The present project addresses the question: What regulates the number of species in an ecological context? This issue has two aspects:

1. The question of the number of species that can co-exist in a given area.
2. How these particular species have been able to disperse in to the area.

Global and local processes

It is well-known that the number of vascular plant species declines from the equator towards the poles, even though the structuring processes behind this pattern are still a subject of debate (Gaston et al. 1998). Several large scale processes have been proposed to explain this pattern of biogeographical history after the last glaciation, e.g., productivity, area effects, and evolutionary rate under the constraints of low temperature. However, many of these proposed processes change along the same latitudinal gradient (Rodhe 1992) and the importance of each one is difficult to address separately. In addition, this does not answer the question of what regulates the number of species on a smaller scale in Arctic environments. Local processes, such as competition (or rather the lack of competition) have been put forward as important factors determining the number of species in the Arctic (Young 1971, Stevens 1989; but see Rodhe 1992). If competition, as proposed, is an important structuring factor for the number of species, and if competition intensity decreases with latitude, we would expect to find changes in the pattern of species richness along this gradient on a local scale.

Species richness and scale

The number of species found depends on the scale of the area investigated. The larger the area the more individuals there are, and more individuals means greater species richness in a given area. The greater the species number present, the greater the probability of obtaining high species diversity in the sampling. The processes affecting species richness may also differ between spatial scales. Competition, for example (i.e., the interaction between individuals), may be most important on smaller scales, whereas other processes such as evolution or the long distance dispersal of some species may be more important on larger ones.

Longitudinal patterns

Besides the change in species richness latitudinally, there might also be a change along a longitudinal gradient. If such a gradient exists in the Canadian Arctic, it may be explained by climatic differences between maritime and continental climate and/or differences in biogeographical history (e. g. glaciation history). Another important factor may be isolation from larger land areas in combination with differences in dispersive abilities among species.

Our study

The aim of this study is to contribute to the discussion about local and regional processes as well as to the debate on species richness along a latitude gradient in the Arctic. We have made inventories of plant species richness on different scales in 13 localities in the Canadian Arctic and it will be related to both latitude and longitude. A comparison of how it is distributed along these gradients on different scales will say something about the structuring processes in Arctic plant communities. We will also look at how species richness is distributed within different areas.

We have inventoried species richness at a total of 13 sites in the Canadian Arctic, between 62° and 78.6°N (the magnetic North Pole) and between 66.5°W in the eastern Canadian Arctic and Ivvavik at 139°W in the west, near the border with Alaska. Inventories have been focused on mesic plant communities at all sites, since this is a dominant habitat in the Canadian Arctic. In addition to the mesic plots, dry habitats were inventoried in the same manner at five of the sites. Inventories were conducted in ten 25 dm² quadrates and twenty 1 dm² quadrates randomized within a 20×20 m grid at each site. All species have been noted and the number of individuals, vegetative individuals, shoots, or percentage of cover have been counted/estimated. The actual methods employed for this work depended on the growth habits of the different species. The number of individuals is, as mentioned above, important when dealing with species richness.

The total number of species in the 20 by 20m area and the total number of species in the immediate vicinity (of the site) have been recorded in ca-operation with other activities in the biodiversity theme.

Species richness will be analysed both in relation to latitude and to longitude. If possible we will also correlate richness with climatic data (e.g. precipitation, annual mean temperature, etc.). Local species richness will be correlated with the regional species pool (species richness on a larger scale). Several studies have shown that local diversity is strongly dependent on the regional species pool (e.g., Pärtel et al. 1996).

Preliminary results

Some preliminary results from the inventoried 25 dm² -squares show that there are significant differences in the mean species richness as well as in the variation of species richness between localities.

This means species richness is not related to latitude. Two localities at 69°N and another at 75°N seem to be by far the richest. These localities have also the largest variation of species richness within the sites. Species richness on this scale is probably dependent on the richness in the region as a whole. We will continue to work with species richness quadrats on other scales, both smaller and larger in the coming analysis and we will also make corrections for number of individuals and biomass. The only conclusion that can be drawn from these preliminary results is that there is no clear linear relation between latitude and species richness on a very small scale.