Population growth rate and its determinants: an overview

Abstract

We argue that population growth rate is the key unifying variable linking the various facets of population ecology. The importance of population growth rate lies partly in its central role in forecasting future population trends; indeed if the form of density dependence were constant and known, then the future population dynamics could to some degree be predicted. We argue that population growth rate is also central to our understanding of environmental stress: environmental stressors should be defined as factors which when first applied to a population reduce population growth rate. The joint action of such stressors determines an organism's ecological niche, which should be defined as the set of environmental conditions where population growth rate is greater than zero (where population growth rate = r = log e (N tϩ1 /N t )). While environmental stressors have negative effects on population growth rate, the same is true of population density, the case of negative linear effects corresponding to the well-known logistic equation. Following Sinclair, we recognize population regulation as occurring when population growth rate is negatively density dependent. Surprisingly, given its fundamental importance in population ecology, only 25 studies were discovered in the literature in which population growth rate has been plotted against population density. In 12 of these the effects of density were linear; in all but two of the remainder the relationship was concave viewed from above.Alternative approaches to establishing the determinants of population growth rate are reviewed, paying special attention to the demographic and mechanistic approaches. The effects of population density on population growth rate may act through their effects on food availability and associated effects on somatic growth, fecundity and survival, according to a 'numerical response', the evidence for which is briefly reviewed. Alternatively, there may be effects on population growth rate of population density in addition to those that arise through the partitioning of food between competitors; this is 'interference competition'. The distinction is illustrated using a replicated laboratory experiment on a marine copepod, Tisbe battagliae. Application of these approaches in conservation biology, ecotoxicology and human demography is briefly considered. We conclude that population regulation, density dependence, resource and interference competition, the effects of environmental stress and the form of the ecological niche, are all best defined and analysed in terms of population growth rate.

Density dependence: an ecological Tower of Babel

“…Alternatively, demographic rates encompass fertility, survival, dispersal or their compound interplay in an overall population rate of change generically known as 'population growth rate'-i.e. change in population size between two consecutive time steps due to losses (mortality, emigration) and gains (fertility, immigration) of individuals (Herrando-Pérez et al 2012;Sibly and Hone 2002). The effects of population size on growth rates are popularly gauged through population growth curves, such as logistic equations, in which the calculation of sustainable yields has a fundamental application in conservation and management (Eberhardt et al 2008;Henle et al 2004;Sabo et al 2004).…”

“…Exact compensation is provided when a population (at density) N 0 above the equilibrium population (at density) N e is brought down in a single step to that level'' (Varley 1963). The entire idea fell within key-factor analysis, where the intensity of compensation was measured as the slope (b) of survivors [mathematically identical to the 'instantaneous growth rate' (Sibly and Hone 2002)] to logarithms of densities before mortality operated. So a population underwent overcompensation (b [ 1), exact compensation (b = 1) or under compensation (0 \ b \ 1) when, following a population increase, a mortality factor brought numbers below, to or above carrying capacity, respectively (Varley and Gradwell 1970;Varley et al 1973) (Table 1).…”

Intra-specific scaling of natural mortality in fish: the paradigmatic case of the European eel

“…Among these, population density, through inter-and intra-specific competition, has a major influence on vital rates (Lorenzen and Enberg 2002). In fish populations, density-dependent mechanisms have a strong influence on juvenile (Myers and Cadigan 1993) and, to a lesser extent, adult mortality (Sibly and Hone 2002).…”

Conservation of a rare plant requires different methods in different habitats: demographic lessons from Actaea elata

“…The ordination for Elk was found slightly outside of that region, very close to the S vertex, meaning that survival is the most crucial component of population growth. Increases in either survival or recruitment are required to increase the population growth rate (Sibly and Hone 2002), and clearly survival has a greater impact on population growth in A. elata than does fecundity. This is consistent with the fact that forest plants generally have longer life spans than those found in more open habitats (Ehrlen and Lehtila 2002).…”