Experimental evidence of density dependent activity pattern of a large herbivore in an alpine ecosystem

Abstract

Density dependent processes affecting foraging strategies may in turn influence vital rates and population regulation in large herbivores. Increased competition may lower both forage availability and quality, but whether the main activity constraint at high density is increased searching time or increased digestion time is poorly investigated. In a fully replicated landscape‐scale experiment, we used long‐term data (2003–2009) from domestic sheep grazing at high and low density (80 and 25 sheep km–2, respectively) on alpine summer ranges to test density dependence in allocation of time to feeding (moving) vs digestion (resting) activities and how this in turn affected body growth. Sheep at high density spent more time actively feeding than sheep at low density, but sheep moved shorter distances while foraging at high density. Increased activity levels at high density suggest that the main activity constraint at high density was availability of high‐quality food increasing searching time and possibly reducing intake rates. Increased movement distances at low density is consistent with a higher selection for more productive vegetation types since high‐quality patches are dispersed in the landscape. The alternative hypothesis, that food processing time increased at high density was not supported as it would have reduced overall activity levels. Individual activity levels increased body growth, but this was not sufficient to fully compensate for lower habitat quality leading to an overall reduced body growth at high density. Our experiment clearly documents changes in activity budgets and movement distances of a large herbivore at high population density, providing one potential behavioural mechanism of density dependent responses observed in vital rates.

Temporal scales of density‐dependent habitat selection in a large grazing herbivore

“…For ten consecutive summers, sheep activity was recorded through direct observations with binoculars (Mobæk et al , ). The methods of data recording were modified and improved following the first field season; therefore, data from that season (2002) were not included in the analyses.…”

“…The ewe's coordinates were recorded on a hand‐held GPS‐device, and sheep activity (grazing, resting or walking) and vegetation type were recorded. The sampling unit was defined as the family group (a ewe and her lambs) because of the limited grouping behaviour of this breed (Mobæk et al ).…”

The response of plant diversity to grazing varies along an elevational gradient

“…There were 3 replicates of 3 sheep density treatments, 0, 25 and 80 sheep km À2 within a randomized 3-block design (Fig. 1a) Mobaek et al 2012), and the sheep were free to graze along the elevational gradient. The site spans the treeline ecotone (Moen 1999;K€ orner & Paulsen 2004), being between the forest and the alpine zone (see data in Speed et al 2010a).…”

Temporal patterns of deer–vehicle collisions consistent with deer activity pattern and density increase but not general accident risk

“…positively density dependent, remains difficult to assess. In general, habitat selection (Mobaek et al, 2009;Blix et al, 2014;van Beest et al, 2014), activity (Mobaek et al, 2012) and home range size (Kjellander et al, 2004) of large herbivores are density dependent. In particular, at high density, more habitats are used, which also forces deer into more urban environments, and one might therefore expect a more than proportional increase in DVCs with increasing density of deer.…”