Harsh conditions (e.g., mortality and stress) reduce population growth rates directly; secondarily, they may reduce the intensity of interactions between organisms. Near-exclusive focus on the secondary effect of these forms of harshness has led ecologists to believe that they reduce the importance of ecological interactions, such as competition, and favor coexistence of even ecologically very similar species. By examining both the costs and the benefits, we show that harshness alone does not lessen the importance of species interactions or limit their role in community structure. Species coexistence requires niche differences, and harshness does not in itself make coexistence more likely. Fluctuations in environmental conditions (e.g., disturbance, seasonal change, and weather variation) also have been regarded as decreasing species interactions and favoring coexistence, but we argue that coexistence can only be favored when fluctuations create spatial or temporal niche opportunities. We argue that important diversity-promoting roles for harsh and fluctuating conditions depend on deviations from the assumptions of additive effects and linear dependencies most commonly found in ecological models. Such considerations imply strong roles for species interactions in the diversity of a community.

Arid environments are characterized by limited and variable rainfall that supplies resources in pulses. Resource pulsing is a special form of environmental variation, and the general theory of coexistence in variable environments suggests specific mechanisms by which rainfall variability might contribute to the maintenance of high species diversity in arid ecosystems. In this review, we discuss physiological, morphological, and life-history traits that facilitate plant survival and growth in strongly water-limited variable environments, outlining how species differences in these traits may promote diversity. Our analysis emphasizes that the variability of pulsed environments does not reduce the importance of species interactions in structuring communities, but instead provides axes of ecological differentiation between species that facilitate their coexistence. Pulses of rainfall also influence higher trophic levels and entire food webs. Better understanding of how rainfall affects the diversity, species composition, and dynamics of arid environments can contribute to solving environmental problems stemming from land use and global climate change.

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Vegetation and soils were sampled in 22 old fields ranging in age from 1 to 56 yr since abandonment. Soil nitrogen concentration increased significantly with field age. Vegetation cover, total aboveground plant biomass, and litter cover increased significantly with soil nitrogen. Light penetration to the soil surface was negatively correlated with total plant biomass. Field age and soil nitrogen concentration were used as independent variables in simple regression and partial correlation analyses to determine the relative importance of such time—dependent processes as dispersals vs. the availability of a limiting resource (nitrogen) as predictors of patterns in species richness or the abundance of various plant groups. Species richness per field and within—field heterogeneity in species composition increased with field age. Local species richness decreased with increasing soil nitrogen. Cover of annuals and introduced species decreased with field age and nitrogen; however, annuals contributed an important part of total vegetative cover even n 25—yr—old fields. Cover of perennials and woody species increased with soil nitrogen and field age. Although the fields were bordered by woods, woody species contributed @<15% cover even in the oldest fields. For several plant groups the relationship between cover and soil nitrogen within individual fields was the opposite of that among all fields. These patterns suggest that while soil nitrogen is an important determinant of local species composition and abundance, dispersal and colonization, which are dependent on field age, determine which species are present in a field.