Recruitment dynamics in complex life cycles

PROPAGATION OF SCALE-DEPENDENT EFFECTS FROM RECRUITS TO ADULTS IN BARNACLES AND SEAWEEDS

The interplay between local and large spatial scale processes in open systems is often dependent upon ecological context and species specific factors such as longevity, dispersal capability, or vulnerability to predation. When disturbance clears patches in open systems, the successful reestablishment of adult colonizers and the trajectory of succession may depend upon both the scale of the disturbance event and scale of life history characteristics. Here we examine the links between the size of a disturbance event and long term patterns of variation in recruitment, density, and percent cover in a relatively short-lived but long-range disperser, the acorn barnacle Semibalanus balanoides, and a relatively long lived and short range disperser, the fucoid alga Fucus vesiculosus. Further, a model of linked scale-dependent processes is developed and we explore how long term patterns of scale-dependent recruitment are related to successful establishment of adults. Recruitment, densities, and cover were monitored for five years for both species in a single experiment using clearings of different sizes spread over two north-facing and two south-facing bays on Swan's Island, Maine, USA. Barnacle recruitment was particularly variable in small clearings and was lower overall at more interior sites within bays. While local-scale factors also strongly influenced patterns of variation in fucoid recruitment, fucoids dominate surface cover in large clearings and in south-facing bays. Recruitment was a poor predictor of density, whereas density itself was a good predictor of percent cover for both species. These results indicate that scale dependencies, location-specific factors, and life history traits contribute to patterns of community development on rocky shores and may ultimately determine whether an open patch converges to or diverges from its initial community state.

The stochastic nature of larval connectivity among nearshore marine populations

Many nearshore fish and invertebrate populations are overexploited even when apparently coherent management structures are in place. One potential cause of mismanagement may be a poor understanding and accounting of stochasticity, particularly for stock recruitment. Many of the fishes and invertebrates that comprise nearshore fisheries are relatively sedentary as adults but have an obligate larval pelagic stage that is dispersed by ocean currents. Here, we demonstrate that larval connectivity is inherently an intermittent and heterogeneous process on annual time scales. This stochasticity arises from the advection of pelagic larvae by chaotic coastal circulations. This result departs from typical assumptions where larvae simply diffuse from one site to another or where complex connectivity patterns are created by transport within spatially complicated environments. We derive a statistical model for the expected variability in larval settlement patterns and demonstrate how larval connectivity varies as a function of different biological and physical processes. The stochastic nature of larval connectivity creates an unavoidable uncertainty in the assessment of fish recruitment and the resulting forecasts of sustainable yields.

Behavioral tradeoff in estuarine larvae favors seaward migration over minimizing visibility to predators

The ability of microscopic larvae to control their fate and replenish populations in dynamic marine environments has been a long-running topic of debate of central importance to understanding the ecology and evolution of life in the sea and managing resources in a changing global environment. After decades of research documenting behaviors that keep larvae close to natal populations, it is becoming apparent that larval behaviors in a broader spectrum of species promote long-distance migrations to offshore nursery grounds. Larvae must exert considerable control over their movements. We now show that larval emigration from estuaries is favored even over minimizing visibility to predators. An endogenous tidal vertical migration that would expedite seaward migration of Uca pugilator larvae was maintained experimentally across two tidal regimes. The periodicity of the rhythm doubled to match the local tidal regime, but larvae ascended to the surface during the daytime rather than at night. This process would conserve larval emigration but increase the visibility to predators across part of the species range. The periodicity of tidal vertical migration by Sesarma cinereum larvae failed to double and was inappropriately timed relative to both environmental cycles in the absence of a diel cycle. The timing system regulating tidally timed behaviors in these two species of crabs evidently differed. Phenotypic plasticity can conserve larval transport of both species when tidal and diel cycles are present. It may be widespread in the sea where diverse habitats are encountered across extensive species ranges.

Is surface orientation a determinant for colonisation patterns of vagile and sessile macrobenthos on artificial reefs?

In order to examine how substratum colonisation can affect community structure, a 1-year study was conducted at the Faro/Ancao artificial reef (Algarve, Portugal). In the study of hard substratum communities, motile species are usually neglected and only the conspicuous species are taken into account. Therefore, the development of vagile and sessile components of the epibiotic community were analysed separately. Differences between assemblages on horizontal surfaces, but not on vertical surfaces, were detected. Multivariate analysis detected differences in macrobenthic community structure either considering sessile or motile components. However, significant differences were only detected for vagile fauna. Moreover, this study suggests that for hard substratum communities, analysis of the vagile fauna is important and should be taken into account in the functioning of the artificial raft.

GENETIC IDENTITY DETERMINES RISK OF POST-SETTLEMENT MORTALITY OF A MARINE FISH

Longitudinal sampling of four cohorts of Neopomacentrus filamentosus, a common tropical damselfish from Dampier Archipelago, Western Australia, revealed the evolution of size structure after settlement. Light traps collected premetamorphic individuals from the water column ("settlers") to establish a baseline for each cohort. Subsequently, divers collected benthic juveniles ("recruits") at 1-3-month intervals to determine the relative impacts of post-settlement mortality during the first three months. Growth trajectories for individual fish were back-calculated from otolith records and compared with nonlinear mixed-effects models. Size-selective mortality was detected in all cohorts with the loss of smaller, slower growing individuals. Three months after settlement, recruits showed significantly faster growth as juveniles, faster growth as larvae, and larger sizes as hatchlings. The timing and intensity of post-settlement selection differed among cohorts and was correlated with density at settlement. The cohort with the greatest initial abundance experienced the strongest selective mortality, with most of this mortality occurring between one and two months after settlement when juveniles began foraging at higher positions in the water column. Significant genetic structure was found between settlers and three-month-old recruits in this cohort as a result of natural selection that changed the frequency of mtDNA haplotypes measured at the control region. The extent of this genetic difference was enlarged or reduced by artificially manipulating the intensity of size-based selection, thus establishing a link between phenotype and haplotype. Sequence variation in the control region of the mitochondrial genome has been linked to mitochondrial efficiency and weight gain in other studies, which provides a plausible explanation for the patterns observed here.

ASSOCIATIONS BETWEEN SHOREBIRD ABUNDANCE AND PARASITES IN THE SAND CRAB, EMERITA ANALOGA, ALONG THE CALIFORNIA COAST

I investigated spatial variation in the prevalence and abundance of 4 species of parasites in the sand crab, Emerita analoga, on 8 sandy beaches along 800 km of the California coast, to assess the importance of bird abundance for the distribution of parasites among sand crab populations. I collected sand crabs and counted shorebirds and gulls at each beach during June and November 1994. Sand crabs served as intermediate hosts for 4 species of parasites, including a trematode, Spelotrema nicolli (Cable and Hunnienen, 1938); an acanthocephalan, Polymorphus kenti (Van Cleave, 1947); a nematode, Proleptus sp., and an unidentified trypanorhynch tapeworm. Among sand crab populations, there was substantial spatial variation in the prevalence and abundance of each parasite species. No latitudinal pattern was apparent for any of the 4 species observed. Temporally, parasite prevalence and abundance was significantly different between dates for all 4 parasites. Specifically, sand crab populations experienced higher trematode, nematode, and trypanorhynch prevalence and abundance in November than in June. In contrast, prevalence and abundance of acanthocephalans were higher in June than in November. There were strong positive associations between bird abundance and prevalence of parasitic infection for trematodes and acanthocephalans for some dates but not for nematodes or trypanorhynchs, which use elasmobranchs as definitive hosts. The spatial variation in prevalence and abundance of trematodes and acanthocephalans observed among sand crab populations may be attributed to the distribution and abundance of shorebirds and gulls that serve as definitive hosts.

Habitat Connectivity and Ecosystem Productivity: Implications from a Simple Model

The import of resources (food, nutrients) sustains biological production and food webs in resource-limited habitats. Resource export from donor habitats subsidizes production in recipient habitats, but the ecosystem-scale consequences of resource translocation are generally unknown. Here, I use a nutrient-phytoplankton-zooplankton model to show how dispersive connectivity between a shallow autotrophic habitat and a deep heterotrophic pelagic habitat can amplify overall system production in metazoan food webs. This result derives from the finite capacity of suspension feeders to capture and assimilate food particles: excess primary production in closed autotrophic habitats cannot be assimilated by consumers; however, if excess phytoplankton production is exported to food-limited heterotrophic habitats, it can be assimilated by zooplankton to support additional secondary production. Transport of regenerated nutrients from heterotrophic to autotrophic habitats sustains higher system primary production. These simulation results imply that the ecosystem-scale efficiency of nutrient transformation into metazoan biomass can be constrained by the rate of resource exchange across habitats and that it is optimized when the transport rate matches the growth rate of primary producers. Slower transport (i.e., reduced connectivity) leads to nutrient limitation of primary production in autotrophic habitats and food limitation of secondary production in heterotrophic habitats. Habitat fragmentation can therefore impose energetic constraints on the carrying capacity of aquatic ecosystems. The outcomes of ecosystem restoration through habitat creation will be determined by both functions provided by newly created aquatic habitats and the rates of hydraulic connectivity between them.

Genetic evidence for kin aggregation in the intertidal acorn barnacle (Semibalanus balanoides)

It is generally assumed that larvae of benthic species are thoroughly mixed in the plankton and distributed randomly at settlement. Yet, it has also been hypothesized that a combination of larval gregarious behaviour coupled with particular oceanographic conditions may prevent larvae from mixing completely, and result in nonrandom spatial distributions following settlement. Using microsatellite markers, the main objective of this study was to investigate the occurrence of statistical connections between relatedness and settlement in the intertidal acorn barnacle from the Gulf of St Lawrence, Canada. A second objective was to test the hypothesis that patches of kin-related individuals came from a common parental site. Our results indicated that a significant number of barnacles within a given sample were more closely related than expected by chance despite the enormous potential for admixture during the planktonic phase. Thus, eight out of 37 samples analysed had relatedness values significantly higher than expected from random settlement. Moreover, analyses of sibship network construction and network complexity tests provided evidence for the occurrence of networks within several samples that were characterized by strong connections among individuals. Thus, nonrandom planktonic dispersal associated with relatively stable oceanic currents, as well as additional ecological factors to be rigorously investigated (e.g. behavioural mechanisms), may be more important in determining patterns of genetic structure in marine benthic invertebrates than generally assumed. Therefore, documenting genetic patterns associated with kin aggregation should be a fruitful and an important avenue for future studies in marine invertebrates.

CONCORDANCE BETWEEN SHORT-TERM EXPERIMENTS AND LONG-TERM CENSUSES IN TIDE POOL FISHES

Sixteen years of annual censuses (1990-2005) of an assemblage of tide pool sculpins at Tatoosh Island, Washington, USA, revealed relative constancy in the numbers of recruits and adults. However, the dominant species as a recruit (Clinocottus globiceps) was always replaced numerically as an adult by another species (Oligocottus maculosus). When mechanisms of coexistence were evaluated, little support was found for either a lottery model, because competitive interactions are hierarchical, or a storage-effect model of coexistence, because the relative ranking of recruitment varied little among years and did not covary among species with environmental variables such as ocean temperature, upwelling indices, or an estimator of ocean productivity (oyster condition index). There was also little evidence of niche partitioning based on habitat affinities. Additionally, predation-mediated coexistence had little support, given that the competitive dominant did not have the greatest rates of mortality. Instead, a competition-colonization trade-off may contribute to the coexistence of these species, where C. globiceps always recruits in the greatest numbers, while O. maculosus dominates the adult assemblage via competitive processes. The importance of post-recruitment processes in this assemblage is further suggested by some negative associations among species, the presence of density dependence, some habitat affinities, and previously published experimental work that demonstrated the competitive dominance of O. maculosus over C globiceps.

Spatial scaling of consumer-resource interactions in advection-dominated systems

Ecologists studying consumer-resource interactions in advection-dominated systems such as streams and rivers frequently seek to link the results of small-scale experiments with larger-scale patterns of distribution and abundance. Accomplishing this goal requires determining the characteristic scale, termed the response length, at which there is a shift from local dynamics dominated by advective dispersal to larger-scale dynamics dominated by births and deaths. Here, we model the dynamics of consumer-resource systems in a spatially variable, advective environment and show how consumer-resource interactions alter the response length relative to its single-species value. For one case involving a grazer that emigrates in response to high predator density, we quantify the changes using published data from small-scale experiments on aquatic invertebrates. Using Fourier analysis, we describe the responses of advection-dominated consumer-resource systems to spatially extended environmental variability in a way that involves explicit consideration of the response length. The patterns we derive for different consumer-resource systems exhibit important similarities in how component populations respond to spatial environmental variability affecting dispersal as opposed to demographic parameters.

Molecular detection of marine invertebrate larvae

The ecological patterns of many invertebrate larvae remain an ongoing mystery, in large part owing to the difficult task of detecting them in the water column. The development of nucleic-acid-based technology has the potential to resolve this issue by direct identification and monitoring of embryonic and larval forms in situ. We report herein on the successful development and application of nucleic-acid-based sandwich hybridization assays that detect barnacles using rRNA-targeted probes with both group-(order Thoracica) and species-(Balanus glandula) specificity. Primary results include the determination of target 18S rRNA sequences and the construction of "capture" probes for detection of larvae using hybridization techniques. In addition, we modified existing protocols for whole cell hybridization of invertebrate larvae as confirmation of the sandwich hybridization results. We used both hybridization techniques successfully in the laboratory on a plankton time series collected over 3 months, as well as a week-long in situ deployment of the technique in Monterey Bay, CA. The adaptability of this technology promises to be further applicable to various organisms and could be used to enhance our understanding of larval presence in the world's oceans.

Scaling of Connectivity in Marine Populations

Defining the scale of connectivity, or exchange, among marine populations and determining the factors driving this exchange are pivotal to our understanding of the population dynamics, genetic structure, and biogeography of many coastal species. Using a high-resolution biophysical model for the Caribbean region, we report that typical larval dispersal distances of ecologically relevant magnitudes are on the scale of only 10 to 100 kilometers for a variety of reef fish species. We also show the importance of the early onset of active larval movement mediating the dispersal potential. In addition to self-recruitment, larval import from outside the local area is required to sustain most populations, although these population subsidies are very limited in particular systems. The results reveal distinct regions of population isolation based on larval dispersal that also correspond to genetic and morphological clines observed across a range of marine organisms.

Variation in larval growth can predict the recruitment of a temperate, seagrass-associated fish

Understanding the factors leading to inter-annual variation in recruitment of animals with complex life cycles is a key goal for ecology and the sustainable management of animal resources, such as fisheries. We used otolith microstructure to determine larval growth rates of post-larval King George whiting, Sillaginodes punctata, in seagrass beds of Port Phillip Bay, Australia. Inter-annual variation in growth determined early in the pelagic, offshore larval-stage was highly correlated with post-larval abundance (a predictor of fishery recruitment). Sea surface temperature measured near the presumed spawning area off Western Victoria was significantly correlated with larval growth, and was also significantly correlated with post-larval abundance in Port Phillip Bay. Increased water temperature would have had the direct effect of increasing larval growth and therefore contributing to larval survival, but may also have been indicative of enhanced physical transport and/or plankton productivity. Overall, larval growth rate of King George whiting is a very strong predictor of post-larval abundance, which in turn will influence fishery recruitment in 3-5 years' time.

Scales of benthic-pelagic coupling and the intensity of species interactions: from recruitment limitation to top-down control

Large and usually unpredictable variation in species interaction strength has been a major roadblock to applying local experimental results to large-scale management and conservation issues. Recent studies explicitly considering benthic-pelagic coupling are starting to shed light on, and find regularities in, the causes of such large-scale variation in coastal ecosystems. Here, we evaluate the effects of variation in wind-driven upwelling on community regulation along 900 km of coastline of the southeastern Pacific, between 29 degrees S and 35 degrees S during 72 months. Variability in the intensity of upwelling occurring over tens of km produced predictable variation in recruitment of intertidal mussels, but not barnacles, and did not affect patterns of community structure. In contrast, sharp discontinuities in upwelling regimes produced abrupt and persistent breaks in the dynamics of benthic and pelagic communities over hundreds of km (regional) scales. Rates of mussel and barnacle recruitment changed sharply at approximately 32 degrees -33 degrees S, determining a geographic break in adult abundance of these competitively dominant species. Analysis of satellite images demonstrates that regional-scale discontinuities in oceanographic regimes can couple benthic and pelagic systems, as evidenced by coincident breaks in dynamics and concentration of offshore surface chlorophyll-a. Field experiments showed that the paradigm of top-down control of intertidal benthic communities holds only south of the discontinuity. To the north, populations seem recruitment-limited, and predators have negligible effects, despite attaining similarly high abundances and potential predation effects across the region. Thus, geographically discontinuous oceanographic regimes set bounds to the strength of species interactions and define distinct regions for the design and implementation of sustainable management and conservation policies.

Coral Reef Fish Larvae Settle Close to Home

Population connectivity through larval dispersal is an essential parameter in models of marine population dynamics and the optimal size and spacing of marine reserves. However, there are remarkably few direct estimates of larval dispersal for marine organisms, and the actual birth sites of successful recruits have never been located. Here, we solve the mystery of the natal origin of clownfish (Amphiprion polymnus) juveniles by mass-marking via tetracycline immersion all larvae produced in a population. In addition, we established parentage by DNA genotyping all potential adults and all new recruits arriving in the population. Although no individuals settled into the same anemone as their parents, many settled remarkably close to home. Even though this species has a 9-12 day larval duration, one-third of settled juveniles had returned to a 2 hectare natal area, with many settling <100 m from their birth site. This represents the smallest scale of dispersal known for any marine fish species with a pelagic larval phase. The degree of local retention indicates that marine reserves can provide recruitment benefits not only beyond but also within their boundaries.

Diversification before the most recent glaciation in Balanus glandula

A deep genetic cline between southern populations of the barnacle Balanus glandula (from about Monterey Bay southward) and northern populations (from northern California through Alaska) has recently been described. If this pattern is due to historical isolation and genetic drift, we expect it to have formed recently and represent a transient, nonequilibrium state. However, this cline appears to have formed well before the last glacial maximum. Our assays of sequence diversity at a region of mitochondrial cytochrome oxidase I, combined with coalescent estimators of the time of separation for these two regions, suggest that a late Pleistocene event more than 100 thousand years ago may be responsible for the initial separation. This suggests that either strong oceanographic mechanisms or natural selection have maintained the cline, because there has clearly been adequate time for this cline or polymorphism to resolve itself by genetic drift and migration. However, reliance on only a single mitochondrial marker for which the substitution rate has been estimated still limits the resolution of our analysis.

Habitat Size, Recruitment, and Longevity as Factors Limiting Population Size in Stage‐Structured Species

Surprisingly little research has evaluated how habitat size may limit the population size of species that use different habitats at different stages of their lives. Here we develop simple discrete-time models to describe the population dynamics of species that use separate juvenile and adult habitats. Analytic solutions, model simulations, and elasticity and sensitivity analyses show that adult abundance is only limited by the size of the juvenile habitat when both adult habitat size and recruitment are much larger than juvenile habitat size. Juvenile habitat plays a marginally greater role in limiting population size for species with closed populations, where recruitment is proportional to adult abundance, versus open populations. Because adult populations often accumulate pulses of juveniles, adult habitat size can strongly limit population size over a broad range of parameter values, an effect that increases as the longevity of a species increases. Limited empirical research from a range of taxa supports these model predictions, although few studies were designed to actually test the limiting role of juvenile versus adult habitat. Future research must carefully evaluate whether and how processes at the juvenile stage affect adult abundance, and conservation efforts may be able to use this model to evaluate the cost-effectiveness, vis-a-vis increasing adult abundance, of time and money allocated to protecting juvenile habitats.

Developmental dimorphism: consequences for larval behavior and dispersal potential in a marine gastropod

Specific effects of alternative developmental programs on swimming and settlement behavior for marine larvae have not been identified experimentally. A major impediment to this research has been the rarity of species with variable development. Here, we compared traits related to movement and habitat selection for different ontogenetic stages of long-lived, feeding larvae (planktotrophic) and short-lived, nonfeeding larvae (lecithotrophic) of the herbivorous gastropod Alderia modesta. Newly hatched planktotrophic larvae swam in meandering paths with equal rates of upward and downward movement. As planktotrophic larvae developed towards competence (physiological ability to metamorphose), their swimming paths became straighter, faster, and increasingly directed towards the bottom, traits shared by newly hatched lecithotrophic larvae. Despite differing in developmental history, competent planktotrophic (32-d-old) and lecithotrophic larvae (competent upon hatching) exhibited qualitatively similar swimming behaviors and substrate specificity. However, lecithotrophic larvae moved downward at twice the speed of competent planktotrophic larvae, potentially producing a 5-fold higher rate of contact with the bottom in natural flows. Competent larvae swam downwards rather than passively sinking, even though sinking rates were faster than swimming speeds; active swimming may allow larvae to keep the velum extended, permitting rapid response to chemical settlement cues and promoting successful habitat colonization. Differences between larvae of the two development modes may reflect fine-tuning by selection of traits important for dispersal and settlement into patchy adult habitats.

Strong genetic clines and geographical variation in gene flow in the rocky intertidal barnacle Balanus glandula

A long-standing issue in marine biology is identifying spatial scales at which populations of sessile adults are connected by planktonic offspring. We examined the genetic continuity of the acorn barnacle Balanus glandula, an abundant member of rocky intertidal communities of the northeastern Pacific Ocean, and compared these genetic patterns to the nearshore oceanography described by trajectories of surface drifters. Consistent with its broad dispersal potential, barnacle populations are genetically similar at both mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1-alpha) loci across broad swaths of the species' range. In central California, however, there is a striking genetic cline across 475 km of coastline between northern and southern populations. These patterns indicate that gene flow within central California is far more restricted spatially than among other populations. Possible reasons for the steep cline include the slow secondary introgression of historically separated populations, a balance between diversifying selection and dispersal, or some mix of both. Geographic trajectories of oceanic drifters closely parallel geographical patterns of gene flow. Drifters placed to the north (Oregon; approximately 44 degrees N) and south (Santa Barbara, California; approximately 34 degrees N) of the cline disperse hundreds of kilometers within 40 days, yet over the long-term their trajectories never overlapped. The lack of communication between waters originating in Oregon and southern California probably helps to maintain strong genetic differentiation between these regions. More broadly, the geographical variation in gene flow implies that focusing on species-level averages of gene flow can mask biologically important variance within species which reflects local environmental conditions and historical events.