Demographic response of plant populations to habitat fragmentation and temporal environmental variability

The effect of habitat transformation on a twig epiphytic orchid: Evidence from population dynamics

The tropical Andean landscape has been dramatically transformed over the last century with remaining native forest limited to small fragments within a heterogeneous matrix of crops, cattle pastures, and urban environments. We aimed to explore the impact of habitat transformation on the population dynamics in an endemic twig epiphytic orchid located within the undisturbed forest and within modified matrix habitat in two regions with contrasting landscape structures: with a dominant shade coffee matrix and a dominant grassland matrix. Over 2 years, we surveyed 4,650 individuals of the Colombian endemic orchid, Rodriguezia granadensis. We undertook four post-breeding censuses in three sites in each region in both native forest and pasture sub-sites (12 sub-sites; 48 censuses in total), and constructed demographic transition matrices (n = 36). The transition probabilities were calculated using a Bayesian approach and population grow rates were evaluated using asymptotic models and elasticities using transient dynamics. Between regions, higher population growth rate and inertia (defined as the largest or smallest long-term population density with the same initial density distribution) was seen in the shade coffee-dominated landscape. Additionally, population growth rate and damping ratio was higher in forest compared with pasture, with lower convergence time for the forest subsites. These demographic patterns reveal the contrasting levels of population resilience of this orchid in different landscape structures with the more connected shade-coffee dominated landscape permitting some healthier populations with greater population growth and survival in forest than pasture. This study highlights that twig epiphyte colonization of isolated phorophytes in pastures should not be interpreted as a sign of a healthy population but as a temporal transitory period.

Modeling temporal dynamics of genetic diversity in stage-structured plant populations with reference to demographic genetic structure

Predicting temporal dynamics of genetic diversity is important for assessing long-term population persistence. In stage-structured populations, especially in perennial plant species, genetic diversity is often compared among life history stages, such as seedlings, juveniles, and flowerings, using neutral genetic markers. The comparison among stages is sometimes referred to as demographic genetic structure, which has been regarded as a proxy of potential genetic changes because individuals in mature stages will die and be replaced by those in more immature stages over the course of time. However, due to the lack of theoretical examination, the basic property of the stage-wise genetic diversity remained unclear. We developed a matrix model which was made up of difference equations of the probability of non-identical-by-descent of each life history stage at a neutral locus to describe the dynamics and the inter-stage differences of genetic diversity in stage-structured plant populations. Based on the model, we formulated demographic genetic structure as well as the annual change rate of the probability of non-identical-by-descent (denoted as η). We checked if theoretical expectations on demographic genetic structure and η obtained from our model agreed with computational results of stochastic simulation using randomly generated 3,000 life histories. We then examined the relationships of demographic genetic structure with effective population size N_e, which is the determinants of diversity loss per generation time. Theoretical expectations on η and demographic genetic structure fitted well to the results of stochastic simulation, supporting the validity of our model. Demographic genetic structure varied independently of N_e and η, while having a strong correlation with stable stage distribution: genetic diversity was lower in stages with fewer individuals. Our results indicate that demographic genetic structure strongly reflects stable stage distribution, rather than temporal genetic dynamics, and that inferring future genetic diversity solely from demographic genetic structure would be misleading. Instead of demographic genetic structure, we propose η as an useful tool to predict genetic diversity at the same time scale as population dynamics (i.e., per year), facilitating evaluation on population viability from a genetic point of view.

Site-specific temporal variation of population dynamics in subalpine endemic plant species

Endemic plants in high mountains are projected to be at high risk because of climate change. Temporal demographic variation is a major factor affecting population viability because plants often occur in small, isolated populations. Because isolated populations tend to exhibit genetic differentiation, analyzing temporal demographic variation in multiple populations is required for the management of high mountain endemic species. We examined the population dynamics of an endemic plant species, Primula farinosa subsp. modesta, in four subalpine sites over six years. Stage-based transition matrices were constructed, and temporal variation in the projected population growth rate (λ) was analyzed using life table response experiments (LTREs). The variation in λ was primarily explained by the site × year interaction rather than the main effects of the site and year. The testing sites exhibited inconsistent patterns in the LTRE contributions of the vital rates to the temporal deviation of λ. However, within sites, growth or stasis had significant negative correlations with temporal λ deviation. Negative correlations among the contributions of vital rates were also detected within the two testing sites, and the removal of the correlations alleviated temporal fluctuations in λ. The response of vital rates to yearly environmental fluctuations reduced the temporal variation of λ. Such effects manifested especially at two sites where plants exhibited higher plasticity than plants at other sites. Site-specific temporal variation implies that populations of high mountain species likely exhibit asynchronous temporal changes, and multiple sites need to be evaluated for their conservation.

Soil-related variations in the population dynamics of six dipterocarp tree species with strong habitat preferences

Differences in the density of conspecific tree individuals in response to environmental gradients are well documented for many tree species, but how such density differences are generated and maintained is poorly understood. We examined the segregation of six dipterocarp species among three soil types in the Pasoh tropical forest, Malaysia. We examined how individual performance and population dynamics changed across the soil types using 10-year demographic data to compare tree performance across soil types, and constructed population matrix models to analyze the population dynamics. Species showed only minor changes in mortality and juvenile growth across soil types, although recruitment differed greatly. Clear, interspecific demographic trade-offs between growth and mortality were found in all soil types. The relative trade-offs by a species did not differ substantially among the soil types. Population sizes were projected to remain stable in all soil types for all species with one exception. Our life-table response experiment demonstrated that the population dynamics of a species differed only subtly among soil types. Therefore, species with strong density differences across soil types do not necessarily differ greatly in their population dynamics across the soil types. In contrast, interspecific differences in population dynamics were large. The trade-off between mortality and growth led to a negative correlation between the contributions of mortality and growth to variations in the population growth rate (λ) and thus reduced their net contributions. Recruitment had little impact on the variation in λ. The combination of these factors resulted in little variation in λ among species.

Long-term population demography of Trillium recurvatum on loess bluffs in western Tennessee, USA

BACKGROUND AND AIMS

Understanding the demography of long-lived clonal herbs, with their extreme modularity, requires knowledge of both their short- and long-term survival and ramet growth patterns. The primary objective of this study was to understand the dynamics of a clonal forest herb, Trillium recurvatum, by examining temporal and small-scale demographic patterns. We hypothesized: (i) there would be more variability in the juvenile age class compared with non-flowering adult and flowering adult classes due to year-to-year fluctuations in recruitment; (ii) rates of population growth (λ) and increase (r) would be highest in non-flowering ramets due to a combination of transitions from the juvenile stage and reversions from flowering adults; and (iii) inter-ramet distances would be most variable between flowering and juvenile ramets due to a combination of clonal growth, seed dispersal by ants and ramet death over time.

METHODOLOGY

Census data were collected on the total number of stems in the population from 1990 to 2007, and placed within one of three life stages (juvenile, three-leaf non-flowering and three-leaf flowering). Modified population viability equations were used to assess temporal population viability, and spatial structure was assessed using block krigging. Correlations were performed using current and prior season weather to current population demography.

PRINCIPAL RESULTS

The first hypothesis was rejected. The second hypothesis was supported: population increase (r) and growth rate (λ) were highest in non-flowering ramets. Finally, the third hypothesis was rejected: there was no apparent density dependence within this population of Trillium and no apparent spatial structure among life stages.

CONCLUSIONS

Overall population density fluctuated over time, possibly due to storms that move soil, and prior year's temperature and precipitation. However, density remained at some dynamic stable level. The juvenile age class had greater variability for the duration of this study and population growth rate was greatest for non-flowering ramets.

Estimating covariation between vital rates: a simulation study of connected vs. separate generalized linear mixed models (GLMMs)

Covariation between vital rates is recognized as an important pattern to be accounted for in demographic modeling. We recently introduced a model for estimating vital rates and their covariation as a function of known and unknown effects, using generalized linear mixed models (GLMM's) implemented in a hierarchical Bayesian framework (Evans et al., 2010) In particular, this model included a model-wide year effect (YEAR) influencing all vital rates, which we used to estimate covariation between vital rates due to exogenous factors not directly included in the model. This YEAR effect connected the GLMMs of vital rates into one large model; we refer to this as the "connected GLMMs" approach. Here we used a simulation study to evaluate the performance of a simplified version of this model, compared to separate GLMMs of vital rates, in terms of their ability to estimate correlations between vital rates. We simulated data from known relationships between vital rates and a covariate, inducing correlations among the vital rates. We then estimated those correlations from the simulated data using connected vs. separate GLMMs with year random effects. We compared precision and accuracy of estimated vital rates and their correlations under three scenarios of the pervasiveness of the exogenous effect (and thus true correlations). The two approaches provide equally good point estimates of vital rate parameters, but connected GLMMs provide better estimates of covariation between vital rates than separate GLMMs, both in terms of accuracy and precision, when the common influence on vital rates is pervasive. We discuss the situations where connected GLMMs might be best used, as well as further areas of investigation for this approach.