Topic 9.8 Notes – Invasive Species

What Invasive Species Are

An invasive species is a non-native species that establishes and spreads in a new area and threatens native species. It can be a plant, animal, fungus, or microorganism.

Here are the terms that get mixed up a lot:

• Native species lives naturally in a region.
• Non-native species (also called introduced, alien, or exotic) was moved there by human activity.
• Established or naturalized species has a self-sustaining population in the new place.
• Invasive species is non-native and causes ecological harm or threatens natives.

The big trap is thinking all non-native species are invasive. They are not. Some stay confined or are useful, like crops or ornamentals. APES cares about the ones that harm ecosystems.

This usually involves crossing natural barriers because of people, not a species slowly shifting range on its own. Disturbance and human-caused environmental change also make ecosystems easier to invade.

How Invasions Happen and Why Some Species Succeed

A biological invasion tends to follow the same path:

1. Transport
   The organism gets moved beyond its normal range.
2. Introduction
   It is released, escapes, or arrives in the new ecosystem.
3. Establishment
   It survives and reproduces enough to form a stable population.
4. Spread
   It expands into more habitat.
5. Impact
   It begins harming native species or ecosystem processes.

Stages of biological invasion

Common pathways

That first part of the sequence usually happens through a few major human pathways.

• Ballast water from ships
• Organisms attached to ship hulls
• Wood packaging, firewood, nursery plants, agricultural products
• Seeds in soil, feed, clothing, vehicles, machinery
• Boats, trailers, fishing gear, bait containers
• Intentional release of ornamental plants, pets, aquarium species, game species, biocontrol species

Spread gets easier with roads, canals, disturbed land, connected waterways, and repeated transport.

Why some species take over

Many invasive species are:

• Generalists
  They can use many food sources or habitats and tolerate many conditions.
• r-selected species
  They mature early, reproduce often, have many offspring, grow fast, and disperse well.

Other helpful traits include disturbance tolerance, vegetative reproduction, and strong seed or larval dispersal.

Enemy release matters too. In the new range, the invader may leave behind predators, parasites, and pathogens that kept it in check.

Ecosystems that are especially vulnerable include:

• Disturbed habitats
• Island ecosystems
• Isolated lakes
• Systems with native species that have narrow niches, small populations, or slow reproduction

How Invasive Species Harm Ecosystems

Competition

Invaders may outcompete natives for food, water, nutrients, light, space, nesting sites, or shelter, which lowers native survival and reproduction.

Predation and Herbivory

Some invaders eat native species or heavily graze plants with few defenses.
Burmese pythons in the Florida Everglades are the classic example. They prey on native mammals and birds and are hard to control.

Disease and Parasites

Introduced species can bring pathogens or parasites that native species have little resistance to.

Habitat and Ecosystem Change

They can change:

• nutrient cycling
• decomposition
• soil chemistry
• water flow and clarity
• erosion
• fire frequency or intensity
• habitat structure
• food webs

Some create positive feedbacks that help themselves spread, such as invasive grasses that increase fire and then regrow quickly after fire.

Biodiversity Loss and Food-Web Effects

Invasive species reduce native populations and species richness. They can also trigger trophic cascades. This is one of the major causes of biodiversity loss.

Economic and Human Effects

They can damage agriculture, fisheries, forestry, infrastructure, water-intake pipes, boats, and recreation. Some spread disease or cause toxins or injuries.

Representative example

Zebra mussels were introduced to North America in ballast water. They spread through the Great Lakes, connected waterways, and on boats/equipment.

Zebra mussel introduction and spread by ballast water

The map highlights the route into the Great Lakes and then spread through inland waterways. The ship diagrams at the bottom connect that spread to ballast water transport.

Their impacts include:

• competing with native filter feeders
• smothering native mussels
• changing water clarity and food webs
• clogging pipes
• fouling boats and docks

Controlling Invasive Species

Control works best when matched to the invasion stage.

Prevention

Usually the most effective and least expensive option.

• inspections and quarantine
• import restrictions
• ballast-water treatment
• hull cleaning
• clean, drain, dry boats
• no pet, aquarium, or bait release
• limit firewood movement
• public education

Early Detection and Rapid Response

This means finding the species while the population is still small and acting immediately.

• surveys
• traps
• environmental sampling
• public reports

Once spread happens, control gets much harder and more expensive.

Control Methods

• Physical/mechanical
  Hand-pulling, digging, cutting, trapping, hunting, barriers, harvesting
• Chemical
  Herbicides, insecticides, molluscicides, rodenticides. These can work fast but may harm non-target species or pollute.
• Biological
  Predators, parasites, or pathogens. These may give long-term suppression but can become a new problem.
  Cane toad in Australia is the warning example of failed biocontrol.
• Habitat modification/restoration
  Restore native conditions, reduce reinvasion, and replant natives.

Integrated Management

The best plans combine methods based on the species, ecosystem, invasion stage, and non-target risks. For zebra mussels, that could mean inspecting and drying boats, monitoring larvae and adults, and treating small local infestations.

How to Evaluate a Control Plan

A strong plan should identify:

• the species
• how it causes harm
• the stage of invasion
• the intervention
• how the intervention reduces introduction, survival, reproduction, or spread
• effects on native species
• cost, feasibility, and unintended consequences
• a monitoring plan

Evidence that a plan is working includes:

• lower invasive abundance or density
• slower spread
• less area occupied
• reduced reproduction
• recovery of native species or species richness
• less infrastructure or economic damage
• minimal non-target effects

Short-term decline alone is not enough. APES wants long-term suppression plus ecosystem recovery.