Potential for Infestation: Life History Traits & Population Growth

Cockroach and dandelion icons
Cockroaches and dandelions are easily associated with infestation. These icons will be used to represent a species’ potential for infestation. The outline without colour represents low risk of infestation while the red colour represents high risk of infestation.

What factors influence the environments that species thrive in and how do the conditions in those environments influence the characteristics of the species? Can we predict the likelihood that a species will become a problem very quickly and result in an infestation? What will happen if we wipe out almost all of the population but leave a few individuals behind? These are the questions that must be answered when managing plant and animal populations. In the context of this website, we are interested in these questions from the perspective of trying to prevent populations from establishing or spreading. Although a little dramatic (for television of course), this video that outlines some key elements of issues that result in infestation and the nature of species that can be a real challenge to manage.

Life history traits are an effective way to assess the ecology of a species and anticipate the consequences of its introduction to a new environment. Seven important life history traits that can predict the potential for rapid population growth and the probability of infestation include: competitive ability, rate of development, age of reproduction, adult body size, number of reproductive cycles, and reproductive output (Pianka, 1970).

Dog Strangling Vine, Cynanchum rossicum, is a common invasive plant in southern Ontario. It grows rapidly, produces a lot of seeds and thrives in disturbed environments. These are the traits of most successful invaders as their population size can increase very rapidly even when only a few individuals establish in an area. (Photo: Feltham, 2012)

Some species such as dandelions and cockroaches have traits that promote rapid population growth from only a few founding individuals. The letter “r” is used to represent the population growth rate in ecology. Organisms with these traits have frequent population cycles of population explosion followed by rapid decline. Rats, for example, mature in 21 days, reproduce at 21 days of age, have a small body size, can reproduce every 21 days for 1-2 years, and they can produce an average litter of 8-12 young with each reproductive effort. A few simple calculations will illustrate just how quickly one pregnant female rat can become a serious problem. First the assumptions:

  • Litter size of 8 rat pups
  • Half of the litter is female = 4 female rats/litter
  • Half of them survive and reproduce = 2 female rats/litter

And now the math:

  • Generation 1 = 8 rat pups of which 2 females survive and reproduce
  • Generation 2 (21 days later) = 3 females (original + 2) produce 3 litters of 8 rats of which 6 females survive to reproduce
  • Generation 3 (42 days later) = 9 females (original +2 + 6) produce 9 litters of 8 rats of which 18 females survive to reproduce
  • Generation 4 (63 days later) = 27 females (original + 2 + 6 + 18) produce 27 litters of 8 rats of which 54 females survive to reproduce
  • Generation 5 (84 days later) = 81 females (original + 2 + 6 + 18 + 54) produce 81 litters of 8 rats of which 162 females survive to reproduce
  • Add 162 to represent all the males that would have survived if their survival rate was the same as that of females and you have 324 rats in 84 days!

These species are known for their rapid rate of population growth and are referred to as r-strategists and they tend to do well in disturbed unstable environments where resources are unpredictable. Disturbed environments are a product of many human activities and human actions often prevent these disturbed environments from becoming stable ecosystems.

Turtles are very slow to mature and produce moderate clutch sizes with low survival rates for the hatchlings. These traits make species such as this Blanding’s Turtle, Emydoidea blandingii, slow to recover from population decline and unlikely to establish a large population if introduced to a non-native environment. (Photo: Feltham, 2013)

Species such as oak trees and turtles, have traits that favour a long life, slow population growth and stable populations. These species usually have populations that are near carrying capacity. Carrying capacity is the number of individuals of a particular species that can be supported by the ecosystem. In ecology, carrying capacity is represented by the letter K; hence species such as oak trees and turtles are referred to as k-strategists which tend to do well in stable environments with predictable resource availability. These species rarely cause issues as non-native invaders if only a few individuals are introduced to the new environment. However, k-strategists are often listed as threatened and endangered in their native environment which results in concerted efforts to restore populations. In some cases, these restored populations can become source of conflict as in the case of Gray Wolves praying on livestock after their successful re-introduction to Yellowstone National Park. Sandhill Cranes are another example of a successful recovery of an endangered species that resulted in conflict when migrating flocks descend upon agricultural fields and feed on seeds and newly germinated crops. How do you manage situations where protected species are also the source of conflict? Read the posts and management strategies presented on this site to find out about some innovative solutions to these unique challenges.