SN2 - Ecology and Evolution: Threats to Biodiversity Study Notes

Biogeochemical Cycles: Mechanisms and Processes # The Water (Hydrological) Cycle * The water cycle describes the movement of water starting from the atmosphere and falling to the surface of the planet into either aquatic or terrestrial ecosystems through the process of precipitation. * Once at the surface, water is either absorbed into biotic organisms or remains within abiotic systems. * In abiotic systems, water integrates into aquatic ecosystems or infiltrates into terrestrial ecosystems as ground water via subsurface flow and infiltration. * Water absorbed by organisms is eventually released back into the atmosphere as water vapor through the processes of transpiration (from vegetation) and respiration. * Water in abiotic components is released as water vapor into the atmosphere via evaporation from the ocean, land, reservoirs, soil, and other water surfaces. * As warm, moist air rises higher in the atmosphere, water vapor undergoes condensation, returning to a completely liquid state. * The cycle restarts as this liquid falls back to the ground as precipitation. * Three major human impacts on the water cycle include: 1. Deforestation (affects recycled rainfall and run-off). 2. Pollution (point sources like wastewater treatment plants and factories; nonpoint sources like airborne nutrients and pesticides). 3. Climate Change (resulting in changes to rainfall patterns, as projected by NOAA/GFDL CM2.1). # The Carbon Cycle and Human Impact * Starting from the atmosphere, Carbon Dioxide ($CO_2$) is captured via photosynthesis by plants, algae, and photosynthetic bacteria. * These organisms use carbon to build complex organic molecules, releasing Oxygen ($O_2$) as a byproduct. * Organic compounds are then broken down via cellular respiration in primary producers and consumers, or through decomposition, which releases $CO_2$ back into the atmosphere. * Carbon reservoirs include: 1. Solutes in oceans. 2. Plant and animal biomass. 3. Atmosphere ($CO_2$). 4. Soils. * Human activities have increased atmospheric $CO_2$, contributing to global warming and climate change because carbon acts as a greenhouse gas. * Consequences include rising temperatures, increased drought, and stronger storms that damage ecosystems and limit their ability to function as carbon sinks. * Increased $CO_2$ leads to ocean acidification, causing stored Calcium Carbonate ($CaCO_2$) to dissolve. # The Nitrogen Cycle and Eutrophication * The nitrogen cycle begins with atmospheric Nitrogen ($N_2$). Nitrogen-fixing bacteria located in the roots and nodules of legumes, or free-living in the soil, convert atmospheric nitrogen into ammonia ($NH_3/NH_4^+$) via fixation. * Nitrifying bacteria convert ammonia into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$) through nitrification. * Abiotic factors like lightning and volcanoes create nitrites in the atmosphere, which dissolve in rainwater and enter the soil during precipitation. * Nitrates are broadly used by plants and absorbed through the roots via assimilation. * Nitrogen incorporated into organic molecules is broken down by decomposers (aerobic and anaerobic bacteria and fungi) into ammonia via ammonification. * Alternatively, denitrifying bacteria can convert nitrates back into atmospheric $N_2$. * Human-induced Eutrophication occurs when nutrient overload (nitrogen and/or phosphorus) promotes excessive algae growth. * Process: 1. Nutrient load-up (excessive fertilizers flushed into water). 2. Plants and algae flourish. 3. Algae blooms block sunlight, preventing photosynthesis in deeper plants; oxygen is depleted. 4. Decomposition of dead plants by bacteria further exhausts oxygen levels. 5. Death of the ecosystem occurs as oxygen reaches levels where life is impossible (dead zones). # The Phosphorus Cycle * The phosphorus cycle begins with exposed sedimentary rock. Over time, wind causes weathering and erosion, producing phosphate-rich dusts. * This dust enters terrestrial and aquatic systems through precipitation. * In terrestrial systems, phosphate ($PO_4^{3-}$) is absorbed by plants, which are eaten by consumers. Decomposition of plants, consumers, and their wastes allows for the reuptake of phosphorus, though some is lost to leaching into waterways. * In aquatic systems, phosphate is taken up by plankton, consumed, and decomposed. Most phosphate is reabsorbed, while some is lost to sedimentation. * Sediment eventually turns back into sedimentary rock, which is re-exposed through geological uplift. # Long-Term Ecological Research: Hubbard Brook * Established in 1955 in New Hampshire, this long-term study focuses on: 1. Hydrology (water flow, snowfall, ice-in/ice-out). 2. Environmental factors for tree growth and deforestation effects on mineral flux. 3. Impacts of changes on bird behavior and insect populations (reproductive capacity). 4. Acid rain effects on soil mineral changes and plant root micro-environments. 5. Cycling of Nitrogen, Sulfur, Phosphorus, Mercury, Calcium, and Carbon. # Biodiversity and the Anthropocene * The "Big Five" threats to Earth's biodiversity are: 1. Changes in land and sea use (50%). 2. Species overexploitation (24%). 3. Invasive species and disease (13%). 4. Pollution. 5. Climate Change. * Levels of Biodiversity: 1. Genetic diversity: Variation within a population; loss stunts microevolution and adaptation. 2. Species diversity: Variety across ecosystems; loss includes extirpation (local) and extinction (global). 3. Ecosystem diversity: Variety of environments across the biosphere. * The Anthropocene is a proposed geological epoch starting in the mid-20th century, characterized by radical human alterations to Earth (carbon emissions, forest fires, ocean acidification, species extinction, melting ice). * Mass Extinctions throughout time: 1. Ordovician (85% death rate, 450 million years ago). 2. Devonian (70% death rate). 3. Permian (95% to 96% marine/70% terrestrial death rate). 4. Triassic (80% to 85% death rate). 5. K-T (80% death rate, 65 million years ago, asteroid impact). # Habitat Loss and Fragmentation * Habitat loss is the primary cause of extinction globally, driven largely by agricultural expansion. * The species-area relationship dictates that larger areas support more species. * Edge Effects: Fragmentation creates edges where light, temperature, and moisture conditions change (increased sunlight/warmth, decreased moisture, increased wind/fire risk). This affects soil invertebrates and nutrient cycling. * Biological Dynamics of Forest Fragments Project (BDFFP): Started in 1979 near Manaus, Brazil, to study edge effects. Found that fragmented forests lose species depending on the surrounding matrix. * Roads and barriers impose chemical pollutants (dust, heavy metals, lead) within $10-200 \times \text{m}$ of the surface and restrict animal movement, leading to genetic drift and decreased diversity. # Overexploitation and Invasive Species * Overexploitation involves intense harvesting resulting in decline, such as the 1992 collapse of the Atlantic Cod industry ( < 1 \times \text{%} of previous catches) due to industrial bottom-trawling. * Bison were historically overexploited both for resources and as a tool of government policy to starve indigenous populations. * Invasive species are exotic/non-native species that become established and disrupt ecosystems. * Characteristics of invasives: High reproductive rates, long-lived, rapid dispersal, rapid growth, generalists, high tolerance, and resistance to predation. * Mechanisms of introduction: Accidental transport (Ballast water), agriculture/aquaculture, biological control, and intentional release (e.g., European Starlings released in New York in the 1890s, now numbering over 200 million). * Control strategies: Physical (barriers, culling), Chemical (herbicides), and Biological (predators/parasites). Prevention and biosecurity are the most cost-effective. # Pollution and Bioaccumulation * Pollution includes industrial chemicals, fertilizers, plastics, and endocrine disruptors. * Bioaccumulation: Accumulation of contaminants in an organism over time. * Biomagnification: Increasing concentration of toxins at higher trophic levels (e.g., PCBs in Herring gull eggs are 5,000 times higher than in phytoplankton). * Plastics: Uniquely mark the Anthropocene, forming "plastiglomerates." They break down into microplastics that interfere with hormones and reproduction. Recent studies show microplastics in the human brain olfactory bulb and human testes. * Endocrine disruptors: Chemicals that mimic hormones (like estrogen) and bind to receptors, inhibiting development, decreasing germ cells, and persisting in the environment.