APES 9.5-9.6: Invasive Species and Human Impacts on Diversity

Invasive species

An introduced species that establishes in a new environment and causes ecological, environmental, economic, or human-health harm. Invasive species are a harmful subset of established non-native species.

How does an introduced species differ from an invasive species?

An introduced species is transported outside its native range, usually by humans, whereas an invasive species becomes established and produces significant harm in its new environment.

Why is the term "invasive species" considered subjective?

There is no universally accepted definition of invasion or harm. Different definitions may emphasize effects on biodiversity, ecosystems, economies, human health, or even native species altered by human activity.

What is propagule pressure, and why does it increase invasion success?

Propagule pressure is the repeated or large-scale arrival of individuals, seeds, larvae, or other reproductive units to a new location. Repeated introductions increase the probability that enough organisms survive, reproduce, and establish a viable population.

Why can an introduced species remain unnoticed before becoming invasive?

At low population density, reproduction and survival may be insufficient to sustain the population. A lag period or multiple introductions may occur before establishment and rapid growth.

How can disturbance promote biological invasions?

Disturbances such as fire, land clearing, roads, and soil movement alter resource availability and reduce competition. Species that regenerate rapidly or tolerate disturbed conditions can then colonize more successfully than native species.

What role do limiting nutrients play in invasions after ecosystem disturbance?

Nitrogen and phosphorus are often limiting resources. Disturbance can change their availability, allowing a rapidly growing introduced species to capture nutrients and outcompete native plants.

Ecological niche

The set of environmental conditions, resources, and interactions that allow a species to survive and reproduce. An introduced species may become invasive if it occupies an unused niche or creates a new opportunity in the recipient ecosystem.

How can an introduced species succeed when native species already occupy most available niches?

It may use resources that native species cannot access, such as nutrients in unusual soils or water reached by a deep taproot. It may also alter the environment, suppress competitors chemically, or avoid predators and diseases present in its native range.

Edge effect

A change in ecological conditions and species interactions at the boundary between two habitats, such as intact forest and cleared agricultural land. Edge habitats can create new opportunities for introduced species.

Why may islands be especially vulnerable to invasive species?

Island species often evolved with fewer competitors and predators, leaving ecological niches relatively open. Their isolation can also produce endemic species with limited defenses against newly introduced predators, competitors, or pathogens.

How can geographic isolation both increase and decrease invasion risk?

Isolation can reduce the arrival of non-native organisms, delaying colonization. Once an invader arrives, however, island communities may be highly vulnerable because native species often lack strong competitors or predators.

How can intensive management of fenced areas unintentionally favor invasive plants?

Clearing, mowing, and soil disturbance reduce native competition and expose bare soil. Fences may exclude herbivores that would otherwise consume invasive plants, while wind, water, birds, and small animals can still transport propagules into the area.

Which traits commonly contribute to the success of invasive species?

Common traits include rapid growth, high reproductive rates, vegetative reproduction, resilience, association with humans, broad environmental tolerance, and a history of successful invasions. These traits are useful predictors but do not identify every invasive species.

How can competition promote the spread of an introduced species?

If native competitors and predators are absent or less effective in the new environment, the introduced species may obtain resources with little resistance. This can produce rapid population growth, although competitive exclusion of natives is not inevitable.

How can an invasive species modify its environment to benefit itself or other organisms?

It may release allelopathic chemicals, change nutrient cycling or abiotic conditions, alter fire frequency, or create shelter and microclimates. These changes can suppress native species or facilitate additional introduced species.

Ecological facilitation in biological invasions

A process in which one species physically or chemically changes the environment in a way that improves conditions for another species. For example, structures produced by invasive mussels can create habitat for benthic invertebrates.

How can invasive plants alter wildfire dynamics?

Fire-adapted plants can produce abundant dry material that increases fire frequency or intensity. Frequent fires may prevent fire-sensitive native plants from recovering, reinforcing the invader's dominance.

How can small founding populations evolve rapidly after introduction?

Founder effects and population bottlenecks reduce population size and genetic variation, but the remaining variation may favor traits suited to dispersal or novel stressors. Natural selection, phenotypic plasticity, and post-introduction evolution can then increase fitness in the new environment.

Enemy release hypothesis

An introduced species may become invasive because it leaves behind important predators, parasites, pathogens, or competitors from its native range. Reduced biological control allows rapid population growth in the recipient ecosystem.

Vector in invasion biology

A pathway or agent that transports organisms to a new location. Vectors include ships, ballast water, fouled hulls, vehicles, firewood, pets, recreation equipment, horticultural materials, and the intentional movement of organisms.

Why are human activities the dominant vectors for many non-native species?

Humans transport organisms more rapidly and across greater distances than most natural dispersal processes. International trade, tourism, shipping, agriculture, horticulture, and the pet trade repeatedly create opportunities for introduction.

How can biological control introductions produce unexpected ecological damage?

A species introduced to control a pest may fail to target only that pest or may be poorly matched to its activity pattern. It can then establish a large population and prey on native organisms or create new ecological problems.

How do ballast water and hull fouling spread aquatic invasive species?

Organisms can attach to the external surfaces of vessels or survive in water taken aboard for stability and later discharged in another port. These mechanisms transport marine and freshwater organisms between otherwise separated ecosystems.

Why can recreation be an underestimated pathway for biological invasions?

Seeds, spores, larvae, and pathogens can hitchhike on boats, boots, waders, bicycles, pets, vehicles, and firewood. Recreational equipment can move organisms farther and faster than natural dispersal.

How might climate change increase invasion risk during transport and after introduction?

Warmer conditions can allow organisms to survive in regions that were previously unsuitable and can create new species interactions. Temperature stress during transport may also select for genotypes that tolerate later warming in the introduced range.

How can native species act as a biological barrier to invasion?

In large, relatively undisturbed systems, native species may occupy available niches and consume potential invaders. This can limit establishment in main habitats even if the invader persists in isolated or disturbed areas.

How do invasive species reduce biodiversity?

They can displace native species through competition, alter habitats, prey on native organisms, spread pathogens, or hybridize with related species. These effects can cause local extinctions and make communities in different regions more similar, a process called biotic homogenization.

What is genetic pollution caused by invasive species?

Genetic pollution is the unintended transfer of genes from introduced organisms into native populations through hybridization and introgression. It can reduce locally adapted genetic differences and potentially replace or eliminate native genotypes.

How can invasive species affect ecosystem functions rather than only species numbers?

Invasives can change fire regimes, nutrient cycling, hydrology, sediment structure, light penetration, and food-web interactions. These functional changes may persist even when the invader's direct effects on individual species are not immediately obvious.

Why can removing an invasive species sometimes harm an ecosystem?

After becoming dominant, an invasive species may provide habitat, food, or physical structure that other organisms have begun to use. Sudden removal can therefore disrupt established interactions, so management must consider both eradication benefits and possible secondary effects.

How can invasive species affect human economies?

They may damage crops, forests, fisheries, infrastructure, and timber resources, while creating costs for monitoring, quarantine, control, and eradication. Economic impacts can occur even when ecological damage is localized.

How can invasive species threaten human health?

Introduced organisms may directly harm people or act as vectors for pathogens. For example, invasive insects can transport plant diseases, and newly introduced organisms can alter disease transmission or exposure.

Why is early detection and prevention generally important in invasion management?

Control is usually more feasible when a population is localized and rare than after it becomes widespread and dominant. Inspection, quarantine, cleaning equipment, regulating transport, and limiting repeated introductions reduce propagule pressure before establishment.

Why are invasive species not always identified solely by their biological traits?

Traits such as rapid growth and reproduction occur in both invasive and noninvasive species. Invasion success depends on interactions among organismal traits, propagule pressure, disturbance, resource availability, community resistance, and conditions in the recipient environment.

How does habitat loss reduce biodiversity?

Converting natural ecosystems to farms, cities, roads, or other human uses removes the resources and conditions species need to survive. Habitat fragmentation also isolates populations, increases edge effects, and reduces viable population sizes.

How does overexploitation reduce biodiversity?

Harvesting organisms for food, trade, medicine, or recreation faster than populations can reproduce causes population declines. Persistent overexploitation can produce ecological imbalances and drive vulnerable species toward extinction.

How does pollution reduce biodiversity?

Pollutants can poison organisms, disrupt reproduction and development, degrade habitat quality, and alter food webs. Nutrient pollution can cause eutrophication, algal blooms, oxygen depletion, and the loss of aquatic species.

How does climate change reduce biodiversity?

Changing temperatures, precipitation patterns, sea levels, and disturbance regimes can shift species ranges and breeding schedules. Species that cannot migrate or adapt quickly enough may experience habitat loss, disrupted interactions, population decline, or extinction.