C4.1.4 Capture–mark–release–recapture and the Lincoln index Notes

Estimating Population Size of Motile Organisms: Capture–Mark–Release–Recapture and the Lincoln Index

1. For motile organisms, such as birds, fish, insects, and mammals direct counting is impractical because they move constantly and can be difficult to track.
2. Ecologists use the capture–mark–release–recapture (CMR) method to estimate population sizes.
3. This method provides a statistically valid estimate of population size using a representative sample, allowing researchers to track species over time and assess conservation status, migration patterns, and population dynamics.

The Capture–Mark–Release–Recapture Method

This method involves four key steps:

1. Capture:
   1. A sample of the population is caught using traps, nets, or other capture techniques.
   2. The number of captured individuals (M) is recorded.
2. Mark:
   1. Each individual is marked in a harmless and recognizable way (e.g., colored bands for birds, non-toxic dye for insects, or electronic tags for mammals).
   2. The mark must not affect survival, behavior, or make the animal more vulnerable to predators.
   3. Durable enough to remain until the next capture event.
3. Release: The marked individuals are returned to their habitat and given time to mix back into the population.
4. Recapture: After a set period, another sample of the population is captured. Scientists record how many marked vs. unmarked individuals are found.

Using the Lincoln Index to Estimate Population Size

Ecologists use the Lincoln Index to estimate population size:

\[
\text{Estimated Population Size} =
\]
\[
\frac{\text{Number of Recaptured Individuals that were Marked} \times \text{Total in Second Capture}}{\text{Number Marked in First Capture}}
\]

Limitations of the Capture-Mark-Release-Recapture Method

Despite being useful, this method has limitations:

1. Assumptions May Not Hold: The method assumes no births, deaths, or migration occur between samples, but real populations are constantly changing.
2. Marking Challenges: Some species are difficult to mark without affecting their survival or behavior.
3. Sampling Bias: If marked individuals are more or less likely to be recaptured, the estimate may be inaccurate.

Why Use the Lincoln Index?

1. Non-Invasive & Ethical → Allows population estimation without harming individuals.
2. Useful for Conservation → Tracks endangered species (e.g., tigers, whales).
3. Efficient for Motile Species → Works well for animals that move, unlike quadrat sampling for sessile organisms.

Reflection and Broader Implications

1. The capture–mark–release–recapture method and the Lincoln Index are essential for wildlife management and conservation biology.
2. By estimating population sizes, scientists can:
   • Monitor endangered species and track conservation progress.
   • Assess the impact of habitat destruction on animal populations.
   • Manage pest species by estimating their growth rates.