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Patterns in the past

The LDG is a noticeable pattern among modern organisms that has been described qualitatively andquantitatively. It has been studied at various taxonomic levels, through di�erent time periods and acrossmany geographic regions. The LDG has been observed to varying degrees in Earth's past, possibly due todi�erences in climate during various phases of Earth's history. Some studies indicate that the LDG wasstrong, particularity among marine taxa, while other studies of terrestrial taxa indicate the LDG had littlee�ect on the distribution of animals.

Hypotheses for pattern

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Although many of the hypotheses exploring the latitudinal diversity gradient are closely related andinterdependent, most of the major hypotheses can be split into three general hypotheses.

Spatial/Area hypothesesThere are five major hypotheses that depend solely on the spatial and areal characteristics of the tropics.

Mid-domain e�ectUsing computer simulations, Colwell and Hurtt and Willig and Lyons first pointed out that if species’latitudinal ranges were randomly shu�led within the geometric constraints of a boundedbiogeographical domain, species' ranges would tend to overlap more toward the center of the domainthan towards its limits, forcing a mid-domain peak in species richness. Colwell and Lees called thisstochastic phenomenon the mid-domain e�ect, presented several alternative analytical formulations forone-dimensional MDE, and suggested the hypothesis that MDE might contribute to the latitudinalgradient in species richness, together with other explanatory factors considered here, including climaticand historical ones. Because "pure" mid-domain models attempt to exclude any direct environmental orevolutionary influences on species richness, they have been claimed to be null models. On this view, iflatitudinal gradients of species richness were determined solely by MDE, observed richness patterns atthe biogeographic level would not be distinguishable from patterns produced by random placement ofobserved ranges. Others object that MDE models so far fail to exclude the role of environment at thepopulation level and in setting domain boundaries, and therefore cannot be considered null models.Mid-domain e�ects have proven controversial. While some studies have found evidence of a potentialrole for MDE in latitudinal gradients of species richness, particularly for wide-ranging species othersreport little correspondence between predicted and observed latitudinal diversity patterns.

Geographical area hypothesisAnother spatial hypothesis is the geographical area hypothesis. It asserts that the tropics are the largestbiome and that large tropical areas can support more species. More area in the tropics allows species tohave larger ranges, and consequently larger population sizes. Thus, species with larger ranges are likelyto have lower extinction rates. Additionally, species with larger ranges may be more likely to undergoallopatric speciation, which would increase rates of speciation. The combination of lower extinction ratesand high rates of speciation leads to the 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

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biomes, successive biomes north of the tropics all have about the same area. Thus, if the geographicalarea hypothesis is correct these regions should all have approximately the same species richness, whichis not true, as is referenced by the fact that polar regions contain fewer species than temperate regions.To explain this, Rosenzweig suggested that if species with partly tropical distributions were excluded, therichness gradient north of the tropics should disappear. Blackburn and Gaston 1997 tested the e�ect ofremoving tropical species on latitudinal patterns in avian species richness in the New World and foundthere is indeed a relationship between the land area and the species richness of a biome oncepredominantly tropical species are excluded. Perhaps a more serious flaw in this hypothesis is somebiogeographers suggest that the terrestrial tropics are not, in fact, the largest biome, and thus thishypothesis is not a valid explanation for the latitudinal species diversity gradient. In any event, it wouldbe di�icult to defend the tropics as a "biome" rather than the geographically diverse and disjunct regionsthat they truly include.

The e�ect of area on biodiversity patterns has been shown to be scale dependent, having the strongeste�ect among species with small geographical ranges compared to those species with large ranges whoare a�ected more so by other factors such as the mid-domain and/or temperature.

Species-energy hypothesisThe species energy hypothesis suggests the amount of available energy sets limits to the richness of thesystem. Thus, increased solar energy at low latitudes causes increased net primary productivity. Thishypothesis proposes the higher the net primary productivity the more individuals can be supported, andthe more species there will be in an area. Put another way, this hypothesis suggests that extinction ratesare reduced towards the equator as a result of the higher populations sustainable by the greater amountof available energy in the tropics. Lower extinction rates lead to more species in the tropics.

One critique of this hypothesis has been that increased species richness over broad spatial scales is notnecessarily linked to increased number of individuals, which in turn is not necessarily related toincreased productivity. Additionally, the observed changes in the number of individuals in an area withlatitude or productivity are either too small to account for the observed changes in species richness. Thepotential mechanisms underlying the species-energy hypothesis, their unique predictions and empiricalsupport have been assessed in a major review by Currie et al..

The e�ect of energy has been supported by several studies in terrestrial and marine taxa

Climate harshness hypothesisAnother climate-related hypothesis is the climate harshness hypothesis, which states the latitudinaldiversity gradient may exist simply because fewer species can physiologically tolerate conditions athigher latitudes than at low latitudes because higher latitudes are o�en colder and drier than tropicallatitudes. Currie et al. found fault with this hypothesis by stating that, although it is clear that climatictolerance can limit species distributions, it appears that species are o�en absent from areas whoseclimate they can tolerate.

Climate stability hypothesisSimilarly to the climate harshness hypothesis, climate stability is suggested to be the reason for thelatitudinal diversity gradient. The mechanism for this hypothesis is that while a fluctuating environmentmay increase the extinction rate or preclude specialization, a constant environment can allow species tospecialize on predictable resources, allowing them to have narrower niches and facilitating speciation.The fact that temperate regions are more variable both seasonally and over geological timescalessuggests that temperate regions are thus expected to have less species diversity than the tropics.

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

Historical/Evolutionary hypothesesThere are three main hypotheses that are related to historical and evolutionary explanations for theincrease of species diversity towards the equator.

The historical perturbation hypothesisThe historical perturbation hypothesis proposes the low species richness of higher latitudes is aconsequence of an insu�icient 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

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of historical perturbations such as glaciation. This hypothesis suggests that diversity in the temperateregions have not yet reached equilibrium, and that the number of species in temperate areas willcontinue to increase until saturated.

The evolutionary rate hypothesisThe evolutionary rate hypothesis argues higher evolutionary rates in the tropics have caused higherspeciation rates and thus increased diversity at low latitudes. Higher evolutionary rates in the tropicshave been attributed to higher ambient temperatures, higher mutation rates, shorter generation timeand/or faster physiological processes. Faster rates of microevolution in warm climates have been shownfor plants, mammals and amphibians. Based on the expectation that faster rates of microevolution resultin faster rates of speciation, these results suggest that faster evolutionary rates in warm climates almostcertainly have a strong influence on the latitudinal diversity gradient. More research needs to be done todetermine whether or not speciation rates actually are higher in the tropics. Understanding whetherextinction rate varies with latitude will also be important to whether or not this hypothesis is supported.

The hypothesis of e�ective evolutionary timeThis hypothesis assumes that diversity is determined by the evolutionary time under which ecosystemshave existed under relatively unchanged conditions, and by evolutionary speed directly determined bye�ects of environmental energy on mutation rates, generation times, and speed of selection. It di�ersfrom most other hypotheses in not postulating an upper limit to species richness set by various abioticand biotic factors, i.e., it is a nonequilibrium hypothesis assuming a largely non-saturated niche space. Itdoes accept that many other factors may play a role in causing latitudinal gradients in species richnessas well. The hypothesis is supported by much recent evidence, in particular the studies of Allen et al. andWright et al..

Biotic hypothesesBiotic hypotheses claim ecological species interactions such as competition, predation, mutualism, andparasitism are stronger in the tropics and these interactions promote species coexistence andspecialization of species, leading to greater speciation in the tropics. These hypotheses are problematicbecause they cannot be the proximate cause of the latitudinal diversity gradient as they fail to explainwhy species interactions might be stronger in the tropics. An example of one such hypothesis is thegreater intensity of predation and more specialized predators in the tropics has contributed to theincrease of diversity in the tropics. This intense predation could reduce the importance of competitionand permit greater niche overlap and promote higher richness of prey. However, as discussed above,even if predation is more intense in the tropics, as it cannot be the ultimate cause of species diversity inthe 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.These studies suggest the intensity of species interactions are not correlated with the change in speciesrichness with latitude.

Synthesis and conclusions

There are many other hypotheses related to the latitudinal diversity gradient, but the above hypothesesare 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

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hypotheses are similar to and dependent on one another. For example, the evolutionary hypotheses areclosely dependent on the historical climate characteristics of the tropics.

Short descriptionThe increase in species richness or biodiversity that occurs from the poles to the tropics, o�en referred toas the latitudinal diversity gradient, is one of the most widely recognized patterns in ecology. Put anotherway, in the present day localities at lower latitudes generally have more species than localities at higherlatitudes. The LDG has been observed to varying degrees in Earth's past. Explaining the latitudinaldiversity gradient is one of the great contemporary challenges of biogeography and macroecology. Thequestion “What determines patterns of species diversity?” was among the 25 key research themes for thefuture identified in 125th Anniversary issue of Science. There is a lack of consensus among ecologistsabout the mechanisms underlying the pattern, and many hypotheses have been proposed and debated.A recent review noted that among the many conundrums associated with the LDG the causal relationshipbetween rates of molecular evolution and speciation has yet to be demonstrated. Understanding theglobal distribution of biodiversity is one of the most significant objectives for ecologists andbiogeographers. Beyond purely scientific goals and satisfying curiosity, this understanding is essential forapplied issues of major concern to humankind, such as the spread of invasive species, the control ofdiseases and their vectors, and the likely e�ects of global climate change on the maintenance ofbiodiversity. Tropical areas play a prominent role in the understanding of the distribution of biodiversity,as their rates of habitat degradation and biodiversity loss are exceptionally high.

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