BIOLOGY Volume 2 - A Guide to General Biology - 2004

11. QUANTITATIVE ECOLOGY

11.2. Analysis of the Biota

11.2.3. Methods for Estimating Population Size

In quantitative ecological research, it is essential to estimate with reasonable accuracy the number of organisms inhabiting a unit of space (area or volume). In most cases, this is equivalent to determining the population size. Naturally, the estimation Methods depend on the size and lifestyle of the organisms being surveyed, as well as the scale of the studied area. The number of plants and sessile or slow-moving animals can be counted directly, or the percentage cover of different species can be determined to compare their Abundance. Indirect methods are used to survey fast-moving organisms over large areas. In habitats where observing organisms is difficult due to their Behavior and lifestyle, removal or capture-recapture (mark-release-recapture, population dilution) methods are employed. Depending on the approach taken, all quantitative censuses are divided into objective and subjective methods.

Objective Methods

Direct objective methods include quadrat sampling, direct observation, and photography, whereas indirect methods are based on specimen removal and capture-recapture techniques.

QUADRAT SAMPLING. By counting the number of organisms in A number of quadrats representing a known fraction of the surveyed area, the results can be easily extrapolated. This method allows the determination of three parameters related to the spatial distribution of species.

1. Population Density (Abundance). Population density is the number of individuals of a given species per unit of space. On land, the number of organisms is counted within randomly distributed quadrats. The advantage of this method lies in obtaining absolute, precise estimates that enable comparisons between different species and habitats. Its disadvantages include being labor-intensive and, in some cases, the ambiguity of the term "individual." For instance, plants often form numerous shoots connected by underground parts; in practice, determining whether one is dealing with a single genetic individual or multiple ones can be very difficult. It is even harder to decide whether such individuals, which sometimes spread over a large area, should be counted as multiple individuals or just as a single one.

2. Frequency of Occurrence. This is essentially a measure of the probability (chance) of finding a particular species within a randomly placed quadrat. For example, if a species is recorded in only one out of ten quadrats, its frequency of occurrence is 10%. Determining this requires only recording presence or absence—the number of individuals does not matter. However, the quadrat size must be chosen carefully, as the results depend upon it. Furthermore, the general problem of working with quadrats remains—how to deal with specimens that fall only partially within the sampling area (for example, a creeping stem rooted outside the quadrat boundary). The advantage of this method is its simplicity, which allows for the rapid survey of vast areas, such as extensive forests. The disadvantages are that the resulting frequency value is influenced by quadrat size, Organism size, and their spatial distribution patterns (random, uniform, or clumped — Vol. 3, Sec. 2.4.2).

11.2. What is the frequency of occurrence of a species if it is recorded in 86 out of 200 quadrats?

3. Percentage Cover. This metric indicates what percentage of the surveyed area is occupied by a given species—either by the bases of its individuals or by the ground projections of all their parts. Cover can be measured directly in the field or from photographs, estimated using a Levy bridge (Fig. 11.15), or simply approximated visually. This method is useful because it allows one to assess the relative role of different species within a community. It is particularly convenient when the number of individual specimens is difficult to count and even theoretically impossible to define (e.g., in grasses). However, as a rule, such measurements are either overly labor-intensive or prone to subjectivity.

11.3. If a 10-spoke Levy apparatus was used 10 times and recorded 36 contacts with plants, what is the percentage of its projective cover in this habitat?

DIRECT OBSERVATION. Direct counting can be applied not only to sessile organisms, but also to fast-moving large animals such as deer, wild ponies, lions, birds, and bats.

PHOTOGRAPHY. By directly counting individuals on aerial photographs, it is possible to determine the population sizes of large mammals and seabirds gathering in open areas. One can also use "camera traps" installed along animal trails; the camera shutter is triggered automatically when an animal's body breaks a light beam directed at a control photocell.

REMOVAL METHOD. This method is useful for estimating the abundance of small organisms, such as insects, in a given area or volume of Water. Using a standardized Procedure (e.g., taking a specific number of sweeps with a net of a fixed size), a certain number of animals are captured, counted, and retained until the end of the study rather than being released. The procedure is repeated several times, with the number of captured animals decreasing each time. Based on these data, a graph is plotted (Fig. 11.16). Extrapolating this graph yields the total population size, which corresponds to the point where animals cease to be caught (the zero ordinate), meaning that theoretically all individuals of the given species have been captured and counted.

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Fig. 11.16. Graph of the number of individuals in the sample versus the cumulative number of previously removed individuals. Extrapolating the line to its intersection with the abscissa (i.e., to a zero size for the next sample) gives the total size of the studied population.

CAPTURE-RECAPTURE METHOD. This method involves capturing an animal, marking it in a harmless way, and returning it to its original population. For example, aluminum discs are attached to the opercula of net-caught fish, and captured birds are ringed. Small mammals are marked with dye or by clipping a patch of fur in a specific pattern; Arthropods are also marked with dye. In all cases, a specific code should be used to allow the identification of individual specimens. After a certain period, a recapture is performed, during which the marked individuals are "diluted" by unmarked ones caught for the first time. The population size is calculated using the formula:

This population size estimate is known as the Lincoln index. Its accuracy depends on a number of assumptions listed below.

1. Organisms in the population mix randomly. This is not always true, as the population may be divided into groups, and different groups may be encountered by the researcher during different trapping sessions.

2. Sufficient time has elapsed between trapping sessions to allow random mixing. The less mobile the species, the longer this period must be.

3. The method is applicable only to a resident population occupying a limited area.

4. Organisms are evenly distributed across the studied area.

5. Changes in population size resulting from individual migration, birth, and death rates are negligibly small.

6. Marking does not alter the mobility of organisms or affect their survival rate (e.g., their vulnerability to predators).

11.4. When estimating the trout population in a lake, 625 fish were caught, marked, and released. A week later, 873 trout were caught, of which 129 were found to be marked. What is the approximate size of this population?

In the case of plants or small animals, such as barnacles, direct counting is far too labor-intensive and, depending on the desired level of accuracy, can be replaced by an approximate estimation of projective cover or abundance within a quadrat. At the beginning of the study, to "get your eye in", it is recommended to use a gridded frame (see Fig. 11.12). Various scales are used to estimate projective cover and abundance, which are often quite subjective.

Subjective methods

These methods are based on the observer's visual estimation rather than direct measurements and counts. For example, Crisp and Southward developed the following scale to estimate the abundance of limpets on rocky shores:

The figures here correspond to the approximate percentage cover of the sampling quadrat, but this is only visually estimated rather than measured. Obviously, even with a well-trained eye, the results obtained are difficult to compare: an "abundant" species might cover 51% or over 90% of the area, and so on. Furthermore, small organisms, even when highly abundant, will not yield a large projective cover and will thus appear less "abundant" than large ones that are present in much smaller numbers. Consequently, theoretically, a separate scale should be used for each species. Nevertheless, such a five-point system can be convenient for presenting data in the form of, for example, kite diagrams (Sec. P2.3.3).