Population persistence in the face of advection

Many populations live in ‘advective’ media, such as rivers, where flow is biased in one direction. In these environments, populations face the possibility of extinction by being washed out of the system, even if the net reproductive rate (R) is greater than one. We propose a formal condition for population persistence in advective systems: a population can persist at any location in a homogeneous habitat if and only if it can invade upstream. This leads to a remarkably simple recipe for calculating the minimal value for the net reproductive rate for population persistence. We apply this criterion to discrete-time models of a semelparous population where dispersal is characterized by a mechanistically derived kernel. We demonstrate that persistence depends strongly on the form of the kernel’s ‘tail’, a result consistent with previous literature on the speed of spread of invasions. We apply our theory to models of stream invertebrates with a biphasic life cycle, and relate our results to the ‘colonization cycle’ hypothesis where bias in downstream drift is offset by upstream bias in adult dispersal. In the absence of bias in adult dispersal, variability in the duration of the larval stage and in oviposition sites have a large effect of the persistence condition. The minimization calculations required in our approach are very straightforward, indicating the feasibility of future applications to life history theory.     

Drift and benthic invertebrate responses to stepwise and abrupt increases in non-scouring flow

We conducted two experiments to assess drift and benthic invertebrate responses to stepwise and abrupt changes in non-scouring flow in gravel-bed experimental streams. Intuitively, a stepwise flow increase should allow aquatic invertebrates more time to seek refuges than would an abrupt increase. We hypothesized that abrupt flow increases would result in larger increases in taxon richness and in the number of invertebrates in the drift, and a larger decrease in benthic density than would stepwise flow increases. Two kinds of drift response to flow increases were observed in the stepwise experiment: (1) no response (e.g. Caenissp. [Ephemeroptera] and Sphaeriidae [Pelecypoda]); (2) threshold response of some aquatic (e.g. Crangonyx pseudogracilis[Amphipoda]) and semiaquatic (e.g. Ormosiasp. [Diptera]) taxa. Drift richness and drift density in both experiments declined after reaching a peak. The peak was reached almost immediately in the abrupt treatment and later in the stepwise treatment. Maximum richness of taxa and taxon composition in the drift were similar in both experiments. Despite significant increases in drift, stepwise and abrupt increases in flow did not have a significant effect on benthic density. However, relative to reference streams, the percentage of total benthic invertebrates in the drift increased 10× in the stepwise experiment and 33× in the abrupt experiment. These non-scouring increases in flow were non-trivial. Our results suggest that several high flow events of the same magnitude (i.e. 2.5–3.0 fold increases) may cause considerable losses of benthic populations to the drift. The rate of increases in flow appears to be important: abrupt increases in flow had a stronger effect on invertebrate drift than did stepwise increases.     

Predator–prey systems in streams and rivers

Many predator–prey systems are found in environments with a predominantly unidirectional flow such as streams and rivers. Alterations of natural flow regimes (e.g., due to human management or global warming) put biological populations at risk. The aim of this paper is to devise a simple method that links flow speeds (currents) with population retention (persistence) and wash-out (extinction). We consider systems of prey and specialist, as well as generalist, predators, for which we distinguish the following flow speed scenarios: (a) coexistence, (b) persistence of prey only or (c) predators only (provided they are generalists), and (d) extinction of both populations. The method is based on a reaction–advection–diffusion model and traveling wave speed approximations. We show that this approach matches well spread rates observed in numerical simulations. The results from this paper can provide a useful tool in the assessment of instream flow needs, estimating the flow speed necessary for preserving riverine populations.     

Invertebrate drift and benthic community dynamics in a lowland neotropical stream, Costa Rica

In this study we quantified invertebrate drift and related it to the structure of the benthic community, over a 6–8 month period, in a 4th-order tropical stream in Costa Rica. Relative to reports from similar-sized temperate and tropical streams, drift densities were high (2-fold greater: mean 11.2 m⁻³; range 2.5–25 m⁻³), and benthic insect densities were relatively low (>3-fold lower: mean 890 m⁻²; range 228–1504 m⁻²). Drift was dominated by larval shrimps that represented more than 70% of total drift on any given date; the remaining 30% was composed of 54 insect taxa. Among insects, Simuliidae and Chironomidae (Diptera) and Baetidae, Leptohyphes and Tricorythodes (Ephemeroptera) comprised 24% of total drift. Drift periodicity was strongly nocturnal, with peaks at 18:00 h (sunset) and 03:00 h. Our results, and those of previous experiments in the study stream, suggest that nighttime drift is driven by the presence of predatory diurnal drift-feeding fishes and nocturnal adult shrimps. There were no clear seasonal patterns over both ‘dry’ and wet seasons, suggesting that benthic communities are subject to similar stresses throughout the year, and that populations grow and reproduce continuously. This revised version was published online in July 2006 with corrections to the Cover Date.     

Benthos and macroinvertebrate drift in six streams differing in alkalinity

The productive capacity of aquatic systems often is equated with the ‘chemical richness’ of the water. A primary objective of the present study was to relate macroinvertebrate benthos and drift to a streams' productive capacity as indicated by absolute levels of alkalinity. We tested this relationship in six 2nd–3rd order tributaries of the Salmon River, Idaho that ranged in alkalinity from 50 to 360 mg 1⁻¹. Benthic density and biomass, drift biomass, and benthic organic matter increased with increasing levels of alkalinity, although not all relationships were significant. The proportion of drift biomass to benthic biomass was similar among study streams suggesting that drift was primarily passive during the study period. The data suggest that spatial variations in landscape-scale geology may indirectly affect spatial patterns of macroinvertebrate benthic and drift standing crops among streams within a single river basin by mediating lotic chemical richness as found among tributaries of the Salmon River basin. Author for correspondence     

Effects of Demographic Stochasticity on Population Persistence in Advective Media

Many populations live and disperse in advective media. A fundamental question, known as the “drift paradox” in stream ecology, is how a closed population can survive when it is constantly being transported downstream by the flow. Recent population-level models have focused on the role of diffusive movement in balancing the effects of advection, predicting critical conditions for persistence. Here, we formulate an individual-based stochastic analog of the model described in (Lutscher et al., SIAM Rev. 47(4):749–772, 2005) to quantify the effects of demographic stochasticity on persistence. Population dynamics are modeled as a logistic growth process and dispersal as a position-jump process on a finite domain divided into patches. When there is no correlation in the interpatch movement of residents, stochasticity simply smooths the persistence-extinction boundary. However, when individuals disperse in “packets” from one patch to another and the flow field is memoryless on the timescale of packet transport, the probability of persistence is greatly enhanced. The latter transport mechanism may be characteristic of larval dispersal in the coastal ocean or wind-dispersed seed pods.     

Recent evolutionary history predicts population but not ecosystem‐level patterns

In the face of rapid anthropogenic environmental change, it is increasingly important to understand how ecological and evolutionary interactions affect the persistence of natural populations. Augmented gene flow has emerged as a potentially effective management strategy to counteract negative consequences of genetic drift and inbreeding depression in small and isolated populations. However, questions remain about the long‐term impacts of augmented gene flow and whether changes in individual and population fitness are reflected in ecosystem structure, potentiating eco‐evolutionary feedbacks. In this study, we used Trinidadian guppies (Poecilia reticulata) in experimental outdoor mesocosms to assess how populations with different recent evolutionary histories responded to a scenario of severe population size reduction followed by expansion in a high‐quality environment. We also investigated how variation in evolutionary history of the focal species affected ecosystem dynamics. We found that evolutionary history (i.e., gene flow vs. no gene flow) consistently predicted variation in individual growth. In addition, gene flow led to faster population growth in populations from one of the two drainages, but did not have measurable impacts on the ecosystem variables we measured: zooplankton density, algal growth, and decomposition rates. Our results suggest that benefits of gene flow may be long‐term and environment‐dependent. Although small in replication and duration, our study highlights the importance of eco‐evolutionary interactions in determining population persistence and sets the stage for future work in this area.     

Hydro-morphologically related variance in benthic drift and its importance for numerical habitat modelling

Numerical fish-habitat modelling on various scales is considered to be state of the art in river management. However, most of the concepts applied use steady-state hydraulic parameters such as flow velocity and water depth. Herein we present analysis and discussion of the possibility of including a drift-feeding parameter (SI_F) into habitat evaluations based on multiplying suitability indices. “Sources” and “sinks” of benthic drift were identified according to both the zero-crossing and hydraulic-threshold methods in an alpine gravel-bed river. Minor differences could be determined between the two methods in a well-developed riffle–pool section. Macroinvertebrates, used for simulating benthic drift, were collected by multi-habitat sampling and appraised according to their critical threshold (τ _cr) for motion on the bed surface and sinking velocity (v _s). The findings of the calculation of drift rates using one- (1D) and two-dimensional (2D) hydrodynamic numerical models highlight a specification of best feeding position for drift-feeding fish (i.e. brown trout, grayling) considering the SI_F parameter. Riffle–pool sequences are characteristic of pristine alpine streams; our findings underline their importance as production (riffles) and consumption areas (pools) in terms of holistic river function. Moreover, the results indicate that (artificial) lateral obstruction (e.g. dams) may lead to a reduced transport rate of benthic organisms due to low bottom shear stress (<0.25 N m⁻²). Thus, deposition of drifting macroinvertebrates occurs in backwaters, with downstream impacts on benthic and fish communities.     

Temporal patterns of benthic community development in an Arctic fjord (Kongsfjorden, Svalbard): results of a 24-year manipulation study

We investigated temporal patterns of recolonisation and disturbance in a benthic hard bottom community in high-arctic Kongsfjorden from 1980 to 2003 through annual photographic surveys. A manipulative sampling design was applied, where half of the study area (treatment areas) was cleared at the beginning of the study. Twenty-three different taxa and groups of benthic epifauna were found in the photographs. The benthic community structures of treatments and controls converged within the first decade, but significant differences prevailed until ≤13 years after the start of the study. We could distinguish between three different time intervals with increased inter-annual changes. While the observed differences during the first two intervals could be attributed to recolonisation and succession, the changes in interval 3 were mostly due to increased external forcing and characterised by low inter-group and high inter-annual differences. During this interval, brown algae (mainly Desmarestia) and the sea urchin Strongylocentrotus droebachiensis emerged in high densities, while sea anemone populations declined. Different recolonisation patterns for individual species were related to life span, rate of maturity, predators and larval settlement. We could not find a climax stage in the succession for the benthic community at Kvadehuken, presumably due to the constant level of disturbances at the site.     

Seasonal influences on population spread and persistence in streams: spreading speeds

The drift paradox asks how stream-dwelling organisms can persist, without being washed out, when they are continuously subject to the unidirectional stream flow. To date, mathematical analyses of the stream paradox have investigated the interplay of growth, drift and flow needed for species persistence under the assumption that the stream environment is temporally constant. However, in reality, streams are subject to major seasonal variations in environmental factors that govern population growth and dispersal. We consider the influence of such seasonal variations on the drift paradox, using a time-periodic integrodifferential equation model. We establish upstream and downstream spreading speeds under the assumption of periodically fluctuating environments, and also show the existence of periodic traveling waves. The sign of the upstream spreading speed then determines persistence. Fluctuating environments are characterized by seasonal correlations between the flow, transfer rates, diffusion and settling rates, and we investigate the effect of such correlations on the population spread and persistence. We also show how results in this paper can formally connect to those for autonomous integrodifferential equations, through the appropriate weighted averaging methods. Finally, for a specific dispersal function, we show that the upstream spreading speed is nonnegative if and only if the critical domain size exists in this temporally fluctuating environment.     

Genetic variability and structure of the remnant natural populations of <Emphasis Type="Italic">Cedrus libani</Emphasis> (Pinaceae) of Lebanon

Cedrus libani of Lebanon is a valuable natural resource and the dominant species in its natural ecosystem. Intense and diverse anthropogenic pressures over historical times raised concerns about its genetic vigor and continued survival. Our investigation of the genetic diversity included samples from all remnant natural populations. Assessment of the genetic diversity using random amplified polymorphic DNA markers revealed the persistence of considerable variation distributed within populations with low population differentiation corroborated by Bayesian and analysis of molecular variance estimates (G _ST = 0.07, Φ _ST = 0.09). Individual assignment tests were carried out to investigate measures of gene flow. Inferences concluded that this natural heritage is not currently threatened by inbreeding or by random genetic drift. Correlation studies investigated possible effects of spatial distribution and environmental conditions on genetic structure. A climatic trend corresponding to a temperature–humidity gradient correlated significantly with the level of genetic diversity, while the edaphic variation did not.     

The relation between invertebrate drift and two primary controls,discharge and benthic densities,in a large regulated river

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Limiting inbreeding in disjunct and isolated populations of a woody shrub

Pollen movements and mating patterns are key features that influence population genetic structure. When gene flow is low, small populations are prone to increased genetic drift and inbreeding, but naturally disjunct species may have features that reduce inbreeding and contribute to their persistence despite genetic isolation. Using microsatellite loci, we investigated outcrossing levels, family mating parameters, pollen dispersal, and spatial genetic structure in three populations of Hakea oldfieldii, a fire‐sensitive shrub with naturally disjunct, isolated populations prone to reduction in size and extinction following fires. We mapped and genotyped a sample of 102 plants from a large population, and all plants from two smaller populations (28 and 20 individuals), and genotyped 158–210 progeny from each population. We found high outcrossing despite the possibility of geitonogamous pollination, small amounts of biparental inbreeding, a limited number of successful pollen parents within populations, and significant correlated paternity. The number of pollen parents for each seed parent was moderate. There was low but significant spatial genetic structure up to 10 m around plants, but the majority of successful pollen came from outside this area including substantial proportions from distant plants within populations. Seed production varied among seven populations investigated but was not correlated with census population size. We suggest there may be a mechanism to prevent self‐pollination in H. oldfieldii and that high outcrossing and pollen dispersal within populations would promote genetic diversity among the relatively small amount of seed stored in the canopy. These features of the mating system would contribute to the persistence of genetically isolated populations prone to fluctuations in size.     

Commercial potential of seaweeds from St Lawrence Island,Alaska

This study assessed the possibility of using drift and subtidal seaweeds from St Lawrence Island, Alaska (lat. 63°N) for sale by the native population after simple processing. Over 125 km of coastline were surveyed for distribution of both drift and subtidal seaweeds. Drift seaweed wet weight densities ranged from 0.2 to over 9 kg m⁻², with an average of over 4 t km⁻¹ in the areas sampled. Attached, benthic seaweed densities ranged from 0.15 to 0.32 kg m⁻². Thirty and 35% of the biomass was composed of Agarum cribrosum and species of Laminaria, respectively, both as drift and as benthic seaweed. Data from tagged Laminaria indicated that growth rates were relatively slow for most of the year. The drift seaweed resource on the coasts south and west of the city of Gambell appeared to have good potential for a small-scale commercial harvest. (*author for correspondence)     

Genetic drift during the spread phase of a biological invasion

Recent theoretical and experimental models have revealed the role played by evolution during species spread, and in particular have questioned the influence of genetic drift at range edges. By investigating the spread of an aquatic invader in patchy habitats, we quantified genetic drift and explored its consequences for genetic diversity and fitness. We examined the interplay of gene flow and genetic drift in 36 populations of the red swamp crayfish, Procambarus clarkii, in a relatively recently invaded wetland area (30 years, Brière, northwest France). Despite the small spatial scale of our study (15 km²), populations were highly structured according to the strong barrier of land surfaces and revealed a clear pattern of colonization through watercourses. Isolated populations exhibited small effective sizes and low dispersal rates that depended on water connectivity, suggesting that genetic drift dominated in the evolution of allele frequencies in these populations. We also observed a significant decrease in the genetic diversity of isolated populations over only a 2‐year period, but failed to demonstrate an associated fitness cost using fluctuating asymmetry. This study documents the possible strong influence of genetic drift during the spread of a species, and such findings provide critical insights into the current context of profound rearrangements in species distributions due to global change.     

Does genetic variation and gene flow vary with rarity in obligate seeding Persoonia species (Proteaceae)?

Theory predicts that genetic variation is a determinant of persistence, and that the abundance and distribution of variation is strongly dependent on genetic drift and gene flow. Small, isolated populations are expected to be less diverse and more differentiated than large, inter-connected populations. Thus rare species may be more at risk of extinction. We used 389 putative AFLP loci to compare genetic variation and structuring in two pairs of closely-related common (large populations geographically widespread) and rare (small populations spatially restricted) Persoonia species. We genotyped 15–22 adult plants, from four populations, covering the geographic range of each species. Although genetic diversity was low for all four species (for long-lived outcrossing perennials), we found significantly more diversity within populations of the rare species than within those of the common species. AMOVA revealed significant levels of structure both among species (21%) and populations (15%). The proportion of inter-population variation within species did not vary consistently with rarity (Pair 1 rare 21.1% versus common 16.5%; Pair 2 rare 15.8% versus common 20.6%). However populations of the rare species were more differentiated than common species with similar geographic separation, suggesting greater gene flow between populations of the common species. Therefore, even relatively small genetically isolated populations of rare Persoonia species were more diverse than large populations of common Persoonia species. We hypothesise that common Persoonia species have undergone a rapid range expansion from a narrow gene pool, while genetic diversity is maintained in the soil seed-bank of rare remnants.     

Extinction and recolonization of maritime Antarctica in the limpet Nacella concinna (Strebel, 1908) during the last glacial cycle: toward a model of Quaternary biogeography in shallow Antarctic invertebrates

Quaternary glaciations in Antarctica drastically modified geographical ranges and population sizes of marine benthic invertebrates and thus affected the amount and distribution of intraspecific genetic variation. Here, we present new genetic information in the Antarctic limpet Nacella concinna, a dominant Antarctic benthic species along shallow ice‐free rocky ecosystems. We examined the patterns of genetic diversity and structure in this broadcast spawner along maritime Antarctica and from the peri‐Antarctic island of South Georgia. Genetic analyses showed that N. concinna represents a single panmictic unit in maritime Antarctic. Low levels of genetic diversity characterized this population; its median‐joining haplotype network revealed a typical star‐like topology with a short genealogy and a dominant haplotype broadly distributed. As previously reported with nuclear markers, we detected significant genetic differentiation between South Georgia Island and maritime Antarctica populations. Higher levels of genetic diversity, a more expanded genealogy and the presence of more private haplotypes support the hypothesis of glacial persistence in this peri‐Antarctic island. Bayesian Skyline plot and mismatch distribution analyses recognized an older demographic history in South Georgia. Approximate Bayesian computations did not support the persistence of N. concinna along maritime Antarctica during the last glacial period, but indicated the resilience of the species in peri‐Antarctic refugia (South Georgia Island). We proposed a model of Quaternary Biogeography for Antarctic marine benthic invertebrates with shallow and narrow bathymetric ranges including (i) extinction of maritime Antarctic populations during glacial periods; (ii) persistence of populations in peri‐Antarctic refugia; and (iii) recolonization of maritime Antarctica following the deglaciation process.     

Genetic structure is influenced by environmental barriers: empirical evidence from the common voleMicrotus arvalis populations

One of the major challenges in population biology is the identification of barriers to gene flow and/or secondary contacts between differentiated entities. The level of genetic differentiation among eight populations of the common voleMicrotus arvalis (Pallas, 1779) around the Biebrza Wetlands, NE Poland was examined by analyzing seven microsatellite loci for 140 voles and testing for the presence of barriers to gene flow. Overall population differentiation was moderate and significant (F _ST = 0.081,p < 0.001) and there was no correlation between geographical and genetic distances among populations. We found a relatively high level of genetic variability within the populations studied. This could be explained by male bias in dispersal, a phenomenon recently found inM. arvalis. Patterns of genetic structure visualized in synthetic genetic maps showed clear gradients along a southeast-northwest axis across the study area, as well as the presence of a potential barrier to dispersal. The position of a barrier to gene flow identified using Monmonier’s maximum difference algorithm likely corresponds to humid habitats of the Biebrza Wetlands. These results suggest that the presence of environmental barriers to gene flow and drift may be responsible for the observed spatial genetic structure ofM. arvalis in the Biebrza Valley. Institute of Biology, University of Białystok, OEwierkowa 20 B, 15-950 Białystok, Poland,     

How to leave or stay on the substratum when you can’t swim? Evidence of the role of mucus thread secretion by postlarvae of Pectinaria koreni (Malmgren) in still water and flume experiments

Through an experimental approach we investigate the role of mucus secretion in postlarvae of Pectinaria koreni (tubicolous polychaete) on their ability to drift within the benthic boundary layer or to stay at the water–substratum interface. Fall velocity measurements were conducted with either living or dead postlarvae which were allowed to sink into a 2 m long Plexiglas cylindrical chamber. Five groups of increasing size-classes were tested ranging from the very first benthic stage (1 mm < Tube_length < 2 mm: membranous tube present accounting for more than 75% of the total tube length) to older stages (6 mm < Tube_length < 10 mm: membranous tube absent). We used these results to propose the first estimates of dispersal distances by several post-larval stages secreting mucus or sinking passively through the water column. Experiments were carried out in the HYCOBENTHOS flume to determine values of critical shear velocity (u _*c) inducing bedload transport and further resuspension of postlarvae of increasing sizes. The influence of mucus secretion by recruits on their ability to stay or quit a ‘suitable’ substratum was investigated by using either living or dead individuals. Results showed that: (a) the ability to secrete mucus rapidly is limited to the younger stages; (b) fall velocity of postlarvae is drastically lowered by mucus secretion (5 orders of magnitude) and is higher for the older stages; (c) dead recruits behave similarly to ‘passive’ recruits; (d) horizontal distances of drift dispersal may be considerable (up to 800 m for a single 22 min trip); (e) mucus secretion may be used by the postlarvae to anchor themselves to the substratum. Cost-benefit of using the mucus secretion as a tool for recruitment and the related spatial scales are discussed.     

Run and Tumble Chemotaxis in a Shear Flow: The Effect of Temporal Comparisons,Persistence, Rotational Diffusion,and Cell Shape

Escherichia coli is a motile bacterium that moves up a chemoattractant gradient by performing a biased random walk composed of alternating runs and tumbles. This paper presents calculations of the chemotactic drift velocity v _d (the mean velocity up the chemoattractant gradient) of an E. coli cell performing chemotaxis in a uniform, steady shear flow, with a weak chemoattractant gradient at right angles to the flow. Extending earlier models, a combined analytic and numerical approach is used to assess the effect of several complications, namely (i) a cell cannot detect a chemoattractant gradient directly but rather makes temporal comparisons of chemoattractant concentration, (ii) the tumbles exhibit persistence of direction, meaning that the swimming directions before and after a tumble are correlated, (iii) the cell suffers random re-orientations due to rotational Brownian motion, and (iv) the non-spherical shape of the cell affects the way that it is rotated by the shear flow. These complications influence the dependence of v _d on the shear rate γ. When they are all included, it is found that (a) shear disrupts chemotaxis and shear rates beyond γ≈2 s⁻¹ render chemotaxis ineffective, (b) in terms of maximizing drift velocity, persistence of direction is advantageous in a quiescent fluid but disadvantageous in a shear flow, and (c) a more elongated body shape is advantageous in performing chemotaxis in a shear flow. J.T. Locsei is supported by an Oliver Gatty Studentship from the University of Cambridge.