and western Pacific marine prosobranch gastropods per degree of latitude.
Source: Roy et al. (1998), figure 1, p. 3700. © National Academy of Sciences, USA.
Using an extensive database of published gradients, Hillebrand (2004) found that the overall strength and slope of the gradient for marine organisms were significantly negative and of similar magnitude compared with gradients for terrestrial organisms. The marine gradients were stronger than those for freshwater organisms, although there were clear differences in gradients between biogeographical regions. At the local scale, gradients were weakest for autotrophs and strongest for carnivores. The gradient parameters also differed between oceans and between different habitats, with steeper declines related to the open ocean pelagic rather than coastal benthic fauna. Weaker gradients were found for the Pacific than in the Atlantic, although the Pacific is older than the Atlantic. The Indian Ocean demonstrated an interesting pattern: the gradient was very steep, but the strength was weak. This latter pattern conforms to the notion that biodiversity hotspots dominate the spatial pattern of diversity in the Indian Ocean from which diversity radially decreases. Radial rather than latitudinal gradients and high overall diversity correspond well to the steep slope and weak strength of the latitudinal gradient.
Several marine organisms were characterised by weak gradients due to low body mass (diatoms, protozoa), organism type (macrophytes), and life form (infauna). Nevertheless, marine gradients were as strong as or even stronger than terrestrial gradients. Body mass is an important factor influencing the strength and slope of the latitudinal gradient, and this may be related to dispersal chance, energetic constraints, and/or population size. The Analysis of Reef Life survey data for 4127 marine species at 2406 coral and rocky sites worldwide confirms that total richness of vertebrate species peaks in the low latitudes between 15°N and 15°S, but richness of large mobile invertebrates is highest at high latitudes (Edgar et al. 2017). Species richness correlated with temperature for fish and nutrients for macroinvertebrates. The pattern for mobile macroinvertebrates was attributed to constraints imposed by temperature‐mediated fish predation and herbivory across the tropics.
Not all organisms of different kingdoms or phyla show clear latitudinal trends. For example, pelagic marine microbial assemblages did not show a latitudinal gradient in one study (Moss et al. 2020), while another study found a clear increase in diversity towards the tropics for planktonic archaea, bacteria, eukaryotes, and major virus clades (Ibarbalz et al. 2019). These conflicting results underscore the fact that there are limits to our understanding of the latitudinal gradient and microbial speciation.
The causal mechanisms of the latitudinal diversity gradient have been the subject of intense debate (Roy et al. 1998; Brown 2014; Veron et al. 2015; Pontarp et al. 2018). Hypotheses proposed to explain latitudinal biodiversity gradients include temperature, primary productivity, area, natural disturbance regime, climatic stability, fragmentation and connectivity, and human disturbance (Edgar et al. 2017). Recent discussions have centred around two main phenomena: phylogenic niche conservatism and ecological productivity. The former refers to the fact that tropical species appear to occur in narrower niches than species of higher latitude. The latter refers to the fact that productivity (and subsequent metabolism) of a species tends to be greater at higher temperatures. These two factors undoubtedly play important roles, but accumulating evidence suggests that the single most important factor is kinetics, that is, the temperature dependence of ecological and evolutionary rates as relatively higher temperatures in the tropics generate and maintain high diversity. Nonetheless, over 32 different hypotheses have been advanced to explain the latitudinal gradient (Brown 2014; Pontarp et al. 2018). The search for a primary cause has led to some consideration that there may not be any one cause but that many causes co‐vary depending on location and more importantly, with taxonomic group.
Compelling evidence exists that most lineages originated in the tropics via reticulate evolution (Jablonski et al. 2013; Veron et al. 2015). What is fairly clear is that (i) rates of origination of new species are highest in the tropics, (ii) higher rates of speciation than extinction generate high diversity of species and clades within the tropics, (iii) most species and clades of tropical origin remain confined to the tropics due to environmental constraints, (iv) a small number of tropical species overcome such constraints to expand their ranges into higher latitudes, and (v) high rates of extinction result in low standing stocks of species and clades at higher latitudes (Brown 2014).
A central causal theory proposed by Brown (2014) offers that there is a framework showing how historical events and environmental conditions affect the dynamics of fundamental ecological and evolutionary processes to generate and maintain variation in standing stocks of biodiversity. The fossil record, phylogenetic reconstructions, analyses of variation in genomes and niche traits, and metabolic theory all play a role in the search for a central explanatory hypothesis. Brown (2014) states that the latitudinal gradient is so ancient and pervasive because “The relationship of the earth to the sun and the variation in solar energy input creates a gradient of environmental temperature. Temperature affects the rate of metabolism and all biological activity including the rates of ecological interactions and coevolution. Diversity begets diversity because the Red Queen runs faster when she is hot.” What this means most simply is that species diversity is higher in the tropics than towards the poles because temperatures are warmer in the lower latitudes and that productivity and speciation are correspondingly greater. This is the simplest explanation and does not preclude other causal mechanisms from operating to explain the observed patterns of latitudinal diversity.
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