Technical Specification
Learning Objectives
- Describe how invasive species disrupt ecosystems using before-and-after data from real invasion case studies.
- Calculate invasion front velocity and identify acceleration patterns in a species' spread.
- Explain the role of limiting factors in controlling population growth.
Data Protocol
Safety & Integrity Guidelines
- Record every raw measurement before computing derived metrics.
- Carry units at every step —
km/yr, ind/m², cells/mL — never bare numbers. - Cite the source of any dataset you reuse (here: Urban et al., 2008, Nature).
- Never fabricate a data point to force a trend. Mark unknowns as
N/A.
The Hook
In 1935, scientists released 102 cane toads in Queensland, Australia to control beetle pests in sugarcane. Today, over 200 million cane toads inhabit 1.2 million km² of the continent. Meanwhile, kudzu — a vine introduced from Japan as an ornamental — now covers an estimated 3 million hectares of the southeastern United States, growing up to 30 cm/day in peak season.
Both introductions seemed harmless at the time. Neither was. This module answers the core question: why do some organisms explode in number when introduced somewhere new, and what does that mean for everything else living there?
Core Definitions
An invasive species is an organism introduced outside its native range that causes ecological, economic, or human harm. Two ranges matter:
- Native range — where a species evolved; held in check by co-evolved predators, parasites, competitors.
- Introduced range — where humans transported it and it established a reproducing population.
Limiting Factors — the missing brakes
In their native ranges, predators and competition act as brakes on growth. Introduce a species somewhere those brakes don't exist and you get the invasion pattern:
Release from limiting factors → exponential growth → ecosystem disruption.
The common threads across both case studies:
- No natural predators — nothing eats kudzu in N. America; nothing eats cane toads in Australia.
- No competition — kudzu fills a fast-growing-vine niche no native plant occupies.
- High reproductive rate — kudzu roots at every node; female cane toads lay
8,000–30,000 eggs, twice per year. - Climate matching — both released into climates similar to their native range.
- Enemy release hypothesis — removal of native enemies grants a temporary competitive advantage.
Vocabulary — predict, then reveal
Invasive SpeciesThe kudzu vine is considered invasive because it was introduced to the SE United States and now covers over 3 million hectares, smothering native forests.
An organism introduced outside its native range that causes ecological, economic, or human harm.
Native RangeIn its native range of Japan and SE China, kudzu is held in check by natural herbivores and diseases that did not travel with it to the US.
The geographic area where a species naturally evolved and exists without human introduction.
Introduced RangeAustralia is part of the cane toad's introduced range — brought from Hawaii in 1935, it now covers 1.2 million km².
An area where a species has been transported by humans and established a reproducing population.
Limiting FactorsPredators and competition act as limiting factors on cane toads in Central America, but in Australia those factors are largely absent.
Environmental conditions — predators, disease, space, nutrients — that restrict population growth.
Competitive AdvantageKudzu gains a competitive advantage because it grows up to 30 cm/day and roots at every node, outcompeting natives for sunlight.
A trait that lets one species outcompete another for resources.
Invasion VelocityThe cane toad's invasion velocity rose from 4 km/yr in the 1940s to 15 km/yr by 2010 as front toads evolved longer legs.
The rate at which a species' geographic range expands, typically in km/yr.