Ecosystems, Biomes, and Biogeochemical Cycles Comprehensive Notes

An ecosystem is a complex and dynamic community of living organisms, including plants, animals, fungi, and microorganisms, that interact with one another and with the non-living components of their environment. These interactions form intricate relationships, enabling the system to function cohesively and sustain life. Each ecosystem possesses unique characteristics that are influenced by various factors, including climate, geography, and the biotic and abiotic elements present within it.

Ecosystem Components

• Biotic Factors: These components constitute the living or once-living elements within an ecosystem that interact with each other in various ways, creating a complex web of relationships.

  • Producers (autotrophs): Organisms, such as plants and algae, that produce their own food through photosynthesis, forming the base of the food web. They harness solar energy and convert it into chemical energy.

  • Herbivores (primary consumers): Animals that feed on producers to obtain energy. They play a critical role in transferring energy from plants to higher trophic levels.

  • Carnivores (secondary/tertiary consumers): Organisms that consume other animals. Secondary consumers eat primary consumers, while tertiary consumers may eat secondary consumers. They help regulate herbivore populations and maintain balance in the ecosystem.

  • Omnivores: Species that consume both plants and animals, thus playing a versatile role within the food web.

  • Detritivores: Organisms that feed on dead organic matter, including decomposing plants and animals, contributing to nutrient cycling in ecosystems.

  • Decomposers: Fungi, bacteria, and other microorganisms that break down dead organic material, returning essential nutrients to the soil and facilitating the continuation of life cycles.

• Abiotic Factors: The non-living physical and chemical elements that impact ecosystems, shaping their structure and function.

  • Sunlight: The primary source of energy for almost all ecosystems, influencing photosynthesis, plant growth, and seasonal changes.

  • Temperature: A significant abiotic factor that affects organism distribution, metabolism, and survival. Temperature variations influence climatic zones and ecosystem types.

  • Precipitation: Essential for water availability, it affects soil moisture, plant growth, and the types of organisms that can thrive in an ecosystem. Patterns of rainfall can lead to various biome types (e.g., deserts, rainforests).

  • Moisture / Water: Vital for all living organisms, influencing physiological processes and habitat availability.

  • pH: The acidity or alkalinity of soil or water, which can affect nutrient availability and the types of organisms that can inhabit an area.

  • Soil: Composes the primary substrate for terrestrial organisms. Soil quality and composition influence vegetation types and higher trophic levels.

Species Interactions

• Predator-Prey Relationships: This biotic interaction involves predators hunting and consuming prey, forming a crucial dynamic that regulates population sizes and community structure.

• Symbiotic Interactions: Close, long-term interactions between different biological species can be categorized into three main types:

  • Mutualism ($+/+$): A symbiotic relationship where both organisms benefit and often enhance each other's survival and reproduction. For example, the relationship between ants and acacia trees, where ants protect the trees in return for food and shelter, benefits both species.   

  • Commensalism ($+/0$): In this relationship, one organism benefits while the other remains unaffected. Examples include jackals that scavenge predator kills or owls nesting in tree hollows, demonstrating how species can coexist beneficially.

  • Parasitism ($+/-$): A relationship where one organism (the parasite) benefits at the expense of another (the host), often harming the host in the process. Examples include fleas and ticks that live off mammals and intestinal tapeworms that reside in hosts, indicating the delicate balance of life and survival strategies.

Competition and Resource Partitioning

• Resource Partitioning: This occurs when competing species utilize shared resources differently—through variations in timing, space, or resource preferences—to minimize direct competition, enabling coexistence and promoting biodiversity.

• Resource Competition: Species vie for limited environmental resources, such as food, water, and space. Over time, species may adapt their resource use, influencing overall species diversity and community structure.

Terrestrial Biomes

Terrestrial biomes are expansive geographical areas defined by climate patterns, vegetation types, and distinctive animal life. These characteristics primarily vary with temperature, precipitation, and latitude. Climatographs illustrate monthly averages of temperature ($^\text{C}$) and precipitation (mm) throughout the year, aiding in long-term ecological comparisons. Furthermore, temperature structure shifts systematically with elevation ($\text{m}$).

Primary Productivity and Biodiversity Trends

• Primary productivity: Forest biomes in warm, humid regions yield significantly greater primary productivity and higher biodiversity compared to cooler, arid habitats. This high productivity is essential for sustaining a rich diversity of species.

• Ecosystem Dynamics: Biomes are not static; they undergo temporal changes and geographic shifts in response to climate variability, such as global temperature changes of $1^\text{C}$ to $2^\text{C}$, often leading to shifts in species distributions and ecosystem functions.

Summary of Major Terrestrial Biomes

• Tundra:

  • Vegetation / Features: Characterized by tundra meadow biomes featuring low-growing plants like mosses and lichens.

  • Geographic Locations: Primarily found in regions of Canada and Russia.

  • Climate & Soil: Experiences a harsh, cold climate with permafrost; soils are poorly developed and nutrient-deficient, with a temperature range from approximately $-15^\text{C}$ to $5^\text{C}$.

• Taiga (Boreal Forest):

  • Vegetation: Dominated by coniferous trees such as spruce, fir, and pine, which are adapted to cold climates.

  • Geographic Locations: Common across Canada and Russia.

  • Human Disturbances: Practices like logging and tar sands extraction pose significant threats to this biome, with a temperature range of approximately $5^\text{C}$ to $20^\text{C}$.

• Temperate Deciduous Forest:

  • Vegetation: Composed of broadleaf deciduous trees such as oak and maple that exhibit seasonal leaf loss.

  • Geographic Locations: Predominantly found in Eastern North America and Europe.

  • Human Disturbances: Heavily logged and affected by urbanization, with a temperature range of approximately $20^\text{C}$ to $30^\text{C}$.

• Shrubland:

  • Geographic Locations: Found in the Mediterranean region, notable for its scrubby foothills and arid regions.

  • Human Disturbances: Extensive human habitation and agriculture driven by favorable climatic conditions for crops, shifting native ecosystems.

• Temperate Grassland:

  • Vegetation / Features: Primarily comprised of prairie vegetation, characterized by rich soils suitable for agriculture.

  • Geographic Locations: Prominent in the American corn belt and other regions.

  • Human Disturbances: Large-scale agricultural conversion drastically alters ecosystems, causing habitat fragmentation.

• Desert:

  • Characteristics: Characterized by arid climate conditions with low annual precipitation and significant temperature variations between day and night.

• Savanna:

  • Vegetation: Characterized by a mix of tropical woodlands and grasslands, often supporting a diverse array of herbivores and carnivores.

  • Characteristics: Distinct wet and dry seasons that influence species behaviors and migration patterns.

• Tropical Rainforest:

  • Geographic Locations: Found in areas such as the Amazon Basin, Congo Basin, and Southeast Asia, home to unparalleled biodiversity.

  • Status & Disturbances: While the Amazon and Congo rainforests remain relatively intact, major threats arise from deforestation due to timber harvesting and agricultural expansion in regions like West Africa and Southeast Asia. Temperature remains consistently warm, around $20^\text{C}$ to $30^\text{C}$.

Aquatic Biomes and Stratification

Aquatic biomes can be categorized into freshwater and marine environments based on their salinity levels, offering an array of habitats that support rich biodiversity. Different abiotic conditions influence the characteristics and species compositions present in these systems.

Categorization of Aquatic Biomes

• Freshwater Environments:

  • Rivers and Streams: Flowing water bodies that support diverse ecosystems and physical habitats.

  • Ponds and Lakes: Standing water bodies where stratification occurs, affecting temperature and oxygen levels.

  • Freshwater Wetlands:

    • Marshes: Characterized by herbaceous plants that thrive in saturated soils.

    • Swamps: Rich in woody vegetation and often home to diverse wildlife.

    • Bogs: Unique ecosystems characterized by acidic peat and slow decomposition, important for carbon storage.          

• Marine Environments:

  • Intertidal Zones: Areas that experience regular tidal changes and support specialized organisms.

  • Estuaries: Nutrient-rich regions where freshwater meets the sea, fostering diverse biological communities.

  • Coral Reefs: Complex and highly productive ecosystems that host a wide range of marine life, crucial for maintaining marine biodiversity.

  • Salt Marshes: Coastal wetlands characterized by salt-tolerant plants that provide habitat and protect shorelines.

  • Mangrove Swamps: Coastal ecosystems that serve as nurseries for many marine species.

  • Open Ocean (Pelagic Zone): Vast areas of the ocean that support a unique array of organisms adapted to deep-sea conditions.          

Comparison of Abiotic Indicators (Terrestrial vs. Aquatic)

• Terrestrial Abiotic Indicators:

  • Annual average precipitation: Impacts vegetation types and species diversity.

  • Annual average temperature: Affects metabolic rates and community structure.

  • Soil type: Determines the types of plants that grow, influencing the entire ecosystem structure.

  • Latitude and Altitude: Significant factors in climate and ecosystem types.

  • Topography: Influences microclimates and moisture availability.

  • Wind speed: Affects pollination and seed dispersal.

• Aquatic Abiotic Indicators:

  • Depth: Influences light availability and temperature gradients.

  • Sunlight availability: Essential for photosynthesis and primary productivity.

  • Velocity (current speed): Affects sediment transport and aquatic organism habitats.

  • Salinity: Influences organism distribution and community dynamics.

  • Nutrients (nitrates and phosphates): Critical for primary production and ecosystem health.

  • Suspended matter (silt / turbidity): Affects light penetration and photosynthesis in aquatic habitats.

  • Bottom substrate type (muddy, sandy, rocky): Influences benthic organisms and habitat types.

Freshwater Lake Stratification

• Littoral Zone: This shallow, sunlit area near the shore is rich in biodiversity, supporting rooted aquatic plants and various organisms that thrive in these nutrient-rich waters.

• Limnetic Zone: Characterized by open water where light penetrates, dominated by plankton that serve as food sources for larger fish.   

• Photic Zone: The upper layer receiving sufficient sunlight for photosynthesis, crucial for plant and algae growth.

• Profundal Zone: Located beneath the photic zone, this dark water area lacks light and contains unique organisms adapted to cold, deep-water conditions.

• Benthic Zone: The bottom layer of lakes where decomposers break down organic material, presenting a critical role in nutrient cycling and oxygen consumption.

Marine Stratification and Dynamics

• Marine Zones:

  • Intertidal Zone: The dynamic region affected by tidal changes, supporting organisms capable of withstanding fluctuating conditions.

  • Pelagic Zone: Features the open ocean, rich in nutrients and diverse marine life.

  • Photic Zone: Sunlit surface waters where photosynthesis occurs, supporting primary producers.

  • Aphotic Zone: Deep, dark waters lacking light, where alternative energy sources, such as chemosynthesis, can occur.

  • Benthic Zone: Comprising the ocean floor, the home for many bottom-dwelling organisms.          

• Limiting Factors: Sunlight and nutrient availability are pivotal in determining species distributions in marine environments, often varied by depth, temperature, and salinity.

• Productivity & Biodiversity Hotspots: Coral reefs and estuaries are recognized as areas of high biodiversity and primary productivity, reflecting the health of marine ecosystems.

• Nutrient Concentration: Nitrogen and phosphorus concentrations are highest along coastlines due to terrestrial runoff, highlighting the coupling between land and marine ecosystems.

• Upwellings: Ocean currents that bring nutrient-rich waters to the surface, fostering high phytoplankton populations and supporting diverse marine life.

• Biomass Sinking & Decomposition: When marine organisms die, their remains sink to the ocean floor, where decomposers recycle nutrients similarly to terrestrial ecosystems.

• Secondary Productivity: Cold ocean waters support higher secondary productivity due to increased dissolved oxygen concentrations, supporting diverse marine species.

Biogeochemical Cycles

Biogeochemical cycles represent the continuous, self-regulating movements of chemical elements and molecules through biotic and abiotic sources, playing a critical role in ecosystem functioning and stability. These cycles are intrinsically linked, and disruptions often arise from human activities.

The Carbon Cycle

The carbon cycle is pivotal for life on Earth, involving the exchange of carbon-containing compounds between biotic and abiotic components.

Fast Carbon Cycling (FCC)

• Stemming from biological processes, it revolves around photosynthesis and cellular respiration. Biological primary production and decomposition govern this pathway.

  • Photosynthesis Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

  • Cellular Respiration Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

• Primary Reservoir: The atmosphere acts as the main reservoir for fast carbon cycling, albeit with a brief residence time.

• Role of Decomposers: Critical for returning carbon dioxide back to the atmosphere, thus facilitating nutrient cycling through decomposition processes that enhance soil organic matter and nutrient availability.

Carbon Sequestration, Sedimentation, and Long-Term Reservoirs

• The deep burial and sedimentation of organic material sequester carbon, contributing to geological carbon storage over time.

• Largest Carbon Reservoirs: Deep marine sediments and fossil deposits, formed through geological processes over millions of years, represent the most substantial carbon reserves.

• Release Pathways:

  • Rapid return: The combustion of fossil fuels releases carbon into the atmosphere swiftly, enhancing greenhouse gas concentrations.

  • Slow return: Geological weathering processes slowly convert solid carbonate rocks back into gaseous carbon dioxide, reintroducing carbon into the cycle over extended periods.

The Nitrogen Cycle

The nitrogen cycle is essential for life, delineating the transformation and transfer of nitrogen atoms between various environmental reservoirs and biological entities.

Biological and Chemical Steps of the Nitrogen Cycle

1. Fixation: Conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$) by soil microbes or through abiotic processes such as lightning, making nitrogen accessible to living organisms.

2. Ammonification: The decomposition of organic matter into ammonium ($NH_4^+$), facilitated by bacteria, returning nitrogen to the soil.

3. Nitrification: The oxidation of ammonium to nitrite ($NO_2^-$) and subsequently to nitrate ($NO_3^-$) by specialized soil bacteria, enabling plant uptake.

4. Assimilation: Nitrogen is absorbed by plants through their root systems as nitrates, which are then incorporated into proteins; animals obtain nitrogen by consuming plant material.

5. Denitrification: This process, conducted by denitrifying bacteria, converts nitrates back into atmospheric nitrogen gas ($N_2$), completing the nitrogen cycle and returning nitrogen to its atmospheric reservoir.

Key Nitrogen Cycle Characteristics

• Primary Reservoir: The atmosphere, retaining 98\text{%} of nitrogen gases found in the troposphere.

• Storage Duration: Nitrogen reservoirs generally have shorter storage durations compared to carbon, emphasizing its biological cycling through rapid microbial processes.

• Main Driver: The nitrogen cycle is primarily driven by the activities of microbes, facilitating essential transformations and supporting ecosystem health.

The Phosphorus Cycle

The phosphorus cycle describes the movement of phosphorus through environmental and biological systems, vital for living organisms.

• Key Characteristics:

  • No Atmospheric Phase: Unlike carbon and nitrogen, phosphorus does not have a gaseous state under normal environmental conditions, primarily cycling through water and soil.

  • Cycle Speed: The phosphorus cycle is characterized by its slow velocity, primarily governed by geological processes such as weathering and erosion.

  • Ecological Significance: Due to the absence of an atmospheric phase, phosphorus is a critical limiting nutrient for primary producers, influencing entire food webs.          

The Hydrologic (Water) Cycle

The hydrologic cycle illustrates the continual movement of water in its various forms (solid, liquid, gas) through the environment.

• Primary Driver: Solar energy directly powers all hydrologic processes, leading to transformations and movement through evaporation, condensation, and precipitation.

• Primary Reservoir: Oceans serve as the largest reservoir of water, predominantly containing saline water.

• Freshwater Reservoirs: Ice caps, glaciers, groundwater aquifers, and surface water bodies constitute essential freshwater reserves critical for sustaining ecosystems and human populations.

• Key Hydrologic Processes:

  • Evaporation: The transformation of liquid water into vapor.

  • Transpiration: The release of water vapor from plant leaves into the atmosphere.

  • Condensation: The conversion of vapor into liquid droplets in the atmosphere, forming clouds.

  • Precipitation: The falling of water in liquid or solid form to the Earth’s surface, replenishing water bodies.

  • Sublimation: The transition of ice directly into water vapor without passing through a liquid phase.

  • Infiltration: The process of water seeping into the ground, recharging aquifers.

  • Groundwater recharge: Sustains underground aquifers and influences surface water flows vital for ecosystem functioning.     

Environmental Applications and AP Practice Questions

Phosphorus Overload in the Everglades Ecosystem

Human Activities Contributing to Elevated Phosphorus Levels

Human activities significantly impact aquatic ecosystems such as the Everglades, introducing excess phosphorus through several mechanisms:

• Agricultural and residential runoff containing synthetic fertilizers, pesticides (particularly organophosphate pesticides).

• Seepage from residential septic systems located close to waterways, contributing to nutrient loading.

• Direct effluent discharge from wastewater treatment facilities lacking advanced nutrient removal processes.

• Combined Sewer Overflows (CSOs) during heavy rainfall events, leading to untreated waste entering water bodies.

• Agricultural runoff containing concentrated animal waste from feedlots, exacerbating nutrient inflows.

• The use of phosphate-containing detergents in domestic and industrial applications, further enriching water systems.

Adverse Ecological Consequences of Phosphorus Overload

Elevated phosphorus levels trigger significant ecological consequences in sensitive ecosystems like the Everglades:

• Cultural Eutrophication: Increased nutrient levels lead to eutrophication, resulting in harmful algal blooms that block sunlight and deplete oxygen, causing hypoxic conditions detrimental to aquatic life.

• Displacement of Endemic Species: Native species evolved to thrive in low-nutrient conditions face competition from non-native or invasive species, altering community dynamics.

• Specific Ecological Example: Aggressive invasive species, such as cattails, outcompete and replace native flora like sawgrass, leading to declines in overall biodiversity and disrupting ecological balance.