Drivers of β-diversity along latitudinal gradients revisited

Abstract

Aim Ecologists have generally agreed that β‐diversity is driven at least in part by ecological processes and mechanisms of community assembly and is a key determinant of global patterns of species richness. This idea has been challenged by a recent study based on an individual‐based null model approach, which aims to account for the species pool. The goal of the present study is twofold: (1) to analyse data sets from different parts of the world to determine whether there are significant latitude–β‐diversity gradients after accounting for the species pool, and (2) to evaluate the validity of the null model. Location Global. Methods A total of 257 forest plots, each being 0.1 ha in size and having 10 0.01‐ha subplots, were used. We conducted four sets of analyses. A modified version of Whittaker's β‐diversity index was used to quantify β‐diversity for each forest plot. A randomization procedure was used to determine expected β‐diversity. Results The number of individuals per species, which characterizes species abundance distribution, alone explains 56.8–84.2% of the variation in observed β‐diversity. Species pool (γ‐diversity) explained only an additional 2.6–15.2% of the variation in observed β‐diversity. Latitude explains 18.6% of the variation in raw β deviation in Gentry's global data set, and explains 11.0–11.6% of the variation in standardized β deviation in the global and three regional analyses. Latitude explains 33.2–46.2% of the variation in the number of individuals per species. Main conclusions Species abundance distribution, rather than species pool size, plays a key role in driving latitude–β‐diversity gradients for β‐diversity in local forest communities. The individual‐based null model is not a valid null model for investigating β‐diversity gradients driven by mechanisms of local community assembly because the null model incorporates species abundance distributions, which are driven by mechanisms of local community assembly and in turn generate β‐diversity gradients.

Late winter-to-summer change in ocean acidification state in Kongsfjorden, with implications for calcifying organisms

“…Such low X a may be detrimental for the aragonite-forming organisms (Bednaršek and Ohman 2015). Laboratory experiments on Limacina helicina found reduced calcification and larval dissolution at X a \ 1 (Comeau et al 2009(Comeau et al , 2010Lischka et al 2011;Lischka and Riebesell 2012), although it was also evident that calcification continued even at aragonite-undersaturated conditions (Comeau et al 2010). Projected shoaling of the aragonite saturation horizon due to ocean acidification may change the depth of their diel vertical migration, as well as the overwintering depth, and could impact the length of time that these organisms are exposed to aragonite-undersaturated water (Comeau et al 2010(Comeau et al , 2011.…”

“…Furthermore, synergistic effects of high winter temperatures (2-7°C) and high fCO 2 ([650 latm) in experiments have shown shell degradation of the L. helicina, in particular for overwintering juveniles (Lischka and Riebesell 2012). Since the life stage during winter is the most vulnerable stage of the L. helicina, in case of warmer winters, shell damage due to the contributing effect of increased temperature and increased fCO 2 could be critical for successful recruitment to the adult population.…”

“…During a 24-h cycle, they migrate from surface to deeper water layers down to [200 m, especially during the night, likely to avoid predators (Comeau et al 2010;Lischka and Riebesell 2012). In Kongsfjorden, L. helicina has a 1-year life cycle (Lischka et al 2011) and overwinters at depths below 200 m (Gannefors et al 2005;Lischka et al 2011).…”

“…They become adults in early summer and reproduce in July/August and their veligers grow and become juveniles in autumn (Lischka et al 2011). The juveniles also overwinter at depths below 200 m and continue to grow in spring to become adults in early summer (Gannefors et al 2005;Lischka and Riebesell 2012). Most studies confirm that early stages of L. helicina are the most vulnerable (e.g., Kurihara 2008;Comeau et al 2010;Lischka et al 2011) since they have not fully developed with shells (Kobayashi 1974;Fabry et al 2008).…”

“…Despite these conditions, the juvenile L. helicina is presently able to persist this sensitive period of their life cycle at low food availability, although they do appear to have limited growth at this time. Indeed, there is evidence that L. helicina (and L. retroversa) cease growth in winter (Lischka and Riebesell 2012). Some organisms can survive low food periods such as winter on stored lipids.…”