Reptile populations in decline
The Living Planet Index (LPI) is a measure of the state of the world's biological diversity based on population trends of vertebrate species from terrestrial, freshwater and marine habitats. The LPI has been adopted by the Convention of Biological Diversity (CBD) as an indicator of progress towards its 2011-2020 target to 'take effective and urgent action to halt the loss of biodiversity. The LPI is based on trends of thousands of population time series collected from monitored sites around the world. This online portal allows registered participants to search for and contribute data.
The LPI a global biodiversity indicator, built from aggregated abundance trends of vertebrate species populations, and is used to communicate biodiversity trends and monitor progress toward the Aichi Biodiversity Targets set by the Convention on Biological Diversity. The index aggregates individual time series of vertebrate population sizes or proxies from around the world to track average changes in abundance through time; it does this by averaging the change in abundance of species over time and conventionally begins in 1970. This method has also been applied to produce LPIs by biogeographic realm and at regional and national scales (e.g., Mediterranean wetland species; Arctic vertebrates). Furthermore, the LPI has been used to investigate the effectiveness of conservation management by measuring species abundance trends in protected areas across Africa and to test policy scenarios.
Underlying the LPI is a database containing over 18,000 population time series from more than 3,600 species. Reptiles are represented by 194 species and 549 population time series within the database: this equates to 1.85% of all described reptile species as of August 2016 and 6% of described vertebrates. Taxonomic coverage in the database is a consequence of the approach to data collection. Because the database is populated using data available in the public domain, the content is neither completely random, as it is subject to biases inherent in ecological and conservation literature, nor is it targeted, as the aim is to collect all of the available data on any vertebrate species from as broad a geographic coverage as possible. Coupled with this is the temporal disparity among the monitored populations in the database—population time series cover a large range of time periods, from 2–70 yr. Any indicators produced from this database require an understanding of the taxonomic, geographic, and temporal representation of the underlying data. Bias is inherent in global biodiversity databases but can be dealt with if impacts of biases on biodiversity indicators such as the LPI are recognized and corrected where possible.
In a forthcoming article in the Journal of Herpetology, Saha et al. (2018) investigate global population trends and data gaps for reptiles using the LPI method. This represents the first time the LPI has been used to analyze in detail the global trends for a particular taxon. The authors analyze trends in reptile populations over time globally and in taxonomic and spatial subsets of the data. Saha et al. identify data gaps by examining the taxonomic and geographic representativeness of the data set and test for bias toward threatened species. They also assess the sources and quality (completeness and length) of reptile population time series to provide suggestions for reptile monitoring and conservation and establish priorities for expanding the current reptile data set.
Using data for 549 reptile populations representing 194 species from the Living Planet database, we provide the first detailed analysis of this database for a specific taxonomic group. We estimated an average global decline in reptile populations of 54–55% between 1970 and 2012. Disaggregated indices at taxonomic, system and biogeographical levels showed trends of decline, often with wide confidence intervals because of a prevalence of short time series. We assessed gaps in our reptile time-series data and examined what types of publication they primarily originated from to provide an overview of the range of data sources captured in the Living Planet database. Data were biased toward crocodilians and chelonians, with only 1% and 2% of known lizard and snake species represented, respectively. Population time-series data stemmed primarily from published ecological research (squamates) and data collected for conservation management (chelonians and crocodilians). We recommend exploration of novel survey and analytical techniques to increase monitoring of reptiles, especially squamates, over time. Open access publication and sharing of data sets are vital to improving knowledge of reptile status and trends, aided by the provision of properly curated databases and data-sharing agreements. Such collaborative efforts are vital to effectively address global reptile declines.
Saha A, McRae L, Dodd CK, Gadsden H, Hare KM, Lukoschek V, Bohm M. 2018. Tracking Global Population Trends: Population Time-Series Data and a Living Planet Index for Reptiles. Journal of Herpetology 52(3):259-268.