scholarly journals Pushed beyond the brink: Allee effects, environmental stochasticity, and extinction

2014 ◽  
Author(s):  
Gregory Roth ◽  
Sebastian Schreiber
Keyword(s):  
High Probability ◽  
Geometric Mean ◽  
Environmental Stochasticity ◽  
Initial Population ◽  
Positive Density ◽  
Allee Effects ◽  
Population Densities ◽  
Density Dependent ◽  
Environmental Fluctuations ◽  
Predation Rates

To understand the interplay between environmental stochasticity and Allee effects, we analyze persistence, asymptotic extinction, and conditional persistence for stochastic difference equations. Our analysis reveals that persistence requires that the geometric mean of fitness at low densities is greater than one. When this geometric mean is less than one, asymptotic extinction occurs with high probability for low initial population densities. Additionally, if the population only experiences positive density-dependent feedbacks, conditional persistence occurs provided the geometric mean of fitness at high population densities is greater than one. However, if the population experiences both positive and negative density-dependent feedbacks, conditional persistence only occurs if environmental fluctuations are sufficiently small. We illustrate counter-intuitively that environmental fluctuations can increase the probability of persistence when populations are initially at low densities, and can cause asymptotic extinction of populations experiencing intermediate predation rates despite conditional persistence occurring at higher predation rates.

scholarly journals When higher carrying capacities lead to faster propagation

2018 ◽  
Author(s):  
Marjorie Haond ◽  
Thibaut Morel-Journel ◽  
Eric Lombaert ◽  
Elodie Vercken ◽  
Ludovic Mailleret ◽  
...  
Keyword(s):  
Carrying Capacity ◽  
Invasion Biology ◽  
Reaction Diffusion Equations ◽  
Positive Density ◽  
Allee Effects ◽  
Demographic Stochasticity ◽  
Positive Dependence ◽  
Density Dependent ◽  
Temporal Models ◽  
Density Dependent Dispersal

AbstractThis preprint has been reviewed and recommended by Peer Community In Ecology (https://dx.doi.org/10.24072/pci.ecology.100004). Finding general patterns in the expansion of natural populations is a major challenge in ecology and invasion biology. Classical spatio-temporal models predict that the carrying capacity (K) of the environment should have no influence on the speed (v) of an expanding population. We tested the generality of this statement with reaction-diffusion equations, stochastic individual-based models, and microcosms experiments with Trichogramma chilonis wasps. We investigated the dependence between K and v under different assumptions: null model (Fisher-KPP-like assumptions), strong Allee effects, and positive density-dependent dispersal. These approaches led to similar and complementary results. Strong Allee effects, positive density-dependent dispersal and demographic stochasticity in small populations lead to a positive dependence between K and v. A positive correlation between carrying capacity and propagation speed might be more frequent than previously expected, and be the rule when individuals at the edge of a population range are not able to fully drive the expansion.

scholarly journals Experimental demonstration of an Allee effect in microbial populations

2016 ◽  
Vol 12 (4) ◽  
pp. 20160070 ◽  
Author(s):  
RajReni B. Kaul ◽  
Andrew M. Kramer ◽  
Fred C. Dobbs ◽  
John M. Drake
Keyword(s):  
Allee Effect ◽  
Vibrio Fischeri ◽  
Microbial Populations ◽  
Critical Threshold ◽  
Initial Population ◽  
Positive Density ◽  
Allee Effects ◽  
Low Carbon ◽  
Positive Density Dependence ◽  
The Relationship

Microbial populations can be dispersal limited. However, microorganisms that successfully disperse into physiologically ideal environments are not guaranteed to establish. This observation contradicts the Baas-Becking tenet: ‘Everything is everywhere, but the environment selects’. Allee effects, which manifest in the relationship between initial population density and probability of establishment, could explain this observation. Here, we experimentally demonstrate that small populations of Vibrio fischeri are subject to an intrinsic demographic Allee effect. Populations subjected to predation by the bacterivore Cafeteria roenbergensis display both intrinsic and extrinsic demographic Allee effects. The estimated critical threshold required to escape positive density-dependence is around 5, 20 or 90 cells ml −1 under conditions of high carbon resources, low carbon resources or low carbon resources with predation, respectively. This work builds on the foundations of modern microbial ecology, demonstrating that mechanisms controlling macroorganisms apply to microorganisms, and provides a statistical method to detect Allee effects in data.

scholarly journals A cryptic Allee effect: spatial contexts mask an existing fitness–density relationship

2015 ◽  
Vol 2 (6) ◽  
pp. 150034 ◽  
Author(s):  
Akira Terui ◽  
Yusuke Miyazaki ◽  
Akira Yoshioka ◽  
Shin-ichiro S. Matsuzaki
Keyword(s):  
Allee Effect ◽  
Local Level ◽  
Freshwater Mussel ◽  
Fertilization Rate ◽  
Positive Density ◽  
Allee Effects ◽  
Flow Conditions ◽  
Conspecific Density ◽  
Sperm Density ◽  
Density Relationship

Current theories predict that Allee effects should be widespread in nature, but there is little consistency in empirical findings. We hypothesized that this gap can arise from ignoring spatial contexts (i.e. spatial scale and heterogeneity) that potentially mask an existing fitness–density relationship: a ‘cryptic’ Allee effect. To test this hypothesis, we analysed how spatial contexts interacted with conspecific density to influence the fertilization rate of the freshwater mussel Margaritifera laevis . This sessile organism has a simple fertilization process whereby females filter sperm from the water column; this system enabled us to readily assess the interaction between conspecific density and spatial heterogeneity (e.g. flow conditions) at multiple spatial levels. Our findings were twofold. First, positive density-dependence in fertilization was undetectable at a population scale (approx. less than 50.5 m 2 ), probably reflecting the exponential decay of sperm density with distance from the sperm source. Second, the Allee effect was confirmed at a local level (0.25 m 2 ), but only when certain flow conditions were met (slow current velocity and shallow water depth). These results suggest that spatial contexts can mask existing Allee effects.

Estimation of partial resistance in potato genotypes against Meloidogyne chitwoodi

Nematology ◽  
2011 ◽  
Vol 13 (4) ◽  
pp. 477-489 ◽  
Author(s):  
Thomas Been ◽  
Corrie Schomaker ◽  
Patrick Norshie
Keyword(s):  
Partial Resistance ◽  
Damage Threshold ◽  
Internal Quality ◽  
Yield Reduction ◽  
Initial Population ◽  
Multiplication Rate ◽  
Population Densities ◽  
Meloidogyne Chitwoodi ◽  
Industrial Processing ◽  
Final Population

AbstractThree new potato genotypes, designated AR 04-4107, AR 04-4096 and AR 04-4098, with resistance towards Meloidogyne chitwoodi, and the susceptible cv. Désirée were grown at a range of population densities of M. chitwoodi in a climate-controlled glasshouse in order to establish the presence and degree of partial resistance. Tuber parts of about 12 g were planted at densities (Pi) of 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 and 256 second-stage juveniles (J2) (g dry soil)−1. The plants were allowed to grow for a period of 105 days. Tomato cv. Moneymaker was included and inoculated at Pi = 2 J2 (g soil)−1 to verify the quality of the inoculum by measuring the multiplication rate. Plant height was measured weekly over 11 weeks. At harvest, fresh shoot, root and tuber weights, and number of tubers were measured to express yield. Final population densities (Pf) were calculated as the total number of nematodes found in soil and roots. Tubers were scored for visible symptoms and a root-knot index was calculated. The relation between pre-plant population densities (Pi) and nematode densities at harvest (Pf) was fitted using R. The multiplication rate a of M. chitwoodi on AR 04-4107, AR 04-4096, AR 04-4098 and cv. Désirée was 0.55, 0.27, 0.91 and 32, respectively. Partial resistance rsa of AR 04-4107, AR 04-4096 and AR 04-4098 was 1.7%, 0.8% and 2.8%, respectively. Partial resistance expressed as rsM was 0.2%, 0.2% and 0.1%, respectively. It can be concluded that AR 04-4107, AR 04-4096 and AR 04-4098 are strongly partially resistant to M. chitwoodi. Also, the population dynamics curves run almost parallel between both the tested genotypes and the reference cultivar, indicating that a simple and cheap partial resistance test is feasible. When tuber yields were fitted to the Seinhorst model for yield reduction, cv. Désirée showed a minimum yield (m) of 0.86, while all three resistant genotypes suffered no yields losses at all (m = 1), which indicates that the observed resistance was associated with tolerance. As a result of the remarkably high partial resistance, quality damage was low compared with cv. Désirée. The root-knot index, which takes into account internal quality damage of the potato tuber, was below 10 for all genotypes with partial resistance, the lower damage threshold used for industrial processing of consumption potatoes. Visible symptoms on the tuber skin were absent up to densities of 32 J2 (g soil)−1 for genotypes AR 04-4098 and AR 04-4096 and 2 J2 (g soil)−1 for AR 04-4107, and significantly reduced at higher densities when compared with the susceptible cv. Désirée. However, when tuber peels were investigated, egg masses were detected in tubers at almost all initial population densities.

Population growth and density dependence

2019 ◽  
pp. 63-80
Author(s):  
Gary G. Mittelbach ◽  
Brian J. McGill
Keyword(s):  
Population Growth ◽  
Density Dependence ◽  
Growth Model ◽  
Species Interactions ◽  
Single Species ◽  
Positive Density ◽  
Allee Effects ◽  
Species Population ◽  
Exponential Growth Model ◽  
Single Species Population

This chapter reviews the basic mathematics of population growth as described by the exponential growth model and the logistic growth model. These simple models of population growth provide a foundation for the development of more complex models of species interactions covered in later chapters on predation, competition, and mutualism. The second half of the chapter examines the important topic of density-dependence and its role in population regulation. The preponderance of evidence for negative density-dependence in nature is reviewed, along with examples of positive density dependence (Allee effects). The study of density dependence in single-species populations leads naturally to the concept of community-level regulation, the idea that species richness or the total abundance of individuals in a community may be regulated just like abundance in a single-species population. The chapter concludes with a look at the evidence for community regulation in nature and a discussion of its importance.

Life cycle and damage of the root-knot nematode Meloidogyne minor on potato, Solanum tuberosum

Nematology ◽  
2014 ◽  
Vol 16 (2) ◽  
pp. 185-192 ◽  
Author(s):  
Wim M.L. Wesemael ◽  
Lirette M. Taning ◽  
Nicole Viaene ◽  
Maurice Moens
Keyword(s):  
Life Cycle ◽  
The Netherlands ◽  
Second Generation ◽  
The United States ◽  
Initial Population ◽  
Base Temperature ◽  
Root Knot Nematode ◽  
Population Densities ◽  
Damage Development

Meloidogyne minor is a root-knot nematode reported in Belgium, Ireland, The Netherlands, Portugal, United Kingdom, Chile and the United States. It is found in sport fields and golf courses where it causes the yellow patch disease. However, M. minor has also been detected in potato fields in The Netherlands and the UK and may pose a threat for potato cultivation. Therefore, the life cycle and damage of M. minor on potato cv. Bintje were examined under controlled conditions. To assess its life cycle, young potato plants were inoculated with freshly hatched second-stage juveniles (J2). The developmental stages of M. minor were recorded at weekly intervals after inoculation until second generation J2 were detected. One week after inoculation, only vermiform juveniles were found in the roots. All juveniles were swollen after 3 weeks and the first adult females were observed. Egg masses were seen after 6 weeks together with second generation J2. The number of degree days for M. minor to complete its life cycle was calculated using a base temperature of 5°C (DD5); between 606 and 727 DD5 were needed to complete the life cycle. Damage development of M. minor on potato was examined in a pot experiment with different inoculation densities. Symptoms (galling on the tubers) were similar to those caused by M. chitwoodi and M. fallax. At initial population densities () of 10 J2 (100 cm3 soil)−1 and more, tubers showed galls. Severely damaged potato tubers were observed at (100 cm3 soil)−1 and a damage threshold of 41 J2 (100 cm3 soil)−1 was calculated. An in vitro test showed that five, commonly grown, potato cultivars were good hosts for M. minor. Based on our results, M. minor is able to develop on potato and cause severe damage at low initial population densities. Therefore, further spread of this nematode in agricultural fields should be avoided.

scholarly journals Evidence for positive density-dependent emigration in butterfly metapopulations

Oecologia ◽  
2011 ◽  
Vol 167 (3) ◽  
pp. 657-665 ◽  
Author(s):  
Piotr Nowicki ◽  
Vladimir Vrabec
Keyword(s):  
Positive Density ◽  
Density Dependent
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