User:Magellanic83/sandbox

The increase in species richness or biodiversity that occurs from the poles to the tropics, often referred to as the latitudinal diversity gradient (LDG), is one of the most general rules and widely recognized patterns in ecology. Put another way, in the present day localities at lower latitudes generally have more species than localities at higher latitudes. The LDG has been observed to varying degrees in Earth's past.

Explaining what causes the latitudinal diversity gradient is a major unresolved scientific question. “What determines patterns of species diversity?” was among the 25 key research themes for the future identified in 125th Anniversary issue of Science. Beyond satisfying scientific curiosity, this understanding is essential for applied issues of major concern to humankind, such as predicting the effects of global climate change on biodiversity, and controlling the spread of invasive species and diseases. Furthermore, this is a question of high priority, as tropical areas play a prominent role in addressing this question due to their high species richness, while their rates of habitat degradation and biodiversity loss are exceptionally high.[4]

There is a lack of consensus among ecologists about the mechanisms underlying the pattern, and many hypotheses have been proposed and debated. Understanding the global distribution of biodiversity is one of the most significant objectives for biogeography and macroecology.

Hypotheses for the Pattern
Although many of the hypotheses exploring the latitudinal diversity gradient are closely related and interdependent, most of the major hypotheses can be split into three general categories: Spatial hypotheses, Temporal hypotheses, and Biotic Hypotheses

Spatial Hypotheses
There are four major hypotheses that explain regional species diversity by invoking spatial and geographic characteristics.

Mid-Domain Effect
The mid-domain effect is a geometric artifact which asserts that if species’ geographic ranges were randomly shuffled within geometric constraints of a bounded domain (e.g. continents, for terrestrial species), then species ranges would tend to overlap more toward the center of the domain than towards its limits, causing a peak of species richness in the middle of the domain. This has been demonstrated using computer simulations, where species ranges are randomly generated and show a peak in the center of a continent (or other domain).

“Pure” mid-domain models attempt exclude any direct environmental or evolutionary influences of species richness, lending them to be considered a null model of species diversity. In this view, if latitudinal gradients of species richness were determined solely by the geometry of the continent, then observed richness patterns at biogeographic scales would be indistinguishable from patterns produced by random placement of observed ranges. Thus, mid-domain models are a tool which allows for inferences on the strength environmental influences on species ranges by comparing the observed pattern’s deviance from what would be expected by random assortment alone. Mid-domain models have been controversial, with some suggesting that they are unsuitable for use as a null model because they fail to exclude the role of environment at the population level.

While some studies have found evidence of a potential role for mid-domain effect in latitudinal gradients of species richness, particularly for wide ranging species    , others report little correspondence between mid-domain predicted and observed latitudinal diversity patterns   , suggesting alternative mechanisms for the latitudinal diversity gradient.

Geographical Area Hypothesis
The geographical area hypothesis asserts that tropical areas support more species because the tropics are the largest biome by area. This idea relies on the species-area curve, which generally says that the larger the area, the more species that area can support. One possible mechanism for why larger areas support more species is that it allows species to attain larger ranges, and consequently larger population sizes. Species with larger ranges experience lower extinction rates. Additionally, species with larger ranges may be more likely to undergo allopatric speciation, which would increase rates of speciation. The combination of lower extinction rates and higher rates of speciation would lead to high levels of species richness in the tropics.

A critique of the geographical area hypothesis is that even if the tropics is the most extensive of the biomes, successive biomes north of the tropics all have about the same area. Thus, if the geographical area hypothesis is correct these regions should all have approximately the same species richness, which is not true, as is referenced by the fact that polar regions contain fewer species than temperate regions. Perhaps a more serious flaw in this hypothesis is some biogeographers suggest that the terrestrial tropics are not, in fact, the largest biome, and thus this hypothesis is not a valid explanation for the latitudinal species diversity gradient. It is also difficult to defend all of the tropics as one cohesive biome, as the tropical ecosystems are rather geographically disjunct, especially in comparison to the boreal biome.

The effect of area on biodiversity patterns has been shown to be scale dependent as well, having the strongest effect among species with small geographical ranges in comparison to species with large ranges, the latter of which are more affected by other factors such as the mid-domain effect.

Climate Harshness Hypothesis
The climate harshness hypothesis states the latitudinal diversity gradient may exist simply because fewer species can physiologically tolerate conditions at higher latitudes than at low latitudes, because higher latitudes are often colder, drier, and more seasonal than tropical latitudes. This has also been called the environmental filter hypothesis, as tropical species would be ‘filtered’ out from temperate latitudes if they could not handle such 'stressful' or harsh climates. A key assumption of this hypothesis is that there are more available strategies in the tropics due to relative benign climates, however, recent work has shown that for both mammals and trees there are more available strategies (measured by traits) represented in the temperate zones than the tropics. Another fault with the climate harshness hypothesis is that although it is clear that climatic tolerance can limit species distributions, it appears that species are often absent from areas whose climate they can tolerate.

Critiques for this hypothesis include the fact that there are many exceptions to the assumption that climate stability means higher species diversity. For example, low species diversity is known to occur often in stable environments such as tropical mountaintops. Additionally, many habitats with high species diversity do experience seasonal climates, including many tropical regions that have highly seasonal rainfall.

Species-Energy Hypothesis
The species energy hypothesis suggests the amount of available energy sets limits to the species richness in a system. Thus, increased solar energy (with an abundance of water) at low latitudes causes increased net primary productivity (or photosynthesis). This hypothesis proposes the higher the net primary productivity the more individuals can be supported, and the more species there will be in an area. 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. Lower extinction rates lead to more species in the tropics. The potential mechanisms underlying the species-energy hypothesis, their unique predictions, and empirical support have been assessed in major reviews, which has been supported in terrestrial and marine taxa.

One critique of this hypothesis has been that increased species richness over broad spatial scales is not necessarily linked to increased number of individuals, which in turn is not necessarily related to increased productivity. Additionally, observed changes in the number of individuals in an area with latitude or productivity are often either too small (or in the wrong direction) to account for the observed changes in species richness.

Temporal Hypotheses
There are four main temporal hypotheses relating to environmental and evolutionary history explanations for the increase of species diversity towards the equator.

Historical Perturbation Hypothesis
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. This hypothesis suggests that diversity in the temperate regions have not yet reached equilibrium, and that the number of species in temperate areas will continue to increase until saturated.

Environmental Stability Hypothesis
The environmental stability suggests that the increased stability of tropical environments are the reason for the latitudinal diversity gradient. The mechanism for this hypothesis is that a fluctuating environment may increase the extinction rate or prevent specialization, a constant environment can allow species to specialize on predictable resources, allowing for narrower niches and facilitating speciation. The fact that temperate regions are more variable both seasonally and over geological timescales suggests that temperate regions are thus expected to have less species diversity than the tropics. This hypothesis is closely related to the Historical Perturbation Hypothesis

Evolutionary Rate Hypothesis
The evolutionary rate hypothesis argues higher evolutionary rates in the tropics have caused higher speciation rates and thus increased diversity at low latitudes. Higher evolutionary rates in the tropics have been attributed to higher ambient temperatures, higher mutation rates, shorter generation time and/or faster physiological processes. Faster rates of microevolution in warm climates (i.e. low latitudes and altitudes) have been shown for plants, mammals , and amphibians. Based on the expectation that faster rates of microevolution result in faster rates of speciation, these results suggest that faster evolutionary rates in warm climates almost certainly have a strong influence on the latitudinal diversity gradient. More research needs to be done to determine whether or not speciation rates actually are higher in the tropics. Understanding whether extinction rate varies with latitude will also be important to whether or not this hypothesis is supported.

Effective Evolutionary Time Hypothesis
This hypothesis assumes that diversity is determined by the effective evolutionary time under which ecosystems have existed under relatively unchanged conditions, and by evolutionary speed directly determined by effects of environmental energy (temperature) on mutation rates, generation times, and speed of selection. 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. It does accept that many other factors may play a role in causing latitudinal gradients in species richness as well. The hypothesis has been supported by recent evidence.

Biotic hypotheses
Biotic hypotheses claim biotic interactions such as competition, predation, mutualism, and parasitism are stronger in the tropics and these interactions promote species coexistence and specialization, leading to greater species richness. These hypotheses are problematic because they often fail to explain why species interactions might be stronger in the tropics. An example of one such hypothesis is the greater intensity of predation and more specialized predators in the tropics has contributed to the increase of diversity in the tropics. This intense predation could reduce the importance of competition (see competitive exclusion) and permit greater niche overlap and promote higher richness of prey. However, as discussed above, even if predation is more intense in the tropics (which is not certain), as it cannot be the ultimate cause of species diversity in the tropics because it fails to explain what gives rise to the richness of the predators in the tropics. Several recent studies have failed to observe consistent changes in ecological interactions with latitude suggesting the intensity of species interactions are not correlated with the change in species richness with latitude.

Another biotic hypothesis is that tropical species richness is driven by increased niche overlap, where there are many highly similar, but specialized, species coexisting with more tightly packed narrower niches than in the temperate zones.

Synthesis and conclusions
There are many other hypotheses related to the latitudinal diversity gradient, but the above hypotheses are a good overview of the major ones still cited today. It is important to note that many of these hypotheses are similar to and dependent on one another. For example, the evolutionary hypotheses are closely dependent on the historical climate characteristics of the tropics.

The generality of the latitudinal diversity gradient
A meta-analysis of nearly 600 latitudinal gradients tested the generality of the latitudinal diversity gradient across different organismal, habitat and regional characteristics. Results indicate the latitudinal gradient occurs in marine, terrestrial, and freshwater ecosystems, in both hemispheres. The gradient is steeper and more pronounced in richer taxa (i.e. taxa with more species), larger organisms, in marine and terrestrial versus freshwater ecosystems, and at regional versus local scales. 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. Both animals and plants show the trend, yet there are notable exceptions to the trend in each (explained below).

Patterns in the Past
In order to find how general the latitudinal gradient is, some have looked to see if the latitudinal diversity gradient is observed in the geologic past. This has been studied at various taxonomic levels, through different time periods, and across many geographic regions. The LDG has been observed to varying degrees in Earth's past, possibly due to differences in climate during various phases of Earth's history. Some studies indicate that the LDG was strong, particularity among marine taxa, while other studies of terrestrial taxa indicate the LDG had little effect on the distribution of animals.

Salamanders
The richness peak of salamanders is in the southern Appalachian Mountains, and salamanders are distinctly absent from much of the tropical latitudes, with only Central America and a small portion of Northern South America having salamander species. This has been attributed to salamander's sensitivity to temperature and a general lack of ability to handle anything warmer than moderate temperatures. However, despite not having a wide geographic extent in the tropics, it has been argued that the neotropical salamanders may account for up to 40% of all known salamander species.

Aquatic macrophytes
The diversity of aquatic plants in Costa Rica illustrates the reverse pattern to the general latitudinal gradient of species diversity. There is a higher level (almost equal) of diversity of aquatic macrophytes in cool temperate latitudes (northeastern North America; 145 species) compared to warm temperate latitudes (Southeastern North America; 122 species) or tropical aquatic habitats (Costa Rica; 120 species) where diversity is lowest.

Marine benthic algae
Marine benthic algae of the Atlantic coast of Europe follow the latitudinal gradient of species diversity of high diversity at low latitudes and low diversity at high latitudes. However, marine benthic algae of the Pacific coast of South America illustrate the opposite pattern of higher species diversity toward high latitudes.

Data robustness
One of the main assumptions about LDGs and patterns in species richness is that the underlying data (i.e. the lists of species at specific locations) are complete. However, this assumption is not met in most cases. For instance, diversity patterns for blood parasites of birds suggest higher diversity in tropical regions, however, the data may be skewed by sampling biases underestimating in rich faunal areas such as Southeast Asia and South America. 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. At a 3° (approx. 350 km2) spatial resolution, less than 1.8% of the world's oceans have above 80% of their fish fauna currently described.

Conclusion
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. While some ecologists continue to search for the ultimate primary mechanism that causes the latitudinal richness gradient, many ecologists suggest instead this ecological pattern is likely to be generated by several contributory mechanisms. 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.