Global reef fish richness gradients emerge from divergent and scale-dependent component changes

How does variation in total and relative abundance contribute to gradients of species diversity?

Patterns of biodiversity provide insights into the processes that shape biological communities around the world. Variation in species diversity along biogeographical or ecological gradients, such as latitude or precipitation, can be attributed to variation in different components of biodiversity: changes in the total abundance (i.e., more-individual effects) and changes in the regional species abundance distribution (SAD). Rarefaction curves can provide a tool to partition these sources of variation on diversity, but first must be converted to a common unit of measurement. Here, we partition species diversity gradients into components of the SAD and abundance using the effective number of species (ENS) transformation of the individual-based rarefaction curve. Because the ENS curve is unconstrained by sample size, it can act as a standardized unit of measurement when comparing effect sizes among different components of biodiversity change. We illustrate the utility of the approach using two data sets spanning latitudinal diversity gradients in trees and marine reef fish and find contrasting results. Whereas the diversity gradient of fish was mostly associated with variation in abundance (86%), the tree diversity gradient was mostly associated with variation in the SAD (59%). These results suggest that local fish diversity may be limited by energy through the more-individuals effect, while species pool effects are the larger determinant of tree diversity. We suggest that the framework of the ENS-curve has the potential to quantify the underlying factors influencing most aspects of diversity change.

Volatility in coral cover erodes niche structure, but not diversity, in reef fish assemblages

The world's coral reefs are experiencing increasing volatility in coral cover, largely because of anthropogenic environmental change, highlighting the need to understand how such volatility will influence the structure and dynamics of reef assemblages. These changes may influence not only richness or evenness but also the temporal stability of species' relative abundances (temporal beta-diversity). Here, we analyzed reef fish assemblage time series from the Great Barrier Reef to show that, overall, 75% of the variance in abundance among species was attributable to persistent differences in species' long-term mean abundances. However, the relative importance of stochastic fluctuations in abundance was higher on reefs that experienced greater volatility in coral cover, whereas it did not vary with drivers of alpha-diversity. These findings imply that increased coral cover volatility decreases temporal stability in relative abundances of fishes, a transformation that is not detectable from static measures of biodiversity.

A multiscale framework for disentangling the roles of evenness, density, and aggregation on diversity gradients

Disentangling the drivers of diversity gradients can be challenging. The Measurement of Biodiversity (MoB) framework decomposes scale‐dependent changes in species diversity into three components of community structure: species abundance distribution (SAD), total community abundance, and within‐species spatial aggregation. Here we extend MoB from categorical treatment comparisons to quantify variation along continuous geographic or environmental gradients. Our approach requires sites along a gradient, each consisting of georeferenced plots of abundance‐based species composition data. We demonstrate our method using a case study of ants sampled along an elevational gradient of 28 sites in a mixed deciduous forest of the Great Smoky Mountains National Park, USA. MoB analysis revealed that decreases in ant species richness along the elevational gradient were associated with decreasing evenness and total number of species, which counteracted the modest increase in richness associated with decreasing spatial aggregation along the gradient. Total community abundance had a negligible effect on richness at all but the finest spatial grains, SAD effects increased in importance with sampling effort, and the aggregation effect had the strongest effect at coarser spatial grains. These results do not support the more‐individuals hypothesis, but they are consistent with a hypothesis of stronger environmental filtering at coarser spatial grains. Our extension of MoB has the potential to elucidate how components of community structure contribute to changes in diversity along environmental gradients and should be useful for a variety of assemblage‐level data collected along gradients.

Plant species-area relationships are determined by evenness, cover and aggregation in drylands worldwide

AIM

Species-area relationships (also known as 'species-area curves' and 'species accumulation curves') represent the relationship between species richness and the area sampled in a given community. These relationships can be used to describe diversity patterns while accounting for the well-known scale-dependence of species richness. Despite their value, their functional form and parameters, as well as their determinants, have barely been investigated in drylands.

LOCATION

171 drylands from all continents except Antarctica.

TIME PERIOD

2006-2013.

MAJOR TAXA STUDIED

Perennial plants.

METHODS

We characterized species-area relationships of plant communities by building accumulation curves describing the expected number of species as a function of the number of sampling units, and later compared the fit of three functions (power-law, logarithmic and Michaelis-Menten). We tested the prediction that the effects of aridity, soil pH on SAR are mediated by vegetation attributes such as evenness, cover, and spatial aggregation.

RESULTS

We found that the logarithmic relationship was the most common functional form (c.50%), followed by Michaelis-Menten (c.33%) and power-law (c.17%). Functional form was mainly determined by evenness. Power-law relationships were found mostly under low evenness, logarithmic relationships peaked under intermediate evenness and the Michalis-Menten function increased in frequency with increasing evenness. The SAR parameters approximated by the logarithmic model ('small-scale richness' (b₀ ) and 'accumulation coefficient' (b₁ )) were determined by vegetation attributes. Increasing spatial aggregation had a negative effect on the small-scale richness and a positive effect on the accumulation coefficient, while evenness had an opposite effect. In addition, accumulation coefficient was positively affected by cover. Interestingly, aridity decreased small scale richness but did not affect the accumulation coefficient.

MAIN CONCLUSIONS

Our findings highlight the role of evenness, spatial aggregation and cover as main drivers of species area relationships in drylands, the Earth's largest biome.