Consumers 



Herbivore populations and grazing effects 



Steve Mac Lean* 

 Frank Pitelka* 

 Guy Cameron 

 Pete Escherich 

 Russ Graham 

 Herb Melchior* 



University of Illinois 

 University of California 

 University of California 

 University of California 

 University of California 

 San Diego State College 



Steve Temple 

 Harry Coulombe 

 George West 

 Paul Whitney 

 John Coady* 



San Diego State College 

 San Diego State College 

 University of Alaska 

 University of Alaska 

 University of Alaska 



One of the conspicuous features of the coastal tundra ecosystem is the dominance of a single 

 grazing species, the brown lemming, Lemmus trimucwnatus. Population density of this species 

 varies in a cycle of large amplitude over a 3 to 5-year period. In the absence of other significant 

 grazers, this results in a cyclic grazing process. 



In 1970 lemming density was very low. The results of standard kill-trap lines, which have been 

 run at Barrow by Pitelka since 1955 (Fig. 2) allow us to compare lemming abundance with that record- 

 ed in previous seasons. The 1970 results are given below: 



Lemmus trimucronatus 

 Dicrostonyx groenlandicus 



Total 



6 



33 



The totals for Lemmus trimucronatus are among the lowest ever recorded. The total for the 

 normally rare collared lemming, Dicrostonyx groenlandicus, is higher than in previous seasons and 

 includes a number of captures in habitats in which this species has never previously been trapped 

 at Barrow. Most significant of these is the low, very wet habitat of central marsh. However, the 

 combined numbers of both species are quite low and indicate negligible grazing pressure in 1970. 



Vegetational dominance, cover, and height were observed for all trap lines. Observations 

 were made at 6-m intervals along each trap line. The results will be used to correlate trapping 

 success (hence habitat use) with cover characteristics throughout the cycle. 



The objective of quantitative modeling of the tundra ecosystem requires that an estimate be 

 made of consumer populations in terms of number (and biomass) per unit area of tundra. Attempts 

 were made to produce such data for lemmings by mark and release techniques on live-trap grids. 

 Estimates of actual population densities made by such means could then be used to calibrate the 

 kill-trap lines, so that kill-trap results could be used to estimate population density. Two plots 

 were established (each 61 x 61 m) to accomplish this calibration. One was laid out near site 2 

 (Fig. 3), the other between trap lines indicated as Tj (Fig. 2). 'Each consisted of 100 live traps 

 placed within a meter of the grid intersections at 6-m intervals; in addition, 20 live traps were 

 placed outside the live-trap plot along a midline which bisected the plot and extended 61 m beyond 

 both boundaries. This line was to be snap-trapped following the live-trapping program. 



Each plot was live-trapped once during the summer, for a period of 6 days, with three checks 

 each day. The plot near trap line Tj was live-trapped between 28 July and 3 August; no lemmings 

 were captured. Immediately following this a snap-trap calibration line, consisting of 30 stations 

 at 6-m intervals, with three traps per station, was run through the plot. One-third of this line was 

 located within the live-trap plot, while two-thirds of the line extended beyond the plot on either 

 side. The snap-trap line was run for three days (4-6 August), with two checks each day; no lemmings 

 were captured. 



♦Principal authors. 



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