Small Founder Population Research Articles

What does the ‘closed herd’ really mean for Australian breeding companies and their customers?

The perception that the genetic background of the Australian pig population is limiting for genetic improvement of commercial pigs in Australia is considered in the context of well established theory combined with practical evidence. The diversity of pig breeds used in modern commercial pig-breeding programs is diminished worldwide relative to all the pig breeds available. Australia is no different in this respect. The use of predominantly three main breeds (Large White, Landrace, Duroc) and synthetic lines, with contributions from other minor breeds to form the basis of a cross-breeding system for commercial pig production is well established internationally. The Australian concern of relatively small founder populations is potentially of relevance, from a theoretical perspective, for (1) the prevalence of defects or the presence of desirable alleles, and (2) the loss of genetic variation or increase in inbreeding depression resulting from increased inbreeding in closed nucleus lines, potentially reducing response to selection. However, rates of response achieved in Australian herds are generally commensurate with the performance recording and selection emphasis applied, and do not appear to be unduly restricted. Moreover, favourable alleles present in unrepresented breeds are frequently present in the three major breeds elsewhere, and therefore would be expected to be present within the Australian populations. Wider testing would provide confirmation of this. Comparison of estimates of effective population size of Australian populations with experimental selection lines overseas (e.g. INRA) or other intensely selected species (e.g. Holstein cattle) suggest adequate genetic diversity to achieve ongoing genetic improvement in the Australian pig industry. However, fitness traits should be included in breeding goals. What remains to be seen is whether novel phenotypes or genotypes are required to meet future challenges, which might be imposed by changes in the environment (e.g. climate change, disease) or market needs. Given probable overlap in genetic merit across Australian and foreign populations for unselected attributes, we suggest that sufficient genetic resources are already present in Australian herds to continue commercial progress within existing Australian populations that have adapted to Australian conditions.

Variation in the Striated Heron Butorides striatus in Australasia

Some populations of Butorides striatus are dimorphic. Rufous birds, which predominate over grey only in coastal mid-western Australia, appear to be morphs in which phaeomelanin replaces eumelanin in greater than usual concentrations. In normally pigmented (grey) forms there is geographical variation in depth of colour and in size. Apart from the usual trend for northern populations to be smaller than those farther south, variation is irregular and of the chequerboard pattern: populations in eastern Australia and the Solomon Islands are large and dark, those from western New Guinea to the Moluccas small and dark and those in north-western Australia, south-eastern New Guinea and the Meervlakte of northern New Guinea small and pale; populations of the second and third group intergrade in the Trans-Fly of southern New Guinea. Thus, it is difficult to trace geographical relations and routes of dispersal in the region; the patchwork variation in colour is thought to have resulted from random fixation in small founding populations. The following subspecies are accepted: stagnatilis in coastal mid- and north-western Australia east to the Gulf of Carpentaria; macrorhynchus in coastal eastern Australia north to Cape York Peninsula; flyensis in coastal south-eastern New Guinea; moluccarum in western New Guinea and adjacent islands west to the Moluccas; idenburgi in the Meervlakte of north-western New Guinea: and solomonensis in the Solomon Islands and New Ireland.

Evolution: Insect invasions and natural selection.

Observations of a real-time invasion of Australia by Asian honeybees demonstrate how natural selection can allow a small founding population to overcome the genetic odds stacked against success.

Genome-Wide Meta-Analysis of Sciatica in Finnish Population.

Sciatica or the sciatic syndrome is a common and often disabling low back disorder in the working-age population. It has a relatively high heritability but poorly understood molecular mechanisms. The Finnish population is a genetic isolate where small founder population and bottleneck events have led to enrichment of certain rare and low frequency variants. We performed here the first genome-wide association (GWAS) and meta-analysis of sciatica. The meta-analysis was conducted across two GWAS covering 291 Finnish sciatica cases and 3671 controls genotyped and imputed at 7.7 million autosomal variants. The most promising loci (p<1x10-6) were replicated in 776 Finnish sciatica patients and 18,489 controls. We identified five intragenic variants, with relatively low frequencies, at two novel loci associated with sciatica at genome-wide significance. These included chr9:14344410:I (rs71321981) at 9p22.3 (NFIB gene; p = 1.30x10-8, MAF = 0.08) and four variants at 15q21.2: rs145901849, rs80035109, rs190200374 and rs117458827 (MYO5A; p = 1.34x10-8, MAF = 0.06; p = 2.32x10-8, MAF = 0.07; p = 3.85x10-8, MAF = 0.06; p = 4.78x10-8, MAF = 0.07, respectively). The most significant association in the meta-analysis, a single base insertion rs71321981 within the regulatory region of the transcription factor NFIB, replicated in an independent Finnish population sample (p = 0.04). Despite identifying 15q21.2 as a promising locus, we were not able to replicate it. It was differentiated; the lead variants within 15q21.2 were more frequent in Finland (6–7%) than in other European populations (1–2%). Imputation accuracies of the three significantly associated variants (chr9:14344410:I, rs190200374, and rs80035109) were validated by genotyping. In summary, our results suggest a novel locus, 9p22.3 (NFIB), which may be involved in susceptibility to sciatica. In addition, another locus, 15q21.2, emerged as a promising one, but failed to replicate.

Diversity and linkage disequilibrium in farmed Tasmanian Atlantic salmon.

Farmed Atlantic salmon (Salmo salar) is a globally important production species, including in Australia where breeding and selection has been in progress since the 1960s. The recent development of SNP genotyping platforms means genome-wide association and genomic prediction can now be implemented to speed genetic gain. As a precursor, this study collected genotypes at 218132 SNPs in 777 fish from a Tasmanian breeding population to assess levels of genetic diversity, the strength of linkage disequilibrium (LD) and imputation accuracy. Genetic diversity in Tasmanian Atlantic salmon was lower than observed within European populations when compared using four diversity metrics. The distribution of allele frequencies also showed a clear difference, with the Tasmanian animals carrying an excess of low minor allele frequency variants. The strength of observed LD was high at short distances (<25kb) and remained above background for marker pairs separated by large chromosomal distances (hundreds of kb), in sharp contrast to the European Atlantic salmon tested. Genotypes were used to evaluate the accuracy of imputation from low density (0.5 to 5K) up to increased density SNP sets (78K). This revealed high imputation accuracies (0.89-0.97), suggesting that the use of low density SNP sets will be a successful approach for genomic prediction in this population. The long-range LD, comparatively low genetic diversity and high imputation accuracy in Tasmanian salmon is consistent with known aspects of their population history, which involved a small founding population and an absence of subsequent introgression. The findings of this study represent an important first step towards the design of methods to apply genomics in this economically important population.

Managing wolves in the Yellowstone area: Balancing goals across jurisdictional boundaries

ABSTRACTGray wolf (Canis lupus) restoration in the Greater Yellowstone Ecosystem began in 1995 with a small founder population in Yellowstone National Park, USA, which increased and contributed to a fully restored population in the northern Rocky Mountains by 2003. Upon removal as a federally listed, threatened species, wolf management outside the park was conveyed to Idaho, Montana, and Wyoming (USA) during 2009–2012; though wolves were relisted in Wyoming in 2014. Subsequent harvests elicited substantial negative reactions from wolf advocates when wolves that lived primarily in the park moved into surrounding states and were lawfully harvested. Conversely, many game managers and hunters advocated for larger harvests of wolves, including those in the park. These divergent viewpoints merit consideration when developing management plans for wolves that move between preserves and areas where hunting is permitted. We describe the history of wolf restoration and hunting in the ecosystem, contrast National Park Service and state management objectives, and characterize the risk to wolves living primarily in the park from hunting in surrounding areas. We recommend a framework for trans‐boundary wolf management that considers population, social structure, and ecosystem objectives on public lands, potential influences of harvests on population growth, depredation risks to livestock, and opportunities for hunters and wildlife watching. We also consider human attitudes and social norms, thereby allowing for differences in values in our prescriptions. This framework has been implemented in a key hunting area along the northern boundary of Yellowstone and is broadly applicable elsewhere for the management of wildlife species held in the public trust. © 2016 The Wildlife Society.

A genetic assessment of the English bulldog

BackgroundThis study examines genetic diversity among 102 registered English Bulldogs used for breeding based on maternal and paternal haplotypes, allele frequencies in 33 highly polymorphic short tandem repeat (STR) loci on 25 chromosomes, STR-linked dog leukocyte antigen (DLA) class I and II haplotypes, and the number and size of genome-wide runs of homozygosity (ROH) determined from high density SNP arrays. The objective was to assess whether the breed retains enough genetic diversity to correct the genotypic and phenotypic abnormalities associated with poor health, to allow for the elimination of deleterious recessive mutations, or to make further phenotypic changes in body structure or coat. An additional 37 English bulldogs presented to the UC Davis Veterinary Clinical Services for health problems were also genetically compared with the 102 registered dogs based on the perception that sickly English bulldogs are products of commercial breeders or puppy-mills and genetically different and inferior.ResultsFour paternal haplotypes, with one occurring in 93 % of dogs, were identified using six Y-short tandem repeat (STR) markers. Three major and two minor matrilines were identified by mitochondrial D-loop sequencing. Heterozygosity was determined from allele frequencies at genomic loci; the average number of alleles per locus was 6.45, with only 2.7 accounting for a majority of the diversity. However, observed and expected heterozygosity values were nearly identical, indicating that the population as a whole was in Hardy-Weinberg equilibrium (HWE). However, internal relatedness (IR) and adjusted IR (IRVD) values demonstrated that a number of individuals were the offspring of parents that were either more inbred or outbred than the population as a whole. The diversity of DLA class I and II haplotypes was low, with only 11 identified DLA class I and nine class II haplotypes. Forty one percent of the breed shared a single DLA class I and 62 % a single class II haplotype. Nineteen percent of the dogs were homozygous for the dominant DLA class I haplotype and 42 % for the dominant DLA class II haplotype. The extensive loss of genetic diversity is most likely the result of a small founder population and artificial genetic bottlenecks occurring in the past. The prominent phenotypic changes characteristic of the breed have also resulted in numerous large runs of homozygosity (ROH) throughout the genome compared to Standard Poodles, which were phenotypically more similar to indigenous-type dogs.ConclusionsEnglish bulldogs have very low genetic diversity resulting from a small founder population and artificial genetic bottlenecks. Although some phenotypic and genotypic diversity still exists within the breed, whether it is sufficient to use reverse selection to improve health, select against simple recessive deleterious traits, and/or to accommodate further genotypic/phenotypic manipulations without further decreasing existing genetic diversity is questionable.

The minimum founding population in dispersing organisms subject to strong Allee effects

Summary Dispersal can push population density below strong Allee thresholds ensuring the demise of small founding populations. As a result, for isolated populations of dispersing organisms, the minimum founding population size that enables establishment can be quite different from the Allee threshold. We derive an expression for the minimum founding population size for a general integrodifference equation model of a dispersing population with a strong Allee effect. We demonstrate the utility of the general model by using it to approximate minimum founding populations for the mountain pine beetle. The minimum founding population of the general model increases linearly with the mean squared displacement of the dispersing organism. Transient dynamics of the general model suggest that population density at the point of introduction will often decrease before increasing, even when the minimum founding population size is exceeded. Calculation of minimum founding population sizes may prove useful for predicting invasion success based on propagule size and for maximizing the success of species reintroductions when strong Allee effects exist.

The History of Elk (&lt;i&gt;Cervus canadensis&lt;/i&gt;) Restoration in Ontario

Elk (Cervus canadensis) historically inhabited southern Quebec and central Ontario, but, by the early 1900s, the species was extirpated from this region. Attempts to re-establish an Elk population in Ontario during the first half of the 20th century had limited success. We reviewed historical documents, population census records, and a previous study pertaining to Elk reintroduced to Ontario in the early 1900s for clues to the cause(s) of their limited population growth. After an apparent rapid population increase in the 1940s followed by unregulated hunting during the subsequent 3 decades, Elk abundance in Ontario had not appreciably changed from 1970 to 1997, most likely because of the small founding population, unsustainable hunting, and accidental mortality. After the abolition of legal hunting in 1980, natural mortality appeared to be the main limiting factor. A limited sample of pregnancy and calf recruitment rates, body measurements, and physical condition parameters collected in 1993–1997, suggested that adults were healthy, reproducing successfully, and not limited by food availability; thus, it was concluded that remnant Elk populations could be augmented by introducing additional animals. A renewed Elk restoration effort, conducted from 1998 to 2001, imported 443 Elk from Elk Island national Park in Alberta to 4 release areas across central Ontario (Lake of the Woods, Lake Huron North Shore, Nipissing/French River, and Bancroft/North Hastings), resulting in a provincial population of about 800 Elk by 2013.

A SURVEY OF DISEASES IN CAPTIVE RED WOLVES (CANIS RUFUS), 1997-2012.

Conservation efforts to preserve the red wolf (Canis rufus) have been in progress since the 1970s through the U.S. Fish and Wildlife Service Red Wolf Recovery Program and the Association of Zoos and Aquarium's Red Wolf Species Survival Plan. An ongoing part of this project has been to monitor mortality trends, particularly to look for potential genetic conditions resulting from inbreeding given the small founding population of only 14 individuals. An initial survey was conducted in the 1990s but a comprehensive assessment of the population has not been done since then. This retrospective review evaluates mortality in the population from 1997 to 2012 through analysis of gross necropsy and histology records provided by cooperating institutions that housed red wolves during the time period of interest. Of the 378 red wolves that died during this 15-yr period, 259 animals had gross necropsy records, histology records, or both that were evaluated. The major causes of neonatal death were parental trauma, stillbirth, or pneumonia. Overall, juveniles had very low mortality rates with only 12 wolves aged 30 days to 6 mo dying during the study period. The most common cause of death within the adult populations was neoplasia, with epithelial neoplasms, carcinomas, and adenocarcinomas being the most common types reported. Gastrointestinal disease was the second most common cause of death, particularly gastric dilation and volvulus, inflammatory bowel disease, and gastrointestinal perforations. These findings are in stark contrast to causes of mortality in the wild population, which are primarily due to human-related activities such as vehicular trauma, gunshot, or poisoning. Overall, the captive population has few health problems, but an increase in inflammatory bowel disease in particular warrants further investigation.

The evolutionary origin and population history of the grauer gorilla.

Gorillas living in western central Africa (Gorilla gorilla) are morphologically and genetically distinguishable from those living in eastern central Africa (Gorilla beringei). Genomic analyses show eastern gorillas experienced a significant reduction in population size during the Pleistocene subsequent to geographical isolation from their western counterparts. However, how these results relate more specifically to the recent biogeographical and evolutionary history of eastern gorillas remains poorly understood. Here we show that two rare morphological traits are present in the hands and feet of both eastern gorilla subspecies at strikingly high frequencies (>60% in G. b. graueri; ∼28% in G. b. beringei) in comparison with western gorillas (<1%). The intrageneric distribution of these rare traits suggests that they became common among eastern gorillas after diverging from their western relatives during the early to middle Pleistocene. The extremely high frequencies observed among grauer gorillas-which currently occupy a geographic range more than ten times the size of that of mountain gorillas-imply that grauers originated relatively recently from a small founding population of eastern gorillas. Current paleoenvironmental, geological, and biogeographical evidence supports the hypothesis that a small group of eastern gorillas likely dispersed westward from the Virungas into present-day grauer range in the highlands just north of Lake Kivu, either immediately before or directly after the Younger Dryas interval. We propose that as the lowland forests of central Africa expanded rapidly during the early Holocene, they became connected with the expanding highland forests along the Albertine Rift and enabled the descendants of this small group to widely disperse. The descendant populations significantly expanded their geographic range and population numbers relative to the gorillas of the Virunga Mountains and the Bwindi-Impenetrable Forest, ultimately resulting in the grauer gorilla subspecies recognized today. This founder-effect hypothesis offers some optimism for modern conservation efforts to save critically endangered eastern gorillas from extinction.

Saving wild ungulate diversity through enhanced management and sperm cryopreservation.

Wild ungulates throughout the world face the impending risk of extinction. Small founding population size, lack of interest in exhibiting wild ungulates and declining space in zoos are not sustaining ex situ populations. Animals managed in ex situ collections continue to experience >20% neonate loss globally. To ensure population sustainability there is a critical need to: (1) manage ungulates in large herds, increasing mate choice and reproductive efficiency; (2) improve husbandry and genetic management; and (3) develop consistent assisted reproductive technologies, including sperm cryopreservation and AI. Recently, new models in the management of ungulates have begun to emerge. Animal managers and researchers are also beginning to exploit advances in genomics to improve genetic management of their collections. Furthermore, the past decade has witnessed significant advances particularly in semen collection and cryopreservation in numerous species. Advances in gonadal tissue cryopreservation now offer additional opportunities to preserve male genomes. The new knowledge generated is enabling the creation of genetic (sperm) banks to rescue and enhance reproductive management of wild ungulates. The present paper reviews the threats to ungulate populations, the status and relevance of animal management and biomaterial banking efforts to ensure long-term survival of these charismatic species.

Population genetics of the invasive giant hogweed (Heracleum sp.) in a northern European region

Heracleum mantegazzianum is a successful invader in many European countries. Besides its impact on biodiversity, it represents a health hazard because of its phototoxic sap. Population genetic studies may help to predict which species are most likely to become invasive. In this study, we investigated the population genetic variation and structure of Heracleum sp., identified as H. mantegazzianum in Finland, i.e., within its northern range of distribution, using microsatellite markers. Our aims were to reveal the genetic characteristics and evolutionary processes that have resulted in observed patterns. The amount of genetic variation measured as expected heterozygosity was modest (mean H E = 0.406), which may be due to relatively small founder populations and/or inbreeding (mean F IS = 0.268). Mean allele numbers per population ranged from 2.0 to 4.7. The pairwise F ST values were generally high, and the mean F ST among all populations equalled 0.498. Based on F ST values, populations were significantly differentiated from each other, indicating restricted gene flow among the populations. The Bayesian structure analysis provided additional information on population genetic structure. The analysis divided the eight populations into two main clusters. Genetic analyses indicate that H. mantegazzianum has been introduced to Finland multiple times. Distinct population genetic patterns indicate that human-mediated dispersal is important.

Phylogenetic analysis of the endangered takin in the confluent zone of the Qinling and Minshan Mountains using mtDNA control region

The takin (Budorcas taxicolor) is an Endangered ungulate. We analyzed the variation within mtDNA control region sequences of takin populations in the Qinling Mountains, the Minshan Mountains and the confluence of these two mountain ranges. We did not find any shared haplotypes among the populations. We observed apparent variation in the control region length among the three populations, and independent population expansions in the late of Pleistocene, which suggests these populations may have independent evolutionary histories. We found only one haplotype, and the lowest measures of genetic diversity (h = 0; π = 0) in the population from the confluent zone, which suggests populations in the confluent zone may have grown from small founder populations and gene flow with other populations has ceased. Based on their phylogenetic relationships, we concluded that the takin population in the confluent zone was in the same clade as the Tangjiahe population, which suggests that these takin populations are Sichuan takin (Budorcas taxicolor tibetana).

Local Virus Extinctions following a Host Population Bottleneck.

A small number of African green monkeys (AGMs) were introduced into the Caribbean from West Africa in the 1600s. To determine the impact of this population bottleneck on the AGM virome, we used metagenomics to compare the viral nucleic acids in the plasma of 43 wild AGMs from West Africa (Gambia) to those in 44 AGMs from the Caribbean (St. Kitts and Nevis). Three viruses were detected in the blood of Gambian primates: simian immunodeficiency virus (SIVagm; in 42% of animals), a novel simian pegivirus (SPgVagm; in 7% of animals), and numerous novel simian anelloviruses (in 100% of animals). Only anelloviruses were detected in the Caribbean AGMs with a prevalence and levels of viral genetic diversity similar to those in the Gambian animals. A host population bottleneck therefore resulted in the exclusion of adult-acquired SIV and pegivirus from the Caribbean AGMs. The successful importation of AGM anelloviruses into the Caribbean may be the result of their early transmission to infants, very high prevalence in African AGMs, and frequent coinfections with as many as 11 distinct variants. The extent to which viruses can persist in small isolated populations depends on multiple host, viral, and environmental factors. The absence of prior infections may put an immunologically naive population at risk for disease outbreaks. Isolated populations originating from a small number of founder individuals are therefore considered at increased risk following contact with populations with a greater variety of viruses. Here, we compared the plasma virome of West African green monkeys to that in their descendants after importation of a small number of animals to the Caribbean. A lentivirus and a pegivirus were found in the West African population but not in the Caribbean population. Highly diverse anelloviruses were found in both populations. A small founder population, limited to infants and young juvenile monkeys, may have eliminated the sexually transmitted viruses from the Caribbean AGMs, while anelloviruses, acquired at an earlier age, persisted through the host population bottleneck.

Convergence of generalized urn models to non-equilibrium attractors

Generalized Polya urn models have been used to model the establishment dynamics of a small founding population consisting of k different genotypes or strategies. As population sizes get large, these population processes are well-approximated by a mean limit ordinary differential equation whose state space is the k simplex. We prove that if this mean limit ODE has an attractor at which the temporal averages of the population growth rate is positive, then there is a positive probability of the population not going extinct (i.e. growing without bound) and its distribution converging to the attractor. Conversely, when the temporal averages of the population growth rate are negative along this attractor, the population distribution does not converge to the attractor. For the stochastic analog of the replicator equations which can exhibit non-equilibrium dynamics, we show that verifying the conditions for convergence and non-convergence reduces to a simple algebraic problem. We also apply these results to selection–mutation dynamics to illustrate convergence to periodic solutions of these population genetics models with positive probability.

Transposable elements as agents of rapid adaptation may explain the genetic paradox of invasive species.

Rapid adaptation of invasive species to novel habitats has puzzled evolutionary biologists for decades, especially as this often occurs in the face of limited genetic variability. Although some ecological traits common to invasive species have been identified, little is known about the possible genomic/genetic mechanisms that may underlie their success. A common scenario in many introductions is that small founder population sizes will often lead to reduced genetic diversity, but that invading populations experience large environmental perturbations, such as changes in habitat and environmental stress. Although sudden and intense stress is usually considered in a negative context, these perturbations may actually facilitate rapid adaptation by affecting genome structure, organization and function via interactions with transposable elements (TEs), especially in populations with low genetic diversity. Stress-induced changes in TE activity can alter gene action and can promote structural variation that may facilitate the rapid adaptation observed in new environments. We focus here on the adaptive potential of TEs in relation to invasive species and highlight their role as powerful mutational forces that can rapidly create genetic diversity. We hypothesize that activity of transposable elements can explain rapid adaptation despite low genetic variation (the genetic paradox of invasive species), and provide a framework under which this hypothesis can be tested using recently developed and emerging genomic technologies.

Numbat nirvana: conservation ecology of the endangered numbat (Myrmecobius fasciatus) (Marsupialia : Myrmecobiidae) reintroduced to Scotia and Yookamurra Sanctuaries, Australia

Despite a vigorous reintroduction program between 1985 and 2010, numbat populations in Western Australia are either static or declining. This study aimed to document the population ecology of numbats at two sites that are going against this trend: Scotia Sanctuary in far western New South Wales and Yookamurra Sanctuary in the riverland of South Australia. Scotia (64 659 ha) and Yookamurra (5026 ha) are conservation reserves owned and managed by the Australian Wildlife Conservancy and where numbats were reintroduced in 1999 and 1993 respectively. Both sites have large conservation-fence-protected introduced-species-free areas where there are no cats (Felis catus) or red foxes (Vulpes vulpes). Numbats were sourced from both wild and captive populations. From small founder populations, the Scotia numbats are now estimated to number 169 (113–225) with 44 at Yookamurra. Radio-collared individuals at Scotia were active between 13 and 31°C. Females had home ranges of 28.3 ± 6.8 ha and males 96.6 ± 18.2 ha, which leads to an estimated sustainable population or carrying capacity of 413–502 at Scotia. Captive-bred animals from Perth Zoo had a high mortality rate upon reintroduction at Scotia due to predation by raptors and starvation. The habitat preferences for mallee with a shrub understorey appear to be driven by availability of termites, and other reintroduced ecosystem engineers appear to have been facilitators by creating new refuge burrows for numbats. This study shows that numbats can be successfully reintroduced into areas of their former range if protected from introduced predators, and illustrates the difficulties in monitoring such cryptic species.

Overcoming Allee effects through evolutionary, genetic, and demographic rescue

Despite the amplified threats of extinction facing small founder populations, successful colonization sometimes occurs, bringing devastating ecological and economic consequences. One explanation may be rapid evolution, which can increase mean fitness in populations declining towards extinction, permitting persistence and subsequent expansion. Such evolutionary rescue may be particularly important, given Allee effects. When a population is introduced at low density, individuals often experience a reduction in one or more components of fitness due to novel selection pressures that arise from diminished intraspecific interactions and positive density dependence (i.e. component Allee effects). A population can avoid extinction if it can adapt and recover on its own (i.e. evolutionary rescue), or if additional immigration sustains the population (i.e. demographic rescue) or boosts its genetic variation that facilitates adaptation (i.e. genetic rescue). These various forms of rescue have often been invoked as possible mechanisms for specific invasions, but their relative importance to invasion is not generally understood. Within a spatially explicit modelling framework, we consider the relative impact of each type of rescue on the probability of successful colonization, when there is evolution of a multi-locus quantitative trait that influences the strength of component Allee effects. We demonstrate that when Allee effects are important, the effect of demographic rescue via recurrent immigration overall provides the greatest opportunity for success. While highlighting the role of evolution in the invasion process, we underscore the importance of the ecological context influencing the persistence of small founder populations.

Allee effects and population regulation: a test for biotic resistance against an invasive leafroller by resident parasitoids

AbstractResident natural enemies can impact invasive species by causing Allee effects, leading to a reduction in establishment success of small founder populations, or by regulating or merely suppressing the abundance of established populations. Epiphyas postvittana, the Light Brown Apple Moth, an invasive leafroller in California, has been found to be attacked by a large assemblage of resident parasitoids that cause relatively high rates of parasitism. Over a 4‐year period, we measured the abundance and per capita growth rates of four E. postvittana populations in California and determined parasitism rates. We found that at two of the sites, parasitism caused a component Allee effect, a reduction in individual survivorship at lower E. postvittana population densities, although it did not translate into a demographic Allee effect, an impact on per capita population growth rates at low densities. Instead, E. postvittana populations at all four sites exhibited strong compensatory density feedback throughout the entire range of densities observed at each site. As we found no evidence for a negative relationship between per capita population growth rates and parasitism rates, we concluded that resident parasitoids were unable to regulate E. postvittana populations in California. Despite a lack of evidence for regulation or a demographic Allee effect, the impact of resident parasitoids on E. postvittana populations is substantial and demonstrates significant biotic resistance against this new invader.