Global Population Dynamics Database Research Articles

The frequency and severity of catastrophic die‐offs in vertebrates

AbstractRare bouts of extreme environmental perturbations (catastrophes) have been predicted to have a major influence on the probability of extinction. Yet very little information is available on the frequency and severity of catastrophes. Improving the available information concerning catastrophe parameters would allow for an evaluation of their effect and a start towards understanding their causes. We used the Global Population Dynamics Database to determine the frequency and severity of die‐offs in 88 species of vertebrates. We define a catastrophe as any 1‐year decrease in population size of 50% or greater. The data yielded three findings. (1) The frequency of severe die‐offs in vertebrate populations is strongly related to the generation length of the organism. (2) The probability of a severe die‐off for a particular population is approximately 14% per generation. (3) The frequency of die‐off severity can be modelled as a modified power function with the frequency of die‐offs decreasing with increasing magnitude of effect. The distribution is not consistent with catastrophes stemming from environmental sources different than those responsible for smaller fluctuations, but seems to represent the tail of a continuous distribution of environmental perturbations.

Investigating long-term ecological variability using the Global Population Dynamics Database.

The Global Population Dynamics Database (GPDD) is an important new source of information for ecologists, resource managers, and environmental scientists interested in the dynamics of natural populations. It comprises more than 4500 time series of population abundance for over 1800 animal species across many taxonomic groups and geographical locations. The GPDD offers great potential for asking comparative questions about the nature of population variability. We illustrate this by characterizing some critical features of ecological variability, variance growth, and spectral reddening.

Characterizing population vulnerability for 758 species

We investigate relationships between life history traits and the character of population dynamics as revealed by time series data. Our classification of time series is according to ‘extinction category,’ where we identify three classes of populations: (i) weakly varying populations with such high growth rates that long‐term persistence is likely (unless some extreme catastrophe occurs); (ii) populations with such low growth rates that average population size must be large to buffer them against extinction in a variable environment; and (iii) highly variable populations that fluctuate so dramatically that dispersal or some other refuge mechanism is likely to be key to their avoidance of extinction. Using 1941 time series representing 758 species from the Global Population Dynamics Database, we find that, depending on the form of density dependence one assumes, between 46 and 90% of species exhibit dynamics that are so variable that even large carrying capacities could not buffer them against extinction on a 100‐year time horizon. The fact that such a large proportion of population dynamics are so locally variable vindicates the growing realization that dispersal, habitat connectedness, and large‐scale processes are key to local persistence. Furthermore, for mammals, simply by knowing body size, age at first reproduction, and average number of offspring we could correctly predict extinction categories for 83% of species (60 of 72).

The macroecology of population dynamics: taxonomic and biogeographic patterns in population cycles

Regular cycles in population abundance are fascinating phenomena, but are they common in natural populations? How are they distributed among taxa? Are there differences between different regions of the world, or along latitudinal gradients? Using the new Global Population Dynamics Database we analysed nearly 700 long (25 + years) time series of animal field populations, looking for large‐scale patterns in cycles. Nearly 30% of the time series were cyclic. Cycle incidence varied among taxonomic classes, being most common in mammal and fish populations, but only in fish did cycle incidence vary among orders. Cycles were equally common in European and North American populations, but were more common in Atlantic fish than Pacific fish. The incidence of cycles increased with latitude in mammals only. There was no latitudinal gradient in cycle period, but cycle amplitude declined with latitude in some groups of fish. Even after considering the biases in the data source and expected type I error, population cycles seem common enough to warrant ecological attention.