Diversity Plots Research Articles

Supersaturated and Relaxing Island Faunas: A Critique of the Species-Age Relationship

We critically examine Wilcox's recent study which purports to prove dynamic equilibrium theory by demonstrating a species diversity-age relationship for lizard faunas on seventeen Baja California 'supersaturated' islands. We show both conceptual and technical problems with his analysis and propose an alternative explanation for his results. The proposition that species diversity results from balanced rates of species immigrations and extinctions was popularized by Preston (1962) and MacArthur & Wilson (1963, 1967). The experiments of Simberloff & Wilson (1970) and subsequently many others (see references in Simberloff, 1974) have been construed as conclusive support of this hypothesis. For several years, the hypothesis received little criticism. This 'dynamic equilibrium theory' states that species number remains constant because of observable but balanced species extinctions and immigrations, while species composition constantly changes. The existence of immigration, extinction, and 'turnover' (changes in species composition in the absence of changes in species number) is crucial to this theory. Lynch & Johnson (1974) criticized Diamond's (1969) conclusion that species 'turnover' is significant. Subsequently, Smith (1975) suggested that the high apparent rate of turnover was a problem of scale, and that as 'extinction' and 'immigration' has been defined, the equilibrium hypothesis was trivial. Simberloff (1976) recalculated his estimates of turnover for mangrove island arthropods and concluded that although turnover consisting of population extinctions occurred, far more of the original estimates represented either transient intra-population movement or death of individuals which never produced populations in the first place. Thus, the very experiments cited as the foundation for the dynamic equilibrium hypothesis were found not to evidence much turnover. Recently, Wilcox (1978) has contended that if 'non-equilibrium, supersaturated' island faunas are 'relaxing' (decreasing in species numbers), this can be construed as support for dynamic equilibrium theory. Were he to demonstrate such a relationship, it could be taken as support for Diamond's (1972) and Terborgh's (1975) ideas of long relaxation times. However, it has no bearing on dynamic equilibrium theory, since, as mentioned above, the critical requirement for dynamic equilibrium is observable, balanced rates of species immigrations and extinctions with species composition changes in the absence of changes in species number. Not only are Wilcox's (1978) conclusions irrelevant to the controversy involving dynamic equilibrium theory, but his analysis of the island agediversity relationship for lizard faunas on the islands in the Baja California region is beset with conceptual and technical problems that undermine its implication of long relaxation times. Given the omnipresent species-area relationship (Connor & McCoy, 1979) one would expect changes in island area resulting from sea level fluctuations to affect island 0305-0270/79/1200-0311 $02.00 ? 1979 Blackwell Scientific Publications 311 20 This content downloaded from 207.46.13.134 on Sun, 25 Sep 2016 05:00:36 UTC All use subject to http://about.jstor.org/terms 312 Stanley H. Faeth and Edward F. Connor species numbers. Diamond (1972), Terborgh (1975), Simpson (1974), and now Wilcox (1978) suggest that changes in species numbers resulting from such events do not occur instantaneously, but lag several thousand years behind changes in an island's physiographic characteristics, particularly area. Relaxation curves showing this trajectory of species loss over time (Terborgh, 1975) are usually a virtual mirror image of the familiar colonization curve (MacArthur & Wilson, 1963; Simberloff & Wilson, 1970). However, relaxation times are said to be extremely protracted, unlike those for colonization. Relaxing islands are thus supersaturated with species and are losing species in an inexorable return to the 'proper' number of species for a given island's area, and in Wilcox's example (1978), latitude. Wilcox presumes the islands in the Baja California region to be supersaturated, since when the islands were connected to the mainland the resident numbers of lizard species would have been some larger subset of the large mainland pool. Now isolated, the islands presumably retain more species than they 'should' have for their size and latitude. Specifically, Wilcox relates lizard species diversity to island age on seventeen Baja California land bridge islands, as well as to island area, elevation, isolation, distance from mainland, and latitude. He reasons that the observation of an inverse relationship between island age and diversity implies that young islands are still supersaturated and relaxing. He explores this relationship in three ways: (1) simple correlation between island area and diversity, (2) stepwise multiple regression techniques using species diversity as the dependent variable with island age and other physical parameters as explanatory variables, and (3) a type of partial correlation analysis where, after the effects of area and latitude are 'factored out,' residual species diversity is plotted against age. He then construes the plot of residual species diversity versus age as proof that species diversity is 'relaxing' toward an equilibrium value consistent with the present size and latitude of the island.

Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic

Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators. Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata. High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation.

Abundance, diversity and temporal variability in a California intertidal nudibranch assemblage

A 40-month study of a nudibranch assemblage within a 250 m2 intertidal area at Asilomar State Beach, California, USA, indicated that dominance was shared among 9 species which comprised 87% of the total number of individuals enumerated. These 9 species also showed high frequency of occurrence in the area. The number of nudibranch species counted per month was shown to be independent of weather and tide conditions, but the numbers of individuals and diversity values were correlated with wave action, lower values and numbers being tabulated during times of storms. Cumulative plots of diversity and species number based upon 15 min enumeration times indicated adequate assessment of diversity and species number after 60 min of sampling. The number of species and individuals and the diversity varied from month to month over the 40 months, but these variations were shown to be statistically insignificant or to be correlated with weather. Hence, it was concluded that the assemblage was a stable one, structured primarily by the 9 dominant species. No statistically significant seasonal or annual changes were observed in the assemblage. Diversity was found to correlate most highly with species number and less with number of individuals. The average number of species found per month was 13, the average number of individuals 104, and the average monthly diversity (HH}) was 1.82. Significant positive and negative correlations between abundances of certain species were found. These were attributed to occurrence of prey items or to synchrony of reproduction. There was no evidence of migration, and resident status for one dominant species was established. Comparison of the present study with earlier work suggested that a stable nudibranch assemblage was not merely a local phenomenon.

Discussion

D r A. L ord stated that the concentration of Foraminiferida by means of carbon tetrachloride or other heavy liquids is susceptible to error for a number of reasons and counts of such assemblages can introduce serious discrepancies into subsequent statistical studies. Did Mrs Lees examine the heavy residues after the carbon tetrachloride flotations and if so how reliable was the technique in this particular case? T he A uthor stated that she examined a limited number of residues when first undertaking the flotation technique with the samples. The counts for the residues were added to those for the original flotations, but the differences found were not significant and therefore no further residues were examined. The Start Bay Foraminiferida are fresh and therefore the tests are usually empty. There is a minimal amount of infill material such as pyrite or glauconite, for example, which might prevent floating. M r N. J. B. H award asked whether the author had looked at hydroids in the Recent material. Mrs L ees replied that no hydroids were found but the substrate in the Bay is mainly sand and is perhaps not as suitable as, say, rock exposures for hydroid attachment. P rofessor P. C. S ylvester -B radley asked whether the microfauna other than Foraminiferida had been examined with a view to the evidence they could give on the environments postulated? T he A uthor noted that many ostracod species are present but these have not yet been studied. D r M. D. B rasier observed that the contradiction between the diversity and triangular plots in terms of