The titi monkey in fragmented landscapes 919


TABLE 1 Patch and landscape metrics used as predictor variables in univariate and compound models of occurrence of the arboreal black- handed titi monkey Callicebus melanochir in a fragmented landscape of Atlantic Forest, Bahia State, Brazil (Fig. 2).


Metric Patch area Patch quality Description & calculation Area (ha) of the study fragment.


Mean enhanced vegetation index of the study fragment, as a proxy of habitat quality.We overlaid a 90 × 90mgrid on each study fragment&extracted one value of the index for each grid cell, using 10-m resolution Sentinel satellite images, to obtain the mean index. The number of values obtained per fragment (1–3,309) depended on the area of the fragment. The index is a measure of vegetation reflectance developed for tropical forests; it performs well under heavy aerosol & burning biomass conditions (Miura et al., 1998), is sensitive to gross primary production & plant biomass (Ogaya et al., 2015), which in turn are higher in old growth forests (or high quality habitats) than in early successional forests (or low-quality habitats) (Gatti et al., 2015; Nyirambangutse et al., 2017),&is positively related to canopy structure (Huete et al., 2002).


Patch visibility


Sum of the area (ha) of visible neighbouring forest fragments within a 500-m buffer of the focal fragment, ex- cluding the area of the latter. This is a measure of connectivity of forest fragments within the dispersal ability of the species. A 3-dimensional raster of relief, generated using a 30-m resolution digital elevation model in ArcGIS, overlaid on the 90 × 90 m grid (see patch quality, above), with visibility of surrounding fragments assessed following Silva et al. (2015). A neighbouring fragment was visible if it could be seen from at least one of the grid cells in the focal fragment.


Landscape connectivity


Sum of the total area (ha) of forest within the 500-m buffer, excluding that of the focal fragment (Martensen et al., 2008, 2012; Ribeiro et al., 2009).


FIG. 2 Fragmented landscapes of the study area on the north margin of the Jequitinhonha River in Itapebi and Itarantim municipalities, Bahia, Brazil.


All four patch–landscape metrics showed a positive rela-


tionship with the occurrence of our model species in gener- alized linear models, as we expected. The best-supported univariate model was patch area, demonstrating that occur- rence of our model species is primarily driven by the area of forest (Fig. 3a, Table 2, Supplementary Table 2). The results from compound model selection (Table 2, Supplementary Table 2) showed an additive effect of other variables beyond the effect of patch area, demonstrating that species occurrence is also influenced by patch quality and visibility, and landscape connectivity. However, the additive variables had a weak contribution in the compound models. In Fig. 3b–d we present the response surface for the pre- dicted occurrence of titi monkeys as a function of the com- pound models.


Discussion


Our findings confirmthat patch area has amajor effect on the occurrence of our arboreal model species C. melanochir:the larger the forest fragment, the higher is the probability of


occurrence. The relationship between the area of forest frag- ments and the occurrence of arborealmammals is well docu- mented (Harcourt & Doherty, 2005;Magioli et al., 2015), with evidence from studies on primates (Arroyo-Rodríguez et al., 2008;Sharmaetal., 2013;Silva et al., 2015;Carretero-Pinzón et al., 2016), carnivores (Michalski & Peres, 2005; Nagy-Reis et al., 2017), marsupials and rodents (Nupp & Swihart, 2000; Linnell et al., 2017), bats (Muylaertetal., 2016), and ungulates and shrews (Lawes et al., 2000). Additionally, forest quality has a positive effect on the oc-


currence of our model species, suggesting that forest quality is an important trait of forest fragments, balancing the effect of reduced area. Therefore, patch quality could be useful for predicting occurrence of arboreal mammals and for managing their conservation, especially in fragments of,100 ha, which are common in tropical forest hotspots (Turner & Corlett, 1996; Ribeiro et al., 2009). To measure habitat quality from the perspective of a


single forest-dependent species is challenging, and species- specific measurements of habitat quality can rarely be extra- polated to other taxa (Mortelliti et al., 2010). To overcome


Oryx, 2021, 55(6), 916–923 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319001522


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