Gradients In Species Composition Research Articles

Environmental correlates of vegetation patterns and species richness in the northern Grampians, Victoria

Abstract Plant species cover‐abundance and density data were collected for 94 sample plots across a gradient from rocky uplands to sandy outwash plains in the northern part of Grampians (Gariwerd) National Park in western Victoria. Detrended correspondence analysis (DCA) was used to identify dominant gradients in species composition. A range of static (e.g. substrate type, soil depth, microclimate indicators) and dynamic (e.g. elapsed time since last fire) environmental variables were measured. Correlations were sought between these variables and vegetation patterns including those for richness (R) and Shannon‐Weiner diversity (H′).The dominant gradient of vegetation change identified by DCA separated rocky sites and sites near ephemeral streams, from well‐drained, sandy sites. Secondary gradients identified time since last fire as important for sandy sites, and altitude and aspect‐related microclimate for rocky sites. Diversity was highest in the first 2 years after fire but showed no further decline in older sites. Overall, R and H' were negatively correlated with soil nutrient concentrations. On sandy sites R was high, but was low on rocky sites and near streams. Within the rocky sites, R was highest on cool, moist south and east slopes, and lowest on hot, dry north and west slopes. Explanations of diversity patterns based on inhibition of competitive exclusion due to stress and recurrent disturbance best fit the results presented here.

Neutral models for the spatial distribution of organisms: implications for paleoenvironmental interpretation

A fundamental assumption of paleoecology is that spatial variability in species distributions and associations reflect underlying spatial variability in physical environmental parameters. For example, gradients in species composition are usually interpreted as relating to gradients in variables such as water depth, temperature, or soil moisture. Numerous examples support the idea that when an environmental gradient is known to exist, there is a corresponding non-random community gradient. We suggest, however, that the reverse may not necessarily be true; i.e., a community gradient may exist even in the absence of an underlying physical gradient.We have developed a series of models for the spatial distribution of organisms on landscapes (spatially heterogenous areas ranging in size from hectares to many square kilometers). In these models, a landscape is represented by a two-dimensional lattice, where each node of the lattice is a habitat site. Each habitat site is randomly assigned to one of a set of multiple habitat types (generally 3-6). A map of the lattice would thus show a random distribution of habitat types.Habitat sites are competed for by sessile organisms that complete their life cycle in one time step (e.g., annual plants). “Seeds” are spread to a site from its eight next nearest neighbors. Each species' seeds successfully germinate on only a subset of the available habitat types (its “niche”), with one or more types being optimal (highest probability of germination). These subsets can overlap, so that different species can potentially occupy the same site. The probability of a given species taking over a site depends on the number of neighboring sites it occupies and whether the site is optimal. A site can be taken over by a suboptimal species if it locally more abundant.Each organism's niche directly controls its ability to spread (“percolate”) through the landscape. Under the next-nearest neighbor rule, a species must be able to live on >40% of all possible sites in order to spread. Competition, by reducing the number of available sites, may prevent a species from reaching all habitable sites, even those that are optimal.In one model, five habitat types are randomly distributed on a 201 × 201 matrix. Five species, each of which has an optimal habitat corresponding to one of the five habitats but can live on two additional habitats, are randomly introduced at one edge of the map and are allowed to spread. When the resulting distribution maps are examined, preferred species associations and gradients are observed, although there is no underlying habitat gradient. Instead, the observed patterns result from a combination of biotic interactions and historical contingencies, such as founder effects.This and related models show how random processes can produce biotic patterns in the absence of environmental patterns. They provide appropriate neutral models for the interpretation of paleoenvironmental patterns based solely on species distributions.

Gradient Analysis of Latitudinal Variation in Southern Rocky Mountain Forests

Vegetation samples collected near Taos, New Mexico, Wet Mountain Valley, Colorado, and Rocky Mountain National Park, Colorado, were used to analyse latitudinal variation in Southern Rocky Mountain forests. Objectives were: (1) to examine latitudinal shifts in species and community positions with respect to local environmental gradients; (2) to compare between regions the factors correlated with local gradients in species composition; and (3) to illustrate the power of gradient-based methods for the analysis of large-scale geographic variation in vegetation. Dominant compositional gradients at the three localities were related to elevation, site moisture status, and potential solar radiation. The average elevation of floristically similar stands declined with increasing latitude between Taos and Wet Mountain Valley (-46.5 m per degree latitude), and between Wet Mountain Valley and Rocky Mountain National Park (-106.6 m per degree latitude). Elevation shifts between localities varied with the reference stand's position along the elevation and moisture gradients. Smaller elevation shifts between localities were found for widespread species. Species were, in general, most widely distributed along the elevation gradient at Rocky Mountain National Park and along the moisture gradient at Taos. High-elevation Picea engelmanniilAbies lasiocaipa forests were comparatively similar in the three regions, although the number of recognizable communities per region increased with latitude. In contrast, there was considerable latitudinal change in the composition of lowelevation forests. Several commonly accepted geographic patterns showed considerable complexity when examined with change in latitude. Although species richness (species per 0.1 ha) is generally considered to decline with increasing latitude, highest species richness was found at Wet Mountain Valley, the middle locality on the latitudinal sequence. However, relative species richness patterns along the elevation and moisture gradients were similar for all three localities.

Fine root production of astringent phenolics.

Chemical quality of fine roots (<1 mm diameter) was determined over a gradient of species composition in the Mixed Mesophytic Forest Region. Ash-free nitrogen, calcium, and phosphorus concentrations of roots declined by 49, 41, and 72%, respectively, over a gradient of increasing soil acidity (pH 5.3 to 4.7). Lignin concentration was unrelated to either the vegetation gradient or any of the soil changes it encompassed; however, astringent phenolics increased by 275% over the same gradient. Trends in the chemical constituency of fine roots suggest that the production of phenolics in below-ground plant parts is increased on nutrient-poor sites. This response is best related to changes in species composition, especially increasing importancy of Quercus spp.

Selecting representative reserve systems in remote areas: A case study in the Nullarbor region, Australia

We present a procedure for the selection of a network of reserves representing the biological diversity of a large and biologically poorly known region. The quantitative analysis gives equal weighting to a wide array of different species: mobile and sessile, long-lived and ephemeral, heterothermic and homeothermic, etc. Sampling was based on quadrats that were positioned through the region using a stratified random strategy. This provided a presence-absence matrix of the species composition at each quadrat. Numerical pattern analysis was used to identify 14 species assemblages and to re-order the sites and species in the data matrix. The geographic pattern of each assemblage throughout the region was described by contouring assemblage richness, the isolines being the proportion of the number of species in each assemblage. The data matrix was re-examined. Some assemblages exhibit several gradients in species composition. For example, a 0·1 isoline in the east may represent a different 10% of species than a 0·1 isoline in the west. Other assemblages exhibit only a single gradient in species composition. The results were used to select the optimum positions of reserves needed to represent the compositional diversity of each of the 14 species assemblages. Limited field checking confirmed the predicted isolines in assemblage compositional richness to a satisfactory extent.

The possible significance of long-range dispersal for the biogeography of seaweeds

Indirect evidence of the existence of long-distance dispersal of seaweeds is provided by the fact that mid-oceanic islands of volcanic origin are inhabited by well-developed seaweed floras which could reach these islands only overseas from continental donor areas. For instance, the flora of Tristan da Cunha (S. Atlantic Ocean) was established by long-distance dispersal in less than 1 million years (the approximate age of the island); the seaweed flora of the Faeroes (N. Atlantic Ocean) could be constituted in less than 10,000 years (the end of the Pleistocene ice cover of these islands). There is no evidence for either supporting or discounting the possible role of planktonic stages of seaweeds (spores, propagules, zygotes) in the long-distance dispersal of seaweeds. There is, however, some evidence of long-distance dispersal as floating plants, or as plants attached to floating objects (including floating algae). There are a few examples of “artificial” long-range dispersal by man (possibly on ship hulls, oysters, in ballast water). Long-range dispersal of seaweeds does exist, but it is an exception rather than the rule. If it were the rule, the world’s seaweed floras would show similar latitudinal gradients in species composition in the oceans and on both hemispheres. This is, however, not the case.

Lichens in Relation to the Four Corners Power Plant in New Mexico

In the region around the Four Corners Power Plant of New Mexico, 159 lichen species in 40 genera are recorded. Although the power plant has operated over 10 years, no marked gradient in species composition or lichen cover data were evident in relation to the power plant. Eight desert lichen species are shown to be sensitive to 0.5 ppm SO/sub 2/ when moist but are insensitive when dry. Because of the aridity of the climate the lichens are probably rarely susceptible to SO/sub 2/ injury. In addition the lack of pattern probably reflects the dominance of resistant crustose growth forms in the lichen flora and the fact that ground level SO/sub 2/ concentrations are relatively low even though SO/sub 2/ emissions are relatively high.

Species Diversity and Longitudinal Succession in Stream Fishes

A quantitative survey was made of the distribution and abundance of fishes in Owego Creek, New York. Four of the five headwaters species occurred throughout the area and two of these species dominated the fauna in all areas. Thirty—one species were found in the area. Succession took the form of additions to the headwaters assemblage and replacement was of minor importance. Regression analyses show that the number of species in any area was correlated most strongly with stream depth although an effect of position was also significant. Species diversity (information theoretic) was independent of position and depended on depth alone. Behavioral observations support the importance of the depth factor.