Adding fluctuations to models of resource competition provides one solution to Hutchinson's paradox. Fundamental to fluctuation dependent mechanisms of coexistence are differential species' responses to environmental fluctuations through time. The covariance between the environment and competition this generates leads to compensatory population dynamics, now thought to be important for community stability. We tested this theory experimentally using freshwater diatoms. We measured the growth of Cyclotella pseudostelligera and Fragilaria crotonensis over a range of temperatures. Cyclotella showed a higher growth rate and higher silicate assimilation rate between 6°C and 18°C, while Fragilaria showed greater competitive ability above 18°C. These results were used to parameterize a resource competition model. Numerical simulation of this model predicted stable coexistence and compensatory dynamics under fluctuating but not constant temperature. A long‐term competition experiment with Cyclotella and Fragilaria confirmed the predictions of the model. These results demonstrate the role abiotic fluctuations may play in maintaining the diversity of natural communities.

B. 2004. Population and community variability in randomly fluctuating environments. Á/ Oikos 106: 105 Á/116.The prediction that environmental fluctuations may destabilise populations and yet stabilise aggregate community properties has remained largely untested. We examined population and community stability under constant and fluctuating temperatures in simple planktonic assemblages of differing algal richness. Temperature dependent resource competition produced a highly asymmetric community structure where algal community biomass was dominated by one species. For a given level of species richness, temperature fluctuations induced lower community covariance and thus stabilised community biomass. However, increasing algal species richness increased the variability of population abundance and growth rates, as well as population and community variability. Consumer dynamics were directly destabilised by environmental fluctuations. These results confirm recent theoretical studies suggesting a stabilising effect of environmental fluctuations at the community level. However, they also support the theoretical prediction that increasing species richness may be of limited value for community stability, most especially in asymmetric communities, when competition directly affects population variability.

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Non‐vascular plant distribution patterns were examined in three microhabitats along an altitudinal gradient on a recent lava flow of the Piton de la Fournaise volcano (La Réunion, Mascarene archipelago). The uniform nature of the lava flow provides an excellent system to study the relationship between altitude and species diversity and distribution, and at the same time avoiding confusing multiple effects of substrate and vegetation heterogeneity. Non‐vascular plants were surveyed with quadrats within an altitudinal range from 250 m to 850 m a.s.l. Fine‐scale variations in bryophyte communities between three ecological microhabitats (the ground and on the rachises of two fern species) were investigated. Three specific questions were addressed: (1) What is the species diversity of bryophyte communities on a 19‐year‐old lava flow? (2) How does altitude influence the diversity and distribution of bryophytes on a lava flow? (3) Does microhabitat variation control bryophyte diversity? In our study, bryophyte diversity increased with altitude. Unexpectedly, species richness was very high; 70 species of bryophytes were recorded including nine new records for the island. Diversity was also controlled by ecological microhabitats. Bryophyte species were structured into six categories according to altitude and microhabitat preferences. Results suggested that the high diversity of these cryptic organisms on this lava flow is fostered in part by their host substrate and their adaptative strategies on new substrates. On a broader scale, it was concluded that lava flows as primary mineral environments are important to conserve, as they support a high diversity of pioneer organisms that constitute the early stages of the development of La Réunion's remnant lowland rainforest.