The Different Methods used for Sampling Animal Populations!

Essay The Different Methods used for Sampling Animal Populations !

The ecological study of animal populations in the ecosystem involves considerably more problems than the study of plants. Animals are harder to see and most are not stationary—they are here one minute and gone the next. The following methods furnish valuable information’s regarding animal populations of different kinds:

1. Trapping and collecting animals:

The sampling of a population involves collecting animals, either alive for marking and release, or dead. For different types of animals different techniques are adopted which are as follows:

(i) Trapping and collecting flying insects:

Aerial nets or heavy duty sweep nets which are designed to withstand the hard- wear encountered when put through grass and woody vegetation, are used for the collection of diurnal insects. For nocturnal insects, traps containing ultra-violet light or a mercury-vapour light are used. Insects then picked off the sheet. For killing the insects, killing jars containing a layer of plaster of paris and potassium cyanide (KCN) on the bottom are used.

(ii) Trapping and collecting aquatic organisms:

For collecting aquatic organisms are used dip nets for organisms in the water bottom nets for scraping along the bottom of ponds, wire-baskets scraper nets, and plankton towing nets. For aquatic collecting from the shore, aquatic throw nets are useful. For collecting bottom organisms in deep water, is used a bottom dredge (bag net) lowered from a boat. Fish, tadpoles, and large crustacean collected with seines (large vertical fishing-nets).

(iii) Trapping and collecting soil organisms:

Different animals of soil are collected by following methods: Soil arthropods are extracted from the soil by means of a Tullgrcn funnel, an improv­ed version of the Berlese funnel (Fig.4.5). It consists of a heat source and a smooth funnel, preferably glass, and a shelf of hard- ware cloth on which to place the sample.

The heat from the lamp then desiccation derives the arthropods downward, until them into the collecting bottle beneath the funnel space is present between the wire and the wall of the funnel.

For collecting soil arthropods, a sample of litter or soil is placed on the hardware cloth shelf, so fitted in the funnel that air space is present between the wire and the wall of the funnel. To begin extraction, the lid of the funnel is opened to 90° and the 100-watt bulb is turned on. After about 16 hours, depending upon the sample size and moisture content, the lid is shut and bulb is changed by 15-watt bulb.

There occur two periods of arthropod exodus, the first wave due to heat and the second wave due to desiccation. The collecting bottle beneath the funnel may contain alcohol, formalin, or water. The animals are then sorted and identified under the microscope.

Larger soil animals like spiders and beetles, can be taken in traps made from funnels and cans set in the soil to ground level. Boards placed on the ground may attract millipedes, centipedes and slugs. Meat bait in small wire traps will attract scavenger insects.

For the collection of earthworms, a dilute solution of forma­lin (25 ml of 40 per cent formalin to 4 541 litres) is applied to a quadrate 61 cm². Within a few minutes worms will come to the sur­face. After earthworm movement to the surface stops, a second application of dilute formalin solution is done.

When worms cease to come to the surface the second time, the quadrate is dig out as deep as necessary. The soil is hand-sorted for maximum recovery of earthworms. Earthworm cocoons can be extracted by the floatation method.

Soil nematodes can be collected by Tullgren funnel method and also by floatation method.

(iv) Trapping and collecting small animals in vegetation:

Sweep nets with stout frames to withstand sweeps close to the ground and in woody growth are useful for collecting many types of insects and even some arboreal amphibians and reptiles. Drag nets consist­ing of light tubular frames, to which are attached canvas bags, are useful on flat ground. Overhead vegetation cart be sampled by beating the limbs with sticks to dislodge the animals, which should fall into canvas collecting trays beneath.

(v) Trapping and collecting birds and mammals:

Birds can be trapped for banding in specially constructed traps, cannon nets for larger game birds, and mist nets. For mammal^ live traps of wood or wire and snap traps are used. These traps can be baited with natural foods, dripping water, etc.

2. Marking animals:

Marking individuals in an animal population is necessary if one wishes to distinguish certain members of a population at some future date, to recognize individuals from their neighbours, to study movements or to estimate number of animals in a population.

Arthropods and snails are best marked with a quick-drying cellulose paint. Aquatic insects and molluscs are marked by ship- fouling paint.

Fishes are usually marked by tagging in several ways. Strap tags of Monel metal (a nickle-base alloy) may be attached to the jaw, the preopercle, or the operculum. Streamer or pennant (a long narrow flag) tags are sometimes attached to various parts of body, usually at the base of the dorsal fin. A plastic tag can be inserted into the body cavity of fish by performing a minor opera­tion. Fish can also be marked by clipping the fins.

Frogs, toads, salamanders and most lizards can be marked by some system of toe clipping which involves the removal of the dis­tal part of one or more toes. Snakes and lizards can be marked by removing scales or patches of scales in certain combinations. Birds are usually marked either by serially numbered aluminium bands, by cellulose and aluminium coloured bands or by dyeing plumage by conspicuous or contrasting colour.

Small mammals may be marked by toe-clipping or by notch­ing the ear. Bear, deer, elk, moose, rabbits and hares can be mar­ked with strap tags or plastic discs attached to the ear. Aluminium bands similar to those used on birds can be attached to the forearm of bats.

Radioactive tracers:

The use of radioactive tracers in mark­ing of animals, is a particularly useful method for studying animals that are secretive in habits, live in dense cover, spend part or all of their lives underground, or that have radically different phases in their life cycle, such as moths and butterflies.

It is found that if animals are fed small traces of gamma-emitt­ing radioactive material along with food, then the radio-active materials are metabolically incorporated into the tissues. The tracer becomes a part of the animal and is passed along to egg or offspring. This technique is useful for studying dispersal, for the identification of specific broods or litters, for obtaining data on population dynamics and natural selection.

3. Aging techniques:

The determination of age structure of wild populations is a difficult task. Aging techniques are known only for few animals such as fishes, birds and mammals.

(a) Fishes:

Fish aging techniques began first when Hoffbaur (1898) published his studies on the scale markings of known-age carp. Since then, this technique becomes more developed and re­fined. It is based on the fact that a fish scale starts as a tiny plate and grows as the fish grows.

A number of microscopic ridges, the circular are laid down about the centre of the scale each year when the fish growing well in summer, the ridges are spaced wide apart. During winter, when growth slows down, the ridges are close together.

This annual check on growth enables the biologists to, determine the age of a fish by counting the number of areas of closed rings, the annuli. Because the growth of a scale continues throughout the life of a fish, it also provides information on the growth rate.

This is obtained by measuring the total radius of the scale, the radius to each year’s growth ring, and the total body length of the fish. Then by simple proportion, the yearly growth rate can be determined.

Other techniques in aging fish include the length-frequency distribution, vertebral development, and rings or growth layer in the otolith or ear stone.

Further, due to identification of the large number of year classes (animals born in a population during a particular year), one can determine dominant year classes, learn the age when fish reach sexual maturity, estimate production, mortality and the effects of fish harvest.

(b) Birds:

The age of birds can be determined by studying the plumage development. In a technique, until moulted, the tail feathers of juvenile waterfowl are notched at tip, in contrast with the normally contoured feathers of the winter plumage. The shape of the primary wing feathers separates adults from young among many gallinaceous game birds. When the whole bird is available, the presence or depth of the bursa of Fabricus, a blind pouch lying dorsal to the caecum and opening into the cloaca, indicates juvenile birds.

(c) Mammals:

Among mammals the examination of repro­ductive organs is useful because the majority do not breed until the second year. The presence of the epiphysal cartilage in rabbits and squirrels identifies juveniles up to 6 or 7 months. Likewise, black bars on the pelage of the underside of the tail of juvenile grey squirrels separate the young from the adults.

Skull measure­ments are useful in beavers and musk rats. Annual growth rings on the roots of canine teeth indicate age for the first few years of life in the fur seal. Growth rings also show up in the horns of mountain sheep. The wear and replacement of teeth in deer and elk permit the determination of different age classes in these mammals.

The age of some animals like cottontail rabbit, the racoon, the black bear and fur seal and birds can be determined by lens- weigh technique. Because the lens of the eye of most mammals and birds grows continuously throughout life, and because there is only slight variation between individuals in lens size and growth, the measurement of the lens is a practicable method for aging in these animals. The technique involves the weighing of the dry lens and comparing its weight against a chart of lens weights of known- age individuals.

4. Sex determination:

The sex of most mammals can be determined by examining external genitalia and the sex of birds by plumage differences. For example, peacock-peahen, cock and hen, male and female Koel (Eudynamys), etc., all have plumage differences in both sexes.

5. Determination of home range and territory:

The home range of wild animals can be determined by following two simple methods: 1. The area, in which all the observations of the move­ments of individuals have been made, is measured arid outlined on a map. If the observations are obtained by trapping, then must assume that animal could have gone half-way toward an ad­jacent trap, especially if the traps were set in a regular grid.

In another method, the fact that small comes readily to bait has been exploited in a manner that does not involve trapping. Bait boxes are made from quart-size paper milk containers and are set out in a 50-ft grid over the study area. These boxes serve as containers for the bait and as receptacles for the feces.

If food at certain stations is stained with a dye, then the visitation by animals that fed on the dyed bait to other bait boxes can be traced through coloured droppings. The distance moved by a particular animal from a station with dyed bait can be determined when coloured scats are recovered at various stations in the grid.

Because territorial boundaries are rather rigidly maintained by birds during the breeding season, territorial boundaries can be map­ped by observing the movements of the birds during the day, by plotting singing perches, by observing location of territorial dis­putes, and on occasion by chasing the bird. When the bird arrives at the boundary of its territory, it generally will double back.

6. Estimation of number of animals in population:

The numbers of animals in wild populations can be estimated by follow­ing three methods: 1. True census, a count of all individuals in a given area; 2. Sampling estimates, derived from counts on sample plots; and 3. Indices, in which the trends of populations from year to year or from area to area are obtained through road­side counts, animal signs, and the like.

(a) True census:

A true census implies a direct count of all individuals in a given area. Direct counts can be made only in large ana conspicuous animals and in areas of their maximum con­centration. Elephants, rhinoceros, deer, etc., in open country, herds of elk and caribou, waterfowl on wintering grounds, rooke­ries, roosts, breeding colonies of birds and mammals permit direct counting usually either from the air or from aerial photographs.

(b) Estimates from sampling:

This involves following methods:

(i) Sample plots:

Relatively immobile forms such as barna­cles, molluscs and cicada emergence holes, can be estimated by the quadrate method, similar to that used for plants. Foliage arthro­pods may be sampled by a number of strokes with a standard sweep over a 10 = m² area. Estimates of zooplankton, obtained by pulling plankton net through a given distance of water at several depths, can be made by filtering a known volume of sample through a funnel using a filter pump.

The filter paper is marked off in equal squares and by the help of a hand lens or a binocular microscope, the organisms in each square are counted. The num­bers are then related back to the total volume of water sampled.

Very small-sized zooplanktons are counted by Rafter plankton- counting cell. This consists of a microscope slide base plate ruled into ten 1 cm squares.

(ii) Mark-recapture method:

In this method, a group of ani­mals in the population are captured and marked with a band or tag for some time. Then they are released back into the population, where they are distributed among unmarked individuals. Later another group of animals is captured, some of which will be mar­ked and some of which are not.

The ratio of marked animals to the number in the sample is assumed to be similar to the ratio of the total animals initially marked to the total number of animals in the entire population.

(iii) Removal method:

This method involves capturing ani­mals and removing them from the study area. If this technique is used over a short time, the number of animals removed per unit effort should get smaller and smaller.

By totaling the accumula­tive catch through time, it is possible to estimate the number in the area, even though we only remove some of the animals. This method is useful where one desires a relative measure or index figure for small-mammal populations in order to compare one habitat with another.

(c) Indices:

Indices arc estimates of animal populations derived from counts of animal signs, calls, road-side counts, and so on. The results do not give estimates of absolute populations, but they do indicate trends of populations from year to year and from habitat to habitat.

Call counts are used for game birds like woodcock, pea cock, quail, etc. Road side counts involve counting of animal tracks, browse, signs, active dens and lodges, etc. Counting pellet or faecal groups is widely used to estimate big-game popula­tions.

7. Measuring mortality:

The measurement of mortality in natural populations is most earnestly required for various ecolo­gical aspects like construction of life tables, life equations, sur­vivorship curves, growth curves, etc. The mortality in a wild population can be measured by following methods: (i) If a num­ber of marked animals are found dead a known time after making, the percentage dying can be plotted against time and a curve drawn through the points. The probability of dying can be read directly from the chart, or it may be calculated by dividing the number of individuals that die during a period of time by the initial population. (ii) If the animals can be readily aged, as is true with deer, rabbits, mountain sheep, and others, and if sufficient lower jaws, eye lenses, or horns can be obtained, then mortality and the probability of dying can be expressed on an age basis.