untitled

increase. These results are consistent with the interpretation that there was com-

petition between large and small granivorous rodents.

Although the above examples produced results consistent with the predictions of

interspecific competition, there was no attempt to measure the competition coeffi-

cients. However, Abramsky et al. (1979) carried out a similar removal experiment on

the shortgrass prairie in Colorado in which a competition coefficient was measured.

In this case voles (M.ochrogaster) were removed and the response of deermice

(P.maniculatus) recorded. Figure 9.5 shows the negative relationship between the

number of deermice present in the removal plot and the number of voles present in

the previous sampling period 2 weeks earlier, as expected if competition were acting.

To measure the competition effect (a) of voles on deermice, the Lotka–Volterra equa-

tion was used. At equilibrium dN 1 /dt=0, and so:

K 1 =N 1 +a×N 2 ×(20.75)

140 Chapter 9

40

30

20

10

0

123 123

Year

Total number captured

Small granivores Small omnivores

Large granivores

Present Absent

67

40

30

20

10

0

Total number captured

Fig. 9.4Exclusion of
large granivorous
rodents resulted in an
increase in the small
granivorous rodent
population relative to
control areas, indicating
competition. Small
omnivorous rodent
numbers did not
increase significantly,
indicating lack of
competition. (Data from
Munger and Brown
1981.)

100

80

60

40

20

0

0246810

Number of deermice

Number of voles

Removal plot

Non-removal plot

Fig. 9.5Number of
deermice (Peromyscus
maniculatus) known to
be alive at time tplotted
against number of voles
(Microtus ochrogaster)
known to be alive at
time t−1 (t−1 is the
sample period 2 weeks
earlier than period t).
(After Abramsky et al.
1979.)