On Models Predicting Abundance of Species and Endemics for the Darwin Finches in the Galápagos Archipelago

Abstract

339In a previous report to this journal (1963) we made conclusions on the processes of speciation and production of endemics among the Darwin finches in the Galapagos Archipelago. The conclusions were inferred from multiple-regression analysis wherein four independent variables of the insular environment were quantified and tested for respective abilities to predict species data. The analysis was an attempt by linear regression to discover (a) the environmental determinants of insular variation in the species numbers (Y ,) and (b) the determinants of such variation for numbers of endemic subspecies (Y2 ) . Insular area (X,) and number of land plant species (X2 ) -presumably indexing ecologic and floristic diversity-were found lacking in power to "push" or "move" the dependent variables (the Y's). Isolation, measured by distance between islands (X3) and by distance from a given island to Indefatigable Island near the center of the archipelago (X,), was the major predictor of number of species and endemics for islands, with X 3 being more important than X, in terms of contribution to Y variances (~y2). For both problem a and b, however, a component (~dy2) of the variance remained unaccounted for by regression of the V's on the X's, and this was attributed to error inherent to the analysis, to factors not considered, or to both.1 Publication supported by the U. S. Public Health Service.To decrease the values of~dy2, we have (i) shifted from use of desk calculator to digital computer analysis; (ii) included in the analysis new environmental factors (Xr., X.) not previously considered; and (iii) utilized new models or estimating equations involving transformations of the arithmetic values of the primary measurements of the V's or X's to their logarithmic counterparts in nonlinear or joint linear-nonlinear variation. Below we summarize results of such a comparative multiple-regression analysis, and, following this, a statement is made of the bearing of the new information on our previous conclusions (Hamilton and Rubinoff, 1963). Data for the V's and the four X's are from our previous report, and the new factors (Xr., X.) are from a comparable study of plant species abundance in the archipelago (Hamilton et al., 1963). The first is multiple linear regression and the last three are examples of multiple nonlinear analyses. With inclusion of new factors, the number of

Centres of speciation and ecological differentiation in the Galapagos land bird fauna

“…The avifauna of the Galapagos Islands has provided a fertile testing ground for evolutionary and biogeographic theory (Swarth, 1934;Lack, 1947Lack, , 1969Bowman, 1961Bowman, , 1963Hamilton and Rubinoff, 1963, 1964, 1967Grant, 1986;Harper, 1987). This avifauna offers the best example of insular divergent evolution for which the entire, or nearly entire, species assemblage resulting from natural processes still exists.…”

The Measurement of Isolation on Archipelagos, and Its Relation to Insular Faunal Size and Endemism

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Rubinoff (1963, 1964) have studied isolation, number of species, and endemism in relation to the Darwin's finches of the Galapagos archipelago, and have concluded that, contrary to the view of Bowman (1961), the contention of Lack (1947) that there is a marked correlation between degree of isolation and production of endemic forms, is upheld. In their studies, two measures of isolation were used: distance from the nearest finchinhabited island ( X , ) , and (as used by Bowman) distance from Indefatigable, an island near the center of the archipelago ( X , ) .

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“…Using the same method, showed that elevation was the major factor influencing numbers of plant species on the Galapagos islands, and (Hamilton et a]., 1964;Hamilton and Armstrong, 1965) that in the case of polyphyletic assemblages of bird species (resulting from repeated colonizations) on archipelagos in the East Indies, East Central Pacific, West Indies and the Gulf of Guinea, area of island is the major predictor of species numbers (equivalent of U,) , when considered along with elevation of island and various measures of isolation.…”

The Measurement of Isolation on Archipelagos, and Its Relation to Insular Faunal Size and Endemism

“…In their studies, two measures of isolation were used: distance from the nearest finchinhabited island ( X , ) , and (as used by Bowman) distance from Indefatigable, an island near the center of the archipelago ( X , ) . By multiple regression analysis, Hamilton and Rubinoff showed that X S explained more of the variation in both total number of finch species on an island ( Y , ) and number of endemies on an island ( Y 2 ) than did X4, when these two factors only were considered; moreover X s was the prime "mover" of Y 1 and Y2 when these two factors were considered together with area of island, and floral diversity of island (the factor regarded as of prime importance by Bowman), or even when elevation of island and area of adjacent island were also included.Using the same method, showed that elevation was the major factor influencing numbers of plant species on the Galapagos islands, and (Hamilton et a]., 1964;Hamilton and Armstrong, 1965) that in the case of polyphyletic assemblages of bird species (resulting from repeated colonizations) on archipelagos in the East Indies, East Central Pacific, West Indies and the Gulf of Guinea, area of island is the major predictor of species numbers (equivalent of …”

“…Using the same method, showed that elevation was the major factor influencing numbers of plant species on the Galapagos islands, and (Hamilton et a]., 1964;Hamilton and Armstrong, 1965) that in the case of polyphyletic assemblages of bird species (resulting from repeated colonizations) on archipelagos in the East Indies, East Central Pacific, West Indies and the Gulf of Guinea, area of island is the major predictor of species numbers (equivalent of…”