Study area. — Our study was conducted on the Maracá-Jipioca Islands (02°01′13″N, 50°30′20″W), located off the coast of the Brazilian Amazon (Fig. 1), which originated from erosional processes and deposits of tertiary sediments on consolidated mud (Allison et al. 1995; Nittrouer et al. 1995). ESEC-MJ is a federally protected area comprised mostly of estuarine habitats (Ferreira et al. 2017). With annual rainfall ranging from 2,300 to 2,800 mm (Am climate type, according to Köppen’s climate classification) and limited variation in elevation (mean altitude = 3 meters), the islands have extensive flooded areas and lagoons (Tavares 2014; Ferreira et al. 2017). The distance from the islands to the continental coast ranges from 6–10 kilometers (Ferreira et al. 2017; Duarte et al. 2022). The vegetation on the islands is primarily composed of mangroves, grasslands, and formations of mixed vegetation (locally called Tesos) consisting of mangrove trees (Rhizophora sp. and Avicennia sp.), bamboo (Guadua sp.), and shrubs (Ferreira et al. 2017). Together, the islands harbor at least 11 vertebrate orders that have been reported as jaguar prey, including tegu (a lizard; Salvator merianae), shorebirds, rodents, and ungulates. However, no other species of terrestrial carnivore inhabits the islands, probably due to spatial limitations and the competitive pressure imposed by jaguar (Duarte et al. 2022). Therefore, the functional role of the jaguar as a predator in this system is irreplaceable.
Field Survey. — Camera traps are a widely used non-invasive method to survey wild felids, as they provide reliable data for assessing space use and occurrence patterns of wildlife, enabling population-level inferences (Hebblewhite and Haydon 2010; Burton et al. 2015; Karanth et al. 2017; Carvalho et al. 2019). Our data on jaguar detectability was gathered from 25 camera-trap stations installed at an average spacing of 2.4 ± 1.1 km, which were active for 90 days, from 25 November 2018 to 22 February 2019 (Duarte et al. 2022). Each station (considered a spatial replicate) hosted two paired camera traps (Bushnell Trophy Cam HD), installed 30–60 cm above the ground. Individual jaguars were identified using the patterns of rosettes along their flanks and spots on other body parts. Due to logistical constraints with installing camera-traps across the islands, we selected a major continuous and accessible region, which was sufficiently representative of the entire island landscape. A minimum convex polygon encompassing our camera trap sites covered 25% of the total area of the islands (Duarte et al. 2022). The cameras functioned continuously and were programmed to take three sequential photos and a 40-s video when triggered. The total sampling effort was 2,250 station-trap days (i.e., the number of stations multiplied by the number of days that they were active; Srbek-Araujo and Chiarello 2007), with all camera traps used in our analyses working throughout the sampling period. We did not deploy baits to avoid biasing the detection probabilities of jaguar and their prey (Duarte et al. 2018).
Characterization of camera-trap stations. — Each camera-trap station was characterized based on a set of environmental variables that best represent the landscape context of the islands and that, according to other studies of jaguars, may influence the detectability and occupancy patterns of this species (Ferreira et al. 2017; Duarte et al. 2022). Accordingly, we determined the habitat composition of each camera-trap station based on four habitat types: wetland (beach and sea, lagoons and channels), grassland, mangrove, and Teso (Ferreira et al. 2017) (Table 1). We used raster information on the islands and the surrounding marine area available from the MapBiomas web platform (MAPBIOMAS 2022) to assess coverage (ha) of each habitat type (Fig. 1). These data were imported to QGis software to assess the proportion (ha) of each habitat type within a buffer of 1 km radius around each monitoring station (Allen et al. 2022; McManus et al. 2022; QGIS Development Team 2022). Buffer size was determined based on the distances between camera trap stations, i.e., ~ 50% of the average distance between each station. In addition, we also used the software Google Earth Engine (Gorelick et al. 2017) to estimate the distance from each monitoring station to the water (sea, lagoons, and channels) (Table 1).
Prey selection. — Data on mammals, reptiles, and birds recorded at more than five of our monitoring stations and previously described or recorded as jaguar prey (see González and Miller 2002; Duarte et al. 2022) were considered as possible drivers of jaguar occupancy patterns. Among all the mammals detected, we considered only four mammals as potential jaguar prey, i.e., agouti (Dasyprocta leporina), black-eared opossum (Didelphis marsupialis), giant anteater (Myrmecophaga tridactyla), and white-tailed deer (Odocoileus virginianus) (Table 1). Tegus were the only species of reptile that we included in our analysis (Table 1). We also included three orders of birds as potential jaguar prey, namely Anseriformes, Gruiformes, and Pelecaniformes (Table 1). We did not consider shorebirds at species level, since they are social and our records correspond to mixed flocks, which would result in super-abundance estimates (Miller 1984; Farmer and Durbian 2006; Lanctot et al. 2008). Despite jaguars being recognized as also consuming crocodilians, turtles, and fishes (Supplementary Information – Table S1), we could not estimate their abundance from our camera-trap data since camera-trapping is not an adequate method for sampling those species and so we have no detection data for those groups. Therefore, they were not considered as drivers in our occupancy model analysis. However, we acknowledge that these groups are present on the islands (personal observation) and in the diet of the islands’ jaguars (Supplementary Information – Table S1), so likely also influence the spatial ecology of this felid.
Data Analysis. — The detectability and occupancy patterns of jaguars and their main prey were assessed using an occupancy modeling approach developed by MacKenzie et al. (2017). This modeling procedure estimates the probability of a site being occupied/used (ψ) by jaguar and/or its prey when detectability is imperfect (a detection probability of < 1, i.e., the species is present but not detected). We structured our model based on 25 sites (camera-trap stations) divided into six groups of 15 days. We also scaled the environmental variables (ha) and prey probability (%) into maximum likelihood estimates (Arnold 2010). Prior to conducting the modeling procedure, we tested for correlations among variables by estimating Spearman correlation coefficients between all independent variables and considered that no significant correlation existed if the coefficient value was between − 0.7 and 0.7 (Dormann et al. 2013).
The modeling procedure was implemented in two phases according to the following approach. First, we tested which factors were more influential in determining target species detectability while maintaining occupancy as constant (i.e., ψ = 1). We tested four environmental variables as drivers of jaguar detectability and for each prey type (Table 1 – Environmental variables). We built models corresponding to all combinations of these four environmental variables and ranked them according to the Akaike Information Criterion (AIC) (Burnham et al. 2011). We also calculated overdispersion factors (c-hat) to assess overdispersion of our best models (MacKenzie et al. 2017). The model with the lowest AIC and ΔAIC = 0 (the difference between the AIC of the model and the lowest AIC value; MacKenzie et al. (2017)) was identified as the best model and the variables included in this best model were considered the most influential variables of the detectability component of all subsequent models (Castro et al. 2022). Secondly, we used the variables from the best detectability model and included them in all models built subsequently to assess the drivers of occupancy. We followed the same approach described above for model building and selection (Castro et al. 2022) to conduct occupancy modeling. However, in this case, we tested three different hypotheses: H1 - jaguar occupancy is mostly determined by environmental variables (i.e., habitat) (Table 1); H2 – jaguar occupancy is more dependent on occupancy patterns of its potential prey (Table 1) (i.e., prey); H3 – jaguar occupancy is determined by the concurrent action of both habitat characteristics and prey occupancy (Table 1) (i.e., habitat + prey). Therefore, we built three sets of models. For H2 (prey), first we built occupancy models for each prey type individually and then identified the best model for each species (using the same procedure as applied for jaguars). We used the predicted occupancy from each of those prey models to estimate the probability of occupancy of each prey type per camera-trap station. Then, we used the percentage probabilities to determine approximate prey availabilities as covariates in H2 and H3. We identified the hypothesis with strongest support as that which presented the lowest AIC value for the best occupancy model (i.e., overall ΔAIC = 0). We identified informative variables by highlighting those whose 95% confidence interval of the coefficient did not include 0 (Arnold 2010), i.e., we were confident that their influence on jaguar occupancy was either positive or negative.
We used the software ‘R’ (R Core Team 2022) and the packages ‘camtrapR’ (Niedballa et al. 2016), ‘unmarked’ (Fiske and Chandler 2011), ‘wiqid’ (Meredith 2020), ‘AICcmodavg’ (Mazerolle and Mazerolle 2017), and ‘MuMIn’ (Barton 2009a; b) for these analyses.
Table 1
—Explanatory variables used (see text for details of habitat classification) to test each of our formulated ecological scenarios about jaguar detectability and occupancy patterns in ESEC-MJ.
Hypothesis | Variable | Data Type | Description |
Environmental variables (H1) | Grassland | Numerical continuous | Proportion in ha of Grassland within a 1 km radius |
Mangrove and teso | Numerical continuous | Proportion in ha of Mangrove and teso within a 1 km radius |
Wetland | Numerical continuous | Proportion in ha of Wetland within a 1 km radius |
Distance to water | Numerical continuous | Distance in m of camera traps to the water (sea, channels, and lagoons) |
Prey variables (H2) | Probability occurrence Agouti | Numerical continuous | Probability of occurrence of Agouti (0–99%) |
Probability occurrence White-tailed deer | Numerical continuous | Probability of occurrence of White-tailed deer (0–99%) |
Probability occurrence Giant anteater | Numerical continuous | Probability of occurrence of Giant anteater (0–99%) |
Probability occurrence Black-eared opossum | Numerical continuous | Probability of occurrence of Black-eared opossum (0–99%) |
Probability occurrence Tegu | Numerical continuous | Probability of occurrence of Tegu (0–99%) |
Probability occurrence Anseriformes | Numerical continuous | Probability of occurrence of Anseriformes (0–99%) |
Probability occurrence Gruiformes | Numerical continuous | Probability of occurrence of Gruiformes (0–99%) |
Probability occurrence Pelecaniformes | Numerical continuous | Probability of occurrence of Pelecaniformes (0–99%) |
Environmental and prey variables (H3) | Environmental variables + Occupancy probability of prey | Numerical continuous | Environmental variables (H1) together with the occupancy probability of each prey (H2) |