I performed a mark-recapture survey at four sites (of two different sizes) for one species of lizard, and now have data for - total survey time, survey area and number of individuals encountered. Is there any way to calculate survey effort from this data?
I intend to revisit the sites and assess if there has been population decline; so quantifying the search effort each year is key to support any findings.
You need randomized species accumulation curves for sites using either individuals or time versus species as measures of sampling effort.
Article Species accumulation curves and the measure of sampling
What may seem like a simple metric can become very complex. The basic number you are looking for is catch per unit effort with effort being time-constrained or area-constrained. An important assumption is that all members of the target population are equally catchable (males, females, juveniles) at all sites (habitats) using whatever technique you used. Factors that affect these numbers include time of year, time of day, and weather conditions. Many population models also assume a closed system, that is births, deaths, immigration, and emigration will not significantly change the population size or demographics. As long as these factors are understood and included in the survey protocol you should be able to generate meaningful numbers.
Within either the time or area constrained methods you must consider technique (e.g., visual encounter surveys, pit falls, box traps, or other device used for capturing target species, etc.). With the visual encounter survey if you were marking and releasing the animals as you encountered them you may need to subtract the time required to mark and collect other data such as sex and size. Not knowing the scope of your survey efforts or the number of animals encountered, I do not know if that is an important factor for your study. If a device is used to capture the animal then the numbers are more straight forward: x number of animals per y length of time (trap-days or trap-hours) assuming equal number of traps per unit area.
In a study we conducted of Bog Turtles at 4 sites in New York we found that we did not capture a sufficient number of individuals on a single survey day to make meaningful population estimates. We pooled data for each site and for all sites combined made on 3 successive surveys conducted approximately 2 weeks apart during the period from peak spring emergence to onset of nesting. We repeated this study over a 4 year period. We choose the Schumacher-Eschmeyer method for our data analysis which is designed to work with data sets that include multiple resample efforts. Because these populations are relatively small only 1 of the 4 sites produced a population estimate within the 95% confidence limits.
What survey technique was applied and how many people applied it? It is then possible to calculate survey effort following standard procedures (time spent, distance traveled, traps set etc).......
For example, if using traps traps per unit area per day open .....
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