Using statistics to design and estimate vital rates in matrix population models for a perennial herb

Matrix population models are widely used to assess population status and to inform management decisions. Despite existing theories for building such models, model construction is often partially based on expert opinion. So far, model structure has received relatively little attention, although it may affect estimates of population dynamics. Here, we assessed the consequences of two published matrix structures (a 4 × 4 matrix based on expert opinion and a 10 × 10 matrix based on statistical modeling) for estimates of vital rates and stochastic population dynamics of the long-lived herb Astragalus scaphoides. We explored the ways in which choice of model structure alters the accuracy (i.e., mean) and precision (i.e., variance) of predicted population dynamics. We found that model structure had a negligible effect on the accuracy and precision of vital rates and stochastic stage distribution. However, the 10 × 10 matrix produced lower estimates of stochastic population growth rates than the 4 × 4 matrix, and more accurately predicted the observed trends in population abundance for three out of four study populations. Moreover, estimates of realized variation in population growth rate due to fluctuations in population stage structure over time were occasionally sensitive to matrix structure, suggesting differential roles of transient dynamics. Our study indicates that statistical modeling for choosing categories in matrix models might be preferable over expert opinion to accurately predict population trends and can provide a more objective way for model construction when the biological knowledge of the species is limited.     

Rapid response to habitat restoration by the perennial Primula veris as revealed by demographic monitoring

The demography of Primula veris, a typical species of the species-rich Mesobromion grasslands, was investigated at two contrasting habitats in Eastern Belgium. Both a forested site and a clear-cut parcel were part of formerly larger calcareous grassland areas. By monitoring the demographic response of the target species, this study attempts to clarify the differences in fecundity, growth and survival between a restored and degraded calcareous grassland site. A study of plant traits showed a decrease in number of flowers, inflorescence stalks and plant size under a closing tree canopy. We surveyed individuals in permanent plots between 1999 and 2001. At the forested site, a first sign of decline included lower proportions of flowering individuals and afterwards an increase in mortality was found. Alternatively, removing canopy resulted in an immediate flowering response and both increased growth and seedling recruitment the year after. At both sites, survival rates strongly depended on the initial state, location and year. Projection matrix analysis revealed large differences in modelled population growth rates between sites and years. Under closing canopy the species showed only slow population decline, but the decrease was larger in the last survey year when higher mortality affected the number of reproductive individuals. Transitions between stages with the highest impact on population growth rate were identified by elasticity analysis. If calcareous grassland is forested, survival of reproductive adults is predicted to be very important for conservation of P. veris populations. However, seedling recruitment needs to be raised to guarantee long-term persistence of the populations. On the other hand, clear-felling of a site for restoration of species-rich calcareous grasslands may result in a rapid recovery of certain species.     

Differential effects of abandonment on the demography of the grassland perennial Succisa pratensis

Abandonment of traditional land-use practices can have strong effects on the abundance of species occurring in agricultural landscapes. However, the precise mechanisms by which individual performance and population dynamics are affected are still poorly understood. To assess how abandonment affects population dynamics of Succisa pratensis we used data from a 4-year field study in both abandoned and traditionally grazed areas in moist and mesic habitats to parameterize integral projection models. Abandoned populations had a lower long-term stochastic population growth rate (λ _S = 0.90) than traditionally managed populations (λ _S = 1.08), while λ _S did not differ between habitat types. The effect of abandonment differed significantly between years and had opposed effects on different vital rates. Individuals in abandoned populations experienced higher mortality rates and lower seedling establishment, but had higher growth rates and produced more flower heads per plant. Population viability analyses, based on a population survey of the whole study area in combination with our demographic models, showed that 32 % of the populations face a high risk of extinction (>80 %) within 20 years. These results suggest that immediate changes in management are needed to avoid extinctions and further declines in population sizes. Stochastic elasticity analyses and stochastic life table response experiments indicated that management strategies would be most effective if they increase survival of small plants as well as seedling establishment, while maintaining a high seed production. This may be achieved by varying the grazing intensity between years or excluding grazers when plants are flowering.     

Population dynamics of diploid and hexaploid populations of a perennial herb

BACKGROUND AND AIMS: Despite the recent enormous increase in the number of studies on polyploid species, no studies to date have explored the population dynamics of these taxa. It is thus not known whether the commonly reported differences in single life-history traits between taxa of different ploidy levels result in differences in population dynamics. METHODS: This study explores differences in single life-history traits and in the complete life cycle between populations of different ploidy levels and compares these differences with differences observed between different habitat types and years. Diploid and hexaploid populations of a perennial herb, Aster amellus, are used as the study system. Transition matrix models were used to describe the dynamics of the populations, and population growth rates, elasticity values and life-table response experiments were used to compare the dynamics between populations and years. KEY RESULTS: The results indicate that between-year variation in population dynamics is much larger than variation between different ploidy levels and different habitat conditions. Significant differences exist, however, in the structure of the transition matrices, indicating that the dynamics of the different ploidy levels are different. Strong differences in probability of extinction of local populations were also found, with hexaploid populations having higher probability than diploid populations, indicating strong potential differences in persistence of these populations. CONCLUSIONS: This is the first study on complete population dynamics of plants of different ploidy levels. This knowledge will help to understand the ability of new ploidy levels to spread into new areas and persist there, and the interactions of different ploidy levels in secondary contact zones. This knowledge will also contribute to understanding of interactions of different ploidy levels with other plant species or other interacting organisms such as pollinators or herbivores.     

The demography of a newly established Osprey Pandion haliaetus population in France

Using long‐term mark–resighting data acquired over 27 years in continental France, we estimated demographic parameters and modelled the dynamics of a newly established population of Ospreys Pandion haliaetus using a life‐history model. We then performed prospective and retrospective analyses to estimate the sensitivity of the population growth rate to demographic parameters, and to quantify their contribution to the observed variation in abundance. The observed population growth rate was estimated at 1.150 (from one to 38 pairs in the period 1985–2011), and the stochastic population growth rate was estimated at 1.156. The number of fledglings per nest made the largest contribution to the variance of the observed population growth rate. Breeding productivity was stable across years. In contrast, the prospective analysis indicated that the sensitivity of the population growth rate was greatest for immigration and adult survival. Our results suggest that the increase of a new and recently established breeding population of Ospreys was mainly driven by local dynamics (high productivity and high proportion of breeding individuals), with no sign of density‐dependence except for juvenile survival. This probably reflects highly favourable conditions for breeding. Our results show that productivity can be a major driver in recovering raptor populations, and conservation work should aim to protect occupied nest‐sites and their surrounding habitat and to maintain highly favourable foraging areas in the vicinity of breeding sites.     

Historical demography of Lantana camara L. reveals clues about the influence of land use and weather in the management of this widespread invasive species

Weather is a general stochastic influence on the life history of weeds. In contrast, anthropogenic disturbance (e.g. land use) is an important deterministic influence on weed demography. Our aim with this study was to investigate the relative contributions of land use and weather on the demography of Lantana camara (lantana), a weed of agricultural and natural habitats, based on the intensive monitoring of lantana populations under three land uses (viz. farm[pasture], and burnt and grazed forests) in subtropical Australia. Lantana populations were growing vigorously across all land uses (asymptotic population growth rate, λ > 3). Examination of historical demography using retrospective perturbation analyses showed that weather was a strong influence on lantana demography with the transition from an El Niño (2008–09) to a La Niña (2009–10) year having a strong positive effect on population growth rate. This effect was most marked at the grazed site, and to a lesser extent at the burnt site, with seedling-to-juvenile and juvenile-to-adult transitions contributing most to these effects. This is likely the result of burning and grazing having eliminated/reduced interspecific competition at these sites. Prospective perturbation analyses revealed that λ was most sensitive to proportionate changes in growth transitions, followed by fecundity and survival transitions. Examination of context-specific patterns in elasticity revealed that growth and fecundity transitions are likely to be the more critical vital rates to reduce λ in wet years at the burnt and grazed forest sites, compared to the farm/pasture site. Management of lantana may need to limit the transition of juveniles into the adult stages, especially in sites where lantana is free from competition (e.g. in the presence of fire or grazing), and this particularly needs to be achieved in wet years. Collectively, these results shed light on aspects of spatial and temporal variation in the demography of lantana, and offer insights on its context-specific management.     

Life history and population size variability in a relict plant. Different routes towards long-term persistence

A central tenet of conservation biology is that population size affects the persistence of populations. However, many narrow endemic species combine small population ranges and sizes with long persistence, thereby challenging this tenet. I examined the performance of three different-sized populations of Petrocoptis pseudoviscosa (Caryophyllaceae), a palaeoendemic rupicolous herb distributed along a small valley in the Spanish Pyrenees. Reproductive and demographic parameters were recorded over 6 years, and deterministic and stochastic matrix models were constructed to explore population dynamics and extinction risk. Populations differed greatly in structure, fecundity, recruitment, survival rate, and life span. Strong differentiation in life-history parameters and their temporal variability resulted in differential population vulnerability under current conditions and simulated global changes such as habitat fragmentation or higher climatic fluctuations. This study provides insights into the capacity of narrow endemics to survive both at extreme environmental conditions and at small population sizes. When dealing with species conservation, the population size–extinction risk relationship may be too simplistic for ancient, ecologically restricted organisms, and some knowledge of life history may be most important to assess their future.     

Reinstatement of traditional mowing regimes counteracts population senescence in the rare perennial Primula vulgaris

Question: Traditional management of grassland verges or ditch banks included mowing as a way to provide additional harvesting of hay. Nowadays, such sites are often left unmanaged, as mowing verges is no longer profitable in modern agricultural systems. Are vulnerable plant species able to withstand competition with the surrounding vegetation and maintain viable populations under these circumstances? How do they respond to reinstatement of traditional mowing regimes? Location: Oedelem, northwestern Belgium. Methods: To investigate the effect of reinstatement of the rare perennial Primula vulgaris, demography and adult plant performance were monitored in a grassland verge between 1999 and 2003 under different mowing regimes. Year transitions between life stages were analysed with matrix population models. To disentangle the contributions of the deviations in different life stage transitions to the variation in overall population growth rate, life table response experiments were used. Results: Both management and year had a strong impact on demographic traits of P. vulgaris. If plots were left unmanaged, lower plant performance and declining population growth rates were observed. While population growth rates differed significantly between mowing regimes, mowing of plots only in July did not differ from mowing in July and October in terms of vegetative and reproductive output of adults. Mowing twice a year appeared to be most efficient in increasing population growth rate both by raising recruitment and growth of individuals into large reproductive adults. Conclusions: Large P. vulgaris populations show a good ability to recover from recent abandonment of traditional management regimes. By mowing twice a year, managers are able to target vital rates that are most influential: growth and flowering of adult individuals.     

Genetic structure of the forest herb Primula elatior in a changing landscape

To investigate whether changes in land use and associated forest patch turnover affected genetic diversity and structure of the forest herb Primula elatior, historical data on landscape changes were combined with a population genetic analysis using dominant amplified fragment length polymorphism markers. Based on nine topographic maps, landscape history was reconstructed and forest patches were assigned to two age classes: young (less than 35 years) and old (more than 35 years). The level of differentiation among Primula populations in recently established patches was compared with the level of differentiation among populations in older patches. Genetic diversity was independent of population size (P > 0.05). Most genetic variation was present within populations. Within-population diversity levels tended to be higher for populations located in older forests compared with those for populations located in young forests (Hj = 0.297 and 0.285, respectively). Total gene diversity was also higher for old than for young populations (Ht = 0.2987 and 0.2828, respectively). The global fixation index FST averaged over loci was low, but significant. Populations in older patches were significantly more differentiated from each other than were populations in recently established patches and they showed significant isolation by distance. In contrast, no significant correlations between pairwise geographical distance and FST were found for populations in recently established patches. The location of young and old populations in the studied system and altered gene flow because of increased population density and decreased inter-patch distances between extant populations may explain the observed lower genetic differentiation in the younger populations. This study exemplifies the importance of incorporating data on historical landscape changes in population genetic research at the landscape scale.     

Modelling the effects of genetics and habitat on the demography of a grassland herb

There is growing evidence that genetic and ecological factors interact in determining population persistence. The demographic effects of inbreeding depression can largely depend on the ecological milieu. We used demographic data of the perennial herb Succisa pratensis from six populations in grazed and ungrazed sites with different soil moisture. We built an individual-based model assessing the demographic consequences of inbreeding depression in populations with different management and habitat. Today this plant has to cope with severe landscape fragmentation, deteriorating habitat conditions in terms of decreasing grazing intensity, and the effects of inbreeding depression. For each population we performed simulations testing two inbreeding depression hypotheses (partial dominance and overdominance) and three epistatic functions among loci. The results indicated stronger inbreeding depression effects for populations in unfavourable sites without grazing or in xeric habitats compared to populations in favourable mesic sites with grazing. Overall, we found stronger effects with overdominance, a result that emphasizes the importance of understanding the genetic mechanisms of inbreeding depression. Hence, management practices can interact with the genetic consequences of inbreeding depression in population dynamics, which may have important implications for plant population ecology and evolutionary dynamics of inbreeding depression.