44.2 Biogeography

Biogeography and Species Distribution

Biogeography studies how and why species are distributed across the planet. It combines biology and geography to explain patterns we see in nature, like why polar bears live in the Arctic but not in the tropics, or why islands often have unique species found nowhere else. Understanding biogeography also helps explain how ecosystems respond to changes in climate and landscape over time.

Concept of Biogeography

Biogeography examines the distribution of species and ecosystems across geographic space and through geological time. It asks a simple but powerful question: why do organisms live where they do?

The answer involves several interacting factors:

• Historical events like continental drift, ice ages, and volcanic activity have shaped where lineages of organisms ended up over millions of years.
• Ecological conditions such as climate, topography, and soil type determine whether a species can survive and reproduce in a given area.
• Biotic interactions like competition, predation, and mutualism further refine which species coexist in a region.

By studying these factors together, biogeography provides insights into evolutionary history and helps predict how species distributions may shift as environments change.

Abiotic Factors in Species Distribution

Abiotic factors are the non-living physical and chemical conditions of an environment. They set the baseline rules for where organisms can survive.

• Temperature directly affects metabolic rates, growth, and survival. Every species has a range of temperatures it can tolerate. Polar bears are adapted to Arctic cold, while cacti thrive in hot desert conditions. Species that end up outside their thermal tolerance range simply can't persist.
• Precipitation controls water availability, which is one of the strongest drivers of biome type. High rainfall supports tropical rainforests; low rainfall produces grasslands or deserts.
• Sunlight provides the energy that drives photosynthesis, forming the base of nearly every food web. Sunlight intensity varies with latitude: tropical regions receive more direct sunlight year-round than polar regions, contributing to differences between biomes like tropical forests and tundra.
• Topography refers to the physical shape of the land. Mountain ranges can block species dispersal and create rain shadows, while valleys may funnel organisms along corridors. Elevation changes also produce microclimates, so a single mountain can host several distinct habitat types from base to summit.
• Soil composition determines which nutrients are available for plant growth, which in turn shapes the animal communities that depend on those plants. Factors like pH and water retention matter too. Serpentine soils, for example, are nutrient-poor and toxic to many plants, so only specially adapted species grow there.
• Dispersal barriers like oceans, mountain ranges, and large deserts limit species movement. These barriers help explain why continents and islands often have very different species assemblages, even when their climates are similar.

Biogeographical Processes and Patterns

Several large-scale processes create the distribution patterns biogeographers study:

• Continental drift has rearranged landmasses over hundreds of millions of years. Species that evolved on a single landmass became separated as continents split apart, which is why marsupials dominate Australia but are rare elsewhere. The breakup of Pangaea is a classic example of how geology shapes biology.
• Island biogeography explains species richness on islands using two main variables: island size and distance from the mainland. Larger islands support more species because they offer more habitats and resources. Islands closer to the mainland receive new colonizers more frequently. This theory, developed by MacArthur and Wilson, also applies to habitat "islands" like isolated patches of forest.
• Ecological succession describes how species composition in a community changes over time after a disturbance (like a fire or volcanic eruption) or in a newly formed habitat (like a retreating glacier). Pioneer species colonize first, gradually giving way to more complex communities until a relatively stable state is reached.

Abiotic Factors and Ecosystem Dynamics

Abiotic Influences: Aquatic vs. Terrestrial

Aquatic and terrestrial ecosystems face different abiotic challenges, so the factors that most strongly shape species distribution differ between them.

Aquatic ecosystems:

• Water availability is obviously not limiting, but dissolved oxygen can be. Oxygen levels vary with temperature, depth, and water movement, and low-oxygen zones exclude many species.
• Temperature stratification divides lakes into distinct layers. The warm upper layer (epilimnion) sits above the cold lower layer (hypolimnion), separated by a zone of rapid temperature change called the thermocline. Different species are adapted to each layer.
• Currents and water chemistry (salinity, pH, nutrient concentrations) strongly influence which organisms live where. Estuaries, where freshwater meets saltwater, and coral reefs, which require warm, clear, nutrient-poor water, are good examples.
• Light penetration decreases with depth. The photic zone near the surface supports photosynthesis, while the deeper aphotic zone and benthic zone (the bottom) rely on organic matter sinking from above.

Terrestrial ecosystems:

• Water availability is often the most limiting factor, especially in arid regions like deserts and dry grasslands.
• Temperature and precipitation patterns together are the strongest predictors of biome type. Tundra forms where it's cold and dry; savannas form where it's warm with seasonal rainfall.
• Soil composition and topography play significant roles. Clay soils retain water differently than sandy soils, and karst landscapes (formed from dissolved limestone) create unique drainage patterns and habitats.
• Light availability is generally not limiting on land, except in the understory of dense forests like tropical rainforests or at extreme polar latitudes during winter.

Abiotic Factors and Primary Productivity

Net primary productivity (NPP) is the rate at which primary producers (mainly plants and algae) convert solar energy into biomass after accounting for their own cellular respiration. NPP determines how much energy is available to support consumers in an ecosystem, so it's a fundamental measure of ecosystem function.

Several abiotic factors control NPP:

Temperature:

1. NPP generally increases with temperature up to an optimum, then declines as heat stress damages enzymes and increases water loss.
2. Moderate-temperature biomes like temperate forests tend to have higher NPP than extremely cold biomes (tundra) or extremely hot, dry ones (deserts).

Precipitation:

1. NPP increases with precipitation because water is essential for photosynthesis and nutrient transport in plants.
2. The biomes with the highest NPP on Earth, tropical rainforests and temperate rainforests, are also among the wettest.

Sunlight:

1. NPP increases with sunlight availability since light energy drives photosynthesis.
2. Some biomes with high sunlight exposure can achieve relatively high NPP even with moderate precipitation. Tropical grasslands and savannas are a good example.

Nutrient availability:

• NPP is often limited by the availability of key nutrients, especially nitrogen and phosphorus. Even with plenty of water, warmth, and sunlight, nutrient-poor soils constrain plant growth.
• Nutrient-rich biomes like temperate grasslands and river floodplains can support high NPP because regular nutrient inputs (from decomposition or flooding) keep the soil fertile.