10 MammalsCharles J. KrebsDepartmen

10 Mammals
Charles J. Krebs
Department of Zoology, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4
Introduction
Census methods for mammals depend critically on the size of the species and its natural history.
If species are diurnal, common and highly visible, the census problem is relatively simple. If
species are nocturnal, rare and difficult to detect, the census problems are most difficult. As in all
ecological census work, you need to decide the purpose of the study and the level of precision
you require. Higher precision bears costs in time and money, and methods that lead to higher
precision might not be practical for some species within a finite budget.
A sequence of decisions to facilitate the choice of methods for a mammal census is outlined
in Figure 10.1 (Table 10.1). Just because many studies of a particular species or group of species
have used a particular method does not mean that you must use this method for your study.
Many studies have not used the best methods in the past, and there is no reason to continue using
sub-optimal techniques that waste time and money.
Total counts
The simplest way to determine how many individuals of a particular species of mammal live
in an area is to count all of them. This census method we might consider the Holy Grail of
mammal-census methods, yet it can hardly ever be achieved. Total counts can be done on large
mammals in restricted areas (Bookhout 1994), but one should always be sceptical of the accuracy
of total counts, since in most cases to date there is a negative bias – estimated numbers are less
than actual numbers. Grey whales have been counted since 1975 as they migrate south along
the coast of central California and, while one might think that a party of observers could count
all the whales moving close to shore, in fact they recorded only 79% of the whales (Rugh et al.
1990). The message is clear: if you think you can do a total count, check your assumption by
double-counting with two independent sets of observers.
Method
If numbers are small, you can count isolated blocks of individuals, but if they are larger you
will need to photograph the groups and count from the photos. This method can be used with
Ecological Census Techniques: A Handbook, ed. William J. Sutherland.
Published by Cambridge University Press. C _ Cambridge University Press 1996, 2006.
351

ผลลัพธ์ (อังกฤษ) 2:

10 Mammals
Charles J. Krebs
Department of Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
Introduction
Census methods for critically mammals Depend on the Size of the species and ITS Natural history.
If species are diurnal, common and highly visible,. the census problem is relatively simple. If
species are Nocturnal, Rare and difficult to detect, the Census Most Problems are difficult. As in all
Ecological Census Work, You Need to Decide the purpose of the Study and the level of Precision
You Require. Higher Precision Bears costs in time and Money, and methods that Lead to Higher
Precision might not be practical for Some species Within a finite Budget.
A Sequence of decisions to Facilitate the Choice of methods for a mammal Census is outlined
in Figure 10.1 (Table 10.1. ). Just because MANY Studies of a particular species or Group of species
have used a particular method does not Mean that You must use this method for your Study.
Many Studies have not used the best methods in the Past, and there is no REASON to Continue using.
Sub-Optimal techniques that waste time and Money.
Total counts
The Simplest Way to Determine How MANY individuals of a particular species of mammal Live
in an Area COUNT is to all of them. We might consider this the Holy Grail Census method of
mammal-Census methods, yet it Can Hardly Ever be achieved. Total counts Can be done on Large
mammals in restricted areas (Bookhout the 1994th), but one should always be Sceptical of the accuracy
of total counts, since in Most Cases to date there is a negative bias - Numbers are estimated Less
than Actual Numbers. Grey whales have been Counted since the 1,975th as they Migrate South along
the coast of Central California and, while one might Think that a party of observers could COUNT
all the whales Moving close to Shore, in Fact they Recorded only 79% of the whales (Rugh. et al.
1,990). The message is Clear: if You Think You Can do a total COUNT, Check your assumption by
double-Counting with Two Independent sets of observers.
Method
If Numbers are Small, You Can COUNT isolated Blocks of individuals, but if they are larger You
Will. need to photograph the groups and count from the photos. Can be used with this method
Ecological Census Techniques: A Handbook, ED. William J. Sutherland.
Published by Cambridge University Press. Cambridge University Press C _ 1996, 2006.
351.