A Comparative Demography of Plants Based Upon Elasticities of Vital Rates

Abstract

Abstract. Elasticities of matrix elements from population projection matrices are commonly used to analyze the relative contributions of different life history transitions (birth, survival, growth) to the finite rate of increase (). Hitherto, comparative demography based on matrix models has relied upon decomposing elasticity matrices into blocks, each containing matrix elements deemed to represent recruitment, stasis, or progression to larger size classes. Elasticities across an entire matrix always sum to unity, and different populations and species can be compared on the basis of the relative proportions of these three variables. This method has been widely used, but it contains a weakness in that the value of matrix elements is a function of more than one vital rate. For example, transitions representing progression to larger size classes involve a survival rate as well as a growth rate. Ideally, then, demographic comparisons between populations should be made using elasticities of vital rates themselves, rather than elasticities of matrix elements that are compounds of those rates. Here, we employ the complete set of general equations for the elasticity of vital rates in an entirely new analysis of matrices for 102 species of perennial plants. The results show a surprising similarity to an earlier analysis based upon matrix element elasticity and provide important confirmation of general patterns of correlation between plant life history and demography. In addition, we show that individual vital rate elasticities cannot, on their own, predict variation in life history. Therefore, all three demographic processes (survival, growth, and reproduction) are necessary to account for life history variation. The new analysis provides a firmer foundation for comparative demography.

Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

“…For example, a recent model for the impact of overhunting on an animaldispersed tree species assumed seed dispersal affected vital rates of only seeds and seedlings and concluded that local extinction of large-bodied dispersers resulted in only slight decreases in tree population growth rate [5]. Similar to that study and others that employ size-structured models for tree population dynamics [14,21], we found high sensitivity of total population density to size-dependent survival, which highlights the importance of adult survival to population dynamics. Our spatially explicit model demonstrates that these previous sensitivity analyses of size-structured populations do not contradict empirical evidence for major impacts of overhunting on tree demography over decadal timescales, including declines in animal-dispersed tree abundance [12] and rapid evolutionary changes in seed size [49].…”

“…The most comprehensive study to date tracked changes in a tree community as hunting increased over a 15-year period and found increased spatial aggregation and decreased sapling recruitment for animaldispersed tree species, leading to an overall decline in sapling biodiversity [12]. While these empirical studies provide convincing evidence of short-term negative impacts of overhunting on tree populations, there is a discrepancy between these empirical results and the general finding of demographic studies that in long-lived plants, population dynamics are minimally sensitive to changes in recruitment [13][14][15]. If the effects of seed disperser loss on tree vital rates are limited to early life stages, overhunting may result in only slight decreases in tree population growth rates [5].…”

“…While the importance of size and stage for structuring tropical tree populations has long been recognized [13,14,21], nearly all research on NDD in tropical forests has quantified effects on growth or survival for only one or two life stages. Most empirical research on NDD in tree species has focused on seeds [18] and seedlings [22], although several studies have also detected NDD for later life stages, including saplings [19,23] and adult trees [24].…”

Fire-mediated disruptive selection can explain the reseeder–resprouter dichotomy in Mediterranean-type vegetation

“…The main uncertainties were around survival, growth and fecundity, which tend to be negatively correlated across plant life histories (Franco and Silvertown 2004). Our alternative parameterisations were one life history with high mortality and fast growth (survival reduced by 10 % compared to baseline and growth increased by 100 %), one life history with high mortality and high fecundity (survival reduced by 10 % and fecundity increased by 150 %), and one life history with high fecundity and slow growth (increased fecundity by 150 % and decreased growth by 60 %).…”

Historical demography of Lantana camara L. reveals clues about the influence of land use and weather in the management of this widespread invasive species

“…Understanding the relative importance of these factors is necessary to formulate context-specific management tactics for such plants (Hobbs & Humphries 1995). Syntheses of demography have offered valuable insights into potentially general approaches to the management of weeds based on the structure of weed life history (Silvertown, Franco, Pisanty, & Mendoza 1993;Franco & Silvertown 2004;Ramula, Knight, Burns, & Buckley 2008). Increasingly, the biotic (e.g.…”