The Physical Environment and Biogeography of Life

Introduction to Ecology and Biological Organization Levels

• Definition of Ecology: Ecology is defined as the scientific study of the complex interrelationships between organisms and their environments. This includes both living elements (biotic) and nonliving elements (abiotic).

• Levels of Biological Organization: Ecology is organized and studied at several distinct hierarchical levels:

  • Individuals: The fundamental unit of study, focusing on single organisms.

  • Populations: Groups of individuals belonging to the same species that inhabit a specific, defined area.

  • Communities: An assemblage consisting of various interacting species that live together within the same place and time.

  • Ecosystems: This level includes the organisms of a particular community in conjunction with the physical and chemical environment they occupy.

  • Landscapes: A broader geographic area that consists of multiple interconnected ecosystems or habitats.

  • Global Biosphere: The most expansive level, encompassing all regions of the Earth—both terrestrial and aquatic—along with the Earth's atmosphere, where organisms are capable of living.

Earth's Climate: Solar Radiation and Atmospheric Composition

• Energy Source: The primary energy driving Earth's climate originates from the Sun in the form of solar radiation.

• Solar Radiation Distribution:

  • Approximately $30\%$ of incoming solar radiation is reflected back into space by clouds, atmospheric gases, and the surface of the Earth.

  • The remaining portion is absorbed by the atmosphere and the Earth's surface.

• Weather vs. Climate:

  • Weather: Refers to the short-term state of atmospheric conditions occurring at a specific time and place.

  • Climate: Refers to the average atmospheric conditions, including the extent of their variation, measured over long durations ranging from years to millennia at a particular location.

• The Atmosphere: A thin layer of gases surrounding the Earth that moderates temperature by trapping heat. Its composition includes:

  • Nitrogen gas ($N_2$): $78\%$

  • Oxygen ($O_2$): $21\%$

  • Argon ($Ar$): $0.9\%$

  • Carbon dioxide ($CO_2$): $0.037\%$

  • Traces of other various gases.

• Greenhouse Gases: Specific gases within the atmosphere, such as carbon dioxide ($CO_2$) and methane ($CH_4$), that allow sunlight to enter the atmosphere but trap the heat radiating back from the Earth's surface, leading to a buildup of heat.

Variations in Global Climate Patterns

• Solar Energy Variation: Climate varies across the globe primarily because of the variation in the amount of solar energy received, which is influenced by latitude and seasonal changes.

  • Higher Latitudes: In these regions, solar energy is distributed over a larger surface area, making it less intense. The radiation strikes at an angle, meaning it must pass through more of the atmosphere, resulting in more energy being absorbed or reflected before reaching the surface.

  • Equatorial Regions: Near the equator, sunlight hits the Earth perpendicularly, making the energy more intense.

• Hemispheric Temperature Difference: The Northern hemisphere is observed to be slightly warmer than the Southern hemisphere. The primary hypothesis for this is that the Northern hemisphere contains more landmass, which heats up more than water.

Atmospheric Circulation and Precipitation

• Solar Input and Air Movement: Solar energy input is the driver for atmospheric circulation and global precipitation patterns.

  • When a portion of air is warmed, it expands, becomes less dense, and begins to rise.

  • As this air rises to higher elevations, it begins to cool.

  • Cool air has a lower capacity to hold water vapor compared to warm air; therefore, as the expanding air cools, it releases moisture in the form of precipitation.

• Differential Heating: The uneven heating of the Earth’s surface creates atmospheric circulation, which in turn influences worldwide atmospheric pressure, air temperatures, and precipitation.

Prevailing Winds and the Coriolis Effect

• Prevailing Winds: These are patterns of air circulation (wind) that move across the surface of the Earth predominantly in a single direction.

  • Trade Winds: Blow from east to west in the tropical regions.

  • Westerlies: Blow from west to east in the mid-latitude regions.

  • Easterlies: Blow from east to west in regions above $60^{\circ}\text{N}$ or $60^{\circ}\text{S}$ latitude.

• Generation of Winds: Global wind and ocean currents are generated by the Earth's spherical shape and its east-to-west rotation.

• Coriolis Effect: The velocity of Earth's rotation is fastest at the equator and slowest near the poles. This difference in rotational speed creates the Coriolis effect, which is the deflection of moving air or water.

Ocean Currents

• Drivers of Movement: Ocean currents are primarily driven by prevailing winds, which move surface water through frictional drag.

• Gyres: These are large-scale, circular ocean currents.

  • Gyres rotate in a clockwise direction in the Northern Hemisphere.

  • Gyres rotate in a counterclockwise direction in the Southern Hemisphere.

Concepts and Classification of Biomes

• Definition of Biomes: Biomes are groupings of ecologically similar organisms that are shaped by the specific environments in which they are found.

• Classification: Terrestrial biomes are classified primarily by the growth forms of their dominant plant species.

• Boundaries: The boundaries between different biomes are somewhat arbitrary; in many geographic locations, biomes merge into one another rather than having sharp divisions.

• Walter Climate Diagrams: This graphic technique is used to visualize the "growing season" of a biome.

  • It plots temperature and precipitation data.

  • A growing season is defined as the months when average temperatures are above freezing ($0\,^{\circ}\text{C}$) and precipitation is sufficient to support plant growth.

  • These diagrams utilize two y-axis scales: one for temperature and one for precipitation.

Detailed Survey of Terrestrial Biomes

• Tropical Rainforest:

  • Located in equatorial regions.

  • Characteristics: High rainfall and high temperatures throughout the entire year.

  • Biological Importance: It is the most productive and species-rich biome on Earth.

  • Nutrient Cycling: While it has the highest overall productivity, most nutrients are tied up in the living vegetation, leaving the soils very poor.

• Desert:

  • Concentrated in two specific belts centered around $30^{\circ}\text{N}$ and $30^{\circ}\text{S}$ latitude.

  • Formation: Found where warm, dry air sinks under conditions of high atmospheric pressure.

  • Flora: Succulent plants are common.

  • Fauna: Pollination and fruit dispersal are often carried out by animals. Many desert animals are adapted to require no supplementary water beyond what they can extract from their food sources.

• Temperate Grassland:

  • Found globally in areas that are relatively dry for much of the year.

  • Climate: Characterized by hot summers and relatively cold winters.

  • Vegetation: Rich in species and supports herds of large grazing mammals.

  • Adaptations: Plants are adapted to fire; they store much of their energy in underground structures and can resprout quickly after being eaten by grazers or burned.

  • Human Impact: Most temperate grasslands have been converted for agricultural use.

• Temperate Deciduous Forest:

  • Located in eastern North America, Europe, and eastern Asia.

  • Climate: Precipitation is evenly distributed throughout the year, but temperatures fluctuate dramatically between the summer and winter seasons.

  • Flora: Dominated by deciduous trees that lose their leaves during cold winters.

  • Fauna: Many animals migrate to escape the winter cold and find food; others hibernate in underground burrows.

• Boreal Forest:

  • Occurs just above $50^{\circ}\text{N}$ latitude, below the Arctic tundra, and at high elevations on temperate-zone mountains (below alpine tundra).

  • Climate: Features long, very cold winters and short, often relatively warm summers.

  • Flora: Dominated by coniferous trees that are capable of photosynthesizing as soon as temperatures rise.

  • Temperate Evergreen Forest: A related biome occurring along continental coasts where winters are mild and wet, and summers are cool and dry.

• Tundra:

  • Found at high latitudes, generally above $65^{\circ}$.

  • Climate: Low temperatures and a very short growing season.

  • Soil: Underlain by permafrost, which is soil permeated with permanently frozen water.

  • Hydrology: Though precipitation is low, the soil remains wet because the underlying permafrost prevents water from draining away.

  • Flora: Vegetation is typically low-growing.

Biogeographic Regions and Processes

• Biogeography: The study of the patterns of distribution of populations, species, and communities.

• Biogeographic Regions: These are distinct regions of biota characterized by different species that occur at a continental scale. There are 7 major regions separated by climatic, topographic, or aquatic barriers:

  1. Palearctic

  2. Nearctic

  3. Neotropical

  4. Ethiopian/Afrotropic

  5. Australasia

  6. Oriental/Indomalaya

  7. Antarctic

• Evolutionary Isolation Processes:

  • Vicariance: The separation of a species' distribution due to the formation of a physical barrier.

  • Dispersal: Occurs when members of a species cross an existing physical barrier and establish a new population in a different area.

Species Diversity and Island Biogeography

• Species-Area Relationship: A pattern where species diversity increases as the area increases.

• Island Patterns: Diversity is most often measured on islands, which show a decrease in species numbers as the distance from the source (species pool) increases.

  • Example: Bird species on islands surrounding New Guinea. Larger islands that were closer to the mainland consistently contained a higher number of species.

• Theory of Island Biogeography: Developed to explain the species-area relationship of islands based on the balance of two processes:

  1. Immigration: The arrival of new species to an island.

  2. Extinction: The loss of species already present on the island.

• Determinants of Species Number:

  • Size/Area of the Island: Smaller islands have fewer resources, leading to higher extinction rates. Larger islands have more resources, resulting in lower extinction rates.

  • Distance of the Island: Islands farther from the species pool have lower immigration rates, while islands closer to the species pool have higher immigration rates.