What Are Survivorship Curves?

Survivorship curves are graphical tools ecologists use to visualize how the likelihood of survival changes with age in a population. There are three classic types:

• Type I: High survival rates in early and middle life, followed by a rapid decline in later years. Typical of many large mammals, including humans.

• Type II: A relatively constant death rate throughout life, seen in some birds and reptiles.

• Type III: Extremely high mortality in early life stages, with survivors living much longer. This curve is typical of many fish, plants, and most importantly, r-selected species such as invasive insects.

Why Invasive Insect Pests Exhibit Type III Survivorship

High Early Mortality

Invasive insect pests lay hundreds or thousands of eggs, but only a small fraction survive to adulthood. The vast majority of offspring die early due to predation, environmental hazards, or lack of resources. This pattern creates a steep drop in the survivorship curve at the youngest ages—defining the Type III curve.

Mass Reproduction and r-Selected Strategy

Invasive insects are classic examples of r-selected species. They maximize reproductive output to ensure that at least a few offspring survive to maturity, even in unpredictable or harsh environments. This strategy is highly effective for colonizing new areas, especially where natural enemies may be absent or less effective.

Adaptation to Disturbed Environments

Type III survivorship is advantageous in disturbed or rapidly changing environments, which are often the first places invaded by pest species. The ability to produce many offspring quickly allows invasive insects to exploit new resources and outpace both native competitors and control measures.

Real-World Examples

• Emerald Ash Borer: This invasive beetle lays numerous eggs, but only a few larvae survive to adulthood due to predation and host tree defenses. Studies show that most mortality occurs in the early larval stages, a hallmark of Type III survivorship.

• Spotted Lanternfly: Another invasive pest, it produces many eggs with high early mortality, but survivors can develop quickly and establish large populations.

Survivorship Curves and Pest Management

Understanding that invasive insect pests follow a Type III survivorship curve has practical implications:

• Control Strategies: Targeting early life stages (eggs and larvae) can be the most effective way to reduce pest populations, as this is when mortality is naturally highest.

• Monitoring: Knowing that adult populations are a small fraction of the total offspring produced helps in designing more accurate monitoring and intervention programs.

• Predicting Outbreaks: The r-selected, Type III strategy means populations can explode rapidly under favorable conditions, making early detection and rapid response critical.

Why Not Type I or II?

• Type I curves are typical of K-selected species, which invest heavily in a few offspring and provide significant parental care—traits rarely seen in invasive insects.

• Type II curves represent a constant mortality rate across all ages, which does not match the life cycle of most insect pests, where early mortality is overwhelmingly high.

Debunking Common Myths

• Not All Invasive Species Are r-Selected: While most invasive insect pests are r-selected with Type III curves, some invasive organisms may have different life history strategies.

• Not Always Human-Introduced: Although humans are the main vector for spreading invasive insects, natural dispersal can also play a role.

Conclusion

The typical survivorship curve for invasive insect pest species is Type III. This pattern reflects their strategy of producing vast numbers of offspring, most of whom die young, with only a few surviving to adulthood and reproducing. Understanding this curve is essential for developing effective pest management and biosecurity strategies. By focusing on the vulnerabilities of these species in their early life stages, we can better protect ecosystems, agriculture, and economies from the threats posed by invasive insect pests.