Limits...
Predicting climate change impacts on polar bear litter size.

Molnár PK, Derocher AE, Klanjscek T, Lewis MA - Nat Commun (2011)

Bottom Line: In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40-73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55-100% if break-up occurs 2 months earlier.Simultaneously, mean litter size would decrease by 22-67% and 44-100%, respectively.Similar litter size declines may occur in over one-third of the global polar bear population.

View Article: PubMed Central - PubMed

Affiliation: Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2G1. pmolnar@ualberta.ca

ABSTRACT
Predicting the ecological impacts of climate warming is critical for species conservation. Incorporating future warming into population models, however, is challenging because reproduction and survival cannot be measured for yet unobserved environmental conditions. In this study, we use mechanistic energy budget models and data obtainable under current conditions to predict polar bear litter size under future conditions. In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40-73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55-100% if break-up occurs 2 months earlier. Simultaneously, mean litter size would decrease by 22-67% and 44-100%, respectively. The expected timeline for these declines varies with climate-model-specific sea ice predictions. Similar litter size declines may occur in over one-third of the global polar bear population.

Show MeSH

Related in: MedlinePlus

The relationship between maternal energy density and litter size.(a) Estimated probabilities for a pregnant female polar bear to have one (p1, dashed line), two (p2, solid line) or three cubs (p3, dotted line) at den emergence as a function of maternal energy density (defined as storage energy relative to lean body mass) at den entry, as determined by multinomial logistic regression. (b) Expected mean litter size at den emergence as a function of maternal energy density at den entry (solid line), calculated as X(E/LBM)=p1(E/LBM)+2p2(E/LBM)+3p3(E/LBM). Data are observed litter sizes at den emergence as a function of maternal energy density at den entry (N=28).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3105343&req=5

f1: The relationship between maternal energy density and litter size.(a) Estimated probabilities for a pregnant female polar bear to have one (p1, dashed line), two (p2, solid line) or three cubs (p3, dotted line) at den emergence as a function of maternal energy density (defined as storage energy relative to lean body mass) at den entry, as determined by multinomial logistic regression. (b) Expected mean litter size at den emergence as a function of maternal energy density at den entry (solid line), calculated as X(E/LBM)=p1(E/LBM)+2p2(E/LBM)+3p3(E/LBM). Data are observed litter sizes at den emergence as a function of maternal energy density at den entry (N=28).

Mentions: We used multinomial logistic regression models30 on data from 28 pregnant females with known litter sizes to test whether litter size at den emergence can be predicted from maternal age (A), storage energy29 at den entry (E), energy density29 at den entry (E/LBM), or certain combinations of these variables (see Table 1 and Methods for details). The model with only energy density was an excellent predictor of litter size at den emergence (likelihood ratio test, P=0.0004), and the regression probabilities of having one, two or three cubs (Fig. 1) are 1 2 3 This model also explained the data significantly better than the regression models with storage energy or age (Table 1), and it was therefore used in subsequent calculations to predict litter size from body condition at den entry. Because females with insufficient energy stores do not enter maternity dens2526, we further augmented the model by assuming an energy density threshold for reproduction3132, setting the probability of not reproducing, p0, to 1 (with p1=p2=p3=0) if E/LBM<20.0 MJ kg−1, and to 0 (with p1, p2 and p3 given by equations (1)–(3)equations (1)–(3)equations (1)–(3)) otherwise (see Methods and Supplementary Note online for details). The threshold was chosen equal to the lowest den entry energy density ever observed for a female that produced at least one cub26. Energy densities in our sample of females were consistent with this parameterization of the reproduction threshold, ranging from 20.2 to 30.8 MJ kg−1 at den entry.


Predicting climate change impacts on polar bear litter size.

Molnár PK, Derocher AE, Klanjscek T, Lewis MA - Nat Commun (2011)

The relationship between maternal energy density and litter size.(a) Estimated probabilities for a pregnant female polar bear to have one (p1, dashed line), two (p2, solid line) or three cubs (p3, dotted line) at den emergence as a function of maternal energy density (defined as storage energy relative to lean body mass) at den entry, as determined by multinomial logistic regression. (b) Expected mean litter size at den emergence as a function of maternal energy density at den entry (solid line), calculated as X(E/LBM)=p1(E/LBM)+2p2(E/LBM)+3p3(E/LBM). Data are observed litter sizes at den emergence as a function of maternal energy density at den entry (N=28).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3105343&req=5

f1: The relationship between maternal energy density and litter size.(a) Estimated probabilities for a pregnant female polar bear to have one (p1, dashed line), two (p2, solid line) or three cubs (p3, dotted line) at den emergence as a function of maternal energy density (defined as storage energy relative to lean body mass) at den entry, as determined by multinomial logistic regression. (b) Expected mean litter size at den emergence as a function of maternal energy density at den entry (solid line), calculated as X(E/LBM)=p1(E/LBM)+2p2(E/LBM)+3p3(E/LBM). Data are observed litter sizes at den emergence as a function of maternal energy density at den entry (N=28).
Mentions: We used multinomial logistic regression models30 on data from 28 pregnant females with known litter sizes to test whether litter size at den emergence can be predicted from maternal age (A), storage energy29 at den entry (E), energy density29 at den entry (E/LBM), or certain combinations of these variables (see Table 1 and Methods for details). The model with only energy density was an excellent predictor of litter size at den emergence (likelihood ratio test, P=0.0004), and the regression probabilities of having one, two or three cubs (Fig. 1) are 1 2 3 This model also explained the data significantly better than the regression models with storage energy or age (Table 1), and it was therefore used in subsequent calculations to predict litter size from body condition at den entry. Because females with insufficient energy stores do not enter maternity dens2526, we further augmented the model by assuming an energy density threshold for reproduction3132, setting the probability of not reproducing, p0, to 1 (with p1=p2=p3=0) if E/LBM<20.0 MJ kg−1, and to 0 (with p1, p2 and p3 given by equations (1)–(3)equations (1)–(3)equations (1)–(3)) otherwise (see Methods and Supplementary Note online for details). The threshold was chosen equal to the lowest den entry energy density ever observed for a female that produced at least one cub26. Energy densities in our sample of females were consistent with this parameterization of the reproduction threshold, ranging from 20.2 to 30.8 MJ kg−1 at den entry.

Bottom Line: In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40-73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55-100% if break-up occurs 2 months earlier.Simultaneously, mean litter size would decrease by 22-67% and 44-100%, respectively.Similar litter size declines may occur in over one-third of the global polar bear population.

View Article: PubMed Central - PubMed

Affiliation: Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2G1. pmolnar@ualberta.ca

ABSTRACT
Predicting the ecological impacts of climate warming is critical for species conservation. Incorporating future warming into population models, however, is challenging because reproduction and survival cannot be measured for yet unobserved environmental conditions. In this study, we use mechanistic energy budget models and data obtainable under current conditions to predict polar bear litter size under future conditions. In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40-73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55-100% if break-up occurs 2 months earlier. Simultaneously, mean litter size would decrease by 22-67% and 44-100%, respectively. The expected timeline for these declines varies with climate-model-specific sea ice predictions. Similar litter size declines may occur in over one-third of the global polar bear population.

Show MeSH
Related in: MedlinePlus