Demographic Sinks Research Articles

Demographic response of osprey within the lower Chesapeake Bay to fluctuations in menhaden stock

Forage fish support the largest fisheries in the world and play a vital role in marine food webs by transferring energy from plankton to consumers within higher trophic levels. Growing commercial demand for these species and concern for the impact of over harvest on predator populations has driven a paradigm shift in management objectives from maximizing economic return to the establishment of ecosystem-based limits on harvest rates. How well current harvest policy supports noncommercial species like piscivorous birds remains poorly understood. We investigated the relationship between osprey breeding performance within the lower Chesapeake Bay and a menhaden stock index during a period (1974-2021) when the menhaden index fluctuated over 35-fold. Reproductive rate (young/pair/year), brood provisioning (fish/10-h) and the proportion of menhaden in the diet all declined during the study period. Indicators of food stress including brood reduction and nest failure increased during the study period. The population transitioned from reproductive surplus (demographic source) to reproductive deficit (demographic sink). A significant relationship between reproductive rate and the menhaden index suggests that osprey population viability requires that the menhaden stock be restored to 1980s levels. Current ecological reference points based on the food requirements of predatory fish are unlikely to protect the osprey population. We suggest the establishment of menhaden or reproductive thresholds designed to allow osprey to meet demographic targets (1.15 young/pair/year).

Trophic interactions between native newts and introduced mosquitofish suggest invaded ponds may act as demographic sinks

Trophic interactions between native newts and introduced mosquitofish suggest invaded ponds may act as demographic sinks

A Bocage Landscape Restricts the Gene Flow of Pest Vole Populations.

The population dynamics of most animal species inhabiting agro-ecosystems may be determined by landscape characteristics, with agricultural intensification and the reduction of natural habitats influencing dispersal and hence limiting gene flow. Increasing landscape complexity would thus benefit many endangered species by providing different ecological niches, but it could also lead to undesired effects in species that can act as crop pests and disease reservoirs. We tested the hypothesis that a highly variegated landscape influences patterns of genetic structure in agricultural pest voles. Ten populations of fossorial water vole, Arvicola scherman, located in a bocage landscape in Atlantic NW Spain were studied using DNA microsatellite markers and a graph-based model. The results showed a strong isolation-by-distance pattern with a significant genetic correlation at smaller geographic scales, while genetic differentiation at larger geographic scales indicated a hierarchical pattern of up to eight genetic clusters. A metapopulation-type structure was observed, immersed in a landscape with a low proportion of suitable habitats. Matrix scale rather than matrix heterogeneity per se may have an important effect upon gene flow, acting as a demographic sink. The identification of sub-populations, considered to be independent management units, allows the establishment of feasible population control efforts in this area. These insights support the use of agro-ecological tools aimed at recreating enclosed field systems when planning integrated managements for controlling patch-dependent species such as grassland voles.

Roads, forest cover, and topography as factors affecting the occurrence of large carnivores: The case of the Andean bear (Tremarctos ornatus)

Global declines in large-bodied terrestrial vertebrates are widely linked to human disturbance and habitat loss. Identifying remaining opportunities to conserve areas wherein such species may persist or might be reintroduced has thus become a key challenge in biodiversity conservation globally. We used multi-year camera trap data and spatial capture-recapture models to test how individually-identified Andean bears (Tremarctos ornatus) responded to roads and a suite of forest cover and topographic variables that may be indicative of food resources in critically-threatened, equatorial dry forest habitats of northern Peru. Bear density averaged 4 individuals per 100 km2 in our 668 km2 study area, but proximity to roads significantly reduced density, possibly because they are avoided and/or facilitate demographic sinks or human hunting. In contrast, the effects of forest cover and topographic variables on bear density varied seasonally, reflecting documented variation in mating behavior that may result from seasonal food resources. Despite large uncertainties in model predictions on the occurrence of bear habitat outside our study area, extrapolating our results suggests that a substantial area of high-quality habitat for Andean bears exists outside the recognized IUCN range, including equatorial dry forest habitats known to harbor many threatened species. Because Andean bears are vulnerable to extinction and are an established flagship species in much of South America, the expanded range and potential habitats that we identify here require urgent validation for possible inclusion in landscape-level plans to conserve equatorial dry forests of the Andes.

Native biodiversity collapse in the eastern Mediterranean.

Global warming causes the poleward shift of the trailing edges of marine ectotherm species distributions. In the semi-enclosed Mediterranean Sea, continental masses and oceanographic barriers do not allow natural connectivity with thermophilic species pools: as trailing edges retreat, a net diversity loss occurs. We quantify this loss on the Israeli shelf, among the warmest areas in the Mediterranean, by comparing current native molluscan richness with the historical one obtained from surficial death assemblages. We recorded only 12% and 5% of historically present native species on shallow subtidal soft and hard substrates, respectively. This is the largest climate-driven regional-scale diversity loss in the oceans documented to date. By contrast, assemblages in the intertidal, more tolerant to climatic extremes, and in the cooler mesophotic zone show approximately 50% of the historical native richness. Importantly, approximately 60% of the recorded shallow subtidal native species do not reach reproductive size, making the shallow shelf a demographic sink. We predict that, as climate warms, this native biodiversity collapse will intensify and expand geographically, counteracted only by Indo-Pacific species entering from the Suez Canal. These assemblages, shaped by climate warming and biological invasions, give rise to a ‘novel ecosystem’ whose restoration to historical baselines is not achievable.

Sex, senescence, sources and sinks

Abstract Why do most organisms age, and why do most of them reproduce sexually? Does sex rejuvenate? We review progress that has been made linking theories of senescence with those of sexual reproduction. We show that there is a dearth of theory against the numerous questions waiting to be answered theoretically or empirically: observed senescence patterns are a result of past selection acting on individuals of different age categories and abundances, modes of reproduction (asexual, sexual, facultatively sexual, via buds or zygotes). Modular organisms present their own challenges. Assigning offspring an unambiguous age of zero at birth is often too simplistic. We also comment on germline mutations as a form of ageing over generations (Lansing effect) and ask whether there is value in reinvigorating an old metaphor (1988, Sex and death in the protozoa. New York, NY, USA: Cambridge University Press) of chairs and their chairmakers who both are subject to deterioration over time, since endogenous repair is never foolproof. Future theory could usefully revisit a known mathematical analogy between selection on senescence across age classes and source–sink theory. Some modes of reproduction (particularly asexuality) may yield offspring that are, in a sense, already aged, with the problem increasing over generations (ultimately leading to a demographic sink). Insofar as sexuality ‘rejuvenates’, it does so through a gamble that, with some frequency, produces ‘exogenously repaired’ individuals that act as a source of genes to future generations. This gamble has been argued to be managed best when life cycles include a unicellular stage (zygote), an argument that could be usefully complemented with an analysis of the relevant economic trade‐offs between offspring size and number. A free Plain Language Summary can be found within the Supporting Information of this article.

Migration, Mortality and Medicalisation: investigating the long-run epidemiological consequences of urbanisation 1600 - 1945

Today life expectancy is generally higher in urban than rural populations, but early modern towns and cities were demographic sinks with extraordinarily high mortality, especially among the young and migrants who were essential for city growth. The breakthrough to urban natural increase that occurred in the late eighteenth century and the eradication of the urban penalty in the 1930s mark the endpoints of major epidemiological transitions. We seek to investigate how and when cities transformed from urban graveyards into promoters of health between 1600 and 1945. We hypothesize that the processes of endemicisation and exogenous disease variation were key to the evolution of both urban and non-urban mortality regimes, especially with respect to: infectious disease among the young, maternal health, adult migrants and their health/immunological status. This project brings together evidence from institutions (including medical and welfare), registrations of deaths and cause-of-death statistics to provide a continuous overview of the long-run pattern of urban life chances in Britain. The experiences of other European and North American populations will also be employed to elucidate the extent to which towns were drivers of epidemiological change during the first demographic transition, and the differing roles of health and welfare provision, industrialisation and migration in these processes.

Persistence, Extinction, and Recolonization of an Epibenthic Gastropod Population on an Intertidal Sandflat: 35-Y Contingent History of a Key Species of the Benthic Community in Metapopulation and Metacommunity Contexts

For marine benthos with planktonic larval stages, demographic metapopulation and empirical metacommunity studies are scarce. Asink nature was shown for a local intertidal sandflat population of the gastropod Umbonium moniliferumin an estuarine to coastal oceanic region in Amakusa, Kyushu, Japan. The gastropod has a 3-day planktonic larval duration (PLD). It inhabited 11 sandflats, which were shared by the ghost shrimp Nihonotrypaea harmandi. The shrimp is a burrow-dwelling bioturbator with a 30-day PLD. On one sandflat (hereafter U0) in Amakusa, the density and distribution of the two species was monitored from 1979 to 2014. All their main local populations were surveyed in 1998. In July 1979, the gastropod and shrimp were distributed on the lower and upper zones of U0 separately, with mean densities of 1,740 and 170 ind/m2. The gastropod population declined to extinction in 1986, associated with the increase in shrimp density to 440 ind/m2 and distribution to the lowest shore. Shrimp sediment destabilization was most likely to inhibit gastropod recruitment. The gastropod population began to recover in 1997 owing to shrimp population shrink. The regional survey results showed that (1) eight local populations (U0–U7 sandflats) of the two species along a 25-km shoreline of Amakusa, presumably, constitute each substantial metapopulation, and (2) their largest populations were at the shoreline opposite ends. These features would be generated by the interaction of speciesspecific larval dispersal traits and shoreline physiography. The threshold shrimp density of 160 ind/m2 precluding gastropod recruitment was detected from U1–U7. In 2001 and 2009, gastropod population density on U0 reached respective peaks comparable to that in 1979. This population fluctuation was due to varying recruit abundance, which corresponded well with the year-to-year change in shrimp population density across that threshold. The recolonization of the U0 gastropod population ought to be made by allochthonous larvae from the U1–U7 populations, especially U6–U7, suggesting a rescue effect in the source—sink relationship. The degree of self-persistency of the U0 gastropod population was estimated by calculating net reproductive rate (R0) based on the parameters about pre-settlement larval return-home and survival rates and post-settlement juvenile and adult survival rates. The availability of data set restricted the estimation to potential R0 for four year-segments, with each 1-y parameter set extrapolated to the gastropod 4-y life span. On U0 in 1979 to 1981, despite with minimum shrimp influences, the gastropod population could still be a demographic sink probably due to a limited larval return-home rate and due to a low post-settlement survival rate of juveniles caused by epibenthic predation or storm for their first 1 y to reach sexual maturity.

Parasite sources and sinks in a patched Ross–Macdonald malaria model with human and mosquito movement: Implications for control

We consider the dynamics of a mosquito-transmitted pathogen in a multi-patch Ross–Macdonald malaria model with mobile human hosts, mobile vectors, and a heterogeneous environment. We show the existence of a globally stable steady state, and a threshold that determines whether a pathogen is either absent from all patches, or endemic and present at some level in all patches. Each patch is characterized by a local basic reproduction number, whose value predicts whether the disease is cleared or not when the patch is isolated: patches are known as “demographic sinks” if they have a local basic reproduction number less than one, and hence would clear the disease if isolated; patches with a basic reproduction number above one would sustain endemic infection in isolation, and become “demographic sources” of parasites when connected to other patches. Sources are also considered focal areas of transmission for the larger landscape, as they export excess parasites to other areas and can sustain parasite populations. We show how to determine the various basic reproduction numbers from steady state estimates in the patched network and knowledge of additional model parameters, hereby identifying parasite sources in the process. This is useful in the context of control of the infection on natural landscapes, because a commonly suggested strategy is to target focal areas, in order to make their corresponding basic reproduction numbers less than one, effectively turning them into sinks. We show that this is indeed a successful control strategy—albeit a conservative and possibly expensive one—in case either the human host, or the vector does not move. However, we also show that when both humans and vectors move, this strategy may fail, depending on the specific movement patterns exhibited by hosts and vectors.

Annual and seasonal patterns in wildlife road-kill and their relationship with traffic density

ABSTRACTHedgehogs (Erinaceous europaeus), rabbits (Oryctolagus cuniculus) and possums (Trichosurus vulpecula) were 93.4% of 1390 mammal carcasses counted during 92 trips along a 63.2 km route, 2009–2014, through peri-urban and farmland habitats. Pukeko (Porphyrio melanotus) and swamp harriers (Circus approximans) constituted 29% of the 466 bird road-kill. Comparisons indicate the recovery of hedgehog and rabbit populations from declines to 2005 and 2009, respectively, and the increased suppression of possum populations, consistent with disease and pest control trajectories. Road-kill density was spatially and temporally aggregated. Seasonal peaks reflected species-specific periodicity in breeding and dispersal. Carcass densities varied positively with traffic volume and did not asymptote with increasing traffic up to 25,000 vehicles per day. Kill rates per vehicle were maintained at the highest traffic volumes. Busier roads were not avoided by wildlife and could be substantial demographic sinks and barriers to their spread. We discuss the implications for wildlife management and conservation.

Demographic response of a neotropical migrant songbird to forest management and climate change scenarios

Demographic models for species sensitive to human activities that are still relatively common are of particular interest to compare the relative influence of human land use and climate on population trends. Yet, data limitations often restrict our ability to interpret the numerical response of species to habitat alteration and climatic change adds to this challenge. In this study, we used habitat-specific demographic information from an individually-marked population of Ovenbird (Seiurus aurocapilla) and a forest timber supply model to project population trends over an 80-year horizon. We modelled changes in Ovenbird abundance, productivity, and population growth rate as a function of harvesting scenarios (no harvest, forestry-as-usual, and increased [10% or 20%] harvesting intensity) and projected impacts of climate change (0%, 10%, and 50% reductions in population size over the 80-year period), as well as contrasting assumptions about population dynamics (i.e. open vs. closed population). Among the many effects of climate change, it has been hypothesized that reductions in annual snow cover will occur, causing deeper and more frequent frost penetration into the soil and, in turn, a reduction in invertebrate (food) abundance during the following breeding season. Our models suggest that the study area currently is a demographic sink (λ=0.920) for Ovenbirds, although some habitat types still act as demographic sources. Over the first 7years, a large decline in abundance of territorial males (∼25%) is projected, unless population levels are maintained through immigration. Interestingly, when we allowed for immigration from outside the study area, population growth rate remained <1 because a larger proportion of the population occupied habitat types acting as sinks. Over an 80-year period, the climate change scenarios we simulated were more likely to have negative impacts (5–49%) than forestry activities, whether we applied the current management plan or more intensive harvesting scenarios. To our knowledge, this study used some of the most detailed habitat-specific demographic information available for a North American forest songbird to model the relative influence of land use, climate, and population dynamics on population trends. Future studies should examine the possibility of synergistic effects between harvesting and climate change, to model their influence on Ovenbird or other species foraging on litter invertebrates.

Bobcats (Lynx rufus) as a Model Organism to Investigate the Effects of Roads on Wide-Ranging Carnivores.

We are using bobcats (Lynx rufus) as a model organism to examine how roads affect the abundance, distribution, and genetic structure of a wide-ranging carnivore. First, we compared the distribution of bobcat-vehicle collisions to road density and then estimated collision probabilities for specific landscapes using a moving window with road-specific traffic volume. Next, we obtained incidental observations of bobcats from the public, camera-trap detections, and locations of bobcats equipped with GPS collars to examine habitat selection. These data were used to generate a cost-surface map to investigate potential barrier effects of roads. Finally, we have begun an examination of genetic structure of bobcat populations in relation to major road networks. Distribution of vehicle-killed bobcats was correlated with road density, especially state and interstate highways. Collision models suggested that some regions may function as demographic sinks. Simulated movements in the context of the cost-surface map indicated that some major roads may be barriers. These patterns were supported by the genetic structure of bobcats. The sharpest divisions among genetically distinct demes occurred along natural barriers (mountains and large lakes) and in road-dense regions. In conclusion, our study has demonstrated the utility of using bobcats as a model organism to understand the variety of threats that roads pose to a wide-ranging species. Bobcats may also be useful as one of a group of focal species while developing approaches to maintain existing connectivity or mitigate the negative effects of roads.

Distribution, abundance and conservation of the highly endemic Coral Pink Sand Dunes tiger beetle, Cicindela albissima Rumpp

The Coral Pink Sand Dunes tiger beetle, Cicindela albissima Rumpp, occurs only within a small area of the Coral Pink Sand Dunes in southwestern Utah. Even within this dune field, most of the population is limited to a small core area about 2 × 0.5 km2 in size (adjacent areas sometimes contain beetles, but these appear to be demographic sinks). The core habitat is a transition zone of transverse dunes located between more dynamic dunes to the south (crests of these move upwards of 3 m/y) and more stabilised ones to the north. A candidate species since 1994, the beetle was recently proposed for listing as a threatened species by the US Fish and Wildlife Service. A conservation agreement in 1997 established two off-highway-vehicle-free conservation areas, one of these supporting most of the population within the core habitat (A) and the other a potential site for establishing a second viable population (B). Annual population estimates of adults from 1999 to 2013 fluctuated significantly from a low of 558 to high of 2944. Despite a progressive increase in recent years, our studies including two population viability analyses confirmed the species remains at risk of extinction because of its localised distribution, fluctuating abundance apparently limited by reduced rainfall and the failure to establish a second population in Conservation Area B. The fate of the species is tied to the five inter-dunal swale rows that support most of the adults and larvae. Consequently, there are few options for conservation of the species. Proposed listing of C. albissima as a threatened species was recently withdrawn by the US Fish and Wildlife Service in lieu of a revised conservation agreement which modestly expands protection in Conservation Area A.

Are Species’ Range Limits Simply Niche Limits Writ Large? A Review of Transplant Experiments beyond the Range

Many species' range limits (RL) occur across continuous environmental gradients without obvious barriers imposing them. Such RL are expected to reflect niche limits (NL) and thus to occur where populations cease to be self-sustaining. Transplant experiments comparing fitness within and beyond species' ranges can test this hypothesis, but interpretive power depends strongly on experimental design. We first identify often overlooked aspects of transplant design that are critical to establishing the causes of RL, especially incorporating transplant sites at, and source populations from, the range edge. We then conduct a meta-analysis of published beyond-range transplant experiments (n = 11 tests). Most tests (75%) found that performance declined beyond the range, with the strongest declines detected when the measure of performance was lifetime fitness (83%), suggesting that RL commonly involve niche constraints (declining habitat quality). However, only 46% supported range limits occurring at NL; 26% (mostly geographic RL) fell short of NL with self-sustaining transplants beyond the range, and 23% (all elevational RL) exceeded NL with range-edge populations acting as demographic sinks. These data suggest an important but divergent role for dispersal, which may commonly constrain geographic distributions while extending elevational limits. Meta-analysis results also supported the importance of biotic interactions at RL, particularly the long-held assertion of their role in causing low-elevation and equatorial limits.

Development of genetic diversity, differentiation and structure over 500 years in four ponderosa pine populations

Population history plays an important role in shaping contemporary levels of genetic variation and geographic structure. This is especially true in small, isolated range-margin populations, where effects of inbreeding, genetic drift and gene flow may be more pronounced than in large continuous populations. Effects of landscape fragmentation and isolation distance may have implications for persistence of range-margin populations if they are demographic sinks. We studied four small, disjunct populations of ponderosa pine over a 500-year period. We coupled demographic data obtained through dendroecological methods with microsatellite data to discern how and when contemporary levels of allelic diversity, among and within-population levels of differentiation, and geographic structure, arose. Alleles accumulated rapidly following initial colonization, demonstrating proportionally high levels of gene flow into the populations. At population sizes of approximately 100 individuals, allele accumulation saturated. Levels of genetic differentiation among populations (F(ST) and Jost's D(est)) and diversity within populations (F(IS)) remained stable through time. There was no evidence of geographic genetic structure at any time in the populations' history. Proportionally, high gene flow in the early stages of population growth resulted in rapid accumulation of alleles and quickly created relatively homogenous genetic patterns among populations. Our study demonstrates that contemporary levels of genetic diversity were formed quickly and early in population development. How contemporary genetic diversity accumulates over time is a key facet of understanding population growth and development. This is especially relevant given the extent and speed at which species ranges are predicted to shift in the coming century.

A hybrid zone between Bathymodiolus mussel lineages from eastern Pacific hydrothermal vents

BackgroundThe inhabitants of deep-sea hydrothermal vents occupy ephemeral island-like habitats distributed sporadically along tectonic spreading-centers, back-arc basins, and volcanically active seamounts. The majority of vent taxa undergo a pelagic larval phase, and thus varying degrees of geographical subdivision, ranging from no impedance of dispersal to complete isolation, often exist among taxa that span common geomorphological boundaries. Two lineages of Bathymodiolus mussels segregate on either side of the Easter Microplate, a boundary that separates the East Pacific Rise from spreading centers connected to the Pacific-Antarctic Ridge.ResultsA recent sample from the northwest flank of the Easter Microplate contained an admixture of northern and southern mitochondrial haplotypes and corresponding alleles at five nuclear gene loci. Genotypic frequencies in this sample did not fit random mating expectation. Significant heterozygote deficiencies at nuclear loci and gametic disequilibria between loci suggested that this transitional region might be a ‘Tension Zone’ maintained by immigration of parental types and possibly hybrid unfitness. An analysis of recombination history in the nuclear genes suggests a prolonged history of parapatric contact between the two mussel lineages. We hereby elevate the southern lineage to species status as Bathymodiolus antarcticus n. sp. and restrict the use of Bathymodiolus thermophilus to the northern lineage.ConclusionsBecause B. thermophilus s.s. exhibits no evidence for subdivision or isolation-by-distance across its 4000 km range along the EPR axis and Galápagos Rift, partial isolation of B. antarcticus n. sp. requires explanation. The time needed to produce the observed degree of mitochondrial differentiation is consistent with the age of the Easter Microplate (2.5 to 5.3 million years). The complex geomorphology of the Easter Microplate region forces strong cross-axis currents that might disrupt self-recruitment of mussels by removing planktotrophic larvae from the ridge axis. Furthermore, frequent local extinction events in this tectonically dynamic region might produce a demographic sink rather than a source for dispersing mussel larvae. Historical changes in tectonic rates and current patterns appear to permit intermittent contact and introgression between the two species.

Managing ecological traps: Logging and sapsucker nest predation by bears

AbstractWe tested the equal preference ecological trap hypothesis for breeding yellow‐bellied sapsuckers (Sphyrapicus varius) along a time‐since‐harvest gradient (1–5 yr, 16–20 yr, 21–25 yr, and &gt;60 yr) in selection system‐logged hardwood forests in Algonquin Provincial Park, Ontario. Yellow‐bellied sapsuckers preferred 1–5 year and &gt;60‐year‐old cuts equally and more than 16–20 year and 21–25‐year‐old cuts. More‐abundant arthropod food and/or higher‐quality sap resources may have attracted yellow‐bellied sapsuckers to 1–5 year and &gt;60‐year‐old cuts. Only 52% of pairs raised fledglings in 1‐ to 5‐year‐old cuts during years when nest predation by American black bears (Ursus americanus) was common, the incidence of which was negatively related to increased availability of American beech (Fagus grandifolia) nuts from the previous autumn. By contrast, 88% of pairs raised fledglings in all years in &gt;60‐year‐old cuts. One‐ to 5‐year‐old cuts were demographic sinks that represent equal‐preference ecological traps in years when nest predation by bears was common, whereas &gt;60‐year‐old cuts were always demographic sources. High‐quality habitat cues for nesting yellow‐bellied sapsuckers appear to be retained for 1–5 years after selection system logging but fail to deliver safe nest sites. Cavities excavated in heart‐rot‐infected nest trees are least likely to be depredated because cavity walls are typically harder and deter entry by depredating bears. Retaining more potential nest trees per ha at harvest (especially American beech with heart‐rot) may increase the proportion of sapsucker nests that are excavated in bear‐resistant trees, thereby reducing nest predation and increasing fecundity. © 2012 The Wildlife Society.

Evidence of source–sink metapopulations in a vulnerable native galaxiid fish driven by introduced trout

Introduced predators with patchy distributions can create demographic sinks within native prey populations. Such invasions may give rise to source-sink metapopulations if there are still sources of native species colonists in the landscape. In New Zealand, introduced brown trout (Salmo trutta) and rainbow trout (Oncorhychus mykiss) are linked with declines in native non-diadromous galaxiids but co-occur with these galaxiids in some locations. We investigated whether trout create sinks in Galaxias vulgaris populations, and whether trout-free reaches could act as sources, allowing persistence in the sink habitat. We conducted quantitative seasonal monitoring of G. vulgaris population structure across two subcatchments of the Waimakariri River, South Island. Two trout-free and seven trout-invaded sites in the Porter River catchment and two trout-free and five trout-invaded sites in the Broken River catchment were monitored over two winters and the adjoining summer. Spatially continuous monitoring of young-of-the-year (YOY) galaxiid distributions and apparent survival across the Broken River catchment was also undertaken. Galaxias vulgaris YOY recruitment was high in trout-free reaches, indicating positive population growth. Galaxias vulgaris was absent from three trout-invaded sites, and the remaining invaded sites had significantly depleted juvenile recruitment. Information-theoretic model selection indicated that trout, rather than habitat, drove recruitment failure. Trout-invaded sites could be divided into "sinks" that retained no YOY galaxiids, indicating no local recruitment, and "pseudosinks," which had very few recruits. Absence of small G. vulgaris at sink sites suggested population maintenance through immigration of adults from sources, whereas pseudosink sites appear capable of self-recruitment at low carrying capacities. Trout-free reaches appear to act as sources in a river network but are susceptible to future invasions by trout. Thus, not only may invasive species cause source-sink metapopulations in native species, but also the potential of refugia for natives (sources) to become future sinks highlights the vulnerability of these metapopulations when invasive predators are the principal demographic driver.

Using occupancy estimates to fine-tune conservation concerns

Using occupancy estimates to fine-tune conservation concerns

The niche, limits to species' distributions, and spatiotemporal variation in demography across the elevation ranges of two monkeyflowers.

Understanding the processes that create and maintain species' geographic range limits has implications for many questions in ecology, evolution, and conservation biology. Many expectations for the ecological and evolutionary dynamics of populations at the range margin rest on the concordance of geographic limits and the limits of a species' ecological niche. If range limits are coincident with niche limits, then marginal populations should have lower and/or more variable vital rates and population growth rates than central populations. Using data from 8 annual censuses of marked individuals, I investigated the demography of Mimulus cardinalis and Mimulus lewisii across the species' elevation ranges. Central and marginal populations exhibited striking demographic differences, but only for one species were differences in expected directions. Marginal populations from the M. lewisii lower elevation range limit had lower and more variable survival than central populations and appeared to be demographic sinks. In contrast, marginal populations from the M. cardinalis upper elevation limit had higher fecundity and higher population growth rates than central populations. Although the species differed with respect to central-marginal patterns, they were concordant with respect to elevation; that is, both species had higher fitness in higher reaches of their examined ranges. Potential explanations for these patterns include source-sink dynamics, with asymmetrical gene flow mediated by river currents, and climate change, with recent warming shifting the species' climatic envelopes to higher elevations. Hence, assessment of spatiotemporal variation in both demography and dispersal is necessary to fully understand the relationship between the niche and species' distributions.