Population modelling of Gelidium sesquipedale (Rhodophyta, Gelidiales)

A matrix population model of Gelidium sesquipedale, a commercial agarophyte from the Northeast Atlantic, was developed based on demographic data obtained during two years in a commercial stand of Cape Espichel, Portugal. G. sesquipedale individuals were classified into categories such as life cycle phase, spores, juveniles and adult frond size, because the species vital rates, fecundity, fertility, survival, growth and breakage depend on them. We also exemplify the use of a user-friendly modelling software, Stella, to develop a structured-population model. This is the first time this software has been used to model the demography of seaweed populations. The Stella model developed here behaved very similarly to the matrix model, because of its particular construction, which causes the forcing functions to be discrete rather than continuous. The relative importance of spore recruitment and vegetative growth of new fronds in both population growth and population structure was investigated. Elasticity analysis suggests that vegetative recruitment is the most important demographic parameter controlling population growth together with survival and transitions between juveniles (1–6 cm fronds) and class 1 fronds (6–9 cm fronds). On the other hand, sexual reproduction may, by itself, efficiently control the relative proportion of gametophytes and tetrasporophytes in the population, even though its contribution to recruitment is extremely small. A 40% difference in the growth rates of gametophyte and tetrasporophyte submatrices resulted from natural differences in spore recruitment rates. This revised version was published online in June 2006 with corrections to the Cover Date.     

Less favourable climates constrain demographic strategies in plants

Correlative species distribution models are based on the observed relationship between species’ occurrence and macroclimate or other environmental variables. In climates predicted less favourable populations are expected to decline, and in favourable climates they are expected to persist. However, little comparative empirical support exists for a relationship between predicted climate suitability and population performance. We found that the performance of 93 populations of 34 plant species worldwide – as measured by in situ population growth rate, its temporal variation and extinction risk – was not correlated with climate suitability. However, correlations of demographic processes underpinning population performance with climate suitability indicated both resistance and vulnerability pathways of population responses to climate: in less suitable climates, plants experienced greater retrogression (resistance pathway) and greater variability in some demographic rates (vulnerability pathway). While a range of demographic strategies occur within species’ climatic niches, demographic strategies are more constrained in climates predicted to be less suitable.     

Life history and population dynamics of a tree species in tropical semi‐arid climate: A case study with Cordia oncocalyx

In semi‐arid climates, plant population dynamics are strongly influenced by the amount and temporal distribution of rainfall. We monitored a population of the tree species Cordia oncocalyx (Boraginaceae) for 24 months in the dry thorny woodland of semi‐arid northeastern Brazil, to investigate which life‐history traits allow this tree to be locally dominant. We used horizontal life tables and a Lefkovitch matrix and tested for relationships among demographic parameters of seedling, infant, juvenile, immature, virginile and reproductive ontogenetic stages with rainfall and canopy openness. Germination and recruitment occurred in the rainy months, and dry‐season mortality occurred only in seedlings (76% and 100%, first and second years, respectively) and infants (3% and 6%). Juveniles showed greater height growth under more open canopies (Spearman correlation coefficient = 0.24), suggesting that light availability influences growth. The population growth rate was λ = 1.0336, and the highest sensitivity occurred in the infant‐juvenile transition. Our results show light as a restrictive growth factor for plants in the juvenile stage and confirm the strong influence of rainfall on the dynamics of trees in a seasonally dry environment. The formation of a persistent seed bank with germination concentrated at the rainfall onset but spreading over the rainy season are strategies that hedge bets before establishment. The formation of a bank of infants, which can resume growth as soon as there is water, hedges bets after establishment. We attribute the positive population growth rate of Cordia oncocalyx to survival strategies allowing bet‐hedging both before and after establishment.     

A demographic approach to study effects of climate change in desert plants

Desert species respond strongly to infrequent, intense pulses of precipitation. Consequently, indigenous flora has developed a rich repertoire of life-history strategies to deal with fluctuations in resource availability. Examinations of how future climate change will affect the biota often forecast negative impacts, but these—usually correlative—approaches overlook precipitation variation because they are based on averages. Here, we provide an overview of how variable precipitation affects perennial and annual desert plants, and then implement an innovative, mechanistic approach to examine the effects of precipitation on populations of two desert plant species. This approach couples robust climatic projections, including variable precipitation, with stochastic, stage-structured models constructed from long-term demographic datasets of the short-lived Cryptantha flava in the Colorado Plateau Desert (USA) and the annual Carrichtera annua in the Negev Desert (Israel). Our results highlight these populations'' potential to buffer future stochastic precipitation. Population growth rates in both species increased under future conditions: wetter, longer growing seasons for Cryptantha and drier years for Carrichtera. We determined that such changes are primarily due to survival and size changes for Cryptantha and the role of seed bank for Carrichtera. Our work suggests that desert plants, and thus the resources they provide, might be more resilient to climate change than previously thought.     

An integrated modeling approach to estimating Gunnison sage‐grouse population dynamics: combining index and demographic data

Evaluation of population dynamics for rare and declining species is often limited to data that are sparse and/or of poor quality. Frequently, the best data available for rare bird species are based on large‐scale, population count data. These data are commonly based on sampling methods that lack consistent sampling effort, do not account for detectability, and are complicated by observer bias. For some species, short‐term studies of demographic rates have been conducted as well, but the data from such studies are typically analyzed separately. To utilize the strengths and minimize the weaknesses of these two data types, we developed a novel Bayesian integrated model that links population count data and population demographic data through population growth rate (λ) for Gunnison sage‐grouse (Centrocercus minimus). The long‐term population index data available for Gunnison sage‐grouse are annual (years 1953–2012) male lek counts. An intensive demographic study was also conducted from years 2005 to 2010. We were able to reduce the variability in expected population growth rates across time, while correcting for potential small sample size bias in the demographic data. We found the population of Gunnison sage‐grouse to be variable and slightly declining over the past 16 years.     

Feeding location affects demographic performance of cabbage aphids on winter canola

The cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae), is a perennial pest that specializes on plants of the Brassicaceae family, attacking winter canola (Brassica napus L.) mainly during and after flowering. Under field conditions, cabbage aphid colonizes the upper flowering canopy. Population dynamics of aphids in the flowering canopy could be regulated by differences in either plant quality (bottom‐up) or predatory (top‐down) forces. The goal of our study was to determine the effect of feeding location on cabbage aphid demography. A stage‐structured matrix population model was constructed for aphids restricted to reproductive or vegetative plant tissues of canola. We found that feeding location had a large impact on demography of cabbage aphid; the finite rate of increase (λ ± SEM) was higher when aphids were restricted to reproductive tissues, compared to aphids feeding on vegetative tissues: 1.25 ± 0.01 vs. 1.17 ± 0.01 (leaves). Aphids confined to reproductive tissues with higher λ exhibited shorter generation times (T = 14.2 ± 0.2 days) and 53–75% higher net reproductive rates (R₀ = 23.3 ± 1.7) than aphids feeding on vegetative tissues. Prospective analyses showed that there was a nymph‐skewed stable stage distribution, and elasticity values revealed that λ is most sensitive to changes in stasis of adults staying in the adult stage and to adult survival. Retrospective analyses indicated that variation in adult fecundity (value of 0.05) had the largest effect on population dynamics but collectively, growth of nymphal stage 2–3, 3–4, and 4 to adult accounted for most of the difference in λ between the treatments. Monitoring programs should target adults and penultimate instars colonizing reproductive tissues of canola plants in the field as aphids on these plant structures contribute most to population growth.