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101 results for “population fluctuation”
Data from: Accounting for interspecific competition and age structure in demographic analyses of density dependence improves predictions of fluctuations in population size
Understanding species coexistence has long been a major goal of ecology. Coexistence theory for two competing species posits that intraspecific density dependence should be stronger than interspecific density dependence. Great tits and blue tits are two bird species that compete for food resources and nesting cavities. Based on long-term monitoring of these two competing species at sites across Europe, combining observational and manipulative approaches, we show that the strength of density regulation is similar for both species, and that individuals have contrasting abilities to compete depending on their age. For great tits, density regulation is driven mainly by intraspecific competition. In contrast, for blue tits, interspecific competition contributes as much as intraspecific competition, consistent with asymmetric competition between the two species. In addition, including age-specific effects of intra- and interspecific competition in density-dependence models improves predictions of fluctuations in population size by up to three times.
Data from: Hidden survival heterogeneity of three common eider populations in response to climate fluctuations
(1) Understanding how individuals and populations respond to fluctuations in climatic conditions is critical to explain and anticipate changes in ecological systems. Most such studies focus on climate impacts on single populations without considering inter- and intra-population heterogeneity. However, comparing geographically dispersed populations limits the risk of faulty generalizations and helps to improve ecological and demographic models. (2) We aimed to determine whether differences in migration tactics among and within populations would induce inter- or intra-population heterogeneity in survival in relation to winter climate fluctuations. Our study species was the Common eider (Somateria mollissima), a marine duck with a circumpolar distribution, which is strongly affected by climatic conditions during several phases of its annual cycle. (3) Capture-mark-recapture (CMR) data were collected in two arctic (northern Canada and Svalbard) and one subarctic (northern Norway) population over a period of 18, 15 and 29 years, respectively. These three populations have different migration tactics and experience different winter climatic conditions. Using multi-event and mixture modelling, we assessed the association between adult female eider survival and winter conditions as measured by the North Atlantic Oscillation index. (4) We found that winter weather conditions affected survival of female eiders from each of these three populations. However, different mechanisms seemed to be involved. Survival of the two migrating arctic populations was impacted directly by changes in the NAO, whereas the subarctic resident population was affected by the NAO with time lags of two to three years. Moreover, we found evidence for intra-population heterogeneity in the survival response to the winter NAO in the Canadian eider population, where individuals migrate to distinct wintering areas. (5) Our results illustrate how individuals and populations of the same species can vary in their responses to climate variation. We suspect that the found variation in survival response of birds to winter conditions is partly explained by differences in migration tactic. Detecting and accounting for inter- and intra-population heterogeneity will improve our predictions concerning the response of wildlife to global changes.
Data from: Strong, nonlinear selection against fluctuating asymmetry in wild populations of a marine fish
Theoretical links between fluctuating asymmetry (FA) and fitness have led many to use FA as a proxy for average fitness. However, studies examining whether asymmetry actually correlates with individual fitness in wild populations are relatively rare and often use simple measures of association (e.g., correlation coefficients). Consequently, the pattern of selection on asymmetry in the wild is seldom clear. We examined selection on FA of pectoral fin morphology in two wild populations of a marine fish (the kelp perch; Brachyistius frenatus). As expected, variance in signed FA in each initial sample was significantly greater than that found in the surviving population, indicating selection against FA. Our estimate of the fitness surface confirmed perfect symmetry as the phenotypic optimum and indicated strong, nonlinear selection against asymmetry. No difference in the form of selection was detected between populations. However, the level of FA in the initial samples varied among populations, leading to an overall difference in the level of selective mortality. Our results suggest that selection on asymmetry in wild populations may be strongly nonlinear, and indicate that the demographic costs of asymmetry may play a substantial role in the dynamics of populations.
Data from: Mechanism matters: the cause of fluctuations in boom-bust populations governs optimal habitat restoration strategy
Many populations exhibit boom-bust dynamics in which abundance fluctuates dramatically over time. Past research has focused on identifying whether the cause of fluctuations is primarily exogenous, e.g., environmental stochasticity coupled with weak density dependence, or endogenous, e.g., over-compensatory density dependence. Far fewer studies have addressed whether the mechanism responsible for boom-bust dynamics matters with respect to at-risk species management. Here, we ask whether the best strategy for restoring habitat across a landscape differs under exogenously versus endogenously driven boom-bust dynamics. We used spatially explicit individual-based models to assess how butterfly populations governed by the two mechanisms would respond to habitat restoration strategies that varied in the level of resource patchiness – from a single large patch to multiple patches spaced at different distances. Our models showed that the restoration strategy that minimized extinction risk and boom-bust dynamics would be markedly different depending on the governing mechanism. Exogenously governed populations fared best in a single large habitat patch, whereas for endogenously driven populations, boom-bust dynamics were dampened and extinction risk declined when the total restored area was split into multiple patches with low to moderate inter-patch spacing. Adding environmental stochasticity to the endogenous model did not alter this result. Habitat fragmentation lowered extinction risk in the endogenously driven populations by reducing their growth rate, precluding both "boom" phases and, more importantly, "bust" phases. Our findings suggest that: 1) successful restoration will depend on understanding the causes of fluctuations in at-risk populations; 2) the level and pattern of spatiotemporal environmental heterogeneity will also affect the ideal management approach; and 3) counter-intuitively, for at-risk species with endogenously governed boom-bust dynamics, lowering the intrinsic population growth rate may decrease extinction risk.
FIGURE 5 in Seasonal Fluctuation In The Population Of Harmonia Axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) And Co-Occurrence With Other Coccinellids In The Federal District Of Brazil
FIGURE 5: Diversity profile of coccinellid communities collect- ed among organic vegetable crops in two regions of the Federal District.
FIGURE 1 in Seasonal Fluctuation In The Population Of Harmonia Axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) And Co-Occurrence With Other Coccinellids In The Federal District Of Brazil
FIGURE 1: Coloration patterns of Harmonia axyridis collected in the region of the Federal District, Brazil. The scale bar above the photos of the specimens corresponds to 4 mm.
FIGURE 4 in Seasonal Fluctuation In The Population Of Harmonia Axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) And Co-Occurrence With Other Coccinellids In The Federal District Of Brazil
FIGURE 4: Estimated richness of species or genera of the coccinellid community through rarefaction curves in vegetable crops in two regions of the Federal District.
FIGURE 3 in Seasonal Fluctuation In The Population Of Harmonia Axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) And Co-Occurrence With Other Coccinellids In The Federal District Of Brazil
FIGURE 3: Seasonal variation in the population of Harmonia axyridis and other coccinellid (Cycloneda sanguinea, Hippodamia convergens, Eriopis connexa, Scymnus spp. Azya luteipes, Hyperaspis festiva, Olla v-nigrum and Psyllobora sp.) collected in an area of vegetable intercropping in organic systems from February 2009 to January 2010 in Taguatinga, Federal District.
FIGURE 2 in Seasonal Fluctuation In The Population Of Harmonia Axyridis (Pallas, 1773) (Coleoptera: Coccinellidae) And Co-Occurrence With Other Coccinellids In The Federal District Of Brazil
FIGURE 2: Seasonal pattern of rainfall distribution and variation in mean maximum and minimum temperatures in the Federal District, (A) 2008 and (B) 2009.
Data from: Linking genetic diversity and temporal fluctuations in population abundance of the introduced feral cat (Felis silvestris catus) on the Kerguelen Archipelago.
Linking temporal variations of genetic diversity, including allelic richness and heterozygosity, and spatio-temporal fluctuations in population abundance has emerged as an important tool for understanding demographic and evolutionary processes in natural populations. This so-called 'genetic monitoring' was conducted across 12 consecutive years (1996-2007) at three sites for the feral cat, introduced onto the Kerguelen Archipelago fifty years ago. Temporal changes in allelic richness and heterozygosity at 18 microsatellite DNA loci were compared to temporal changes in the adult population abundance index, obtained by typical demographic monitoring. No association was found at the island spatial scale but we observed an association between genetic diversity and adult population indices from year to year within each study site. More particularly, the magnitude of successive increases or decreases in the adult population abundance index appeared to be the major factor linking the trajectories of genetic diversity and adult population abundance indices. Natal dispersal and/or local recruitment, both facilitated by high juvenile survival when the adult population size is small, are proposed as the major demographic processes contributing to such an observed pattern. Finally, we suggested avoiding the use of the harmonic mean as an estimator of long-term population size to study the relationships between demographic fluctuations and heterozygosity in populations characterized by strong multi-annual density fluctuations.
Data from: Trait-demography relationships underlying small mammal population fluctuations
Large-scale fluctuations in abundance are a common feature of small mammal populations and have been the subject of extensive research. These demographic fluctuations are often associated with concurrent changes in the average body mass of individuals, sometimes referred to as the 'Chitty effect'. Despite the long-standing recognition of this phenomenon, an empirical investigation of the underlying coupled dynamics of body mass and population growth has been lacking. Using long-term life-history data combined with a trait-based demographic approach, we examined the relationship between body mass and demography in a small mammal population that exhibits non-cyclic, large-scale fluctuations in abundance. We used data from the male segment of a 25-year study of the monogamous prairie vole, Microtus ochrogaster, in Illinois, USA. Specifically, we investigated how trait–demography relationships and trait distributions changed between different phases of population fluctuations, and the consequences of these changes for both trait and population dynamics. We observed phase-specific changes in male adult body mass distribution in this population of prairie voles. Our analyses revealed that these changes were driven by variation in ontogenetic growth, rather than selection acting on the trait. The resulting changes in body mass influenced most life-history processes, and these effects varied among phases of population fluctuation. However, these changes did not propagate to affect the population growth rate due to the small effect of body mass on vital rates, compared to the overall differences in vital rates between phases. The increase phase of the fluctuations was initiated by enhanced survival, particularly of juveniles and fecundity, whereas the decline phase was driven by an overall reduction in fecundity, survival and maturation rates. Our study provides empirical support, as well as a potential mechanism, underlying the observed trait changes accompanying population fluctuations. Body size dynamics and population fluctuations resulted from different life-history processes. Therefore, we conclude that body size dynamics in our population do not drive the observed population dynamics. This more in-depth understanding of different components of small mammal population fluctuations will help us to better identify the mechanistic drivers of this interesting phenomenon.
Environmental drivers of annual population fluctuations in a trans-Saharan insect migrant
<p>Many latitudinal insect migrants, including agricultural pests, disease vectors and beneficial species, show huge fluctuations in the year-to-year abundance of spring immigrants reaching temperate zones. It is widely believed that this variation is driven by climatic conditions in the winter-breeding regions, but evidence is lacking. We identified the environmental drivers of the annual population dynamics of a cosmopolitan migrant butterfly (the painted lady Vanessa cardui), using a combination of long-term monitoring, climate and atmospheric data within the western part of its Afro-Palearctic migratory range. Our population models show that a combination of high winter NDVI (normalised difference vegetation index) in the Savannah/Sahel of sub-Saharan Africa, high spring NDVI in the Maghreb of North Africa, and frequent favourably-directed tailwinds during migration periods, are the three most important drivers of the size of the immigration to Western Europe, while our atmospheric trajectory simulations demonstrate regular opportunities for windborne trans-Saharan movements. The effects of sub-Saharan vegetative productivity and wind conditions confirm that painted lady populations either side of the Sahara are linked by regular mass migrations, making this the longest annual insect migration circuit so far known. Our results provide the first quantification of the environmental drivers of large annual population fluctuations of an insect migrant, and hold much promise for predicting invasions of migrant insect pests, disease vectors and beneficial species.</p>
Figure 4 in Seasonal population fluctuation and spatial distribution of Orthoptera in two grassland areas of Attica - Greece
Figure 4. Mean number of Chorthippus bornhalmi per sample in the mountain station through a 2-year study.
Figure 3 in Seasonal population fluctuation and spatial distribution of Orthoptera in two grassland areas of Attica - Greece
Figure 3. Mean number of Dociostaurus maroccanus per sample in the lowland station through a 2-year study.
Figure 1 in Seasonal population fluctuation and spatial distribution of Orthoptera in two grassland areas of Attica - Greece
Figure 1. Mean number of individuals per sample of Acrididae, Tettigoniidae and Gryllidae families in the lowland station through a 2-year study.
Figure 2 in Seasonal population fluctuation and spatial distribution of Orthoptera in two grassland areas of Attica - Greece
Figure 2. Mean number of individuals per sample of Acrididae, Tettigoniidae and Gryllidae families in the mountain station through a 2-year study.
Figure 1 in Fluctuating asymmetry in populations of the South American frog Physalaemus cuvieri (Leptodactylidae) in areas with different degrees of disturbance
Figure 1. Ecological Station of Minas Gerais Federal University (EEUFMG) and Serra do Cipó National Park (PNSC), located at Minas Gerais State, southeastern Brazil. State boundaries are represented by black lines, while rivers, lakes and sea are represented in grey. State capitals are represented by white circles.
Growth, stress and acclimation responses to fluctuating temperatures in field and domesticated populations of Manduca sexta
<p>Diurnal fluctuations in temperature are ubiquitous in terrestrial environments, and insects and other ectotherms have evolved to tolerate or acclimate to such fluctuations. Few studies have examined whether ectotherms acclimate to diurnal temperature fluctuations, or how natural and domesticated populations differ in their responses to diurnal fluctuations. We examine how diurnally fluctuating temperatures during development affect growth, acclimation and stress responses for two populations of Manduca sexta: a field population that typically experiences wide variation in mean and fluctuations in temperature, and a laboratory population that has been domesticated in nearly constant temperatures for more than 300 generations. Laboratory experiments showed that diurnal fluctuations throughout larval development reduced pupal mass for the lab but not the field population. The differing effects of diurnal fluctuations were greatest at higher mean temperature (30 °C): here diurnal fluctuations reduced pupal mass and increased pupal development time for the lab population, but had little effect for the field population. We also evaluated how mean and fluctuations in temperature during early larval development affected growth rate during the final larval instar as a function of test temperature. At an intermediate (25 °C) mean temperature, both the lab and field population showed a positive acclimation response to diurnal fluctuations, in which subsequent growth rate was significantly higher at most test temperatures. In contrast at higher mean temperature (30 °C), diurnal fluctuations significantly reduced subsequent growth rate at most test temperatures for the lab population, but not for the field population. These results suggest that during domestication in constant temperatures, the lab population has lost the capacity to tolerate or acclimate to high and fluctuating temperatures. Population differences in acclimation capacity in response to temperature fluctuations has not been previously demonstrated, but they may be important for understanding the evolution of reaction norms and performance curves. </p>
Data and scripts from: Experimental evidence of size-selective harvest and environmental stochasticity effects on population demography, fluctuations, and nonlinearity
<p class="MsoNormal">Theory and analyses of fisheries datasets indicate that harvesting can alter population structure and destabilize nonlinear processes, which increases population fluctuations. We conducted a factorial experiment on the population dynamics of <em>Daphnia magna</em> in relation to size-selective harvesting and stochasticity of food supply. Harvesting and stochasticity treatments both increased population fluctuations. Timeseries analysis indicated that fluctuations in control populations were nonlinear, and nonlinearity increased substantially in response to harvesting. Both harvesting and stochasticity induced population juvenescence, but harvesting did so via depletion of adults whereas stochasticity increased the abundance of juveniles. A fitted fisheries model indicated that harvesting shifted populations towards higher reproductive rates and larger-magnitude damped oscillations that amplify demographic noise. These findings provide experimental evidence that harvesting increases nonlinearity of population fluctuations and that both harvesting and stochasticity increase population variability and juvenescence.</p>
Growth, stress and acclimation responses to fluctuating temperatures in field and domesticated populations of Manduca sexta
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