Projecting forest cover in Madagascar 159
FIG. 3 Lemur subpopulations occurring in different-sized forest blocks in Madagascar in 2017 and 2050, assuming the same deforestation rates as recorded during 2015–2017. Size classes double from one class to the next. Values on the x-axis are the midpoints in each category (e.g. 5 km2 represents blocks of 0–9.99 km2, 15 km2 represents blocks of 10–19.99 km2, etc.).
Lemur subpopulations in protected areas of different sizes
If all lemur species were present in all forest blocks of any given protected area (which is unlikely), the protected areas would be home to 1,299 subpopulations of the various lemur species (Supplementary Table 3). Of these 1,299 mostly isolated subpopulations, 471 (36.3%) would occur in forest blocks ,10 km2 (Fig. 3).
Change in protected forest areas for lemur subpopulations until 2050
The projection of the 2015–2017 deforestation rates to 2050 is restricted to analysis at the level of forest blocks. The num- ber of lemur subpopulations in forest blocks of the smallest size of up to 10 km2 would increase by c. 10% (from 471 to 519). The mode remains at 80–160 km2. According to this analysis, the sizes of lemur subpopulations would be re- duced, but in 2050 most species would still remain within the size classes of forest blocks they occupied in 2017 (Supplementary Table 3).
Discussion
Application of the forest analysis to lemur occurrences illus- trates the importance of obtaining more detailed informa- tion regarding the actual situation of protected areas for conservation evaluations. If the total size of protected areas was used to estimate the size of lemur subpopula- tions, most species would be assumed to occur in areas of 320–640 km2. However, when one considers the forested
areas and fragmentation of forest areas, the majority of for- est blocks within protected areas are,10 km2. Any species occurring in only one of these forest blocks would be categorized as Critically Endangered according to the B2 cri- terion of the IUCN Red List threat categories (area of occu- pancy,10 km2; IUCN, 2001). When not considering these smallest forest fragments, the mode of forest blocks is pre- dicted to decline from 320–640 km2 for protected areas as a whole to 80–160 km2. As these isolated forest blocks com- prise the actual forests within the protected areas, they re- flect a decline in area for continuous subpopulations to c. 25% of their size in 2017. This reduction in size is relevant not only for subpopulations but also for communities as a whole. Species–area relationships predict a continuous de- cline in species numbers with decreasing area of suitable habitat (MacArthur &Wilson, 1967). The slope of this rela- tionship varies widely amongst taxa and ecosystems (Brown & Lomolino, 1998), and, apart from analyses on national and continental scales (Brown, 1995; Cowlishaw, 1999), most studies address this issue using forest fragments of much smaller size and thus do not allow predictions of the viability of lemur subpopulations in relation to the size of larger forest blocks (Harcourt, 2002; Fahrig, 2017; Kling et al., 2020; Strier, 2021). However, a fourfold differ- ence in size from a mean area of c. 480 to c. 120 km2 would be associated with a change in community processes and substantial extinction debt (i.e. a delayed extinction of species until the number of species associated with a certain area is reached again according to the species–area relationships deriving from the biogeography of the island; MacArthur & Wilson, 1967). In the long term, the reduction in size of the forest blocks would result in a re- duction of individuals per subpopulation, possibly with subsequent extinction and finally reduced species num- bers per forest block. As data on the viability of different- sized lemur populations are lacking (Ganzhorn et al., 2000), the predicted reduced forest block size adds a new con- ceptional dimension and challenge to conservation plan- ning (Kuussaari et al., 2009; Laurance et al., 2012). Although lemur subpopulations suffer significantly from large-scale forest destruction and hunting (Schwitzer et al., 2014; Randriamady et al., 2021; Borgerson et al., 2022; Kappeler et al., 2022), fragmentation effects do not yet seem to be significant at the scale of forest blocks .10 km2 (Steffens et al., 2022). Even the species with the largest body mass and thus the lowest population densities and numbers of individuals seem to maintain viable populations in re- latively small forest blocks if human pressure is controlled (Jolly et al., 2002). Although genetic deficits because of genetic erosion or inbreeding could become relevant (Mon- tero et al., 2019), most of these forest blocks are projected to persist up to and beyond 2050, providing a time buffer of more than one human generation for establishing effective conservation measures.
Oryx, 2024, 58(2), 155–163 © The Author(s), 2023. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323001175
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