Abstract
Since 1994, IUCN Red List assessments apply globally acknowledged standards to assess species distribution, abundance and trends. The extinction risk of a species has a major impact on conservation science and international funding mechanisms. Great ape species are listed as Endangered or Critically Endangered. Their populations are often assessed using their unique habit of constructing sleeping platforms, called nests. As nests rather than apes are counted, it is necessary to know the time it takes for nests to disappear to convert nest counts into ape numbers. However, nest decomposition is highly variable across sites and time and the factors involved are poorly understood. Here, we used 1,511 bonobo (Pan paniscus) nests and 15 years of climatic data (2003–2018) from the research site LuiKotale, Democratic Republic of the Congo, to investigate the effects of climate change and behavioural factors on nest decay time, using a Bayesian gamma survival model. We also tested the logistic regression method, a recommended time-efficient option for estimating nest decay time. Our climatic data showed a decreasing trend in precipitation across the 15 years of study. We found bonobo nests to have longer decay times in recent years. While the number of storms was the main factor driving nest decay time, nest construction type and tree species used were also important. We also found evidence for bonobo nesting behaviour being adapted to climatic conditions, namely strengthening the nest structure in response to unpredictable, harsh precipitation. By highlighting methodological caveats, we show that logistic regression is effective in estimating nest decay time under certain conditions. Our study reveals the impact of climate change on nest decay time in a tropical remote area. Failure to account for these changes would invalidate biomonitoring estimates of global significance, and subsequently jeopardize the conservation of great apes in the wild.
Highlights
In the past 50 years, a marked increase in global mean temperature due to anthropogenicinduced climate change has affected tropical rainforests inhabited by the great apes, orangutans (Pongo spp.), gorillas (Gorilla spp.), chimpanzees (Pan troglodytes subspp.) and bonobos (Pan paniscus) [1,2,3]
By looking at the decrease in expected log predictive density (ELPD) values given by removing a particular variable, we evaluated the importance of each factor in driving nest decomposition time
We found that nests constructed in species such as Anonidium mannii (129 days) or Plagiostyles africana (113 days) lasted considerably longer than those constructed in trees such as Scorodophloeus zenkeri (84 days) or Trichoscypha arborescens (77 days; only used in per month in 2016–2018 (P2)) (S1 Table)
Summary
In the past 50 years, a marked increase in global mean temperature due to anthropogenicinduced climate change has affected tropical rainforests inhabited by the great apes, orangutans (Pongo spp.), gorillas (Gorilla spp.), chimpanzees (Pan troglodytes subspp.) and bonobos (Pan paniscus) [1,2,3]. It is generally recommended to apply site- and time-specific conversion factors to data generated from a particular survey [35], ideally obtained by monitoring large samples of nests (representative of the survey period) until full decomposition [35] As such studies could last several months, Great ape nest decay and climate change more time-efficient methods have been developed, such as the retrospective estimation of nest decay with a single revisit of a marked nest, using logistic regression [57], or hidden Markov chain [58] analysis. If we do not use values obtained for the specific survey period and location in question, we cannot account for climate-related changes in mean decomposition time, hindering our ability to correctly estimate population trends
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