the arrival of early placental mammals to Australia.
A land route was reestablished between North and South America during the Pliocene. This bridge permitted a diversity of placental mammals to invade the southern continent. Simultaneously, marsupial carnivores and placental ungulates went extinct in South America, while S. American rodents and Xenarthrans (armadillo-like mammals) became much reduced in diversity. Only a few S. American species were able to reach and survive the temperate North America. The modern continents were well separated by this time. As the landmasses became more isolated, global climate changes compressed the tropical zone toward the equator and produced a gradient of climatic zonation. This affected the distribution of some African species such as the giraffe and elephant southward (sub- Saharan) as the climate became drier.
One important consequence of continental drift is that some species do not exist everywhere they might be well adapted for. A good example of this is the distribution of placental and marsupial mammals. The introduction of placental mammals into most land masses resulted in mass extinctions of marsupial mammals. However, placental mammals were unable to reach Australia, and therefore marsupial mammals dominate that continent. In recent times, however, humans have introduced placental mammals to Australia which in many cases have outcompeted the native marsupials. This has occurred with the dingo (Canis familiaris dingo) which has outcompeted native predators such as the Tasmanian devil.
Fluctuations in sea level have created and removed land bridges connecting continents and islands. This allows migration of species into areas previously unavailable to them, or conversely, isolates populations of the same species into discrete populations, which allows allopatric speciation to occur. Fluctuations in sea level occur as a result of glaciation. Sea level falls during ice ages, reducing the extent of warm inland seas, where much of the marine species live.
Glaciers also affected current species distributions in another way. Glacial advance can cause extinctions in cases where a species cannot migrate faster than the glacier (mostly with plants) or where no suitable habitat exists below the glacial front. Glaciers can also leave relict populations at high altitudes that remain ice free.
Rates of climate change can affect speciation. South America was subject to extreme climate changes during the Pleistocene and endured major alternations of wet and dry periods. Tropical rainforest may have undergone cycles of expansion and contraction. Many birds of tropical lowland forests are reluctant to cross ecological barriers so speciation by geographic isolation may occurred
Current species distributions likewise are limited by the climate in the areas they inhabit. Some species are able to exist in a wide variety of habitats, while others may tolerate relatively low amounts of climate variation. Major climatic factors are the amount of timing of rain, and seasonality, variability, and predictability of weather. Rain may either be constant throughout the year, or be seasonal. Species adapted to survive in areas with seasonal rainfall must be able to deal with these extremes. Variability of sunlight energy varies least in tropical areas, whereas seasonal extremes become greater with increasing distance from the equator.
Effects of climatic patterns on vegetation include:
Climate influences animal distributions both directly and indirectly through effects of vegetation. Warm temperatures can lead to the inactivation or denaturation of enzymes, can increase metabolic rates in plants so that they respire more than they photosynthesize (in effect starve), and can lead to dehydration. At cold temperatures species can be killed at temperatures <1 degree Celsius unless they have special adaptations, metabolism can slow down or stop, or cold weather can reduce fitness to a point where other factors (disease, parasitism) can then take hold.
endotherms regulate temperature by the production of heat within their own bodies, and can survive in cold areas
ectotherms rely on external sources of heat, and therefore have ranges which are limited by temperature
Allen's rule: animals from cold climates usually have shorter extremities than animals with otherwise similar characteristics from warmer climates
Bergmann's rule: animals with a wide distribution are larger in the colder areas of their range
Many ecologists believe that species ranges are shaped in large part by biotic interactions. Predation and competition may be important on the small scale to limit populations.
The theory of island biogeography states that the number of species on an island is determined by 2 factors: distance from the mainland and the size of the island. Distance from the mainland determines immigration rates, while the size of the island determines extinction rates (species area curves)
Large islands provide larger targets for dispersing animals and support more varied habitats. They also can maintain a larger population size. Larger populations are less subject to random fluctuations that would reduce the population to zero.
This model is important in determining the effective sizes of areas to preserve. With increasing development and deforestation, we are left with patches of habitat that are in effect islands.
| Factors increasing the number of species | Factors decreasing the number of species |
|---|---|
| fragmentation of continents | coalescence of areas |
| heterogeneity of environment | homogeneity |
| topographic relief | lack of relief |
| benign climate | harsh climate |
| intermediate disturbance | severe disturbance |
Vulnerability to extinction:
| Time period | million years ago | % genera extinct | % species extinct |
|---|---|---|---|
| Cretaceous-Tertiary | 65 | 47 | 76 |
| Triassic-Jurassic | 208 | 47 | 76 |
| Permian-Triassic | 245 | 84 | 96 |
| late Devonian | 367 | 55 | 82 |
| late Ordovician | 438 | 61 | 85 |
These rates are a few magnitudes higher than the background extinction rate, of 1-2 species per million species present per year.