The Perils of Having Tasty Neighbors: Grazing Impacts of Large Herbivores at Vegetation Boundaries

Overview and Theoretical Framework

• Vegetation Boundaries as Focal Points: The boundaries between distinct vegetation patches are critical zones for dynamic interactions, particularly in grazed ecosystems. Variations in the acceptability and palatability of different vegetation types to herbivores drive these dynamics.

• Concept of Apparent Competition: The study highlights "apparent competition," where key vegetation resources attract herbivores, leading to higher impact on surrounding, less preferred vegetation. This occurs when two plant species share a common herbivore, and the presence of a preferred species increases the grazing pressure on its neighbor.

• Study Objective: To examine how the distribution of preferred grass patches within a landscape dominated by a less preferred dwarf shrub, heather ($Calluna vulgaris$), influences spatial variation in utilization by free-ranging red deer ($Cervus elaphus$) and sheep ($Ovis aries$).

• Key Finding: Vegetation-herbivore interactions are highly localized. Hot spots at grass-heather boundaries are the primary drivers of vegetation dynamics, and local utilization (at the scale of individual bites) cannot be predicted solely from large-scale parameters like herbivore density.

Study Area and Site Selection

• Location: The Cairngorm Mountains in northern Scotland, UK (approximately $57^{\circ} N$, $3^{\circ} 39' W$).

• Management Units (Estates): Six land management units were studied, ranging in size from $2500$ to $23000\,ha$ (mean $9950\,ha$):

  • Eastern Estates: A, B, and C.

  • Western Estates: X, Y, and Z.

• Red Deer Density Classes: Estates were classified based on census data (Red Deer Commission 1995 and 1999 counts):

  • Low Density: Estates A ($13.7\,deer/km^2$ in 1995), B ($8.3\,deer/km^2$ in 1995), and X ($5.0\,deer/km^2$ in 1999).

  • Moderate Density: Estate Z ($13.9\,deer/km^2$ in 1999).

  • High Density: Estates C ($30.9\,deer/km^2$ in 1995) and Y ($36.1\,deer/km^2$ in 1995).

• Other Herbivores: Domestic sheep (present on estates A, C, and part of B), mountain hares ($Lepus timidus$), and European rabbits ($Oryctolagus cuniculus$).

Hierarchical Sampling Design

• Spatial Scaling: The study utilized a nested hierarchy across five scales:

  1. Estate Level: The overarching management unit.

  2. Location Level: Up to four different locations per estate, with each location containing four square types in close proximity (< 2\,km).

  3. Local Scale (Square): $0.25\,km^2$ squares based on the Ordnance Survey National Grid of Great Britain.

  4. Neighborhood Scale: The 8 surrounding $0.25\,km^2$ squares (totaling $2.25\,km^2$).

  5. Transect Scale: $10\,m$ transects running perpendicular to grass-patch edges.

  6. Quadrat Scale: Individual sampling points along each transect.

• Stratification of Squares: Squares were selected if they contained > 50\% heather cover. They were classified using Land Cover Map (LCM) data based on grass proportion:

  • Low Local Grass: $1\%$ to $8\%$ grass.

  • High Local Grass: > 12\% grass.

  • Neighborhood Classification: The same thresholds applied to the mean proportion of grass in the eight surrounding squares.

Vegetation and Herbivore Sampling Procedures

• Heather Utilization Measurement: Utilization was recorded at distances of $0$, $0.25$, $0.5$, $0.75$, $1$, $1.5$, $2$, $3$, $4$, $5$, $7$, and $10\,m$ from the grass-patch edge. Shoots were classified into four browse levels:

  • $N_0$: Not browsed.

  • $N_1$: Browsed to less than half its length.

  • $N_2$: Browsed to more than half its length.

  • $N_3$: Browsed into the previous season's growth.

• Utilization Formula:     $\text{percentage utilization} = 100 \times \frac{0.3N_1 + 0.8N_2 + 1.2N_3}{N_0 + N_1 + N_2 + N_3}$

• Vegetation Characteristics:

  • Heather Canopy Height: Measured using a sward stick at each quadrat.

  • Heather Growth Stage: Categorized as pioneer, building, mature, or degenerate (Watt 1955).

  • Grass Patch Species: Classified as bent/fescue ($Agrostis/Festuca$ spp.), purple moor grass ($Molinia caerulea$), or mat grass ($Nardus stricta$).

  • Patch Size Class: Estimated by length and width, ranging from < 0.001\,ha to > 0.1\,ha.

• Herbivore Indices: Presence/absence of dung for deer, sheep, hare, and rabbit was recorded within $5\,m$ of the transect origin in spring 1999.

Statistical Analysis and Data Modeling

• Models Used: Linear mixed models (LMM) and generalized linear mixed models (GLMM) were fitted using SAS PROC MIXED and the GLIMMIX macro.

• Transformations: Utilization ($U$) was transformed as $\log_e(U + 1)$. Distance ($d$) was transformed as $\log_e((\text{d} + 25) / 25)$.

• Variance Components: Variance was Partitioned across the hierarchical scales (Estate, Location, Square, Transect, Quadrat). Residual maximum likelihood (REML) was used to fit random effects.

• Standardization: To account for topography and age, heather heights were analyzed as a log ratio of the mean height in the "edge zone" ($0-50\,cm$) versus the "distant zone" ($5-10\,m$).

Patterns of Heather Utilization and Spatial Scales

• Distance Effect: There was a sharp, inverse square-root decline in heather utilization with increasing distance from the grass-patch edge. Mean utilization at the edge ranged from $16.5\%$ to $25.4\%$, while at $5\,m$ distance, it dropped to between $3.5\%$ and $11.5\%$.

• Small-Scale Dominance: The greatest contribution to variance in utilization occurred at the smallest spatial scale (quadrat level). This indicates that herbivores interact with vegetation primarily at the scale of individual foraging decisions move by move.

• Intermediate Scale Impact: The proportion of grass in the local landscape ($0.25\,km^2$) and the neighborhood ($2.25\,km^2$) significantly increased heather utilization in both the edge and distant zones.

• Landscape Scale Limitations: Large-scale parameters, such as average deer density per estate, were poor predictors of local utilization levels. High variability between locations within the same estate suggests that land managers cannot rely on crude herbivore counts to predict grazing impact hotspots.

Effects of Vegetation Composition and Patch Characteristics

• Grass Species Preference Hierarchy: Heather utilization was significantly affected by the dominant grass species in the adjacent patch within the edge zone ($0-50\,cm$):

  1. Highest Utilization: Around $Agrostis/Festuca$ patches (most preferred by herbivores).

  2. Intermediate Utilization: Around $Nardus$ patches.

  3. Lowest Utilization: Around $Molinia$ patches (least preferred).

• Patch Size: Grass-patch size did not have a significant effect on heather utilization in this free-ranging study, contrasting with some previous paddock-based experiments.

• Heather Growth Stage: Utilization was generally higher in "building" heather compared to "mature" heather at both local and neighborhood scales, though some of this effect was attributed to observer subjectivity.

Impacts on Heather Canopy Structure

• Height Gradient: Heather was consistently shorter near grass-patch edges ($1-2\,m$ zone) compared to areas further away. Around $Agrostis/Festuca$ patches, heather was on average $32\%$ shorter at the edge than in the distant zone.

• Cumulative Effects: The height difference reflects long-term utilization patterns. Proportional height differences between zones increased with the age/growth phase of the heather, suggesting a cumulative reduction in annual height increment due to grazing as the heather matures.

• Competitive Implication: Lower canopy height near grass edges reduces the competitive advantage heather normally holds over grasses (which rely on shade-avoidance), potentially facilitating a positive-feedback loop where grass expands at the expense of heather.

Discussion and Ecological Implications

• Mechanism of Apparent Competition: The study confirms that the presence of high-quality food (grass) "spills over" grazing pressure onto low-quality neighbors (heather). This is a definitive example of apparent competition mediated by herbivore behavior.

• Management Challenges: Moorlands ($Calluna$-dominated) are declining globally due to grazing, pollutants, and forestry. Predicting damage requires understanding at which scales herbivores select resources. Since impacts are localized, management based on estate-wide herbivore densities is often insufficient.

• Potential Feedback Loops: High shoot consumption (> 25\%) leads to heather cover decline. In hotspots where grazing exceeds this threshold, grass gradually replaces heather, further attracting herbivores and creating a self-reinforcing cycle of vegetation change.

• Stability of Heather: Large areas of heather persist because much of the landscape remains beyond the $5\,m$ "danger zone" of grass patches. Additionally, acidic, low-nutrient soils in many moorlands naturally favor heather growth over most coexisting grasses.