Forest Fragmentation Alters the Population Dynamics of a Late‐successional Tropical Tree

Tropical late‐successional tree species are at high risk of local extinction due to habitat loss and fragmentation. Population‐growth rates in fragmented populations are predicted to decline as a result of reduced fecundity, survival and growth. We examined the demographic effects of habitat fragmentation by comparing the population dynamics of the late‐successional tree Poulsenia armata (Moraceae) in southern Mexico between a continuous forest and several forest fragments using integral projection models (IPMs) during 2010–2012. Forest fragmentation did not lead to differences in population density and even resulted in a higher population‐growth rate (λ) in fragments compared to continuous forests. Habitat fragmentation had drastic effects on the dynamics of P. armata, causing the population structure to shift toward smaller sizes. Fragmented populations experienced a significant decrease in juvenile survival and growth compared to unaltered populations. Adult survival and growth made the greatest relative contributions to λ in both habitat types during 2011–2012. However, the relative importance of juvenile survival and growth to λ was highest in the fragmented forest in 2010–2011. Our Life Table Response Experiment analysis revealed that positive contributions of adult fecundity explained most of the variation of λ between both habitats and annual periods. Finally, P. armata has a relatively slow speed of recovery after disturbances, compromising persistence of fragmented populations. Developing a mechanistic understanding of how forest fragmentation affects plant population dynamics, as done here, will prove essential for the preservation of natural areas.     

Interactive life‐history traits predict sensitivity of plants and animals to temporal autocorrelation

Temporal autocorrelation in demographic processes is an important aspect of population dynamics, but a comprehensive examination of its effects on different life‐history strategies is lacking. We use matrix population models from 454 plant and animal populations to simulate stochastic population growth rates (log λ_s) under different temporal autocorrelations in demographic rates , using simulated and observed covariation among rates. We then test for differences in sensitivities, or changes of log λ_s to changes in autocorrelation among two major axes of life‐history strategies, obtained from phylogenetically informed principal component analysis: the fast‐slow and reproductive‐strategy continua. Fast life histories exhibit highest sensitivities to simulated autocorrelation in demographic rates across reproductive strategies. Slow life histories are less sensitive to temporal autocorrelation, but their sensitivities increase among highly iteroparous species. We provide cross‐taxonomic evidence that changes in the autocorrelation of environmental variation may affect a wide range of species, depending on complex interactions of life‐history strategies.     

Population Dynamics of the Andean Lizard Anolis heterodermus: Fast‐slow Demographic Strategies in Fragmented Scrubland Landscapes

Habitat fragmentation and loss affect population stability and demographic processes, increasing the extinction risk of species. We studied Anolis heterodermus populations inhabiting large and small Andean scrubland patches in three fragmented landscapes in the Sabana de Bogotá (Colombia) to determine the effect of habitat fragmentation and loss on population dynamics. We used the capture‐mark‐recapture method and multistate models to estimate vital rates for each population. We estimated growth population rate and the most important processes that affect λ by elasticity analysis of vital rates. We tested the effects of habitat fragmentation and loss on vital rates of lizard populations. All six isolated populations showed a positive or an equilibrium growth rate (λ = 1), and the most important demographic process affecting λ was the growth to first reproduction. Populations from landscapes with less scrubland natural cover showed higher stasis of young adults. Populations in highly fragmented landscapes showed highest juvenile survival and growth population rates. Independent of the landscape's habitat configuration and connectivity, populations from larger scrubland patches showed low adult survivorship, but high transition rates. Populations varied from a slow strategy with low growth and delayed maturation in smaller patches to a fast strategy with high growth and early maturation in large patches. This variation was congruent with the fast‐slow continuum hypothesis and has serious implications for Andean lizard conservation and management strategies. We suggest that more stable lizard populations will be maintained if different management strategies are adopted according to patch area and habitat structure.     

A demographic analysis of vole population responses to fragmentation and destruction of habitat

 Fragmentation and destruction of natural habitats is currently considered to be the major threat to wildlife populations. We here perform a comprehensive analysis of the demographic effects of habitat fragmentation and destruction on 14 populations of the root vole. The experiment was divided into two consecutive periods. During the first period, we contrasted populations with the same initial size and structure in continuous and fragmented habitat. During the second period, we fragmented the continuous habitat into the same configuration as the permanently fragmented habitat so that the effect of habitat destruction could be evaluated. We estimated survival and fecundity parameters and combined them into population projection matrices to evaluate their relative impact on population growth. In the first period of the experiment there was no difference in population growth rate between fragmented and continuous populations, although litter size was significantly higher in the continuous populations. In the second period, we found higher population growth rates in populations that had experienced habitat destruction. By applying the transition matrix model to empirical estimates of demographic parameters, we demonstrate that the difference in population growth rate in the second period of the experiment was the result of a nonsignificant difference in adult survival. Movements out of the habitat patches were significantly lower in populations that had experienced habitat destruction. We conclude that predator-caused mortality of animals moving out of the habitat patches was the main determinant of demographic variation in this system. Received: January 31, 2002 / Accepted: March 25, 2003     

Metapopulation-level adaptation of insect host plant preference and extinction-colonization dynamics in heterogeneous landscapes

Species living in highly fragmented landscapes typically occur as metapopulations with frequent turnover of local populations. The turnover rate depends on population sizes and connectivities, but it may also depend on the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host plant species, which show variation in their relative abundances at two spatial scales: locally among individual habitat patches and regionally among networks of patches. Female butterflies in turn exhibit spatial variation in genetically determined host plant preference within and among patch networks. Emigration, immigration and establishment of new populations have all been shown to be strongly influenced by the match between the host plant composition of otherwise suitable habitat patches and the host plant preference of migrating butterflies. The evolutionary consequences of such biased migration and colonization with respect to butterfly phenotypes might differ depending on spatial configuration and plant species composition of the patches in heterogeneous patch networks. Using a spatially realistic individual-based model we show that the model-predicted evolution of host plant preference due to biased migration explains a significant amount of the observed variation in host plant use among metapopulations living in dissimilar networks. This example illustrates how the ecological extinction-colonization dynamics may be linked with the evolutionary dynamics of life history traits in metapopulations.     

Spatial data replacing temporal data in population viability analyses: An empirical investigation for plants

In conservation management, there is an urgent need for estimates of population viability and for knowledge of the contributions of different life-history stages to population growth rates. Collection of long-term demographic data from a study population is time-consuming and may considerably delay the start of proper management actions. We examined the possibility of replacing a long-term temporal data set (demographic data from several years within a population) with a short-term spatial data set (demographic data from different populations for the same subset of two continuous years) for stochastic estimates of population viability. Using matrix population models for ten perennial plant species, we found that the matrix elements of spatial data sets often deviated from those of temporal data sets and that matrix elements generally varied more spatially than temporally. The appropriateness of replacing temporal data with spatial data depended on the subset of years and populations used to estimate stochastic population growth rates (log λ_s). Still, the precision of log λ_s estimates measured as variation in the yearly change of logarithmic population size rarely differed significantly between the spatial and temporal data sets. Since a spatiotemporal comparison of matrix elements and their variation cannot be used to assess whether spatial and temporal data sets are interchangeable, we recommend further research on the topic.     

Incorporating the temporal autocorrelation of demographic rates into structured population models

Population dynamics are typically temporally autocorrelated: population sizes are positively or negatively correlated with past population sizes. Previous studies have found that positive temporal autocorrelation increases the risk of extinction due to ‘inertia’ that prolongs downward fluctuations in population size. However, temporal autocorrelation has not yet been analyzed at the level of life cycle transitions. We developed an R package, colorednoise, which creates stochastic matrix population projections with distinct temporal autocorrelation values for each matrix element. We used it to analyze long-term demographic data on 25 populations from the COMADRE and COMPADRE databases and simulate their stochastic dynamics. We found a broad range of temporal autocorrelation across species, populations and life cycle stages. The number of stage-classes in the matrix strongly affected the temporal autocorrelation of the growth rate. In the plant populations, reproduction transitions had more negative temporal autocorrelation than survival transitions, and matrices dominated by positive temporal autocorrelation had higher extinction risk, while in animal populations transition type was not associated with noise color. Our results indicate that temporal autocorrelation varies across life cycle transitions, even among populations of the same species. We present the colorednoise package for researchers to analyze the temporal autocorrelation of structured demographic rates.     

Population dynamics of the rare plant Kosteletzkya pentacarpos (Malvaceae): a nine-year study

Rare plant species have extremely narrow distributions that can be reduced to a single or few populations. The rare long-lived plant Kosteletzkya pentacarpos is one such species because only two extant localities are known in the western Mediterranean. In this study, we analyse the population dynamics over nine years of the only population known in north-east Spain, which is located at the Llobregat delta (Barcelona). We collected basic demographic data to build a transition matrix model. We computed population growth rates λ and their confidence intervals for each year of study. We conducted elasticity and variance decomposition analyses to determine the relative importance of vital rates to overall population dynamics. On average, the K. pentacarpos population exhibited an increasing dynamics. Survivorship of adult plants contributed the most to each λ, whereas temporal variance in fecundity and juvenile fate explained the observed variation in λ. Despite the increasing dynamics of K. pentacarpos , important reductions in fecundity resulting from biotic agents and recruitment owing to habitat limitations are constraints for population growth. We conclude that the knowledge generated in this long-term study should be used to create new K. pentacarpos populations at the Llobregat delta in order to minimize the risk of extinction following catastrophic events that are nearly impossible to predict.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 455–462.     

Site‐to‐site variation in the demography of a fire‐affected perennial. Acacia suaveolens,at Ku‐ring‐gai Chase National Park,New South Wales,Australia

Abstract Acacia suaveolens (Sm.) Willd is a perennial shrub that forms even‐aged stands, recruited from a soil seed‐bank following fire. It has previously been subject to demographic studies, which used a space‐for‐time substitution to investigate temporal patterns following fire. In the present study the potential for spatial variation across sites was investigated by sampling at several similarly aged populations in Ku‐ring‐gai Chase National Park, northern Sydney, Australia. Significant variation in mean size and fecundity of A. suaveolens individuals was observed among sites, over a 2‐4.6‐fold range in plant size, and a sevenfold range in mean fecundity. The observed variation at 3 years after fire encapsulated most of the variation previously observed among sites 0‐17 years since fire, emphasizing the importance of spatial variation in this species. For each site a two‐stage (seed, plant) matrix model was constructed, and projected from 3 to 25 years following fire. Population growth was measured as number of seeds per 3‐year‐old plant, and found to vary 1.4‐fold across models for different sites. This site‐to‐site variation, as well as that in size, fecundity and survival, was statistically significant. Variation in projected seeds per plant could mostly be attributed to differences in fecundity rather than plant survival. Sensitivity analyses emphasized the biological significance of the variation in fecundity. Whereas previous studies have focused on temporal variation, this work demonstrates the importance of extending our understanding of a species to include the spatial component of population dynamics.     

The generality of management recommendations across populations of an invasive perennial herb

Demographic models are widely used to produce management recommendations for different species. For invasive plants, current management recommendations to control local population growth are often based on data from a limited number of populations per species, and the assumption of stable population structure (asymptotic dynamics). However, spatial variation in population dynamics and deviation from a stable structure may affect these recommendations, calling into question their generality across populations of an invasive species. Here, I focused on intraspecific variation in population dynamics and investigated management recommendations generated by demographic models across 37 populations of a short-lived, invasive perennial herb (Lupinus polyphyllus). Models that relied on the proportional perturbations of vital rates (asymptotic elasticities) indicated an essential role for plant survival in long-term population dynamics. The rank order of elasticities for different vital rates (survival, growth, retrogression, fecundity) varied little among the 37 study populations regardless of population status (increasing or declining asymptotically). Summed elasticities for fecundity increased, while summed elasticities for survival decreased with increasing long-term population growth rate. Transient dynamics differed from asymptotic dynamics, but were qualitatively similar among populations, that is, depending on the initial size structure, populations tended to either increase or decline in density more rapidly than predicted by asymptotic growth rate. These findings indicate that although populations are likely to exhibit transient dynamics, management recommendations based on asymptotic elasticities for vital rates might be to some extent generalised across established populations of a given short-lived invasive plant species.     

Can temporal covariation and autocorrelation in demographic rates affect population dynamics in a raptor species?

Theoretical studies suggest that temporal covariation among and temporal autocorrelation within demographic rates are important features of population dynamics. Yet, empirical studies have rarely focused on temporal covariation and autocorrelation limiting our understanding of these patterns in natural populations. This lack of knowledge restrains our ability to fully understand population dynamics and to make reliable population forecasts. In order to fill this gap, we used a long‐term monitoring (15 years) of a kestrel Falco tinnunculus population to investigate covariation and autocorrelation in survival and reproduction at the population level and their impact on population dynamics. Using Bayesian joint analyses, we found support for positive covariation between survival and reproduction, but weak autocorrelation through time. This positive covariation was stronger in juveniles compared with adults. As expected for a specialized predator, we found that the reproductive performance was strongly related to an index of vole abundance explaining 86% of the temporal variation. This very strong relationship suggests that the temporally variable prey abundance may drive the positive covariation between survival and reproduction in this kestrel population. Simulations suggested that the observed effect size of covariation could be strong enough to affect population dynamics. More generally, positive covariation and autocorrelation have a destabilizing effect increasing substantially the temporal variability of population size.     

Local population dynamics and the impact of scale and isolation: a study on different little owl populations

The understanding of how variation of demographic rates translates into variation of population growth is a central aim in population ecology. Besides stochastic and deterministic factors, the spatial extent and the isolation of a local population may have an impact on the contribution of the different demographic components. Using long-term demographic data we performed retrospective population analyses of four little owl ( Athene noctua ) populations with differential spatial extent and degree of isolation to assess the contribution of demographic rates to the variation of the growth rate (λ) of each local population and to the difference of λ among populations. In all populations variation of fecundity contributed least to variation of λ, and variation of adult survival contributed most to variation of λ in three of four populations. Between population comparisons revealed that differences mainly stem from differences of immigration and juvenile local survival. The relative importance of immigration to λ tended to decrease with increasing spatial extent and isolation of the local populations. None of the four local populations was self-sustainable. Because the local populations export and import individuals, they can be considered as open recruitment systems in which part of the recruited breeding birds are not produced locally. The spatial extent and the degree of isolation of a local population have an impact on local population dynamics; hence these factors need to be considered in studies about local population dynamics and for deriving conservation measures.     

Effects of regional climate and small‐scale habitat quality on performance in the relict species Ramonda myconi

Abstract. The Mediterranean Basin harbours paleo‐endemic species with a highly restricted and fragmented distribution. Many of them might also be of the remnant type, for which the regional dynamics depends on the persistence of extant populations. Therefore, a key issue for the long‐term persistence of these species is to assess the variability and effects of ecological factors determining plant performance. We investigated the spatio‐temporal variability in plant traits and ecological factors of Ramonda myconi, a preglacial relict species with remnant dynamics, in 5 populations over 4–7 yr. Ecological factors contributing to fecundity showed a high degree of between‐year variability. Pre‐dispersal fruit predation had a minor influence on total reproductive output, and most of the variability was found among individuals within populations and years. Spatio‐temporal variability in growth and survival was rather low but significant, whereas recruitment showed important between‐population variability. Among‐year variability in fecundity and growth was related to climatic fluctuations on a regional scale, notably rainfall and temperature in a particular period, while the spatial variability in survival and recruitment was explained by within‐population (patch) habitat quality. Although R. myconi is able to withstand repeated periods of drought, water availability seems to be the most important factor affecting plant performance in all the study populations. These findings suggest that the long‐term persistence of species showing remnant population dynamics in habitats under the influence of Mediterranean climate might be threatened by increased aridity as a result of climate change.     

Population size,habitat fragmentation,and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.     

Spatial Variation in Lesser Prairie-Chicken Demography: A Sensitivity Analysis of Population Dynamics and Management Alternatives

ABSTRACT The lesser prairie-chicken (Tympanuchus pallidicinctus) is currently considered a candidate for protection under the Endangered Species Act. To identify potential limiting factors for lesser prairie-chicken populations, we developed an age-based matrix model of lesser prairie-chicken population dynamics to compare the relative importance of components of reproduction and survival, and determine if various management alternatives stabilize or increase rates of population change. We based our analyses on an intensive 6-year population study from which demographic rates were estimated for each age class in Kansas. We used deterministic models and elasticity values to identify parameters predicted to have the greatest effect on the rate of population change (λ) at 2 study sites. Last, we used life-stage simulation analysis to simulate various management alternatives. Lambda was <1 for both populations (site 1: λ = 0.54, site 2: λ = 0.74). However, we found differences in sensitivity to nest success and chick survival between populations. The results of the simulated management scenarios complemented the lower-level elasticity analysis and indicated the relative importance of female survival during the breeding season compared with winter. If management practices are only capable of targeting a single demographic rate, changes to either nest success or chick survival had the greatest impact on λ at site 1 and 2, respectively. Management that simultaneously manipulated both nest success and chick survival was predicted to have a greater effect on λ than changes in survival of adult females. In practice, our demographic analyses indicate that effective management should be based on habitat conservation measures to increase components of fecundity.     

Genetic structure among remnant populations of a migratory passerine,the Northern Wheatear Oenanthe oenanthe

Continuous animal populations often become fragmented due to anthropogenic habitat alterations. These small, fragmented populations are fragile due to demographic and genetic factors, whereas immigration can enhance their long‐term viability. Previously, we showed that high philopatry affected the local dynamics of three small and remnant subpopulations of Northern Wheatears in The Netherlands. Here, we show that these three populations together with an additional larger population in the European lowlands are highly genetically differentiated based on 22 microsatellite markers. In contrast, we found no evidence for differentiation using two mitochondrial DNA markers. An IMa2 analysis indicates that gene flow has occurred regularly among our sampled populations. As immigration of colour‐ringed birds among our sampled populations is rare at best, our results suggest that the populations have recently become isolated from one another. Low dispersal rates in highly mobile birds may occur when suitable habitat becomes highly fragmented, and will accentuate stochastic demographic processes and inbreeding, both reducing population viability. As dispersal rates are low among populations of Northern Wheatears in The Netherlands, there is only a small probability of recolonization of habitat patches where populations have become locally extinct.     

Demographic consequences for a threatened plant after the loss of its only disperser. Habitat suitability buffers limited seed dispersal

Seed dispersal links the end of a plant's reproductive cycle with the establishment of new recruits. Dispersal over short distances may lead to the local aggregation of individuals, slower population growth and, ultimately, to lower population densities. In this study, we analyse the demographic consequences for the shrub Daphne rodriguezii after the loss of its only seed disperser in an island ecosystem (Menorca Island, western Mediterranean). During a period of 8–10 years, we collected demographic data from five populations, four where the disperser is extinct (disrupted) and the only one in which it still persists (undisrupted). We calculated basic deterministic variables, analysed life table response experiments (LTRE) and their covariation among demographic traits, and simulated future population vulnerability. Population growth rate (λ) was either stable or negative and independent of whether the population was disrupted or not. Current and past population dynamics were similar in the two largest populations (one being the undisrupted), which suggests that the environmental conditions allow them to be stable regardless of seed disperser presence. Variation in λ was dependent on rainfall variability and was highly influenced by stasis and growth. There also existed tradeoffs between the former life traits and fecundity, which indicate strong competition when resources are limiting (e.g. high plant aggregation due to limited seed dispersal or low rainfall), and that could ultimately affect high‐elasticity demographic traits. Our study suggests that the population dynamic of D. rodriguezii is stable under the current conditions, and that where dispersal is limiting, important environmental changes (e.g. in habitat suitability and/or rainfall regime) might lead to local extinctions.     

Effects of climate on population fluctuations of ibex

Predicting the effects of the expected changes in climate on the dynamics of populations require that critical periods for climate‐induced changes in population size are identified. Based on time series analyses of 26 Swiss ibex (Capra ibex) populations, we show that variation in winter climate affected the annual changes in population size of most of the populations after accounting for the effects of density dependence and demographic stochasticity. In addition, precipitation during early summer also influenced the population fluctuations. This suggests that the major influences of climate on ibex population dynamics operated either through loss of individuals during winter or early summer, or through an effect on fecundity. However, spatial covariation in these climate variables was not able to synchronize the population fluctuations of ibex over larger distances, probably due to large spatial heterogeneity in the effects of single climate variables on different populations. Such spatial variation in the influence of the same climate variable on the local population dynamics suggests that predictions of influences of climate change need to account for local differences in population dynamical responses to climatic conditions.     

How does herbivory affect individuals and populations of the perennial herb Paeonia officinalis?

In this study we quantified variability in foliage herbivory and pre-dispersal seed predation and its effects on plant performance and demography in populations of a rare and protected perennial herb, Paeonia officinalis. An individual-based survey was performed during four years in four populations, which contained plants in both open habitat and woodland. We detected marked spatial and temporal variation among and within populations in foliage herbivory (by insects) and pre-dispersal seed predation (by insects, rodents and Roe Deer). Foliage herbivory decreased with plant demographic stages in open habitats, from seedlings to reproductive individuals, but no significant trend was detected in woodland habitats. This may be due to different demographic origin of larger vegetative plants in this habitat. Depending on demographic stage, herbivory was higher in open habitats or not significantly different between habitats. This suggests differences in herbivore abundance in different habitats within sites. Pre-dispersal seed predation remained weak and did not depend on habitat. We did not detect any consequence of foliage herbivory on seedling mortality and individual growth in our study. Our results illustrate the need to investigate plant-herbivore interactions over several years in distinct populations in order to more accurately evaluate herbivore impact on plant population dynamics.     

Accounting for stochasticity in demographic compensation along the elevational range of an alpine plant

Demographic compensation arises when vital rates change in opposite directions across populations, buffering the variation in population growth rates, and is a mechanism often invoked to explain the stability of species geographic ranges. However, studies on demographic compensation have disregarded the effects of temporal variation in vital rates and their temporal correlations, despite theoretical evidence that stochastic dynamics can affect population persistence in temporally varying environments. We carried out a seven‐year‐long demographic study on the perennial plant Arabis alpina (L.) across six populations encompassing most of its elevational range. We discovered demographic compensation in the form of negative correlations between the means of plant vital rates, but also between their temporal coefficients of variation, correlations and elasticities. Even if their contribution to demographic compensation was small, this highlights a previously overlooked, but potentially important, role of stochastic processes in stabilising population dynamics at range margins.