Modelling the effect of human-caused mortality on a lion sub-population using spreadsheets

Herrmann, Eric (2004-03)

Thesis (MScFor)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: Free-ranging lions (Panthera leo) in the Kgalagadi Transfrontier Park (KTP) have been subject to persecution by farmers following livestock depredation in adjacent grazing areas. In recent years at least one adult female was killed annually from a sub-population of five boundary prides that have home ranges adjoining these livestock grazing areas. While no long-term records of human-caused mortality are available, the impact of current rates of human-caused mortality is uncertain. Female-based, age-structured models were thus used to estimate the long-term viability of the KTP lion sub-population subjected to human-caused mortality under deterministic and stochastic environmental conditions. Population parameters incorporated in the models included age-class specific natural mortality, female fecundity and birth sex ratio. In so doing sustainable threshold rates of persecution were established, so that effective conservation measures can be taken, if required, to ensure the continued survival of boundary prides in the park. Sensitivity analyses of natural survival rates indicated that adult female survivorship is the most important population parameter with respect to maintaining population viability, compared to younger female age-classes, fecundity or birth sex ratio. Hence adults were also the most sensitive age-class with respect to human-caused mortality, as adult survival repeatedly acts upon individuals with the highest reproductive value. In the deterministic model, with the most optimistic survival parameter values, fecundity and birth sex ratio (female-biased) estimates, the sub-population is only able to sustain an annual persecution of three adult females, before the sub-population exhibits a sustained decline. In the worst-case scenario, where fecundity and sex ratio estimates are at their lower extremes, the maximum sustainable age-class specific persecution rate is zero, for all age-classes. Whilst these hypothetical scenarios are unrealistic, they do highlight the extreme thresholds of potentially sustainable persecution rates. Under the most optimistic scenario using the stochastic model, the highest achievable survival probability of the sub-population, when subjected to a persecution rate of one adult annually, was 78%. Although increased fecundity and birth sex ratio biased towards females may increase the survival probability, these parameters are generally at their mean values in the long-term, and may thus not necessarily prevent a sustained population decline. The models therefore suggest that the current persecution rate of one adult annually (or 4% of the adult sub-population), appears unsustainable in the longterm. To ensure the survival of existing boundary prides and to maintain a viable subpopulation, adult lionesses should, as far as possible, be afforded protection from persecution.

AFRIKAANSE OPSOMMING: Vrylewende leeus (Panthera leo) in die Kgalagadi Oorgrenspark (KTP) is onderworpe aan vervolging deur boere as gevolg van predasie op vee in aangrensende weidingsgebiede. In onlangse jare is ten minste een volwasse wyfie uit ‘n subpopulasie van vyf troppe met loopareas wat aan die weidingsgebiede grens, jaarliks uitgewis. Geen langtermyn rekords van vrektes as gevolg van menslike oorsake is beskikbaar nie, en die impak van vrektes wat deur mense veroorsaak word, is dus onseker. Wyfie-gebaseerde, ouderdomgestruktureerde modelle is daarom gebruik om te voorspel wat die langtermyn lewensvatbaarheid is van die KTP leeu subpopulasie wat onderworpe is aan vrektes deur menslike invloede onder deterministiese en stogastiese omgewingsfaktore. Bevolkingsfaktore wat deur die modelle in ag geneem is, sluit ouderdomsgroep-spesifieke natuurlike vrektes, aanwas van wyfies en geboorte geslagsverhouding in. Sodoende is volhoubare uitwissingstempos bepaal sodat, indien nodig, effektiewe bewaringsmeganismes toegepas kan word om die voortbestaan van troppe in die grensgebiede van die park te verseker. Sensitiwiteitsanalises van natuurlike oorlewingstempos het aangetoon dat volwasse wyfies se oorlewing die belangrikste bevolkingsfaktor is om die bevolking se lewensvatbaarheid te volhou vergeleke met jonger wyfie ouderdomsgroepe, aanwas of geslagsverhouding by geboorte. Daarom was volwassenes ook die sensitiefste vir vrektes as gevolg van menslike invloede, omdat die oorlewing van volwassenes herhaaldelik inwerk op individue met die hoogste reproduktiewe waarde. In die deterministiese model met die mees optimistiese oorlewing, aanwas en geboorte verhouding (wyfie-gebasseerde) beramings, is die subpopulasie slegs in staat om ‘n jaarlikse uitwissing van drie volwasse wyfies te onderhou, voordat die sub-populasie ‘n volgehoue afname toon. In die uiterste geval waar aanwas en geslags verhouding skattings op die laagste is, is die maksimum volhoubare ouderdomsklasspesifieke beramings nul vir alle ouderdomsklasse. Terwyl hierdie hipotetiese senario’s onrealisties is, onderstreep dit die uiterste vlakke van potensiële volhoubare uitwissingstempos. Onder die mees optimistiese senario – ‘n uitwissingstempo van een volwassene jaarliks – met die gebruik van die stogastiese model, was die hoogste haalbare oorlewingswaarskynlikheid van die sub-populasie 78%. Alhoewel ‘n toename in aanwas en geboorte geslagsverhouding (met oorhelling na wyfies), ‘n toename in oorlewingswaarskynlikheid tot gevolg kan hê, is hierdie faktore oor die algemeen gemiddeld oor die langtermyn en sal dit nie noodwendig ‘n volgehoue afname in die populasie verhoed nie. Die modelle dui daarop dat die huidige uitwissingstempo van een volwassene (of 4% van die volwasse sub-populasie) op ‘n jaarlikse grondslag onvolhoubaar is oor die langtermyn. Om die oorlewing van bestaande troppe in die grensgebiede, asook ‘n lewensvatbare subpopulasie, te verseker, moet volwasse leeuwyfies so ver as moontlik teen vervolging beskerm word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50025
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