Latitudinal Gradients in Species Richness - Essay Example

Latitudinal Gradients, Biodiversity Literature Review: Latitudinal Gradients in Species Richness Sumanta Sanyal www.academia-research.com Dated: August 25, 2005 Abstract This study actively promotes hypotheses put forward in recent times in explanation of latitudinal gradients in species richness. First it lightly touches upon biodiversity as a whole to set parameters within which the study topic can dealt with. It defines biodiversity in the most modern manner taking into account genetic variation, which is the most specific means by which variations within different taxa can be successfully explained. Next it goes on to introduce the various nuances inherent within the biogeographical term "Latitudinal Gradients in Species Richness". It spells out the various complexities within the term and the inherent difficulties present in determining and explaining species variation across the globe using hypotheses based on it. It next posits certain singular hypotheses that support latitudinal gradient in species richness and points out the strengths and failings inherent within each such hypothesis. It concludes the study with a revision of the six hypotheses explaining latitudinal gradients in species richness that are currently favoured among biogeographers. What is Biodiversity Biodiversity is the variety of life on Earth. It is distributed heterogeneously across the Earth. Some areas like moist rain forests and coral reefs teem with variant life and are taken to have high biodiversity while others like hot dry deserts and polar regions are almost devoid of life and are taken to have very low biodiversity. Most other regions fall in between. Explaining why such differences occur is a major objective of ecologists and biogeographers. A fine definition of biological diversity is available from a book "Biodiversity" edited by Edmund Wilson, 1988, wherein it is put as - "Totality of hereditary variations in life forms, across all levels of biological organization from genes and chromosomes within individual species to the array of species themselves and finally at the highest level to the living communities of ecosystems, such as forests and lakes".(Biodiversity, latitudinal gradients, Para. 2) For the purpose of this present study this definition is sufficient as genetic variation remains the most specific basis for determining variations within and among species'. Latitudinal Gradient Introduction This Earth has a varied biome incidence in certain patterns and processes and the broadest pattern that a geographical survey should delve into to determine and explain successfully the species diversity across the different biomes is the latitudinal gradient in species richness. It can successfully explain in most cases incidence in species variations across varying physical environments as well as such variations across the notional temporal scale. This study focuses on the various recent hypotheses that have been put forward advocating study of this eminently revealing pattern. Though the latitudinal gradient is a particular spatial pattern it also often accounts in large part for such other spatial patterns as elevation and depth gradients across land and water. At the very onset of this study it must be admitted that although much emphasis has been placed on latitudinal gradients in species richness not much is known about the variations in genes, individuals and populations along latitudinal gradients. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) The prevailing hypothesis, widely accepted, is that species richness increases from the low latitudes, the tropics, to the high ones, the Polar Regions. This holds true for both terrestrial and freshwater species and in some measure marine species. This has a few exceptions in some taxa as in the case of the high incidence and diversity of conifers and some species of wasps in the temperate regions and the penguins and the seals in the high latitudes but the latitudinal gradient holds true for most major taxa (classes, orders and family) of microbes, plants and animals. It holds true not only for present forms of life but also can be construed as having been such from fossil records. Crane and Lidgard (1989) estimate that there has been a latitudinal gradient for land plant life for the last 110 million years. (Species Diversity: Spatial Patterns and Scales, Tzung-Su Ding, March 16, 2003) In the marine environment open-ocean pelagic and deep-sea taxa also show broad latitudinal gradient patterns in species richness though there is some debate in the incidence patterns of shallow-water non-calcareous taxa. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) The asymmetries in the pattern prompt biogeographers to be cautious in not analysing latitudinal gradients in species richness in total isolation from other causal environmental factors such as chance, historical perturbation, environmental stability, habitat heterogeneity, productivity and interspecific interactions. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) Latitudinal Gradients: Hypotheses The Null Model Hypothesis The mechanisms mentioned above that determine biodiversity are not always mutually exclusive and in various combinations pose problems of resolution to biogeographers. Thus, to solve such a knotty case, a model has been put forward based on the physical structure of the Earth. It is called the "Null Model" and it assumes that there are no environmental gradients but only a rough latitudinal association between the dimensions and occurrence (midpoint values) of the geographical range of species. It predicts a peak in species richness at the tropical latitudes. It posits very plausible explanations for this conclusion. It holds that the latitudinal range of a species within a taxonomic group is bounded to the north and south by certain constraints like physical ones such as a continental edge or climatic ones such as critical temperatures or optimum precipitation. In this model the manner in which the ranges are distributed changes systematically. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) The null model is also successfully applicable in instances of large species incidence in territories bounded physically as in the case of the large island of Madagascar off the east coast of Africa. While lemurs, an infraorder within the primate order, have died out elsewhere on the globe they thrive on Madagascar and have subsequently speciated into 22 species to exploit the varied habitat and food resources on the island. (Robert Jurmain et al, Introduction to Physical Anthropology, Page 120.) The null model displays certain difficulties in imposition on other kinds of constraints where such constraints are dependent on the inclusiveness of the species being surveyed. The Area Model Hypothesis Another model which seeks to explain latitudinal gradients in species richness based on the physical characteristics of the Earth is the "Area Model". This model observes that the tropics have a larger area under similar climatic conditions than any other latitudinal band. This is because: The surface area of the latitudinal bands decrease as they proceed towards the poles The temperature gradient between the equator and the poles is non-linear with the mean temperature being relatively constant between S and N The regions of similar climates immediately north and south of the equator are continuously adjacent The larger areas with climatic similarities in the tropics ensure that species richness for particular species is at large mean geographical range levels. The large area also ensures that there is more scope for further speciation and less probability of extinction. These in turn make the tropics more species rich than any other extra-tropical zone. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) Nevertheless, the area model cannot solely account for latitudinal gradients in species richness when it is held in isolation to other biodiversity factors for than it only can hold true for species richness in the tropics (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) Species/Energy Relationships (A Hypothesis) Historically the lower latitudes have always had a high productivity level at organism levels than the higher latitudes simply because of the higher levels of insolation that ensures higher energy provisions that species can subsequently utilise to grow and specialise. Kevin J. Gaston in his excellent article in Nature magazine reports that at a relatively local scale with spatial resolution and extent there is a marked tendency towards a hump-shaped relationship between species richness and available environmental energy. When a sufficient number of energy levels are sampled it is found that species richness increases from low to moderate levels of energy available and starts to decline as the energy available reaches a high. This has a simple explanation in that insolation levels do have lower and upper optimum limits - desert regions with high annual temperature ranges have traditionally low species richness as do Polar Regions with low annual temperature ranges Nevertheless, it may be said that species/energy relationship depends much upon the taxa being surveyed and their net requirements of energy that may vary substantially from taxa to taxa. (Global Patterns in Biodiversity, Kevin J. Gaston, May 11, 2000) Productivity Hypothesis This hypothesis is an extension of the available energy hypothesis because productivity is directly linked to the availability of energy. It is a combination of the energy availability and water availability factors Rapoport Rescue Hypothesis This hypothesis postulates that mean sizes of species range decline towards the equator. This hypothesis does explain diversity trends by positing that immigration across geographical ranges increases species richness but the trend is rather contrary as species migration away from the tropics is more likely than towards them. Evolutionary Speed Hypothesis This hypothesis postulates that high energy/productivity levels in the tropics reduces generation times and allows for greater rates of speciation that, in turn, increases species richness Geometric Constraints Hypothesis This hypothesis postulates that geographic barriers like the isolated Australian landmass promote species richness as in the case of the diversity of the marsupials (Global Biodiversity: Patterns and Processes, Meffe and Carroll, Chapter 4.) Some Affecting Factors Topographic Variance Another simple but very effective explanation for the incidence of high species richness in the tropics is that the variance in topographic features has much more environmental effects in the tropic or the lower latitudes than at the upper latitudes. This means that adiabatic lapses in the lower latitudes due to elevation can lead to a more diverse eco-climate than at the upper latitudes. High elevation can create temperate and even artic conditions in the tropics while such elevations in the cooler upper latitudes have no such disparity other than varying degrees of hostile coldness that is not distinguishable in terms of species richness above the eco-climate prevailing at sea levels. This uniqueness induces a more varied eco-climate range across the tropics that is not available in the high latitudes. Hemispheric disparity The disparity between the latitudinal gradients in species richness in the Northern and Southern Hemispheres can be explained by the disparity in landmass. The Southern Hemisphere has less landmass and more ocean area than the Northern. So disparities in terrestrial species richness may be made up by disparities in marine species richness. Conclusion Nevertheless, to conclude, as Kevin J. Gaston observes, latitudinal gradients in species richness cannot be explained on the basis of any single factor but rather a combination of biodiversity factors have to be made use of to satisfactorily do the job. So, no single hypothesis, in absolute isolation to the others, can be successfully in explaining the pattern though the null model hypothesis, the area hypothesis and the ambient energy/ productivity hypotheses have much to recommend within their scopes and ambiences. References Biodiversity, Latitudinal Gradients. Extracted on August 22, 2005, from: http://geography.uoregon.edu/whitlock/geog323/mitch/lectures/lec02.htm Global Biodiversity: Patterns and Processes, Meffe and Carroll, Chapter 4. Extracted on August 22, 2005, from: http://venus.uwindsor.ca/courses/biology/macisaac/55-437/lecture3.htm Global patterns in biodiversity, Kevin J. Gaston, May 11, 2000. Extracted from Nature.com website on August 22, 2005, from: http://www.nature.com/cgi-taf/DynaPage.taffile=/nature/journal/v405/n6783/full/405220a0_fs.html Introduction to Physical Anthropology, Robert Jurmain et al, Page 120. Wadsworth Thomson Learning, Eighth Edition, 1999, ISBN: 0 534 51444 8 (paperback edition) Species Diversity: Spatial Patterns and Scales, Tzung-Su Ding, March 16, 2003. Extracted on August 22, 2005, from: tp://ceiba3.cc.ntu.edu.tw/course/c98081/handout3.htm Read More