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Population fluctuations

  • Biodiversity

  • Function


Biodiversity stabilises ecosystems because different species respond to environmental perturbations in different ways, which stabilises overall ecosystem function (de Mazancourt et al. 2013, Tilman 1996).

Aggregation of population trends from multiple species can give rise to misleading overall trends due to random population fluctuations (Buschke et al. 2021). The overall population trend from aggregate indices can mask a combination of increasing and declining populations (Buschke et al. 2021).

It is challenging to use a simple population metric to determine whether a species is at risk of extinction, as this depends on the factors influencing population variability (Melbourne and Hastings 2008). In small populations random population fluctuations are an important driver of extinction risk, whereas environmental variability affects a greater range of population sizes (Melbourne and Hastings 2008). Population extinction risk at a given site is strongly influenced by spatial considerations such as connectivity and migration of source populations (Engen et al. 2002). Population size is a more important determinant of extinction risk than population trends, although trends can be useful in identifying endangered species (O’Grady et al. 2004).

Methodology summary

Two of the most accepted variability metrics are (Heath 2006):

  • Standard deviation of log transformed abundances = sd[log(N)]
  • Coefficient of variation (CV) is the ratio of the standard deviation to the mean of abundance per year; CV = (sd of abundance)/(mean of abundance) (Franzen et al. 2013)

Sampling design and grain (resolution) and extent (scope) of sampling determine the scale at which a dataset is relevant. Data collected at the site level can give an idea of population fluctuations for a species within a site only if it is reasonable to expect that reproductive and foraging activities are strongly influenced by the site.

Heath 2006 proposes a new metric – population variability (PV) – which quantifies the variability of the average percent difference between all combinations of observed abundances. PV is expected to be more accurate at estimating long term variability even when data is collected on short timescales.

See metrics on Relative/absolute abundance, Vegetation biomass, Mammal biomass, Invertebrate biomass for methods on estimating species abundance, these can be tracked over multiple years to monitor population trends.

Metric threshold or direction of change

Generally a persistent downward trend is undesirable.


  • Agricultural
  • Forest
  • Grassland
  • Heathland
  • Other
  • Peatland
  • Saltmarsh
  • Wetland


  • Population


  • High


  • Future

Technical expertise

  • High

Standardised methodology

  • No