Res. Popul. Ec0l.(1969) XI, 1--13

INQUIRY INTO EFFECTS OF PREBAITING ON REMOVAL

CENSUS IN A VOLE POPULATION'

Ryo TANAKA ~ and Masaomi KANAMORI ~le

Zoologica| Laboratory, Kochi Women's University, Kochi, Japan *~ Sugadaira Biological Laboratory, Tokyo Kyoiku University,

Sugadaira, Sanada-machi, Chiisagata-gun, Nagano Pref., Japan

IMTRODUCTION

At present there is a growing tendency that prebaiting is executed prior to a

census trapping by the removal method in small mammals aiming at raising the

removal rate and taking a good sample covering the whole study area. The trend

has been promoted by Polish ecologists offering their standard minimum method, as

is stated thus; a plot as large as over 5 ha in area is laid out with a grid consisting

of 256 stations, spaced 15 m apart, two snap traps being set on each, and the traps

are run by checking twice a day for 5 days after five-day prebaiting (ANDRZEJEWSKI

1967). So far, it is well appreciated by themselves through their field tests, still

there are some inconsistencies in result, causing some different comments on it,

among themselves. Further, the main part of the method has been accepted as the

standard of IBP by participants in the symposium of Small Mammal Working Group

at Oxford in July of 1968, and their method deserves comments from abroad (GENTRY

et al. 1968).

The senior author has long worked on census methods for small mammals and

now we are going on with the same study under IBP in Japan. In the summer of

1968, we performed a field work planned for investigating the utility of prebaiting

in the removal census of a vole population. Perhaps few studies devised with this

aim have ever been encountered. The plot size was much smaller than in Poland,

nevertheless the single-species population in the study area was of so high density

that a sufficient sample was available for study.

METHODS AND STUDY AREA

On a waste grassland at Sugadaira located 1300 m high in Nagano Prefecture,

two plots were laid out at a 60 m interval, which is far beyond the maximum

length of home range of our voles (TANAKA 1962), SO as to offer assurance that

home range of any individual seldom covers both plots, viz. two samples can be

taken independently of each other from those. The vegetation, besides grasses, in

the plots and their nearby area was predominantly formed of forbs, such as Trifolium

repens, T. pratense, Rumex japonicus, Erigeron annus, Oenothera odorata and so on;

1 Contribution from JIBP-PT No. 55, carried out by the grant from the expenditure of Education Department to the specific study on "Dynamics of Biosphere"

it suggests that both plots were provided with about the same habitat for the vole.

A single snap-trap, baited with a raw peanut, was set at each station, spaced

5 m apart, of a grid on either plot, 50 • m in size, hence having a trap number

121 in total. The census trapping was conducted by checking once a day as usual

for five days. One (D) of the plots was prebaited for three days with peanuts

attached to trap-sized cardboards placed on trap-stations, while another (C) did not

undergo any prebaiting at all.

All the captures, amounting to 405 in number, were composed of the vole,

Microtus montebeUi, suggested a very high density of the simple population present

in the area.

TRAPPING RECORDS AND EQUATION TO BE APPLIED

The catch records of the vole are shown in Table 1. On applying the regression

census-equation (1)

C , = ( N - S , , - 1 ) p .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (1)

for n=I , II, . . . . . . V days, where C, is catch on nth day, S,-1 sum previously caught

and p probability of capture in time unit (one day), to trapping records obtained

Table 1. Trapping records of the vole in Plot C (not prebaited) and D (prebaited).

Plot C Plot D

Date Number of- Date Number of (1968) captures (1968) captures

12 August ( I ) 13 ( II ) 14 (m) 15 (IV) 16 (v)

21 26 43 31 21

17 August ( I ) 18 (II) 19 (III) 20 (IV) 21 (V)

99 80 40 29 15

Total 142 263

under a single.catch trap plan, we should devote our first attention to the effect of

multiple collisions on sampling (TANAKA 1963, TANAKA & KANAMORI 1967). The

effect is expected to be significant, when the number of animals is considerably

great as compared with the trap number used. The condition appears to be just

available to the present case in view of the situation built up by the population

itself and the trapping design adoped. In 1963, it was demonstrated, by examining

several exampes of catch records gained by the removal method, that a significant

effect of multiple collisions could not observed in cases with the ratio of maximum

catch (C~) to trap number (T) being 0. 24 or less (TANAKA 1963). The ratio (C ,JT)

is as much ~as 0.82 with Plot D and yet 0.36 with C; therefore it is positively

necessary for us to use Equation (2) (z-equation), devised by LESLIE & DAVIS

(1939) for assessing the absolute number of rats under the influence of multiple

col l i s ions , ins tead of Equat ion (1).

z= (N-S,~-Op; z ~ T { l o g ( T - T t) - l o g ( T - T ' -C, , )} . . . . . . . . . . . . . . . (2)

where T ' is n u m b e r of t r aps sp rung ineffect ively (nega t ive ly or by non-object ive

an imals ) .

W e are r ead i ly a w a r e of the fact in Tab le 1 tha t the ca tches on ea r l i e r two days

in C seem to be too smal l to fol low the same rule to which the subsequent ser ies

of ca tch is wel l subject , whereas any l ike inord ina te ca tches a re not found in D.

F o r such da t a cons idered a l toge the r in C, however , one migh t imag ine some shape

of cu rv i l inea r r eg ress ion caused by s t rong neophobia or by the in tense effect of

mul t ip le col l is ions (TANAKA 1963), bu t z-equat ion wil l su re ly be unava i l ab le to the

whole ser ies of ca tch because the m a x i m u m of C~ lies on the th i rd day. Besides, a

cu rv i l inea r r eg ress ion due to an increase of the t r appab l e popula t ion (N2-) b y t u rn ing

of the non- t rappab le an imals into the t rappable , as was sugges ted by us (TANAKA

& KANAMORI 1967, KANAMORI ~r TANAKA 1968), would be conceived (Fig. 2). In

advance of us, KIKKAWA (1964) touched upon the t r ans f e r f rom the un t r appab le to

the t r appab l e pa r t of a popula t ion wi thou t r e f e r r ing to the curv i l inear regress ion.

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Fig. 1. Distribution map of trap station where capture occurred in variable frequencies (denoted with circles) on earlier two days (I, II) together and on later three days (III~V) together in either plot (C above, D below). Broken lines within each plot delineate borders of the internal square.

F r o m Fig. 1, i t s eems l ike ly tha t the inord ina te ca tches in C a re re fe rab le to

he te rogeneous sampl ing occur red in such a w a y tha t cap tu re took place in a va s t

m a j o r i t y of cases on the half a r ea in the sou th -wes t of the plot on ea r l i e r two days ,

bu t i t is appa ren t t ha t cap tu re did a lmos t al l over the plot a rea on la te r th ree days.

W e should not, however , cons ider an ent i re absence of any res iden t an ima l s because

no cap tu re occur red in the o ther half a r ea on the ear l ie r days . On the o ther hand,

a good sample cover ing al l the plot a rea was t aken th rough the whole per iod in D.

T h e d i s t r ibu t ion pa t t e rn of cap tu re can be inves t iga ted s t a t i s t i ca l ly by means of

L - - i n d e x (MoRISITA 1962), which has been es tab l i shed on the bas is of Poisson

dis t r ibut ions .

In case, however , a b inomina l d i s t r ibu t ion is expec ted as is the case wi th our

examples where the f requency of cap tu re (0, 1, 2 . . . . . . n) m a y occur for a g iven

days of n, the index IB should be used ins tead :

IB=I~(n--1/T) / ( n - - l ) T T

where, pu t t ing N= ~ x,, L = T ~ , x , ( x , - 1 ) / N ( N - 1 ) and E ( L ) = ( n - l ) / ( n - 1 / T ) i=l i=1

in b inomina l d is t r ibut ions . The ca lcu la ted va lues for these indexes a re g iven in

Tab le 2. T h e values of I~ a re all more or less g rea t e r than unity, which case m a y

r ep re sen t difference in p robab i l i t y of occur rence of cap ture among t raps . The va lue

is l a rges t on ea r l i e r days in C as is cons is ten t wi th a c lumped d i s t r ibu t ion of

cap tu re (Fig. 1), and, as c o m p a r e d wi th that , i t is much sma l l e r and nea re r to un i ty

on the co r respond ing days in D so as to sugges t nea r ly r a n d o m cap tu re in each t rap.

T h e f ac t t ha t the va lue on the whole per iod is sma l l e r than tha t on e i ther ea r l i e r or

la te r days in each plot m a y be in t e rp re t ed as mean ing tha t t r aps suffered cap tu re

Table 2. Number of traps where capture occurred in different frequencies (x) during the respectively designated periods.

Plot C Plot D Number of captures in atrap Earlier Later Whole Earlier Later Whole

(x) days days period days days period (I, If) (III~V) (I~V) (I, II) (III~V) (I~V)

81 33 7

55 41 21 4

39 40 28 10 4 0

13 37 71

58 45 15 3

10 23 43 29 13 3 J

i

Total number of 121 121 121 121 121 121 traps (T) Total number of 47 95 142 179 I 84 263 captures (N)

I~ O. 7835 O. 8943 O. 9911 O. 5393 O. 8330 O. 8359

E (I~)* 0. 5021 0. 6685 0. 8013 0. 5021 0. 6685 0. 8013

IB 1. 5604 1. 3378 1. 2369 1. 0741 1. 2465 1. 0432

* Expectation of I, in binominal distributions.

on earlier days were to some degree compensated for by those suffered no capture

on later days and vice versa. Thus the catches on the earlier two days have proved worthy of being ruled

out from the census data in C, and then the rest on the later three days was used

to count N1H that is the population estimate at the time of day III by applying

Equation (2), the estimate (N) of the desired population on the initial day being

gained by adding the previously removed number (47) to /VHz. As to D, the same

equation was applied to all the data to count N. According to LESLIE & DAVIS

(1939), the least square method was followed to rate parameters, and standard

errors of their estimates were counted by means of the formulae they presented

ibidem.

C: 2"r177 23]+47=178• 23 (175~181)

1~=0. 4060:k0. 0010 (0. 3933~0. 4187)

D: -~r=259• 8 (231~287)

/~=0. 7920• 0483 (0. 6394~0. 9446)

There are 95% fiducial limits of the estimates denoted in parentheses. In the calcu-

lation, we presumed that T' is approximate zero, for all the ca tch was restricted to

the vole and the number of traps negatively sprung was negligibly small, and the

linearity of regression of z was assured by the ze-test also following their method

as below:

C: n=3, a2= 0.05, 2a~2/n= 44.60, z~/ (~-2) =0. 0011, 0. 95~ P <0. 98

D: n=5, a2=98.16, Sa~2/r~=179.12, ze/(n-2)=0.548, 0 . 5 0 < P < 0 . 8 0

The calculated linear or curvilinear regressions of z or C~ on S~-~ and their

observed values are represented in Fig. 2. It is just obvious that the distribution of

realized catches is more suitable to a curve than to a right line in D. If one were

to apply Equation (1) by thinking nothing of its curvilinearity, a considerable

overestimation would be produced. In this case, it turns out probable that multiple

C~ z 50 ~ C lOO

5O

I I 0 50 100 S~-, 150 200 250

Fig. 2. Regression lines of z (straight broken lines) and Cn (curved solid lines) on Sn-1 fitted to actual values (asterisk for z, double circle for Cn) on later three days in C and on whole period in D respectively.

collisions exerted a strong effect on sampling, from the evidence that the expected

ratio of traps suffering 0, 1, 2 . . . . . . effective collisions under a multiple-catch trap

design is counted as 0. 183, 0. 311, 0. 264 . . . . . . respectively from the Poisson series A

with rn=pN/T that is 1.70, and then two or more collisions, giving rise to only

one capture each under the actual trap design, took place in as many as some half

number of traps. In spite of so many animals expected to have collided with

pre-occupied traps (ineffective collisions), few of them must have practically been

captured in remaining unoccupied ones within a time unit, for z-equation has been

statistically proved to be valid to our present data.

As contrasted with D, the effect was not so much intense on day III in C, the

aforesaid expected ratio being counted from m=p-Nzzz/T= 0.439 as 0.645 for 0,

0.283 for 1, 0.062 for 2 . . . . . . and thus two or more collisions happened in

only 7 percent traps. Correspondingly the observed catches are all just on a right

line, and if we dare to apply Equation (1), the maximum likelihood estimate (/VzH t)

works out at 144, no much larger than /VHz, but its assymptotic variance, obtained

by the formula of ZIPeIN (1956), proves to be as large as 900, perhpas due to small

degree of freedom, so that it scarcely serves for a strict test of significant difference

between N~u and -~zH t in C or between /Vs in both plots.

So long as the results by the least square method are concerned, however, it is

clearly demonstrated in view of the fiducial limits of each estimate that either the

population or the probability of capture was significantly larger in D than in C.

Though the used variance of estimates is not on the assymptotic basis, the statistical

test will well be assured of its reliability, since sufficient samples were available.

TEST OF RANDOM SAMPLING

Natural populations like those concerned consist of different groups in age and

sex which may behave or response toward traps in different ways among those,

and it is desirable to inquire into how the different behaviors, if any, inflicted

influence upon our trap-sampling for census. If we are forced to adopt a community

composed of populations of several species, predominant in abundance, as object of

trapping, sampling at random out of each population will be very difficult to do

successfully at least from the view-point of the fact that there exist behavioral

ranks different among species of small mammals. Recently many students have

attached importance to the subject (CALHOUN 1959, TUR~EK 1967, GENTRY et al. 1968), which does not matter to sampling in this study so that no further discussion

on it is needed, still it deserves mention that the last authors offered adverse criti-

cisms against Polish census method from the standpoint of the subject as a result of

their field test.

Statistical tests of independence among samples taken from the D population

were exercised in respect to the following items:

a. Whether or not the body weight distribution in the whole sample is differential

(1) (2) (3) (4) according to sex; the sample was divided into four

classes, i. e. (1) 16 g or less, (2) 17~25g, (3) 26 Male 23 40 32 16 Female 16 34 65 29 40 g and (4) 41 g or more, by gross body weight,

among which adolescence arises at size range of (2), z~=12. 66, P~O. Ol

hence ( I ) i s reduced to juveniles. The result of

analysis shows that females were significantly in excess of males in size distribution.

The reverse relation is usually observed in small mammals, nevertheless, since

there is found no evidence that small-sized voles are more apt to be caught in males

than in females, the then structure of the population must have been in reality in

the unusual trend by uncertain reasons.

b. Whether or not the size distribution is different between samples on earlier

and later days in either sex; I, II . . . . . . V in the association table refer to the day

numbers as in Table 1. The result affords the proof that large voles were trapped

earlier than were small voles, mainly represented by juveniles, in either sex. The

same proof was presented by DAVIS & EMLEN (1956) in rat populations, but they

could not conclude that males tend to be caught prior to females.

Male Female

(I) (2) (3), (4) (I) (2) (3), (4)

I, II 6 29 35 5 24 75 III.-~V 17 11 13 11 10 19

Z2~13.42, P<0.01 Z2>13.66, P<0.01

KIKKAWA (1964) detected dominant-subordinate relations in trap response sometimes

occurred among observed voles, in which large males were often dominant.

ANDRZEJEWSKI et al. (1967) evidenced in a vole population by means of recapture

data of combined sexes that trap-prone voles are decidedly larger in body weight

than trap-shy voles. In general, for small mammals it is conceivable and feasible

that trap-addicted individuals, viz. with higher probability of capture, are apt to

occupy traps in advance of trap-shy ones. The difference in the trap response will

be reduced to congenital factors as well as to behavioral causes involving social

rank, size of home range and amount of activity (KIKKAWA 1964), besides a previous

trap-experience may be an important cause as far as the capture-recapture method

is concerned (TANAKA • KANAMORI 1967). If we think much of social rank among

those, from our result and the above cited literature it follows that small-sized

individuals such as juvenile, subadult or young adult are generally in lower social

order than large adult for small mammals.

c. Whether or not both sexes were trapped every day according to the same

rule; the association table shows that both sexes were caught in the same way

during the whole period, but from the result we can not always derive the conclusion

I II III IV, V

Male 35 35 20 21 Female 59 45 20 20

Z~=3.22, 0.50~P~0. 20

that there was no sexual difference in social rank;

when we follow the probable assumption that

males are predominant over females in social

rank, the result is interpreted as indicating that

the preponderance of females in size distribution

over males (a) was balanced by the superiority of large voles in social order over

small voles (b), so as to have induced the same trapping way of both sexes.

At any rate, the results may be summed up by saying that our sampling was

nearly reasonable as for sex but not fully at random so far as the juvenile group is

concerned.

Additionally, the test was made for either plot whether or not the daily catch

of combined sexes in the external belt (outer two rows and lines of traps) and in

the internal spuare (Fig. 1) occurred in the same way on earlier and later days. As

a result, it is proved that the catch was realized according to the same rule through

D C

I II III IV V I, II I I I ~ V

External 56 45 17 15 7 29 60 Internal 43 35 23 14 8 18 35

~2=2.88, 0.80~P~0.50 Z2=0.034, 0.90~P~0.8G

earlier and later days in both external and internal areas within either plot. The

average total catch per trap was 1.94 for external and 2. 51 for internal with D and

1.24 for external and 1.08 for internal with C. It follows that the tendency that

the catch was increased on the border zone of the plots on later days, which may

be important evidence for invasion from surroundings, could not be found at all.

Consequently we may safely think that the immigration, as is seen occasionally in

removal trappings, if any, took place to a negligible degree in both plots.

Comparing the trappability per trap for a vole between the external belt and

the internal square within plots, ADAMCZUK & RYSZKOWSKI (1968) represented the

fact that twice as many voles were caught in the external as in the internal, the

difference being markedly increased on the last two days out of five days as census

period. They interpreted the margin effect as the result of immigration and other

factors, and yet we would like to lay major emphasis on immigration as cause of

the phenomenon.

JANION et al. (1968) introduced a new idea of census method, that is after all

reduced to the formulae given by ZIPPIN (1956), aiming at the population size diluted

by immigration for a given census period, say n days, instead of the initial size.

In the distribution of number of days (t) from the initial to the time the presence

of resident animals is revealed, the expectation of t for t = 0, 1, 2 . . . . . . n - 1 is

stated as follows:

E(t) =q/p-nq~/(1-q~)

Herein q = l - p as usual, and E(t) is expressed with R in terms of realized catches

as follows: R - 1•215 . . . . . . + ( n - l ) Cn-~

By putting E(t)=R, p is estimated, and then N is counted from XCn=N(1-q ~) for

n = l , 2 . . . . . . n.

Their theory is founded on the premise that each resident has a constant proba-

bility of capture and each animal, which immigrated into the plot during the period,

had escaped capture with the same probability before its immigration. In census

works by the removal method, however, daily catches may fluctuate on account of

various sorts of factors (TANAKA 1963, 1966), and the invasion is but one of those,

so their method will be quite invalid for such data as was affected intensely by

factors other than invasion; such is the case with the present examples under the

pronounced effect of multiple collisions.

EVALUATION OF PREBAITING

On the prebaited plot (D) the probability of capture estimated (p~ was some

twofold as high as on the not prebaited (C) despite the fact that the trap spacing of

5m was adopted on either plot; this is sure to be one of the beneficial effects of

prebaiting. Though a convincing proof is lacking, another will be that a consistent

sampling could be performed through the census period with D, as is induced by the

subsequent reasoning.

The population density per acre is calculated at 189 for C and 275 for D from

by means of the usual method of DICE (1938) relying on the average size of home

range. We used a range length of 13 m, corresponding to the observed range

length (STICKEL 1954), as average diameter of home range, though it was the

value observed in 1967 when the density was of the order of 70~80 (TANAKA KANAMORI 1967). Since it is a certain fact that the range size is reversely dependent

on density, a true range length must have been still less than 13 m in 1968.

Thus, besides the very high density, it is found out in 1968 that in D there were

about one and a half times as many voles as in C; so much difference in density

between two adjacent areas with habitat similar to each other as is seen in this

case is feasible to occur. Therefore, also because of lacking in any proof that a

marked invasion happened to either plot, it is unlikely that the 3-day prebaiting

elicited invasion so as to result in the hightened density on the part of D.

Too long a prebaiting is not only labor-expensive but it may cause us to fear

that immigration is promoted (PELIKAN 1967), and on the contrary too short (say

one day) a prebaiting may raise heterogeneity in probability of capture among indi-

viduals of a population, because it is highly probable that only some of them get

accustomed to traps and baits.

10

In our study in 1967, we considered that the condition in terms of "Nw-Nr", where Nw represents the whole population, had been established already on the initial

day of the census trapping preceded by no prebaiting on the plot of 5 m trap-spacing.

If the condition had been likewise satisfied in the present case where traps were

also spaced 5 m apart on either plot, no inordinate catches might have appeared in

C. As a matter of fact, however, the home range in 1968 was inferred to be somewhat

smaller than in 1967, and thus even the spacing as close as 5 m must have not

been enough to establish the condition on the initial day. Therefore we may suppose

that the 3-day prebaiting in D caused most of non-trappable voles to get trappable

and made altogether the trappable voles learn to visit baited traps (cardboards) so

that the good sampling with the enhanced rate, in terms of p-values, much greater

than in C could be executed consistently through the census period, while in C some

of non-trappable voles were present for earlier two days, with the result that there

took place the heterogeneous sampling and inordinate catches, yet followed with the

regular sampling on later.

Nevertheless, the phenomenon of multiple collisions gave strong influence to

sampling on account of the excessive abundance of resident voles relative to the trap

number especially with D. Hence it follows that ~r for D must have been approxi-

mate to Nw but 2~r for C might have been somewhat less than Nw. By way of compensation we would like to remark that as for the curvilinear

formula, presented by us in 1967, to esimate Nw from capture-recapture data, we

thought of the possible case in which only a small portion of the non-trappable

group consists of animals escaping capture by ineffective collisions.

Consequently we may say that only if a census work is devised just enough to

build up the aforesaid condition at the initial time of trapping, any prebaiting will

not always be required, but still a prebaiting continued adequately long, say 3 days

or longer, can enhance the rate of sampling. The effect of prebaiting should be

evaluated from the view-point of interrelation among population density, size of home

range and trap spacing, for the basic utility of prebaiting consists in that it may

help toward attainment of our ultimate purpose to estimate Nw. Therefore we come

to think that the trap spacing as wide as 15 m adopted by Polish workers makes it

difficult for them to attain the desired purpose even by their intensive trapping at

least for the vole (Clethrionomys glareolus) dealt with by them.

Lately KOTT (1965) put forward evidence in favor of our supposition as a result

of field work accomplished on a large scale with Microtus pennsylvanicus. His con-

clusions pertaining to the present subject are to the effect that:

The population estimate rises as the trap density is hightened by increase of trap

number due to reduction of trap spacing until the estimate is levelled off when the

spacing becomes about 25 feet, but the estimate rises only slightly with increase in

trap density induced by increment of trap number on each station.

Hence he suggested that a regular grid with traps spaced as close as 12~15 feet

11

gives a good coverage of the area sampled. Since his census methods always relied

on the assumption that the population size is kept constant f rom the initial t ime of

trapping onward, his words "a good coverage of the area sampled" can be interpreted

as meaning that the condition "Nw-Nr" may be established on the initial day, and

his suggestion implies that so close spacing is desirable that we may attain the

estimate of N~r, for his result proved that only an increase of t rap number on each

station without shortening the interval between stations serves little the desired

purpose.

SUMMARY

A field work with two plots of grid, a snap trap being set on each station spaced

5 m apart, was executed in the summer of 1968 to evaluate prebaiting in census

trapping by comparing the result in one plot, prebaited for three days, with that in the

other not prebaited. Since the population was as high as some 230 per acre on the

average in density and formed of the vole, Microtus montebelli, alone, sufficient

samples were gathered irrespective of the plot size as small as 50 • m.

Owning to the circumstances, multiple collisions inflicted so intense influence on

sampling especially in the prebaited plot that z-equation for census adjusted to the

effect was well applicable to the data in either plot. In sampling, the fact that small

voles are apt to be caught later than large voles was statistically evidenced in either

sex, and yet any proof that males tend to be caught prior to females was not offered.

I t was ascertained in either plot that the daily catch was realized according to the

same rule through the whole period of trapping in both external belt and internal

square within the plot; hence it follows that no considerable immigration occurred.

One of the beneficial effects of prebaiting is sure to be that the probability of

capture was markedly enhanced in the prebaited plot, and a second is supposed,

though inconclusively, to be that a good sampling could be executed consistently

through the census period giving rise to no inordinate catches perhaps due to hetero-

geneous sampling as was seen in earlier days in the not prebaited plot. The

supposition has derived f rom the condition that most of the whole population is

trappable, which is established by interrelation among population density, size of

home range and trap spacing. It was suggested that the effect of prebaiting should

be evaluated from the view-point of the interrelation, because the basic utility of

prebaiting consists in that it may help to our utl imate purpose to estimate the whole

population.

REFERENCES

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biological productivity investigations. Scc. Prod. Terr. Ecos. (ed. K. PETRUSSWmZ), I: 275-281.

12

ANDRZEJEWSKI, R., K. PETRUSEWlCZ and J. WASZKIEWIcz-GLIwICZ (1967) The trappability of Clethrio-

nomys glareolus and other ecological parameters obtained by the CMR capture method. Ekol.

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TANAKA, R. (1963) Examination of the routine census equation by considering multiple collisions

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TANAKA, R. (1966) Simulated removal trapping considered in connection with actual census data

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TANAKA, R. and M. KANAMORI (1967) New regression formula to estimate the whole population for

recapture-addicted small mammals. Res. Popul. Ecol., 9: 83-94.

TUR~SK, F .J . (1967) Some methodical aspects of quantative studies of vertebrates. Part II: small

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Z~pPIN, C. (1956) iAn evaluation of the removal method of estimating animal populations. Biome- trics, 12: 163-189.

13

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