Invasive species Knowledge-Based Study

Study notes

1. What Are Invasive Species?

   In ecology, a native species is one that naturally occurs in a specific ecosystem due to natural evolutionary and geographic processes. In contrast, an introduced (or exotic) species is an organism transported by human activity—either intentionally or accidentally—beyond its native geographical range.

   Not all introduced species become invasive. An introduced organism is classified as an invasive species only when it establishes a self-sustaining population and actively causes environmental, economic, or public health harm in its new environment.

   Key Distinction

   An introduced plant in a backyard garden that cannot survive outside cultivation is non-native, but it is not invasive. It only gains invasive status when it escapes cultivation, spreads aggressively, and outcompetes local organisms.

   Why Do Certain Species Become Invasive?

   Invasive species often possess particular ecological and physiological adaptations that facilitate rapid colonisation:

   • High Reproductive Output: Producing vast quantities of seeds, eggs, or offspring in short reproductive cycles.
   • Broad Ecological Tolerance: The ability to thrive across diverse temperature ranges, moisture levels, soil types, and food resources.
   • Phenotypic Plasticity: Morphological or physiological flexibility that allows quick acclimatisation to changing environments.
   • Effective Dispersal Mechanisms: Adaptations such as wind-borne seeds, vegetative fragmentation, or mobile larvae that permit rapid geographical expansion.

   A fundamental ecological principle explaining their success is the Enemy Release Hypothesis. In their native ranges, population densities are strictly regulated by specialised predators, parasites, and pathogens. When transported to novel ecosystems, they often escape these co-evolved natural enemies, freeing more resources for growth and reproduction.

   Impacts on Biodiversity and Ecosystem Function

   Invasive species represent one of the primary drivers of global biodiversity loss. Their ecological impacts manifest through several interconnected pathways:

   • Competitive Exclusion: Invasive organisms frequently deplete limiting resources (such as light, water, nutrients, or nesting cavities) more efficiently than native species, driving vulnerable local taxa toward local extinction.
   • Direct Predation and Herbivory: Native prey often lack co-evolved anti-predator behaviours (known as ecological naivety) to detect or evade invasive predators. A classic example is the brown tree snake (Boiga irregularis), which decimated native forest bird populations in Guam.
   • Alteration of Ecosystem Processes: Some invasive species modify physical habitat conditions. For instance, invasive cheatgrass (Bromus tectorum) changes fire regimes in western North America by increasing the frequency and intensity of wildfires, inhibiting native shrub regeneration.
   • Hybridisation: Introduced taxa can interbreed with related native species, diluting the native gene pool and potentially causing the loss of rare, distinct genotypes.

   Approaches to Managing Invasive Species

   Ecologists employ a hierarchical framework to control biological invasions:

   1. Prevention: The most cost-effective management strategy, relying on strict biosecurity protocols, quarantine inspections at borders, and regulations on ballast water discharge.
   2. Early Detection and Rapid Response (EDRR): Surveillance programmes designed to locate and eliminate emerging non-native populations before widespread establishment occurs.
   3. Eradication: Complete removal of the target population, which is usually feasible only in isolated habitats, such as small islands, or at very early stages of invasion.
   4. Long-term Containment and Control: Applied when eradication is no longer practical. Control methods include:
      • Mechanical/Physical Control: Manual removal, trapping, or physical barriers.
      • Chemical Control: Application of targeted herbicides or pesticides, balanced against potential non-target toxicity.
      • Biological Control (Biocontrol): The deliberate introduction of a co-evolved natural enemy from the invasive species' native range. Biocontrol requires extensive safety trials to ensure the introduced biocontrol agent does not shift hosts and attack native taxa.