Rare species drive local trait diversity in two geographically disjunct examples of a naturally rare alpine ecosystem in New Zealand

Abstract

Question Is there evidence for similar community assembly processes in two geographically disjunct examples of a rare alpine ecosystem? Location Two alpine granite gravel fields 610 km apart along a fault line in western South Island, New Zealand – the Lookout Range and Mt Titiroa. Methods Plot‐based vascular plant composition and traits for 86 species (height, seed length, leaf size and nutrient concentrations) were used to examine community structure reflecting assembly processes. We examined species richness, trait values averaged across species and plots, relationships between trait pairs using standardized major axis (SMA) regression and phylogenetically independent contrasts (PIC), relationships between abundance and trait values, and functional diversity indices against null models. Lastly, we partitioned taxonomic, functional and phylogenetic diversity within and across alpine ranges. Results Functional and phylogenetic turnover of diversity between the two locations was low (2.9% and 6.3%, respectively) relative to turnover of taxonomic diversity (75%). Species‐level traits were similar between the two locations, except leaf P, which was higher at Mt Titiroa. Plot‐level average traits were all significantly higher on Mt Titiroa. Relationships between species‐level traits were typically non‐significant at both locations, or significant only at a single location. In contrast, relationships between plot‐level trait values were frequently significant and consistent across the two locations. At both locations, dominant species had a narrow range of similar trait values, while rare species had a wide range of values. Within plots, we found both trait convergence and divergence. Associations of trait values among dominant and rare species were largely non‐random but inconsistent between the two locations. Conclusions The two locations had a similar pool of trait values but often differed in how those traits were assembled. Results demonstrate that dominant species are subject to rigorous filters during community assembly leading to trait convergence, but rare species contributed to trait diversity and trait divergence. Models of community assembly can often predict traits of dominant species but must consider the importance of rare species as a source of local trait diversity, even in species‐poor communities under severe environmental conditions.

Rare Species Support Vulnerable Functions in High-Diversity Ecosystems

“…Indeed, rare species may perform functions complementary to those delivered by other, even closely related, species as a result of their distinct functional traits [19]. In turn, those rare species may increase the functional diversity of local communities [20], sustain ecosystem functioning [21], and provide functional traits able to support the main ecosystem processes under future environmental conditions [18].…”

“…Recent studies that investigated the contribution of rare species to functional diversity reached inconsistent conclusions, but were restricted to local samples of a limited number of species [20],[34]–[36]. The question of whether species with unusual combinations of functional traits, which are likely to support vulnerable ecological functions, are overwhelmingly rare is still unresolved in species-rich regional assemblages and at large scales.…”

Expanding an existing classification of New Zealand vegetation to include non-forested vegetation

“…Slopes are relatively gentle; (x slope = 15º, range 2-32º) and often south-facing. Mt Titiroa and the Lookout Range (see next alliance) are distinctive in supporting the two extensive granite gravel fields in New Zealand, an ecosystem type that is considered naturally uncommon (Williams et al 2007;Richardson et al 2012 Fig. 3).…”

“…New Zealand non-forest vegetation ranges from alpine tussock grasslands dominated by Chionochloa species to exotic-dominated grasslands at lower elevations (Cockayne 1928;Wardle 1991;Mark 1993;Mark & McLennan 2005) to low-statured types of unusual environments such as coastal turfs and alpine granite gravel fields (Williams et al 2007;; Richardson et al 2012). At the New Zealand scale, classifications of non-forest vegetation have been broad and non-formalised, focusing on readily recognised physiognomic (e.g.…”

How to design trait-based analyses of community assembly mechanisms: Insights and guidelines from a literature review

“…3A). The pool can also be defined based on operational submatrices consisting of environmentally uniform groups of sites, such as types of vegetation (Ding et al, 2012;Pipenbaher et al, 2013) or locations (Richardson et al, 2012), also termed geographical units (Cornell and Harrison, 2014). These options specifically addresses the influence of local niche-based mechanisms while controlling the influence of environmental filtering playing at a larger habitat scale, i.e.…”