Interacting effects of predation risk and resource level on escape speed of amphibian larvae along a latitudinal gradient

Fast‐growing genotypes living in time‐constrained environments are often more prone to predation, suggesting that growth‐predation risk trade‐offs are important factors maintaining variation in growth along climatic gradients. However, the mechanisms underlying how fast growth increases predation‐mediated mortality are not well understood. Here, we investigated if slow‐growing, low‐latitude individuals have faster escape swimming speed than fast‐growing high‐latitude individuals using common frog (Rana temporaria) tadpoles from eight populations collected along a 1500 km latitudinal gradient. We measured escape speed in terms of burst and endurance speeds in tadpoles raised in the laboratory at two food levels and in the presence and absence of a predator (Aeshna dragonfly larvae). We did not find any latitudinal trend in escape speed performance. In low food treatments, burst speed was higher in tadpoles reared with predators but did not differ between high‐food treatments. Endurance speed, on the contrary, was lower in high‐food tadpoles reared with predators and did not differ between treatments at low food levels. Tadpoles reared with predators showed inducible morphology (increased relative body size and tail depth), which had positive effects on speed endurance at low but not at high food levels. Burst speed was positively affected by tail length and tail muscle size in the absence of predators. Our results suggest that escape speed does not trade‐off with fast growth along the latitudinal gradient in R. temporaria tadpoles. Instead, escape speed is a plastic trait and strongly influenced by the interaction between resource level and predation risk.     

Incorporating environmental change over succession in an integral projection model of population dynamics of a forest herb

Despite seemingly obvious effects of environmental drivers, mechanisms behind long‐term changes in plant population sizes over time are often poorly known. We investigated how soil potassium concentration and seed predation are likely to change over time as a result of succession from deciduous forest to spruce forest, and how this affects population trajectories of Actaea spicata. Observations and addition experiments showed that high soil potassium concentration increased individual growth rates. Among‐site comparisons showed that soil potassium concentration was lower where proportion spruce was higher. Incorporation of a gradual increase in spruce over time in an integral projection model where individual growth depended on potassium suggested a net decrease in A. spicata population sizes over forest succession. This result suggests that small changes in factors with small effects on individual performance can influence patterns of species occupancy along successional gradients. We incorporated also density independent and density dependent effects of pre‐dispersal seed predation over succession into the same model. Seed predation influenced the tree composition at which A. spicata population growth was positive. However, significant effects of A. spicata population size on seed predation intensity did not translate into important feedback effects on population growth trajectories over succession. Our results illustrate how demographic models can be used to gain understanding of the mechanisms behind effects of environmental change on species abundances and distributions by the simultaneous inclusion of changing abiotic and biotic factors.     

The Effects of Predation Risk, Food Abundance, and Population Size on Group Size of Brown-Headed Cowbirds (Molothrus ater)

Social and ecological conditions can influence flock formation (e.g. number of flocks, flock size, etc.) depending on the degree of social attraction of a species. We studied group formation in brown‐headed cowbirds (Molothrus ater) over short time periods (30 min) in two semi‐natural experiments conducted under controlled conditions. First, we determined the shape of the relationship between intake rate and flock size by manipulating group size in a single enclosure. Second, we assessed the role of population size, food abundance, and predation risk, and their interactions, in flock size formation in a system of four enclosures (two with and two without food) connected to a central refuge patch. In the first experiment, we found that pecking rates peaked at intermediate flock sizes (three to six individuals), which was influenced by greater availability of foraging time and more aggressive interactions in large groups. In the second experiment, flock sizes in the patches with food increased with population size likely due to the benefits of patch exploitation in groups. Flock size decreased after predator attack probably because refuge availability reduced perceived predation risk more than flocking in larger groups. Food abundance had minor effects, varying flock sizes between the two patches with food, under high food availability conditions when population size was high, probably due to social cohesion effects. Our results suggest that: (1) this species has an inverted‐U food intake–group size relationship with a range of intake‐maximizing flock sizes rather than a single peak, (2) the presence of a near refuge modifies the expected benefits of group patch exploitation under high predation risk, and (3) an increase in population size would more likely be translated into rapid increases in the size of the flocks rather than in more new flocks.     

Spatiotemporal variation in predispersal seed predation intensity

The effect of predispersal seed predation by Bruchus atomarius (Bruchidae, Coleoptera) on individual performance and population dynamics of the perennial forest herb, Lathyrus vernus (Leguminosae), was investigated in 11 permanent plots over 4 years. Seed predation and parameters describing intra-specific neighbour distance, plant size, inflorescence size, flowering phenology and current and previous herbivore damage were measured on all plants. In addition, demographic information from all plots was analysed using transition matrix population models in order to estimate the influence of seed predation on population growth rates. Predispersal seed predation rates differed significantly among years. Plot averages ranged from 0 to 83.7%. However, most of the variation occurred among individuals. Within individuals there was no consistency in predation rates among years. Exposure to herbivory, plant size and flowering phenology did not affect predation rates but individuals with larger inflorescences suffered from significantly higher predation. Seed predation in L. vernus was not influenced by neighbour distances of individual plants but it was positively correlated with the average density of seeds within plots, suggesting that seed predation is density dependent at the patch level. The reduction in population growth rate due to seed predation ranged from 0 to 7.6%. The sensitivity of population growth rate to reductions in seed production varied considerably among years and plots. This variation was mainly due to differences in the reproductive value of seeds and seedlings. The intensity of seed predation over the range found was not correlated with changes in population growth rate. The results of this study suggest that the influence of external factors, like seed predation, on population growth rate largely depends on the demographic transition rates in the investigated population.     

A review of the population dynamics of mule deer and black‐tailed deer Odocoileus hemionus in North America

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Recruitment rates exhibit high elasticity and high temporal variation in populations of a short-lived perennial herb

Empirical studies for different life histories have shown an inverse relationship between elasticity (i.e. the proportional contribution to population growth rate) and temporal variation in vital rates. It is accepted that this relationship indicates the effect of selective pressures in reducing variation in those life‐history traits with a major impact on fitness. In this paper, we sought to determine whether changes in environmental conditions affect the relationship between elasticity of vital rates and their temporal variation, and whether vital rates with simultaneously large elasticity and temporal variation might represent a characteristic life‐history strategy. We used demographic data on 13 populations of the short‐lived Hypericum cumulicola over 5–6 years, in three time‐since‐fire classes. For each population of each time‐since‐fire, we computed the mean matrix over years and its respective elasticity matrix, and the coefficients of variation in matrix entries over study years as an estimate of temporal variability. We found that mean elasticity negatively significantly correlated with temporal variation in vital rates in populations (overall eight out of 13) included in each time‐since‐fire. However, seedling recruitment exhibited both high elasticity and high temporal variation in almost all study populations. These results indicated that (1) the general relationship between elasticity and temporal variation in vital rates was not modified by environmental changes due to time‐since‐fire, and (2) high elasticity and high temporal variation in seedling recruitment in H. cumulicola is a particular trait of the species' life history. After seed survival in the soil seed bank, seedling recruitment represents the most important life‐history trait influencing H. cumulicola population growth rate (and fitness). The high temporal variability in seedling recruitment suggests that this trait is determined by environmental cues, leading to an increase in population size and subsequent replenishment of the seed bank in favorable years.     

A natural antipredation experiment: predator control and reduced sea ice increases colony size in a long‐lived duck

Anthropogenic impact on the environment and wildlife are multifaceted and far‐reaching. On a smaller scale, controlling for predators has been increasing the yield from local natural prey resources. Globally, human‐induced global warming is expected to impose severe negative effects on ecosystems, an effect that is expected to be even more pronounced in the scarcely populated northern latitudes. The clearest indication of a changing Arctic climate is an increase in both air and ocean temperatures leading to reduced sea ice distribution. Population viability is for long‐lived species dependent on adult survival and recruitment. Predation is the main mortality cause in many bird populations, and egg predation is considered the main cause of reproductive failure in many birds. To assess the effect of predation and climate, we compared population time series from a natural experiment where a trapper/down collector has been licensed to actively protect breeding common eiders Somateria mollissima (a large seaduck) by shooting/chasing egg predators, with time series from another eider colony located within a nature reserve with no manipulation of egg predators. We found that actively limiting predator activity led to an increase in the population growth rate and carrying capacity with a factor of 3–4 compared to that found in the control population. We also found that population numbers were higher in years with reduced concentration of spring sea ice. We conclude that there was a large positive impact of human limitation of egg predators, and that this lead to higher population growth rate and a large increase in size of the breeding colony. We also report a positive effect of warming climate in the high arctic as reduced sea‐ice concentrations was associated with higher numbers of breeding birds.     

Size‐Assortative Shoaling in the Guppy (Poecilia reticulata): The Role of Active Choice

Many fish species exhibit size‐assortative shoaling, which is often thought to be driven by predation risk. Recent fieldwork has revealed that guppies (Poecilia reticulata) are more size assorted in high‐predation populations than in low‐predation ones. However, size assortment does nonetheless occur in some low‐predation populations, suggesting that predation is unlikely the sole driving force behind size‐assortment. Here, we investigated in the laboratory the potential role of active choice in size‐assortative shoaling in wild‐caught female guppies originating from two populations of the same river system in Trinidad. Small or large focal females from each population were offered a binary choice of shoaling with either four small female conspecifics or four large ones. Observed shoaling preferences depended on the body size of the focal fish, suggesting phenotype‐mediated conflict over group composition. Large focal fish preferred to shoal with the size‐matched stimulus shoal of large fish. In contrast, small focal fish did not shoal assortatively but also preferred to shoal with larger females. Our results suggest that size‐assortative shoaling in female guppies is likely to be due to factors other than active choice, such as habitat segregation and sexual harassment.     

Biotic resistance on the increase: native predators structure invasive zebra mussel populations

1. Abundant native predators, parasites and pathogens that switch to consuming a hyper‐successful exotic species may be able to control the invasive population. Native predators may, however, need time to adapt to feed effectively on an exotic resource. In this case, mortality on an exotic population from native predators could increase over time even without a numerical increase in the predator population. 2. We measured mortality of zebra mussels (Dreissena polymorpha) in the Hudson River both in controls open to predation and in exclosures that excluded large predators to estimate mortality of zebra mussels from large predators and other causes. 3. We found that predation by the blue crab (Callinectes sapidus), and perhaps other predators, causes high mortality on zebra mussels in the Hudson River estuary. This predation apparently led to increased mortality and altered population structure in the invader over time. 4. Long‐term data from the Hudson River suggest that components of the invaded ecosystem, like rotifers, are recovering through predator‐caused release from zebra mussel grazing. Increased mortality on hyper‐successful exotic populations over time may be a common phenomenon with both ecological and management implications.     

Selection for life‐history traits to maximize population growth in an invasive marine species

Species establishing outside their natural range, negatively impacting local ecosystems, are of increasing global concern. They often display life‐history features characteristic for r‐selected populations with fast growth and high reproduction rates to achieve positive population growth rates (r) in invaded habitats. Here, we demonstrate substantially earlier maturation at a 2 orders of magnitude lower body mass at first reproduction in invasive compared to native populations of the comb jelly Mnemiopsis leidyi. Empirical results are corroborated by a theoretical model for competing life‐history traits that predicts maturation at the smallest possible size to optimize r, while individual lifetime reproductive success (R₀), optimized in native populations, is near constant over a large range of intermediate maturation sizes. We suggest that high variability in reproductive tactics in native populations is an underappreciated determinant of invasiveness, acting as substrate upon which selection can act during the invasion process.     

Rapid increase in southern elephant seal genetic diversity after a founder event

Genetic diversity provides the raw material for populations to respond to changing environmental conditions. The evolution of diversity within populations is based on the accumulation of mutations and their retention or loss through selection and genetic drift, while migration can also introduce new variation. However, the extent to which population growth and sustained large population size can lead to rapid and significant increases in diversity has not been widely investigated. Here, we assess this empirically by applying approximate Bayesian computation to a novel ancient DNA dataset that spans the life of a southern elephant seal (Mirounga leonina) population, from initial founding approximately 7000 years ago to eventual extinction within the past millennium. We find that rapid population growth and sustained large population size can explain substantial increases in population genetic diversity over a period of several hundred generations, subsequently lost when the population went to extinction. Results suggest that the impact of diversity introduced through migration was relatively minor. We thus demonstrate, by examining genetic diversity across the life of a population, that environmental change could generate the raw material for adaptive evolution over a very short evolutionary time scale through rapid establishment of a large, stable population.     

Predicting which species will benefit from corridors in fragmented landscapes from population growth models

Connecting isolated patches of habitat in fragmented landscapes with corridors is a popular conservation strategy. This strategy is also controversial in large part because of uncertainty about what characteristics of a species and its environment promote corridor use. In this article we address the question, For what types of species will populations benefit from corridors? We asked this question using a model of two logistically growing populations connected by migration in which both emigration and migration success were determined by the presence or absence of a corridor. We found that in the short run (e.g., during recovery from disaster), corridors are most effective for species with fast-growing populations that have low survivorship when dispersing through unsuitable (matrix) habitat. We also found that emigration rates and habitat-specific mortality rates are key determinants of the effects of corridors on population size. In the long term, corridors are most likely to benefit species with slow-growing populations that have low survivorship when dispersing through matrix habitat. Our results confirm the major conclusions from previous empirical studies of corridor benefits. However, most studies fail to consider the most appropriate questions to determine the potential benefits of habitat corridors. First, what is the time scale of the conservation goal? Corridors have positive effects on different suites of species in the short and long term. Second, is the major threat of local extinction due to sustained population decline or boom-bust cycles? Third, what is the migration rate through the matrix? Fourth, what fraction of migrants dispersing through the matrix successfully immigrate to another patch?     

Dynamics of an age‐structured population drawn from a random numbers table

I constructed age‐structured populations by drawing numbers from a random numbers table, the constraints being that within a cohort each number be smaller than the preceding number (indicating that some individuals died between one year and the next) and that the first two‐digit number following 00 or 01 ending one cohort’s life be the number born into the next cohort. Populations constructed in this way showed prolonged existence with total population numbers fluctuating about a mean size and with long‐term growth rate (r) ≈ 0. The populations’ birth rates and growth rates and the females’ per capita fecundity decreased significantly with population size, whereas the death rates showed no significant relationship to population size. These results indicate that age‐structured populations can persist for long periods of time with long‐term growth rates of zero in the absence of negative‐feedback loops between a population’s present or prior density and its birth rate, growth rate, and fecundity, contrary to the assumption of density‐dependent regulation hypotheses. Thus, a long‐term growth rate of zero found in natural populations need not indicate that a population’s numbers are regulated by density‐dependent factors.     

The adaptive significance of population differentiation in offspring size of the least killifish,Heterandria formosa

We tested the hypothesis that density‐dependent competition influences the evolution of offspring size. We studied two populations of the least killifish (Heterandria formosa) that differ dramatically in population density; these populations are genetically differentiated for offspring size, and females from both populations produce larger offspring when they experience higher social densities. To look at the influences of population of origin and relative body size on competitive ability, we held females from the high‐density population at two different densities to create large and small offspring with the same genetic background. We measured the competitive ability of those offspring in mesocosms that contained either pure or mixed population treatments at either high or low density. High density increased competition, which was most evident in greatly reduced individual growth rates. Larger offspring from the high‐density population significantly delayed the onset of maturity of fish from the low‐density population. From our results, we infer that competitive conditions in nature have contributed to the evolution of genetically based interpopulation differences in offspring size as well as plasticity in offspring size in response to conspecific density.     

Cannibals and parasites: conflicting regulators of bimodality in high latitude Arctic char, Salvelinus alpinus

Unexploited populations of Arctic char (Salvelinus alpinus) sampled in autonomous lake ecosystems in northern Svalbard demonstrate extraordinary catch curves with age and size frequency distributions characterized by discrete bimodality. Analyses of size‐age relationship, summer diet and food‐related intestinal parasite intensities of modal char groups revealed a pattern of discrete ontogenetic niche shifts. Life‐history changes at age 10–15 and size 200–300 mm/50–300 g involved shifting from an initial mode of small‐sized, slow‐growing and sexually mature individuals feeding on micro‐crustaceans and aquatic insects (Chironomidae, Trichoptera), to a terminal mode of large‐sized and fast‐growing cannibals. Cannibalism, however, was found to result in accumulation of cestodan parasites, of which Diphyllobothrium ditremum increases age‐related mortality rates and may be lethal at 1500–2000 plerocercoids. Genetically allopatric populations with cannibalism demonstrated a female‐biased sex ratio, primarily in the initial mode, suggesting sexual asynchrony in their ontogeny. By contrast, a small population of large‐sized, non‐cannibalistic Arctic char feeding exclusively on the large amphipod Gammaracanthus lacustris, demonstrated unimodal size and age frequency distribution, faster growth, an excess of males and lower parasite burden. Seasonal prey shortage and slow juvenile growth in association with fitness components favoring large body size is a suggested mechanism for inducing cannibalism. Although not the basic cause of bimodality as such, it is concluded that ontogenetic niche shift by cannibalism reinforces discrete age modal divergence resulting in the numerical preponderance of large‐sized individuals in these marginal char populations. Cannibalism is thus considered an important strategy for survival of landlocked Arctic char in the High Arctic. As a conflicting cost to the more efficient use of available energy by larger individuals, the accumulation of cestodan parasites in cannibals, however, will reduce the survival rate of old individuals and accelerate their termination within this modal group.     

Scaling mercury biodynamics from individuals to populations: Implications of an herbivorous fish on mercury cycles in streams

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Climate change disrupts local adaptation and favours upslope migration

Contemporary climate change is proceeding at an unprecedented rate. The question remains whether populations adapted to historical conditions can persist under rapid environmental change. We tested whether climate change will disrupt local adaptation and reduce population growth rates using the perennial plant Boechera stricta (Brassicaceae). In a large‐scale field experiment conducted over five years, we exposed > 106 000 transplants to historical, current, or future climates and quantified fitness components. Low‐elevation populations outperformed local populations under simulated climate change (snow removal) across all five experimental gardens. Local maladaptation also emerged in control treatments, but it was less pronounced than under snow removal. We recovered local adaptation under snow addition treatments, which reflect historical conditions. Our results revealed that low elevation populations risk rapid decline, whereas upslope migration could enable population persistence and expansion at higher elevation locales. Local adaptation to historical conditions could increase vulnerability to climate change, even for geographically widespread species.     

Optimal reproductive strategies in age-structured populations of zooplankton

SUMMARY. We describe a model of zooplankton population dynamics that accounts for differences in mortality and physiology among animals of different ages or sizes. The model follows changes in numbers of individuals and changes in individual and egg biomass through time and it expresses mortality and net assimilation as functions of animal size.
We investigated the effect of egg size, age at first reproduction, and size at first reproduction on the per capita growth rates of populations growing under different conditions. In the absence of predation or when exposed to vertebrate predators that prefer large prey, populations achieve maximum growth rates when animals hatch from small eggs and reach maturity quickly at small sizes. Populations exposed to invertebrate predators that concentrate on small animals may increase r in two different ways. One way is for animals to increase juvenile survivorship by hatching from large eggs and by shortening the juvenile period. An alternative strategy is for animals to hatch from small eggs and to postpone maturity until they grow beyond the range of sizes available to their predators. Certain life history strategies maximize r if animals continue to grow after they reach maturity. By growing larger, non-primiparous females are able to hatch larger clutches and thereby increase the overall rate of population growth.
The model analysis shows how to assess age-dependent mortality rates from field data. The net rate of population increase and the age distribution of eggs together provide specific, quantitative information about mortality.     

The evolution of growth trajectories: what limits growth rate?

According to life‐history theory, growth rates are subject to strong directional selection due to reproductive and survival advantages associated with large adult body size. Yet, growth is commonly observed to occur at rates lower than the maximum that is physiologically possible and intrinsic growth rates often vary among populations. This implies that slower growth is favoured under certain conditions. Realized growth rate is thus the result of a compromise between the costs and advantages of growing rapidly, and the optimal rate of growth is not equivalent to the fundamental maximum rate. The ecological and evolutionary factors influencing growth rate are reviewed, with particular emphasis on how growth might be constrained by direct fitness costs. Costs of accelerating growth might contribute to the variance in fitness that is not attributable to age or size at maturity, as well as to the variation in life‐history strategies observed within and among species. Two main approaches have been taken to study the fitness trade‐offs relating to growth rate. First, environmental manipulations can be used to produce treatment groups with different rates of growth. Second, common garden experiments can be used to compare fitness correlates among populations with different intrinsic growth rates. Data from these studies reveal a number of potential costs for growth over both the short and long term. In order to acquire the energy needed for faster growth, animals must increase food intake. Accordingly, in many taxa, the major constraint on growth rate appears to arise from the trade‐off between predation risk and foraging effort. However, growth rates are also frequently observed to be submaximal in the absence of predation, suggesting that growth trajectories also impact fitness via other channels, such as the reallocation of finite resources between growth and other traits and functions. Despite the prevalence of submaximal growth, even when predators are absent, there is surprisingly little evidence to date demonstrating predator‐independent costs of growth acceleration. Evidence that does exist indicates that such costs may be most apparent under stressful conditions. Future studies should examine more closely the link between patterns of resource allocation to traits in the adult organism and lifetime fitness. Changes in body composition at maturation, for example, may determine the outcome of trade‐offs between reproduction and survival or between early and late reproduction. A number of design issues for studies investigating costs of growth that are imposed over the long term are discussed, along with suggestions for alternative approaches. Despite these issues, identifying costs of growth acceleration may fill a gap in our understanding of life‐history evolution: the relationships between growth rate, the environment, and fitness may contribute substantially to the diversification of life histories in nature.     

Population growth lags in introduced species

When introduced to new ecosystems, species'' populations often grow immediately postrelease. Some introduced species, however, maintain a low population size for years or decades before sudden, rapid population growth is observed. Because exponential population growth always starts slowly, it can be difficult to distinguish species experiencing the early phases of slow exponential population growth (inherent lags) from those with actively delayed growth rates (prolonged lags). Introduced ungulates provide an excellent system in which to examine lags, because some introduced ungulate populations have demonstrated rapid population growth immediately postintroduction, while others have not. Using studies from the literature, we investigated which exotic ungulate species and populations (n = 36) showed prolonged population growth lags by comparing the doubling time of real ungulate populations to those predicted from exponential growth models for theoretical populations. Having identified the specific populations that displayed prolonged lags, we examined the impacts of several environmental and biological variables likely to influence the length of lag period. We found that seventeen populations (47%) showed significant prolonged population growth lags. We could not, however, determine the specific factors that contributed to the length of these lag phases, suggesting that these ungulate populations'' growth is idiosyncratic and difficult to predict. Introduced species that exhibit delayed growth should be closely monitored by managers, who must be proactive in controlling their growth to minimize the impact such populations may have on their environment.