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Changes in mean body size in an expanding population of a threatened species.

ABSTRACT: With some taxa, a reduction in the mean size of individuals may reflect over-harvesting and/or trophy hunting. However, we show that in sea turtles, a reduction in the mean size of breeding individuals may be part of the good news story of an expanding population. We describe a 70-fold increase in annual nest numbers on the island of Sal (Cape Verde, North Atlantic) between 2008 and 2020 (from 506 to 35 507 nests), making this now one of the largest loggerhead (Caretta caretta) nesting aggregations in the world. We use 20 128 measurements of the size of nesting turtles to show that their mean annual size has decreased by about 2.4 cm, from 83.2 to 80.8 cm. This decrease in the mean size of nesting turtles was not caused by the removal of larger turtles, for example by selective harvesting. Rather we develop a theoretical model to show than this decrease in mean size can be explained by an influx of first-time nesters, combined with a decrease in the size of those first-time nesters over time. A reduction in mean size of nesting turtles has been reported across the Atlantic, Pacific and Indian Oceans, and may be a common feature of population recoveries in sea turtles.

SUBMITTER: Hays GC

PROVIDER: S-EPMC9174726 | biostudies-literature | 2022 Jun

REPOSITORIES: biostudies-literature

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Changes in mean body size in an expanding population of a threatened species.

Hays Graeme C GC Taxonera Albert A Renom Berta B Fairweather Kirsten K Lopes Anice A Cozens Jacquie J Laloë Jacques-Olivier JO

Proceedings. Biological sciences 20220608 1976

With some taxa, a reduction in the mean size of individuals may reflect over-harvesting and/or trophy hunting. However, we show that in sea turtles, a reduction in the mean size of breeding individuals may be part of the good news story of an expanding population. We describe a 70-fold increase in annual nest numbers on the island of Sal (Cape Verde, North Atlantic) between 2008 and 2020 (from 506 to 35 507 nests), making this now one of the largest loggerhead (<i>Caretta caretta</i>) nesting ag ...[more]

PMID: 35673864

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Project description:Conservation efforts are leading to demographic growth and spatial expansion of some previously endangered species. However, past population bottlenecks or population size fluctuations can have lasting effects on effective population size (N e), even when census size (N c) appears large or recovered. The UK metapopulation of Eurasian otters (Lutra lutra) has a well-documented history of population recovery over recent decades, with indicators of presence (faeces and footprints) increasing in distribution and number over successive national surveys. To determine whether this increase in N c is reflected in increased N e, we analysed a large-scale microsatellite dataset (21 years: 1993-2014; 407 individuals) for signals of recent N e change using BOTTLENECK and LDNe, and evaluated potential biases associated with unaccounted spatial genetic structuring and inclusion of admixed genotypes. We obtained clear bottleneck signals in East England, and signals of recent population expansion in Wales and South West England in some analyses, consistent with national otter surveys and recent findings from whole-genome sequencing. Analyses that did not account for spatial genetic structuring yielded strong spurious signals of United Kingdom-wide population expansion, and N e estimates from these analyses were suppressed by a factor of 3-4. Inclusion of admixed individuals had weaker impacts on N e estimates, with overlapping 95% confidence intervals from different analyses. Notably, total N e summed across regions was small and well below the N e = 500 size deemed necessary for long-term population viability (sum of river basin district groups: 170.6, 95% C.I.: 102.1-348.3). Conclusions drawn from UK otter surveys, which had suggested a robust population close to panmixia, are therefore not supported by our genetic evidence. Our study highlights the value of including genetic monitoring of endangered or recovering species in monitoring plans, while also providing methodologically important information about N e estimation from real-world datasets.

Project description:Alternative metrics exist for representing variation in plant body size, but the vast majority of previous research for herbaceous plants has focused on dry mass. Dry mass provides a reasonably accurate and easily measured estimate for comparing relative capacity to convert solar energy into stored carbon. However, from a "plant's eye view", its experience of its local biotic environment of immediate neighbors (especially when crowded) may be more accurately represented by measures of "space occupancy" (S-O) recorded in situ-rather than dry mass measured after storage in a drying oven. This study investigated relationships between dry mass and alternative metrics of S-O body size for resident plants sampled from natural populations of herbaceous species found in Eastern Ontario. Plant height, maximum lateral canopy extent, and estimated canopy area and volume were recorded in situ (in the field)-and both fresh and dry mass were recorded in the laboratory-for 138 species ranging widely in body size and for 20 plants ranging widely in body size within each of 10 focal species. Dry mass and fresh mass were highly correlated (r2 > .95) and isometric, suggesting that for some studies, between-species (or between-plant) variation in water content may be unimportant and fresh mass can therefore substitute for dry mass. However, several relationships between dry mass and other S-O body size metrics showed allometry-that is, plants with smaller S-O body size had disproportionately less dry mass. In other words, they have higher "body mass density" (BMD) - more dry mass per unit S-O body size. These results have practical importance for experimental design and methodology as well as implications for the interpretation of "reproductive economy"-the capacity to produce offspring at small body sizes-because fecundity and dry mass (produced in the same growing season) typically have a positive, isometric relationship. Accordingly, the allometry between dry mass and S-O body size reported here suggests that plants with smaller S-O body size-because of higher BMD-may produce fewer offspring, but less than proportionately so; in other words, they may produce more offspring per unit of body size space occupancy.

Dataset Information

Changes in mean body size in an expanding population of a threatened species.

Publications

Changes in mean body size in an expanding population of a threatened species.

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