Biogeography

Biogeography

Biogeography is an ecological field of interest that focuses on the distribution of organisms and the abiotic factors that affect them.

Learning Objectives

Explain the role of biogeography in the analysis of species distribution

Key Takeaways

Key Points

  • The composition of plant and animal communities change as abiotic factors, which include temperature and altitude, start to vary.
  • Some species exist only in specific geographical areas while others can thrive in a variety of areas; however, no single species can be found everywhere in the world.
  • Studying an area where a species is not found is also of importance to ecologists in determining unique patterns of species distribution.
  • As with animals, plant species can also be either endemic, usually found in isolated land masses, or generalists, found in many regions.

Key Terms

  • biogeography: the study of the geographical distribution of living things
  • generalist: species which can thrive in a wide variety of environmental conditions
  • endemic: unique to a particular area or region; not found in other places

Biogeography

Biogeography is the study of the geographic distribution of living things and the abiotic (non-living) factors that affect their distribution. Abiotic factors can include temperature, moisture, nutrients, oxygen, and energy availability, as well as disturbances from events such as wind and fire. Differences in temperature and rainfall are primarily based on latitude and elevation. As these abiotic factors change, the composition of plant and animal communities also changes. For example, if you were to begin a journey at the equator and walk north, you would notice gradual changes in plant communities. At the beginning of your journey, you would see tropical wet forests with broad-leaved evergreen trees, which are characteristic of plant communities found near the equator. As you continued to travel north, you would see these broad-leaved evergreen plants eventually give rise to seasonally-dry forests with scattered trees. You would also begin to notice changes in temperature and moisture. At about 30 degrees north, these forests would give way to deserts, which are characterized by low precipitation.

Moving farther north, you would see that deserts are replaced by grasslands or prairies. Eventually, grasslands are replaced by deciduous temperate forests. These deciduous forests give way to the boreal forests found in the subarctic, the area south of the Arctic Circle. Finally, you would reach the Arctic tundra, which is found at the most northern latitudes. This trek north reveals gradual changes in both climate and the types of organisms that have adapted to environmental factors associated with ecosystems found at different latitudes. However, different ecosystems exist at the same latitude due in part to abiotic factors such as jet streams, the Gulf Stream, and ocean currents. If you were to hike up a mountain, the changes you would see in the vegetation would parallel those as you move to higher latitudes.

Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere. For example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size. Other species are generalists, living in a wide variety of geographic areas. The raccoon, for example, is native to most of North and Central America.

Species distribution patterns are based on biotic and abiotic factors and the influences these factors have had during the very long periods of time required for species evolution. Therefore, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century. Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and animals. Approximately 3/4 of living plant and mammal species are endemic species found solely in Australia.

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Endemic species of Australia: Australia is home to many endemic species. The (a) wallaby (Wallabia bicolor), a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial. The (b) echidna (Tachyglossus aculeatus) is an egg-laying mammal.

Sometimes ecologists discover unique patterns of species distribution by determining where species are not found. Hawaii, for example, has no native land species of reptiles or amphibians and has only one native terrestrial mammal, the hoary bat. Most of New Guinea, as another example, lacks placental mammals.

Plants can be endemic or generalists. Endemic plants are found only in specific regions of the earth, while generalists are found in many regions. Isolated land masses, such as Australia, Hawaii, and Madagascar, often have large numbers of endemic plant species. Some of these plants are endangered due to human activity. The forest gardenia (Gardenia brighamii), for instance, is endemic to Hawaii; only an estimated 15–20 trees are thought to exist.

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Endangered forest gardenia: Listed as federally endangered, the forest gardenia is a small tree with distinctive flowers. It is found only in five of the Hawaiian Islands in small populations consisting of a few individual specimens.

Energy Sources

The availability of energy and nutrient sources affects species distribution and their adaptation to land or aquatic habitats.

Learning Objectives

Assess how energy availability affects species distribution within an ecosystem

Key Takeaways

Key Points

  • In land habitats, plant adaptations include life cycles that are dependent on the availability of light; for example, species will flower or grow at varying times to ensure they capture enough available light suitable to their needs.
  • In aquatic ecosystems, species growth and distribution are adapted to deal with the sometimes-limited availability of light due to its absorption by water, plants, suspended particles, microorganisms, and water depth.
  • Ocean upwelling and spring and fall turnovers are important processes regulating the distribution of nutrients in an aquatic ecosystems.
  • Nutrient availability is connected to the energy needs of organisms in aquatic ecosystems since sequestered energy is reused by living organisms from dead ones.

Key Terms

  • ephemeral: lasting for a short period of time
  • upwelling: the oceanographic phenomenon that occurs when strong, usually seasonal, winds push water away from the coast, bringing cold, nutrient-rich deep waters up to the surface
  • thermocline: a layer within a body of water or air where the temperature changes rapidly with depth

Energy Availability & Nutrient Cycling as Abiotic Factors

Energy Sources

Energy from the sun is captured by green plants, algae, cyanobacteria, and photosynthetic protists. These organisms convert solar energy into the chemical energy needed by all living things. Light availability can be an important abiotic force directly affecting the evolution of adaptations in photosynthesizers. For instance, plants in the understory of a temperate forest are shaded when the trees above them in the canopy completely leaf out in the late spring. Not surprisingly, understory plants have adaptations to successfully capture available light. One such adaptation is the rapid growth of spring ephemeral plants, such as the spring beauty. These spring flowers achieve much of their growth and finish their life cycle (reproduce) early in the season before the trees in the canopy develop leaves.

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Ephemeral plant: The spring beauty is an ephemeral spring plant that flowers early in the spring to avoid competing with larger forest trees for sunlight.

In aquatic ecosystems, the availability of light may be limited because sunlight is absorbed by water, plants, suspended particles, and resident microorganisms. Toward the bottom of a lake, pond, or ocean, there is a zone that light cannot reach. Photosynthesis cannot take place there and, as a result, a number of adaptations have evolved that enable living things to survive without light. For instance, aquatic plants have photosynthetic tissue near the surface of the water. The broad, floating leaves of a water lily cannot survive without light. In environments such as hydrothermal vents, some bacteria extract energy from inorganic chemicals because there is no light for photosynthesis.

Nutrient Cycling

The availability of nutrients in aquatic systems is also an important aspect of energy or photosynthesis. Many organisms sink to the bottom of the ocean when they die in the open water. When this occurs, the energy found in that organism is sequestered for some time unless ocean upwelling occurs. Ocean upwelling is the rising of deep ocean waters that occurs when prevailing winds blow along surface waters near a coastline. As the wind pushes ocean waters offshore, water from the bottom of the ocean moves up to replace this water. As a result, the nutrients once contained in dead organisms become available for reuse by other living organisms.

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Upwelling: Ocean upwelling is an important process that recycles nutrients and energy in the ocean. As wind (green arrows) pushes offshore, it causes water from the ocean bottom (red arrows) to move to the surface, bringing up nutrients from the ocean depths.

In freshwater systems, the recycling of nutrients occurs in response to air temperature changes. The nutrients at the bottom of lakes are recycled twice each year: in the spring and fall turnover, which recycles nutrients and oxygen from the bottom of a freshwater ecosystem to the top of a body of water. These turnovers are caused by the formation of a thermocline: a layer of water with a temperature that is significantly different from that of the surrounding layers. In wintertime, the surface of lakes found in many northern regions is frozen. However, the water under the ice is slightly warmer, while the water at the bottom of the lake is warmer yet at 4 °C to 5 °C (39.2 °F to 41 °F). Water is densest at 4 °C; therefore, the deepest water is also the densest. The deepest water is oxygen poor because the decomposition of organic material at the bottom of the lake uses up available oxygen that cannot be replaced by means of oxygen diffusion into the water due to the surface ice layer.

In springtime, air temperatures increase and surface ice melts. When the temperature of the surface water begins to reach 4 °C, the water becomes heavier and sinks to the bottom. The water at the bottom of the lake, displaced by the heavier surface water, rises to the top. As it rises, the sediments and nutrients from the lake bottom are brought along with it. During the summer months, the lake water stratifies, or forms layers, with the warmest water at the lake surface.

As air temperatures drop in the fall, the temperature of the lake water cools to 4 °C; this causes fall turnover as the heavy cold water sinks and displaces the water at the bottom. The oxygen-rich water at the surface of the lake then moves to the bottom of the lake, while the nutrients at the bottom of the lake rise to the surface (). During the winter, the oxygen at the bottom of the lake is used by decomposers and other organisms requiring oxygen, such as fish.

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Nurient recycling in freshwater systems: The spring and fall turnovers are important processes in freshwater lakes that act to move the nutrients and oxygen at the bottom of deep lakes to the top. Turnover occurs because water has a maximum density at 4 °C. Surface water temperature changes as the seasons progress, causing denser water to sink.

Temperature and Water

Temperature and water are important abiotic factors that affect species distribution.

Learning Objectives

Describe species adaptations to temperature fluctuations and the availability of water

Key Takeaways

Key Points

  • Temperature is a factor that influences species distribution because organisms must either maintain a specific internal temperature or inhabit an environment that will keep the body within a temperature range that supports their metabolism.
  • Many species have developed adaptations, such as migration, hibernation, and estivation, to deal with temperature fluctuations in the environments in which they live.
  • Water retention is vital to all living beings; adaptations have evolved within both terrestrial and aquatic species to minimize water loss.

Key Terms

  • osmosis: The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane
  • estivate: to go into stasis or torpor in the summer months
  • extremophile: an organism that lives under extreme conditions of temperature, salinity, etc; commercially important as a source of enzymes that operate under similar conditions
  • torpor: a state of being inactive or stuporous
  • hibernation: a state of inactivity and metabolic depression in animals during winter

Temperature & Water as Abiotic Influences

Temperature

Temperature affects the physiology of living things as well as the density and state of water. It exerts an important influence on living organisms because few can survive at temperatures below 0 °C (32 °F) due to metabolic constraints. It is also rare for them to survive at temperatures exceeding 45 °C (113 °F). This is a reflection of evolutionary response to typical temperatures. Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur at higher temperatures. Therefore, organisms must either maintain an internal temperature or inhabit an environment that will keep the body within a temperature range that supports metabolism. Some animals have adapted to enable their bodies to survive significant temperature fluctuations, as seen in hibernation or reptilian torpor. Similarly, some bacteria have adapted to survive in extremely-hot temperatures found in places such as geysers. Such bacteria are examples of extremophiles: organisms that thrive in extreme environments.

Temperature can limit the distribution of living things. Animals faced with temperature fluctuations may respond with adaptations, such as migration, in order to survive. Migration, the movement from one place to another, is common in animals, including many that inhabit seasonally-cold climates. Migration solves problems related to temperature, locating food, and finding a mate. In migration, for instance, the arctic tern (Sterna paradisaea) makes a 40,000 km (24,000 mi) round trip flight each year between its feeding grounds in the southern hemisphere and its breeding grounds in the Arctic Ocean. Monarch butterflies (Danaus plexippus) live in the eastern United States in the warmer months, but migrate to Mexico and the southern United States in the wintertime. Some species of mammals also make migratory forays: reindeer (Rangifer tarandus) travel about 5,000 km (3,100 mi) each year to find food. Amphibians and reptiles are more limited in their distribution because they lack migratory ability. Not all animals that can migrate do so as migration carries risk and comes at a high energy cost.

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Arctic tern: The arctic tern is an example of a species that must migrate yearly to deal with temperature fluctuations that exist in the regions where it is found.

Some animals hibernate or estivate to survive hostile temperatures. Hibernation enables animals to survive cold conditions, while estivation allows animals to survive the hostile conditions of a hot, dry climate. Animals that hibernate or estivate enter a state known as torpor, a condition in which their metabolic rate is significantly lowered. This enables the animal to wait until its environment better supports its survival. Some amphibians, such as the wood frog (Rana sylvatica), have an antifreeze-like chemical in their cells, which retains the cells’ integrity and prevents them from bursting.

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Wood frog: The wood frog, like all other amphibians and reptiles, cannot migrate; as a result, the species survives extreme temperature changes through the antifreeze-like chemical found in their cells.

Water

Water is required by all living things because it is critical for cellular processes. Since terrestrial organisms lose water to the environment by simple diffusion, they have evolved many adaptations to retain water.

Examples of adaptations used by terrestrial and aquatic species include the following:

  • Plants have a number of interesting features on their leaves, such as leaf hairs and a waxy cuticle, that serve to decrease the rate of water loss via transpiration.
  • Freshwater organisms, surrounded by water, are constantly in danger of having water rush into their cells because of osmosis. Many adaptations of organisms living in freshwater environments have evolved to ensure that solute concentrations in their bodies remain within appropriate levels. One such adaptation is the excretion of dilute urine.
  • Marine organisms are surrounded by water with a higher solute concentration than the organism and, thus, are in danger of losing water to the environment because of osmosis. These organisms have morphological and physiological adaptations to retain water and release solutes into the environment. For example, marine iguanas (Amblyrhynchus cristatus) sneeze out water vapor that is high in salt in order to maintain solute concentrations within an acceptable range while swimming in the ocean and eating marine plants.
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Marine iguanas: Marine iguanas have a special, salt-secretion adaption that allows them to minimize bodily water loss.

Inorganic Nutrients and Other Factors

Soil structure, oxygen availability, wind, and fire are abiotic factors that have influences on species distribution and quantity.

Learning Objectives

Identify further abiotic factors that affect species distribution and abundance

Key Takeaways

Key Points

  • Soil structure, pH, and its nutrient content affect the distribution of plants, which in turn influences the distribution of the animals that feed on them.
  • Oxygen availability is an important abiotic factor affecting species in both aquatic and terrestrial environments.
  • Wind and fire impose physical disturbances that species must be adapted to in order to live in affected areas.

Key Terms

  • inorganic: relating to a compound that does not contain carbon
  • transpiration: the loss of water by evaporation in terrestrial plants, especially through the stomata; accompanied by a corresponding uptake from the roots

Other Important Abiotic Factors

Inorganic nutrients, soil structure, and aquatic oxygen availability are further abiotic factors that affect species distribution in an ecosystem. The same is true for terrestrial factors, such as wind and fire, which can impact the types of species that inhabit regions exposed to these types of disturbances.

Inorganic Nutrients and Soil

Inorganic nutrients, such as nitrogen and phosphorus, are important in the distribution and the abundance of living things. Plants obtain these inorganic nutrients from the soil when water moves into the plant through the roots. Therefore, soil structure (the particle size of soil components), soil pH, and soil nutrient content play an important role in the distribution of plants. Animals obtain inorganic nutrients from the food they consume. Therefore, animal distributions are related to the distribution of what they eat. In some cases, animals will follow their food resource as it moves through the environment.

Oxygen Availability

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Jack pine cones: The mature cones of the jack pine (Pinus banksiana) open only when exposed to high temperatures, such as during a forest fire. A fire will probably kill most vegetation, so a seedling that germinates after a fire is more likely to receive ample sunlight than one that germinates under normal conditions.

Some abiotic factors, such as oxygen, are important in aquatic ecosystems as well as terrestrial environments. Terrestrial animals obtain oxygen from the air they breathe. Oxygen availability can be an issue for organisms living at very high elevations, where there are fewer molecules of oxygen in the air. In aquatic systems, the concentration of dissolved oxygen is related to water temperature and the speed at which the water moves. Cold water has more dissolved oxygen than warmer water. In addition, salinity, water current, and tide can be important abiotic factors in aquatic ecosystems.

Other Terrestrial Factors

Wind can be an important abiotic factor because it influences the rate of evaporation and transpiration. The physical force of wind is also important because it can move soil, water, or other abiotic factors, as well as an ecosystem’s organisms.

Fire is another terrestrial factor that can be an important agent of disturbance in terrestrial ecosystems. Some organisms are adapted to fire and, thus, require the high heat associated with fire to complete a part of their life cycle. For example, the jack pine, a coniferous tree, requires heat from fire for its seed cones to open. Through the burning of pine needles, fire adds nitrogen to the soil and limits competition by destroying undergrowth.

Abiotic Factors Influencing Plant Growth

The two most important abiotic factors affecting plant primary productivity in an ecosystem are temperature and moisture.

Learning Objectives

Identify the abiotic factors that affect plant growth

Key Takeaways

Key Points

  • Primary production, on which almost all of life on earth is dependent, occurs through either photosynthesis or chemosynthesis.
  • Annual biomass production, used to estimate net primary productivity by plants in an area, is directly influenced by an environment’s abiotic factors, which include temperature and moisture.
  • Warm and wet climates have the greatest amount of plant biomass because they offer conditions in which photosynthesis, plant growth, and the resulting net primary productivity are highest.

Key Terms

  • biomass: the total mass of all living things within a specific area, habitat, etc.
  • eco-region: a region, smaller than an ecozone, that contains a distinct biodiversity of flora and fauna
  • chemosynthesis: the production of carbohydrates and other compounds using the oxidation of chemical nutrients as a source of energy rather than sunlight; it is limited to certain bacteria and fungi

Abiotic Factors Influencing Plant Growth

Temperature and moisture are important influences on plant production (primary productivity) and the amount of organic matter available as food (net primary productivity). Primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of chemical compounds as its source of energy. Almost all life on earth is directly or indirectly reliant on primary production. The organisms responsible for primary production, known as primary producers or autotrophs, form the base of the food chain. In terrestrial eco-regions, these are mainly plants, while in aquatic eco-regions, they are mainly algae.

Net primary productivity is an estimation of all of the organic matter available as food. It is calculated as the total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration. In terrestrial environments, net primary productivity is estimated by measuring the aboveground biomass per unit area, which is the total mass of living plants, excluding roots. This means that a large percentage of plant biomass which exists underground is not included in this measurement. Net primary productivity is an important variable when considering differences in biomes. Very productive biomes have a high level of aboveground biomass.

Annual biomass production is directly related to the abiotic components of the environment. Environments with the greatest amount of biomass have conditions in which photosynthesis, plant growth, and the resulting net primary productivity are optimized. The climate of these areas is warm and wet. Photosynthesis can proceed at a high rate, enzymes can work most efficiently, and stomata can remain open without the risk of excessive transpiration. Together, these factors lead to the maximal amount of carbon dioxide (CO2) moving into the plant, resulting in high biomass production. The aboveground biomass produces several important resources for other living things, including habitat and food. Conversely, dry and cold environments have lower photosynthetic rates and, therefore, less biomass. The animal communities living there will also be affected by the decrease in available food.

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Primary productivity and biomass production: The magnitude and distribution of global primary production varies between biomes. However, warm and wet climates have the greatest amount of annual biomass production.