Question

Elton (1958) showed that introduced species often increase enormously and then subside to a more static, lower density level. How might this occur in a species that was not the subject of introductions for biological control? How could you distinguish this case from a decline that followed the introduction of some parasitoids for biological control?

Short answer

Without biological control, populations stabilize after exponential growth due to natural limits. Declines with parasitoid introduction are often more immediate and inverse to the predator population increase.

Step-by-step solution

Understand Introduction Dynamics

When a new species is introduced to an ecosystem where it has no natural predators, its population can grow rapidly due to lack of competition and predation. This phase is often referred to as the exponential growth phase.

Recognize Limiting Factors

Eventually, the population growth slows as the species encounters environmental limitations such as food shortages, habitat space, or disease. These limitations cause the population to stabilize at a lower density.

Consider Natural Population Control

In cases where biological control agents such as parasitoids are not introduced, the population may still exhibit declines through natural regulatory mechanisms, such as increased predation, competition, or disease outbreaks.

Compare With Biological Control Scenarios

If parasitoids were introduced for biological control, you would expect to see a decline directly correlated with their introduction. Observing population dynamics at the same time would show an inverse relationship between predator (parasitoid) and prey populations.

Distinguish Between Natural and Introduced Controls

Examine the timeline of population changes. A decline due to natural factors would not have a sudden onset or tight correlation with the introduction of an external agent. It would typically show a slower, more gradual adjustment compared to a decline post-parasitoid introduction.

Key concepts

Natural Regulatory Mechanisms

Every ecosystem has its own set of checks and balances. These are known as natural regulatory mechanisms. They keep populations in check, preventing any single species from overwhelming the ecosystem. For introduced species, these mechanisms often kick in after an initial phase of rapid growth. This is because introduced species generally lack predators and diseases initially. As time goes by:

• Predation: Predators might adapt or arrive, starting to control the population.
• Competition: Other species might compete for resources like food and space, limiting growth.
• Disease: With higher population densities, disease outbreaks become more common, naturally reducing the population.

These factors act together to transition the species from a rapid growth phase to a more stable population level. Each mechanism helps bring an introduced species back to an equilibrium within the new environment.
Natural regulatory mechanisms ensure that ecosystems maintain their intricate balance over time. Without them, new species might continue to spread unchecked, potentially disrupting other species and the resources available.

Biological Control

Biological control is a method where specific natural enemies are introduced to control a species' population. This is particularly useful when a species becomes too burdensome, threatening other native species or causing ecological or economic harm.

• Parasitoids: Insects that lay their eggs on or in other insects. When the eggs hatch, the larvae consume the host, effectively reducing its population.
• Predators: Animals that are known to target the invasive species directly, decreasing their numbers.
• Pathogens: Diseases that specifically affect the target species, giving an indirect method of control.

Introducing these controls requires careful study to avoid unforeseen consequences since they can become invasive themselves or harm non-target species. Biological control creates a forced balance, activating new predatory or competitive pressures that might not naturally occur. When successful, it helps restore balance in the ecosystem, reducing the population of troublesome species to manageable levels.

Population Growth and Stabilization

Populations typically exhibit phases of growth followed by stabilization. When a species first enters a new environment, its population can grow exponentially. The conditions seem perfect, with plenty of resources and little competition.

• Initial Growth: Rapid population increase; also known as exponential growth.
• Environmental Limits: Eventually, resources become limited; food shortages, lack of space, and other constraints apply.
• Stabilizing Phase: As the species reaches the environment's carrying capacity, growth slows and stabilizes.

In the case of introduced species, this pattern can sometimes appear dramatic when these populations, after a phase of significant growth, settle into an equilibrium. This balance between the species' needs and available resources is crucial for maintaining ecosystem stability and ensuring that no single species dominates the entire environment. Over time, the population adopts a stable level where growth rates balance out with mortality and emigration, allowing for a healthy dynamic within the ecosystem.