Irruptive potential in roe deer: Density-dependent effects on body mass and fertility

We examined the irruptive potential of a roe deer (Capreolus capreolus) pop ulation on a 10-km(2) island 2 km off the coast of central Norway (63 degre es 40' N). As roe deer density increased from 10 to 34.5 animals/km(2) from spring 1991 to spring 1994, reproductive status was determined in 33 2-yea r old and 66 adult, radiocollared does. Body mass was known for 19 adults a nd 12 2-year old females. The studied population had a high overall rate of increase, r = 0.409, although this decreased with increasing population de nsity. Density only explained 2.8% and 6.7% of the variation in maternal bo dy mass of 2-year olds and adults, respectively, and an examination of indi vidual trajectories showed that density did not affect litter size either. Body mass did not affect reproductive status in the 2 age groups; however, body mass affected the number of fawns produced. Females with above average body mass had 40% higher productivity than females with below average body mass. The ability of female roe deer to maintain high body condition and h igh reproductive rates at high density means that monitoring of vital rates alone will not allow managers to detect these potential rapid increases in density until they have irrupted. Therefore, direct monitoring of populati on size or an index of population abundance is needed to allow rapid change s in harvest level that can prevent irruptions.