Abstract
Winnowing of poorly-adapted species from local communities causes shifts/declines in species richness, making ecosystems increasingly ecologically depauperate. Low diversity can be associated with marginality of environments, which is increasing as climate change impacts ecosystems globally. This paper demonstrates the demographic mechanisms (size-specific mortality, growth, fertility; and metapopulation connectivity) associated with population-level changes due to thermal stress extremes for five zooxanthellate reef-coral species. Effects vary among species, leading to predictable changes in population size and, consequently, community structure. The Persian/Arabian Gulf (PAG) is an ecologically marginal reef environment with a subset of Indo-Pacific species, plus endemics. Local heating correlates with changes in coral population dynamics and community structure. Recent population dynamics of PAG corals were quantified in two phases (medium disturbed MD 1998-2010 and 2013-17, severely disturbed SD 1996/8, 2010/11/12) with two stable states of declining coral frequency and cover. The strongest changes in life-dynamics, as expressed by transition matrices solved for MD and SD periods were in Acropora downingi and Porites harrisoni, which showed significant partial and whole-colony mortality (termed “shrinkers”). But in Dipsastrea pallida, Platygyra daedalea, Cyphastraea microphthalma the changes to life dynamics were more subtle, with only partial tissue mortality (termed “persisters”). Metapopulation models suggested recovery predominantly in species experiencing partial rather than whole-colony mortality. Increased frequency of disturbance caused progressive reduction in coral size, cover, and population fecundity. Also, the greater the frequency of disturbance, the more larval connectivity is required to maintain the metapopulation. An oceanographic model revealed important local larval retention and connectivity primarily between adjacent populations, suggesting that correlated disturbances across populations will lead to winnowing of species due to colony, tissue, and fertility losses, with resultant insufficient dispersal potential to make up for losses – especially if disturbances increase under climate change. Variable extinction thresholds exists based on the susceptibility of species to disturbance (“shrinkers” versus “persisters”), determining which species will be winnowed from the community. Besides projected changes in coral community and population structure, no species are projected to increase in cover. Increased marginality due to climate change will lead to a net loss of coral cover and novel communities in PAG.
Highlights
Gradients in latitude, habitat, and anthropogenic disturbance characterize the distribution of biodiversity in general, and of zooxanthellate reef-building shallow-water corals (Reaka et al, 2001; Schluter and Pennell, 2017)
The present study explores changes in demographics of the most common reef corals in PAG (Sheppard and Sheppard, 1993; Riegl, 1999) over the last decade
PAG bleaching thresholds were at 35.7◦C water temperature for 1 day and 3 weeks of daily mean temperatures at 35◦C (Riegl et al, 2011) suggesting a steady increase of hot days at Sharjah (45◦C daily max. air temperature correlates with ∼35◦C daily max. water temperature at Ras Ghanada) will increase the likelihood of bleaching and result in a shorter temporal spacing between bleaching events on the Ras Ghanada reef
Summary
Habitat, and anthropogenic disturbance characterize the distribution of biodiversity in general, and of zooxanthellate reef-building shallow-water corals (Reaka et al, 2001; Schluter and Pennell, 2017). Increased excursions from a typical tropical environmental envelope seems to determine the limits of adaptation (Veron, 1995; but see discussion concerning the Mid Domain Effect; Colwell et al, 2004; Connolly, 2005; Hawkins et al, 2005; Currie and Kerr, 2008) These mechanisms are evident along latitudinal gradients but increasingly along temporal and anthropogenic gradients (vanHooidonk et al, 2013; Riegl and Purkis, 2015; Hughes et al, 2017a,b; Schluter and Pennell, 2017). Even within highest-diversity zones, gradients in environmental quality cause gradients of species richness, with species winnowed from zones beyond their physiological or ecological optima (Veron, 1995)
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