Abstract
One of the few laws in ecology is that communities consist of few common and many rare taxa. Functional traits may help to identify the underlying mechanisms of this community pattern, since they correlate with different niche dimensions. However, comprehensive studies are missing that investigate the effects of species mean traits (niche position) and intraspecific trait variability (ITV, niche width) on species abundance. In this study, we investigated fragmented dry grasslands to reveal trait‐occurrence relationships in plants at local and regional scales. We predicted that (a) at the local scale, species occurrence is highest for species with intermediate traits, (b) at the regional scale, habitat specialists have a lower species occurrence than generalists, and thus, traits associated with stress‐tolerance have a negative effect on species occurrence, and (c) ITV increases species occurrence irrespective of the scale. We measured three plant functional traits (SLA = specific leaf area, LDMC = leaf dry matter content, plant height) at 21 local dry grassland communities (10 m × 10 m) and analyzed the effect of these traits and their variation on species occurrence. At the local scale, mean LDMC had a positive effect on species occurrence, indicating that stress‐tolerant species are the most abundant rather than species with intermediate traits (hypothesis 1). We found limited support for lower specialist occurrence at the regional scale (hypothesis 2). Further, ITV of LDMC and plant height had a positive effect on local occurrence supporting hypothesis 3. In contrast, at the regional scale, plants with a higher ITV of plant height were less frequent. We found no evidence that the consideration of phylogenetic relationships in our analyses influenced our findings. In conclusion, both species mean traits (in particular LDMC) and ITV were differently related to species occurrence with respect to spatial scale. Therefore, our study underlines the strong scale‐dependency of trait‐abundance relationships.
Highlights
One of the few laws in ecology is that we find only a few common but many rare taxa in a community (McGill et al, 2007; Preston, 1948)
We predicted that (a) at the local scale, species occurrence is highest for species with intermediate traits, (b) at the regional scale, habitat specialists have a lower species occurrence than generalists, and traits associated with stress-tolerance have a negative effect on species occurrence, and (c) intraspecific trait variability (ITV) increases species occurrence irrespective of the scale
Theory postulates that species with intermediate trait values are the most abundant ones, because these species are those that are best adapted to the specific environmental conditions and may have the highest competitive ability (Grime, 2006; Laughlin et al, 2012; Rolhauser et al, 2019)
Summary
One of the few laws in ecology is that we find only a few common but many rare taxa in a community (McGill et al, 2007; Preston, 1948). Species mean traits were shown to be suitable predictors for species assembly along environmental gradients (Bergholz et al, 2017; Cornwell & Ackerly, 2009; Grime, 2006; May et al, 2013a). They predict species abundance at a specific site (Laughlin et al, 2012). Solid empirical evidence for an unimodal relationship between species mean traits and abundance remains scarce with studies reporting linear and/ or opposing relationships (Cornwell & Ackerly, 2010; Lauterbach et al 2013; Liu et al, 2017; Read et al, 2017)
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