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Research Of Why Mountains Are Higher In The Tropics

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As the human population on Earth surpasses 7 billion, anthropogenic effects on the environment are becoming ever more pronounced. The consumption of natural resources by humans has instigated rapid worldwide climate change, habitat fragmentation, and invasion by non-native species. These changes have prompted a significant decrease in biodiversity, increase in extinction rates, and dramatic shifts in community assemblages and species ranges. Therefore, studying the natural biodiversity rate and the climatic tolerance of species is now more important and urgent than ever. These natural processes are often much more poorly understood than the artificial trends, but are no less critical to evaluating the increasing anthropogenic effects on our environment. Mountain slopes are, in general, much more vulnerable to human disturbance and climate change than flatlands.

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Aside from the effects of human activities, species diversity and community composition varies widely on both latitudinal and elevational gradients. In 1967, Daniel Janzen linked those two gradients by introducing the “mountains are higher in the tropics” hypothesis (mentioned hereafter as Janzen’s hypothesis). He stated that because climatic factors including temperature and precipitation vary much less in the tropics than in the higher latitudes, and because elevational changes cause changes in these climatic factors, mountains and high elevations provide more effective barriers to dispersal in tropical regions than they do in temperate and polar regions. From this hypothesis, inferences can be made about diverse biological topics including biodiversity, speciation, species ranges, and future effects of climate change. Although the amount of literature relating latitudinal and elevational gradients to these topics has increased dramatically in recent years, most of the research has been conducted on a regional rather than global scale, and on a single taxon instead of multiple taxa.

Since Janzen’s hypothesis was published in 1967, corroborating evidence from subsequent research generally supports the fundamental ideas behind it. However, recent research suggests that some of his assumptions may not be accurate. For example, Janzen’s hypothesis is much more difficult to apply to locations in the Southern Hemisphere, which are on average more affected by the climatic-mediating effect of oceans than locations in the Northern Hemisphere. Furthermore, compounding variables including vegetation structure and treeline are also highly correlated with elevation and latitude, and could profoundly influence vertebrate ecological factors such as diversity and dispersal. Recently, there have been many attempts to broaden the scope of Janzen’s hypothesis, and to apply it to many different taxa and biogeographic regions.

However, the crux of his hypothesis has largely remained untouched: the related climate-variability hypothesis (CVH) and its relationship with the thermal tolerance limits of terrestrial organisms. The climate-variability hypothesis, proposed by Stevens (1989), states that because the annual climate in the tropics varies less than in temperate and polar regions, tropical species may have narrower thermal tolerance limits than species at higher latitudes. To examine this interaction, Gutierrez-Pesquera et al. (2016) measured the breadth of thermal tolerance in tropical and temperate tadpoles. They observed a significant positive correlation between latitude and tolerance range in those tadpoles, supporting both the CVH and Janzen’s hypothesis, although local variation in tolerance range was more likely to be explained by microclimate temperatures. This trend was also observed in New World salamanders and other amphibians. These studies suggest that the CVH is well-supported by data on ectotherms. However, more research may be needed on endotherms, which have different sets of climatic requirements and dispersal limitations, to properly examine the applicability of the CVH to Janzen’s hypothesis.

As our climate warms, and specialist species are forced to shift their ranges, the climate variability hypothesis, and, by extension, the “mountains are higher in the tropics” hypothesis, remains of paramount importance. Species with narrower thermal tolerance limits, living in the tropics or in mountainous temperate regions, cannot disperse or shift their ranges as easily as those in flatland temperate regions, and thus may be more susceptible to future climate change. Because the majority of vertebrate biodiversity lies in tropical regions, it is important to understand how future climate shifts will affect tropical communities. Greater community disassembly was observed in tropical regions than in temperate regions, implying that a warming climate will affect tropical communities disproportionately. The polar regions are also some of the most vulnerable to climate change, as permafrost melts, erosion rates increase, and total snow and ice cover declines. Unfortunately, the public perception of the polar regions as homogenous and devoid of life has influenced public policies relating to preservation of Arctic and Antarctic regions. Research relating Janzen’s hypothesis and climate trends in biogeography to polar regions has been relatively rare, and new research examining the biodiversity of the polar regions and community response to climate change would be instrumental in introducing high latitudes into discussions about Janzen’s hypothesis and its assumptions. In addition, research on arctic biodiversity has the potential to shift public perception as tundra and ice cap regions degrade at an accelerating rate.

In addition to warming the Earth’s climate with greenhouse gas emmissions, human resource consumption and population growth have fundamently altered the biotic community in several other ways. Examining the effects of human use on the biodiversity and vegetation structure of dry tropical forests in India, Agarwala et al. (2016) found a decrease in ecological variables such as abundance and richness with increasing population density. Furthermore, the community structures of areas with high human habitation were significantly different than communities in areas of low human habitation. The same trends of range shifting and community disassembly were observed in North American mammals. It is clear that humans are changing ecosystems throughout the world, and especially in polar, tropical, and montane ecosystems.

Another important assumption of Janzen’s hypothesis lies in the ability of temperate species to disperse more easily than tropical species. Species with limited dispersal ability will not be able to cross barriers such as mountain ranges effectively. There has been evidence both supporting and opposing this idea. On a lacustrine island in Panama, many species of tropical understory birds were extremely limitated in their dispersal ability, with several species studied unable to fly more than 200 m across open water. This trend is consistent in mammal distributions. Niche occupancy models of both volant and non-volant mammals in higher latitudes were more likely to be entirely filled than occupancy models of low-latitude species, providing support for the limited dispersal of tropical species. However, the relatively small ranges of tropical species and the homogenous climate of tropical regions indicate that while abiotic factors such as climate are likely to limit species distributions in temperate regions, biotic factors including competition and niche partitioning are more accurate predictors of tropical species ranges.

Smith and Klicka (2010) likewise found negligible effects of elevation on the dispersal of tropical Mesoamerican birds. These contradictory results show that much more research is necessary to fully examine the relationships between dispersal ability, elevation, and latitude. Specifically, the interaction between abiotic and biotic factors in shaping terrestrial species ranges must be examined more thoroughly for a clearer understanding of these relationships. Regardless of the specific predictor variables on species range sizes, species that are able to disperse more easily have larger range sizes, and as a result, temperate species tend to occupy larger ranges in general than tropical species. The positive correlation between latitude and range size has been well-documented as Rapoport’s Rule, although the ubiquity of such as rule has been subject to considerable debate. Unfortunately, few studies have linked Rapoport’s Rule to Janzen’s hypothesis by studying elevational range sizes, or the breadth of elevations occupied by a species, specifically. These studies are uncommon partially because of the immense amount of resources required to conduct a large-scale latitudinal study of range sizes and topography. However, some studies have synthesized previous research to work around those resource contraints. Using 80 years of prior field studies, one study was able to examine the reported elevational distributions of 16,500 montane species across four vertebrate classes.

In support of Janzen’s hypothesis and the CVH, this study discovered decreasing elevational range sizes with decreasing latitude in all taxonomic groups except rodents and non-breeding birds. Similar studies on Andean passerine birds and mayflies in Colorado and Ecuador likewise found support for both Rapoport’s Rule and Janzen’s hypothesis. They reported strong positive correlations between the elevational range of the organisms and their latitudinal maxima. However, Rahbek et al. (1997) found limited support for Rapoport’s Rule in South American birds. The general increase in vertebrate range sizes with increasing latitude may also have to do with the interaction between body size of the vertebrates themselves and their dispersal ability. However, this relationship has not yet been thoroughly examined. Body size was generally a poor predictor of range size in mammals in Mexico, and was not significantly correlated with the elevational range sizes of Andean passerine birds. However, experimental research on the potential correlation between body size and dispersal ability (especially in non-volant terrestrial animals) could solidify these arguments, and increase our understanding of physiological trends in latitude and elevation. Janzen’s hypothesis did not specifically reference biodiversity. However, the CVH has been used by many researchers to explain speciation and therefore changes in diversity with increasing latitude and elevation.

Although the causes of speciation on elevational-latitudinal gradients are not clear, the relationship between elevation and overall diversity with increasing latitude is much better understood. Beta diversity measurements of adjacent communities in quadrats throughout North America revealed that, as latitude increases towards the Arctic, the elevational effects on mammalian beta diversity decrease. Similarly, the topography of arctic regions has an inconsequential effect on mammalian and avian herbivore diversity. The diversity in mammals along elevational gradients suggest that the peak of maximum diversity is at lower elevations in the tropics than in higher latitudes (McCain et al., 2005). This evidence supports Janzen’s hypothesis, suggesting that varying topography provides a more complete barrier in the tropics for mammals than in temperate and polar regions. However, many of these studies fail to account for the varying morphological diversity between taxa. These studies mainly base their results on research conducted a priori, and as a result, may only recognize species that have unique, charismatic morphological characteristics. Therefore, diversity of cryptic species is often underrepresented in CVH-diversity studies.

Comparing cryptic species of mayflies between temperate Colorado and tropical Ecuador, Gill et al. (2016) reported significant correlations between elevational range, diversity, and latitude. Unfortunately, few, if any, other researchers have attempted to relate cryptic species diversity with Janzen’s hypothesis. Much of the data collected to date have supported an increasing effect of elevation on diversity with decreasing latitude, but more research incorporating cryptic species and regions or taxa underrepresented in past studies is necessary to examine those relationships further. As our climate warms, and specialist species are forced to shift their ranges, the climate variability hypothesis, and, by extension, the “mountains are higher in the tropics” hypothesis, remains of paramount importance. Species with narrower thermal tolerance limits, living in the tropics or in mountainous temperate regions, cannot disperse or shift their ranges as easily as those in flatland temperate regions, and thus may be more susceptible to future climate change. Because the majority of vertebrate biodiversity lies in tropical regions, it is important to understand how future climate shifts will affect tropical communities (Sheldon et al., 2011). Greater community disassembly was observed in tropical regions than in temperate regions, implying that a warming climate will affect tropical communities disproportionately. The polar regions are also some of the most vulnerable to climate change, as permafrost melts, erosion rates increase, and total snow and ice cover declines.

Unfortunately, the public perception of the polar regions as homogenous and devoid of life has influenced public policies relating to preservation of Arctic and Antarctic regions (Barrio et al., 2016). Research relating Janzen’s hypothesis and climate trends in biogeography to polar regions has been relatively rare, and new research examining the biodiversity of the polar regions and community response to climate change would be instrumental in introducing high latitudes into discussions about Janzen’s hypothesis and its assumptions. In addition, research on arctic biodiversity has the potential to shift public perception as tundra and ice cap regions degrade at an accelerating rate.

In addition to warming the Earth’s climate with greenhouse gas emmissions, human resource consumption and population growth have fundamently altered the montane biotic community in several other ways. Examining the effects of human use on the biodiversity and vegetation structure of dry tropical forests in India, Agarwala et al. (2016) found a decrease in ecological variables such as abundance and richness with increasing population density. Furthermore, the community structures of areas with high human habitation were significantly different than communities in areas of low human habitation. The same trends of range shifting and community disassembly were observed in North American mammals (Guralnik, 2007).

It is clear that humans are changing ecosystems throughout the world, and especially in polar, tropical, and montane ecosystems. In light of these anthropogenic changes, it is now more urgent than ever to explore questions about the natural effects of climate, landscape, geography, and ecology. Janzen’s hypothesis has provided researchers with a framework to explore these questions thoroughly. Studies referencing this hypothesis, along with research on the CVH, the dispersal ability of montane and tropical species, and the role of elevation in speciation and biodiversity, will remain at the forefront of biogeography as our climate warms and specialist species are forced to shift their ranges.

Further examininations of the shortcomings and limitations of Janzen’s hypothesis, such as its dependency on taxa from the Northern Hemisphere and on studies examining research conducted a priori, will be necessary in order to ensure the validity of latitudinal and elevational gradients in ecology. However, Janzen’s hypothesis remains one of the most critical foundations for studying ecology in a changing world. As the human impacts on Earth’s ecology and biodiversity are becoming more profound and may soon be insurmountable and irreversible, examining the natural trends of elevation and latitudinal gradients through Janzen’s hypothesis and its correlaries is vital to the sustainability of biodiversity and natural communities on Earth.

References

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01 February 2021

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