Introduction - Introduction to Ecological Sampling - page 4

animals removed). Both the removal method and the change in ratio method
are closely related to the mark-recapture methods described in Chapter 7.
Chapter 6, “Plotless Sampling,” is concerned with methods for estimating
the density of objects, such as the trees in a defined area, without dividing
the area into plots of a certain size and then randomly sampling the plots
or sampling the area by one of the other methods discussed in Chapters 2
to 5. Instead, points are selected in the area of interest either randomly or
with a systematic pattern, then the distance to the nearest object is measured
for each point and possibly the distance from that object to its nearest or
k th-nearest neighbor. There are various methods that have been proposed
for that type of sampling, and Chapter 6 describes two of these in some
detail. The first is T-square sampling. In this case, once a point in the area
of interest is selected for sampling, then the distance from this point to the
nearest object is measured. A line is then set passing through the position
of the object at a right angle to the line from the initial point to the object.
The distance from the object to its nearest neighbor on the side of the line
away from the initial point is then measured. Based on the two measured
distances (from the point to the first object and the first object to its near-
est neighbor) for a number of points in the area of interest, it is possible to
estimate the density of objects in the whole area, assuming that the location
of objects is at least approximately random in the area. The second method
described in Chapter 6 is wandering quarter sampling. This starts with a
randomly selected point in the region of interest. A direction is then selected,
such as west, and the nearest object within the 90° angle from southwest to
northwest is found. The distance from this object to the next object in the
southwest-to-northwest direction is then measured. This procedure is con-
tinued until n distances have been measured and it is possible to estimate the
density of points in the sampled area based on these distances.
Chapter7, “Introduction to Mark-Recapture Sampling and Closed-
Population Models,” covers these methods for closed populations (with no
losses or gains of animals during the sampling period). With closed popula-
tion methods, there is a first sample time when captured animals are marked
and released, and then one or more further samples are taken, with captured
animals again marked and released. Then, it is possible to estimate the pop-
ulation size subject to certain assumptions about the capture process.
Chapter 8, “Open-Population Mark-Recapture Models,” covers situations
when the population size can change between sample times because of losses
(deaths and emigration) and gains (births and immigration). With open pop-
ulations, the sampling process is similar to that for closed populations, but
there are generally more samples. It becomes possible to estimate the popu-
lation size at the sample times other than the first and last, the survival rates
between sample times except between the last two samples, and the number
of new entries to the population between the sample times except between
the last two samples. This is again subject to certain assumptions about the
capture process and the survival rates and the new entries to the population.