American Society of Naturalists

Guillem Masó, Arpat Ozgul, and Patrick S. Fitze (Jan 2020)

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Climatic predictability: consistent small non-significant survival differences add up to important population decline

First robust evidence that environmental predictability can explain important population decline

Global climate change is exposing animal populations to novel conditions that might affect species persistence and increase their susceptibility to become extinct. Global climate change is known to affect average environmental conditions. For example, it leads to higher average temperatures and in some localities, it reduces precipitation. However, despite its effect on average conditions, it also affects environmental predictability (i.e. the correlation among measures of time series). For example, a decrease in precipitation predictability means that using precipitation falling today we can predict the likelihood that precipitation falls the next day, and this likelihood is smaller when predicting the next day’s precipitation in one month. While most studies tackle the effects of differences in average conditions, hardly any studies investigated whether animal population are affected by changes in environmental predictability, and how they will respond.

To understand whether and how changes in environmental predictability might affect species persistence, a research team from the Natural Museum of Natural Sciences in Madrid (MNCN-CSIC), the Pyrenean Institute of Ecology (IPE-CSIC), and the University of Zürich (UZH) tested the effects of differences in precipitation predictability on the European common lizard (Zootoca vivipara, subspecies louislantzi). They exposed lizard populations to more and less predictable precipitation over three years, and measured the lizard’s survival and reproduction, the latter using molecular paternity assignment. Thereafter, they used complex statistical population models to test for effects of precipitation predictability on population dynamics and the mechanisms underlying these effects.

How do populations respond to differences in environmental predictability? The team found that reduced precipitation predictability negatively affected population growth, which leads to a decrease in population size and increased extinction risk.

Which mechanisms are underlying this population decline? The study demonstrated that the prime suspect parameters (e.g. effects on reproduction) were not responsible for the observed decline. On the contrary, the team showed that small, but consistent differences in survival explained the negative effect on population growth. More specifically, mainly slight differences in adult survival, the least vulnerable age class, rather than survival of the competitively inferior yearlings and juveniles were responsible for the observed population decline.

The study shows that changes in environmental predictability might have major consequences on population dynamics and that the decrease in an environment’s predictability forecasted by climatic models may exacerbate the rate of the massive population declines currently observed in lizards, amphibians, and birds. Finally, the study as well shows that despite these massive effects, their detection under field conditions requires decades of research, since only complex analyses based on sophisticated and tricky to get data, allow to unravel them over short time intervals. This may explain why in many cases population declines are detected late and not until they are massive, which importantly affects their conservation.

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Abstract

Global climate change is leading to decreased climatic predictability. Theoretical work indicates that changes in the climate’s intrinsic predictability will affect population dynamics and extinction, but experimental evidence is scarce. Here, we experimentally tested whether differences in intrinsic precipitation predictability affect population dynamics of the European common lizard (Zootoca vivipara) by simulating more (MP) and less predictable (LP) precipitation in 12 semi-natural populations over 3 years and measuring different vital rates. A seasonal age-structured matrix model was parametrized to assess treatment effects on vital rates and asymptotic population growth (λ). There was a non-significant trend for survival being higher in MP than LP precipitation, and no differences existed in reproductive rates. Small non-significant survival differences in adults explained changes in λ and survival differences among age-classes were in line with predictions from cohort resonance. As a result, λ was significantly higher in MP than LP. This experimentally shows that small effects have major consequences on λ, that forecasted decreases in precipitation predictability are likely to exacerbate the current rate of population decline and extinction, and that stage-structured matrix models are required to unravel the aftermath of climate change.