Ecology: Determining Population Size

Study Notes

Determining population size, denoted as 'N', is a crucial aspect in numerous fields of ecology.
It plays a vital role in the development of population models, the control of pests, the management of resources like fisheries, and the preservation of biodiversity through conservation biology.

Animal movement poses a significant challenge to accurately counting populations.
Methods like quadrats and transects, typically used for stationary species, are not suitable for highly mobile animals.
Specialized techniques have been developed to address the issue of animal movement.

A population census involves a complete enumeration of all individuals within a population.
This approach is generally feasible only for large animals confined to a restricted area.
One example is the Soay sheep population on the island of Hirta in the St Kilda Archipelago, Scotland.

In most situations, population size is estimated from samples.
It's essential that these samples accurately represent the entire population to avoid bias.

This technique, named after Danish biologist C.G.J. Petersen, is widely used to estimate population size.
It involves capturing, marking, and releasing a sample of individuals.
A second sample is captured later, and the proportion of marked individuals in the second sample is used to estimate the overall population size.
The formula used for this estimation is: Proportion marked in the second sample = Proportion of the population caught in the first sample.

The precision of the mark-recapture estimate depends on the proportion of the population marked and the proportion of the area sampled.
A larger proportion of marked individuals leads to greater certainty in the estimate.
Ideally, at least 50% of the population should be marked.
Sampling the entire area occupied by the population is essential to avoid underestimating the population size.
The method assumes:
- All individuals have an equal chance of being captured.
- The population remains stable between captures with no births, deaths, immigration, or emigration.
- Marks are not lost or overlooked.

This assumption might not hold true due to individual differences in size, behavior, or other factors that could influence susceptibility to capture.
These differences can be corrected for if the relevant factors, known as covariates, are recorded and accounted for.
Behavioral adaptations to trapping, such as "trap shyness," can also influence catchability over time.
Minimizing these effects through careful trap selection and minimizing capture duration can address this issue.

The method assumes a closed population, meaning no individuals enter or leave the population between captures.
This assumption is violated when immigration, emigration, births, or deaths occur.
To minimize this violation, the time interval between recaptures should be short while still allowing for thorough mixing of marked and unmarked individuals.
For open populations, more sophisticated multi-capture methods, like the Cormack-Jolly-Seber (CJS) method, can be used.

It is critical to avoid marking methods that could negatively impact survival rates.
Traditional practices like toe clipping in amphibians have been abandoned due to their detrimental effects on survival rates.
Robust tagging methods that minimize loss or shedding are crucial.

The assumption of permanent marks is crucial.
If tags are lost, population size may be overestimated.
The use of durable and secure tags is essential.
Double tagging, where multiple tags are applied, can also minimize errors from tag loss.