Environmental Engineering Reference
In-Depth Information
2.4.3.1 Using count data
A direct measure of productivity is given by the ratio of young to adults in the pop-
ulation (Box 2.14). The necessary data can be obtained in one of three ways,
depending on the practicalities of different field methods:
●
Using population abundance estimates separated by age;
●
Using sample counts of the population separated by age;
●
Using counts of offspring, eggs, embryos, placental scars (Martin
et al
. 1976),
fruit or seeds associated with a sample of potentially breeding individuals.
Box 2.14
Measuring productivity using count data.
Given estimated or counted numbers of offspring,
y
, and adults,
a
, productivity
is simply the ratio of the two:
y
a
P
ˆ
The precision of the estimate depends on the sampling method. Where
y
and
a
represent abundance estimates with their own associated variances:
SE(
P
ˆ
)
P
ˆ
CV(
y
)
2
CV(
a
)
2
Where a sample of
n
individuals is counted without direct connection between
individual breeders and their offspring:
P
ˆ
(1
P
ˆ
)
2
n
SE(
P
ˆ
)
Finally, where
n
individuals each produce
y
offspring, the population variance
is used (see Box 2.1):
s
2
(
y
)
n
SE(
P
ˆ
)
In the last two cases, if it is known that no individuals have been counted more
than once (sampling without replacement), and the total population,
N
, is also
known, the standard error can be reduced according to the proportion of the
population not sampled:
n
N
SE(
P
ˆ
)
SE(
P
ˆ
)
1
Note that these methods can be used to estimate age or sex specific productivity
rates in cases where such structure is likely to be important (see for example Milner
et al
. 2007), simply by applying the equations to data separated by age or sex. They
can also be adapted to the estimation of other population ratios, such as sex ratio.
