Comprehensive Study Guide: Ecology, Biodiversity, Evolution, and Ecosystem Services
Ecosystem Responses to Disturbances
Definition of Ecosystem Disturbance:
An ecosystem disturbance is an event initiated by physical, chemical, or biological agents that results in pronounced changes in population size or community composition.
Categories of Disturbances:
Natural Disturbances: Naturally occurring physical or biological events, including:
Hurricanes
Ice storms
Tsunamis
Tornadoes
Volcanic eruptions
Forest fires
Anthropogenic Disturbances: Human-induced disruptions directly altering natural systems, including:
Human settlements
Agriculture
Air pollution
Forest clear-cutting
Mountaintop removal for coal mining
Ecological Impact and Nutrient Cycling:
Disturbances are not inherently disastrous for ecological communities.
Certain events, such as low-intensity forest fires, provide critical ecological benefits by liberating nutrients trapped within dead organic matter and returning them to the soil for reuse by living organisms.
Ecosystem Stability: Resistance and Resilience
Resistance:
Definition: A measure of the extent to which a disturbance can affect the overall flows of energy and matter within an ecosystem.
Key Characteristics: An ecosystem displays high resistance if a disturbance takes place but the net primary productivity () and nutrient flows remain essentially unchanged.
Example: A scenario in which a disturbance eliminates an entire producer species, yet overall ecosystem remains stable and unchanged.
Resilience:
Definition: The rate at which an ecosystem returns to its pre-disturbance original state following a disruptive event.
High Resilience: The ecosystem recovers its baseline functions and structure relatively quickly.
Example: An area subjected to severe drought where half of the resident species are eliminated, but energy and matter flows completely recover to normal levels within the following year.
Low Resilience: The ecosystem requires many decades or centuries to recover its original structure and flows.
Examples: The Florida Everglades and the Chesapeake Bay.
Intermediate Disturbance Hypothesis
Core Principle:
Ecosystems subject to intermediate levels of disturbance sustain higher species diversity than ecosystems characterized by either low or high disturbance regimes.
Mechanisms Driving Diversity Across Disturbance Regimes:
Low Disturbance Frequency or Intensity:
Generates intense competitive interactions among resident species.
Highly competitive, dominant species systematically outcompete and exclude weaker species through competitive exclusion, resulting in lower overall diversity.
High Disturbance Frequency or Intensity:
Imposes severe, frequent environmental stress and destruction.
Only species possessing exceptionally high population growth rates and rapid colonization capabilities can survive and avoid local extinction, leading to low overall diversity.
Intermediate Disturbance Frequency or Intensity:
Mortality rates caused by disturbances prevent competitive dominants from monopolizing resources, while disturbance rates remain low enough that vulnerable species are not eradicated.
This dynamic balance maximizes coexistence across diverse taxa, fostering the highest overall species richness.
Watershed Dynamics and Environmental Management
Watershed Fundamentals:
A watershed (alternatively termed a drainage basin or catchment) is a defined land area where all surface water drains and converges at a single exit point, such as a river, lake, or ocean.
Serves as an ideal localized landscape unit for ecologists to quantify biogeochemical nutrient cycling, hydrological flows, and broader ecosystem functions.
Runoff and Natural Filtration:
Surface runoff washes across terrestrial landscapes, gathering and transporting pollutants into surrounding aquatic bodies.
Intact, healthy watersheds serve as natural filtering systems, immobilizing sediments and contaminants before water reaches larger aquatic ecosystems.
Mountain Runoff Management:
Effective runoff mitigation requires the preservation and active restoration of native vegetation along slopes.
Forests and riparian plant life function as natural sponges by:
Intercepting precipitation before it impacts bare soil.
Decreasing overland water velocity.
Providing root channels and porous ground cover that facilitate water infiltration into the soil table.
Deforestation and Riparian Degradation:
Clear-cutting or removing trees directly along waterways strips the water column of protective canopy shade.
This loss of shade exposes surface water to direct solar radiation, driving up water temperatures and reducing dissolved oxygen while simultaneously impairing terrestrial soil infiltration.
Quantifying Earth's Biodiversity
Global Species Richness Scope:
Taxonomists have formally cataloged and named approximately distinct species.
Approximately new species were formally discovered and described in alone.
Insects contain a greater number of documented species than most other taxonomic groups.
Global estimates for the true total number of species on Earth range from to .
Components of Species Diversity:
Relying solely on raw species counts is inadequate for diagnosing ecosystem health or assessing human disruption; two distinct metrics are required:
Species Richness:
The total count of distinct species inhabiting a given sampling area (e.g., a pond, meadow, or defined forest plot).
Tropical rainforests display exceptionally high species richness, which corresponds to elevated , highly diversified food webs, and an abundance of specialized ecological niches.
Species Evenness:
A quantitative measure of the relative abundance of individuals distributed among each of the different species present in a community.
High evenness indicates that all species possess roughly similar individual population abundances.
Low evenness indicates that one or a few dominant species constitute the vast majority of the community's individuals.
Anthropogenic Impacts:
Human environmental disturbances typically depress both species richness and species evenness simultaneously.
Ecological Sampling Techniques
Field Sampling Applications:
Because counting every individual within a landscape is unfeasible, standardized statistical sampling tools are deployed to evaluate abundance and spatial distribution.
Standard Methodologies:
Quadrats:
Standardized square sampling frames placed randomly across an ecosystem.
Organisms within each quadrat are counted, and statistical extrapolation is used to determine total population density and distribution across the larger landscape.
Transect Lines:
Measured, straight sampling lines established across an environmental gradient or habitat.
Researchers traverse the line to document the presence, abundance, and spatial layout of organisms at regular intervals.
Phylogeny and Evolutionary Relationships
Phylogenetic Concepts:
Phylogeny is the study of the evolutionary history and genealogical relatedness among distinct biological taxa.
Phylogenetic Trees (Cladograms):
Branching diagrams that represent hypotheses about how modern and ancestral species diverged from shared common ancestors over evolutionary time.
Evolutionary proximity is dictated by shared traits: species sharing a higher proportion of derived characteristics are considered more closely related.
Evidence Bases for Constructing Phylogenies:
Comparative morphology (physical anatomical structures)
Ethology (behavioral patterns)
Molecular genetics (DNA and protein sequence alignments)
Fundamentals of Genetics and Microevolution
Evolutionary Foundations:
Evolution: A directional or non-directional change in the genetic composition of a biological population across successive generations.
Microevolution: Evolutionary change occurring at a fine temporal scale, shifting allele frequencies from one single generation to the next.
Genetic Terminology:
Gene: The physical and functional unit of heredity located at a specific locus on a chromosome; composed of a DNA sequence encoding instructions for protein synthesis.
Allele: A specific alternate variant or version of a gene.
Genotype: The complete collection of alleles and genes present inside a specific individual organism.
Phenotype: The physical, physiological, and behavioral traits expressed by an organism.
Phenotypes are determined by interactions between the underlying genotype and environmental conditions.
Example: Incubation temperatures of clutches of eggs dictate offspring sex determination in multiple turtle and crocodile species.
Primary Sources of Genetic Diversity
Mutation:
An unexpected change in the nucleotide sequence of the genetic code, typically caused by uncorrected errors during DNA replication preceding cell division.
Most mutations are neutral, while some are lethal to the organism.
Rarely, a mutation confers a selective advantage, such as mutations in mosquitoes that reduce their physiological vulnerability to chemical insecticides.
The continuous accumulation of viable mutations within a gene pool serves as the raw material for evolutionary novelty.
Genetic Recombination:
The reassortment and shuffling of alleles occurring during sexual reproduction.
Does not generate novel alleles, but creates novel combinations of existing alleles, generating diverse phenotypes.
Modes of Evolution: Selection and Random Processes
Artificial Selection:
Occurs when human selection pressures determine which individual organisms survive and reproduce, targeting predetermined, desirable phenotypic traits.
Domestic Examples:
Modern domesticated dog breeds evolved through deliberate selective breeding of the gray wolf ancestral stock.
Agricultural livestock breeds (horses, cattle, sheep, pigs, chickens) were systematically modified through targeted artificial selection.
Unintended Consequences:
Heavy agricultural spraying of glyphosate herbicides (e.g., Roundup) has selected for herbicide-resistant superweeds.
Widespread antibiotic overprescription and overuse has selected for drug-resistant bacterial strains.
Natural Selection:
A mechanism formulated by Charles Darwin wherein prevailing environmental pressures dictate which individuals survive and successfully reproduce.
Fitness and Adaptations:
Fitness refers to an individual's relative capability to survive and produce fertile offspring.
Adaptations are heritable physical or behavioral traits that increase an individual organism's fitness in a specific environment.
Core Postulates Required for Natural Selection:
Phenotypic traits must vary among individuals within a population.
The advantageous phenotypic variations must be inheritable and capable of transmission to subsequent generations.
Random Evolutionary Processes:
Gene Flow (Gene Migration):
The migration and physical transfer of genetic material between distinct populations, modifying local allele frequencies.
Sustained gene flow between populations reduces genetic divergence and prevents speciation.
Can counteract inbreeding depression; for example, introducing Texas panthers into a critically inbred Florida panther population increased population numbers and sharply diminished morphological defects.
Genetic Drift:
Fluctuations in allele frequencies from generation to generation driven entirely by chance events rather than fitness advantages.
Exerts pronounced evolutionary impacts primarily on small, isolated populations.
Bottleneck Effect:
A dramatic, sudden collapse in population size brought about by habitat destruction, natural cataclysms, overharvesting, or abrupt environmental change.
The surviving cohort retains only a small subset of the ancestral population's original genetic diversity.
Extreme bottlenecks can lead to species extinction; for example, cheetahs experienced a severe bottleneck approximately , leaving the species with low genetic diversity and high disease susceptibility.
Founder Effect:
The establishment of a new colony by an exceptionally small group of individuals migrating away from a larger parent population (e.g., a small flock of birds colonizing an isolated oceanic island).
The resulting gene pool diverges substantially from the original mainland population due strictly to sampling error.
Valuation of Ecosystems: Intrinsic vs. Instrumental Worth
Intrinsic Value:
The inherent, autonomous worth an ecosystem holds simply because it exists, entirely detached from its practical utility or benefits to humanity.
Preserved out of ethical or philosophical necessity, treated as an end in itself comparable to human rights, freedom, or happiness.
Instrumental Value:
The tangible and intangible value of an ecosystem measured by its utility as an instrument or resource to advance human well-being.
Quantified formally through the categorization of ecosystem services.
Categories of Ecosystem Services
Provisioning Services:
Tangible goods and materials directly harvested, extracted, or collected from natural ecosystems for direct human use.
Core Examples:
Food resources: cultivated crops, domesticated livestock, and wild ocean fisheries.
Water supplies: fresh water collected for residential consumption and crop irrigation.
Raw biomass: timber for structural lumber and plant fuel for burning.
Medicinal biochemicals: specialized compounds derived from wild plants for pharmaceuticals.
Regulating Services:
Benefits provided by the natural buffering and maintenance of physical environments and biogeochemical cycles.
Core Examples:
Water purification via contaminant adsorption in wetland soils.
Air quality moderation through particulate capture and gas exchange by terrestrial flora.
Pollination of agricultural crops carried out by wild bees, insects, and birds.
Flood buffering and soil stabilization supplied by wetland and mangrove roots.
Climate regulation and biological control of disease-transmitting vectors.
Supporting Services:
Baseline ecological processes essential for the perpetuation, health, and functionality of all other ecosystem services.
Core Examples:
Biogeochemical nutrient cycling, including decomposition and mineralization.
Pedogenesis and soil formation via geological breakdown and humic decay.
Primary production generating biomass via autotrophic photosynthesis.
Global hydrological cycling through precipitation, surface evaporation, and plant transpiration.
Critical habitat provision for diverse biological life.
Cultural Services:
Non-material, experiential, educational, and intellectual contributions ecosystems provide to human society.
Core Examples:
Outdoor recreation and eco-tourism, including hiking, camping, and animal watching.
Aesthetic inspiration and religious or spiritual reverence for natural landscapes.
Scientific inquiry and outdoor educational opportunities.
Limits of Economic Valuation Frameworks and Sustainability
Risks of Exclusively Instrumental Valuation:
Monetizing natural systems solely through market-based instrumental metrics incentivizes the pursuit of short-term economic gains over long-term environmental sustainability.
Overextracting provisioning resources (e.g., intensive clear-cutting or overfishing) compromises the supporting and regulating processes that underpin those resources.
Neglecting the supporting foundations leads to ecosystem collapse, permanently eroding the future availability of resources.