Species Area Relationships Speciation and Phylogeny

An important theoretical problem in ecology and biogeo-graphy is to explain the species-area relationship, that is, the increase in species richness with an increase in sample area, usually plotted on a log-log scale. The species-area problem requires the spatially explicit version of the UNT. According to the UNT, the shape and steepness of the species-area curve for nested sample areas is controlled by the fundamental biodiversity number 0 and the dispersal rate m. The UNT predicts shallow species-area curves when dispersal is rapid relative to the speciation rate, leading to fewer, widespread, and abundant species and communities. It predicts steeper curves when specia-tion rates are high relative to dispersal, leading to a patchwork of communities having many local endemics. It predicts that for a given 0, total metacommunity diversity will decrease with an increase in dispersal rate. This happens because abundant species then sweep out local endemics due to a mass effect.

The UNT also predicts that, when all spatial scales are considered, species-area relationships will be triphasic, exhibiting three scaling regions. At small spatial scales, the UNT predicts that the species-area relationship will be curvilinear because common species are sampled more quickly on average than rare species, and as the remaining uncollected species become rarer, it takes ever larger samples to find them. On intermediate spatial scales, the UNT predicts a linear log-log species-area relationship and the slope of this log-log relationship will be a function of 0 and m. On the largest spatial scales, the UNT predicts that the species-area curve will become steeper, increasing to a limiting slope of unity. The UNT provides a measure of the mean size of the metacommunity, which is given by the metacommunity size corresponding to the area corresponding to the upward inflection in the species-area curve. Above this spatial scale, the dynamics of speciation, dispersal, and extinction in the adjacent metacommunity become more and more dynamically independent. The UNT predicts that this upward inflection will occur due to decoupling of biogeographic processes at large spatial scales even in the absence of barriers to dispersal between metacommunities.

Considering speciation from the perspective of neutral theory, what matters is the size of the founding population because this influences the time to extinction of the new species. Two modes of speciation were studied in the original development of the UNT, point-mutation and random-fission speciation. Species arising under point mutation have very small initial population sizes because they are lineages founded by single mutant individuals. Under random-fission speciation, new species form by the random cleavage of an ancestral species into two daughter species. Average species lifespans are short under point-mutation speciation, because the small populations founded by single individuals usually go extinct quickly. In contrast, average species lifespans are much longer under random-fission speciation because of their larger initial population sizes. Because species live longer under random-fission speciation, the UNT predicts that the steady state species richness in the metacommunity will be greater than under point-mutation speciation.

The predictions of the UNT for phylogeny and coalescence theory are different from the results of the early explorations by Raup and colleagues of neutral phyloge-netic models, and the later analytical models of Nee and colleagues. Under the previous neutral models, lineages had preassigned equal probabilities of speciating or going extinct, and lineage abundance was ignored. Under the UNT, however, lineages per se have no preassigned spe-ciation and extinction rates. Instead, the probability of speciating or going extinct is determined by lineage abundance, the distribution of which is dictated by the fundamental biodiversity number, 0. The UNT leads to a number of different predictions for phylogeny. First, globally very abundant species are expected to be much older on average than rare species. Second, these globally abundant and widespread metacommunity species are expected to be the ancestors of many more modern species than are rare and local species. This is a consequence not only of their much longer expected life spans, but also of their much higher total birth rate per unit time (opportunities for speciation) than in rare species. This effect will make certain branches of phylogenetic trees much more speciose than expected from pure random-branching processes. Third, the UNT produces a genuine diversity steady state at equilibrium between speciation and extinction, which is not true of the Raup-Nee models, which produce nearly exponential growth in the number of lineages. There is evidence that globally abundant taxa have longer evolutionary life spans, at least during normal extinction times, but this pattern breaks down during mass extinctions. There is also evidence that the periods of relative stability in diversity between punc-tuational events are dynamic steady states with continual species turnover Finally, under point-mutation specia-tion, phylogenies in which extinct lineages have been pruned out are expected to exhibit a fractal geometry, and the fractal dimension of the phylogeny is proportional to the fundamental biodiversity number, 0.

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