Academic journal article The American Midland Naturalist

Evaluating Relationships between Spatial Heterogeneity and the Biotic and Abiotic Environments

Academic journal article The American Midland Naturalist

Evaluating Relationships between Spatial Heterogeneity and the Biotic and Abiotic Environments

Article excerpt

ABSTRACT.-

While most studies of heterogeneity have focused on describing patterns of species or communities, few have focused on the relationships between biotic and abiotic environmental landscape-level gradients. Our study was designed to determine relationships between grazing (heavy, moderate, ungrazed), topographic position (upland vs. riparian), vegetation structure and the thermal environment (i.e., soil-surface temperatures) and determine the influence on landscape patterns of heterogeneity. Biotic and abiotic patterns of heterogeneity were evaluated by establishing 200-m transects that were centered on and perpendicular to a riparian zone so that spatial patterns of variability could be determined along each transect which transcends the maximum level of landscape heterogeneity. Vegetation cover and structure and soil-surface temperatures were recorded at 1-m intervals along the transect. Bare ground increased and leaf litter, grass cover, vegetation heights and angle of obstruction decreased with grazing intensity. However, mean soil-surface temperatures did not differ between grazing treatments. Tree canopy cover associated with riparian areas generally reduced soil-surface temperatures 20 C below that of upland temperatures. In fact, 96% of observations of riparian soil-surface temperature were ≤ 39 C, while 94% of upland soil-surface temperatures were ≥ 40 C regardless of grazing intensity. Vegetation characteristics and soil-surface temperatures were correlated (P < 0.05), but correlation coefficients were small because soil-surface temperature was highly variable. Grazing influenced patterns of landscape heterogeneity, but effects were inconsistent among biotic and abiotic variables. Although grazing had little influence on moderating mean soil-surface temperatures, results suggest that grazing intensity influences thermal heterogeneity at a variety of spatial scales. For instance, thermal heterogeneity (in moderately grazed treatments) is highest at smaller (lag distances ≤ 20 m) and larger (lag distances ≥ 48 m) spatial scales but was lowest at moderate scales (lag distances 22-45 m) . For all variables, other than soil-surface temperature and forb cover, semi-variances of moderately grazed sites generally lie intermediate between heavy and ungrazed sites. Nearly all ungrazed vegetation characteristics, except leaf litter, fit a spherical model that reached a sill at a lag distance ≤ 20 m and became spatially independent thereafter, while heavily and moderately grazed sites typically fit an exponential model, indicating a high degree of continuity. Patterns of thermal variability (on uplands) are not related directly to any one vegetation variable, hence, landscape patterns based on vegetation parameters alone are of limited value since patterns of thermal variability are effected by the integration of vegetation and environmental variables within the ecosystem.

INTRODUCTION

Many ecologists are beginning to recognize the role of heterogeneity in ecological systems by acknowledging its influence on population dynamics and biodiversity (MacArthur and Pianka, 1966; Wiens, 1976; Turner and Gardner, 1991; Sarnelle et al, 1993). In fact, it has been suggested that heterogeneity is actually the root of biological diversity at all levels of ecological organization and should serve as the foundation for conservation and ecosystem management (Christensen, 1997; Ostfeld et al, 1997; Wiens, 1997). Concern regarding the role of spatial heterogeneity in influencing population and community dynamics has gained recent attention (Levin, 1992; Wiens, 1997; White and Walker, 1997; Sanderson and Harris, 2000; Fuhlendorf and Engle, 2001). Spatially discrete disturbance patterns are described historically as a shifting mosaic (Kay, 1998; Fuhlendorf and Engle, 2001), where the presence of alternative habitat types may have provided (1) complementary resources (i.e., food availability, cover from predators or climate) that improved habitat quality for some species and/or (2) unique habitat types capable of supporting a variety of species, both of which presumably enhance biodiversity. …

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