Understanding the causes, patterns, and consequences of biodiversity is critical to protecting biodiversity and to understanding the consequences of changing diversity. Two important influences on arctic species diversity are energy availability (summer temperatures) and habitat variability (Rannie 1986, Edlund and Alt 1989, Walker 1995). These influences operate at different scales, with summer temperatures limiting the number of species able to survive in a region, and habitat variability determining the patterns of species diversity within regions of similar energy resources.

This study examines the correlation between species richness (a measure of plant diversity), mean July temperatures, and landscape heterogeneity in order to determine the relative influence of temperature and the physical landscape on an observable gradient in species richness along an arctic river. It will help in understanding how species diversity varies with separate components of landscape heterogeneity within a riparian ecosystem.

The area under investigation is the Hood River, which runs for 185 km through the Northwest Territories of Canada (Fig. 1). The Hood flows east and north through low tundra vegetation of dwarf birch, willow and ericacious shrubs, sedge meadows and barren rock lichen fields. It provides a good area to test hypotheses regarding the correlation of species richness and landscape and environmental factors as it has had little anthropogenic influence, a relatively simple flora, and a distinct gradient in vascular plant species richness. The study focused on the riparian zone (that area which shows evidence of past or present influence by the river) because approximately 80% of the regional diversity is concentrated in that area.

Sampling was done at seventeen sites, evenly spaced along the river. Numbers of vascular plant species, summer temperatures and landscape heterogeneity were measured at each site. Plant species richness was recorded by noting species presence within a set of long narrow plots at each study site. Plots were randomly located within the riparian zone and provided a measure of alpha or point diversity of each site. July air temperatures were recorded hourly at stations set at a uniform height above ground, at similar distances from the river, and in similar landform and vegetation types. Landscape heterogeneity was measured as the number of states of a set of environmental variables found at each intensive study site. These variables included the number of types of substrates, moisture regimes, slopes, aspects, surface ages, and range of soil pH. An index of environmental heterogeneity characterized the site heterogeneity.

The vascular flora for the study area includes 201 species (103 forbs, 71 grasses and sedges, 19 deciduous woody shrubs and 8 evergreen woody shrubs). Species richness at each site ranged from 70 to 109 vascular plant species. Sites with the lowest species richness were those in the upper portions of the river, with diversity generally increasing towards the coast. Vascular species diversity is strongly correlated with mean July temperature in many arctic sites; for example Rannie (1986) recorded a loss of twenty-five species for each 1 degree C mean July temperature decrease. Therefore we postulated that the gradient in species richness found along the Hood might be due to a temperature gradient associated with decreasing elevation from the headwaters to the coast (356 meters). Mean July temperatures measured along the Hood ranged from 9.6 to 10.3 degrees C. Although there was a slight trend towards warmer temperatures with decreasing elevation, mean July temperatures were not correlated with species richness at individual sites. Variation in species richness from site to site is strongly correlated with measures of environmental heterogeneity (Fig. 2). Range in pH value is the most significant component of the index of environmental heterogeneity in terms of predicting species richness at a site (Fig. 3). The observed variation in species richness along the river is correlated with both an increase in the range of soil pH and with a trend towards higher pH as the river nears the coast. The primary cause of higher soil pH is the presence of uplifted marine sediments occurring at elevations of less than 150 meters. This abrupt transition from more acidic glacial deposits to less acidic uplifted marine deposits is characteristic of many rivers in this region of Canada.

Figure 2. Correlation of species richness and an index of environmental heterogeneity (EH). The index ranges from 0 to 1 with maximum heterogeneity at 1. EH incorporates measures of six environmental variables at each site.

Figure 3. Correlation of species richness and range of pH values at each site. The x-axis is the number of pH classes found at each site. Classes include pH ranges of 4.1 to 5.0, 5.1 to 5.5, 5.6Ü6.0, 6.1 to 6.5, 6.6 to 7.0, 7.1 to 7.5, 7.6 to 8.0, 8.1 to 8.5, and 8.6 to 9.0.

EDLUND, S.A. AND B.T. ALT. 1989. Regional congruence of vegetation and summer climate patterns in the Queen Elizabeth Islands, Northwest Territories, Canada. Arctic. 42:3-23.

RANNIE, W.F. 1986. Summer air temperature and number of vascular species in Arctic Canada. Arctic. 39:133-137.

WALKER, M.D. 1995. Patterns and causes of arctic plant community diversity. In Chapin, F.S. and C. Korner, eds. Arctic and Alpine Biodiversity: Patterns, Causes, and Ecosystem Consequences. Springer-Verlag, Ecological Studies, 113:1-18.