Variation among Individuals and Reduced Demographic Stochasticity

Area of Science:

• Ecology
• Conservation Biology
• Population Dynamics

Background:

• Population viability analysis (PVA) uses stochastic demographic models to predict extinction risk.
• Higher variance in demographic rates generally increases extinction risk.
• Current PVA models assume identical expected fates for all individuals, modeling demographic stochasticity via distributions like the binomial.

Purpose of the Study:

• To investigate how individual variation in expected survival affects demographic stochasticity in PVA.
• To determine if current PVA models overestimate extinction risk due to this assumption.

Main Methods:

• Development of a simple conceptual model to explore individual variation effects.
• Analysis of demographic stochasticity in survival using Jensen's inequality.
• Consideration of individual variation in fecundity and its impact on extinction risk.

Main Results:

• Existing PVA models overestimate demographic stochasticity in survival when individual variation in expected survival exists.
• This overestimation is linked to Jensen's inequality and the concave nature of the binomial variance function.
• The impact of individual variation on fecundity-driven stochasticity is uncertain but could reduce extinction risk.

Conclusions:

• Standard PVA models may inflate extinction probabilities by not accounting for individual heterogeneity in survival.
• Incorporating individual variation in survival could lead to more accurate extinction risk assessments.
• Further research is needed to understand individual variation in fecundity for comprehensive PVA.