Red grouse

The explanation for cycles in the dynamics of the red grouse (Lagopus lagopus scoticus) in the United Kingdom has been a matter of disagreement for decades. Some have emphasized an extrinsic factor, the parasitic nematode Trichostrongylus tenuis (Dobson & Hudson, 1992; Hudson et al., 1998). Others have emphasized an intrinsic process through which increased density leads to more interactions between non-kin male birds and hence to more aggressive interactions. This leads in turn to wider territorial spacing and, with a delay because this is maintained into the next year, to reduced recruitment (Watson & Moss, 1980; Moss & Watson, 2001). Both viewpoints, therefore, rely on a delayed density dependence to generate the cyclic dynamics (see Section 10.2.2), though these are arrived at by very different means.

We have already seen in Sections 12.7.4 and 14.4.2 that even field-scale parasites? experiments have been unable to determine the role of the nematodes with certainty. There seems little doubt that they reduce density, and the results of the experiment are consistent with them generating the cycles, too. But the results are also consistent with the nematodes determining the amplitude of the cycles but not generating them in the first place.

extrinsic and intrinsic factors autocorrelation function analysis

Autocorrelation Function Analysis

Figure 14.12 (a) Coupled oscillations in the abundance of the moth Plodia interpunctella and its parasitoid Venturia canescens (P and V, respectively) and, on the right, an autocorrelation function (ACF) analysis of those data (host above, parasitoid below). Sloping lines show the levels the bars must exceed for statistical significance (P < 0.05). The cycle periods (l) are 6-7 weeks, with significant correlations at l, 2l, etc. and significant negative correlations at 0.5l, 1.5l, etc. (After Begon et al., 1996.) (b) Time series for the abundance of the sawfly Euura lasiolepis (left) and an ACF analysis of those data (right). There is a hint of an 8-year cycle (positive correlation with a lag of 8 years; negative correlation with a lag of 4 years), but this does not come close to statistical significance (exceeding the lines). (After Turchin & Berryman, 2000; following Hunter & Price, 1998).

Figure 14.12 (a) Coupled oscillations in the abundance of the moth Plodia interpunctella and its parasitoid Venturia canescens (P and V, respectively) and, on the right, an autocorrelation function (ACF) analysis of those data (host above, parasitoid below). Sloping lines show the levels the bars must exceed for statistical significance (P < 0.05). The cycle periods (l) are 6-7 weeks, with significant correlations at l, 2l, etc. and significant negative correlations at 0.5l, 1.5l, etc. (After Begon et al., 1996.) (b) Time series for the abundance of the sawfly Euura lasiolepis (left) and an ACF analysis of those data (right). There is a hint of an 8-year cycle (positive correlation with a lag of 8 years; negative correlation with a lag of 4 years), but this does not come close to statistical significance (exceeding the lines). (After Turchin & Berryman, 2000; following Hunter & Price, 1998).

In another field experiment, aspects of the alternative, 'kinship' or 'territorial behavior' hypothesis were tested (Mougeot et al., 2003). In experimental areas, established males were given testosterone implants at the beginning of the fall, when territorial contests take place. This increased their aggressiveness (and hence the size of their territories) at densities that would not normally generate such aggression. By the end of the fall, it was clear that, relative to the control areas, the increased aggression of the older males had reduced the recruitment of the younger males: testosterone treatment had significantly reduced male densities and had particularly reduced the ratio of young (newly recruited) to old males, though there was no consistent effect on female densities (Figure 14.13a).

Moreover, in the following year, even though the direct effects of the testosterone had worn off, the young males had not returned (Figure 14.13a). Also, because young relatives had been driven out, levels of kinship were likely to be lower in experimental than in control areas. Hence, the kinship hypothesis predicts that recruitment and density in the experimental areas should have remained lower through the following year: that is, lower kinship leads to more aggression, which leads to larger territories, which leads to lower recruitment, which leads to lower density. These predictions were borne out (Figure 14.13b).

kinship, territories and aggression?

Population 1

Population 2

Population 1

Population 2

10 I_I ■■■I■■■I ■ ■■ I ■ ■■ I ■ ■■ I_ _I ■ ■■ I ■ ■■ I ■ ■■I■■■I■■■ I_10

Aug Dec Aug Dec Apr Aug Dec Aug Dec Apr 2000 2001 2002 2000 2001 2002 Year

Control area Testosterone area

er P

Population 1

2001 2002 Year

te rat

i1.0 ta

er P

Population 2

2001 2002 Year

Figure 14.13 (a) Changes in grouse numbers (males (cocks); the young : old cock ratio; and females (hens)) in control (o) and testosterone-implant experimental areas (•) in two populations. The gray bar represents the period of time over which the males were given implants. (b) Per capita recruitment in the two populations was higher in the control areas than in the experimental areas, both in 2001, immediately after treatment, and 1 year later. (After Mougeot et al., 2003.)

Thus, these results establish, at least, the potential for intrinsic processes to have (delayed) density-dependent effects on recruitment, and thus to generate cycles in the grouse. In a companion paper, Matthiopoulos et al. (2003) demonstrate how changes in aggressiveness can cause population cycles. As Mougeot et al. themselves note, though, the possibility remains that both the parasites and territorial behavior contribute to the observed cycles. Indeed, the two processes may interact: parasites, for example, reduce territorial behavior (Fox & Hudson, 2001). Certainly, it is far from guaranteed that either of the alternative explanations will ultimately be declared the 'winner'.

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment