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Matrix models for size-structured populations: unrealistic fast growth or simply diffusion?

Picard N, Liang J - PLoS ONE (2014)

Bottom Line: For 53 tree species from a tropical rain forest in the Central African Republic, the diffusion resulting from common matrix dimensions was much stronger than would be realistic.Moreover, λ could either increase or decrease when class width increased depending on the species.Overall, even if the class width should be kept small enough to limit diffusion, it had little impact on the estimate of λ for tree species.

View Article: PubMed Central - PubMed

Affiliation: UPR Biens et services des écosystèmes forestiers tropicaux (BSEF), Centre de coopération internationale en recherche agronomique pour le développement (CIRAD), Montpellier, France.

ABSTRACT
Matrix population models are widely used to study population dynamics but have been criticized because their outputs are sensitive to the dimension of the matrix (or, equivalently, to the class width). This sensitivity is concerning for the population growth rate (λ) because this is an intrinsic characteristic of the population that should not depend on the model specification. It has been suggested that the sensitivity of λ to matrix dimension was linked to the existence of fast pathways (i.e. the fraction of individuals that systematically move up a class), whose proportion increases when class width increases. We showed that for matrix population models with growth transition only from class i to class i + 1, λ was independent of the class width when the mortality and the recruitment rates were constant, irrespective of the growth rate. We also showed that if there were indeed fast pathways, there were also in about the same proportion slow pathways (i.e. the fraction of individuals that systematically remained in the same class), and that they jointly act as a diffusion process (where diffusion here is the movement in size of an individual whose size increments are random according to a normal distribution with mean zero). For 53 tree species from a tropical rain forest in the Central African Republic, the diffusion resulting from common matrix dimensions was much stronger than would be realistic. Yet, the sensitivity of λ to matrix dimension for a class width in the range 1-10 cm was small, much smaller than the sampling uncertainty on the value of λ. Moreover, λ could either increase or decrease when class width increased depending on the species. Overall, even if the class width should be kept small enough to limit diffusion, it had little impact on the estimate of λ for tree species.

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Related in: MedlinePlus

Variance of the dbh growth rate versus one-year dbh increment for 53 tree species.Notice that axes are in logarithmic scale. The line is the regression line on log-transformed data.
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pone-0098254-g004: Variance of the dbh growth rate versus one-year dbh increment for 53 tree species.Notice that axes are in logarithmic scale. The line is the regression line on log-transformed data.

Mentions: The relationship across species between the variance of the growth rate and the one-year dbh increment could be modeled by a power relationship: (Figure 4). Combining the Usher/Courant-Friedrichs-Lewy condition (8), condition (15) and this power relationship gives the following approximate interval for the class width:


Matrix models for size-structured populations: unrealistic fast growth or simply diffusion?

Picard N, Liang J - PLoS ONE (2014)

Variance of the dbh growth rate versus one-year dbh increment for 53 tree species.Notice that axes are in logarithmic scale. The line is the regression line on log-transformed data.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4048208&req=5

pone-0098254-g004: Variance of the dbh growth rate versus one-year dbh increment for 53 tree species.Notice that axes are in logarithmic scale. The line is the regression line on log-transformed data.
Mentions: The relationship across species between the variance of the growth rate and the one-year dbh increment could be modeled by a power relationship: (Figure 4). Combining the Usher/Courant-Friedrichs-Lewy condition (8), condition (15) and this power relationship gives the following approximate interval for the class width:

Bottom Line: For 53 tree species from a tropical rain forest in the Central African Republic, the diffusion resulting from common matrix dimensions was much stronger than would be realistic.Moreover, λ could either increase or decrease when class width increased depending on the species.Overall, even if the class width should be kept small enough to limit diffusion, it had little impact on the estimate of λ for tree species.

View Article: PubMed Central - PubMed

Affiliation: UPR Biens et services des écosystèmes forestiers tropicaux (BSEF), Centre de coopération internationale en recherche agronomique pour le développement (CIRAD), Montpellier, France.

ABSTRACT
Matrix population models are widely used to study population dynamics but have been criticized because their outputs are sensitive to the dimension of the matrix (or, equivalently, to the class width). This sensitivity is concerning for the population growth rate (λ) because this is an intrinsic characteristic of the population that should not depend on the model specification. It has been suggested that the sensitivity of λ to matrix dimension was linked to the existence of fast pathways (i.e. the fraction of individuals that systematically move up a class), whose proportion increases when class width increases. We showed that for matrix population models with growth transition only from class i to class i + 1, λ was independent of the class width when the mortality and the recruitment rates were constant, irrespective of the growth rate. We also showed that if there were indeed fast pathways, there were also in about the same proportion slow pathways (i.e. the fraction of individuals that systematically remained in the same class), and that they jointly act as a diffusion process (where diffusion here is the movement in size of an individual whose size increments are random according to a normal distribution with mean zero). For 53 tree species from a tropical rain forest in the Central African Republic, the diffusion resulting from common matrix dimensions was much stronger than would be realistic. Yet, the sensitivity of λ to matrix dimension for a class width in the range 1-10 cm was small, much smaller than the sampling uncertainty on the value of λ. Moreover, λ could either increase or decrease when class width increased depending on the species. Overall, even if the class width should be kept small enough to limit diffusion, it had little impact on the estimate of λ for tree species.

Show MeSH
Related in: MedlinePlus