Persistence Of Local Populations Research Articles

Microsatellite DNA analysis of coastal populations of bull trout (Salvelinusconfluentus) in British Columbia: zoogeographic implications and its application to recreational fishery management

Microsatellite DNA variation was assayed among 383 bull trout (Salvelinus confluentus) from 20 Pacific coastal localities from the Skeena River (central British Columbia) to the Olympic Peninsula (western Washington State). An average of 1.7 alleles was resolved per population and heterozygosity averaged 0.35. Twenty-six fish were identified as bull trout × Dolly Varden (Salvelinus malma) hybrids. Population subdivision was substantial (θ = 0.33), but subdivision was higher (θ = 0.46) when interior populations (N = 37) were included, indicating a major genetic distinction between "coastal" and "interior" bull trout. Bull trout populations north of the Squamish River were genetically more similar to interior bull trout than to other more southern coastal populations, suggesting that they had been founded by headwater transfers from interior populations. Individual assignment of bull trout averaged 53.4% correct assignment among populations (range: 12%–95%). Mixture analysis indicated that most fish from the lower Fraser River recreational fishery originated from major nearby tributaries (e.g., Pitt, upper Lillooet, and Chilliwack rivers). Our results substantiate the existence of two major evolutionary lineages of bull trout and highlight the importance of tributary habitats for the persistence of local populations, as well as for those that forage in downstream areas on the lower Fraser River.

Sex, death and tragedy

The population consequences of sexual conflict are relatively unexplored. In a recent paper, Le Galliard et al. now show that males of the common lizard Lacerta vivipara cause such damage to females that male-biased populations decrease in size, posing a real risk to the persistence of local lizard populations. Their study reveals surprising parallels between sexual conflict and the tragedy of the commons, where selfish competition over females destroys the very resource (i.e. the females) over which the males are fighting.

Evaluating hypotheses about dispersal in a vulnerable butterfly

AbstractSound management of species requires reliable estimates of dispersal. Indeed, dispersal of individuals among local populations is a key factor in the biology and persistence of local populations and metapopulations. Here, the small‐scale dispersal pattern of a vulnerable species, the endemic Sardinian chalk hill blue butterfly, was studied by applying capture–recapture multistate models and a model selection based on AIC values. Model parameters were survival, capture and movement probabilities. The model selection showed that (a) survival probability of individuals varied between sexes, (b) capture probability varied between sexes and among patches, and (c) movement probability varied with direction. The probability of movement among adjacent local populations was generally low and ranged from 0.009 to 0.212. Movement probabilities were subsequently modeled using data on interpatch distance and donor patch population size or area. The ultrastructural “biology‐based” models turned out to be the most appropriate models for inference, showing that dispersal decreases with increasing interpatch distance and increasing donor patch population size or area, and suggesting that butterfly dispersal is affected by patch geometry and the presence of conspecifics. The application of multistate models, the model selection approach, and ultrastructural modeling allowed testing the validity of some general hypotheses related to dispersal in metapopulations and helped elucidate the butterfly small‐scale dispersal pattern.

Relationships between local population persistence, local abundance and regional occupancy of species: distribution patterns of diatoms in boreal streams

AbstractAims We have two aims: (1) to examine the relationship between local population persistence, local abundance and regional occupancy of stream diatoms and (2) to characterize the form of the species–occupancy frequency distribution of stream diatoms.Location Boreal streams in Finland. There were three spatial extents: (1) across ecoregions in Finland, (2) within ecoregions in Finland, and (3) within a single drainage system in southern Finland.Methods Diatoms were sampled from stones (epilithon), sediment (epipelon) and aquatic plants (epiphyton) in streams using standardized sampling methods. To assess population persistence, diatom sampling was conducted monthly at four stream sites from June to October. The relationships between local population persistence, local abundance and regional occupancy were examined using correlation analyses.Results There was a significant positive relationship between local persistence and abundance of diatoms in epilithon, epipelon and epiphyton. Furthermore, local abundance and regional occupancy showed a significant positive relationship at multiple spatial extents; that is, across ecoregions, within ecoregions and within a drainage system. The relationships between occupancy and abundance did not differ appreciably among impacted and near pristine‐reference sites. The occupancy–frequency distribution was characterized by a large number of satellite species which occurred at only a few sites, whereas core species that occurred at most sites were virtually absent.Main conclusions The positive relationship between local population persistence and abundance suggested that a high local abundance may prevent local extinction or that high persistence is facilitated by a high local cell density. High local persistence and local abundance may also positively affect the degree of regional occupancy in stream diatoms. The results further showed that anthropogenic effects were probably too weak to bias the relationship between occupancy and abundance, or that the effects have already modified the distribution patterns of stream diatoms. The small number of core species in the species–occupancy frequency distribution suggested that the regional distribution patterns of stream diatoms, or perhaps unicellular microbial organisms in general, may not be fundamentally different from those described previously for multicellular organisms, mainly in terrestrial environments, although average global range sizes may differ sharply between these two broad groups of organisms.

Widening the window of persistence in seasonal pathogen–host systems

Local instability of exploiter–victim systems is well-known in both theory and in nature. Victims can be too sparse to support exploiter reproduction (under-exploitation) or they can be too readily driven to extinction (over-exploitation). Exploiters of seasonal resources face the additional challenge of surviving periods when victims are rare or unavailable. We formulate a fully stochastic model of highly seasonal pathogen–host dynamics and explore the interactions between an entomopathogenic nematode and its lepidopteran host. Our model suggests that if nematode populations experience the high rates of mortality predicted by short-term laboratory experiments, the paired threats of under- and over-exploitation should preclude the long-term persistence of this exploiter–victim system. We measured nematode mortality rates in the field and found that long-term mortality is lower than that predicted by short-term experiments. Incorporation of this new data into our model produces long-term persistence of local nematode populations across a range of initial nematode densities.

A model for range expansion of coastal algal species with different dispersal strategies: the case of Fucus serratus in northern Spain

A model was used to investigate the nature of the distributional boundary of the brown alga Fucus serratus in northern Spain. The model explored the colonisation of a shore by invaders with contrasting dispersal modes: an exponentially bounded mode, as an example of short-range dis- persal; an extreme, long-range dispersal mode; and a mixed mode with variable proportions of short- and long-distance dispersers. The organisms were dioecious, had a limited life-span and reproduced only when plants of different sexes were within a short distance. For species with exponentially bounded dispersal and interannual variability in the environment, the model reproduced the basic features of the distribution of F. serratus: survival and reproduction beyond the distributional bound- ary, sharp boundary and displacements of the boundary through time. Species with exclusively long- range dispersal exhibited continually accelerating rates of spread under favourable conditions. How- ever, long-range dispersers might fail to invade or become extinct when exposed to a short series of unfavourable years. Regional abundance is critical for the persistence of local populations. For long- range dispersers, no distinct distributional boundary was recognised. At the opposite extreme, invaders with exponentially bounded dispersal had small and decelerating rates of spread, high resistance to disturbances and distinct distributional boundaries; the persistence of populations mainly relies upon local abundance. Mixed strategies, with most of the propagules having exponen- tially bounded dispersal and a small fraction having long-range dispersal, appear to be the superior strategy; they combine rapid colonisation rates and high resistance to disturbances. The combination of Allee effects and propagule pressure (i.e. number of propagules arriving at a given locality) explains the differences among dispersal strategies.

A model for behavioral regulation of metapopulation dynamics

Habitat fragmentation can decrease local population persistence by reducing connectivity, which is a function of dispersal of individuals among habitat fragments. Dispersal is often treated as diffusion in population models, even though for many species it is a result of a series of behavioral decisions. We developed a metapopulation model to explore the potential importance of dispersal behaviors in driving metapopulation dynamics. We incorporated types of behavior that affect dispersal—colonization inhibiting, colonization enhancing, extinction inhibiting, extinction enhancing, rescue enhancing, rescue inhibiting—into Levins’ (1969) metapopulation model and projected occupancy rates for a variety of parameter values. Examples from the literature of behaviors associated with each of these parameters are provided. Our model simplifies into previously published metapopulation models that incorporate only a single behavior, and we present a density-dependent rescue function that leads to multiple non-zero equilibria. We found a variety of behavioral effects on metapopulations. Rescue enhancement fills patches faster than does colonization enhancement or extinction inhibition, and declines in patch occupancy are moderate with extinction enhancement, but colonization inhibition causes metapopulation extinction. We also found that with colonization and extinction inhibitions, equilibrium patch occupancy is inversely related to patch turnover rate. With density-dependent rescue, persistence depends not only on the strength of the strong rescue effect, but also on having a sufficient initial fraction of patches occupied; the stronger the rescue effect, the lower this fraction can be. This study suggests that dispersal behavior can have strong influences on metapopulation dynamics. It confirms the importance of understanding the relationship between landscape structure and dispersal behavior in understanding population persistence.

Gap dynamics in perennial subalpine grasslands: trends and processes change during secondary succession

Summary 1 Revegetation of gaps may affect the floristic composition of subalpine grasslands during secondary succession. Six hypotheses concerning overall effects, gap-size and edge effects, and changes in these during secondary succession, are tested using principal response curves (PRCs), a recent derivative of partial redundancy analysis. 2 In a field experiment, large (625 cm 2 ) and small (39 cm 2 ) gaps and controls were established in a replicated successional series (0, 10 and 40 years after abandonment). Colonization modes were quantified in year 1, and the floristic composition was monitored twice yearly over four growing seasons. 3 Micro-successions in gaps account for 8.8–12.2% of the compositional variance. Size effects add 3.3–5.5% per successional stage, while edge effects appear only in the mid-successional stage (adding 1.6%). Seed colonization rates are comparable with those observed in temperate systems, and the effects on community composition are considerable (initial gap response, size effect). In contrast, changes in local competitive regimes have weaker effects (revegetation dynamics after the first year, edge effects). This may be attributed to the slow growth rates of alpine plants and the short growing season in subalpine climates, as well as to the infertile soils of the semi-natural grasslands. 4 A majority (74%) of the species are affected by gaps, but their responses are not consistent across successional stages. Species that recruit into gaps primarily as seedlings, and locally rare species, become relatively more dependent on gaps for local population persistence during succession. 5 These experiments give insight into the effects of gap revegetation processes for community composition and population persistence in these perennial grasslands. During secondary succession, changes in disturbance regime affect the gap revegetation processes and the probability for local population persistence for gap-enhanced species.

Local population persistence as a pre-condition for large-scale dispersal of Idotea metallica (Crustacea, Isopoda) on drifting habitat patches

Idotea metallica establishes self-sustaining populations exclusively on objects drifting at the sea surface. Large-scale transport of drift material with surface currents results in an efficient dispersal of the species. Two types of drifting objects are utilised (biotic and abiotic), providing quite different conditions of life. Ephemeral biotic substrata (mainly uprooted macroalgae) may be used for transport and food, however, resulting habitat destruction from feeding must be a major threat for local population persistence of I. metallica. Abiotic substrata or wood represent efficient vectors for long-distance dispersal due to their resistance to biodegradation, but do not provide food for this herbivorous species. In laboratory experiments, the spatially-limited conditions of drifting substrata were simulated in microcosms. Idotea metallica established persistent populations on both types of substrata. On abiotic substrata, however, where the animals were fed only on Artemia larvae, high variations in density and a reduced intrinsic rate of population growth increased the risk of population extinction. Idotea metallica avoids habitat destruction by limited feeding on macroalgae. In contrast, the coastally distributed congener Idotea baltica destroyed algal habitats by feeding about 10 times faster than I. metallica.

Dispersal and metapopulation dynamics of an oceanic seabird, the wandering albatross, and its consequences for its response to long‐line fisheries

Summary We measured for the first time in a pelagic seabird the dispersal rate of wandering albatrosses and show that while adults are highly philopatric to breeding sites the dispersal rate of young birds is not negligible, even at large distances. We show that the wandering albatross has metapopulation dynamics, i.e. migration events are sufficient to influence the dynamics of populations at other islands. The spatial dynamics of wandering albatross has important consequences when analysing and making projections of the effects that incidental by‐catch related to long‐line fishery activities at different spatial scales can have on the persistence of local populations of wandering albatross.

Regional-scale demography of Ramonda myconi: Remnant population dynamics in a preglacial relict species

Remnant population dynamics permit many plant species to persist timespans extending from decades up to several millenia. The regional-scalepersistence of these plant species strictly depends on the persistence of localpopulations within the region. This type of dynamics can explain the existenceof preglacial relict species in the Mediterranean today. We studied thepopulation dynamics of the long-lived iteroparous herb Ramondamyconi, a preglacial relict species with a fragmented distributioninMediterranean mountains, to evaluate the regional-scale persistence of thespecies. Demographic data were collected from 5 populations placed at LaCerdanya Pyrenean region for up to 6 years. The main life-history features ofthis species are the great longevity of adult plants and the high mortality ofseedlings. Matrix population models were used to investigate its demography.Overall, the population growth rate (λ) ranged from a low of 0.79 to ahigh of 1.06. However, λ did not differ significantly from theequilibrium point, as indicated by their confidence intervals, except for onepopulation in one year. Despite the small between-year variation in λ,variation in climatic conditions at La Cerdanya from year to year explained animportant part of such variation. Elasticity analyses were performed toevaluatethe relative importance of demographic parameters for population growth. Stasistransitions made the greatest contribution to λ. Finally, the long-termdynamics of R. myconi populations were analysed byincorporating environmental stochasticity into the models. Projectionsindicatedthat local R. myconi populations tend to decline over timebut with a long time to extinction, so the persistence ofR. myconi all over La Cerdanya is determined by thepersistence of its remnant local populations.

The Effects of Dispersal Patterns on Marine Reserves: Does the Tail Wag the Dog?

The concept of marine reserves as a method of improving management of fisheries is gaining momentum. While the list of benefits from reserves is frequently promoted, precise formulations of theory to support reserve design are not fully developed. To determine the size of reserves and the distances between reserves an understanding of the requirements for persistence of local populations is required. Unfortunately, conditions for persistence are poorly characterized, as are the larval dispersal patterns on which persistence depends. With the current paucity of information regarding meroplanktonic larval transport processes, understanding the robustness of theoretical results to larval dispersal is of key importance. From this formulation a broad range of dispersal patterns are analyzed. Larval dispersal is represented by a probability distribution that defines the fraction of successful settlers from an arbitrary location, the origin of the distribution, to any other location along the coast. While the effects of specific dispersal patterns have been investigated for invasion processes, critical habitat size and persistence issues have generally been addressed with only one or two dispersal types. To that end, we formulate models based on integrodifference equations that are spatially continuous and temporally discrete. We consider a range of dispersal distributions from leptokurtic to platykurtic. The effect of different dispersal patterns is considered for a single isolated reserve of varying size receiving no external larvae, as well as multiple reserves with varying degrees of connectivity. While different patterns result in quantitative differences in persistence, qualitatively similar effects across all patterns are seen in both single- and multiple reserve models. Persistence in an isolated reserve requires a size that is approximately twice the mean dispersal distance and regardless of the dispersal pattern the population in a patch is not persistent if the reserve size is reduced to just the mean dispersal distance. With an idealized coastline structure consisting of an infinite line of equally spaced reserves separated by regions of coastline in which reproduction is nil, the relative settlement as a function of the fraction of coastline and size of reserve is qualitatively very similar over a broad range of dispersal patterns. The upper limit for the minimum fraction of coastline held in reserve is about 40%. As the fraction of coastline is reduced, the minimum size of reserve becomes no more than 1.25 times the mean dispersal distance.

Tree seed dispersal among forest fragments: II. Dispersal abilities and biogeographical controls

AimTo investigate the medium to long‐distance dispersal abilities of temperate hardwood tree species and the ecological controls on dispersal, including distance to and connectivity with seed sources, seed source strength, and species dispersal mechanism and seed size.LocationA fragmented forest system in the Long Point region, Southern Ontario.MethodsPine plantations were the `seed traps' in which seedlings of hardwood species were enumerated to indicate past dispersal events. The influence of distance to seed sources, dispersal mechanism and seed mass on the probability of dispersal to plantations were evaluated using logistic analysis. Regression analysis was used to determine the effect of seed source strength (mature tree abundance within 150 m of plantations) on seedling density in plantations. Connectivity was assessed by comparing the strength of correlations between seedling abundance in plantations and the abundance of mature trees around plantations in connected vs. unconnected source areas.ResultsSeedling presence in plantations decreased significantly with distance from the nearest potential seed source for species grouped according to dispersal mechanism. Probabilities of seedling presence were ≥0.8 at 25 m distances, decreasing to under 0.3 at 175 m distances. While twenty‐seven of twenty‐nine species were present in at least one plantation with a seed source within 25 m, only thirteen of twenty‐seven species occurred in plantations with a seed source ≥100 m away, and only nine of twenty‐four species in plantations ≥150 m from seed sources, indicating limited potentials for interfragment migration. Seed source strength was significantly related to seedling density in plantations for twelve of fifteen species tested, indicating the importance of species commonness to interfragment migration. Connectivity was not related to dispersal frequency in the system, but this finding applies to a relatively well‐connected system of forests and plantations and is expected to differ for systems with greater patch isolation. In addition to these general controls, dispersal was related to species dispersal mechanism. Seed size was negatively related to dispersal frequency, but only within the rodent dispersal spectra. Bird‐dispersed species appeared to have superior interfragment dispersal abilities, closely followed by lighter seeded rodent‐ and wind‐dispersed species. Large‐seeded rodent‐dispersed species (Juglans spp., Quercus macrocarpa) and species lacking well‐developed adaptations for dispersers were infrequent in plantations generally, and with the exception of Q. macrocarpa, were absent from plantations more than 50 m from seed sources. Species dispersal abilities were ranked according to dispersal mechanism and seed size such that: bird > lighter‐seeded rodent = wind > larger‐seeded rodent = unspecialized.Main conclusionsDistance to seed sources appears to be a key determinant of patch colonization. Infrequent dispersal over distances of >100–150 m for most species in this system raises concerns about the abilities of tree species to be sustained in fragmented forests. For some species, distances of as little as 50 m appear to be isolating, and these, in particular, risk regional extinctions over time scales depending on their local population persistence. Artificial introductions may be needed to maintain fragmented tree populations, particularly for rare species and those with poor dispersal indicated in this study.

A selective advantage to immigrant genes in a Daphnia metapopulation.

Immigrants to habitats occupied by conspecific organisms are usually expected to be competitively inferior, because residents may be locally adapted. If residents are inbred, however, mating between immigrants and residents results in offspring that may enjoy a fitness advantage from hybrid vigor. We demonstrate this effect experimentally in a natural Daphnia metapopulation in which genetic bottlenecks and local inbreeding are common. We estimate that in this metapopulation, hybrid vigor amplifies the rate of gene flow several times more than would be predicted from the nominal migration rate. This can affect the persistence of local populations and the entire metapopulation.

A demographic analysis of a southern snowshoe hare population in a fragmented habitat: evaluating the refugium model

The allegedly noncyclic dynamics of southern snowshoe hare (Lepus americanus) populations may be explained by a model invoking habitat fragmentation and facultative predation (the refugium model) under which animals dispersing from patches of preferred habitat fail to establish themselves because of predation by facultative carnivores. We compared the refugium model with a revised model invoking heavy on-site predation in preferred habitat as the proximal mechanism responsible for the stability of southern snowshoe hare populations. The survival and movements of hares in a fragmented habitat in central Idaho were monitored via radiotelemetry on 6 sites differing in habitat quality (indexed by understory cover) from 1998 to 2000. In support of the revised model, predation rates were high irrespective of cover availability or hare density, and predators did not kill dispersing animals disproportionately. Furthermore, predation was focused on small hares, suggesting that poor recruitment of juveniles may be the mechanism ultimately responsible for the damped dynamics of southern snowshoe hare populations. The low survival rates we measured suggest that the population under study was undergoing a marked decline. However, the observed decline, determined by comparing study-site population estimates, was less severe, implying that the persistence of local snowshoe hare populations in some areas of the species' southern range may be influenced by metapopulation dynamics. Specifically, southern snowshoe hare populations in small patches of usable habitat may be prevented from going extinct by the arrival of immigrants from similar nearby patches.

The spatial dynamics of White-browed Babbler groups in a fragmented agricultural landscape

White-browed Babbler Pomatostomus superciliosus groups occupying linear strips of vegetation had breeding territories that were smaller in area and had longer linear dimensions than those occupying patches. A group's non-breeding home range was larger than its breeding territory. Groups occupying linear/patch home ranges expanded the linear extent and area of their home ranges more than those within other home range configurations. Some groups moved during the non-breeding season and this was more likely to occur if the group occupied a remnant with a low abundance of invertebrates during summer. Some groups that moved returned prior to the next breeding season, but the majority were never seen again. New groups moved into the study sites and established in vacant home ranges. This suggests that those groups that left the study sites may have established new home ranges elsewhere. Breeding site fidelity was lower in groups that had failed in previous breeding attempts. Therefore, group movements were influenced by the feeding and breeding quality of the habitat. However, the configuration of the local population also influenced group movements with those groups on the edge of a local population being more likely to move than those in the interior. New groups were formed by two processes; group dispersal, where groups generally filled a vacant home range, and group budding, which involved the splitting of a large group. Group dispersal maintained group densities while group budding increased the density of groups in a local population. These two processes were common, producing localized fluctuations in the density of groups. Since babbler groups contain only one breeding pair, changes in group density represent changes in effective population size. Therefore, group dynamics may be important to the persistence of local populations of White-browed Babblers, especially in landscapes that have suffered from habitat loss and fragmentation.

Habitat requirements and local persistence of three damselfly species (odonata: coenagrionidae)

Habitat requirements and population persistence were investigated in three damselfly species, all coastal plain pond specialists: Enallagma recurvatum, E. laterale, and E. pictum. Because of geographic restriction, two are of special concern to conservation, E. recurvatum and E. laterale. We surveyed more than 70 ponds on Cape Cod, Massachusetts, and collected adult presence-absence data during the summers of 1999 and 2000. We achieved a detection rate approaching 100% for each species by visiting each pond up to three times. We looked for relationships between the presence of each damselfly species and presence of specific aquatic vegetation, the presence of the other Enallagma species, and the number of ponds within various distances of the 72 surveyed ponds. Using stepwise logistic regression, we found the following significant associations: E. recurvatum with the rush Juncus militaris; E. laterale with water lilies (Nuphar variegatum and Brasenia schreberi) the damselfly E. pictum, and the number of ponds within 2 km; and E. pictum with the water lily Nymphaea odorata, the damselfly E. laterale, and the number of ponds within 1.5 km and 2.5 km. Presence-absence data were used to calculate turnover and local extinction rates for each species between the two years. E. recurvatum's turnover and local extinction rates (33.3% and 41% respectively) were much higher than either E. laterale (9.8%, 11.5%) or E. pictum (7.7%, 10%). These results suggest that E. recurvatum occurs in a metapopulation, and that patch colonization rates might be important to local population persistence.

Metapopulation processes and persistence in remnant water vole populations

We examined the spatial distribution of water vole populations in four consecutive years and investigated whether the regional population processes of extinction, recolonisation and migration influence distribution and persistence. We examined how such regional processes are influenced by spatial variation in habitat quality. In addition, we assessed the relevance of metapopulation concepts for understanding the dynamics of species that deviate from classical metapopulation assumptions and developing conservation measures for them. Populations were patchy and discrete, and the patchy distribution was not static between years. Population turnover occurred even in the absence of predatory mink, which only influenced the network of populations at the end of the study. Most populations were clustered close together in the upper tributaries. Local population persistence was predominantly influenced by population size: large populations were more persistent. Recolonisation rates were influenced by isolation and habitat quality. The isolation estimates which best explained the distribution of water vole populations incorporated straight‐line distances, suggesting water voles disperse overland. The distribution of recolonised sites indicated that dispersing voles actively selected habitat on the basis of its quality. Water voles depart from some of the assumptions made by frequently used metapopulation models. In particular there is no clear binary distinction between suitable and non‐suitable habitat. Accounting for variation in habitat quality before investigating temporal changes in population distribution allowed us to demonstrate that the key metapopulation processes were important. The significance of regional population processes relative to local population processes may have increased in declining, fragmented populations compared to pristine regional populations. We hypothesise that although mink predation is likely to eventually cause regional extinction in many areas, metapopulation processes have delayed this decline. Consequently, conservation measures should take into account mink predation rates and regional population processes, before considering aspects of habitat quality.

Spatio-temporal variation of reproductive success and conservation of the narrow-endemic Centaurea corymbosa (Asteraceae)

The viability of a metapopulation of the rare, cliff-dwelling Centaurea corymbosa depends critically on the persistence of extant local populations, as it has a very low colonising ability. The variation in components of seed production in space and time was investigated and compared with the variation of supposedly causal plant and population traits. Temporal variation in the number of capitula per plant and in the number of ovules per capitulum were related to resource limitation between years and flowering periods, respectively. The seed-to-ovule ratio also varied widely, because of variation in fertilization rate, and, to a lesser extent, in abortion rate; predation being negligible. Isolated individuals had lower fertilization rates than individuals within dense patches. But despite a high density, the smallest studied population showed the lowest fertilization rates and highest abortion rates. This Allee effect on seed production represents a possible threat for small and/or low-density populations. Reinforcing populations of low density and supplying the smallest populations with new genotypes, by seed deposition into clefts of rocks, are suggested as possible conservation management strategies for C. corymbosa.

Dynamics of Intermittent Stream Habitat Regulate Persistence of a Threatened Fish at Multiple Scales

Traditionally, threatened species management has emphasized conservation of individual populations and has assumed that abundance within suitable habitats is primarily governed by local environmental factors. However, recent research has revealed that landscape-level processes such as disturbance, dispersal, and habitat patch mosaic structure may also strongly influence local populations. We studied the population and habitat dynamics of a threatened fish, the Arkansas darter (Etheostoma cragini) at four spatial scales (pool, reach, segment, watershed) in two intermittent Colorado plains streams. At each scale, information on hydrology, habitat, and fish populations was combined to evaluate factors influencing darter persistence. At the pool scale, Arkansas darters persisted in most permanent pools during summer drought, tolerated extremes of hyperthermia and hypoxia, and were extirpated only when pools dried. Deeper pools had a higher probability of persisting during summer drought. At the reach scale, survival measured during an intensive mark–recapture study was high in a stenothermal spring pool, but low in associated downstream pools that froze during winter and had harsh, variable thermal regimes during summer. However, juvenile darters hatched earlier, grew faster, and were relatively more abundant in the warmer downstream pools compared to juveniles in the spring pool, where older fish dominated. Movement rates between the two habitats were relatively low, but adult darters were more mobile, dispersing out of the spring pool in early spring and returning upstream before the onset of summer drought. At the segment scale, flow variation among seasons and years controlled habitat connectivity, thereby affecting dispersal and reproduction and shaping the distribution and age composition of darter populations. At the watershed scale, nonnative northern pike (Esox lucius) preyed on darters, reducing their distribution and abundance in 13 km of Big Sandy Creek and isolating upstream darter populations from those downstream. Large floods scoured new spring-pool refugia and filled other pools, altering the mosaic of physical habitats. A comprehensive picture of factors controlling Arkansas darter populations and their habitat emerged only when physical and biotic processes were integrated across all four scales. To be effective, recovery efforts for species inhabiting variable environments must consider multiple scales, from landscape-level processes that create and maintain refugia to fine-scale factors that regulate the abundance and persistence of local populations.