Harvested amphibians are often assessed only by how many adults are present or how much is caught. But for conservation, that is only part of the story. To understand whether a population can persist under harvest pressure, it is more useful to ask which life stages most strongly shape population growth. For the African bullfrog Hoplobatrachus occipitalis in Dangbo, Benin (Figure 1), our demographic analysis provides a practical answer: the population depends most on reproduction and juvenile recruitment.

Map of the Commune of Dangbo, Ouémé Department, Benin, the study's sampling area, with an inset locating it within Benin
Figure 1. Commune of Dangbo in Benin. The sampling area of the exploited African bullfrog Hoplobatrachus occipitalis was the Commune of Dangbo (shaded polygon), Ouémé Department, Republic of Benin (inset map). Labels adjacent to Dangbo Commune refer to adjacent communes. Map border labels refer to UTM, Zone 31 projection. Photo: Hyppolite Agadjihouèdé

A two-stage model for a harvested frog

We used a simple two-stage matrix population model with juveniles and adults (Figure 2). Adult survival came from capture–mark–recapture analysis, juvenile survival was assumed from available information, and fecundity was estimated from field reproductive output. Following our earlier work on frog exploitation in the Ouémé valley (1), and using standard field handling guidance for amphibians (2), we estimated a population growth rate of lambda = 0.5756, indicating decline under current conditions.

Diagram of a two-stage projection matrix model separating juvenile and adult African bullfrogs
Figure 2. Stage-structure projection matrix for the exploited African bullfrog Hoplobatrachus occipitalis. The two-stage model separates juveniles and adults and was used to estimate population growth rate and elasticity. It provides the demographic basis for identifying conservation priorities. Photo: Thierry Matinkpon Agblonon Houélomè

What the elasticity analysis revealed

The elasticity analysis gave the most important result (Figure 3). Population growth was equally sensitive to adult reproduction and juvenile maturation and survival, with elasticity values of 0.4543 for each of those transitions. By contrast, adult survival had much lower influence, with an elasticity of 0.0913. In simple terms, improving the number of young frogs produced and the number that survive to adulthood would have a much larger effect on population recovery than increasing adult survival alone.

Elasticity heatmap showing population growth is most sensitive to adult reproduction and juvenile recruitment
Figure 3. Elasticity heatmap for Hoplobatrachus occipitalis in Dangbo, Benin. The heatmap shows that population growth is most sensitive to adult reproduction and juvenile recruitment, while adult survival has a smaller effect on lambda. This highlights breeding and nursery habitats as the key management targets. Photo: Imaculé Schadrac Baglo

This matters because the most obvious conservation action is not always the most effective one. In species like H. occipitalis, adults are important, but the whole population can still decline if breeding success is poor or juveniles do not survive well enough to replace adults. Our scenario testing showed that increasing juvenile survival from 0.2 to 0.3 raised lambda to 0.9591, which is close to stability (Table 1; Figure 4). That is a strong signal that modest improvements in juvenile conditions could make a meaningful difference.

Tadpole→juvenile rate (r)Juvenile survival (sⱼ)Growth rate (λ)Population status
0.050.20.5756Declining (current estimate)
0.100.30.9591Stable / near stable
0.200.31.3356Growing
0.050.050.3151Rapid decline

Table 1. Population viability scenarios for Hoplobatrachus occipitalis in Dangbo, Benin. Modest gains in juvenile survival and the tadpole-to-juvenile transition rate shift the population from decline toward stability.

Line chart of predicted population growth rate under different juvenile survival scenarios
Figure 4. Predicted population growth rate under different juvenile survival scenarios. Scenario testing shows that modest improvements in juvenile survival can move the population close to stability, confirming juvenile recruitment as a major conservation lever. Photo: Houénafa Aimé Chrysostome Gansa

What this means for conservation and harvest

For practitioners, the message is straightforward. Conservation should focus first on breeding wetlands and juvenile nursery habitats. Shallow wetlands, sheltered breeding sites, and areas with reduced disturbance likely play a major role in recruitment. Protecting these habitats will support egg production, tadpole development, and juvenile survival, all of which are critical for long-term persistence.

This also has implications for sustainable harvest. If harvest pressure removes too many breeding adults, reproduction can fall sharply. If juvenile habitats are degraded, few young frogs will survive long enough to replenish the population. A harvest strategy that ignores those two stages may look effective in the short term but still allow the population to keep declining. The most sensible approach is therefore to combine habitat protection with community-based monitoring and harvest regulation.

Our findings also point to one clear research need: juvenile survival should be measured directly in the field. In this study, juvenile survival was assumed rather than estimated from capture data, so future work should refine that parameter. Better estimates will make population models more realistic and help managers set stronger harvest and habitat-protection measures.

In summary, Hoplobatrachus occipitalis in Dangbo appears vulnerable under current demographic conditions, but the model also shows where action can be most effective. Protecting breeding adults is important, but protecting the next generation may matter even more. For this exploited amphibian, the path toward sustainability runs through the wetlands that support reproduction and juvenile survival.

Acknowledgments

We are grateful to the field assistants and local community members in Dangbo for their collaboration, and to the Amphibian Survival Alliance for funding. All primary data, habitat maps, and R scripts used for the analyses are available from the corresponding author upon request.

References

  1. Gansa, H.A.C., Agadjihouèdé, H. & Hounkanrin, M.B. Exploitation des grenouilles dans la vallée de l'Ouémé, République du Bénin (Afrique de l'Ouest). International Journal of Sciences: Basic and Applied Research 59(1), 49–62 (2021).
  2. Herpetological Animal Care and Use Committee. Guidelines for the Use of Live Amphibians and Reptiles in Field Research. (2004).