Planet Earth/7g. Earth’s Biomes and Communities

7f. Heredity: Gregor Mendel’s Game of Cards

Planet Earth
7g. Earth’s Biomes and Communities

Biogeography

The best way to study the Earth is to travel. The study of the numerous varieties of life-forms on Earth is only appreciated when one travels across the surface, and notes the differences they see on their journey among the plants and animals they witness. Hence the study of life on Earth is linked to the physical geography of the planet in respect to the occurrence of different plants and animals, and the physical environment that they live within. This is largely due to specific adaptions that organisms exhibit to deal with the physical environment of each region. Hence, life in the dry deserts will exhibit different types of animals and plants than cold polar regions, while hot lush rain forests will exhibit a different diverse group of plants and animals unique to each region. The study of the geography of life is the fascinating field of biogeography.

The study of biogeography likely has its origins in the work of Alexander von Humboldt, a wealthy German explorer who lived 200 years ago. Alexander von Humboldt hope to travel the world, but the raging Napoleonic wars in Europe had caused travel to be difficult, both for funds and permission to travel across warring nations. He had hoped to join a research ship to journey around the world, but the war caused the voyage to be canceled. He requested to join Napoleon Bonaparte in Egypt, but the French authorities in the army refused to bring him along. In 1799, Humboldt’s luck changed when Charles IV of Spain authorized his travel of the Americas. Between 1799 to 1804 Humboldt traveled across South and North America and wrote about the experience in his detailed scientific notes. When he arrived in the United States in 1804, he was summoned by Thomas Jefferson at the White House. Jefferson had just completed the Louisiana Purchase from France but knew little of the regional geography. He was eager to consult Humboldt. Humboldt had traveled extensively in Mexico, although his travel into the interior was somewhat limited, they both shared an interest in the animals and plants that might exist in these unexplored regions. The work of Humboldt had a major influence on the idea of nature; indeed biographer Andrea Wulf attributes Alexander von Humboldt as the inventor of the modern notion of nature itself (see Wulf, 2016). Humboldt and many of his age viewed two worlds on Earth, first the world of man containing farms and urban centers in the cities and towns and second the world of nature, the wild and unexplored regions of the Earth. This two-world view of Earth is still prevalent in the modern age. The primary reason for these early explorations funded by governments, was that plants and particularly the raw production of goods from those plants, such as cotton, sugar, coffee, tea, and various crops, were becoming increasingly in demand by growing populations. The discovery of tomatoes, potatoes and corn in the Americas made these early botanic explorations highly lucrative, particularly for the European monarchies that oversaw these colonies in the Americas.

There was intense interest in understanding the distributions of plants and animals on Earth, as they were viewed as an important raw resource, from furs brought back by trappers and timber from lumbermen to food and exotic spices. Biological exploration was tied to understanding the major biomes of the planet particularly during the years of colonization. A biome is a distinct biological community of plants and animals that have formed in response to a shared physical climate. Early geographers of the distribution of plants on Earth, such as the Danish naturalist Fredrick Schouw, provided early classifications of these various regions. Botanists like Franz Meyen in the 1830s were well aware that the local climate, and in particular the average and extreme temperatures and amount of moisture, were very important to explain the unique distributions of various species of plants and animals on Earth. The concept of zones or isotherms, using Humboldt’s term, are areas of similar climates, and hence tend to produce similar plant and animal species. Most important, these zones could dictate which types of plants and animal can be grown in each region of Earth.

The history of sugar cane is an excellent case as to the importance of understanding biomes. Sugar cane, which is today a major source of sugar, originated in New Guinea, and was introduced into Southeastern Asia. The plant was then introduced to parts of the Middle East, but because of the dry climate, did not grow particularly well. Sugar was a rare medicine for much of Europe, and only eaten on rare occasions. With the discovery of the Americas in 1492 by Europeans, it was quickly recognized that the wetter and warmer climate of the Caribbean Islands and South America would be equally suitable for growing and harvesting sugar cane. The earliest sugar plantations were first established as far back as the early 1500s, and with the rapid rise of sugar cane in the New World, it became a very common import into Europe and across the Americas, particularly in places too cold for sugar cane to be grown. This shift resulted in the enslavement of millions of people, particularly from Africa for the continued harvest and cultivation of this plant in the Americas. Soon people in Europe where consuming sugar at much higher levels than ever before, with great demand for sugar in places where it was too cold for the plants to grow. Early biomes were mostly defined on a general description of the local climate, such as deserts, polar regions, or wet tropics, but others defined them based on particular types of plants that grow within them. For example, palm trees, sage brush, or cactus regions on maps.

In the 1860s, Wladimir Köppen traveled frequently on the newly constructed train from his family’s home in the warm southern Crimean city of Simferopol to the far cold north city of Saint Petersburg in northern Russia. The train journey of 2,110 kilometers (1,305 miles) is a nearly direct north-south transit across eastern Europe crossing many different climatic regions along the way. During the journey he would watch from the train the slow changes that occurred among the plant communities that he saw out the window, as they crossed each particular climatic zone. Later in his life, he would use the experience to develop the Köppen climate classification that was first introduced in 1884, and has been modified over the years into a geographic map of Earth that classifies the geographic zones of each of the major terrestrial biomes of the Earth. It is today referred to as the Köppen-Geiger climate classification based on improvements made by Ruldolf Geiger.

The map is a precursor to the United States USDA Plant Hardiness Zone Map that has become in standard use by gardeners and farmers in the United States to determine if a particular plant species would be successfully grown in their garden, depending on the average climate of that region. For example, the state of Utah spans zones 9 through 4, with the southern city of Saint George within the warmer 9 and 8 growing zones, while Salt Lake City is within the 7 and 6 growing zones, and the higher elevation and cooler and drier cities like Vernal and Heber being within the 5 and 4 growing zones. Each zone is demarcated by the average annual minimum winter temperature, divided into 10-degree zones. Unlike the Köppen-Geiger climate classification system, the USDA Plant Hardiness Zone Map is based only on temperature, rather than a combination of temperature and moisture. Both maps and classifications must be updated as each region’s climate changes over time. Furthermore, both classification systems are based on the climate, rather than the occurrence of plants and animals. Other classifications have used the types of soils found in each region to demarcate each biome based on the soil type found in the area. These of course only describe the major biomes of Earth’s terrestrial ecosystems, leaving out the various ecosystems of the Earth’s oceans, which are related to other physical conditions such as salinity, water temperature, light and water depth. These biomes describe the physical abiotic environment and don’t define them on the occurrence of plants or animals as part of their definitions. For example, such systems would not recognize altered environments or biomes, such as agricultural lands and urban city centers that have been heavily altered by human intervention, yet may have similar climatic conditions as naturally preserved lands that are protected.

A generalized map of Earth's major terrestrial biomes, based on vegetation and types of plants found in each region of Earth.

Beginning in the 1970s the botanist David Goodall lead a team of biologists, ecologists, conservationists and land managers to summarize the major biomes on planet Earth in a multivolume book series, entitled Ecosystems of the World. They defined 30 major biomes on Earth, including biomes rarely defined in other systems or classifications, for example subterranean biomes like those found in caves. The Goodall’s classification also divided the Earth into natural biomes from those of human or managed biomes, such as field crops, managed grasslands and industrial urban landscapes. Hence two theories emerge on how to classify the major regions of Earth into biomes, one that is based on the occurrence of living organisms and one relies on the physical climate.

Support for the inclusion of specific biological species in the biome classification system comes from the central idea of keystone species. Keystone species are biological species within an environment that disproportionately affect a large number of other species in the same environment, and their removal from the region would completely alter the entire ecosystem drastically. A keystone species is sometimes not the most abundant, but one that influences the ecosystem the most and helps to maintain it. For example, many apex predators like wolves and lions are often considered a keystone species, because they have a significantly large influence on the occurrence of prey, as well as the plants that those prey animals might depend on.

Although many biomes are simply defined by the basis of the most abundant species, or the species with the largest amount of biomass in the environment. Biomass is the total mass of a particular species of organisms in a given area or volume. For example, the grassland biomes are defined by the large amount of grass plants that live within the region, while conifer forest biomes are defined by the large amount of biomass of conifer and pine trees. Hence biomes can be defined based on the most abundant and common of the biological organisms within a particular region on Earth.

The Major Biomes of Earth

There are many ways in which the Earth has been divided into major biomes. This list is not comprehensive, but includes most of the major biomes that are widely distributed on Earth, and easily recognized by the occurrence of species and climatic and physical conditions found within them.

Terrestrial Biomes

Tundra Biome

The tundra biome occurs at high latitudes near the north and south pole, and represents cold regions on Earth, which are also some of the driest regions on Earth. In these regions the soils are frozen throughout the year as permafrost, with only a thin melt during the height of the summer. These soils are very ancient with only minor yearly growth of lichens and other arctic plants. The very short growing season and low light levels during the winter result in low species richness. The landscape is dominated by lichens and mosses, with grasses and other low laying plants adapted to the cold. Many migratory animals live in these regions of Earth including birds and caribou, as well as polar bears, that often move great distances across the landscape.

Boreal Forest Biome

The great taiga or boreal forests span much the upper latitudes of Canada and Russia. The boreal forest is dominated by cold winters and short growing seasons, with a forest of mostly coniferous trees, which remain green through the long winters. Conifers are a group of trees (gymnosperms) that evolved early in Earth’s history, bearing cones and wind-blown pollen, rather than bearing flowers and fruit. They are perfectly adapted to these seasonally cold regions of Earth, allowing their wind-blown pollen and cone bearing seeds to germinate in the cooler climates. The region contains many mammals, such as wolves, foxes, rabbits, moose, deer, and bison, but fewer reptiles like lizards, snakes and crocodiles, due to the cold climate.

Alpine Biome

The alpine biome is sometimes included in the boreal and tundra biomes depending on the elevation of the landscape. These are topographically high regions on Earth that are also seasonally cold, due to their high elevation. Increasing altitude on Earth decreases the average yearly temperatures, resulting in a similar profile as observed with the higher latitude tundra and boreal biomes. Alpine biomes are unique in that they are isolated on these alpine peaks and plateaus, and while having a similar climate to tundra and boreal regions of Earth they are unique in their isolation. Some alpine biome regions like the Himalayan Plateau are fairly extensive. One important aspect of the alpine biome is called the tree line. The tree line or timberline is the upper boundary of forests on a mountain peak, above which trees are unable to survive. This elevation varies between 10,000 feet (3.0 Km) to 12,000 feet (3.7 Km) above sea level across the United States, and depends on the amount of moisture and annual temperatures, but the tree line can be much lower in elevation the closer one travels toward the poles (higher latitude) as the climate becomes colder in these regions. These mountain top regions are often covered in ice and snow, both as semi-permanent glaciers as well as winter snow and ice that remains long into the summer months.

Temperate Rain Forest Biome

Temperate rain forests are found in the pacific northwestern coast of North America and South Eastern Australia, which are characterized by higher levels of moisture along these coastal regions. They often are filled with large evergreen trees including some of the largest trees that grow on Earth today. The heavy rain, and somewhat milder climate results in a lush forest landscape and hosts many plants that need high levels of moisture to survive including ferns and horsetails. They also host many epiphytes. These are plants that live on other plants like orchids, vines, ferns and moss. These biomes have a high diversity of amphibians, as well mammals and birds when compared to colder and drier biomes. These biomes are a major source of lumber, and affected by industrial forestry.

Temperate Deciduous Forest Biome

This biome includes large regions of the eastern United States, most of northern Europe, and Eastern China, where the climate alternates between mild winters and warm summers, with a fair amount of rain and snow. Temperate deciduous forests are characterized by trees that have adapted to the seasonal changes by losing their leaves in the fall, and re-growing them each year during the spring. This prevents the branches from breaking with the weight of snow in the winter months, but also allowing a greater surface area for photosynthesis in the warmer summer months. These forests are dominated by maple, oak, birch, and elm trees supported by rich soils. These regions are often cleared for farms and agriculture and host major urban centers because of the biological productivity of the soils for growing crops. Temperate deciduous forest are likely more dominate in Earth’s past, but have changed their distribution as the Earth has undergone various glacial and interglacial periods during the last several ice ages.

Tropical Rain Forest Biome

Tropical rain forest biomes are the most species rich biomes on Earth, and exhibit an enormous amount of different species of both plants and animals. This is a result of the lack of seasons, and the continued large amounts of rainfall. Tropical rain forests occur on land below the atmospheric Intertropical Convergence Zone (ITCZ), in which rain nearly continuously falls due to the atmospheric circulation and low pressure as a result of the equatorial position of these forests. This stable warm and wet climate allows trees to remain green throughout the year, and grow much more slowly. This also means that the forest is not shedding significantly large amounts of biomass onto the forest floor like in temperate deciduous forest, and the soils are organically poor, often sandy and leached of nutrients by the abundance of rain. Tropical rain forests support a tall forest canopy that is enclosed year-round, making the forest floor particularly dark. Closed forest canopies prevent sunlight from reaching the forest floor, while open forest canopies, where the trees are more widespread allow more sunlight to reach the forest floor. In close canopy tropical rainforests, sunlight is the limiting resource for plants. Tropical rainforests exhibit poor soils, and when cleared for agriculture often lead to muddy or sandy barren regions of dirt, since the soil is lacking organic carbon molecules and nutrients for plants to re-establish themselves. The rapid deforestation of rain forests has led to significant changes to these biomes, including the occurrence of forest fires. Forest fires in tropical rainforests result when famers and industrial agricultural agents clear the land. The cut wood is stacked and dried, and often set on fire to remove from it, but even if the wood is harvested or clear cut from the forest and removed by trucks, and the waste of wood shavings, chips, branches, twigs and saw dust is burned in large piles. These fires can set the surrounding virgin forest on fire, especially when they blow into these parts of the forest. Over the last decade there has been significant destruction of the tropical rainforest biome. In August of 2019 an estimated 3,500 square miles, or 9,060 square kilometers of the Amazon forest burned in fires across Brazil, Paraguay and Bolivia.

Tropical Deciduous Forest Biome

Tropical deciduous forests are characterized by seasonal wet and dry seasons, within warmer or tropical climates. These are sometimes referred as monsoon forests, as they are influenced by changes in the amount of rainfall. The forests across India, as well as Southeastern Asia, and parts of South America and Africa are greatly influenced by these cycles of rain. These regions rarely if ever see snow, as their temperatures remain warm throughout the year. A result of the monsoon cycle is that the soils often are red in color, due to the oxidation of iron, and tend to be less rich in nutrients than temperature deciduous forests because of leaching. Trees go dominate during the dry seasons and have a specific growing season.

Grassland Biome

Grasslands are broad regions of prairie or grassland steppes that lack significant numbers of trees due to the lack of water. They often have temperate climates, with cold winters and warm summers, and host a wide diversity of grass plants and other low laying scrubs. Grasslands undergo intense growing seasons in the summer, and fall dominate in the colder winters, leading to rich soils, despite the lack of trees. Grasslands host large mammals that feed on the grass covered landscape, including bison and prong horn antelope here in North America, but also have abundant insect and rodent populations, as well as snakes and lizards. Much of the world’s grassland biomes has been replaced by either grazing land or agricultural farms for the growth of wheat and corn. A few protected areas remain, established after the Dust Bowl years in the 1930s, the United State Government established national grasslands although much of this land is open to be leased for cattle, but are protected from agriculture to help maintain the soil.

Grazing Land Biome

Land that hosts large numbers of cattle, sheep, goats and horses for grazing is this biome, which incorporates aspects of grassland, savanna and desert biomes, but could include cleared forest land. Much of the grazing land biome is privately owned as small fenced fields to large ranches, but also includes government managed land that is leased to ranchers for access to grazing. Both cattle and sheep have been introduced to these areas, where they are raised for their skins and wool, as well as for their meat around the world. Grazing land depends on a rich source of grass vegetation, and often cultivate invasive species of plants such alfalfa or lucerne (Medicago sativa), cheat grass (Bromus tectorum) and knapweed (Centaurea diffusa).

Agricultural Biome

The agricultural is the biome of farms and irrigated lands that grow any type of crop. Often these are in regions that have a source of water that can be used on the crops to increase the growth. They typically have very low species richness and biodiversity of any biome because of the heavy use of pesticides and other deterrents for pests that might eat the crops. Corn, wheat, rice, and hay are the major crops on agricultural lands, incorporating various climates from wet rice paddies to dry wheat or alfalfa fields. Agricultural land covers much of the habitable terrestrial land on Earth, and relies on healthy soils and sources of water for the continued growth of food. It can host various mammals and birds, but often has limited biodiversity in plant life, which is found on the periphery of the biome.

Savanna Biome

Savannas are a mix of grassland and forest, which are composed of a scattered distribution of trees. They tend to have high temperatures, and more drier climates, with very seasonal rainy seasons. Savanna biomes are found across the African Sahel in northern Africa, and extending into Southern Africa, both places where precipitation is much lower than the more tropical equatorial regions. Savannas are biologically diverse, supporting large herds of mammals, and host animals from elephants, lions, zebras, antelope and wildebeests.

Chaparral Biome

Chaparrals are very dry biomes that support low scrubby evergreen bushes, and short drought-resistant trees. They are found in Southern California, parts of Arizonia, as well as around the Mediterranean Sea and Middle East and parts of Australia. A result of the vulnerably to frequent droughts, the chaparral biome is affected by frequent wild fires. Plants also tend to have large root structures that can tap into sources of water deep underground, and re-establish themselves after fires.

Desert Biome

Deserts are regions with low-precipitation and temperate to extremely hot climates. As arid environments, deserts have less total biomass, and depend on infrequent rain or snow. Plants are often adapted to the low availability of water, with some deserts nearly lacking plants altogether, with the landscape covered by blowing sand dunes. Many types of deserts exist, in Utah, the desert is a continental interior desert, in that low precipitation is due to the rain shadow of the more coastal mountain ranges in Nevada and California, which prevents significant rain compared to the coasts. Located within the center of the continent, these regions exhibit more extreme warm or hot summers, without the heat sink of the ocean. Deserts are found along the narrow strip of land between 30 to 40 degrees latitude, which due to the Hadley Cell atmospheric circulation pattern are under high pressure most of the time, and tend to exhibit dry air descending at these latitudes. This push of high pressure across these regions on Earth leads to air with low humidity and is undersaturated in respect to the availability of water. As the Earth warms the extent of Earth’s desert biomes is expect to expand poleward, as the atmosphere increases in temperature, leading to increased risk of drought in higher latitudes.

Cave Biome

Cave biomes are the unique life that lives underground in caves. Exhibiting a low species richness and diversity, caves offer a unique biome for life to live outside of the light, and utilized the cave temporarily as shelter. Cave biomes are interesting places to study the unique microscopic and bacterial life forms that are able to live in a biome without light. They often utilize sources of nutrients brought into caves, from such creatures as bats and small mammals that seek them out for shelter.

Urban Biome

The urban biome is likely the one that you are most familiar with since it encapsulates much of the landscape within city and town centers where most people live. This biome includes roads, parks, gardens, golf-courses, parking lots, homes, shopping centers, and warehouses. It is dominated by cultivated plants interspersed with asphalt roads and concrete sidewalks, with structures and buildings. They tend to have a higher biodiversity than agriculture lands due to the cultivation of a wider range of plant species, but like agricultural lands often relies on irrigation. The urban biome is home to many domesticated animals, such as cats and dogs, as well as humans, but also are frequented by raccoons, rats, and mice. The urban biome is rapidly expanding as new housing developments are built, which often grow out into the surrounding biomes. Urban biomes have high levels of plastic waste and other types of human refuse.

Aquatic Biomes

Freshwater River and Creek Biome

Rivers, streams and creeks and flowing freshwater on Earth are an important biome for both aquatic fish and insects, as well as the animals and plants that use these as sources of water. Flowing freshwater is an important source of well oxygenated waters that are utilized as spawning grounds for migratory fish. Large standing water bodies of water are prone to lose oxygen, especially in warm climates, and the flowing nature of freshwater rivers and streams provide an important biome for fish and the animals that depend them for food. Rivers provide a biome that is complex, as these ribbons of water move across different climates and elevations offering a path toward the interior of continents for aquatic life, as well as plants and animals that need access to water. The study of the freshwater river biome or ecosystem is called limnology.

Freshwater Lake and Pond Biome

Freshwater lakes and ponds are equally important biome of the freshwater system and interconnect rivers and streams. Lakes and ponds are biologically rich areas for supporting both aquatic animals and plants, as well as animals and plants that depend on them as a source of water. Many plants live in the shallow littoral zone on the edge of lakes including cattails, horsetails, and reeds. These regions are important for migrating birds, such as ducks and other waterfowl. Just like the ocean, lakes can become stratified, and can lose oxygen through eutrophication. Such events result in the death of fish and other animals that rely on well oxygenated waters. These events mark episodes of major disturbances to the biome, which can also occur during droughts as water levels drop, decreasing the extent of the biome.

Wetlands, Swamps and Marshes Biome

Wetlands are broadly defined as any land that is wet or covered by water during any part of the annual season. Mostly they are found in low poorly drained basins, where water will accumulate during rainy seasons. The amount of water and its depth various, but this more ephemeral source of water is important for amphibian, crocodilians and snakes, as well as birds and mammals. Trees often grow within these damp biomes, such as bald cypress (Taxodium distichum), that can live in standing water found in swamps in the southeastern United States. Many wetlands have been drained for use of the water, and various governmental agencies have worked hard to protect this biome.

Estuaries, Deltas and Tidal Flats Biome

This is a transitional environment between the ocean and land, and connects rivers and freshwater sources of water with saline ocean waters. This produces a biome similar to swamps and marshes, but with the water having a mix of salinity. These biomes are influenced by the daily and monthly tides, as well as longer term rise and fall of sea levels, which can flood regions along the coast. These regions are held together often by mangrove forests, which help trap sediment and nutrients, but can tolerate the salty water. Many invertebrate animals make use of the tides and water by burrowing into the muddy substrate, and filter feeding as the water moves in and out during the tidal cycles. Bivalves (such as clams and mussels) and barnacles as well as bryozoans, often encrust the rocks and wood to filter out food from the water. They are important biomes for birds, crocodilians, turtles as well as fish.

Coastal Lines and Beaches Biome

Beaches are also referred to as the littoral zone of the ocean, as it includes both the sandy coastline as well as the shallow near shore. The breaking of waves, and everchanging tides make beaches an interesting biome for animals and plants. Kelp (or seaweed) of the brown algae class Phaeophyceae, grows in the water, buoyed up by air sacs that help keep the kelp floating within the photic zone. Echinoderms, such as starfish and sea cucumbers lives in this zone, as well as gastropods (snails). Other mollusks such as bivalves dig down into the sandy substrate to filter feed from the passing water. The high energy of the beach coast line and crashing waves, make this region a seemly constantly changing environment, but it is rather stable over time when compared other biomes.

Open Ocean (Pelagic zones) Biome

This is the open ocean biome of swimming and floating plants and animals. The pelagic zone is divided into the photic zone near the surface, and the deeper aphotic zone below, which is dark. Animals live within this zone by freely swimming (like fish, sharks, and marine mammals and various invertebrates (like shrimp and cuttlefish), and those animals that float in the water (jellyfish, plankton, such as tiny dinoflagellates, diatoms, and foraminifera). The Open Ocean Biome is the largest biome on Earth and covers the most volume on Earth. This large volume is because the open ocean or pelagic zone includes a wide range of depth within the ocean waters. This biome is also the least visited by humans, and remains poorly understood given the challenges of studying these ocean creatures.

Ocean Floor (Benthic zones) Biome

The finally biome is the ocean floor, the animals that live by crawling around or sessile (attached to the ocean floor). The benthic zone covers about 70% of Earth, and host a wide variety of biomes, from near shore coral reefs, lagoons, to deep abyssal and hadal zones in the deepest and darkest parts of the Earth. The ocean floor is a unique biome because it depends heavily on the input for organic nutrients from the pelagic zone above, and much of the ocean floor lacks access to sunlight for energy. Most of the organisms that live on the ocean floor utilize the organic waste that rains down, making most of these organisms scavengers or decomposers with them utilizing chemosynthesis.

Measuring Biodiversity

Biodiversity today is often defined by the total variety and variability of life on Earth. It is often used in discussions of its importance for the preservation and protection of key biomes on Earth that support the many biological species. In measuring biodiversity ecologists define two indexes that can be used to compare biomes or regions. The first is richness.

Species richness

Species richness is simply the number of species that have been documented in a certain region or area, while family richness is defined more broadly as the number of families or closely related groups of species in a region or area. Species richness is determined by counting the number of different species within a geographic region, from a single country, or county to a broader ecosystem or biome. The various biomes on Earth have varying degrees of species richness. Tropical rain forest biomes of the Amazon and Congo Rainforests contain very high species richness, with nearly 20,000 species of vertebrates in these areas, while tundra and boreal forests biomes near the poles have low species richness with about 1,000 or less species of vertebrates. The cold and open tundra biome contains less resources for food and shelter, and a harsh environment for biological species. The complexity of the tropical forest environment and uniform climate allows for numerous environmental resources and a stable environment to live in. Species richness is correlates to latitude, with low latitude, equatorial regions having high species richness, while high latitude polar regions having low species richness. Moisture, and the average annual rainfall also effects species richness, but less than the variability of the season temperatures. In the ocean’s warm coral reefs, lagoons and estuaries all exhibit high species richness, while dark cold deep ocean water and regions with high salinity exhibit low species richness, due to limitation of resources within these regions of Earth.

One observation is that animals that live near the poles at high latitudes tend to be larger than animals that live near the equator. This is referred to as Bergman’s rule, named after Carl Bergman, a German a professor of anatomy and physiology in the mid-1800s. One explanation for this pattern is physiological, animals that live in colder climates will be larger to maintain body temperature during colder months of the year (low surface to body ratio), while warmer climates will favor smaller animals that are able to disperse heat more effectively with high surface to body ratios. This applies only to endothermic animals, like mammals and birds.

Determining species richness requires numerous surveys in which each organism is identified to its unique species. This is in practice is difficult to accomplish, although various government agencies have supported such efforts, often relying on the volunteer work of experts in various groups of organisms, from botanist, entomologists, herpetologist, ornithologists, and mammologists, each identifying the species in a given area. Species richness relies on a statistical process called rarefication. For example, each time you visit an area there may be different numbers of species present and observed. Rarefication is the continued sampling of individual organisms and measuring the likelihood of a new and novel species being counted. It looks at the curve of the increasing number of species, and how likely a new species will be encountered on a subsequent visit to the area. This is important, because many species are rare or only visit the region at certain times of the year. Another challenge in studying species richness is that many species, such as microscopic life, are often overlooked in such surveys. It is likely that many biomes are much richer in biological species than currently understood.

Biological diversity

Biological diversity is a measure of the representative abundances of each species in an environment. A biological diverse environment will have more species, but also a more evenly distributed abundance of these species in that environment. One of the measurements to determine biological diversity is the Shannon-Wiener index.

H'=-\sum _{i=1}^{R}p_{i}\ln p_{i}

Let us use a simple example of two groups and compare their Shannon-Wiener index using this formula above. One environment contains the following species:

Survey One
Species A = 25 individuals
Species B = 3 individuals
Species C = 1 individuals
Species D = 2 individuals
Species E = 3 individuals

Here we have 5 species represented by 34 individuals. First, we calculate the proportion of each species by dividing the number of individuals of that species with the total number to get proportions, for example Species A has 25 individuals out of 34, so it represents a proportion of 73.5% in the environment, while Species A with a single individual only represents 1.03% of the individuals. We take the natural log of each of these proportions, multiple them by the proportions, and total them up, divide by the number of species to get a mean and take the negative of the resulting index. So, for this first group the Shannon-Wiener index diversity is 0.185. Let us look at a second group.

Survey Two
Species A = 3 individuals
Species B = 3 individuals
Species C = 3 individuals
Species D = 3 individuals
Species E = 3 individuals

Notice in this group has 5 species, so the species richness is the same between these two groups, but now we have only 15 individuals, but each of those have similar proportions of 3.

We take the proportion of each of these species, which are all 32% and take the natural log, and multiply the result by the proportion, and take the mean by dividing the total by 5, then take the negative. In this cause the resulting index is higher 0.322. That is because there is a more equal proportion of each species abundance to each other. We can say that the first group is less diverse than the second group, despite the fact that there are more individuals in the first group. Such measures of biodiversity are very important because a single species can come to dominate a region, this is particularly an issue with invading species and in particular agricultural and grazing lands where plants or animals of a certain type predominate at the expense of other species. This process results in very low biodiversity in these regions, and often is reflected in species richness numbers as well.

One of the central issues in the study of biodiversity is understanding the mechanisms for promoting a biological diverse ecosystem and maintaining it. Both ecological and paleoecologically studies suggest two major factors come into play in maintaining a biological diverse biome. First is a stable physical environment, with a climate remaining seasonally stable for long periods of time, and second that the environment is left undisturbed, avoiding short term events such forest fires, asteroid impacts, urban and industrial development and other short-term calamities that result in an alteration to the landscape. One aspect of disturbing a landscape is the introduction of a rapidly reproducing invasive species, which can take over a region and dominate the biodiversity of the area. But most disturbances to the landscape are done by humans directly, from building a road, to plowing a field.

Ecological Succession

Ecological succession is the process under which biological communities of plants and animals recover from a major disturbance. These disturbances can be natural, like landslides, volcanic eruptions, or wildfires, or be human induced such as clearing a forest for agriculture, mining, or industrialization of the landscape such as oil and gas drilling, building a parking lot or road and driving across the land. All these disturbances leave behind major altered biological communities, often with highly reduced species richness and biodiversity than before the disturbance. Ecologists study how long an area recovers from a particular disturbance. Ecologist begin by defining a Climax Community. A climax community is a stable community that is self-perpetuating and in equilibrium with the physical habitat of the area, with high levels of species richness and biodiversity, with low to no decline of species occurrences within the area. A climax community can be regarded as a stable community with high biodiversity, that would have existed prior to the disturbance. Early naturalists, like Henry David Thoreau wrote about the succession of forest trees from farms that had been cleared, but subsequently left fallow, and were eventually replaced by trees that reclaimed the farm field after numerous years had passed. One of the key scientists in the study of ecological succession, who also helped coined the term ecology, was Henry Chandler Cowles. In the late 1800 he studied areas that were covered by wind-blown sand dunes and studied how plants re-established themselves over the course of multiple years. Volcanic eruptions and intense wild fires have also proven interesting case studies to understand the succession of plants following the recovery of disturbance.

The idea of ecological succession governs much of land management practices, particularly in the United States where land disturbance is widely regarded as transitory and temporary, and given enough time, the area will recover to its pre-disturbance state. However, land disturbance by human activities often are not a rare occurrence, but frequent, cyclic and cumulative over time, limiting the full recovery. Often a prior climax community may be unknown, as the area may historically have been over grazed and clear cut to begin with. With frequent ground disturbances over hundreds of years, the original species richness and biodiversity of the area may not have been recorded prior to the initiation of these disturbances. Disturbed areas are susceptible to invasive species of both plants and animals, often which take a foothold into the area and alter the area differently than prior to the disturbance. Land managers may alter the environment on a continued basis, such the massive removal of juniper and pinyon trees across Utah to provide grazing land for cattle on public lands or efforts to start controlled burns to prevent major wildfires that might destroy economical valuable structures. Reseeding programs, where disturbed lands from mining and oil and gas drilling on public lands are re-seeded lead to different replacement vegetation, as those that existed prior to the ground disturbances. Contamination of ground water, or the draining of ponds and lakes for use on crops, can lead to long term changes to the land, altering the availability of water making a succession toward a climax community not likely. Furthermore, a consequence of a changing climate over time, and the long-term process of ecological succession, means that prior climax communities existed under a cooler wetter climate, and these plants and animals can no longer be established in the area because of climate change. In truth, the concept of Earth’s biomes is in flux, and changing, with examples of natural biomes becoming limited to protected lands on national parks, and a growing expanse of disturbed lands (such as Urban, Agricultural, and Grazing Biomes), within new sets of biome definitions.

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f. Gregor Mendel’s Game of Cards: Heredity.	g. Earth’s Biomes and Communities.	h. Soil: Living Dirt.