Nordic Society Oikos

On Island Biogeography and ConservationAuthor(s): Dennis D. Murphy and Bruce A. WilcoxSource: Oikos, Vol. 47, Fasc. 3 (Nov., 1986), pp. 385-387Published by: Wiley on behalf of Nordic Society OikosStable URL: http://www.jstor.org/stable/3565453 .

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Sutherland, W. J. and Watkinson, A. R. 1986. Do plants evolve differently? - Nature, Lond. 320: 305.

Tuomi, J., Salo, J., Haukoja, E., Niemela, P., Hakala, T. and Mannila, R. 1983. The existential game of individual self- maintaining units: selection and defence tactics of trees. - Oikos 40: 369-376.

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On island biogeography and conservation

Dennis D. Murphy and Bruce A. Wilcox, Center for Conservation Biol., Dept of Biol. Sci., Stanford Univ., Stanford, CA 94305, USA

In a recent note Lahti and Ranta (1985) discuss the mer- its of applying island biogeographic theory to conser- vation practice, adding yet another paper to the long list of those that have questioned the utility of the theory (Simberloff and Abele 1976, 1982, and many others in between). Previous criticism has focussed on the value of the equilibrium theory in predicting whether a single large or several small reserves of the same total area will protect more species (referred to by the acronym SLOSS). Wilcox and Murphy (1985) countered this crit- icism, arguing that SLOSS is neither a valid test of the utility of island biogeographic theory nor a practical tool for conservation because it oversimplifies the problem of protecting biotic diversity in habitat fragments. A problem far more compelling than whether a single large or several small reserves protect more species is that of the decay of species diversity precipitated by the fragmentation of a large area into several small ones. This process of biotic "relaxation" is predicted by island biogeographic theory and supported by empirical evi- dence. That prediction and numerous others generated by island biogeographic theory have convinced biolo- gists, land managers, and policy makers to consider the long-term consequences of the loss and isolation of habitat; and island biogeographic theory has provided an invaluable conceptual framework in which to do so (Wilcox 1984, Salwasser et al. 1985).

Lahti and Ranta question the utility of island biogeo- graphic theory by presenting evidence that appears to demonstrate that area is not always a good predictor of species diversity. Specifically, they show that although the number of bird species encountered on Finnish peatlands is correlated with area, the number of endan- gered vascular plant species is not. The explanation given is that while the habitat diversity relevant to birds

obviously increases with peatland area, such a rela- tionship is less clear for endangered plants which re- spond to the "trophic status of the habitat". Although we are uncertain about the meaning of trophic status of the habitat, we agree that exceptions to the species-area relationship do exist, as do examples of a lack of con- cordance of species number in different taxa in the same set of areas.

Wilcox et al. (1986) and Murphy and Wilcox (1986), for example, found a weak relationship between the number of butterfly species and area, as well as poor concordance among butterflies, birds, and mammals in an archipelago of montane habitat islands. The latter finding, however, is explained by the differing ecologies of these organisms, and is not necessarily at variance with island biogeographic theory. Characteristic differ- ences in vagilities, susceptibility to extinction, resource requirements, degrees of specialization, and other fac- tors produce differing responses among taxa to the ef- fects of area and isolation (Schoener 1976). This is read- ily apparent in intra-archipelago comparisons of differ- ent taxa (Diamond and Mayr 1976, Brown 1978, Wilcox 1980a, b, Wright 1981).

Despite the subjectivity involved in assessing the characteristics which might determine rates of extinc- tion and recolonization, predictable rankings of z-values (species-area slopes) are the rule in comparisons of tax- onomic groups. One of many intra-archipelago com- parisons of the species-area relationships of widely dif- fering taxa is presented in Tab. 1. The rankings of these California Channel Islands taxa easily might be predic- ted on the basis of relative vagilities alone. The volant taxa, birds and butterflies, have the lowest z-values. Rank order of the nonvolant taxa parallels overwater dispersal power; with reptiles more capable of "rafting"

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Tab. 1. Channel Islands species-area analysis. Regression analysis done on a log-log model: log(area) = z x log(species number) + C. (Data taken from Miller 1984, Power 1972, Rentz and Weissman 1982, von Bloecker 1967, Wilcox 1980b).

Island A B M R BF 0

San Miguel 37 10 4 2 6 8 Santa Rosa 217 22 4 3 20 26 Santa Cruz 249 32 9 6 34 40 Anacapas 2.9 16 2 2 15 14 San Nicholas 58 10 4 2 10 10 Santa Barbara 2.6 13 2 1 8 8 Santa Catalina 194 27 9 8 24 32 San Clemente 145 23 7 2 12 11

Slope (z) .14 .29 .26 .17 .21 l0i'P (10) 10.32 1.46 .97 7.24 6.78 r- .34 .81 .53 .30 .39

A = Area M = Mammals R = Reptiles B = Birds 0 = Orthoptera BF = Butterflies

than mammals. Similar arguments can be made for rela- tive susceptibility to extinction. Mammals, by virtue of large body size and high metabolic rates associated with endothermy, and thus lower population densities, should have the highest extinction rates per area, all else being equal, followed by birds, reptiles, and the two insect taxa. The apparent inconsistency in the rank position of birds is certainly due to their superior disper- sal power, which appears to more than compensate for relatively high extinction susceptibility. These "hetero- geneous" results in Tab. 1 are in fact consistent with, not counter to, island biogeographic theory (albeit they do not necessarily support the equilibrium theory).

Despite variation among taxa in the strength of the ef- fect of area on species number (the z-value), the spe- cies-area relationship remains one of the most validated rules of ecology. This appears to be due to two non- exclusive mechanisms, greater habitat diversity and les- sened likelihood of stochastic extinction with increasing area, that must operate to some degree in virtually every community (Wilcox et al. 1986). Regardless of the causal mechanism(s) underlying the species-area rela- tionship, island biogeography offers conservationists important practical advice about the design of nature re- serves: all else being equal, larger reserves will support more species. That exceptions occur when all else is not equal does not invalidate this principle. In fact, the ex- ceptions are themselves instructive.

The data presented by Lahti and Ranta are a case in point. Their species-area plot for endangered plants (their Fig. IB) does not even remotely suggest a positive relationship and at a glance resembles a log-normal dis- tribution. This result should not be surprising for at least three reasons. First, endangered plants are a bio- logically heterogeneous group. The label "endangered" is based on the imminent threat of extinction, a situ- ation which may have more to do with human demo-

graphics and resource exploitation than with plant ecol- ogy or taxonomic relationships. Second, such a large number of habitat areas could be distributed over a wide geographic area, introducing substantial habitat heterogeneity into the area comparisons. Third, smaller areas arguably should tend to have disproportionately more endangered species. Such areas often are small as a result of the loss of surrounding habitat, resulting in the concomitant reduction and fragmentation of the populations of their constituent species (some of which may have been rare before fragmentation). Thus, Lahti and Ranta's data could be interpreted as arguing for the protection of large continuous habitat islands to prevent the endangerment of additional species.

Recent studies of species-area relationships for grass- land communities illustrate how differences in the inter- pretation of data, not the data per se, produce disparate "results" in reference to the merits of large versus small reserves. Murphy and Ehrlich (1986), studying Califor- nia's native grassland remnants, found low z-values for herbaceous plants and for diurnal Lepidoptera. They suggest that since small remnants appear to preserve a substantial fraction of grassland plant diversity, small, as well as large, remnants should be targeted for pro- tection. In contrast, Simberloff and Gotelli (1984) pres- ent similar results from midwestern prairie grasslands as supportive of their argument that several small island refuges are a superior conservation strategy to a single large refuge.

We would interpret their data more conservatively. Simberloff and Gotelli [p. 38] note that by "virtue of spatial separation [a group of remnants] will encompass more habitats than will a single refuge of equal total area..." This necessarily places the argument in a differ- ent context than that in which the SLOSS issue was originally conceived (Simberloff and Abele 1976), bias- ing the comparison in favor of the set of small parcels. (While that comparison provides one explanation for why sets of small, scattered reserves often support more species, the comparison sheds no light on the effects of habitat size and isolation on species diversity.) Ironica- lly, it is the ubiquity of increasing habitat diversity with greater geographic area that is frequently cited to dis- credit the role of area sensu equilibrium theory. Both area and habitat diversity undoubtedly play varying roles in structuring communities, depending on the taxa involved and geographic scale. The demonstration that habitat area can be a less important determinant of spe- cies number than habitat diversity is itself a useful ob- servation, but it does not undermine the validity of is- land biogeography.

Differences in the interpretation of island biogeo- graphic data explain much of the confusion surrounding the application of island biogeographic theory to con- servation. Basing conservation decisions solely on area considerations where field data demonstrate a weak species-area relationship for taxa targeted for protec- tion is, of course, inadvisable. In such cases, one should

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first proceed by determining which areas have the high- est conservation value (e.g., the largest number of en- dangered plants), then, applying theory, select from among the areas those which maximize the likelihood of survival for the endangered species. Specifically, this in- volves designing reserves (or reserve systems) based on criteria for minimizing the probability of population ex- tinction due to demographic or genetic stochasticity, en- vironmental variation, or natural catastrophes, employ- ing theory which mostly comes from island biogeogra- phy (see Shaffer 1981, Wilcox 1984, Wilcox and Murphy 1985, Salwasser et al. 1985, Soule and Simberloff 1986).

Lahti and Ranta's data (again their Fig. 1B) show one particular peatland island with more than 20 "endan- gered" vascular plant species. This parcel is an obvious target for conservation, its size and number of non-en- dangered species notwithstanding. Where a choice of one among several habitat areas of varying sizes is avail- able (again all else being equal), the area to be afforded protection should be as large and intact as possible. Where there is no choice, as in most cases where frag- mentation is inevitable or has already occurred, ecolo- gists and conservation biologists should address ques- tions of practical significance. How large and intact must a protected area be to achieve a specific conser- vation objective? How many species, which species, and what level of assurance of their persistence can be achieved by alternative design and management strat- egies?

The hypothetical "single large or several small re- serves" problem has little practical relevance here or anywhere in conservation. It tends only to obscure the real problems in preserving the world's growing legion of endangered species. Ironically, the most significant of these is the fragmentation of single large areas into several small areas! This not only follows from island biogeographic theory, but is self-evident. The conclu- sion of Lahti and Ranta (1985) that "there seems not to be very much to learn from the general island-bioge- ography-derived design principles of reserves" appears to stem from a misunderstanding of the purposes of the- ory and a misapplication of the available data. The weight of evidence suggests the opposite conclusion. Is- land biogeographic theory provides those trying to deal with the worldwide extinction epidemic with a useful framework for describing the effects of habitat frag- mentation and for seeking solutions to the challenges of reserve design.

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