Biology PATTERNS OF BIODIVERSITY

### KEY TOPICS

star Latitudinal gradients
star Species Richness in Rainforests
star Species-Area relationships

● The color{violet}("diversity of plants") and color{violet}("animals") is not uniform throughout the world but shows a rather uneven
distribution.

● For many group of color{violet}("animals") or color{violet}("plants"), there are interesting patterns in color{violet}("diversity"), the most well- known being the color{brown}("latitudinal gradient") in color{violet}("diversity.")

● In general, species color{violet}("diversity decreases") as we move away from the equator towards the poles.

● With very few exceptions, tropics (color{violet}("latitudinal range of 23.5° N to 23.5° S")) harbour more species than color{violet}("temperate or polar areas.")

● Colombia located near the equator has nearly color{violet}("1,400 species of birds") while New York at color{violet}("41° N") has color{violet}("105 species") and Greenland at color{violet}("71° N") only color{violet}("56 species.")

● India, with much of its land area in the color{violet}("tropical latitudes"), has more than color{violet}("1,200 species of birds.")

● A forest in a color{violet}("tropical region") like color{violet}("Equador") has up to 10 times as many species of vascular plants as a forest of equal area in a color{violet}("temperate region") like the Midwest of the USA.

● The largely color{violet}("tropical Amazonian rain") forest in South America has the color{violet}("greatest biodiversity") on earth- it is home to more than color{violet}("40,000 species of plants, 3,000 of fishes, 1,300 of birds,") color{violet}("427 of mammals, 427 of amphibians"), color{violet}("378 of reptiles and of more than 1,25,000 invertebrates.")

### SPECIES RICHNESS IN RAINFORESTS

● color{violet}("Scientists estimate") that in these rain forests there might be at least two million insect species waiting to be
color{violet}("discovered") and color{violet}("named.")

● color{violet}("Ecologists") and color{violet}("evolutionary biologists") have proposed various hypotheses; some important ones are:

star color{brown}("Speciation") is generally a function of time, unlike color{violet}("temperate regions") subjected to frequent glaciations in the past, color{violet}("tropical latitudes") have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for color{violet}("species diversification")

star color{brown}("Tropical environments"), unlike temperate ones, are less color{violet}("seasonal, relatively ")more constant and predictable. Such constant environments promote niche color{violet}("specialisation") and lead to a greater species color(violet}("diversity.")

star There is more color{brown}("solar energy") available in the color{violet}("tropics,") which contributes to color{violet}("higher productivity"); this in turn might contribute indirectly to color{violet}("greater diversity.")

### SPECIES-AREA RELATIONSHIPS

● During his color{violet}("pioneering") and color{violet}("extensive explorations") in the wilderness of South American jungles, the great
German naturalist and geographer color{brown}("Alexander von Humboldt") observed that within a region species
richness increased with increasing explored area, but only up to a limit.

● In fact, the relation between species richness and area for a wide variety of color{violet}("taxa (angiosperm plants, birds, bats, freshwater fishes)") turns out to be a color{violet}("rectangular hyperbola.")

● On a color{violet}("logarithmic scale"), the relationship is a color{violet}("straight line") described by the equation
color{violet}("log S = log C + Z log A")
where
color{violet}("S=") Species richness
color{violet}("A=") Area
color{violet}("Z =") slope of the line (regression coefficient)
color{violet}("C = Y")-intercept

● color(violet}("Ecologists") have discovered that the value of color{brown}(Z) lies in the range of color{brown}(0.1) to color{brown}(0.2), regardless of the taxonomic group or the region (whether it is the color{violet}("plants in Britain"), color{violet}("birds in California") or color{violet}("molluscs") in New York state, the slopes of the regression line are amazingly similar).

● But, if we analyse the species-area relationships among very color{violet}("large areas") like the entire continents, we will
find that the color{violet}("slope of the line") to be much steeper (color{violet}(Z) values in the range of 0.6 to 1.2).

● For example, for color{violet}("frugivorous (fruit-eating) birds") and color{violet}("mammals") in the color{violet}("tropical forests") of different continents, the slope is found to be 1.15.