Although many studies have focused on the effects of the environment and area on local patterns of species richness, few studies have demonstrated how to reconcile the availability of more niches with smaller habitat areas in heterogeneous localities. Here, the environmental range that a species prefers was defined as a niche; a space was defined as an available space if the environmental range of the space matches a niche; and the metric ‘environmental range per unit space (ERUS)' was presented to describe the heterogeneity of localities. Because the spaces with stressful environmental ranges, outside the niche, increased with increasing heterogeneity, available spaces did not continue to indefinitely increase, but the proportion of available spaces in spaces was low at high heterogeneous localities. Consequently, the probability of species occurring in their respective available spaces was unimodal. Due to the presence of large spaces in homogenous localities and more spaces in heterogeneous localities, the interval distances between nonadjacent spaces were large in these localities. Together, the changes in the number and proportion of available spaces and distance between spaces determined the unimodal probability of species dispersing into their respective available spaces. Thus, the probability of species coexisting was unimodal because it is important for coexisting species to grow in their respective available spaces. The probability of population extinction increased with increasing heterogeneity because the available spaces became narrower. In this way, the ERUS strongly influence species richness: a unimodal richness along the heterogeneity gradient occurred in suitable, suboptimal and stressful environments and a unimodal algae richness occurred in a lake and river. These results challenge the viewpoint that richness increases with heterogeneity, providing information about the conservation of richness in very homogeneous and heterogeneous regions, and highlighting the importance of balancing the roles of environment and space in understanding and predicting richness.

Breakdown, hyphomycete and invertebrate colonization of beech (Fagus .sjlvcitic.u L.) and poplar (Pop~i/u.s rzigm x c,uroinc.rican Dode juinier) leaves were ctudied in two headwater streams with contrasting water chemistry. Breakdown of both species was slower in stream B, a headwater stream containing herbicides and low concentrations of metal ions. This was related to lower conidial production of aquatic hyphomycetes and lower biomass of G'arnmnr~i.~ sp., a shredder that dominated invertebrate assemblages on leaf packs. Differences in processing ratec, colonization by aquatic hyphomycetes, and shredder assemblages appeared to be related to stream water chemistry. Although the herbicide and metal concentrations in stream B were rather moderate, the activity of aquatic hyphomycetes and shredderc may have been adversely affected.

Although numerous studies have been conducted on niche and neutral theories to learn the drivers of species richness, few of them have demonstrated how to eliminate the influences of unlimited species numbers and absolute species equivalences which are contrary to many observations, and how to link space size with those drivers posited by the two theories. Here we present the environmental gradient per unit space metric that influences niche number, positively correlates with environmental filter and dispersal limitation and negatively correlates with space size occupied by each niche. This metric is incorporated with stochastic abundance and migration and equivalently average birth, death and dispersal of species. The simulation result of a model is consistent with the observation that a unimodal algal richness-water environmental gradient per unit space relationship. Therefore, the environmental gradient per unit space connecting deterministic and stochastic processes is an importantly measurable driver of species richness.