Population density modelling of mixed polymorphic phenotypes: an application of spatial mark‐resight models

Abstract

Melanism is a form of pigmentation polymorphism where individuals have darker colouration than what is considered the ‘wild’ phenotype. In the case of leopards, Panthera pardus, melanism occurs at higher frequencies amongst populations in tropical and subtropical moist forests of south and southeast Asia, presenting a unique challenge in estimating and monitoring these populations. Unlike the wild phenotype that is readily recognizable by its rosette patterns, melanism results in individuals being unidentifiable or ‘unmarked’ through photographic captures obtained using white flash cameras. Spatial mark‐resight (SMR) models that require only a subset of the population to be ‘marked’ offer the opportunity to estimate population density. In this study, we present an application of SMR models to estimate leopard densities using camera trap survey data from three sampling years at Manas National Park (MNP), India. By using an SMR model that allowed us to include captures of unidentified sightings of marked individuals, we were also able to incorporate captures where identity was either not confirmed or only known from a single flank. Following 18 674 trap‐days of sampling across three years, we obtained 728 leopard photo‐captures, of which 22.6% (165) were melanistic. We estimated leopard densities of 4.33, 2.61 and 3.37 individuals/100 km2 across the 3 years. To our best knowledge, these represent the first known estimates of leopard densities from populations comprising both melanistic and wild phenotypes. Finally, we highlight that SMR models present an opportunity to revisit past camera trap survey data for leopards and other species such as Jaguars, P. onca, that exhibit phenotypic polymorphism towards generating valuable information on populations.

Occupancy, density, and activity patterns of a Critically Endangered leopard population on the Kawthoolei‐Thailand border

“…Alternative camera models (see Hedges et al, 2015) may be more suitable for infrared‐based melanistic morph identification. For those populations with spotted and melanistic individuals, using all available captures, rather than only “marked” individuals, is likely to lead to more accurate results (Harihar et al, 2021). Our SMR models, which allowed for the inclusion of melanistic leopards that could not be positively identified, yielded higher density estimates with less uncertainty than standard SECR models.…”

“…Our objectives were (1) to provide the first estimate of leopard population density in Kawthoolei, (2) assess how various anthropogenic and environmental factors affect leopard site occupancy, and (3) to describe the activity patterns of both melanistic and spotted leopards, including any potential differences in activity between these morphs. When melanistic leopards are present, spatial mark‐resight (SMR) density estimation can be advantageous, which only requires a portion of the population to be identifiable (Chandler & Royle, 2013) and allows for the inclusion of all leopard captures, including blurry photographs, single‐flank photos, and melanistic leopards for which markings were indeterminate (Efford & Hunter, 2018; Harihar et al, 2021). Use of standard spatially explicit capture‐recapture (SECR) models can often result in the need to discard data when photographs are unclear, as can be the case for leopard populations in Southeast Asia (Wei et al, 2015).…”

High frequency of an otherwise rare phenotype in a small and isolated tiger population

“…Given recessive transmission and assuming HWE, the census data predicts an allele frequency of ∼0.6, similar to the allele frequency value (0.58) estimated from genetic data in our study ( n = 12). Mark-recapture models based on camera trap data have been used to estimate frequencies of melanistic leopards [e.g., see Harihar et al ( 65 )]. To the best of our knowledge, the seamless integration of genetic data in demographic analyses does not exist so far.…”

Individual Identification of Large Felids in Field Studies: Common Methods, Challenges, and Implications for Conservation Science

“…Such modifications highlight the prospect of camera traps being used to obtain individual identities of species even in situations when their pelage markings might be less prominent. If such camera modifications are impossible, the use of spatial mark-resight (SMR) models that only require a subset of the population to be identified may be used to provide reliable population estimates (Harihar et al, 2021).…”