Ecosystem Dynamics in Ecology
Ecosystem Dynamics
Bioscience Lecture: Overview
This lecture covers essential topics in ecology, focusing on how ecosystems function and interact with various physical processes.
Lecture Outline
Biogeochemical Cycles
The Flow of Energy in Ecosystems
Trophic-Level Interactions
Biodiversity and Ecosystem Stability
Island Biogeography
Ecosystems and Biogeochemical Cycles
Definition of Ecosystem
An ecosystem encompasses all organisms residing in a specific place, along with the abiotic environment with which these organisms interact.
Biogeochemical Cycles
Defined as the movements of chemicals through ecosystems.
Involves both biotic (living) and abiotic (non-living) processes.
These cycles typically cross ecosystem boundaries, meaning one ecosystem might import or export chemicals while interacting with another.
Carbon Cycle
Importance of Carbon
Carbon is a significant component of living organisms.
Carbon fixation: A series of metabolic reactions that convert gaseous compounds into nongaseous forms.
Aerobic cellular respiration: A process that releases carbon dioxide (CO₂) into the atmosphere.
Methanogens: Microorganisms that produce methane (CH₄) through anaerobic respiration.
Changes in the Carbon Cycle
The carbon cycle can accelerate in one direction, leading to significant global consequences over time.
Current reserves of coal and fossil fuels have developed over extensive geological periods.
Human activities, notably the burning of fossil fuels, are causing substantial disruptions in the carbon cycle, including rising atmospheric CO₂ levels annually.
Water Cycle
Availability of Water
Water is vital for all forms of life, constituting approximately 60% of adult human body weight.
The amount of available water directly influences the types and abundance of organisms present within an ecosystem.
Water undergoes synthesis during cellular respiration and is broken down during photosynthesis.
Basic Water Cycle Mechanism
Evaporation: Liquid water from the Earth's surface transforms into vapor, primarily from oceans, lakes, and rivers.
In terrestrial ecosystems, about 90% of evaporation occurs through plant processes.
Resultant vapor condenses and precipitates back to Earth's surface.
Groundwater
Groundwater: Water located beneath Earth's surface.
Aquifers: Permeable underground rock layers storing saturated water.
Important for freshwater supply, accounting for 95% of the fresh water used in the United States.
Divided into:
Upper layers: Known as the water table.
Lower layers: Accessible through wells.
Changes in Water Supply
Fluctuations in water availability can drastically impact ecosystem composition.
For instance, deforestation can disrupt local water cycles, potentially transforming tropical rainforests into semiarid deserts.
Nitrogen Cycle
Importance of Nitrogen
Nitrogen is crucial for the synthesis of proteins and nucleic acids, often being the limiting nutrient within ecosystems.
The atmosphere contains 78% nitrogen (N₂), but most organisms cannot directly utilize it in this gaseous form.
Instead, organisms absorb nitrogen in the forms of ammonia (NH₃) and nitrate (NO₃⁻).
Nitrogen Fixation
This is the conversion of inert N₂ into biologically usable nitrogen compounds, primarily facilitated by nitrogen-fixing microbes that can reside freely or on plant roots.
Human activities, such as nitrogen fertilizer usage, have doubled the rate of nitrogen input to soils and aquatic systems, significantly altering the global nitrogen cycle.
Phosphorus Cycle
Importance of Phosphorus
Phosphorus is essential for all organisms as it is a component of nucleic acids, cellular membranes, and adenosine triphosphate (ATP).
Unlike other cycles, phosphorus has no significant gaseous phase and exists mainly in the form of phosphate (PO4-3) in ecosystems.
Organisms acquire phosphorus through the consumption of plants and algae, which uptake inorganic phosphorus.
Limiting Nutrients
A limiting nutrient is a resource in deficiency relative to the needs of organisms, effectively constraining ecosystem productivity.
Nitrogen and phosphorus are common limiting nutrients in both terrestrial and aquatic ecosystems.
Iron serves as a limiting nutrient for algal populations in approximately one-third of the world's oceans.
Iron as a Limiting Nutrient
Algal populations can thrive when iron-rich dust is transported by wind, provided the iron is in a biologically accessible form.
For example, Saharan dust storms can enhance algal productivity in oceanic waters by introducing iron.
Trophic Levels
Concepts of Trophic Levels
Trophic levels represent the feeding hierarchy within ecosystems.
Autotrophs: Organisms that synthesize their own organic compounds from inorganic sources, categorized as:
Photoautotrophs: Use light energy.
Chemoautotrophs: Derive energy from inorganic oxidation (primarily prokaryotic organisms).
Heterotrophs: Organisms that cannot produce their own food and must consume other organisms.
Producers and Consumers
Primary producers (autotrophs) serve as the foundational energy source.
Consumers (heterotrophs) are further categorized:
Herbivores: First level of consumers, feeding on plants.
Primary carnivores: Consume herbivores.
Secondary carnivores: Can consume either primary carnivores or herbivores.
Detritivores: Feed on decaying organic matter.
Decomposers: Microbial organisms that break down dead matter into simpler substances.
Productivity in Ecosystems
Productivity Metrics
Productivity: The collective rate at which organisms in a particular trophic level synthesize new organic matter.
Primary productivity: The productivity level of primary producers.
Respiration: The rate at which primary producers metabolize organic compounds.
GPP and NPP Explained
Gross Primary Productivity (GPP): The total rate of organic matter synthesis by primary producers.
Net Primary Productivity (NPP): Defined as GPP minus the respiration rate of primary producers, representing the net organic matter available for growth.
Secondary productivity: Refers to the productivity within a heterotroph trophic level.
Energy Processing in Ecosystems
Energy Capture
A small fraction (approximately 1%) of incoming solar radiant energy is trapped by primary producers annually.
This energy is stored in chemical bonds within their biomass and utilized in their own respiration, with losses typically occurring as heat.
Fate of Ingested Energy
Energy transitions through trophic levels are characterized by significant losses:
50% of energetic content is typically egested as feces.
33% is expended on cellular respiration.
Only 17% may be converted into the biomass of the consumer.
The remaining energy may be available to higher trophic levels.
Available Chemical Bond Energy
Over time, the amount of chemical bond energy available at each trophic level is roughly 10% of that available from the previous level.
Limits on Top Carnivores
The number of trophic levels within an ecosystem is constrained by energy availability, establishing limits on top carnivores.
Energy scarcity leads to an exponential reduction of chemical bond energy through trophic chains, implying that only about 1/1000 of the energy captured by photosynthesis carries through to secondary carnivores.
Biodiversity and Ecosystem Stability
Theories of Biodiversity
A prevalent theory posits that increased species richness may enhance the stability of ecosystems.
Empirical evidence indicates that plots with greater biodiversity display less variability in biomass from year to year, notably during drought periods, where biomass decline negatively correlates with species richness.
Controversy Surrounding Biodiversity's Role
The general consensus that ecosystem health relies on biodiversity is not universally accepted.
Critics argue the validity of outcomes observed in experiments:
Introducing more species to a plot may increase the likelihood of one species becoming highly productive, rather than suggesting that diversity directly causes stability.
Plots must demonstrate “overyielding” for the concept of biodiversity to hold value in terms of stability.
Factors Influencing Species Richness
Species richness, or the number of different species in an area, is influenced by several ecosystem characteristics:
Primary productivity: Ecosystems with higher productivity can support greater species richness.
Habitat heterogeneity: Greater ecological variety accommodates more species.
Climatic factors: Seasonality in climate can lead to elevated species coexistence rates.
Latitudinal Gradient in Biodiversity
Biodiversity typically increases as one approaches the equator, with many contributing factors likely explaining this trend.
Tropical regions exhibit the highest biodiversity, demonstrating a cline in species diversity that negatively correlates with latitude.
Conditions in Tropical Regions
High species diversity is observed in tropical regions due to factors including:
Evolutionary age: Longer evolutionary timescales allow for greater speciation.
Increased productivity: Availability of resources is consistently high.
Stability/constancy of environmental conditions: Minimal environmental fluctuation supports diverse organisms.
Predation: Predators can control prey populations, impacting community dynamics.
Spatial heterogeneity: Varied habitat structures facilitate multiple niches and species adaptation.
Review of Community and Ecosystem Dynamics
Define ecosystem and community.
Identify factors determining an organism's niche with examples of biotic and abiotic influences.
Analyze expected temperature distributions across Earth.
List five terrestrial biomes, detailing their typical precipitation and temperature characteristics.
Explore six species interactions, outlining their positive or negative effects, with real-life examples.
Describe a keystone species' role in ecosystems.
Understand and compare species richness and evenness, relating them to biodiversity.
Define primary succession, secondary succession, and climax communities.
Construct a food chain that starts with a producer and concludes with a tertiary consumer, including the role of decomposers.
Explain biomagnification and its effects on wildlife, particularly predatory birds.
Summarize the fundamentals of the water, carbon, nitrogen, and phosphorus cycles in ecosystems.