The relationship of environmental (ED) or habitat (HD) diversity of a landscape with its species richness (S) is of much interest. Based on underlying speciesarea (SA) curves, we show that the standard linear relationship lacks theoretical support. The SED relationship is the product of numerous SA curves per habitat and number of distinct habitats in the landscape. We recognize three basic SED patterns: convex, unimodal, concave, based on three fundamental SA curves: power, logarithmic and sigmoid. The preponderance of positive linear or absence of SED/HD relationships reported so far can be attributed to six causes. These include: only testing for linear relationships; limited data sets that exclude small, unique or isolated habitats; regressions are against non-causal variables; and use of biased data that have not been ground-truthed. Hump-backed SED curves should apply widely in regions with species-rich biota and need to resurrected, provided data collected are sufficiently comprehensive and accurate.
Eight species in the Namib Desert, South Africa were assessed for their leaf area ( A), thickness ( z), saturated ( Q) and dry mass, relative volume of air ( F a), water and dry mass, intrinsic water-use efficiency (based on δ 13C), and N, P and cation (Na+K) contents. As water-storage capacity is a function of Q v and z, this means Q/ A (= Q v • z) is an ideal index of succulence compared with specific-leaf-area and other indices that highlight mass rather than volume. Specific gravity ( ρ l) has a different relationship with the F a of sclero-mesophylls: rising ρ l infers decreasing air content is replaced by water rather than dry matter. The trend among succulent species, including Argentinian/Spanish added to our study, was Q/ A exceeding 1 mg water/mm 2 whose overall slope was ten times that for co-occurring sclerophyll-mesophyll species, and shows the futility of seeking a universal relationship among plants regarding their water-storing properties. (Na+K), N and P concentrations varied on a dry-matter, but not water-volume, basis. W i relationships were essentially functions of variations in z and increased metabolic efficiency. We conclude that z and Q v are keys to the special physiological properties of succulent leaves. Adding succulents would force many current monotonic relationships to dichotomize.
Thousands of plant species produce both extrafloral nectaries (EFNs) on their leaves and nutrient-rich appendages on their diaspores (elaiosomes). Although their individual ecology is well-known, any possible functional link between them has been ignored. Here, we recognized their co-presence in the shrub, Adenanthos cygnorum (Proteaceae), and studied their function and interaction. We observed that ants frequently visit both structures, seeds are attractive to vertebrate granivores but are released into a leafy cup from where they are harvested by ants and taken to their nests, from which seeds, lacking elaiosomes, germinate after fire. We showed that juvenile plants do not produce EFNs and are not visited by ants. We conclude that EFNs are not just an indirect adaptation to minimize herbivory via aggressive ants (or parasitoid wasps) but specifically enhance reproductive success by inducing ants to visit the plant throughout the year, promoting discovery of the seasonally available, elaiosome-bearing seeds on the plant and transporting them to their nests, so avoiding the risk of granivory should seeds fall to the ground.