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4.2: Biogeografia

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    Habilidades para desenvolver

    • Definir biogeografia
    • Listar e descrever os fatores abióticos que afetam a distribuição global de espécies vegetais e animais
    • Compare o impacto das forças abióticas em ambientes aquáticos e terrestres
    • Resuma o efeito dos fatores abióticos na produtividade primária líquida

    Muitas forças influenciam as comunidades de organismos vivos presentes em diferentes partes da biosfera (todas as partes da Terra habitadas pela vida). A biosfera se estende até a atmosfera (vários quilômetros acima da Terra) e até as profundezas dos oceanos. Apesar de sua aparente vastidão para um ser humano individual, a biosfera ocupa apenas um espaço minúsculo quando comparada ao universo conhecido. Muitas forças abióticas influenciam onde a vida pode existir e os tipos de organismos encontrados em diferentes partes da biosfera. Os fatores abióticos influenciam a distribuição dos biomas: grandes áreas de terra com clima, flora e fauna semelhantes.

    Biogeografia

    A biogeografia é o estudo da distribuição geográfica dos seres vivos e dos fatores abióticos que afetam sua distribuição. Fatores abióticos, como temperatura e precipitação, variam com base principalmente na latitude e elevação. À medida que esses fatores abióticos mudam, a composição das comunidades vegetais e animais também muda. Por exemplo, se você começasse uma jornada no equador e caminhasse para o norte, notaria mudanças graduais nas comunidades de plantas. No início de sua jornada, você veria florestas tropicais úmidas com árvores perenes de folhas largas, que são características das comunidades vegetais encontradas perto do equador. Ao continuar viajando para o norte, você veria essas plantas perenes de folhas largas eventualmente dando origem a florestas sazonalmente secas com árvores dispersas. Você também começaria a notar mudanças na temperatura e na umidade. A cerca de 30 graus ao norte, essas florestas dariam lugar a desertos, que são caracterizados pela baixa precipitação.

    Mais ao norte, você verá que os desertos são substituídos por pastagens ou pradarias. Eventualmente, as pastagens são substituídas por florestas decíduas temperadas. Essas florestas decíduas dão lugar às florestas boreais encontradas no subártico, a área ao sul do Círculo Polar Ártico. Finalmente, você alcançaria a tundra ártica, que se encontra nas latitudes mais setentrionais. Essa caminhada para o norte revela mudanças graduais no clima e nos tipos de organismos que se adaptaram aos fatores ambientais associados aos ecossistemas encontrados em diferentes latitudes. No entanto, diferentes ecossistemas existem na mesma latitude devido em parte a fatores abióticos, como correntes de jato, Corrente do Golfo e correntes oceânicas. Se você subisse uma montanha, as mudanças que você veria na vegetação seriam paralelas às que você se movia para latitudes mais altas.

    Ecologistas que estudam biogeografia examinam padrões de distribuição de espécies. Nenhuma espécie existe em todos os lugares; por exemplo, a armadilha de Vênus é endêmica em uma pequena área nas Carolina do Norte e do Sul. Uma espécie endêmica é aquela que é encontrada naturalmente apenas em uma área geográfica específica que geralmente é restrita em tamanho. Outras espécies são generalistas: espécies que vivem em uma grande variedade de áreas geográficas; o guaxinim, por exemplo, é nativo da maior parte da América do Norte e Central.

    Os padrões de distribuição de espécies são baseados em fatores bióticos e abióticos e suas influências durante os longos períodos de tempo necessários para a evolução das espécies; portanto, os primeiros estudos da biogeografia estavam intimamente ligados ao surgimento do pensamento evolutivo no século XVIII. Algumas das assembléias mais distintas de plantas e animais ocorrem em regiões que foram fisicamente separadas por milhões de anos por barreiras geográficas. Os biólogos estimam que a Austrália, por exemplo, tenha entre 600.000 e 700.000 espécies de plantas e animais. Aproximadamente 3/4 das espécies vivas de plantas e mamíferos são espécies endêmicas encontradas apenas na Austrália (Figura\(\PageIndex{1}\)).

    A foto (a) retrata um canguru, membro da família dos cangurus. O canguru é marrom com manchas brancas no pelo e uma barriga marrom clara. Suas mãos estão entrelaçadas. A foto (b) mostra uma equidna. Como um porco-espinho, a equidna tem um corpo compacto coberto por penas marrons e brancas. Tem um focinho longo e delgado.
    Figura\(\PageIndex{1}\): A Austrália abriga muitas espécies endêmicas. O (a) canguru (Wallabia bicolor), um membro de tamanho médio da família dos cangurus, é um mamífero em bolsa ou marsupial. A (b) equidna (Tachyglossus aculeatus) é um mamífero que põe ovos. (crédito a: modificação da obra de Derrick Coetzee; crédito b: modificação da obra de Allan Whittome)

    Às vezes, os ecologistas descobrem padrões únicos de distribuição de espécies determinando onde as espécies não são encontradas. O Havaí, por exemplo, não tem espécies terrestres nativas de répteis ou anfíbios e tem apenas um mamífero terrestre nativo, o morcego. A maior parte da Nova Guiné, como outro exemplo, carece de mamíferos placentários.

    Link para o aprendizado

    Confira este vídeo para observar um ornitorrinco nadando em seu habitat natural em New South Wales, Austrália.

    As plantas podem ser endêmicas ou generalistas: as plantas endêmicas são encontradas apenas em regiões específicas da Terra, enquanto as generalistas são encontradas em muitas regiões. Massas de terra isoladas, como Austrália, Havaí e Madagascar, geralmente têm um grande número de espécies de plantas endêmicas. Algumas dessas plantas estão ameaçadas de extinção devido à atividade humana. A gardênia florestal (Gardenia brighamii), por exemplo, é endêmica do Havaí; acredita-se que existam apenas 15 a 20 árvores (Figura\(\PageIndex{2}\)).

    The photo shows a white flower with seven smooth, diamond-shaped petals radiating out from a yellow center. The flower is surrounded by waxy green leaves.
    Figure \(\PageIndex{2}\): 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. (credit: Forest & Kim Starr)

    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 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 (Figure \(\PageIndex{3}\)). 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.

    This photo shows a white flower with five diamond-shaped petals radiating out from a green center. Faint purple lines radiate out from the center of each petal toward the tip. Five stalk-like stamens with pink-tipped anthers extend from the flower’s green center.
    Figure \(\PageIndex{3}\): The spring beauty is an ephemeral spring plant that flowers early in the spring to avoid competing with larger forest trees for sunlight. (credit: John Beetham)

    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; for example, think of the broad, floating leaves of a water lily—water lilies cannot survive without light. In environments such as hydrothermal vents, some bacteria extract energy from inorganic chemicals because there is no light for photosynthesis.

    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 living 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 (Figure \(\PageIndex{4}\)). 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.

    Arrows in the illustration indicate that the prevailing wind direction is from the coastline toward the open ocean. The wind pushes the surface water away from shore, inducing a current in this direction. A counter-current flows from the depths toward shore, where it meets the surface current. The counter-current brings nutrients from the depths up toward the surface near the shoreline.
    Figure \(\PageIndex{4}\): 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. The spring and fall turnover is a seasonal process that recycles nutrients and oxygen from the bottom of a freshwater ecosystem to the top of a body of water (Figure \(\PageIndex{5}\)). 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, and 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.

    Art Connection

    The illustration shows a cross-section of a lake in four different seasons. In winter, the surface of the lake is frozen with a temperature of 0°C. The temperature at the bottom of the lake is 4°C, and the temperature just beneath the surface is 2°C. During the spring turnover, the surface ice melts and warms to 4°C. At this temperature, the surface water is denser than the 2°C water beneath; therefore, it sinks. In summertime, the surface of the lake is 21°C, and the temperature decreases with depth, to 4°C at the bottom. During the fall turnover, the warm surface water cools to about 10°C; thus, it becomes denser and sinks.
    Figure \(\PageIndex{5}\): 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, and denser water sinks.

    How might turnover in tropical lakes differ from turnover in lakes that exist in temperate regions?

    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 is then displaced by the heavier surface water and, thus, rises to the top. As that water rises to the top, 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; therefore, 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 (Figure \(\PageIndex{5}\)). During the winter, the oxygen at the bottom of the lake is used by decomposers and other organisms requiring oxygen, such as fish.

    Temperature

    Temperature affects the physiology of living things as well as the density and state of water. Temperature exerts an important influence on living things because few living things can survive at temperatures below 0 °C (32 °F) due to metabolic constraints. It is also rare for living things 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 either must maintain an internal temperature or they must 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, such as seen in hibernation or reptilian torpor. Similarly, some bacteria are adapted to surviving in extremely hot temperatures 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 an adaptation found in many 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 and 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: migration carries risk and comes at a high energy cost.

    Some animals hibernate or estivate to survive hostile temperatures. Hibernation enables animals to survive cold conditions, and 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.

    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.

    • 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 are surrounded by water and 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.

    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 (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.

    Other Aquatic Factors

    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, however, 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, 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 (Figure \(\PageIndex{6}\)). Through the burning of pine needles, fire adds nitrogen to the soil and limits competition by destroying undergrowth.

    Photo shows two pine cones that are tightly closed and attached to a branch.
    Figure \(\PageIndex{6}\): The mature cones of the jack pine (Pinus banksiana) open only when exposed to high temperatures, such as during a forest fire. A fire is likely to 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. (credit: USDA)

    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). 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.

    Summary

    Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution. Endemic species are species that are naturally found only in a specific geographic area. The distribution of living things is influenced by several environmental factors that are, in part, controlled by the latitude or elevation at which an organism is found. Ocean upwelling and spring and fall turnovers are important processes regulating the distribution of nutrients and other abiotic factors important in aquatic ecosystems. Energy sources, temperature, water, inorganic nutrients, and soil are factors limiting the distribution of living things in terrestrial systems. Net primary productivity is a measure of the amount of biomass produced by a biome.

    Art Connections

    Figure \(\PageIndex{5}\): How might turnover in tropical lakes differ from turnover in lakes that exist in temperate regions?

    Answer

    Tropical lakes don’t freeze, so they don’t undergo spring turnover in the same way temperate lakes do. However, stratification does occur, as well as seasonal turnover.

    Glossary

    aboveground biomass
    total mass of aboveground living plants per area
    biogeography
    study of the geographic distribution of living things and the abiotic factors that affect their distribution
    biome
    ecological community of plants, animals, and other organisms that is adapted to a characteristic set of environmental conditions
    endemic
    species found only in a specific geographic area that is usually restricted in size
    fall and spring turnover
    seasonal process that recycles nutrients and oxygen from the bottom of a freshwater ecosystem to the top
    net primary productivity
    measurement of the energy accumulation within an ecosystem, calculated as the total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration
    ocean upwelling
    rising of deep ocean waters that occurs when prevailing winds blow along surface waters near a coastline
    thermocline
    layer of water with a temperature that is significantly different from that of the surrounding layers