The power of comparative demography: booms and busts

by Rob Salguero-Gomez on Aug 21, 2016

Jenni McDonald's work on transient dynamics using COMPADRE was recently recognized with the Postdoctoral Excellence Award of the Plant Population Ecology of the Ecological Society of America. Congrats Jenni! Here we leave you with a short summary of her work.   The natural world is not static and populations don’t exist in a vacuum. Environmental variation is inevitable for wild populations. Consequently, the vital rates, stage structures and dynamics of every wild population will change through time. Earlier this year our paper linking stochastic dynamics into contributions from transient dynamics (driven by non-stable stage structures) and asymptotic dynamics (caused by changes in vital rates) was published in Journal of Ecology. The idea of transients being important within stochastic environments was not a new one. However, we built on previous work by exploring absolute dynamics, which accounts for the strength of opposing asymptotic and transient effects. We also used a large-scale comparative database to test this hypothesis, using data from 277 plant populations across 132 species from the COMPADRE Plant Matrix Database. This comparative framework opened up exploration of evolutionary and ecological patterns. Our key result was that transients are ubiquitous in plant populations, contributing to half the dynamics in stochastic environments. Understanding transients is vital for management of both pest species and those of conservation concern. Perturbations in the environment, such as fire, harvesting, disease epidemics and weather changes, will mean that populations are rarely at stable stage structure. Given the contribution of transients to population growth, ignoring non-stable population structure will have implications for management as population growth will be different from that predicted by the long term population growth rate. Consequently, an understanding of transient boom (accelerated population growth) and bust (reduced population growth) could be exploited by managers and conservationists to maintain persistence (or cause extinction) of wild populations. Transients may also provide an explanation as to how some species thrive in a variable landscape, whereas others suffer population declines, for example endangered species potentially may be those who respond poorly to demographic disturbance. Harnessing the power of comparative analysis enabled us to explore evolutionary and ecological patterns and start to shed light on these possibilities. The COMPADRE Plant Matrix Database provides open access to thousands of plant population projection matrices parameterised from empirical data previously dispersed throughout peer-reviewed and grey literature. COMPADRE is the ideal resource to explore transients across populations varying in evolutionary history, growth form and life stage complexity. We found that both transient contributions and asymptotic contributions are influenced heavily by the number of life stages modelled. This could mean that species with complex life histories are able to bounce back from demographic disturbances; alternatively, this observation may be an artefact of modelling design. We found no phylogenetic signal in the contribution of transients to stochastic growth, nor clear patterns related to growth form. Plant populations have a tendency to boom rather than bust in response to variable environments. This raises the future question; have populations evolved to bounce back from disturbance? Our research also highlights the value of large-scale databases. The power of comparative demography allowed us to ask questions regarding the impact of non-equilibrium stage structure on stochastic population dynamics and reveal patterns that would not have been deduced from other means. Empirical data on the life histories of living organisms stored in COMPADRE can contribute to a diverse spectrum of research areas and is of relevance to scientists working in the fields of conservation, ecology and evolution. In addition to the ability to ask new questions, the free and instant access of the database removes any logistical obstacles that many researchers may face in terms of field work and laboratory studies. Such a resource has the potential to inspire new scientific insights, while also embracing the diversity of work life patterns of scientists - undoubtedly a powerful resource. Jenni McDonald Postdoctoral researcher at Exeter University