Lecture 13 Ecology: Nitrogen, Phosphorus Cycles & Biodiversity

The Nitrogen Cycle: Review and Human Impact

  • Review of Nitrogen Cycle Impacts from Human Activities:
    • Land Clearance, Fertilizers, and Agriculture: These activities introduce substantial amounts of nutrients into terrestrial ecosystems.
    • Eutrophication: Nutrients from terrestrial environments leach and run off into water bodies, transferring to aquatic communities. This enrichment of aquatic ecosystems is known as eutrophication.
    • Combustion: Burning fossil fuels significantly increases levels of nitrogen oxides (NO_x) in the atmosphere.
      • NO_x contributes to eutrophication when deposited into terrestrial and eventually aquatic communities.
      • NO_x also remains a component of the atmosphere.
  • Nitrogen Reserves and Fluxes:
    • Atmospheric Reservoir: The largest nitrogen compartment is the atmosphere (N_2), where nitrogen molecules are triple-bonded, making them very stable and difficult to break down and fix into ecosystems.
    • Major Fluxes into Ecosystems:
      • Biological N-fixation: Primarily performed by vegetation.
      • Industrial N-fixation (Haber-Bosch process): An energy-intensive process that converts atmospheric nitrogen into forms usable by plants, extensively used in agriculture.
      • Fossil Fuel Emissions: A significant increase in nitrogen compounds (primarily NOx) from burning fossil fuels. These compounds have a high global warming potential and do not readily flux back into atmospheric N2 form.
    • Importance in Agriculture: Nitrogen, along with phosphorus and potassium (NPK values on fertilizers), are essential limiting nutrients for plant growth, crucial for global food production.

The Phosphorus Cycle

  • Overview of Phosphorus:
    • Limiting Nutrient: Phosphorus (P) is a critical limiting nutrient often added to soils in agricultural practices.
    • Essential Cellular Component: It is a foundational component of various organic molecules, including:
      • ADP (adenosine diphosphate) and ATP (adenosine triphosphate) for energy.
      • DNA and nucleotides (genetic material).
      • Phospholipids (cell membrane structure).
      • Phosphoproteins and coenzymes.
  • Key Forms of Phosphorus in Ecosystems:
    • Phosphate (PO_4^{3-}): The inorganic form, which is soluble and readily taken up by plants. Often referred to as soluble P.
    • Organic Phosphorus (P_org): Phosphorus integrated into organic molecules within living cells and organisms. This form is usable by consumers and decomposers.
    • Bound Phosphorus (P_bound): Phosphate molecules attracted to and bound with soil particles (e.g., clays) or other substrates, rendering it less bioavailable to plants. Microbes can play a role in liberating this bound phosphorus back into the ecosystem.
  • Sources of Phosphorus:
    • Tight Cycling within Ecosystems: Phosphorus is tightly cycled within biomolecules in living organisms.
    • Natural Source: Primarily derived from the chemical weathering of sedimentary rocks and mineral deposits (geological origin).
    • Anthropogenic Sources:
      • Manure: A traditional source of nutrient cycling in agricultural systems.
      • Mined Deposits: The primary source of phosphorus used in modern agricultural fertilizers, extracted from geological deposits.
  • Phosphorus Flow through Terrestrial Ecosystems:
    • Uptake by Primary Producers: Soluble phosphate (PO_4^{3-}) is absorbed by primary producers (plants), representing the entry point into the biological component of ecosystems.
    • Assimilation and Food Webs: Once taken up, it is assimilated into P_org, moving through food chains and food webs as organisms consume one another.
    • Decomposition: Decomposers break down dead organic matter (e.g., fallen leaves), releasing phosphorus back into the soil as phosphate, restarting the cycle.
    • Ecosystem Losses:
      • Erosion and Runoff: Organic phosphorus in organisms or leached from soils can be exported.
      • Binding: Phosphate can become bound to soil minerals, effectively removing it from immediate bioavailability.

The Phosphorus Cycle: Marine Ecosystems and Unique Characteristics

  • Marine Phosphorus Cycle:
    • Entry: Inorganic phosphorus enters the ocean primarily from terrestrial runoff.
    • Surface Ocean Consumption: Almost all phosphorus is consumed in the surface ocean by phytoplankton and algae.
    • Food Web Transfer: It then moves through the marine food web (e.g., zooplankton, whales).
    • Movement to Deep Ocean: A small amount of phosphorus moves down the water column when organisms die or excrete waste.
    • Sediment Burial: Organic phosphorus in the deep ocean is decomposed by bacteria, with some material buried in ocean sediments. Over geological timescales, this sediment can form rock material in the lithosphere, eventually uplifted back to land.
    • Deep Ocean Pump and Upwelling: Over shorter timescales, phosphorus (and other nutrients) is brought back to the surface ocean through upwelling, a process referred to as the deep ocean pump (similar to carbon and nitrogen cycles). This feeds surface ocean productivity, demonstrating that individual atoms are cycled many, many, many, many, many, many times.
  • Distinguishing Characteristics of the Phosphorus Cycle:
    • Source: Unlike nitrogen and carbon, which largely originate from the atmosphere, phosphorus is derived primarily from the chemical weathering of rock (sedimentary and igneous material).
    • No Gaseous Form: Phosphorus does not have a significant gaseous form or atmospheric reservoir, making atmospheric fluxes inconsequential for this class's purposes.
    • Tight Terrestrial/Aquatic Cycling: Phosphorus is a very, very tightly cycled element in biotic compartments. Organisms in these environments rapidly take advantage of its limited availability.
    • Ocean Cycling Timescales: While surface ocean cycling is relatively rapid (around 100 times between surface and deep ocean over short periods), the overall ocean cycle is very slow. After approximately 10 million years, phosphorus can enter the ocean in particulate form or become buried, uplifted, and weathered again.
    • Land Recycling: On land, phosphorus returns to the soil quickly and is re-taken up by plants, leading to repeated usage.

Human Activities and the Phosphorus Cycle

  • Significant Human Impacts:
    • Marine Fishing: Transfers approximately 50 teragrams (Tg) of phosphorus from aquatic communities back to land annually. While not drastically impacting the vast ocean reservoir, it mobilizes phosphorus to terrestrial environments where it can then decay and re-enter ecosystems.
    • Mining: Humans actively mine phosphorus from rock (lithosphere), liberating it for use as fertilizer and applying it to land (biosphere). This mobilizes a considerable amount of phosphorus. Strip mining is a common practice for phosphorus extraction.
    • Waste and Sewage: Excreted waste and sewage contain high levels of phosphorus, which, when released, can be readily taken up by aquatic communities, contributing to eutrophication.
    • Deforestation and Land Cultivation: These activities increase soil erosion, leading to the release and runoff of phosphorus into waterways.
  • Global Fluxes: The largest fluxes orchestrated by humans involve the mining of phosphorus and its application as fertilizers, as well as erosion and marine fishing.
  • Sustainability Concerns: History of environmental disasters and sustainability issues related to phosphorus mining exist, particularly noted on some Pacific islands.

Biodiversity: Definition, Measurement, and Conservation

  • Core Concepts:
    • Definition: Bio refers to biological processes, substances, and life; diversity means different in character, quality, or kind. Biodiversity is fundamentally about differences within life forms.
    • Key Questions: How is biodiversity measured? What are its patterns and drivers? How does it relate to ecosystem services?
  • Historical Conservation Debate (20-30 years ago, tripolar): Conservation policy has historically debated the most effective focus for preserving biodiversity:
    • Specific Taxa: Focusing on threatened, endangered, or genetically unique species.
    • Landscapes and Ecosystems: Conserving entire ecosystems as life support systems for all species, unique and common.
    • Genetic Diversity: Prioritizing broad genetic diversity within and across species, recognizing its role in conferring resilience and adaptability to environmental changes.
  • Multidimensionality of Diversity: Biodiversity can be quantified in various ways, with the appropriate measure depending on the specific research question.
    • Who is there? $\rightarrow$ Taxonomic Diversity
    • Who is related to whom? $\rightarrow$ Phylogenetic Diversity
    • What roles can they play (potential)? $\rightarrow$ Genetic Diversity (traits, characteristics)
    • What roles are they currently playing (actual)? $\rightarrow$ Functional Diversity
    • Genetic Diversity: Acts as the key connector, driving evolutionary changes and making individuals unique through their genetic code.

Measuring Biodiversity: Taxonomic, Phylogenetic, Genetic, and Functional

  • 1. Taxonomic Diversity (Species Richness):
    • Definition: The simplest measure, focusing solely on who's there by counting the number of different species present in an ecosystem, without regard to their abundance.
    • Global Species Estimates (approx. 15 years ago):
      • Terrestrial: Approximately 1.2 million species described, but an estimated 8.7 million species exist.
      • Oceanic: Approximately 194,000 species described, with an estimated 2.2 million species existing. Oceans are considered underdescribed.
      • These discrepancies mean current counts likely underestimate true species diversity.
  • 2. Phylogenetic Diversity (Evolutionary History):
    • Definition: Builds upon taxonomic diversity by incorporating information about the evolutionary relationships and genetic divergence between organisms.
    • Methodology: Uses phylogenetic (evolutionary) trees constructed from genetic data.
    • Interpretation:
      • High PD: Indicates that species in a community are distantly related, suggesting a greater amount of evolutionary history and genetic distinctness.
      • Low PD: Indicates species are closely related.
    • Conservation Significance: Organisms with long evolutionary history (e.g., a species with a unique, long branch on a phylogenetic tree) represent significant evolutionary distinctness and are crucial for conservation efforts (e.g., Phagris grasshoppers of the Murches Islands).
  • 3. Genetic Diversity (Alleles and Adaptability):
    • Definition: The variability of alleles (alternate forms of genes) within a population.
    • Significance: Ensures a population's resilience and ability to adapt to environmental changes (e.g., genes conferring drought resistance).
    • Risk of Low GD: Populations with low genetic diversity are more vulnerable to local extinction when environmental changes are extreme.
    • Examples: Selective breeding in grains (sorghum, millet) for desired traits (protein levels, hardiness) or dog breeding for diverse morphologies and functions. Challenges exist in communicating its importance to the public.
  • 4. Functional Diversity (Ecological Roles):
    • Definition: Describes the various ecological roles organisms play within an ecosystem, based on their adaptations to environmental conditions and trophic positions.
    • Mechanisms: Morphology and behaviors allow organisms to partition and utilize resources (food, habitat) and transfer energy.
    • Functional Traits: Include body size, trophic breadth, taxonomic group, position in the water column (e.g., near surface vs. corals), gregariousness (social behavior), diet, substrate preferences, and habitat complexity preferences.
    • Ecosystem Resilience: Multiple organisms capable of filling the same role can contribute to ecosystem resilience, allowing other species to step in if one type of function is lost.
    • Utility: Helps understand ecosystem processes and their response to disturbances.

Quantifying Taxonomic Diversity: Richness, Evenness, and Scales

  • Richness vs. Evenness:
    • Richness: The simple count of different species present in a community. (e.g., two communities might have the same richness but differ in abundance).
    • Evenness: Describes how close in numbers each species in an environment is, reflecting the balance of species abundances. A community with one super-abundant species and many rare ones has low evenness compared to one where all species are present in similar numbers.
    • Shannon Diversity: A calculation that weights richness by the relative abundance of organisms, combining aspects of both richness and evenness.
    • Class Focus: In this class, the primary focus for taxonomic diversity will be richness (the number of species).
  • Measuring Diversity at Different Scales (Richness Measures):
    • Alpha (α) diversity: The diversity within any individual patch or ecosystem. (e.g., Site 1: 3 species, Site 2: 4 species, Site 3: 2 species).
    • Beta (β) diversity: The diversity among patches, comparing pairs of patches. It focuses on the number of unique species (differences) between sites, not common ones, providing a measure of turnover.
      • Example (from transcript):
        • Site 1 (Species: A, B, C); Site 2 (Species: A, B, D, E); Site 3 (Species: C, F)
        • Alpha Site 1: 3
        • Alpha Site 2: 4
        • Alpha Site 3: 2
        • Beta 1-2 (differences): Unique to S1 (C); Unique to S2 (D, E). Beta = 1+2=3
        • Beta 1-3 (differences): Unique to S1 (A, B); Unique to S3 (F). Beta = 2+1=3
        • Beta 2-3 (differences): Unique to S2 (A, B, D, E); Unique to S3 (C, F). Beta = 4+2=6
    • Gamma (γ) diversity: The total number of species (total richness) across all habitats within a larger area. (e.g., for the above example, Gamma = A, B, C, D, E, F = 6).

Global Patterns and Drivers of Biodiversity

  • 1. Latitudinal Gradient in Species Richness:
    • Pattern: Species richness increases dramatically as one moves from the poles towards the equator, with very few species in cold regions and immense diversity in tropical regions.
    • Hypotheses for the Gradient:
      • Energy and Moisture: More solar energy and moisture in tropical regions lead to higher Net Primary Productivity (NPP), which can support a greater number of species. Polar regions have limited productivity due to extreme cold.
      • Ecosystem Complexity: Tropical ecosystems possess a multitude of microhabitats, fostering diversification, specialization, complex food webs, and high degrees of specialized niches through evolution.
  • 2. Structural Complexity:
    • Observation: Mountainous regions generally support more diversity than flat regions.
    • Explanation: Mountainous areas offer a greater variety of habitats and niches.
      • Niche: A species' specific set of environmental requirements (e.g., temperature, resources) for survival and reproduction.
      • Habitat Partitioning: In complex environments, species can partition into specialized niches, reducing competition and allowing more species to coexist.
    • Example: Research in the Appalachian Mountains (1950s) showed that climatic conditions and habitat preferences for tree species change with altitude and mountain aspect, leading to more variation in species. This highlights niche space and habitat partitioning as drivers of biodiversity.
    • Implication: Habitat loss is a primary driver of biodiversity loss because it destroys these vital niche spaces.
  • 3. Geologic Variation:
    • Observation: Unique geological formations can create specialized habitats that drive the evolution of endemic species.
    • Example: Serpentine Soils in California:
      • Comprise only 1\% of California's land area but support 10\% of its endemic plants.
      • Soil Characteristics: Very low calcium (essential for most plants) and elevated levels of heavy metals (toxic to most plants), making these conditions inhospitable for typical flora.
      • Specialized Adaptations: Plants evolving in these soils exhibit trade-offs, such as larger root systems and shorter stature, allowing them to thrive in these harsh conditions without competing with species from more fertile soils.
  • 4. Species Interactions and Disturbance:
    • Disturbance as a Driver: Disturbances are crucial for maintaining biodiversity.
    • Ecological Disturbance: An abiotic event that kills individuals and frees up resources, effectively resetting the competitive clock.
    • Post-Disturbance Specialization: Different species specialize in various successional stages following a disturbance.
    • Optimal Disturbance Levels: Low to moderate levels of frequent disturbances (e.g., certain fire regimes in California landscapes) can maximize species diversity by preventing competitive exclusion and creating opportunities for various specialists.

Ecosystem Services

  • Definition: Ecosystem services are the resources and processes that ecosystems provide to humans, on which human societies and economies are profoundly dependent.
  • Interdependence: Humans significantly alter ecosystems, simultaneously impacting the very services they provide.
  • Categories of Ecosystem Services:
    • 1. Provisioning Services: Supply tangible products essential for human life.
      • Examples: Food, fresh water, timber, fiber, medicinal resources, air.
    • 2. Regulating Services: The benefits derived from the regulation of ecosystem processes.
      • Examples: Climate regulation (carbon sequestration), water flow regulation (flood control), disease regulation, absorption and detoxification of pollutants, waste decomposition, air quality regulation.
    • 3. Cultural Services: Non-material benefits contributing to human well-being.
      • Examples: Spiritual and religious values, recreational opportunities (fishing, tourism), aesthetic appreciation, mental and physical health benefits, educational and scientific research opportunities.
    • 4. Supporting Services: Fundamental processes that are necessary for the production of all other ecosystem services.
      • Examples: Nutrient cycling (e.g., nitrogen, phosphorus), soil formation and retention, primary production (photosynthesis), pollination (e.g., by insects, critical for food systems).