Effects Of Plant Positive Interactions On Species-area Relationships And Species-time Relationships Along An Environmental Gradient

The species-area relationship(SAR) and the species-time relationship(STR) are two of the few general laws in ecology, which are important for both basic and applied aspects, such as biodiversity conservation. Much work has been done to explore the shapes of SAR and the underlying mechanisms, however, relatively few studies were designed for STR due to the difficulty in gaining high-quality long-term data. In addition, it is ubiquitous that competition and positive interactions simultaneously shape the community composition and structure, to our knowledge, however no studies have investigated the effect of facilitation on SAR and STR. Using an individual-based model incorporating the main processes that structure a community, we tested the general patterns of both SAR and STR along an environmental gradient and assessed the role of facilitation in three model scenarios: CM without facilitation; CFMU, a unimodal relationship between facilitation and the environmental severity; and CFML, a positively linear relationship between facilitation and environmental severity.In three model scenarios, we found that STR displayed similar patterns with SAR. Our results showed that facilitation did influence the patterns of SAR and STR, and the effects of facilitation on SAR and STR depended on the position of the community considered along the environmental gradient and the model employed. Compared with CM, the inclusion of positive interactions(i.e. CFMU and CFML) resulted in higher spatial and temporal scaling exponents at intermediate environmental stress gradients(such as s=0.4 and 0.5); at medium to high severity gradients(such as s=0.7 and 0.8), facilitation decreased the values of the spatial and temporal scaling exponents. However, at low to medium severity levels(such as s=0.1 and 0.2), facilitation had negligible effect on the spatial and temporal scaling exponents. At the ends of the gradients, the spatial and temporal scaling exponents in three scenarios were identical except for the right endpoint in CFML. The study contributes to the understanding of the effects of the biotic(facilitation and competition) and abiotic factors(environmental gradients) on SAR and STR.