Biogeography

Species Richness and the Extinction Crisis

How many species are there? Species richness – the total number of species in an area. 5-10 million is best guess, but may be anywhere from 3-100 million.

Biogeographic Patterns – Latitudinal Gradient The number of species is greatest near the equator and decreases as you move toward the poles.

Latitudinal Gradient Raises these Questions: What determines how many species live in a particular area? Why do some regions have more species than others? Is there any limit to the number of species Earth can support?

Biogeographic Realms & Regions Different portions of the globe frequently have unique biotas. A rainforest in South America has species more closely related to those found in South American prairies than to rainforest species in Africa. This became known as Buffon's Law.

Biogeographic Realms & Regions What determines which species are found in a particular region? Why are some species widely distributed while others have more restricted ranges? How does a species' evolutionary history affect its current distribution?

Biogeographers consider two distinct perspectives to answer all these questions: The first is an ecological perspective, concerned with how short-term interactions among organisms and the physical environment affect a species' current distribution. The second is an historic perspective that focuses on how processes like speciation, extinction and dispersal  affect taxa and biotas.

Sampling Species Richness How do we determine the total number of species in the area. Sampling effort Species accumulation curve

Why do some areas have greater species diversity? Hypothesis #1 The number of species on any wall is determined by the number of of species in the regional species pool. Each time space in a site opens up, it may be settled by any colonist arriving from the regional species pool, the size of which is determined by rates of speciation and extinction.

Why do some areas have greater species diversity? Hypothesis #2 The number of species on any wall is determined by local species interactions. Competition for space and other resources, predator/prey dynamics, etc., drive diversity by determining which species can survive at any given site and whether or not a new species can successfully colonize it.

Hypotheses lead to Predictions In contrast, if species interactions limit the species that can live in a particular place, then as local species richness increases, species interactions will become more and more intense until, eventually, new species are unable to colonize the site. Strong interspecific interactions will place an upper limit on alpha diversity, regardless of the size of the regional species pool.

Species Turnover If you compared two communities within a region, you might find they have few species in common. This turnover in species from one site to the next is also a form of diversity. Biogeographers describe this as beta diversity.

Generalists vs Specialists Generalist species that can reach and occupy just about any habitat type will see any region as one large continuous area full of suitable habitat. In contrast, specialist species that have very narrow niches will see the same area as divided into many distinct habitats, only some of which they can occupy 

Biogeography and Conservation Biology One practical reason to study Earth's biodiversity is so we can judge whether there is really an extinction crisis happening now. 

Human-Related Causes of Extinction Hunting, introduced species, and anthropogenic habitat degradation are big threats to diversity now, just as they were in the past, although technological advances have enabled people to clear forests more quickly and hunt animals more efficiently than their prehistoric ancestors.

Why Protect Biodiversity? Protect the "genetic library" of natural ecosystems. Earth's biodiversity has already provided humanity with food, medicines, and many other resources.

Why Protect Biodiversity? Ecosystems provide a broad set of services that we rely on for our welfare. These ecosystem services include things like climate regulation, water purification, flood control, crop pollination, and soil generation and maintenance. 

Why Protect Biodiversity? People have an aesthetic and ethical obligation to protect our planet and the only other living species known in the universe—even if only so future generations can marvel at the wonderful diversity of life on Earth.

Ecological Biogeography

Species-Area Curves Bigger islands tend to have more species. Easiest to see pattern if data is log-transformed.

“Islands” From an organism's perspective, an island is simply an area of suitable habitat surrounded by an inhospitable landscape matrix. With this definition there are any number of habitats that could be islands, such as caves, desert oases, and the cool habitats of mountain tops.

Island Biogeography An island's species richness is determined by three distinct processes: Immigration  Extinction  Evolution

McArthur & Wilson Equilibrium theory of island biogeography, focused on islands that varied in size and distance from a mainland. They restricted their analysis to ecological time scales, ignoring evolution.

Immigration Rate Immigration rate is affected by: Size of the island Distance from the mainland

Extinction Rate The extinction rate will be affected by the availability of resources. If more species than can be supported by the available resources have immigrated to the island, there will be a high extinction rate.

Equilibrium Theory of Island Biogeography MacArthur and Wilson plotted an island's immigration and extinction rate curves on the same set of axes and made two important predictions about an island's biota.

Equilibrium Theory of Island Biogeography The intersection of the two curves, which occurs when immigration rate equals extinction rate, predicts the equilibrium number of species, S*. When the number of species on an island is greater than S*, the extinction rate exceeds the immigration rate, and the number of species on the island will decrease. Conversely, when the number of species is smaller than S*, immigration exceeds extinction and the number of species will increase.

Equilibrium Theory of Island Biogeography The intersection of the two curves also predicts the rate at which new species replace extinct species, reflecting the dynamic nature of equilibrium on the island. There is one theoretical value of S*, but the composition of S* is always changing. It changes at a rate that MacArthur and Wilson called the equilibrium turnover rate, symbolized as T*.

Support for Turnover Natural experiments supported the concept that species turnover. Krakatau Channel Islands Manipulative field experiments also supported turnover. Florida Keys mangrove islands

Mangrove Experiments Showed the affects of size and distance from the mainland.

Complicating Factors Many communities seldom, if ever, attain an equilibrium number of species. Both natural and anthropogenic disturbances like hurricanes, volcanic eruptions, and clear-cutting can remove species and open up habitat. Evolution can add species to either the island community or to the mainland species pool.

Complicating Factors Islands are not homogenous patches sitting within a featureless seascape. Both islands and their surroundings can have complex environments which can alter immigration and extinction rates

Complicating Factors Species are not interchangeable. Whether or not a species successfully colonizes an island or later goes extinct is affected by its inherent biology and by interactions with other members of the community.

Dispersal For a species to colonize an island, individuals of that species must move there. Movement of individuals between different islands or other patches of good habitat is known as dispersal. Barriers to dispersal prevent organisms from immigrating.

Types of Barriers Types of barriers that species might need to cross in order to successfully move from one patch to another: Corridors are routes through a landscape that all species can cross. Filters are routes that only some species can cross. Sweepstakes routes are those that are nearly impossible to cross except during rare and unpredictable circumstances.

Crossing Barriers Biogeographers recognize three means by which a barrier might be crossed: Jump dispersal occurs when an organism leaps a barrier in a single bound. Diffusion occurs when organisms slowly percolate through a relatively hospitable matrix. Secular migration occurs when populations move so slowly from one area to another that they evolve en route.

Dispersal Example Some 40-45 million years ago, the ancestor species to camels and llamas (family Camelidae) originated in North America. About 3 million years ago some of these animals migrated west across the land bridge from Alaska to Siberia, and from there dispersed across Asia and into Africa (see the arrows on the right). When South America crashed into North America at Panama, other individuals migrated south along the newly formed corridor of land. In this way, the original ancestor species in North America dispersed to four different continents.

Species-Area Curves Both island and mainland species-area curves tend to show the same general pattern. Larger areas contain more species.

Species-Habitat Diversity Hypothesis As you expand sampling area, you will encounter different habitats, and thus different sets of species. This is known as the species-habitat diversity hypothesis.

Island Biogeography & Conservation Biology An early question conservation biologists asked was: Given the ability to set aside a certain amount of land as a reserve, is it better to target single large pieces of land or several small pieces?  Because extinction rates are lower in larger areas, some ecologists argued that making the largest reserve possible would keep the most species from going extinct.   Others argued that several smaller reserves might capture more habitat diversity, as well as protect against major disturbances (e.g., what if a fire burned down all of a single large reserve?).

Historical Biogeography

Evidence of Major Changes Fossils of aquatic animals, such as large sharks, in Kansas indicate that the Earth has changed dramatically over time.

Plate Tectonics Such huge changes in geography happen because Earth's continents slide across the surface of the planet in a process known as plate tectonics. https://youtu.be/Cm5giPd5Uro

Realms & Regions Earth's terrestrial biota can be divided into 12 biogeographic realms that reflect the different evolutionary histories of land masses on different tectonic plates.

Allopatric Speciation Allopatric speciation begins when populations of the same species become geographically isolated. Over time, natural selection and drift cause them to diverge evolutionarily until they are distinct, reproductively isolated species. This may occur as a result of two distinct processes: A vicariance event occurs when a population is physically split by geologic or geomorphic processes. A founder event occurs when a new population is founded by a small number of individuals dispersing from their ancestral range.

A Brief History of Frogs If you could time-travel back onto the Madagascar-India-Seychelles fragment 100 million years ago, you would see frogs that remind you of today's American bullfrog and leopard frog species. What frog history led them from a single species on a fragment of land off Africa to a nearly worldwide distribution today?

A Brief History of Frogs The breakup of the southern continent, Gondwana, created smaller, isolated populations on the new land masses. As the land masses separated, ocean water filled the gap. Animals like frogs could not cross the gaps. This is an example of vicariance.

A Brief History of Frogs According to the model of allopatric speciation, once two populations are physically isolated, the evolutionary processes of mutation, natural selection, and genetic drift would cause mating behavior, feeding strategies, and habitat use to become different between the populations. Over long enough time spans, new, reproductively isolated species were likely to evolve.

Jump Dispersal Geologically-caused vicariance events are one way for populations to become isolated. A different way is through jump dispersal. If individuals from the one population cross a barrier (such as a stretch of ocean) to a new, unoccupied habitat, the new population may be isolated enough to eventually evolve into a new species. Founder Event

A Brief History of Frogs We can make predictions about the pattern of speciation we would expect from dispersal

Sympatric Speciation Sympatric speciation occurs when a population splits into two and eventually speciates without a physical separation.

The Great American Biotic Interchange Approximately 3 million years ago the Isthmus of Panama emerged from the sea, connecting North and South America.  Route for mammals to cross from one continent to another.

Human Accelerated Dispersal Although dispersal and colonization of new habitats is a fundamental biogeographic process, humans have altered dispersal rates for many species. Intentionally and accidentally, people bring species with them as they move from place to place.

Introduced Species - Impacts Introduced species often lead to extinction of native species and loss of biodiversity. Because many of the same species are introduced over and over again, many areas of Earth are losing their distinctive nature. 

Latitudinal Gradients Latitudinal gradients are perhaps the best-known biogeographical pattern. Species diversity tends to be high near the equator and lower near the poles for many taxa. These patterns are well-correlated with a number of climatic variables but are incompletely understood.

Biodiversity Hotspots & Conservation Conservation biologists have attempted to identify hotspots that contain a large number of endemic species that are threatened by human activities. Focusing conservation activities on these hotspots may be an effective use of limited conservation resources.

Global Patterns in Physical Conditions

Global Patterns in Physical Conditions Climate predicts not only the number of species likely to occur in a place, but also the physical appearance of those species.

Differential Energy Input from the Sun Because the Earth is round, the intensity of solar radiation is lower in the poles than near the Equator.

The Tilt and Orbit of the Earth Creates Seasons The tilt of the Earth's axis relative to its orbital plane creates seasonality.

Atmospheric Circulation Differential heating from the Sun across the globe creates air and water currents, such as Hadley cells.

Bands of Wet and Dry Large-scale air circulation patterns tend to create a band of wet areas near the Equator and deserts to the north and south of the Equator.

Terrestrial Biomes Differences in climate and geography give rise to a variety of biomes around the world. These patterns determine a region's climate and, to a large degree, what species can thrive there. 

Temperate Deciduous Forest Temperate deciduous forests receive rain year-round. Cold winters and hot, humid summers. Animals may migrate, hibernate, or survive on scarce available food or stored fat through the winter.

Coniferous Forest Coniferous forests, or taiga, are common in the northern hemisphere. Evergreens dominant Colder, less rain than temperate forests.

Coniferous Forest Mammals that inhabit coniferous forests include deer, moose, elk, snowshoe hares, wolves, foxes, lynxes, weasels, bears. Adapted for long, snowy winters.

Tropical Forest Tropical rain forests receive lots of rain and are generally warm year-round. Stratified Diverse

Tropical Forest Canopy – insectivorous birds and bats fly above the canopy. Fruit bats, canopy birds, and mammals live in the canopy eating leaves & fruit. Middle zones are home to arboreal mammals (monkeys, sloths), birds, bats, insects, amphibians. Climbing animals move along the tree trunks feeding at all levels. Ground level contains larger mammals (capybara, paca, agouti, pigs) as well as a variety of reptiles and amphibians.

Tropical Forest Nutrients in a tropical forest are tied up in living organisms. Soil is poor. Slash and burn agriculture involves removing vegetation to grow crops – but the soil is so poor that the fields must be moved often.

Grassland Temperate grasslands receive seasonal precipitation and have cold winters and hot summers. Prairie

Grassland Grasses and herds of large grazing mammals are dominant. Jackrabbits, prairie dogs, and ground squirrels are common. Predators include coyotes, cougars, bobcats, raptors, badgers, and ferrets.

Grassland Savannas are tropical grasslands with seasonal rainfall.

Grassland Chaparral receives highly seasonal rainfall. Shrubs and small trees are common. Adaptations to fire.

Tundra Tundra has a permanently frozen layer of soil called permafrost that prevents water infiltration. Very cold, short growing season. Little rain

Tundra Tundra is often covered with bogs, marshes, or ponds. Grasses, sedges, and lichens may be common. Lemmings, caribou, musk-oxen, arctic foxes, arctic hares, ptarmigans and other migratory birds.

Desert Deserts have very low precipitation – less than 30 cm/yr. Variable temperatures. Animals often nocturnal and live in burrows. Reptiles and small mammals are common.

Establishing Conservation Priorities Understanding the distribution of the Earth's flora and fauna across the globe can greatly assist conservation efforts, but naming and mapping biomes and ecoregions is not enough. To make the best-informed decisions possible, conservation biologists want to know more about these places, including the threats that they face and the species they contain.