Can environmental constraints determine random patterns of plant species co-occurrence?

Plant community ecologists use the null model approach to infer assembly processes from observed patterns of species co-occurrence. In about a third of published studies, the null hypothesis of random assembly cannot be rejected. When this occurs, plant ecologists interpret that the observed random pattern is not environmentally constrained - but probably generated by stochastic processes. The null model approach (using the C-score and the discrepancy index) was used to test for random assembly under two simulation algorithms. Logistic regression, distance-based redundancy analysis, and constrained ordination were used to test for environmental determinism (species segregation along environmental gradients or turnover and species aggregation). This article introduces an environmentally determined community of alpine hydrophytes that presents itself as randomly assembled. The pathway through which the random pattern arises in this community is suggested to be as follows: Two simultaneous environmental processes, one leading to species aggregation and the other leading to species segregation, concurrently generate the observed pattern, which results to be neither aggregated nor segregated - but random. A simulation study supports this suggestion. Although apparently simple, the null model approach seems to assume that a single ecological factor prevails or that if several factors decisively influence the community, then they all exert their influence in the same direction, generating either aggregation or segregation. As these assumptions are unlikely to hold in most cases and assembly processes cannot be inferred from random patterns, we would like to propose plant ecologists to investigate specifically the ecological processes responsible for observed random patterns, instead of trying to infer processes from patterns

Local distribution patterns of Opuntia echios echios, Bursera graveolens and Scalesia crockeri on Santa Cruz Island, Galapagos

We mapped stems of three plant species in a 2.36 ha plot in the arid zone near the coast of eastern Santa Cruz Island, Galapagos, Ecuador, to determine factors influencing their local distribution. The three species were Opuntia echios var. echios (Cactaceae), a large cactus, Bursera graveolens (Burseraceae), a small tree that dominates dry woodland near the coast, and the shrub Scalesia crockeri (Asteraceae). In our plot, Opuntia was most abundant near the coast, while Bursera and Scalesia increased in density inland and with increased relief. Scalesia also increased in density with increases in Bursera and decreases in other woody plants and was most abundant 200–250 m from the coast. Both Opuntia and Bursera were clumped in the plot as a whole but selected stem size classes were randomly dispersed within homogeneous portions of the sample area. CDF Contribution Number 1012.

Geographical and morphological variation within and between colour phases inCoris julis(L. 1758), a protogynous marine fish

The possible differences between sexes in patterns of morphological variation in geographical space have been explored only in gonochorist freshwater species. We explored patterns of body shape variation in geographical space in a marine sequential hermaphrodite species, Coris julis (L. 1758), analyzing variation both within and between colour phases, through the use of geometric morphometrics and spatially-explicit statistical analyses. We also tested for the association of body shape with two environmental variables: temperature and chlorophyll a concentration, as obtained from time-series of satellite-derived data. Both colour phases showed a significant morphological variation in geographical space and patterns of variation divergent between phases. Although the morphological variation was qualitatively similar, individuals in the initial colour phase showed a more marked variation than individuals in the terminal phase. Body shape showed a weak but significant correlation with environmental variables, which was more pronounced in primary specimens.