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The SLOSS Dilemma: a Butterfly Case Study

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The SLOSS Dilemma: a Butterfly Case Study Biodiversity and Conservation 5, 493-502 (1996) The SLOSS dilemma: a butterfly case study ARTURO BAZ* and ANTONIO GARCIA-BOYERO Departamento de Biologia Animal, Universidad de Alcald, E-28871 Alcalti de Henares, Madrid, Spain. Received 5 December 1994; revised and accepted 6 April 1995 Butterfly species richness is examined on simulated archipelagoes of 2, 3, 4 and 5 holm oak forest fragments in the Guadalajara Province (central Spain). It is shown that there are more species on several small 'islands' than on a single island. Also, species number increases with the number of fragments that form the archipelago, and with the average distance between islands within the archipelago. Thus, we conclude, at least for butterflies in a system of fragmented holm oak forests in central Iberia, that the best strategy in order to maximize the conservation of species richness is the creation of a net of some small and scattered reserves. Keywords: SLOSS; butterflies: simulated archipelagoes; average distance; central Spain Introduction SLOSS is the acronym used to debate whether the best strategy for species survival is to have a single large or several small remnant refuge patches as nature reserves (Diamond, 1975; Wilson and Willis, 1975). The debate has generated considerable controversy (see Wilcox and Murphy, 1985; Murphy and Wilcox, 1986; Lahti and Ranta, 1985, 1986), and among other approaches has been formulated in terms of maximizing species richness or in terms of minimizing extinction rates (Burkey, 1989). Diamond (1976), Jarvinen (1982) and Hubbell and Wright (1983) have questioned the appropriateness of species richness as a measure of conservation success. For some authors, the crucial conservation biology question hinges on whether the function of a nature reserve should be to support more species, or whether it is more important to weight species so that the reserve contains more species that would become extinct in the absence of the reserve. Another factor in the SLOSS argument is that large reserves may have minimal extinctions compared with small reserves. Chance environmental impacts are less likely to cause extinctions in large reserves, yet many small reserves, in contrast, may spread the risk. However, Spellerberg (1991) points out that large reserves are not always the most appropriate and there has been no need to involve theories from island biogeography to make that claim. Immediate conservation objectives and current circumstances will apply, but of greatest importance is the need to conserve, for as long as possible and as much as possible. These considerations have developed into the SLOPP (single large or plentiful patchy) debate which argues that it may be better to divide an area into very small patches each with many individuals, rather than a single large area (see Gilpin, 1988). This paper contains data about butterfly diversity in a forest fragmented system in order to clarify the best strategy for butterfly conservation in biotopes of central Spain from the point of view of maximizing species richness. *To whom correspondence should be addressed. 0960-3115 © 1996 Chapman & Hall 494 Baz and Garcia-Boyero Methods Thirteen holm oak (Quercus rotundifolia Lam.) forest fragments in the Guadalajara Province of central Spain (Fig. la,b) were selected for study on the basis of variety in area, shape and habitat complexity (Fig. lc). Each site was visited once every 15 days from 3 May to 15 September 1991, totalling 9 sample days. Samples were taken on sunny days between 10.00 and 16.00 h. The sampling scheme was based on sampling subunits (40 min) of collecting effort per site per sample (see Viejo, 1984; Baz, 1985). With this procedure, only the individuals that were caught are counted. Butterfly species that were identified while flying but were not caught are counted as one individual, and represent 1% of the total number of individuals. In transect count methods (Pollard, 1977) butterflies were identified in flight. However, the Mediterranean butterfly faunas are diverse, which renders these methods inefficient due to the practical impossibility of recognizing in flight many congeneric species at the species level. Consequently, in this work, we have selected a method more suitable for the peculiarities of the Mediterranean local faunas (see Baz and Garcfa-Boyero, 1995). The validity of the sampling method can be observed in Fig. 2 in which species accumulation curves among patches are presented. Fig. 2 shows that from the sixth day of sampling the number of species remain approximately constant. For test hypotheses about SLOSS we have followed the procedure of Simberloff and Gotelli (1984). Archipelagos of 2, 3, 4 and 5 small 'islands' have been simulated by randomly drawing pairs, trios, quartets and quintets of forest fragments, constraining the total area of each archipelago to less than that of the largest fragment (2115 ha) which has been used as the control. Thus assemblages of 13 pairs, 13 trios, etc of fragments have been established to match the 13 single fragments. Areas of the archipelagos are constrained to insure that, for each number of islands there was a uniform distribution of areas between the smallest possible for that number of islands and the largest possible (see Simberloff, 1986). For each randomly assembled archipelago, we have constructed a species list of all species found anywhere in the archipelago. Moreover, all distances were measured between pairs of islands. Then for every randomly assembled archipelago, average distance between pairs of islands within the archipelago was calculated (Simberloff, 1986). For an archipelago of only one island, the average distance was set to zero. Data about shape, isolation, tree density and structure of vegetation of these forest fragments can be found in Baz and Garcfa-Boyero (1995). Results We obtained a grand total of 2282 individuals of 81 different species of butterflies (see Appendix) whose distribution among the holm oak fragments studied is shown in Table 1. Species number varies from 26 to 43 and no correlation was demonstrated between the number of butterfly species and the area (Ln species number = 3.49 + 0.023 Ln Area: r 2 = 11.02%;p > 0.05) (see Baz and Garc/a-Boyero, 1995 for a more comprehensive discussion of these results in relation with metapopulation dynamics). Fig. 3a shows the relationship between number of species in an archipelago and total area of the archipelago, for archipelagos consisting of from one to five islands. As Wilson and Willis (1975) pointed out, when area and all other factors are held constant, a single SLOSS and butterflies 495 a BRgHLNEGA C [] 1Km, Figure 1. Map of the study site. (a) location of the Guadalajara Province. (b) location of the forest patches. Main roads and villages are also included. (c) Shape and relative size of the forest patches. island will have more species than will an archipelago comprising several islands. However, our data do not show such a pattern as can be observed in Fig. 3a. For all 65 archipelagos the regression equation is: Ln species number = 3.32 + 0.106 Ln Area where r 2 = 46.61%;p < 0.05 The same procedure has been used with the number of islands in an archipelago (see Fig. 496 Baz and Garcia-Bovero 90- • @ -~---e 80-~ ~m i / / ~ ¢48 E Z m 30- 2 20- 10- I T ..... ]F~ - I -- --T ...... ~ .... --T- ....... Y r 1 2 3 4 5 6 7 8 9 Sample day Figure 2. Species accumulation curves among patches. A curve for all localities arc also included. 3b). The correlation between number of species and number of islands is much more than that for area and the number of species. The regression equation is Ln species number = 3.63 + 0.316 Ln islands number where r 2 = 78.25%: p < 0.05 As the regression equation shows, the coefficient of Ln islands number is high and positive when the Wilson and Willis (1975) hypothesis predicts that it wilt be negative. Lastly, the relationship between species number and average distance among islands in an archipelago is defined by the following equation (see Fig, 3c) Ln species number = 3.63 + 0.142 Ln distance where r 2 = 66.59%, p < 0.05 Thus, as the distance among islands increases, the butterfly fauna in an archipelago becomes richer. Since the 65 archipelagos are not all independent of one another (some of them contain SLOSS and butterflies 497 "A) 4.2 °%o oo ! 4 3.8 O0~_ [] 0 [] [] ~ 0 Lj [] 3.6 / [] E] [] 3.4 o 3.2 Log Area 4.4 pm 4.2 .| 4 3.8 3.6 B 3-4 [] 3.2 0 0 3 0.6 0 9 1.2 5 1.8 Log N [number of islands} 4.4 ~"Pl 4-2 0 0 I 4 J 3.8 3.6 B 3.4 [] 3-2 i i , 3 4 Log(n+1) Distance Figure 3. Relationships between numbers of butterfly species on 13 forest patches and 52 simulated archipelagos comprising 2-5 islands each and (a) area; (b) number of islands and (c) average distance. The following symbols are used: [S], 1 island; ~, 2 islands; ~, 3 islands; e, 4 islands; and O, 5 islands. 498 Baz and Garcia-Bovero some of the same islands) the degrees of freedom for the regression are less than 63. However, even with as few as 7, 3 and 4 degrees of freedom respectively (too few for this situation) the coefficient of determination would be significant with p < (I.05 (sec Simberloff and Gotelli, 1984; Simberloff, 1986. By means of stepwise multiple regression we have elaborated a model that allows the prediction of variation in the number of species in the simulated archipelagos, using the pool of 3 variables considered. The regression equation is: Ln species number = 3.47 + 0.03 Ln Area + 0.19 Ln islands number + 0.05 Ln distance.
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