Factors affecting recruitment in red grouse

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Territorial behaviour and population dynamics in red grouse Lagopus lagopus scoticus. I. Population experiments

<p>1: According to the ‘territorial behaviour’ hypothesis, population cycles of red grouse are caused by delayed density-dependent changes in the aggressiveness of territorial cocks. We report here on a replicated population experiment testing assumptions of this hypothesis.</p> <p>2: We used testosterone implants to increase aggressiveness of cocks for 3 months during autumn, when recruitment and territory establishment take place. On two moors located in northern England, and on two 1-km2 areas within each moor, we implanted adult cocks with testosterone on an experimental area and with sham implants on a control area.</p> <p>3: During the first autumn, the testosterone treatment prevented recruitment of young cocks into the territorial populations. This reduced breeding density and altered the age ratio among territorial cocks, and possibly levels of kinship. If so, the ‘kinship’ hypothesis predicted that density and recruitment should continue to differ between testosterone-treated and control areas.</p> <p>4: Grouse density remained significantly lower on the experimental than on the control areas for two consecutive breeding seasons. This confirmed a strong spatial structuring within grouse populations, which prevented immigration from neighbouring higher-density areas. In the second autumn, testosterone was not implanted but the recruitment rate remained significantly lower and cock density continued to decline more on the experimental than on the control areas.</p> <p>5: The results suggest that cocks continued to be aggressive and to maintain large territories for at least a year after aggressiveness was increased experimentally, and therefore that autumn aggressiveness is influenced by previous territorial contests.</p> <p>6: The experiment validates key assumptions of the ‘territorial behaviour’ hypothesis for red grouse cycles. Population models in a subsequent paper demonstrate how changes in aggressiveness can cause population cycles.</p&gt

Territorial behaviour and population dynamics in red grouse Lagopus lagopus scoticus: II. Population models

<p>1. Recent experiments on cyclic red grouse populations discovered that aggressiveness, induced by testosterone implants, depressed population density for more than a year after the implants were exhausted.</p> <p>2. This confirms the observation, also made in previous studies of this territorial species, that aggressiveness can determine population density. Additionally, it hints at a form of social memory that sustains the effect of episodes of high aggressiveness after their cause has ceased to exist.</p> <p>3. We explore the logical consequences of this observation with a simple model of the interaction between population density and aggressiveness. A flexible function describes how aggressiveness changes from year to year as a function of population density. At low densities animals are tolerant to conspecifics and aggressiveness falls from one year to the next. Conversely, at high densities aggressiveness rises.</p> <p>4. In the model, current aggressiveness is set by aggressiveness in the previous year, and modified by last year's population density (first version) or by current population density (second version).</p> <p>5. We assume no particular behavioural mechanism underlying this process but derive conditions under which changes in aggressiveness, effected by density, can generate unstable dynamics.</p> <p>6. The two versions of the model give fluctuations that differ in period and amplitude but have similar conditions for unstable dynamics. Specifically, the more abrupt the transition from tolerant to intolerant behaviour with increasing density, the more likely are cycles to occur.</p> <p>7. We show how a previous model of the kinship hypothesis for red grouse cycles can be recast in the current terminology, and how the new models offer a more general way of examining red grouse population dynamics.</p&gt