DANIEL SIMBERLOFF1 and BRUCE LEVIN2 11 1. Department of Biological Science, Florida State University, Tallahassee, Florida 32306 2. School of Public Health, Division of Biostatistics, Columbia University, 600 West 168th Street, New York, New York 10032 PREDICTABLE SEQUENCES OF SPECIES LOSS WITH DECREASING ISLAND AREA - LAND BIRDS IN TWO ARCHIPELAGOES Summary: We quantify the notion of predictable species loss from progressively smaller islands, and apply the quantification to the indigenous forest-dwelling birds of a series of New Zealand islands and to the passerines of the Cyclades Archipelago in the Aegean Sea. The analysis focuses on the reasons why the species-area relationship deviates from a perfect rank-correlation. For both avifaunas, most species are found remarkably predictably: they approximate a pattern in which each species occupies all those and only those islands larger than some species-specific minimum area. However, a minority of species in each avifauna do not conform to this pattern. Possible reasons for non-conformance include habitat differences among islands, anthropogenous extinctions, and equilibrium turnover. To the extent that the first two forces predominate, it would be far better to base conservation decisions on species lists from particular tracts that are potential refuges than on island occupancy patterns. Keywords: Island biogeography; island avifauna; forest patches; reserves; conservation; species loss; New Zealand; Cyclades. Introduction of sites. In particular, there is very little evidence that It is a commonplace that larger sites, all other things turnover rates are higher on smaller islands than on being equal, tend to contain more species than do large ones (Simberloff, 1976; Rey, 1981), as smaller ones. This "species-area" relationship dates demanded by the equilibrium theory hypothesis. The back to at least 1835 (Connor and McCoy, 1979) oldest hypothesis, that of habitat diversity, derives and is viewed by Schoener (1976) as "one of much support from the careful autecological study of community ecology's few genuine laws," but its basis habitat requirements of particular species, while is poorly understood (Connor and McCoy, 1979; several studies of small marine habitats demonstrate McGuinness, 1984). At least four explanations have the effect posited by the disturbance hypothesis been proposed (McGuinness, 1984): (McGuinness, 1984). a) The random placement hypothesis: If species Conservation implications of the species-area colonize sites analogously to raindrops falling into relationship are murky. Wilson and Willis (1975) buckets, larger sites would have more species. argued that future species loss from reserves can be b) The habitat diversity hypothesis: Larger sites have estimated from species-area curves of island biotae, more habitats, and each habitat has its own complement of species. and that the typical doubling of species richness for c) The equilibrium theory hypothesis: Species ten-fold area increase mandates the preservation of immigrate and go extinct at any site. Smaller sites the largest possible sites. It is difficult to conceive of have smaller populations, so extinction rates are anyone favouring, ceteris paribus, a smaller site for a higher, and more species are thus locally extinct at refuge over a larger one. But all things are never any particular time at small sites. equal. Habitat is never completely identical between d) The disturbance hypothesis: Small sites suffer two sites. For 100 published species-area greater disturbance, and disturbance removes species relationships examined by Connor and McCoy or makes sites less suitable for species. (1979), we find that log (area) explains on average The hypotheses are not mutually exclusive, but only 44.8% of the variation in log (number of even if all forces were operating, one would still like species). If the forces envisioned in the equilibrium to know which is most important. The shape of the theory hypothesis are the primary determinants (or species-area curve cannot weigh in favor of one of those in the random placement or disturbance these hypotheses over the others (McGuinness, hypotheses), then the residual variation in species 1984), and there is little direct study of the causes of richness should probably be viewed as either "noise" the sepcies-area relationship for most archipelagoes resulting from high rates of species turnover, or as New Zealand Journal of Ecology 8: © New Zealand Ecological Society 12 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 8, 1985 effects of island isolation. One would always choose when their minimum habitat size requirements are the larger site, if it were not too isolated. fulfilled. We therefore conclude that the increase in If, on the other hand, the habitat diversity bird species richness can be due to the progressive hypothesis is correct, one might hypothesize that the encountering of different minimum areas." residual variation is primarily caused by habitat differences, and there might be many times when the Statistical Procedures smaller site would be selected. It would depend on Consider a binary matrix of species by islands, with the habitats at the particular sites and the habitat S rows representing species and I columns requirements of particular species of conservation representing islands. Thus a "1" in location i, j interest. This argument is cast in unrealistically represents presence of species i on island j, while "0" Manichaean terms - no-one would argue that represents absence. The islands are in the order habitat should be given no consideration in refuge smallest to largest, while the species are in the order selection. Of course a diverse virgin forest would be least number of islands occupied to greatest number preferable to a recent clearcut for almost all of islands occupied. The matrix for indigenous forest conservation purposes. However, the argument over birds on 26 islands off New Zealand is depicted in relative weights to give area and habitat in Table 1, while the matrix for land birds of the conservation decisions is still a substantive one. To Cyclades Archipelago, fortuitously also available for the extent that the equilibrium theory interpretation 26 islands, is depicted in Table 2. To say that species of the species-area relationship is correct, area per se fall out in predictable sequence as island area should be valued. To the extent that the habitat decreases is to say that, in each row, no "0" will be diversity explanation is valid, area per se should be a rightward of a "1". That is, for each species there is secondary consideration. a minimum island area such that the species is East and Williams (1984) argue that autecological present on all those and only those islands exceeding studies, such as those that will tell us about target that area. All "D's" will be in the upper left of the species' habitat requirements, will often be matrix, and all "1's" in the lower right. unavailable, and that consideration of area alone will How predictable is "predictable"? The hypothesis be a very useful substitute. In particular, for states that, for each species, there should be a line of indigenous forest-dwelling birds on islands off New "D's" followed by a line of "1's". Schoener and Zealand, they conclude that, not only is the number Schoener (1983) call the pattern for each species an of species well predicted by area, but so is the "occurrence sequence". How much does an composition. They detect a predictable, sequential occurrence sequence deviate from this expectation? pattern of species loss as island area decreases - No deviation would consist of exactly the expected each species falls out at a given area, and one can pattern, while maximum deviation would consist of predict which species will be found on each island the opposite pattern: a row of "1 's" followed by a simply from these areas. This predictability, in turn, row of "D's", corresponding to occupation of all the suggests to them that the island biotae would serve small islands and absence from all the large ones as good indicators for the sizes of refuges required (Table 3). Any particular sequence deviates from the on the mainland. expectation by a given number of "transpositions". Here we quantify the notion of a predictable One could transform the observed sequence into the sequence of species loss with decreasing area, and expectation by making a certain number of switches apply the quantification to the New Zealand island between "D's" and "1's". For example, in Table 3, the birds as well as to passerine birds of the Cyclades sequence depicted in row 3 could be transformed Archipelago in the Aegean Sea (Watson, 1964; into the expected sequence by 2 transpositions, while Simberloff, 1985). East and Williams (1984) cite the sequence in row 4 could be transformed to the Diamond (1972), Moore and Hooper (1975), Galli, expected sequence by 3 transpositions. Leck and Forman, (1976), and Whitcomb et at. Suppose species were strewn randomly among (1981) as having shown a similar predictable pattern islands, independently of the islands' areas. How of species loss with area. Since none of these four unlikely would it be, under that random hypothesis, studies lists which species are on which islands, it is for a species to have a sequence as close to the . impossible to assess this claim. None of the cited predicted one as the observed sequence is? Suppose authors themselves make such an assertion, though that a species is found on s of 26 islands. There are Galli et