An approach to determine the extinction risk of exploited populations

Crookes, Douglas J.; Blignaut, James Nelson

doi:10.1016/j.jnc.2019.125750

Cited by 7 publications

(7 citation statements)

References 67 publications

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“…Therefore, the requirement that there is interspecific competition for the model to be applicable is not a strong one, as the data would indicate a zero morality coefficient for the interaction term if there was no interspecific competition. This method of estimating parameters in a population model and using it to determine population trajectories is highly suitable for use in data poor environments (Crookes & Blignaut, 2019). In this study, the mortality coefficient estimated using the home range data and the Gause model was positive, further strengthening the assertion that there is interspecific competition between caracal and blackbacked jackal.…”

Section: Validating the Gaussian Modelsupporting

confidence: 62%

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Ecology, opportunity or threat? Drivers of the caracal (Caracal caracal) population decline in South Africa

Crookes

2023

Afr J Ecol

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Anecdotal evidence suggests that there is interspecific competition between black‐backed jackal (Canis mesomelas) and caracal (Caracal caracal) on farmland in South Africa. Home range data were used across both years (1983–2016) and provinces, as well as polynomial interpolation, to quantitatively assess changes in the population of caracal and black‐backed jackal over time. The jackal abundance estimates had low uncertainty, while the caracal data had high uncertainty. A Gaussian model of interspecific competition was developed, using the properties of the Gaussian model and the more reliable jackal abundance data to reduce the uncertainty associated with the caracal data. It was found that caracals have actually declined in South Africa over the past few decades. Caracals are both beneficial to society in terms of trade and other uses and a threat to farmers in terms of livestock predation. To explore the drivers for hunting further, a predator–prey model was developed. The model found that killing for trade is not the main driver for declines in caracal numbers. However, reducing the persecution of caracal on farmland actually stabilises the number of black‐backed jackal, resulting in ‘win–win’ outcomes for both farmers and caracal. A concerted Africa‐wide response to caracal declines is needed.

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Section: Validating the Gaussian Modelsupporting

confidence: 62%

“…Calibration was the method used to estimate the unknown parameter values in the model. Vensim provides an improved means of calibrating these co‐evolutionary models compared with standard spreadsheets (Crookes & Blignaut, 2019).…”

Section: Methodsmentioning

confidence: 99%

Ecology, opportunity or threat? Drivers of the caracal (Caracal caracal) population decline in South Africa

Crookes

2023

Afr J Ecol

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“…Two species of rhino are endemic to Africa: Diceros bicornis (black rhino) and Ceratotherium simum (white rhino). High levels of poaching, particularly in southern Africa, are putting both of these species at increased risk of extinction despite the implementation of extensive antipoaching strategies (Lindsey and Taylor 2011;Cheteni 2014;Mukwazvure and Magadza 2014;Emslie et al 2019;Haas and Ferreira 2016;Crookes and Blignaut 2019;Ferreira et al 2015). They are currently listed as critically endangered (Emslie 2020b) and near threatened (Emslie 2020a), respectively, on the International Union for Conservation of Nature's Red List (IUCN Red List).…”

Section: Introductionmentioning

confidence: 99%

Effects of dehorning on population productivity in four Namibia sub-populations of black rhinoceros (Diceros bicornis bicornis)

et al. 2022

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The black rhinoceros (rhino) (Diceros bicornis) is critically endangered, with poaching being one of several threats to the species’ survival. Many reserves across several countries, including Namibia, South Africa, and Zimbabwe, now dehorn their rhinos in an attempt to reduce poaching. Historical data collected by the Namibian Ministry of Environment, Forestry, and Tourism and Save the Rhino Trust were used to investigate whether dehorning has an effect on age at first reproduction (AFR), inter-calving interval (ICI), birth sex ratios, calf survival, cause of death, and lifespan. These were assessed in four Namibian sub-populations (hereafter referred to as ‘populations’) of black rhino (denoted A, B, C, and D) which have undergone varying levels of dehorning. No significant difference was found in any of the variables between dehorned and horned individuals. Population was a significant predictor of AFR (LRT = 7.433, p = 0.024) and ICI (LRT = 13.281, p = 0.004), although pairwise comparisons only found populations A and B to be significantly different (AFR: z = −2.736, p = 0.017, ICI: z = −3.705, p = 0.001). Additionally, a significantly higher number of males than females were born in population D (p = 0.021, CI = 0.544, 0.960). The main cause of death across all individuals was poaching, although there was no significant difference in the proportion of rhinos poached between dehorned and horned individuals (X2 = 0.638, p = 0.424, n = 265). No evidence was found to suggest that dehorning has any effect on AFR, ICI, birth sex ratios, calf survival, or lifespan in the black rhino, which is reassuring in the continued use of dehorning as an anti-poaching technique in this species.

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“…where p(x,y), is the price paid to poachers per poached individual, and c(x,y) is the cost of poaching per unit poaching effort. Equation ( 2) is the standard way of modelling harvest effort in theoretical population biology (Hilborn et al 2006;Mansal et al 2014;Holden et al 2018;Crookes and Blignaut 2019). It says poachers increase effort when poaching is profitable and decrease effort when it is unprofitable, with the parameter a controlling how fast poachers adjust effort.…”

Section: Poacher-population Dynamic Modelmentioning

confidence: 99%

Poacher-population dynamics when legal trade of naturally deceased organisms funds anti-poaching enforcement

Holden

Lockyer

2021

Journal of Theoretical Biology

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An approach to determine the extinction risk of exploited populations

Cited by 7 publications

References 67 publications

Ecology, opportunity or threat? Drivers of the caracal (Caracal caracal) population decline in South Africa

Ecology, opportunity or threat? Drivers of the caracal (Caracal caracal) population decline in South Africa

Effects of dehorning on population productivity in four Namibia sub-populations of black rhinoceros (Diceros bicornis bicornis)

Poacher-population dynamics when legal trade of naturally deceased organisms funds anti-poaching enforcement

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