🐼Conservation Biology Unit 3 – Overexploitation & Invasive Species

Key Concepts

• Overexploitation occurs when a resource is harvested at a rate faster than it can be replenished leading to population decline and potential extinction
• Invasive species are non-native organisms that cause harm to the environment, economy, or human health in their introduced range
• Overexploitation and invasive species are major drivers of biodiversity loss and ecosystem degradation worldwide
• Sustainable management practices aim to balance resource use with conservation to prevent overexploitation and minimize the impacts of invasive species
• Ecological factors such as species interactions, habitat alteration, and disturbance regimes influence the susceptibility of ecosystems to overexploitation and invasion
• Economic incentives, cultural practices, and lack of regulations contribute to unsustainable resource use and the spread of invasive species
• Monitoring, early detection, and rapid response are crucial for preventing the establishment and spread of invasive species

Causes of Overexploitation

• Increasing human population size and resource demand drive unsustainable harvest rates of wild populations (fish, timber, wildlife)
• Technological advancements enable more efficient extraction methods and access to previously unexploited areas (deep-sea fishing, remote forests)
  • GPS and sonar improve the ability to locate and track target species
  • Mechanized equipment allows for larger-scale harvesting operations
• Economic incentives encourage short-term profits over long-term sustainability leading to overfishing, poaching, and illegal logging
• Lack of effective regulations, enforcement, and monitoring enables overexploitation to continue unchecked in many regions
• Poverty and limited alternative livelihoods pressure communities to rely on unsustainable resource use for survival
• Cultural practices and traditional medicine create demand for rare and endangered species (rhino horn, tiger bone)
• Climate change and habitat loss exacerbate the impacts of overexploitation by reducing the resilience and recovery capacity of affected populations

Types of Overexploitation

• Overfishing depletes marine fish stocks faster than they can reproduce leading to population collapse and ecosystem imbalance
  • Examples include Atlantic cod, bluefin tuna, and sharks
• Overhunting threatens terrestrial wildlife populations through unsustainable harvest for meat, trophies, or traditional medicine (bushmeat trade, ivory poaching)
• Overlogging degrades forest ecosystems by removing trees at a rate exceeding natural regeneration causing habitat loss and soil erosion
• Overgrazing by livestock can lead to desertification, soil compaction, and loss of plant diversity in grassland and savanna ecosystems
• Overharvesting of non-timber forest products such as medicinal plants, fruits, and seeds disrupts ecological processes and reduces resource availability for wildlife
• Overexploitation of freshwater resources (groundwater pumping, river diversion) alters hydrological cycles and degrades aquatic habitats
• Unsustainable collection of ornamental plants and animals for the pet trade contributes to population declines and local extinctions

Impacts on Ecosystems

• Overexploitation reduces population sizes, alters age and size structure, and decreases genetic diversity of targeted species
• Removal of keystone species or ecosystem engineers (sea otters, beavers) disrupts ecological interactions and modifies habitat structure
• Trophic cascades occur when the loss of top predators releases prey populations from control leading to overgrazing and altered community composition
  • Overfishing of sharks can lead to increases in mesopredators and declines in herbivorous fish
• Selective harvesting of large, reproductively mature individuals reduces the reproductive capacity and resilience of populations to environmental stressors
• Habitat degradation and fragmentation associated with overexploitation (clear-cutting, bottom trawling) reduces ecosystem complexity and biodiversity
• Overexploitation can shift the balance of competitive interactions and facilitate the establishment of invasive species in disturbed ecosystems
• Unsustainable resource use compromises the provision of ecosystem services such as water filtration, carbon sequestration, and nutrient cycling

Invasive Species: Definition and Examples

• Invasive species are non-native organisms that establish, spread, and cause harm in introduced environments
• Invasive species can be plants, animals, fungi, or microorganisms introduced intentionally or accidentally by human activities
• Examples of invasive plants include kudzu (Southeastern US), water hyacinth (global), and Japanese knotweed (Europe, North America)
  • Kudzu grows rapidly, smothering native vegetation and altering soil chemistry
• Invasive animals such as the Burmese python (Florida Everglades), European starling (North America), and zebra mussel (Great Lakes) disrupt food webs and compete with native species
  • Burmese pythons prey on native mammals and birds, reducing their populations
• Invasive fungi like chestnut blight (North America) and Dutch elm disease (Europe, North America) have decimated native tree species
• Invasive microorganisms such as the chytrid fungus (global) and avian malaria (Hawaii) have caused declines and extinctions in amphibian and bird populations

Invasion Process and Factors

• The invasion process involves multiple stages: introduction, establishment, spread, and impact
• Introduction occurs when a species is transported outside its native range through human-mediated pathways (shipping, trade, travel)
  • Ballast water discharge from ships is a major source of aquatic invasions
• Establishment requires the introduced species to survive, reproduce, and form a self-sustaining population in the new environment
  • Factors influencing establishment include propagule pressure, environmental suitability, and biotic resistance
• Spread involves the expansion of the invasive species' range through dispersal and colonization of new areas
  • Spread is facilitated by favorable habitat corridors, human transportation networks, and the species' dispersal abilities
• Impact refers to the negative effects of the invasive species on native biodiversity, ecosystem functioning, and human well-being
• Factors contributing to invasiveness include high reproductive rates, broad environmental tolerances, lack of natural enemies, and ability to alter ecosystem processes
  • Successful invaders often exhibit rapid growth, early maturity, and high fecundity
• Disturbance and habitat modification create opportunities for invasive species to establish by reducing competition from native species

Ecological Effects of Invasive Species

• Invasive species compete with native species for resources such as food, space, and breeding sites leading to reduced growth, reproduction, and survival of native populations
• Predation by invasive species can cause declines and local extinctions of native prey populations that lack evolved defenses
  • Invasive predators like the brown tree snake (Guam) have decimated native bird populations
• Invasive species can hybridize with closely related native species resulting in genetic introgression and loss of locally adapted genotypes
• Ecosystem engineers such as invasive beavers (Tierra del Fuego) and cordgrass (Spartina alterniflora, global) modify physical habitats and alter community structure
• Invasive plants can change soil chemistry, nutrient cycling, and fire regimes creating feedback loops that favor their own persistence (positive feedback)
  • Garlic mustard (North America) releases allelopathic compounds that inhibit native plant growth
• Invasive species can serve as vectors for diseases and parasites that affect native species and humans (West Nile virus, Lyme disease)
• Economic costs of invasive species include damages to agriculture, forestry, and infrastructure as well as expenses for control and eradication efforts

Management Strategies and Solutions

• Prevention is the most cost-effective approach to managing invasive species by reducing the likelihood of introduction and establishment
  • Strategies include border inspections, quarantine measures, and ballast water treatment
• Early detection and rapid response (EDRR) aims to identify and eradicate invasive populations before they become widely established
  • Surveillance, monitoring, and public reporting networks facilitate early detection
• Mechanical control involves physical removal of invasive organisms through methods such as hand-pulling, trapping, and fencing
  • Volunteer programs engage the public in manual removal of invasive plants
• Chemical control uses herbicides, pesticides, and other chemicals to suppress invasive populations
  • Targeted application minimizes non-target impacts, but ecological risks must be considered
• Biological control introduces natural enemies (predators, parasites, pathogens) from the invasive species' native range to reduce its abundance
  • Rigorous testing ensures the specificity and safety of biocontrol agents before release
• Ecosystem management focuses on maintaining and restoring native biodiversity, habitat quality, and ecological processes to enhance resilience against invasions
• Integrated pest management (IPM) combines multiple control methods based on ecological principles to optimize effectiveness and minimize adverse impacts
• International cooperation, trade regulations, and public awareness are essential for preventing the global spread of invasive species and promoting responsible practices