bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of forest clear‐cutting on subtropical bryophyte communities in waterfalls, on dripping walls, and along streams
Authors: Patiño Jairo , Hylander Kristoffer , González-Mancebo Juana M.
Published: June 2010

Forested freshwater ecosystems worldwide are threatened by a number of anthropogenic disturbances, such as water pollution and canalization. Transient or permanent deforestation can also be a serious threat to organisms in forested watersheds, but its effects on different types of freshwater systems has been little studied. We investigated lotic bryophyte communities on rock and soil in subtropical cloud laurel forests on La Gomera Island in the Canary Islands, Spain, and asked whether the response to forest clear‐cutting varied among the communities associated with dripping walls, streams, and waterfalls. We compared three successional forest stages: ancient forests (>250 years), young forests (20–50 years after clear‐cutting), and open stands (5–15 years after clear‐cutting). In each of 56 study sites we sampled general vegetation and substrate data in a 0.01‐ha plot and took composition data of bryophyte species in 3 + 3 subplots of 1 × 1 m. The general pattern of decline in species richness and change in species composition after forest clear‐cutting was stronger for streamside assemblages compared to assemblages on dripping walls and in waterfalls. The change in species numbers on rocks was larger than that on soils, because a guild of species growing on soil (but not on rocks) were favored by disturbance and thus increased in the disturbed sites. Most of the sensitive species could be classified as typical laurel forest species. Mosses were generally more tolerant to forest clear‐cutting than were liverworts. We suggest that streamsides are more sensitive to disturbance than waterfalls and dripping walls because of a larger variation in microclimate before than after clear‐cutting and because they are more easily invaded by early‐successional species (both bryophytes and highly competitive vascular plants). We propose that special care should be taken along small streams within disturbed watersheds if bryophyte assemblages and threatened species should be protected. The susceptibility to anthropogenic pressures is probably rather high in ecosystems that do not regularly experience large‐scale stand‐replacing disturbances, especially on oceanic islands because of isolation and a small total habitat area for focal organisms.
Limited evidence for the demographic Allee effect from numerous species across taxa
Authors: Gregory Stephen D. , Bradshaw Corey J. A. , Brook Barry W. , Courchamp Franck
Published: June 2010

Extensive theoretical work on demographic Allee effects has led to the latent assumption that they are ubiquitous in natural populations, yet current empirical support for this phenomenon is sparse. We extended previous single‐taxon analyses to evaluate the empirical support for demographic Allee effects in the per capita population growth rate of 1198 natural populations spanning all major taxa. For each population, we quantified the empirical support for five population growth models: no growth (random walk); exponential growth, with and without an Allee effect; and logistic growth, with and without an Allee effect. We used two metrics to quantify empirical support, information‐theoretic and Bayesian strength of evidence, and observed top‐rank frequency. The Ricker logistic model was both the most supported and most frequently top‐ranked model, followed by random walk. Allee models had a combined relative support of 12.0% but were top‐ranked in only 1.1% of the time series. Accounting for local climate variation and measurement error caused the loss of top‐ranked Allee models, although the latter also increased their relative support. The 13 time series exhibiting Allee models were shorter and less variable than other time series, although only three were non‐trending. Time series containing observations at low abundance were not more likely and did not show higher support for Allee effect models. We conclude that there is relatively high potential for demographic Allee effects in these 1198 time series but comparatively few observed cases, perhaps due to the influences of climate and measurement error.
Bill size and dimorphism in tidal‐marsh sparrows: island‐like processes in a continental habitat
Authors: Greenberg Russell , Olsen Brian
Published: June 2010

Conditions favoring population divergence in trophic features, such as the low levels of species richness and interspecific competition found on islands, can be similar to conditions that increase their sexual dimorphism or overall variance. Male emberizid sparrows of tidal marshes have undergone parallel evolution of large bills. We tested for parallel increases between dimorphism and overall variation in bill size by comparing three groups totaling 30 sparrow subspecies: tidal‐marsh sparrows, nontidal relatives of tidal‐marsh taxa, and representative sparrow taxa. Bill size (and not other features) showed the following patterns in tidal‐marsh sparrows compared to nontidal relatives or sparrows at large: (1) an increase; (2) a greater increase in males than females; (3) an increase in sexual dimorphism; and (4) greater variation in females. A high degree of sexual dimorphism in bill size is consistent with the hypothesis that low levels of interspecific and high levels of intraspecific competition select for intraspecific niche divergence. Alternatively, increased sexual selection in tidal‐marsh sparrows, vis‐à‐vis high densities and hence increased male–male competition, may account for the differentially large increase in bill size in males. Relaxed natural selection due to high ecosystem productivity and low interspecific competition may explain why, in tidal‐marsh sparrows, female bills have diverged less than males and show higher levels of variability at larger sizes. Both the niche divergence and sexual selection hypotheses depend upon processes, particularly increases in population density, that are similar to those often reported for island passerines. However, the low species diversity and increased intraspecific competition of salt marsh faunas is probably a result of abiotic constraints on colonization (tides and salinity) rather than the isolating distances of island biotas. Thus, both a shift in bill size and increases in its dimorphism and variability may be favored by high productivity and abiotic constraints.
Mycorrhizal networks counteract competitive effects of canopy trees on seedling survival
Authors: Booth Michael G. , Hoeksema Jason D.
Published: June 2010

The dynamics of forest ecosystems depend largely on the survival of seedlings in their understories, but seedling survival is known to be limited by preemption of light and soil resources by overstory trees. It has been hypothesized that “common mycorrhizal networks,” wherein roots of seedlings are linked to overstory trees by symbiotic mycorrhizal fungi, offset some or all of the negative effects of trees on seedlings. Here we report the results of an unambiguous experimental test of this hypothesis in a monodominant Pinus radiata forest. We also tested the hypothesis that adaptive differentiation among plant populations causes local plant genotypes to respond more positively to mycorrhizal networks than nonlocal plant genotypes. Our results demonstrate large positive effects of overstory mycorrhizal networks on seedling survival, along with simultaneous negative effects of tree roots, regardless of whether plant genotypes were locally derived. Physiological and leaf‐chemistry measurements suggest that seedlings connected to common mycorrhizal networks benefited from increased access to soil water. The similar magnitude of the positive and negative overstory effects on seedlings and the ubiquity of mycorrhizal networks in forests suggest that mycorrhizal networks fundamentally influence the demographic and community dynamics of forest trees.
Using expert judgment to estimate marine ecosystem vulnerability in the California Current
Authors: Teck Sarah J. , Halpern Benjamin S. , Kappel Carrie V. , Micheli Fiorenza , Selkoe Kimberly A. , Crain Caitlin M. , Martone Rebecca , Shearer Christine , Arvai Joe , Fischhoff Baruch , Murray Grant , Neslo Rabin , Cooke Roger
Published: June 2010

As resource management and conservation efforts move toward multi‐sector, ecosystem‐based approaches, we need methods for comparing the varying responses of ecosystems to the impacts of human activities in order to prioritize management efforts, allocate limited resources, and understand cumulative effects. Given the number and variety of human activities affecting ecosystems, relatively few empirical studies are adequately comprehensive to inform these decisions. Consequently, management often turns to expert judgment for information. Drawing on methods from decision science, we offer a method for eliciting expert judgment to (1) quantitatively estimate the relative vulnerability of ecosystems to stressors, (2) help prioritize the management of stressors across multiple ecosystems, (3) evaluate how experts give weight to different criteria to characterize vulnerability of ecosystems to anthropogenic stressors, and (4) identify key knowledge gaps. We applied this method to the California Current region in order to evaluate the relative vulnerability of 19 marine ecosystems to 53 stressors associated with human activities, based on surveys from 107 experts. When judging the relative vulnerability of ecosystems to stressors, we found that experts primarily considered two criteria: the ecosystem's resistance to the stressor and the number of species or trophic levels affected. Four intertidal ecosystems (mudflat, beach, salt marsh, and rocky intertidal) were judged most vulnerable to the suite of human activities evaluated here. The highest vulnerability rankings for coastal ecosystems were invasive species, ocean acidification, sea temperature change, sea level rise, and habitat alteration from coastal engineering, while offshore ecosystems were assessed to be most vulnerable to ocean acidification, demersal destructive fishing, and shipwrecks. These results provide a quantitative, transparent, and repeatable assessment of relative vulnerability across ecosystems to any ongoing or emerging human activity. Combining these results with data on the spatial distribution and intensity of human activities provides a systematic foundation for ecosystem‐based management.
Inferential permutation tests for maximum entropy models in ecology
Authors: Shipley Bill
Published: June 2010

Maximum entropy (maxent) models assign probabilities to states that (1) agree with measured macroscopic constraints on attributes of the states and (2) are otherwise maximally uninformative and are thus as close as possible to a specified prior distribution. Such models have recently become popular in ecology, but classical inferential statistical tests require assumptions of independence during the allocation of entities to states that are rarely fulfilled in ecology. This paper describes a new permutation test for such maxent models that is appropriate for very general prior distributions and for cases in which many states have zero abundance and that can be used to test for conditional relevance of subsets of constraints. Simulations show that the test gives correct probability estimates under the null hypothesis. Power under the alternative hypothesis depends primarily on the number and strength of the constraints and on the number of states in the model; the number of empty states has only a small effect on power. The test is illustrated using two empirical data sets to test the community assembly model of B. Shipley, D. Vile, and E. Garnier and the species abundance distribution models of S. Pueyo, F. He, and T. Zillio.
Impact of an invasive oak gall wasp on a native butterfly: a test of plant‐mediated competition
Authors: Prior Kirsten M. , Hellmann Jessica J.
Published: June 2010

Phytophagous insects commonly interact through shared host plants. These interactions, however, do not occur in accordance with traditional paradigms of competition, and competition in phytophagous insects is still being defined. It remains unclear, for example, if particular guilds of insects are superior competitors or important players in structuring insect communities. Gall‐forming insects are likely candidates for such superior competitors because of their ability to manipulate host plants, but their role as competitors is understudied. We investigate the effect of invasive populations of an oak gall wasp, Neuroterus saltatorius, on a native specialist butterfly, Erynnis propertius, as mediated by their shared host plant, Quercus garryana. This gall wasp occurs at high densities in its introduced range, where we stocked enclosures with caterpillars on trees that varied in gall wasp density. Biomass production of butterflies was lower in enclosures on high‐density than on low‐density trees because overwintering caterpillars were smaller, and fewer of them eclosed into adults the following spring. To see if the gall wasp induced changes in foliar quality, we measured host plant quality before and after gall induction on 30 trees each at two sites. We found a positive relationship between gall wasp density and the percentage change in foliar C:N, a negative relationship between gall wasp density and the percentage change in foliar water at one site, and no relationship between the percentage change in protein‐binding capacity (i.e., phenolics) and gall‐wasp density. Additionally, there was a negative relationship between foliar quality and butterfly performance. Our results provide evidence for a plant‐mediated impact of an invasive oak gall wasp on a native butterfly and suggest that gall wasps could act as superior competitors, especially when they occur at high densities.
Interactive effects of land use history and natural disturbance on seedling dynamics in a subtropical forest
Authors: Comita Liza S. , Thompson Jill , Uriarte Maria , Jonckheere Inge , Canham Charles D. , Zimmerman Jess K.
Published: June 2010

Human‐impacted forests are increasing in extent due to widespread regrowth of secondary forests on abandoned lands. The degree and speed of recovery from human disturbance in these forests will determine their value in terms of biodiversity conservation and ecosystem function. In areas subject to periodic, severe natural disturbances, such as hurricanes, it has been hypothesized that human and natural disturbance may interact to either erase or preserve land use legacies. To increase understanding of how interactions between human and natural disturbance influence forest regeneration and recovery, we monitored seedlings in a human‐ and hurricane‐impacted forest in northeastern Puerto Rico over a ∼10‐yr period and compared seedling composition and dynamics in areas that had experienced high‐ and low‐intensity human disturbance during the first half of the 20th century. We found that land use history significantly affected the composition and diversity of the seedling layer and altered patterns of canopy openness and seedling dynamics following hurricane disturbance. The area that had been subject to high‐intensity land use supported a higher density, but lower diversity, of species. In both land use history categories, the seedling layer was dominated by the same two species, Prestoea acuminata var. montana and Guarea guidonia. However, seedlings of secondary‐successional species tended to be more abundant in the high‐intensity land use area, while late‐successional species were more abundant in the low‐intensity area, consistent with patterns of adult tree distributions. Seedlings of secondary‐forest species showed greater increases in growth and survival following hurricane disturbance compared to late‐successional species, providing support for the hypothesis that hurricanes help preserve the signature of land use history. However, the increased performance of secondary‐forest species occurred predominantly in the low‐intensity land use area, suggesting that hurricanes act to homogenize differences in species composition between areas with differing land use histories by increasing secondary‐forest species regeneration in areas that experienced little direct human disturbance. Our results suggest that, through effects on seedling dynamics, hurricanes may extend the signature of land use history beyond the average recovery time of forests not subject to intense natural disturbance events.
Tropical forest soil microbial communities couple iron and carbon biogeochemistry
Authors: Dubinsky Eric A. , Silver Whendee L. , Firestone Mary K.
Published: June 2010

We report that iron‐reducing bacteria are primary mediators of anaerobic carbon oxidation in upland tropical soils spanning a rainfall gradient (3500–5000 mm/yr) in northeast Puerto Rico. The abundant rainfall and high net primary productivity of these tropical forests provide optimal soil habitat for iron‐reducing and iron‐oxidizing bacteria. Spatially and temporally dynamic redox conditions make iron‐transforming microbial communities central to the belowground carbon cycle in these wet tropical forests. The exceedingly high abundance of iron‐reducing bacteria (up to 1.2 × 109 cells per gram soil) indicated that they possess extensive metabolic capacity to catalyze the reduction of iron minerals. In soils from the higher rainfall sites, measured rates of ferric iron reduction could account for up to 44% of organic carbon oxidation. Iron reducers appeared to compete with methanogens when labile carbon availability was limited. We found large numbers of bacteria that oxidize reduced iron at sites with high rates of iron reduction and large numbers of iron reducers. The coexistence of large populations of iron‐reducing and iron‐oxidizing bacteria is evidence for rapid iron cycling between its reduced and oxidized states and suggests that mutualistic interactions among these bacteria ultimately fuel organic carbon oxidation and inhibit CH4 production in these upland tropical forests.
Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover
Authors: Merritt David M. , Nilsson Christer , Jansson Roland
Published: June 2010

The spatial distribution and temporal availability of propagules fundamentally constrain plant community development. This study experimentally tested several hypotheses about the relative roles of wind and water dispersal in colonization and development of riparian communities along rivers. Through controlling the source of propagules (dispersed by wind, water, or both) reaching newly created, bare river margin sites, we isolated the relative roles of dispersal and other factors in plant community development over five years. Replicated treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free‐flowing. Bare river margins receiving only water‐dispersed propagules had significantly higher species richness compared to plots receiving only wind‐dispersed propagules during the initial two years of colonization. Species richness increased annually throughout the study along tranquil and turbulent reaches of the free‐flowing river but reached an asymptote at comparatively low richness after a single year on the impounded river. Propagule source strongly influenced species richness during the initial establishment along both rivers, with richness being significantly higher in plots receiving water‐dispersed seeds. This strong treatment effect continued to be important through time along the regulated river but diminished in importance along the free‐flowing river where other factors such as soil moisture, light availability, and exposure of sites to fluvial disturbance overshadowed the influence of dispersal pathway in mediating species richness. This suggests that hydrochory (plant dispersal by water) may be more important for maintenance of diversity in regulated systems where long‐distance dispersal is absent or negligible, but that the rich local propagule source along free‐flowing rivers supports high species richness. The number of unique species was higher in water‐dispersal plots along both the regulated and free‐flowing rivers. This result suggests that hydrochory may contribute to temporal variability of sites, may enhance richness over time, and may have an important role in meta‐population and meta‐community dynamics of plant communities through long‐distance (and local) dispersal and chance colonization. Our findings provide experimental evidence that water dispersal of plant propagules influences colonization dynamics and is important for long‐term community development in riparian zones.
Unite research with what citizens do for fun: “recreational monitoring” of marine biodiversity
Authors: Goffredo Stefano , Pensa Francesco , Neri Patrizia , Orlandi Antonio , Gagliardi Maria Scola , Velardi Angela , Piccinetti Corrado , Zaccanti Francesco
Published: June 2010

Institutes often lack funds and manpower to perform large‐scale biodiversity monitoring. Citizens can be involved, contributing to the collection of data, thus decreasing costs. Underwater research requires specialist skills and SCUBA certification, and it can be difficult to involve volunteers. The aim of this study was to involve large numbers of recreational divers in marine biodiversity monitoring for increasing the environmental education of the public and collecting data on the status of marine biodiversity. Here we show that thousands of recreational divers can be enrolled in a short time. Using specially formulated questionnaires, nonspecialist volunteers reported the presence of 61 marine taxa encountered during recreational dives, performed as regular sport dives. Validation trials were carried out to assess the accuracy and consistency of volunteer‐recorded data, and these were compared to reference data collected by an experienced researcher. In the majority of trials (76%) volunteers performed with an accuracy and consistency of 50–80%, comparable to the performance of conservation volunteer divers on precise transects in other projects. The recruitment of recreational divers involved the main diving and tour operators in Italy, a popular scientific magazine, and mass media. During the four‐year study, 3825 divers completed 18 757 questionnaires, corresponding to 13 539 diving hours. The volunteer‐sightings‐based index showed that in the monitored area the biodiversity status did not change significantly within the project time scale, but there was a significant negative correlation with latitude, suggesting improved quality in the southernmost areas. This trend could be related to the presence of stressors in the northern areas and has been supported by investigations performed by the Italian Ministry of the Environment. The greatest limitation with using volunteers to collect data was the uneven spatial distribution of samples. The benefits were the considerable amounts of data collected over short time periods and at low costs. The successful development of citizen‐based monitoring programs requires open‐mindedness in the academic community; advantages of citizen involvement in research are not only adding large data sets to the ecological knowledge base but also aiding in the environmental education of the public.
Understanding ecosystem retrogression
Authors: Peltzer Duane A. , Wardle David A. , Allison Victoria J. , Baisden W. Troy , Bardgett Richard D. , Chadwick Oliver A. , Condron Leo M. , Parfitt Roger L. , Porder Stephen , Richardson Sarah J. , Turner Benjamin L. , Vitousek Peter M. , Walker Joe , Walker Lawrence R.
Published: June 2010

Over time scales of thousands to millions of years, and in the absence of rejuvenating disturbances that initiate primary or early secondary succession, ecosystem properties such as net primary productivity, decomposition, and rates of nutrient cycling undergo substantial declines termed ecosystem retrogression. Retrogression results from the depletion or reduction in the availability of nutrients, and can only be reversed through rejuvenating disturbance that resets the system; this differs from age‐related declines in forest productivity that are driven by shorter‐term depression of nutrient availability and plant ecophysiological process rates that occur during succession. Here we review and synthesize the findings from studies of long‐term chronosequences that include retrogressive stages for systems spanning the boreal, temperate, and subtropical zones. Ecosystem retrogression has been described by ecologists, biogeochemists, geologists, and pedologists, each of which has developed somewhat independent conceptual frameworks; our review seeks to unify this literature in order to better understand the causes and consequences of retrogression. Studies of retrogression have improved our knowledge of how long‐term pedogenic changes drive shorter‐term biological processes, as well as the consequences of these changes for ecosystem development. Our synthesis also reveals that similar patterns of retrogression (involving reduced soil fertility, predictable shifts in organismic traits, and ecological processes) occur in systems with vastly different climatic regimes, geologic substrates, and vegetation types, even though the timescales and mechanisms driving retrogression may vary greatly among sites. Studies on retrogression also provide evidence that in many regions, high biomass or “climax” forests are often transient, and do not persist indefinitely in the absence of rejuvenating disturbance. Finally, our review highlights that studies on retrogressive chronosequences in contrasting regions provide unparalleled opportunities for developing general principles about the long‐term feedbacks between biological communities and pedogenic processes, and how these control ecosystem development.
Managing the matrix: decadal responses of eucalypt‐dominated savanna to ambient fire regimes
Authors: Russell-Smith Jeremy , Price Owen F. , Murphy Brett P.
Published: June 2010

Much of our understanding of the response of savanna systems to fire disturbance relies on observations derived from manipulative fire plot studies. Equivocal findings from both recent Australian and African savanna fire plot assessments have significant implications for informing conservation management and reliable estimation of biomass stocks and dynamics. Influential northern Australian replicated fire plot studies include the 24‐year plot‐scale Munmarlary and the five‐year catchment‐scale Kapalga, mesic savanna (>1000 mm/yr of rainfall) experiments in present‐day Kakadu National Park. At Munmarlary, under low‐to‐moderate‐intensity fire treatments, woody vegetation dominated by mature eucalypts was found to be structurally stable. At Kapalga, substantial declines in woody biomass were observed under more intense fire treatments, and modeling assessments implicate early‐season fires as having adverse effects on longer‐term tree recruitment. Given these contrasting perspectives, here we take advantage of a landscape‐scale fire response monitoring program established on three major northern Australian conservation reserves (Kakadu, Litchfield, and Nitmiluk National Parks). Using statistical modeling we assess the decadal effects of ambient fire regime parameters (fire frequency, severity, seasonality, time since fire) on 32 vegetation structure components and abundance of 21 tree and 16 grass species from 122 monitoring plots. Over the study period the mean annual frequency of burning of plots was 0.53, comprising mostly early‐dry‐season, low‐severity fires. Structural and species responses were variable but often substantial, notably resulting in stem recruitment and declines in juveniles, but only weakly explained by fire regime and habitat variables. Modeling of these observations under three realistic scenarios (increased fire severity under projected worsening climate change; modest and significant reductions in fire frequency to meet conservation criteria) indicates that all scenarios have positive and negative structural implications. Effecting significant regional fire regime change (e.g., reduction in frequency and size of severe fires) is demonstrably feasible, but it incurs risks and potentially some undesirable structural consequences. Given recent Australian and African experience, the generality and application of landscape‐scale implications derived from manipulative fire assessments (including variable grazing and browsing regimes) in savanna require more critical assessment.
Direct effects of habitat area on interaction diversity in pollination webs
Authors: Sabatino Malena , Maceira Néstor , Aizen Marcelo A.
Published: June 2010

Island biogeography theory predicts that species richness increases with habitat area and declines with isolation. We expand this framework to address changes in the number of links and species in pollination webs from 12 isolated hills, ranging in area from tens to thousands of hectares, immersed in the agriculture matrix of the Argentine Pampas. We also studied whether total interaction frequency is partitioned more evenly among individual links in richer webs. Our results reveal a direct effect of area on the number of links and species present in each pollination web. However, link richness increased twofold faster than species richness with area. These area effects were not confounded by sampling effort or correlated incidence of exotic species, despite widespread habitat disturbance. Habitat proximity, an inverse measure of isolation, had a marginally significant influence on link but not on species richness. Increased link number was associated with decreasing dominance by any particular interaction and increasing interaction evenness. Despite the strong area effect, a rich pollination web sampled from a small, protected sierra suggests that simple conservation measures, such as reduced grazing and fire suppression, may effectively preserve much local interaction diversity.
Functional trait and phylogenetic tests of community assembly across spatial scales in an Amazonian forest
Authors: Kraft Nathan J. B. , Ackerly David D.
Published: June 2010

Despite a long history of the study of tropical forests, uncertainty about the importance of different ecological processes in shaping tropical tree species distributions persists. Trait‐ and phylogenetic‐based tests of community assembly provide a powerful way to detect community assembly processes but have seldom been applied to the same community. Both methods are well suited to testing how the relative importance of different ecological processes changes with spatial scale. Here we apply both methods to the Yasuní Forest Dynamics Plot, a 25‐ha Amazonian forest with >1100 tree species. We found evidence for habitat filtering from both trait and phylogenetic methods from small (25 m2) to intermediate (10 000 m2) spatial scales. Trait‐based methods detected even spacing of strategies, a pattern consistent with niche partitioning or enemy‐mediated density dependence, at smaller spatial scales (25–400 m2). Simulation modeling of community assembly processes suggests that low statistical power to detect even spacing of traits at larger spatial scales may contribute to the observed patterns. Trait and phylogenetic methods tended to identify the same areas of the forest as being subject to habitat filtering. Phylogenetic community tests, which are far less data‐intensive than trait‐based methods, captured much of the same filtering patterns detected by trait‐based methods but often failed to detect even‐spacing patterns apparent in trait data. Taken together, it appears that both habitat associations and niche differentiation shape species co‐occurrence patterns in one of the most diverse forests in the world at a range of small and intermediate spatial scales.
Allometric scaling predicts preferences for burned patches in a guild of East African grazers
Authors: Sensenig Ryan L. , Demment Montague W. , Laca Emilio A.
Published: June 2010

The high herbivore diversity in savanna systems has been attributed to the inherent spatial and temporal heterogeneity related to the quantity and quality of food resources. Allometric scaling predicts that smaller‐bodied grazers rely on higher quality forage than larger‐bodied grazers. We replicated burns at varying scales in an East African savanna and measured visitation by an entire guild of larger grazers ranging in size from hare to elephant. We found a strong negative relationship between burn preference and body mass with foregut fermenters preferring burns to a greater degree than hindgut fermenters. Burns with higher quality forage were preferred more than burns with lower quality forage by small‐bodied grazers, while the opposite was true for large‐bodied grazers. Our results represent some of the first experimental evidence demonstrating the importance of body size in predicting how large herbivores respond to fire‐induced changes in plant quality and quantity.
Parasitism in a community context: trait‐mediated interactions with competition and predation
Authors: Raffel Thomas R. , Hoverman Jason T. , Halstead Neal T. , Michel Patrick J. , Rohr Jason R.
Published: June 2010

Predation and competition can induce important density‐ and trait‐mediated effects on species, with implications for community stability. However, interactions of these factors with parasitism remain understudied. Here we investigate interactions among competition, predation and parasitism by crossing tadpole density (Bufo americanus), presence of a caged predator (Notophthalmus viridescens), and Echinostoma trivolvis trematodes, experimentally partitioning their effects on tadpole exposure and susceptibility to infection. Predation did not affect E. trivolvis infection but accelerated tadpole development and growth, and decreased activity. The presence of E. trivolvis caused the opposite effects on these three responses and reduced tadpole survival. High conspecific density reduced tadpole survival, growth, and development, and increased tadpole activity. Effects of predation and parasitism on activity were only evident at low tadpole density. High‐density mesocosms also had twice the number of E. trivolvis infections as low‐density mesocosms, despite a lack of evidence for stress‐induced immunomodulation. Instead, this effect was explained by high density delaying tadpole development, which increased both the duration of exposure to cercariae and susceptibility to infection, because tadpoles spent more time in highly susceptible early stages. These results highlight the importance of accounting for trait‐mediated effects, host plasticity, and exposure vs. susceptibility in parasite ecology.
Analysis of pelagic species decline in the upper San Francisco Estuary using multivariate autoregressive modeling (MAR)
Authors: Mac Nally Ralph , Thomson James R. , Kimmerer Wim J. , Feyrer Frederick , Newman Ken B. , Sih Andy , Bennett William A. , Brown Larry , Fleishman Erica , Culberson Steven D. , Castillo Gonzalo
Published: June 2010

Four species of pelagic fish of particular management concern in the upper San Francisco Estuary, California, USA, have declined precipitously since ca. 2002: delta smelt (Hypomesus transpacificus), longfin smelt (Spirinchus thaleichthys), striped bass (Morone saxatilis), and threadfin shad (Dorosoma petenense). The estuary has been monitored since the late 1960s with extensive collection of data on the fishes, their pelagic prey, phytoplankton biomass, invasive species, and physical factors. We used multivariate autoregressive (MAR) modeling to discern the main factors responsible for the declines. An expert‐elicited model was built to describe the system. Fifty‐four relationships were built into the model, only one of which was of uncertain direction a priori. Twenty‐eight of the proposed relationships were strongly supported by or consistent with the data, while 26 were close to zero (not supported by the data but not contrary to expectations). The position of the 2‰ isohaline (a measure of the physical response of the estuary to freshwater flow) and increased water clarity over the period of analyses were two factors affecting multiple declining taxa (including fishes and the fishes' main zooplankton prey). Our results were relatively robust with respect to the form of stock–recruitment model used and to inclusion of subsidiary covariates but may be enhanced by using detailed state–space models that describe more fully the life‐history dynamics of the declining species.
Fire, vital rates, and population viability: a hierarchical Bayesian analysis of the endangered Florida scrub mint
Authors: Evans Margaret E. K. , Holsinger Kent E. , Menges Eric S.
Published: June 2010

Understanding and predicting changes in the abundance of natural populations is a central goal of ecology. These changes are influenced by a variety of exogenous processes (weather, floods, fire); variation in these processes leads to variation in vital rates (survival, fecundity) that may be positively or negatively correlated across the life cycle. We used 20 years of data and a hierarchical Bayesian model to estimate vital rates and their covariation in an endangered plant,Dicerandra frutescensssp.frutescens(Lamiaceae), as a function of time since fire and random year effects. Germination and the number of flowering branches declined with time since fire, and all plants were increasingly likely to become nonreproductive with time since fire. Time since fire had negative effects on survival of seedlings, vegetative plants, and small flowering plants, and positive effects on survival of medium and large flowering plants. Model comparison strongly supported inclusion of time‐since‐fire effects and weakly supported inclusion of year effects influencing all vital rates (“model‐wide” year effects). We used samples from the joint posterior distribution of model parameters to simulate population dynamics as a function of fire regime and year‐to‐year environmental variation. These simulations suggest that populations ofDicerandra frutescensssp.frutescensare least likely to go extinct if the average time between fires is ∼24–30 years. The design of the simulations allowed us to distinguish variation in stochastic population growth associated with process variability (fire, year effects, and demographic stochasticity) from variation associated with parameter uncertainty (finite amounts of data). Even with 20 years of data, half or more of the uncertainty in population growth rates was due to parameter uncertainty. This hierarchical Bayesian population viability analysis illustrates a general analytical framework for (1) estimating vital rates as a function of an exogenous environmental factor, (2) accounting for covariation among vital rates, and (3) simulating population dynamics as a function of stochastic environmental processes while taking into account uncertainty about their effects. We discuss future areas of development for this approach.
Optimal defense: snails avoid reproductive parts of the lichen Lobaria scrobiculata due to internal defense allocation
Authors: Asplund Johan , Solhaug Knut Asbjørn , Gauslaa Yngvar
Published: June 2010

The optimal defense theory (ODT) deals with defensive compounds improving fitness of a particular organism. It predicts that these compounds are allocated in proportion to the risk for a specific plant tissue being attacked and this tissue's value for plant fitness. As the benefit of defense cannot easily be measured in plants, the empirical evidence for ODT is limited. However, lichens are unique in the sense that their carbon‐based secondary compounds can nondestructively be removed or reduced in concentration by acetone rinsing. By using such an extraction protocol, which is lethal to plants, we have tested the ODT by studying lichens instead of plants as photosynthetically active organisms. Prior to acetone rinsing, we found five times higher concentration of meta‐scrobiculin in the reproductive parts (soralia) of Lobaria scrobiculata compared to somatic parts of this foliose epiphytic lichen species. At this stage, the lichen‐feeding snail Cochlodina laminata avoided the soralia. However, after removal of secondary compounds, the snail instead preferred the soralia. In this way, we have successfully shown that grazing pattern inversely reflects the partitioning of the secondary compounds that have a documented deterring effect. Thus our study provides strong and novel evidence for the ODT.
Niche complementarity due to plasticity in resource use: plant partitioning of chemical N forms
Authors: Ashton Isabel W. , Miller Amy E. , Bowman William D. , Suding Katharine N.
Published: June 2010

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less‐used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.
The seaweed Caulerpa racemosa on Mediterranean rocky reefs: from passenger to driver of ecological change
Authors: Bulleri Fabio , Balata David , Bertocci Iacopo , Tamburello Laura , Benedetti-Cecchi Lisandro
Published: June 2010

Disentangling the ecological effects of biological invasions from those of other human disturbances is crucial to understanding the mechanisms underlying ongoing biotic homogenization. We evaluated whether the exotic seaweed, Caulerpa racemosa, is the primary cause of degradation (i.e., responsible for the loss of canopy‐formers and dominance by algal turfs) on Mediterranean rocky reefs, by experimentally removing the invader alone or the entire invaded assemblage. In addition, we assessed the effects of enhanced sedimentation on the survival and recovery of canopy‐forming macroalgae at a relatively pristine location and how their loss affects the ability of C. racemosa to conquer space. C. racemosa did not invade dense canopy stands or influence their recovery in cleared plots. Competition with C. racemosa could not explain the rarity of canopy‐forming species at degraded sites. Removing the assemblages invaded by C. racemosa and preventing reinvasion did not trigger the transition from algal turfs to canopies, but it enhanced the cover of morphologically complex erect macroalgae under some circumstances. Once established, C. racemosa, enhancing sediment accumulation, favors algal turfs over erect algal forms and enables them to monopolize space. Our results show that introduced species that rely on disturbance to establish can subsequently become the main drivers of ecological change.
Disappearing plants: why they hide and how they return
Authors: Gremer Jennifer R. , Sala Anna , Crone Elizabeth E.
Published: June 2010

Prolonged dormancy is a life‐history stage in which mature plants fail to resprout for one or more growing seasons and instead remain alive belowground. Prolonged dormancy is relatively common, but the proximate causes and consequences of this intriguing strategy have remained elusive. In this study we tested whether stored resources are associated with remaining belowground, and investigated the resource costs of remaining belowground during the growing season. We measured stored resources at the beginning and end of the growing season in Astragalus scaphoides, an herbaceous perennial in southwest Montana, USA. At the beginning of the growing season, dormant plants had lower concentrations of stored mobile carbon (nonstructural carbohydrates, NSC) than did emergent plants. Surprisingly, during the growing season, dormant plants gained as much NSC as photosynthetically active plants, an increase most likely due to remobilization of structural carbon. Thus, low levels of stored NSC were associated with remaining belowground, and remobilization of structural carbon may allow for dormant plants to emerge in later seasons. The dynamics of NSC in relation to dormancy highlights the ability of plants to change their own resource status somewhat independently of resource assimilation, as well as the importance of considering stored resources in understanding plant responses to the environment.
Nonnative trout impact an alpine‐nesting bird by altering aquatic‐insect subsidies
Authors: Epanchin Peter N. , Knapp Roland A. , Lawler Sharon P.
Published: June 2010

Adjacent food webs may be linked by cross‐boundary subsidies: more‐productive donor systems can subsidize consumers in less‐productive neighboring recipient systems. Introduced species are known to have direct effects on organisms within invaded communities. However, few studies have addressed the indirect effects of nonnative species in donor systems on organisms in recipient systems. We studied the direct role of introduced trout in altering a lake‐derived resource subsidy and their indirect effects in altering a passerine bird's response to that subsidy. We compared the abundance of aquatic insects and foraging Gray‐crowned Rosy‐Finches (Leucosticte tephrocotis dawsoni, “Rosy‐Finch”) at fish‐containing vs. fishless lakes in the Sierra Nevada Mountains of California (USA). Introduced trout outcompeted Rosy‐Finches for emerging aquatic insects (i.e., mayflies). Fish‐containing lakes had 98% fewer mayflies than did fishless lakes. In lakes without fish, Rosy‐Finches showed an aggregative response to emerging aquatic insects with 5.9 times more Rosy‐Finches at fishless lakes than at fish‐containing lakes. Therefore, the introduction of nonnative fish into the donor system reduced both the magnitude of the resource subsidy and the strength of cross‐boundary trophic interactions. Importantly, the timing of the subsidy occurs when Rosy‐Finches feed their young. If Rosy‐Finches rely on aquatic‐insect subsidies to fledge their young, reductions in the subsidy by introduced trout may have decreased Rosy‐Finch abundances from historic levels. We recommend that terrestrial recipients of aquatic subsidies be included in conservation and restoration plans for ecosystems with alpine lakes.