Detecting and managing an overgrazing-drought synergism in the threatened Echeveria longissima (Crassulaceae): the role of retrospective demographic analysis

Identifying the demographic processes relevant for species conservation in human-impacted areas: does the model matter?

The identification of the demographic processes responsible for the decline in population growth rate (λ) in disturbed areas would allow conservation efforts to be efficiently directed. Integral projection models (IPMs) are used for this purpose, but it is unclear whether the conclusions drawn from their analysis are sensitive to how functional structures (the functions that describe how survival, growth and fecundity vary with individual size) are selected. We constructed 12 IPMs that differed in their functional structure by combining two reproduction models and three functional expressions (generalized linear, cubic and additive models), each with and without simplification. Models were parameterized with data from two populations of two endangered cacti subject to different disturbance intensities. For each model, we identified the demographic processes that most affected λ in the presence of disturbance. Simulations were performed on artificial data and analyzed as above to assess the generality of the results. In both empirical and simulated data, the same processes were identified as making the largest contribution to changes in λ regardless of the functional structure. The major differences in the results were due to misspecification of the fecundity functions, whilst functional expression and model simplification had lesser effects. Therefore, as long as the demographic attributes of the species are well known and incorporated into the model, IPMs will robustly identify the processes that most affect the growth of populations subject to disturbance, making them a reliable tool for developing conservation strategies.

Effects of stochastic herbivory events on population maintenance of an understorey palm species (Geonoma schottiana) in riparian tropical forest

Plant populations can respond to temporal environmental heterogeneity caused by natural disturbances, such as herbivory. Palm individuals of several species are preyed upon by mammals, but the effects of such herbivory events on population dynamics remain poorly known. To evaluate the effects of environmental stochasticity on aGeonoma schottiana(Arecaceae) population, we surveyed annually 40 permanent 20 × 10-m plots in a riparian tropical forest over 5 y (2000–2004) and results were analysed using matrix models. The population growth rate (λ) was in equilibrium during the study period and only one bad year was identified (2002–2003), which had a higher mortality of juvenile individuals due to herbivory. Additionally, the bad year had a higher mortality of reproductive individuals than the other periods. The stasis matrix elements of the later life stages were the vital rates with highest elasticities. The mortality of juvenile and reproductive individuals had a negative contribution to λ in the bad year. Conversely, the growth of infant and juvenile individuals and the clonal growth of juveniles were the vital rates with highest contribution to stability maintenance of λ in the bad year in a life-table response experiment. The palm population had a high individual density, high proportion of the initial life stages, clonal growth, high fertility, abundant seed bank and high seedling recruitment. Despite these traits, if stochastic herbivory events occur frequently over a long period of time, the population will have a negative growth rate and the probability of local extinction will be very high.