Unit 4 Ecology (PCB 3043) - Dr. David Lewis Lecture Objectives

Foundations of Ecological Production and Energy Flow

Conceptual Framework of Production * Energy Flow Diagramming: Energy flow in an ecosystem begins with the capture of energy by autotrophs. For photoautotrophs, the primary source is sunlight. For chemoautotrophs, energy is derived from providing reduced minerals. * Trophic Structure: Energy moves through a sequence of trophic levels: * Primary Producers (Autotrophs): Organisms that capture energy through photosynthesis or chemosynthesis. * Primary Consumers (Herbivores/Grazers): Organisms that consume primary producers. * Secondary Consumers (Primary Predators): Carnivores that consume primary consumers. * Higher Trophic Levels: Secondary predators/tertiary consumers and so on. * Detritus Branch: A significant portion of energy flows into the detritus branch, where dead organic matter is consumed by detritivores. * Energy Termination: All energy flow eventually ends with the loss of energy as heat, according to the laws of thermodynamics.
Productivity Metrics and Partitioning * Gross Primary Productivity (GPP): The total amount of energy captured or carbon fixed by primary producers in an ecosystem. * Primary Producer Respiration ( $R_p$ ): The portion of energy used by plants for their own metabolism and maintenance. * Net Primary Productivity (NPP): The remaining energy stored as biomass after respiration is subtracted from GPP. * Equation for Productivity: $NPP = GPP - R_p$ * Allocation Percentages: Typically, GPP is partitioned such that approximately $50\% - 75\%$ is used for respiration ( $R_p$ ), leaving the remainder (often cited near $25\% - 50\%\text{ depending on the ecosystem and species}$ ) for NPP.
Influences on Primary Production * Climate Drivers: Primary production is primarily influenced by temperature and moisture availability. Warmer temperatures and higher precipitation generally correlate with higher NPP. * Limiting Nutrients: A limiting nutrient is the specific element that, when added, increases primary production. This is identified by adding nutrients experimentally and measuring the growth response. * Lakes: Typically limited by Phosphorus ( $P$ ). * Streams and Rivers: Subject to variable limitation but often $P$ or Nitrogen ( $N$ ). * Estuaries and Oceans: Typically limited by Nitrogen ( $N$ ). * Factorial Design Experiments: Experiments where $N$ and $P$ are added separately and together. * N-limited: Growth increases only when $N$ is added. * P-limited: Growth increases only when $P$ is added. * Co-limited: Growth increases significantly only when both $N$ and $P$ are added.
Species Richness and Regulation * Diversity Effects: Higher species richness of primary producers (plants and algae) leads to higher rates of primary production. This occurs through functional complementarity, where different species use different resources or niches more efficiently. * Herbivore Regulation: Trophic cascades in lakes demonstrate that grazers/herbivores regulate primary production; for example, an increase in piscivores reduces planktivores, allowing zooplankton populations to rise and heavily graze phytoplankton, thereby reducing primary production. * Compensatory Growth: Moderate grazing can actually increase plant productivity. This occurs because the removal of older tissue reduces self-shading, stimulates growth hormones, and reduces water stress, allowing the plant to over-compensate for the lost tissue.
Ecological Efficiency * Definition: The percentage of energy transferred from one trophic level to the next. * Efficiency Range: ecological efficiency is typically low, ranging from approximately $5\% - 20\%$ . * Reasons for Inefficiency: Large amounts of energy are lost through respiration, incomplete consumption (not all tissue is eaten), and incomplete assimilation (energy lost as waste/feces).

Nutrient Cycling and Decomposition

Primary Pools of Organic Matter * Living Biomass vs. Nonliving Material: In most ecosystems, the quantity of energy and organic matter stored in nonliving organic material (detritus/soil) significantly exceeds that stored in living biomass. * Terminology for Decaying Matter: This matter is referred to as detritus, litter, or humus. It is consumed by detritivores (e.g., fungi, bacteria, and macro-invertebrates like earthworms).
The Decomposition Process * Definition: The breakdown of organic matter from complex structures into simpler compounds and eventually inorganic forms. * Mechanisms: Breakdown occurs through physical fragmentation (biological or physical), leaching of water-soluble compounds, and chemical mineralization by microbes. * Controls on Land: Decomposition rates are controlled by climate (faster in warm, moist conditions) and tissue properties (lignin content and $C:N$ ratios). High lignin content slows decomposition. * Controls in Streams: Terrestrial leaf litter decomposition in streams is influenced by the leaf's tissue chemistry and the nutrient concentration ( $N$ and $P$ ) of the stream water; higher water nutrients can accelerate microbial decay.
Mineralization Products * Carbon ( $C$ ): Transformed into Carbon Dioxide ( $CO_2$ ). * Nitrogen ( $N$ ): Transformed into Ammonium ( $NH_4^+$ ) or Nitrate ( $NO_3^-$ ). * Phosphorus ( $P$ ): Transformed into Phosphate ( $PO_4^{3-}$ ).
Animal Influences on Nutrient Dynamics * Burrowing Mammals: Pocket gophers and prairie dogs in grasslands redistribute soil, bringing nitrogen-rich soil to the surface and creating patches that alter light penetration and plant growth. * Grazing Acceleration: Grazing accelerates nutrient cycling. By consuming plants, herbivores turn slow-decaying plant tissue into fast-cycling waste (urine and feces). This stimulates faster plant growth and higher turnover of nutrients from soil to vegetation. * Invasive Species Case Study: In Hawaii, the invasion of Myrica faya (firetree) has significantly altered nitrogen cycling. Because Myrica faya associates with nitrogen-fixing microbes, it increases the total input of $N$ to the ecosystem and accelerates the rate of $N$ cycling within the ecosystem. * Riparian Forests and Salmon: Salmon provide a significant source of marine-derived nitrogen to riparian forests when they return to streams to spawn and die, or are eaten by terrestrial predators.
Nutrient Spiraling in Aquatic Systems * The Concept: In streams and rivers, nutrients do not just cycle in place; they move downstream while cycling, creating a "spiral." * Spiraling Length: The distance a nutrient atom travels to complete one cycle. * Factors Increasing Length: Higher water velocity and lower uptake rates by organisms result in longer spiraling lengths. * Nitrogen Retention: Rapid cycling among organisms and detritus, along with storage in organism tissues, decreases spiraling length and increases nitrogen retention in the stream.
Stoichiometry and Body Composition * Nutritional Needs: Animal body stoichiometry (the ratio of chemical elements) dictates their effect on cycling. If an organism's tissue has a low $N:P$ ratio (meaning it requires a lot of phosphorus), it will strongly retain phosphorus ( $P$ ) and rapidly recycle/excrete nitrogen ( $N$ ) back into the environment.
Disturbance and Ecosystem Budgets * Disturbance Effects: Disturbance typically causes a net loss of nutrients. Evidence from forested watersheds shows increased nitrate ( $NO_3^-$ ) export after clear-cutting. In streams, phosphorus export increases following high-flow disturbance events.

Succession and Ecosystem Development

Types of Succession * Primary Succession: Occurs on newly exposed geological substrates where no soil or previous life existed (e.g., following volcanic eruptions or glacial retreat). * Secondary Succession: Occurs in areas where an existing community has been disturbed, but soil and some organisms remain (e.g., after a forest fire or abandoned farmland).
Research Approaches: Glacier Bay, Alaska * Chronosequence Approach: Studying sites of different known ages (up to $1,500\text{ years}$ ) simultaneously to represent a timeline of succession. * Dynamic Approach: Directly observing a single area ("succession in action") over a long duration (e.g., $138\text{ years}$ ). * Findings: Plant species richness generally increases over time, then stalls or slightly declines at very late stages. Both approaches show similar transitions from pioneer species to shrubs and eventually climax forests, though specific timing and species may vary.
Patterns in Diversity During Succession * General Pattern: Species richness typically follows a "humped" curve, increasing rapidly during early and mid-succession and then stabilizing or slightly decreasing as the community reaches a climax state. * Global Examples: Similar patterns are observed in temperate forest plants and birds, stream benthic communities, and rocky intertidal zones.
Successional Mechanisms * Facilitation: Early species modify the environment in ways that make it more suitable for later species but less suitable for themselves. * Inhibition: Early colonizers make the environment less suitable for all other species; later species can only establish when the early colonizers die. Rocky intertidal experiments have shown evidence for both facilitation and inhibition.
Dynamics and Predictability * Unpredictability: Succession can be nonlinear and unpredictable. Boreal forests may have different end-points; temperate forests may have different starting points; desert streams (algae/insects) show rapid, non-linear pathways post-flooding. * Biomass Changes: Biomass accumulates rapidly in the early phase. In the later phase, stored biomass is large but reaches a "steady state" where the rate of increase approaches zero.
Soil and Nutrient Development * Nitrogen Export: Export from streams declines during succession because maturing forests are more efficient at retaining and storing nitrogen. * Boreal Soil Trends: During succession, soil organic matter and nitrogen ( $N$ ) content increase due to biological fixation and accumulation. However, soil phosphorus ( $P$ ) content decreases because it is weathered from bedrock and is not replenished from the atmosphere. * Hawaii Chronosequence: Due to plate tectonics, geological surfaces to the southeast (near active volcanism) are younger, while surfaces to the northwest are millions of years older ( $4.1\text{ million years}$ ), showing a clear gradient of nitrogen gain and phosphorus loss over time.

Landscape Ecology

Landscape Structure * Components: Landscapes are composed of patches, the matrix (the background habitat), and corridors. * Structural Differences: Two landscapes with the same patch types can differ in structure based on patch density, connectivity, and configuration.
Patch Dynamics: Edge vs. Core * Edge Effects: The edge of a patch has different abiotic and biotic conditions than the interior/core. * Maximizing Core: Large, circular or square patches maximize core area. Small, long, or irregularly shaped patches have higher edge-to-core ratios.
Ecological Responses * Animal Populations: Data indicates that animal dispersal and population density are heavily influenced by the size and connectedness (corridors) of habitat patches. * Watershed Position: In northern Wisconsin, a lake's position determines if it is fed by groundwater (lower in watershed, high mineral/ $Ca$ / $Mg$ input) or precipitation (higher in watershed, low minerals). During drought, lakes dependent on groundwater may experience more dramatic changes in their chemical storage.
Geology and Engineering * Alluvium: Sediment deposited by flowing water. In Arizona bajadas, variation in alluvium age and climate determines soil properties and plant distributions. * Ecosystem Engineers: Organisms that physically alter the landscape (e.g., beavers). Beavers increase landscape heterogeneity and enhance nutrient retention by creating ponds. * Disturbance Regimes: High-frequency, large-scale disturbances produce homogenous landscapes. Infrequent, localized disturbances create complex, heterogeneous (patchy) landscapes.

Biogeography

Insular Habitats * Types: Beyond oceanic islands, this includes mountaintops, fragmented forest patches, lakes, and springs. * Size and Isolation: Diversity increases as island size increases (Area Effect) and decreases as island isolation increases (Distance Effect).
Equilibrium Model of Island Biogeography * Dynamic Equilibrium ( $S$ ): The number of species on an island is a balance between the rate of colonization (immigration) and the rate of local extinction. * Colonization Curve: Decreases as the number of species on the island increases. * Extinction Curve: Increases as the number of species on the island increases. * Turnover: Species turnover is the change in species composition over time; it can be high even if the total species richness ( $S$ ) remains stable.
Biogeographical Patterns * Latitudinal Gradient: Diversity is highest near the equator and decreases toward the poles. * Testing Modern Models: The formation of new islands (e.g., Surtsey or mangrove experiments) shows that species accumulate toward an equilibrium point where colonization and extinction rates eventually balance.

Global Ecology

El Niño Southern Oscillation (ENSO) * Mechanisms: Driven by changes in atmospheric pressure and wind currents. In El Niño conditions, trade winds weaken, stopping the upwelling of cold, nutrient-rich water in the eastern Pacific. Surface water temperatures rise. * Ecological Impacts (1982-83): Increased rainfall in the Galapagos led to high land productivity (ground finches) but crashed ocean primary production, devitalizing fisheries and leading to reproductive failure in sea birds and fur seals. * Great Salt Lake: The event caused increased precipitation, lowering lake salinity and restructuring the food web dynamics.
Global Biodiversity * Definitions: * Endemic Species: Species found in only one specific geographic location. * Biodiversity Hotspot: A region with high levels of biodiversity and many endemic species that is also under threat of destruction. * Plate Tectonics: Continental drift influences dispersal and long-term isolation, creating distinct biogeographical regions. * Extinction Rates: Normal background extinction is low, punctuated by mass extinction events where a massive percentage of global taxa are lost.
The Global Nitrogen Cycle * Human Influence: Human activities fix more nitrogen than all natural (non-human) processes combined. Human activities account for $40\% - 80\%$ of emissions for three reactive nitrogen gases: Nitrous Oxide ( $N_2O$ ), Nitric Oxide ( $NO$ ), and Ammonia ( $NH_3$ ). * N Fixation vs. N Deposition: * Fixation: The process of turning inert $N_2$ gas into biologically reactive forms. * Deposition: The transport of already-fixed nitrogen from the atmosphere to the Earth's surface via rain or dust. * Global Distribution: Maps show that nitrogen deposition is significantly elevated over continental landmasses (especially industrialized regions) compared to background natural rates in unpolluted areas.