Latitudinal gradients in species diversity
Species richness, or biodiversity, increases from the poles to the tropics for a wide variety of terrestrial and marine organisms, often referred to as the latitudinal diversity gradient.
Beyond purely scientific goals and satisfying curiosity, this understanding is essential for applied issues of major concern to humankind, such as the spread of invasive species, the control of diseases and their vectors, and the likely effects of global climate change on the maintenance of biodiversity (Gaston 2000).
Put another way, this hypothesis suggests that extinction rates are reduced towards the equator as a result of the higher populations sustainable by the greater amount of available energy in the tropics.
[10] The potential mechanisms underlying the species-energy hypothesis, their unique predictions and empirical support have been assessed in a major review by Currie et al.
Critiques for this hypothesis include the fact that there are many exceptions to the assumption that climate stability means higher species diversity.
The historical perturbation hypothesis proposes the low species richness of higher latitudes is a consequence of an insufficient time period available for species to colonize or recolonize areas because of historical perturbations such as glaciation (Brown and Lomolino 1998, Gaston and Blackburn 2000).
[15] Faster rates of microevolution in warm climates (i.e. low latitudes and altitudes) have been shown for plants,[16] mammals,[17] birds,[18] fish[19] and amphibians.
[20] Bumblebee species inhabiting lower, warmer elevations have faster rates of both nuclear and mitochondrial genome-wide evolution.
However, recent evidence from marine fish[22] and flowering plants[23] have shown that rates of speciation actually decrease from the poles towards the equator at a global scale.
[13] It differs from most other hypotheses in not postulating an upper limit to species richness set by various abiotic and biotic factors, i.e., it is a nonequilibrium hypothesis assuming a largely non-saturated niche space.
The hypothesis is supported by much recent evidence, in particular, the studies of Allen et al.[14] and Wright et al.[25] The integrated evolutionary speed hypothesis argues that species diversity increases due to faster rates of genetic evolution and speciation at lower latitudes where ecosystem productivity is generally greater.
These hypotheses are problematic because they cannot be the ultimate cause of the latitudinal diversity gradient as they fail to explain why species interactions might be stronger in the tropics.
Overall, these results highlight the need for more studies on the importance of species interactions in driving global patterns of diversity.
The gradient steepness (the amount of change in species richness with latitude) is not influenced by dispersal, animal physiology (homeothermic or ectothermic) trophic level, hemisphere, or the latitudinal range of study.
The study could not directly falsify or support any of the above hypotheses, however, results do suggest a combination of energy/climate and area processes likely contribute to the latitudinal species gradient.
Also, in terrestrial ecosystems the soil bacterial diversity peaks in temperate climatic zones,[36][37] and has been linked to carbon inputs and the microscale distribution of aqueous habitats.
[39] For marine fishes, which are among the most studied taxonomic groups, current lists of species are considerably incomplete for most of the world's oceans.
[40] The fundamental macroecological question that the latitudinal diversity gradient depends on is "What causes patterns in species richness?".
Species richness ultimately depends on whatever proximate factors are found to affect processes of speciation, extinction, immigration, and emigration.
For now, the debate over the cause of the latitudinal diversity gradient will continue until a groundbreaking study provides conclusive evidence, or there is general consensus that multiple factors contribute to the pattern.
