Swetasuria said:
"Tropical latitudes have remained relatively undisturbed for millions of years. Undisturbance gives enough time for species diversification." I read this in my school textbook.
For diversification, shouldn't there be evolution and speciation. And for that shoudn't there be some disturbance. If there isn't any disturbance how will that support species diversification.
Here is the time and area hypothesis. The tropics are older and more stable, so there is more time for new species to evolve. This is the hypothesis that you and I are skeptical of. However, our skepticism doesn’t guarantee that it is wrong.
http://www.ncbi.nlm.nih.gov/pubmed/17355570
“A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet how and why this gradient arose remains unresolved. Here we review two major hypotheses for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that tropical climates are older and historically larger, allowing more opportunity for diversification.”
Here is someone who thinks dispersal is the big reason for the rate of diversity. The organisms don’t evolve quicker in the tropics, or go extinct faster at the poles. Organisms just migrate to the tropics. I doubt it, but who know?
http://wolfweb.unr.edu/~ldyer/classes/arita.pdf
“If speciation and extinction patterns alone cannot fully explain the latitudinal gradient of diversity, dispersal must play a significant role (Goldberg et al. 2005; Jablonski et al.
2006; Roy & Goldberg 2007). The stochastic model generates processes that are contrary to those proposed by the _out of the tropics_ hypothesis, which postulates a
tropical origin for most clades, with a net dispersal to temperate areas (Jablonski et al. 2006). In our simulations, dispersal into the tropics can by itself generate the latitudinal
gradient of diversity, as a consequence of the mid-domain effect (Fig. 2b). Stochastic dispersal, combined with random processes of speciation and extinction, generate strong
latitudinal gradients of diversity (Fig. 4) and particular patterns for surviving species (Figs 6 and 7). Only a small proportion of species originating in the tropics disperse to
extratropical areas, and in fact a very high percentage go extinct within the tropical band (Fig. 5b).”
This link lists three types of mechanisms related to the diversification in the tropics.
http://www.bio.puc.cl/caseb/pdf/prog4/Rivadeneira%20et%20al%20GEB%202011.pdf
While there is an ongoing debate about the underlying mechanisms driving the LGR (Rohde, 1999;Willig et al., 2003), there is growing recognition that ultimately the richness in a region should be determined by a balance between the rates of
extinction, migration (i.e. range dynamics) and origination of species (Wiens & Donoghue, 2004;Mittelbach et al., 2007). This balance can be achieved by different mechanisms that can be broadly classified into three types of hypotheses, as proposed by a recent synthesis (Mittelbach et al., 2007): ecological, historical and evolutionary. Ecological hypotheses suggest that tropical areas harbour greater species richness due to environmental
effects enhancing productivity, carrying capacity or niche breadth (Currie, 1991; Rosenzweig, 1995; Evans et al., 2005). Historical explanations (also referred to as biogeographical; Mittelbach et al., 2007) invoke the tropics as sources of evolutionary novelties (i.e. clades originated in the tropics, but overall
diversification rates are not variable across latitude) and that the LGR is the result of a limited dispersal towards the temperate zones (Wiens & Donoghue, 2004; Wiens & Graham, 2005; Jablonski et al., 2006). Finally, evolutionary hypotheses suggest that the LGR is the result of a latitudinal variation in the diversification rates (although novelties appear evenly across latitude),
with higher speciation and/or lower extinction rates in the tropics (Evans et al., 2005; Weir & Schluter, 2007). Despite the differences between historical and evolutionary hypotheses, they both highlight the importance of long-term dynamics explaining the LGR, so we will refer to both of them as historical/evolutionary hypotheses.
I don't think time and area model can be correct. As you said, disturbance should lead to more diversification. However, there are other theories.
As I understand it, the rate of speciation increases the closer one gets to the equator. However, there often seem to be an increase in individuals of any given species as one approaches the poles. Paleontologists and other biologists can determine these correlations, but can't yet figure out a cause.
As an armchair biologist, allow me to speculate why the correlation.
Maybe the increase in speciation toward the equator is due to the direct sunlight.
Direct sunlight stimulates the growth of plants. There are fewer seasons near the equator, so the sunlight doesn't change that much during the year. However, there is geographical variation due to the attenuation of sunlight in the ocean and the tree canopy's on land. So the plants have to adapt to all these solar environments. Animals differentiate to take advantage of the differentiating plants.
The tropics are warmer. Oxygen is less soluble in warm water. So animals are competing for oxygen in the tropical regions. The variation in oxygen levels may speed up diversification of animals.
I think the number of individuals is often limited by the amount of oxygen available in the water. The solubility of oxygen in water decreases with temperature. So animals in the water have less oxygen to live on near the equator. So there are fewer individuals on the equator, but more species.
At the poles, there is less direct sunlight. However, plants hibernate during the winter because it is also cold. So there is less differentiation between plants.
There is more oxygen in the water. The animals don't have to compete as much for oxygen.
