Abstract
Improving predictions of ecological responses to climate change requires understanding how local abundance relates to temperature gradients, yet many factors influence local abundance in wild populations. We evaluated the shape of thermal‐abundance distributions using 98 422 abundance estimates of 702 reef fish species worldwide. We found that curved ceilings in local abundance related to sea temperatures for most species, where local abundance declined from realised thermal ‘optima’ towards warmer and cooler environments. Although generally supporting the abundant‐centre hypothesis, many species also displayed asymmetrical thermal‐abundance distributions. For many tropical species, abundances did not decline at warm distribution edges due to an unavailability of warmer environments at the equator. Habitat transitions from coral to macroalgal dominance in subtropical zones also influenced abundance distribution shapes. By quantifying the factors constraining species’ abundance, we provide an important empirical basis for improving predictions of community re‐structuring in a warmer world.
Highlights
Amongst the most fundamental questions in ecology is how an organism’s performance is affected by gradients in environmental conditions
Thermal-abundance distributions showed abundant-centre patterns for 25% of species (Fig. 3), and on average, abundance declined by two-thirds of maximum abundance at these species’ thermal range edges
Where species are not limited by geographic boundaries at cool-range limits or ‘niche availability’ limits at warm-range edges, 97% of species display peak maximum abundances away from the edges of species’ thermal distributions (i.e. Topt does not = thermal distribution edges (Tmin) or Tmax)
Summary
Amongst the most fundamental questions in ecology is how an organism’s performance is affected by gradients in environmental conditions. Species’ abundance is expected to be greatest at the centre of their environmental niche if performance declines outside of particular ‘optimal’ environmental conditions (Brown et al 1995; Pironon et al 2016). Assumptions underlying abundant-centre effects have been questioned for decades, and can be violated due to various ecological and evolutionary factors including: (1) fine-scale environmental heterogeneity, (2) local adaptation, (3) physical barriers to dispersal truncating geographic ranges, (4) geographic availability of niche space, (5) habitat gradients and (6) species’ interactions (Sagarin et al 2006). The distribution of abundance across environmental gradients is often complex, and abundance patterns have frequently been inconsistent with the abundant-centre hypothesis (Sagarin & Gaines 2002b; Pironon et al 2016; Dallas et al 2017; Santini et al 2018)
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