bluuebunny/crossref_metadata_2025 huggingface dataset:
DENSITY‐DEPENDENT AGE OF FIRST REPRODUCTION AS A BUFFER AFFECTING PERSISTENCE OF SMALL POPULATIONS
Authors: Ferrer Miguel , Otalora FermÍn , GarcÍa-Ruiz Juan M.
Published: June 2007

We documented a density‐dependent age of first reproduction using a population of Spanish Imperial Eagles (Aquila adalberti) in Doñana National Park (southwestern Spain). Then, a stochastic individual‐based simulation was designed to analyze the effect of variations in the age of first reproduction on the fluctuation of small populations. This is a discrete‐time simulation model that uses the age structure, mortality rates, reproductive rate, and density‐dependent relationships from empirical studies. Age of breeding in the real population was positively related to population density. The probability of persistence of simulated populations was affected by both breeding population ceiling and age of first breeding. Our simulations were compared to the historical evolution of the Doñana population, assuming an age of first reproduction of three years. Density‐dependent variation in age of first breeding increased the mean time of persistence, allowing the population to stay at carrying capacity longer. This buffer mechanism could be crucial for the persistence of small populations.
EFFECT OF ROADSIDE BIAS ON THE ACCURACY OF PREDICTIVE MAPS PRODUCED BY BIOCLIMATIC MODELS
Authors: Kadmon Ronen , Farber Oren , Danin Avinoam
Published: June 2007

Sampling bias is a common phenomenon in records of plant and animal distribution. Yet, models based on such records usually ignore the potential implications of bias in data collection on the accuracy of model predictions. This study was designed to investigate the effect of roadside bias, one of the most common sources of bias in biodiversity databases, on the accuracy of predictive maps produced by bioclimatic models. Using data on the distribution of 129 species of woody plants in Israel, we tested the following hypotheses: (1) that data collected on woody plant distribution in Israel suffer from roadside bias, (2) that such bias affects the accuracy of model predictions, (3) that the road network of Israel is biased with respect to climatic conditions, and (4) that the impact of roadside bias on model predictions depends on the magnitude of climatic bias in the geographic distribution of the road network.

As expected, the frequency of plant observations near roads was consistently greater than that expected from a spatially random distribution. This bias was most pronounced at distances of 500–2000 m from roads, but it was statistically significant also at larger scales. Predictive maps based on near‐road observations were less accurate than those based on off‐road or “rectified” observations (observations corrected for roadside bias). However, the magnitude of these differences was extremely low, indicating that even a strong bias in the distribution of species observations does not necessarily deteriorate the accuracy of predictive maps generated by bioclimatic models. Further analysis of the data indicated that the road network of Israel is relatively unbiased in terms of temperature, and only weakly biased in terms of rainfall conditions. The overall results are consistent with the hypothesis that the impact of roadside bias on model predictions depends on the magnitude of climatic bias in the geographic distribution of the road network. We discuss some theoretical and practical considerations of bias correction in biodiversity databases.
BACKCASTING AND FORECASTING BIOLOGICAL INVASIONS OF INLAND LAKES
Authors: MacIsaac Hugh J. , Borbely Julianna V. M. , Muirhead Jim R. , Graniero Phil A.
Published: June 2007

Human introduction of nonindigenous species constitutes a serious threat to many ecosystems, particularly lakes. Recent attempts to predict invasions have focused on the supply of propagules of nonindigenous species to recipient ecosystems from source populations. Here we develop a spatially explicit “gravity” model to test this concept forBythotrephes longimanus, a crustacean waterflea from Eurasia that is rapidly invading lakes in Ontario, Canada. The gravity model predicted spread ofBythotrephesbased upon seven identified risk factors (e.g., use of contaminated fishing or boat anchor line) that may allow dispersal of either live individuals or their resting eggs from invaded to noninvaded lakes, as well as based on the spatial arrangement of invaded and noninvaded lakes in Ontario. Discriminant analysis of lake gravity scores successfully identified invasion status for 74% of 170 inland lakes. A retrospective analysis of 31 invaded lakes revealed that the order in which lakes were invaded was directly related to the magnitude of vector inflows from invaded sources. Analysis of the dominant vector inflow to each invaded lake revealed a “stepping stone” pattern in which at least five lakes were sequentially invaded from the source population in Lake Huron. One invaded lake (Muskoka) apparently served as an invasion “hub,” resulting in up to 18 additional direct and 17 indirect invasions. Species spread occurred via a combination of dominant, local diffusion (median distance 12.5 km) and rare, long‐distance (>100 km) dispersal. Eleven of 131 lakes that were not invaded in 2000 were reported invaded in 2001. Gravity scores of these lakes were significantly higher than those of other noninvaded systems, indicating that susceptibility to invasion can be related to the magnitude of vector inflows. A GIS model based on gravity scores indicated that distribution ofBythotrephesis expected to expand to eastern and northwestern Ontario, although most new invasions are expected to occur in the central region of the province. Our results indicate that quantitative analysis of human dispersal vectors provides a robust starting point with which to assess vulnerability of discrete ecosystems to invasion. Management efforts focused on reducing the number and magnitude of human‐mediated dispersal vectors may reduce the rate of invasion of new ecosystems.
A MODEL‐FRAMED EVALUATION OF ELEPHANT EFFECTS ON TREE AND FIRE DYNAMICS IN AFRICAN SAVANNAS
Authors: Baxter Peter W. J. , Getz Wayne M.
Published: June 2007

There is a concern that high densities of elephants in southern Africa could lead to the overall reduction of other forms of biodiversity. We present a grid‐based model of elephant–savanna dynamics, which differs from previous elephant–vegetation models by accounting for woody plant demographics, tree–grass interactions, stochastic environmental variables (fire and rainfall), and spatial contagion of fire and tree recruitment. The model projects changes in height structure and spatial pattern of trees over periods of centuries. The vegetation component of the model produces long‐term tree–grass coexistence, and the emergent fire frequencies match those reported for southern African savannas.

Including elephants in the savanna model had the expected effect of reducing woody plant cover, mainly via increased adult tree mortality, although at an elephant density of 1.0 elephant/km2, woody plants still persisted for over a century. We tested three different scenarios in addition to our default assumptions. (1) Reducing mortality of adult trees after elephant use, mimicking a more browsing‐tolerant tree species, mitigated the detrimental effect of elephants on the woody population. (2) Coupling germination success (increased seedling recruitment) to elephant browsing further increased tree persistence, and (3) a faster growing woody component allowed some woody plant persistence for at least a century at a density of 3 elephants/km2. Quantitative models of the kind presented here provide a valuable tool for exploring the consequences of management decisions involving the manipulation of elephant population densities.
USING MATRIX MODELS TO DETERMINE CROPPING SYSTEM EFFECTS ON ANNUAL WEED DEMOGRAPHY
Authors: Davis Adam S. , Dixon Philip M. , Liebman Matt
Published: June 2007

The design of most agricultural systems ignores the possibility that cropping system characteristics may affect weed population dynamics by altering key demographic rates of weeds. We examined legume green manure and tillage timing effects upon giant foxtail (Setaria faberi) demography with prospective and retrospective perturbation analyses of a periodic matrix population model. Demographic data were collected forS. faberigrown in a wheat–corn–soybean crop sequence in the central USA in 2000 and 2001, with either a wheat sole‐crop (W) or wheat/red clover crop mixture (R) in the wheat phase. Wheat phase residues were incorporated either in fall (FT) or spring (ST) to form four cropping system treatments: FT/W, FT/R, ST/W, and ST/R. Demographic rates estimated from the field data included seed survival from October to March and March to October, seedling recruitment, plant survival, fecundity, and seed predation. The variable efficacy of postemergence weed control was modeled by varying the proportion of seedlings surviving to reproductive maturity from 0.025 to 0.20. Deterministic simulations indicated that there was both interannual and management‐induced variation inS. faberipopulation growth rate. Stochastic population growth rate depended upon an interaction between legume green manure and tillage‐timing effects, such that stochastic population growth rate was lowest for fall‐tilled wheat and red clover, greater for fall‐tilled wheat, and greatest for both spring‐tilled management regimes. Within the fall‐tilled regime, the cropping system (wheat vs. wheat plus clover) had a greater influence on relative rates of stochastic population growth as plant survival increased. However, plant survival did not affect the stochastic population growth rates in the spring‐tilled treatments. Elasticity analysis suggested that seed survival through the winter, fecundity, and the proportion of seeds escaping predation were important driving variables for this system. Retrospective perturbation analysis supported these results but also indicated that fecundity and predation levels varied more in response to changing management treatments than winter seed survival, leading to their having a greater influence on differences in population growth among the four management treatments. Perturbation analyses of matrix population models will aid the development of integrated weed‐management systems by elucidating cropping‐system effects upon weed demography.
IDENTIFYING THE MINIMAL DEMOGRAPHIC UNIT FOR MONITORING POND‐BREEDING AMPHIBIANS
Authors: Petranka James W. , Smith Charles K. , Floyd Scott A.
Published: June 2007

Amphibians are declining globally, and efforts are underway to establish long‐term programs to monitor population trends. Because researchers are constrained to sampling a small subset of populations that occur across a landscape, monitoring units should be selected that maximize the statistical independence of population responses to environmental stressors. Breeding sites of pond‐breeding amphibians are often clustered locally, and habitat switching between neighboring ponds could significantly compromise both demographic and statistical independence. This raises a fundamental issue of whether the basic monitoring unit for trend analysis should be an individual pond or a cluster of local ponds. To help resolve the degree of demographic independence within a cluster of local ponds, we conducted a spatial‐scale analysis of breeding‐population synchrony of the wood frog (Rana sylvatica) and spotted salamander (Ambystoma maculatum) in four national parks in Kentucky, North Carolina, and Tennessee, USA. We used the number of egg masses deposited annually from 1993 through 2002 as an index of relative breeding‐population size and used correlation coefficients of annual percentage change in population size as an index of synchrony.

Within parks, synchrony for both species was independent of the distance between ponds and was lower between paired ponds within a local cluster compared with paired comparisons of geographically isolated groups. These patterns indicate a lack of demographic independence within clusters and are consistent with field observations of wholesale shifting of local pond populations to adjoining ponds. Habitat shifting occurred primarily in response to beaver activity that created new ponds or allowed fish to invade fish‐free habitats. Ambystoma maculatum shifted habitats less frequently and exhibited higher levels of within‐cluster population synchrony than R. sylvatica. Turnovers occurred for local pond populations for both species and at higher rates for R. sylvatica. However, no local pond cluster experienced a turnover event during the study. Our data suggest that pond populations that are only a few hundred meters apart are not demographically independent and are best treated as subpopulations of the same monitoring unit. We discuss the implications of these findings for amphibian‐monitoring programs that are designed to assess population trends.
BIOMETRIC AND MICROMETEOROLOGICAL MEASUREMENTS OF TROPICAL FOREST CARBON BALANCE
Authors: Miller Scott D. , Goulden Michael L. , Menton Mary C. , da Rocha Humberto R. , de Freitas Helber C. , Figueira Adelaine Michela e Silva , Dias de Sousa Cleilim Albert
Published: June 2007

We used two independent approaches, biometry and micrometeorology, to determine the net ecosystem production (NEP) of an old growth forest in Pará, Brazil. Biometric inventories indicated that the forest was either a source or, at most, a modest sink of carbon from 1984 to 2000 (+0.8 ± 2 Mg C·ha−1·yr−1; a positive flux indicates carbon loss by the forest, a negative flux indicates carbon gain). Eddy covariance measurements of CO2 exchange were made from July 2000 to July 2001 using both open‐ and closed‐path gas analyzers. The annual eddy covariance flux calculated without correcting for the underestimation of flux on calm nights indicated that the forest was a large carbon sink (−3.9 Mg C·ha−1·yr−1). This annual uptake is comparable to past reports from other Amazonian forests, which also were calculated without correcting for calm nights. The magnitude of the annual integral was relatively insensitive to the selection of open‐ versus closed‐path gas analyzer, averaging time, detrending, and high‐frequency correction. In contrast, the magnitude of the annual integral was highly sensitive to the treatment of calm nights, changing by over 4 Mg C·ha−1·yr−1 when a filter was used to replace the net ecosystem exchange (NEE) during nocturnal periods with u* < 0.2 m/s. Analyses of the relationship between nocturnal NEE and u* confirmed that the annual sum needs to be corrected for the effect of calm nights, which resulted in our best estimate of the annual flux (+0.4 Mg C·ha−1·yr−1). The observed sensitivity of the annual sum to the u* filter is far greater than has been previously reported for temperate and boreal forests. The annual carbon balance determined by eddy covariance is therefore less certain for tropical than temperate forests. Nonetheless, the biometric and micrometeorological measurements in tandem provide strong evidence that the forest was not a strong, persistent carbon sink during the study interval.
CARBON BALANCE AND VEGETATION DYNAMICS IN AN OLD‐GROWTH AMAZONIAN FOREST
Authors: Rice Amy H. , Pyle Elizabeth Hammond , Saleska Scott R. , Hutyra Lucy , Palace Michael , Keller Michael , de Camargo Plínio B. , Portilho Kleber , Marques Dulcyana F. , Wofsy Steven C.
Published: June 2007

Amazon forests could be globally significant sinks or sources for atmospheric carbon dioxide, but carbon balance of these forests remains poorly quantified. We surveyed 19.75 ha along four 1‐km transects of well‐drained old‐growth upland forest in the Tapajós National Forest near Santarém, Pará, Brazil (2°51′ S, 54°58′ W) in order to assess carbon pool sizes, fluxes, and climatic controls on carbon balance. In 1999 there were, on average, 470 live trees per hectare with diameter at breast height (dbh) ≥10 cm. The mean (and 95% ci) aboveground live biomass was 143.7 ± 5.4 Mg C/ha, with an additional 48.0 ± 5.2 Mg C/ha of coarse woody debris (CWD). The increase of live wood biomass after two years was 1.40 ± 0.62 Mg C·ha−1·yr−1, the net result of growth (3.18 ± 0.20 Mg C·ha−1·yr−1 from mean bole increment of 0.36 cm/yr), recruitment of new trees (0.63 ± 0.09 Mg C·ha−1·yr−1, reflecting a notably high stem recruitment rate of 4.8 ± 0.9%), and mortality (−2.41 ± 0.53 Mg C·ha−1·yr−1 from stem death of 1.7% yr−1). The gain in live wood biomass was exceeded by respiration losses from CWD, resulting in an overall estimated net loss from total aboveground biomass of 1.9 ± 1.0 Mg C·ha−1·yr−1. The presence of large CWD pools, high recruitment rate, and net accumulation of small‐tree biomass, suggest that a period of high mortality preceded the initiation of this study, possibly triggered by the strong El Niño Southern Oscillation events of the 1990s. Transfer of carbon between live and dead biomass pools appears to have led to substantial increases in the pool of CWD, causing the observed net carbon release. The data show that biometric studies of tropical forests neglecting CWD are unlikely to accurately determine carbon balance. Furthermore, the hypothesized sequestration flux from CO2 fertilization (<0.5 Mg C·ha−1·yr−1) would be comparatively small and masked for considerable periods by climate‐driven shifts in forest structure and associated carbon balance in tropical forests.
DIEL AND SEASONAL PATTERNS OF TROPICAL FOREST CO2 EXCHANGE
Authors: Goulden Michael L. , Miller Scott D. , da Rocha Humberto R. , Menton Mary C. , de Freitas Helber C. , e Silva Figueira Adelaine Michela , de Sousa Cleilim Albert Dias
Published: June 2007

We used eddy covariance to measure the net exchange of CO2 between the atmosphere and an old‐growth tropical forest in Pará, Brazil from 1 July 2000 to 1 July 2001. The mean air temperature and daily temperature range varied little year‐round; the rainy season lasted from late December to around July. Daytime CO2 uptake under high irradiance averaged 16–19 μmol·m−2·s−1. Light was the main controller of CO2 exchange, accounting for 48% of the half‐hour‐to‐half‐hour variance. The rate of canopy photosynthesis at a given irradiance was lower in the afternoon than the morning. This photosynthetic inhibition was probably caused by high evaporative demand, high temperature, an intrinsic circadian rhythm, or a combination of the three. Wood increment increased from January to May, suggesting greater rates of carbon sequestration during the wet season. However, the daily net CO2 exchange measured by eddy covariance revealed the opposite trend, with greater carbon accumulation during the dry season. A reduction in respiration during the dry season was an important cause of this seasonal pattern. The surface litter was desiccated in the dry season, and the seasonal pattern of respiration appears to be a direct result of reduced forest floor decomposition during drought. In contrast, canopy photosynthesis was not directly reduced by the dry season, probably because deep rooting allows the forest to avoid drought stress
CLIMATE CHANGE EFFECTS ON VEGETATION DISTRIBUTION, CARBON, AND FIRE IN CALIFORNIA
Authors: Lenihan James M. , Drapek Raymond , Bachelet Dominique , Neilson Ronald P.
Published: June 2007

The objective of this study was to dynamically simulate the response of vegetation distribution, carbon, and fire to the historical climate and to two contrasting scenarios of climate change in California. The results of the simulations for the historical climate compared favorably to independent estimates and observations, but validation of the results was complicated by the lack of land use effects in the model. The response to increasing temperatures under both scenarios was characterized by a shift in dominance from needle‐leaved to broad‐leaved life‐forms and by increases in vegetation productivity, especially in the relatively cool and mesic regions of the state. The simulated response to changes in precipitation were complex, involving not only the effect of changes in soil moisture on vegetation productivity, but also changes in tree–grass competition mediated by fire. Summer months were warmer and persistently dry under both scenarios, so the trends in simulated fire area under both scenarios were primarily a response to changes in vegetation biomass. Total ecosystem carbon increased under both climate scenarios, but the proportions allocated to the wood and grass carbon pools differed. The results of the simulations underscore the potentially large impact of climate change on California ecosystems, and the need for further use and development of dynamic vegetation models using various ensembles of climate change scenarios.
SPATIAL SIGNATURE OF ENVIRONMENTAL HETEROGENEITY, DISPERSAL, AND COMPETITION IN SUCCESSIONAL GRASSLANDS
Authors: Seabloom Eric W. , Bjørnstad Ottar N. , Bolker Benjamin M. , Reichman O. J.
Published: June 2007

The fate of individual plants is strongly influenced by their spatial context (e.g., their location relative to competitors or environmental gradients). Recent theoretical developments indicate that spatial structure in plant communities arises as an outcome of environmental heterogeneity, intra‐ and interspecific competition, and localized dispersal. Furthermore, each of these forces is predicted to leave a signature on the spatial and temporal pattern within the community. We track the development of spatial structure in successional grasslands in southern California for five years, using a series of large‐scale field experiments. We compare the observed changes in spatial structure to the predictions of a spatially explicit model of plant competition in heterogeneous landscapes. We conclude that environmental heterogeneity led to the development of aggregation in the first year at scales of 1–2 m. Over time, aggregation was detectable at distances greater than 4 m, and species become segregated as a result of competition and localized dispersal. Large‐scale disturbances (fire) homogenized the community by decreasing the level of aggregation and segregation. In contrast, disturbance by pocket gophers and ground squirrels increased local‐scale heterogeneity by creating annual‐dominated aggregates. The speed with which spatial structure develops in seemingly homogeneous environments illustrates the importance of incorporating spatial context in empirical studies and models of plant community dynamics.
PATTERNS OF HERBIVORY AND DECOMPOSITION IN AQUATIC AND TERRESTRIAL ECOSYSTEMS
Authors: Cebrian Just , Lartigue Julien
Published: June 2007

Describing the relative magnitude and controls of herbivory and decomposition is important in understanding the trophic transference, recycling, and storage of carbon and nutrients in diverse ecosystems. We examine the variability in herbivory and decomposition between and within a wide range of aquatic and terrestrial ecosystems. We also analyze how that variability is associated with differences in net primary production and producer nutritional quality. Net primary production and producer nutritional quality are uncorrelated between the two types of system or within either type. Producer nutritional quality is correlated to the percentage of primary production consumed by herbivores or percentage of detrital production decomposed annually, regardless of whether the comparison is made between the two types of systems or within either type of system. Thus, producer nutritional quality stands out as a consistent indicator of the importance of consumers as top‐down controls of producer biomass and detritus accumulation and nutrient recycling. However, absolute consumption by herbivores and absolute decomposition (both in g C·m−2·yr−1) are often associated with absolute primary production and independent of producer nutritional quality, because the variability in net primary production across systems largely exceeds that in the percentage consumed or decomposed. Thus, primary production often stands out as an indicator of the absolute flux of producer carbon transferred to consumers and of the potential levels of secondary production maintained in the system. These patterns contribute to our understanding of the variability and control of herbivory and decomposition, and implications on carbon and nutrient cycling, in aquatic and terrestrial ecosystems. Furthermore, in view of their robustness, they may offer a template for global change models seeking to predict anthropogenic effects on carbon and nutrient fluxes.
RAPID EVOLUTION OF AN INVASIVE PLANT
Authors: Maron John L. , Vilà Montserrat , Bommarco Riccardo , Elmendorf Sarah , Beardsley Paul
Published: June 2007

Exotic plants often face different conditions from those experienced where they are native. The general issue of how exotics respond to unfamiliar environments within their new range is not well understood. Phenotypic plasticity has historically been seen as the primary mechanism enabling exotics to colonize large, environmentally diverse areas. However, new work indicates that exotics can evolve quickly, suggesting that contemporary evolution may be more important in invasion ecology than previously appreciated. To determine the influence of contemporary evolution, phenotypic plasticity, and founder effects in affecting phenotypic variation among introduced plants, we compared the size, fecundity, and leaf area of St. John's wort (Hypericum perforatum) collected from native European and introduced western and central North American populations in common gardens in Washington, California, Spain, and Sweden. We also determined genetic relationships among these plants by examining variation in amplified fragment length polymorphism (AFLP) markers.

There was substantial genetic variation among introduced populations and evidence for multiple introductions of H. perforatum into North America. Across common gardens introduced plants were neither universally larger nor more fecund than natives. However, within common gardens, both introduced and native populations exhibited significant latitudinally based clines in size and fecundity. Clines among introduced populations broadly converged with those among native populations. Introduced and native plants originating from northern latitudes generally outperformed those originating from southern latitudes when grown in northern latitude gardens of Washington and Sweden. Conversely, plants from southern latitudes performed best in southern gardens in Spain and California. Clinal patterns in leaf area, however, did not change between gardens; European and central North American plants from northern latitudes had larger leaves than plants from southern latitudes within these regions in both Washington and California, the two gardens where this trait was measured. Introduced plants did not always occur at similar latitudes as their most closely related native progenitor, indicating that pre‐adaptation (i.e., climate matching) is unlikely to be the sole explanation for clinal patterns among introduced populations. Instead, results suggest that introduced plants are evolving adaptations to broad‐scale environmental conditions in their introduced range.
DYNAMICS OF SUBARCTIC WETLAND FORESTS OVER THE PAST 1500 YEARS
Authors: Payette Serge , Delwaide Ann
Published: June 2007

Boreal forests at high latitudes are climate‐sensitive ecosystems that respond directly to environmental forcing by changing their position according to latitude or by changing their abundance at local and regional scales. South of the arctic treeline, external forcing (warming, cooling, drought, fire) necessarily results in the changing abundance of the impacted forests; in particular, the deforestation of well‐drained sites through fire is the most important factor. In this study, we examined the changing abundance of wetland forests located at the arctic treeline (northern Québec, Canada) during the last 1500 years, a period of known contrasting climatic conditions. Black spruce (Picea mariana) trees submerged in small lakes and peatland ponds and soil–peat stratigraphy were used concurrently to reconstruct the millennial‐long developmental sequence of wetland stands associated with moisture changes and fire disturbance. Changing lake levels from AD 300 to the present were identified based on radiocarbon‐dated submerged paleosols and tree‐ring cross‐dating of submerged trees distributed in three wetlands from the same watershed. Dead and living trees in a standing position below and above present water level of a small lake (LE Lake) showed direct evidence of past water levels from the 12th century to the present day. Submerged subfossil trees from another lake (LB Lake) and two peatland ponds (PB Peatland) also responded synchronously to changes in soil moisture during the last 1500 years. Regional‐scale catastrophic flooding around AD 1150, inferred from paleosol and subfossil tree data, eliminated riparian peat and wetland trees growing at least since AD 300. Also, the coincidence of events such as the mass mortality of wetland spruce and post‐fire deforestation of a small hill surrounding LE Lake during the late 1500s suggests the impact of local‐scale flooding, probably attributable to greater snow transportation and accumulation on the lake surface after fire disturbance. Massive tree mortality climaxed at ca. 1750, when all wetland trees at LB Lake and PB Peatland died because of permafrost disturbance and soil upthrusting. Lower water levels from AD 300 to 1750 were associated with drier conditions, possibly caused by greater evaporation and/or reduced snow accumulation. Permafrost development in shallow waters occurred during the Little Ice Age, after 1600. It is concluded that the climate at the eastern Canadian treeline was warmer and drier from AD 300 to the onset of the Little Ice Age and promoted tree establishment. The highest water levels were recorded recently (19th and 20th centuries), causing lake and peatland expansion. Any future moisture changes at these subarctic latitudes will result in important spatial rearrangements of wetland ecosystems.
LIMITING RESOURCES, DISTURBANCE, AND DIVERSITY IN PHYTOPLANKTON COMMUNITIES
Authors: Grover James P. , Chrzanowski Thomas H.
Published: June 2007

Phytoplankton diversity, limiting resources, and disturbance were studied in two reservoirs, Eagle Mountain Lake (EML) and Joe Pool Lake (JPL), in north Texas, USA, for three summer growing seasons and two winters. Availabilities of phosphorus, nitrogen, and silicon were measured as chemical concentrations, and availability of light was measured as irradiance during the photoperiod, averaged over the depth of the surface mixed layer. A resource was defined as limiting whenever its availability fell below a threshold, treated as a parameter to be fitted. Depth of the surface mixed layer and wind speed were taken as indices of disturbance associated with episodic mixing. Hydrological disturbance was gauged by rainfall, inflow, and the variation of lake level. We found that diversity was strongly and significantly correlated with the number of limiting resources in one lake (JPL), but not the other (EML). The onset of nitrogen and phosphorus limitation during the summer growing season is associated with increased phytoplankton diversity in JPL. Regression modeling detected decreasing relationships between diversity and disturbance indices in one lake (JPL), but concave‐down relationships in the other (EML), though the peaks of these relationships did not lie within the usual range of disturbance. Predictive regressions for diversity incorporating both limiting resources and disturbance were partitioned into unique and shared effects of these factors. There was a large unique effect of the number of limiting resources in JPL, but otherwise the shared effects were large relative to unique effects.
MORTALITY SENSITIVITY IN LIFE‐STAGE SIMULATION ANALYSIS: A CASE STUDY OF SOUTHERN SEA OTTERS
Authors: Gerber Leah R. , Tinker M. Tim , Doak Daniel F. , Estes James A. , Jessup David A.
Published: June 2007

Currently, there are no generally recognized approaches for linking detailed mortality and pathology data to population‐level analyses of extinction risk. We used a combination of analytical and simulation‐based analyses to examine 20 years of age‐ and sex‐specific mortality data for southern sea otters (Enhydra lutris), and we applied results to project the efficacy of alternative conservation strategies. Population recovery of the southern sea otter has been slow (rate of population increase λ = 1.05) compared to other recovering populations (λ = 1.17–1.20), and the population declined (λ = 0.975) between 1995 and 1999. Age‐based Leslie matrices were developed to explore explanations for the slow recovery and recent decline in the southern sea other population. An elasticity analysis was performed to predict effects of proportional changes in stage‐specific reproductive or survival rates on the rate of population increase. A life‐stage simulation analysis (LSA) was developed to evaluate the impact of changing age‐ and cause‐specific mortality rates on λ. The information used to develop these models was derived from death assemblage, pathology, and live population census data to examine the sensitivity of sea otter population growth to different sources of mortality (e.g., disease and starvation, direct human take [fisheries, gun shot, boat strike, oil pollution], mating trauma and intraspecific aggression, shark bites, and unknown). We used resampling simulations to generate random combinations of vital rates for a large number of matrix replicates and drew on these to estimate potential effects of mortality sources on population growth (λ). Our analyses suggest management actions that are likely and unlikely to promote recovery of the southern sea otter and more broadly indicate a methodology to better utilize cause‐of‐death data in conservation decision‐making.
BOUNDARIES OF OPEN MARINE ECOSYSTEMS: AN APPLICATION TO THE PRIBILOF ARCHIPELAGO, SOUTHEAST BERING SEA
Authors: Ciannelli Lorenzo , Robson Bruce W. , Francis Robert C. , Aydin Kerim , Brodeur Richard D.
Published: June 2007

We applied ecosystem energetics and foraging theory to characterize the spatial extent of the Pribilof Archipelago ecosystem, located in the southeast Bering Sea. From an energetic perspective, an ecosystem is an area within which the predatory demand is in balance with the prey production. From a foraging perspective, an ecosystem boundary should at least include the foraging range of the species that live within it for a portion of their life cycle. The Pribilof Islands are densely populated by species that adopt a central place foraging strategy. Foraging theory predicts that the area traveled by central place foragers (CPF) should extend far enough so that their predatory demands are in balance with prey production. Thus, the spatial extent of an ecosystem, as defined by energetics and the foraging range of constituent species, will require a similar energy balance, and independent assessments should yield similar results. In this study, we compared the area of maximum energy balance, estimated with a food web model during the decade 1990– 2000, with estimates of the foraging range of northern fur seals (the farthest traveling CPF in the Pribilof Islands community) obtained from the literature. From the food web simulations, we estimated that a circle of 100 nautical miles (NM), or 185.2 km, radius encloses the area of highest energy balance and lowest biomass import and that it represents a switch from a piscivorous‐dominated (smaller areas) to a zooplanktivorous‐dominated (larger areas) community. The distance from the breeding site to locations recorded at sea for lactating female fur seals, during the years 1995–1996, ranged from 5.0 to 172.2 NM (9.3–318.9 km), with a median of 97 NM (179.6 km). Thus, ∼50% of the locations recorded for lactating fur seals occurred beyond the area of energy balance estimated by the model, indicating that additional factors can motivate their foraging extent. We propose that energetic constraints set the minimum extent of the Pribilof ecosystem, while the foraging distance of fur seals may indicate the maximum extent. In discussing these results, we highlight the limitations of current definitions of the spatial extent in ecosystems, when related to open oceanic environments, and discuss viable alternatives to characterize boundaries of aquatic systems that are not physically separated from adjacent areas. We believe that these arguments, though controversial, are very timely given the increased emphasis currently placed on the management and protection of entire marine ecosystems.
LONG‐TERM TREND IN THE MATURATION REACTION NORM OF TWO COD STOCKS
Authors: Barot S. , Heino M. , O'Brien L. , Dieckmann U.
Published: June 2007

Average age and size at maturation have decreased in many commercially exploited fish stocks during the last decades. This phenomenon could be either a direct phenotypic response to some environmental variation or the evolutionary consequence of some selective pressure. Traditionally used maturation indices, e.g., the age and size at which 50% of individuals are mature, are not appropriate to assess the causes of changes in maturation because they are influenced, in addition to maturation per se, by growth and survival. To make up for this shortcoming, we use a reaction‐norm‐based approach to disentangle evolutionary changes and phenotypic plasticity. A method is presented to estimate the reaction norm for age and size at maturation from data commonly gathered for the management of fisheries. This method is applied to data on Georges Bank and Gulf of Maine stocks of Atlantic cod (Gadus morhua). The results show that maturation reaction norms in these stocks have shifted significantly downward, resulting in a tendency to mature earlier at smaller size. These findings support the hypothesis that an evolutionary trend, probably caused by high fishing mortalities, is partially responsible for the observed decrease in age and size at maturation in these cod stocks. Two independent reasons justify this interpretation. First, there is no corresponding trend in growth that would suggest that improved feeding conditions could have facilitated maturation. Second, the results are based on maturation reaction norms, from which the known confounding effects of the growth and mortality variation are removed. Consequences of fisheries‐induced evolution for the sustainability of the fishery are discussed.
MARINE RESERVE BENEFITS LOCAL FISHERIES
Authors: Russ Garry R. , Alcala Angel C. , Maypa Aileen P. , Calumpong Hilconida P. , White Alan T.
Published: June 2007

The utility of no‐take marine reserves as fisheries‐management tools is controversial. It is hypothesized that marine reserves will help to sustain fisheries external to them by becoming net exporters of adults (the “spillover effect”) and net exporters of propagules (the “recruitment effect”). Local fishery benefits from spillover will likely generate support from fishing communities for marine reserves. We used underwater visual census to show that biomass of Acanthuridae (surgeonfish) and Carangidae (jacks), two families of reef fish that account for 40–75% of the fishery yield from Apo Island, Philippines, tripled in a well‐protected no‐take reserve over 18 years (1983–2001). Biomass of these families did not change significantly over the same period at a site open to fishing. The reserve protected 10% of the total reef fishing area at the island. Outside the reserve, biomass of these families increased significantly closer to (200–250 m) than farther away from (250–500 m) the reserve boundary over time. We used published estimates of fishery catch and effort, and fisher interviews (creel surveys) to show that the total catch of Carangidae and Acanthuridae combined at Apo Island was significantly higher after (1985–2001) than before (1981) reserve establishment. Hook‐and‐line catch per unit effort (CPUE) at the island was 50% higher during 1998–2001 (reserve protected 16–19 years) than during 1981–1986 (pre‐reserve and early phases of reserve protection). Total hook‐and‐line effort declined by 46% between 1986 and 1998–2001. Hook‐and‐line CPUE of Acanthuridae was significantly higher close to (within 200 m) than far from the reserve. CPUE of Carangidae was significantly higher away from the reserve, possibly reflecting a local oceanographic effect. The benefits of the reserve to local fisheries at the island were higher catch, increased catch rate, and a reduction in fishing effort. The fishery and tourism benefits generated by the reserve have enhanced the living standard of the fishing community.
FISHING FOR LOBSTERS INDIRECTLY INCREASES EPIDEMICS IN SEA URCHINS
Authors: Lafferty Kevin D.
Published: June 2007

Two ecological paradigms, the trophic cascade and the host‐density threshold in disease, interact in the kelp‐forest ecosystem to structure the community. To investigate what happens when a trophic cascade pushes a host population over a host‐threshold density, I analyzed a 20‐year data set of kelp forest communities at 16 sites in the region of the Channel Islands National Park, California, USA. Historically, lobsters, and perhaps other predators, kept urchin populations at low levels and kelp forests developed a community‐level trophic cascade. In geographic areas where the main predators on urchins were fished, urchin populations increased to the extent that they overgrazed algae and starvation eventually limited urchin‐population growth. Despite the limitation of urchin population size by food availability, urchin densities, at times, well exceeded the host‐density threshold for epidemics. An urchin‐specific bacterial disease entered the region after 1992 and acted as a density‐dependent mortality source. Dense populations were more likely to experience epidemics and suffer higher mortality. Disease did not reduce the urchin population at a site to the density that predators previously did. Therefore, disease did not fully replace predators in the trophic cascade. These results indicate how fishing top predators can indirectly favor disease transmission in prey populations.
CRITICAL THRESHOLDS ASSOCIATED WITH HABITAT LOSS FOR TWO VERNAL POOL‐BREEDING AMPHIBIANS
Authors: Homan Rebecca Newcomb , Windmiller Bryan S. , Reed J. Michael
Published: June 2007

A critical threshold exists when the relationship between the amount of suitable habitat and population density or probability of occurrence exhibits a sudden, disproportionate decline as habitat is lost. Critical thresholds are predicted by a variety of modeling approaches, but empirical support has been limited or lacking. We looked for critical thresholds in two pool‐breeding amphibians that spend most of the year in adjacent upland forest: the spotted salamander (Ambystoma maculatum) and the wood frog (Rana sylvatica). These species were selected because of their reported poor dispersal capacities and their dependency on forest habitat when not breeding. Using piecewise regression and binomial change‐point tests, we looked for a relationship between the probability of occupancy of a site and forest cover at five spatial scales, measuring forest cover in radial distances from the pond edge of suitable breeding ponds: 30 m, 100 m, 300 m, 500 m, and 1000 m. Using piecewise regression, we identified significant thresholds for spotted salamanders at the 100‐m and 300‐m spatial scale, and for wood frogs at the 300‐m scale. However, binomial change‐point tests identified thresholds at all spatial scales for both species, with the location of the threshold (percent habitat cover required) increasing with spatial scale for spotted salamanders and decreasing with spatial scale for wood frogs. Thresholds for spotted salamanders occurred at ∼30% forest cover at spatial scales of 100 m or less, with 41% cover at 500 m, and with 51% habitat cover at 1000 m. Thresholds for wood frogs ranged from 88% habitat cover at 30 m from the pond edge, declining to 44% habitat cover within 1000 m. These patterns might be explained by the different winter dispersions of these species. Knowing whether a species has a critical threshold, and at what level of cover and at what spatial scale it exists, would be essential for conservation of habitat‐sensitive species.
POPULATION STRUCTURE AND INBREEDING VARY WITH SUCCESSIONAL STAGE IN CREATED SPARTINA ALTERNIFLORA MARSHES
Authors: Travis Steven E. , Proffitt C. Edward , Ritland Kermit
Published: June 2007

Recruitment patterns in clonal plant populations are predicted to vary with seed dispersal capability and disturbance regime, such that species with small, widely dispersed seeds will become increasingly dominated by vegetative recruitment on disturbed areas following early colonization. Subsequent mortality due to competitive or stochastic effects is then predicted to cause a gradual decline in both clonal diversity and the ability of surviving clones to avoid geitonogamous mating and possible inbreeding depression. We tested predictions of these hypotheses by comparing four adjacent populations of the salt marsh plant, Spartina alterniflora, ranging in age from 2 to ∼50 yr, by measuring fine‐scale genetic structure at the level of both ramets and genets, and the rate of inbreeding. For this purpose, we sampled maternal tissue and seeds from discrete patches in the field and then genotyped both maternal and seedling tissue (germinated in a growth chamber) using standard molecular protocols. As predicted, we observed an increase in clonal diversity (measured as the complement of the Simpson Index corrected for finite sample sizes, 1 − D) up to a maximum of 0.71 within 3‐m2 patches at 16 yr, declining to 0.55 by ∼50 yr. Local recruitment of seedlings was evident as genetic structure occurring at the level of patches, as measured by the fixation index, θ, which was inversely correlated with diversity (R2 > 0.90 at all patch scales). Outcrossing rates were positively associated with clonal diversity, with the highest level (89%) at an intermediate level of 1 − D. The greatest selfing (32%) occurred in young (2‐yr‐old) patches with low diversity. Biparental inbreeding was minimal in all populations, never exceeding 1%. Inbreeding depression was inferred to be severe, as evidenced by near‐zero adult inbreeding coefficients. These results suggest a possible fitness trade‐off between clonal growth and the opportunity for outcrossing. We recommend that restoration plantings of clonal species with limited sexual recruitment capabilities should be designed to ensure adequate clonal diversity for the avoidance of inbreeding and the ability to adapt to subsequent environmental disturbances.