Phenology and cover of plant growth forms predict herbivore habitat selection in a high latitude ecosystem

Abstract

The spatial and temporal distribution of forage quality is among the most central factors affecting herbivore habitat selection. Yet, for high latitude areas, forage quantity has been found to be more important than quality. Studies on large ungulate foraging patterns are faced with methodological challenges in both assessing animal movements at the scale of forage distribution, and in assessing forage quality with relevant metrics. Here we use first-passage time analyses to assess how reindeer movements relate to forage quality and quantity measured as the phenology and cover of growth forms along reindeer tracks. The study was conducted in a high latitude ecosystem dominated by low-palatable growth forms. We found that the scale of reindeer movement was season dependent, with more extensive area use as the summer season advanced. Small-scale movement in the early season was related to selection for younger stages of phenology and for higher abundances of generally phenologically advanced palatable growth forms (grasses and deciduous shrubs). Also there was a clear selection for later phenological stages of the most dominant, yet generally phenologically slow and low-palatable growth form (evergreen shrubs). As the summer season advanced only quantity was important, with selection for higher quantities of one palatable growth form and avoidance of a low palatable growth form. We conclude that both forage quality and quantity are significant predictors to habitat selection by a large herbivore at high latitude. The early season selectivity reflected that among dominating low palatability growth forms there were palatable phenological stages and palatable growth forms available, causing herbivores to be selective in their habitat use. The diminishing selectivity and the increasing scale of movement as the season developed suggest a response by reindeer to homogenized forage availability of low quality.

Figure 1. Study location.

Upper panel: The location of the reindeer district in which the study took place along with a presentation of the hierarchical structure of the study design. A total of 144 sites (black squares) were located within the district by GPS-positions from analyses of tracking data from reindeer (see main text for more detail). Within five days of its localisation each site was analysed for the cover and plant phenology of growth forms. Analysis were conducted within six plots (shaded) of the site. Lower panel: The three maps show the positions of freely ranging reindeer in the early, mid and late season of the summer of 2006.

Figure 2. Change in the ARS-scale through the season.

Grey bars are ARS-scales (i.e. the scale of the local maximum in variance of log FPT) for each 7-days period of the movement path of individual reindeer. The thick black line is the predicted ARS-scale from a gamm model using period as a predictor and log ARS-scale as a response. The thin black lines represent standard error.

Figure 3. Cover and phenology of growthforms.

Boxplots of a) cover (%) and b) vegetative phenology (presented on a relative scale, see Table 2 for actual scale) of each growth form in early, mid and late season. Numbers within panels represent number of sites for which a) the growth form was present and cover was estimated, or b) where phenological measurements of the growth form was possible.

Figure 4. Standardized coefficients of the effects of phenology and cover of growth forms on FPT by reindeer.

Relationships between phenology and cover of growth forms and FPT by reindeer are presented as standardized coefficients (see main text for explanation). Middle point give model estimate, thick lines give ±1 SE (approximately ±68% CI) and thin lines give ±2 SE (approximately ±95% CI). Predictor variables with thin lines that do not cross the central (red) line have a significant relationship to habitat selection by reindeer. Both negative and positive standardized coefficients are related to more selective habitat use by reindeer, with lower or higher values of a predictor variable respectively. For instance, for grasses, younger phenology give higher FPT values, indicating habitat selection for sites where grasses have younger phenology.

Figure 5. FPT-analyses.

Upper panel: Two 7-days periods (second period of the early season and last period of the late season) of the path of the same individual (reindeer #35). Standardized FPT-values were calculated from the respective ARS-scales and are shown as green to red color along the tracks. Lower panel: Variogram of log-transformed FPT values for the two 7-days periods. The ARS-scale was defined at the local maximum in variance.