The Latitudinal Diversity Gradient (LDG) is a fundamental pattern in ecology describing the consistent increase in species richness from the poles toward the equator. This observation establishes that tropical regions harbor far more species than temperate or polar zones. Explaining why life is so much more diverse closer to the tropics has become one of the most enduring and complex challenges in macroecology. This puzzle has led to the development of numerous, sometimes competing, hypotheses that seek to identify the underlying mechanisms responsible for this global distribution of biodiversity.
Observation of the Latitudinal Diversity Gradient
The reality of the LDG is evident across a vast array of life forms in both terrestrial and marine environments. For example, the total number of vascular plant species, mammals, birds, and marine invertebrates shows a clear and steep decline toward higher latitudes. This disparity can be dramatic, such as the comparison between the nearly 1,900 bird species found in Colombia and the fewer than 100 species in the entire Arctic region.
The gradient is often a sharp contrast, with a peak in species richness concentrated between the Tropics of Cancer and Capricorn. This pattern has been confirmed by extensive meta-analyses across nearly 600 studies involving different organisms and habitats. The consistency of this global pattern across diverse taxonomic groups suggests a powerful, overarching set of environmental or historical factors driving diversification and persistence.
Explanations Based on Environmental Resources
One major category of explanation links the diversity gradient directly to the abundant energy and favorable climate conditions in the tropics. The Species-Energy Hypothesis suggests that the amount of available energy limits the richness of a system. Tropical regions receive the most direct solar radiation throughout the year, resulting in warmer temperatures and less seasonal variation.
This high energy input drives the Climate/Productivity Hypothesis, which posits that increased solar energy and high water availability lead to greater net primary productivity (NPP). Higher NPP supports a larger biomass and a greater number of individual organisms. Larger populations tend to have lower extinction rates, allowing more species to persist in a given area.
The Energy-Richness Hypothesis proposes a more direct link between temperature and evolutionary pace through metabolic rates. Warmer temperatures increase the metabolic rates of organisms, which can lead to faster generation times and potentially higher mutation rates. This acceleration of biological processes is thought to increase the overall evolutionary speed, leading to faster rates of genetic divergence and speciation in the tropics.
The climatic stability of the tropics also allows for greater specialization and narrower ecological niches. Since tropical conditions are relatively constant, species do not need to evolve broad physiological tolerances to cope with extreme seasonal shifts. This specialization enables more species to coexist by partitioning resources more finely. These resource-based theories emphasize that the present-day capacity of the tropics to support life is the primary driver of the LDG.
Explanations Based on Historical Stability
A contrasting set of hypotheses focuses on the deep history and long-term consistency of tropical environments. The Time Hypothesis suggests that tropical ecosystems have remained climatically stable for much longer periods than temperate regions. This stability means that tropical lineages have had a greater span of uninterrupted time to accumulate species through diversification.
In contrast, high-latitude regions experienced repeated, massive disturbances from Quaternary glaciations over the past few million years. These recurring ice ages forced species to migrate or face mass extinction, effectively “resetting” the diversification clock in temperate zones. The tropics, largely shielded from these severe climate oscillations, functioned as an undisturbed repository where diversity could continuously build up.
This historical perspective is supported by phylogenetic evidence showing that many temperate taxa are evolutionarily younger and nested within older, more diverse tropical clades. This pattern suggests that the tropics often act as the origin point for new lineages, which later disperse into higher latitudes. Environmental stability is thus viewed as a mechanism that reduces extinction rates and allows for maximum species accumulation.
Explanations Based on Geographical Dynamics
The role of physical space and resulting demographic processes also offers a powerful explanation for the LDG. The Area Hypothesis suggests that the tropical zone constitutes the largest continuous biome on Earth, supporting greater species richness. A larger area generally supports larger population sizes for each species, which reduces the risk of extinction due to random events.
The sheer size of the tropical region also increases the probability of allopatric speciation, where geographic barriers isolate populations and lead to the evolution of new species. Consequently, the balance between speciation and extinction rates favors diversity accumulation more strongly near the equator. The tropics are often described as both a “cradle” and a “museum” of diversity, meaning they have high speciation rates and low extinction rates, respectively.
Palaeontological data support this diversification rate hypothesis by demonstrating that many marine invertebrate groups preferentially originate in the tropics, suggesting a higher origination rate for new taxa. While evidence for a consistently lower extinction rate in the tropics is mixed, the combination of high speciation and at least moderate extinction rates results in a high net diversification rate compared to the poles. This dynamic interplay of geographic size and evolutionary rates provides a final explanation for the planet’s most profound biodiversity pattern.