Publications by authors named "Amy L Freestone"

17 Publications

  • Page 1 of 1

Asymmetry of marine invasions across tropical oceans.

Ecology 2021 08 14;102(8):e03434. Epub 2021 Jul 14.

Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, Maryland, 21037, USA.

Understanding the mechanisms of spatial variation of biological invasions, across local-to-global scales, has been a major challenge. The importance of evolutionary history for invasion dynamics was noted by Darwin, and several studies have since considered how biodiversity of source and recipient regions can influence the probability of invasions. For over a century, the Panama Canal has connected water bodies and biotas with different evolutionary histories, and created a global shipping hot spot, providing unique opportunities to test mechanisms that affect invasion patterns. Here, we test for asymmetry in both the extent of invasions and predation effects, a possible mechanism of biotic resistance, between two tropical oceans at similar latitudes. We estimated nonnative species (NNS) richness for sessile marine invertebrates, using standardized field surveys and literature synthesis, to examine whether invasions are asymmetrical, with more NNS present in the less diverse Pacific compared to the Atlantic. We also experimentally tested whether predation differentially limits the abundance and distribution of these invertebrates between oceans. In standardized surveys, observed total NNS richness was higher in the Pacific (18 NNS, 30% of all Pacific species) than the Atlantic (11 NNS, 13% of all Atlantic species). Similarly, literature-based records also display this asymmetry between coasts. When considering only the reciprocal exchange of NNS between Atlantic and Pacific biotas, NNS exchange from Atlantic to Pacific was eightfold higher than the opposite direction, exceeding the asymmetry predicted by random exchange based simply on differences of overall diversity per region. Predation substantially reduced biomass and changed NNS composition in the Pacific, but no such effects were detected on the Atlantic coast. Specifically, some dominant NNS were particularly susceptible to predation in the Pacific, supporting the hypothesis that predation may reduce the abundance of certain NNS here. These results are consistent with predictions that high diversity in source regions, and species interactions in recipient regions, shape marine invasion patterns. Our comparisons and experiments across two tropical ocean basins, suggest that global invasion dynamics are likely driven by both ecological and evolutionary factors that shape susceptibility to and directionality of invasions across biogeographic scales.
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http://dx.doi.org/10.1002/ecy.3434DOI Listing
August 2021

Stronger predation intensity and impact on prey communities in the tropics.

Ecology 2021 08 13;102(8):e03428. Epub 2021 Jul 13.

Smithsonian Environmental Research Center, Edgewater, Maryland, 21037-0028, USA.

The hypothesis that biotic interactions strengthen toward lower latitudes provides a framework for linking community-scale processes with the macroecological scales that define our biosphere. Despite the importance of this hypothesis for understanding community assembly and ecosystem functioning, the extent to which interaction strength varies across latitude and the effects of this variation on natural communities remain unresolved. Predation in particular is central to ecological and evolutionary dynamics across the globe, yet very few studies explore both community-scale causes and outcomes of predation across latitude. Here we expand beyond prior studies to examine two important components of predation strength: intensity of predation (including multiple dimensions of the predator guild) and impact on prey community biomass and structure, providing one of the most comprehensive examinations of predator-prey interactions across latitude. Using standardized experiments, we tested the hypothesis that predation intensity and impact on prey communities were stronger at lower latitudes. We further assessed prey recruitment to evaluate the potential for this process to mediate predation effects. We used sessile marine invertebrate communities and their fish predators in nearshore environments as a model system, with experiments conducted at 12 sites in four regions spanning the tropics to the subarctic. Our results show clear support for an increase in both predation intensity and impact at lower relative to higher latitudes. The predator guild was more diverse at low latitudes, with higher predation rates, longer interaction durations, and larger predator body sizes, suggesting stronger predation intensity in the tropics. Predation also reduced prey biomass and altered prey composition at low latitudes, with no effects at high latitudes. Although recruitment rates were up to three orders of magnitude higher in the tropics than the subarctic, prey replacement through this process was insufficient to dampen completely the strong impacts of predators in the tropics. Our study provides a novel perspective on the biotic interaction hypothesis, suggesting that multiple components of the predator community likely contribute to predation intensity at low latitudes, with important consequences for the structure of prey communities.
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http://dx.doi.org/10.1002/ecy.3428DOI Listing
August 2021

Predation shapes invertebrate diversity in tropical but not temperate seagrass communities.

J Anim Ecol 2020 02 4;89(2):323-333. Epub 2019 Dec 4.

Department of Biology, Temple University, Philadelphia, PA, USA.

The hypothesis that biotic interactions are stronger at lower relative to higher latitudes has a rich history, drawing from ecological and evolutionary theory. While this hypothesis suggests that stronger interactions at lower latitudes may contribute to the maintenance of contemporary patterns of diversity, there remain few standardized biogeographic comparisons of community effects of species interactions. Using marine seagrasses as a focal ecosystem of conservation importance and sessile marine invertebrates as model prey, we tested the hypothesis that predation is stronger at lower latitudes and can shape contemporary patterns of prey diversity. To further advance understanding beyond prior studies, we also explored mechanisms that likely underlie a change in interaction outcomes with latitude. Multiple observational and experimental approaches were employed to test for effects of predators, and the mechanisms that may underlie these effects, in seagrass ecosystems of the western Atlantic Ocean spanning 30° of latitude from the temperate zone to the tropics. In predator exclusion experiments conducted in a temperate and a tropical region, predation decreased sessile invertebrate abundance, richness and diversity on both natural and standardized artificial seagrass at tropical but not temperate sites. Further, predation reduced invertebrate richness at both local and regional scales in the tropics. Additional experiments demonstrated that predation reduced invertebrate recruitment in the tropics but not the temperate zone. Finally, direct observations of predators showed higher but variable consumption rates on invertebrates at tropical relative to temperate latitudes. Together, these results demonstrate that strong predation in the tropics can have consequential impacts on prey communities through discrete effects on early life stages as well as longer-term cumulative effects on community structure and diversity. Our detailed experiments also provide some of the first data linking large-scale biogeographic patterns, community-scale interaction outcomes and direct observation of predators in the temperate zone and tropics. Therefore, our results support the hypothesis that predation is stronger in the tropics, but also elucidate some of the causes and consequences of this variation in shaping contemporary patterns of diversity.
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http://dx.doi.org/10.1111/1365-2656.13133DOI Listing
February 2020

Multi-trophic native and non-native prey naïveté shape marine invasion success.

PLoS One 2019 6;14(9):e0221969. Epub 2019 Sep 6.

Department of Biology, Temple University, Philadelphia, PA, United States of America.

Invasive predators have caused rapid declines in many native prey species across the globe. Predator invasion success may be attributed to prey naïveté, or the absence of anti-predator behavior between native and non-native species. An understanding of the effects of naïveté at different timescales since introduction and across multiple trophic levels is lacking, however, particularly in marine systems. Given the central role of trophic interactions in invasion dynamics, this knowledge gap limits the ability to predict high impact predator invasions. Naïveté was examined across three trophic levels of marine invertebrates: a native basal prey (hard clam), two non-native intermediate predators (the recently-introduced Asian shore crab and the long-established European green crab), a native intermediate predator (juvenile blue crabs), and a native top predator (adult blue crab). We hypothesized that naïveté would be more pronounced in trophic interactions involving the recently-introduced non-native predator in comparison to the long-established non-native and native intermediate predators. We further hypothesized that the recently-introduced intermediate predator would both benefit from naïveté of the native basal prey and be hindered by higher mortality through its own naïveté to the native top predator. To test these hypotheses, three laboratory experiments and a field experiment were used. Consistent with our hypotheses, basal prey naïveté was most pronounced with the recently-introduced intermediate predator, and this increased the predator's foraging success. This recently-introduced intermediate predator, however, exhibited an ineffective anti-predator response to the native top predator, and was also preyed upon more in the field than its long-established and native counterparts. Therefore, despite direct benefits from basal prey naïveté, the recently-introduced intermediate predator's naïveté to its own predators may limit its invasion success. These results highlight the importance of a multi-trophic perspective on predator-prey dynamics to more fully understand the consequences of naïveté in invasion biology.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0221969PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6730852PMC
March 2020

Habitat fragmentation differentially affects trophic levels and alters behavior in a multi-trophic marine system.

Oecologia 2017 03 20;183(3):899-908. Epub 2016 Dec 20.

Department of Biology, Temple University, Philadelphia, PA, USA.

Seagrass, an important subtidal marine ecosystem, is being lost at a rate of 110 km year, leading to fragmented seagrass seascapes. Habitat fragmentation is predicted to affect trophic levels differently, with higher trophic levels being more sensitive, stressing the importance of a multi-trophic perspective. Utilizing the trophic relationship between the blue crab (Callinectes sapidus) and hard clam (Mercenaria mercenaria), where adult blue crabs prey on juvenile blue crabs, and juvenile blue crabs prey on small hard clams, we examined whether predation rates, abundance, and behavior of predators and prey differed between continuous and fragmented seagrass in a multi-trophic context at two sites in Barnegat Bay, NJ. We tested the hypothesis that fragmented habitats would differentially affect trophic levels within a tri-trophic system, and our results supported this hypothesis. Densities of adult blue crabs were higher in fragmented than continuous habitats. Densities of juvenile blue crabs, the primary predator of hard clams, were lower in fragmented habitats than continuous, potentially due to increased predation by adult blue crabs. Clams experienced lower predation and burrowed to a shallower depth in fragmented habitats than in continuous habitat, likely due in part to the low densities of juvenile blue crabs, their primary predator. Our results suggest that while trophic levels are differentially affected, the impact of habitat fragmentation may be stronger on intermediate rather than top trophic levels in some marine systems.
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http://dx.doi.org/10.1007/s00442-016-3791-2DOI Listing
March 2017

Nonrandom community assembly and high temporal turnover promote regional coexistence in tropics but not temperate zone.

Ecology 2015 Jan;96(1):264-73

A persistent challenge for ecologists is understanding the ecological mechanisms that maintain global patterns of biodiversity, particularly the latitudinal diversity gradient of peak species richness in the tropics. Spatial and temporal variation in community composition contribute to these patterns of biodiversity, but how this variation and its underlying processes change across latitude remains unresolved. Using a model system of sessile marine invertebrates across 25 degrees of latitude, from the temperate zone to the tropics, we tested the prediction that spatial and temporal patterns of taxonomic richness and composition, and the community assembly processes underlying these patterns, will differ across latitude. Specifically, we predicted that high beta diversity (spatial variation in composition) and high temporal turnover contribute to the high species richness of the tropics. Using a standardized experimental approach that controls for several confounding factors that hinder interpretation of prior studies, we present results that support our predictions. In the temperate zone, communities were more similar across spatial scales from centimeters to tens of kilometers and temporal scales up to one year than at lower latitudes. Since the patterns at northern latitudes were congruent with a null model, stochastic assembly processes are implicated. In contrast, the communities in the tropics were a dynamic spatial and temporal mosaic, with low similarity even across small spatial scales and high temporal turnover at both local and regional scales. Unlike the temperate zone, deterministic community assembly processes such as predation likely contributed to the high beta diversity in the tropics. Our results suggest that community assembly processes and temporal dynamics vary across latitude and help structure and maintain latitudinal patterns of diversity.
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http://dx.doi.org/10.1890/14-0145.1DOI Listing
January 2015

Size-energy relationships in ecological communities.

PLoS One 2013 7;8(8):e68657. Epub 2013 Aug 7.

Department of Biology, Temple University, Philadelphia, Pennsylvania, United States of America.

Hypotheses that relate body size to energy use are of particular interest in community ecology and macroecology because of their potential to facilitate quantitative predictions about species interactions and to clarify complex ecological patterns. One prominent size-energy hypothesis, the energetic equivalence hypothesis, proposes that energy use from shared, limiting resources by populations or size classes of foragers will be independent of body size. Alternative hypotheses propose that energy use will increase with body size, decrease with body size, or peak at an intermediate body size. Despite extensive study, however, size-energy hypotheses remain controversial, due to a lack of directly-measured data on energy use, a tendency to confound distinct scaling relationships, and insufficient attention to the ecological contexts in which predicted relationships are likely to occur. Our goal, therefore, was to directly evaluate size-energy hypotheses while clarifying how results would differ with alternate methods and assumptions. We comprehensively tested size-energy hypotheses in a vertebrate frugivore guild in a tropical forest in Madagascar. Our test of size-energy hypotheses, which is the first to examine energy intake directly, was consistent with the energetic equivalence hypothesis. This finding corresponds with predictions of metabolic theory and models of energy distribution in ecological communities, which imply that body size does not confer an advantage in competition for energy among populations or size classes of foragers. This result was robust to different assumptions about energy regulation. Our results from direct energy measurement, however, contrasted with those obtained with conventional methods of indirect inference from size-density relationships, suggesting that size-density relationships do not provide an appropriate proxy for size-energy relationships as has commonly been assumed. Our research also provides insights into mechanisms underlying local size-energy relationships and has important implications for predicting species interactions and for understanding the structure and dynamics of ecological communities.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0068657PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3737256PMC
August 2014

Stronger biotic resistance in tropics relative to temperate zone: effects of predation on marine invasion dynamics.

Ecology 2013 Jun;94(6):1370-7

Department of Biology, Temple University, Philadelphia, Pennsylvania 19122, USA.

Latitudinal patterns of nonnative species richness suggest fewer successful invasions in the tropics, relative to temperate regions. One main hypothesis for this pattern is that biotic resistance to invasion is stronger in the tropics than at higher latitudes. Biotic resistance can limit the distribution and abundance of nonnative species and, in extreme cases, can prevent establishment. We provide the first experimental test of this hypothesis, comparing the strength of biotic resistance in a tropical and a temperate marine ecosystem. Predation is one mechanism of biotic resistance, and since predation can be stronger at lower latitudes, we predicted that predation will serve to increase biotic resistance more in the tropics than at higher latitude. We conducted predator-exclusion experiments on marine epifaunal communities, a heavily invaded system, focusing on nonnative tunicates as a model fauna. The effect of predation on species richness of nonnative tunicates was more than three times greater at sites in tropical Panama than in temperate Connecticut, consistent with the prediction of stronger biotic resistance in the tropics. In Connecticut, predation reduced the abundance of one nonnative tunicate but did not affect the abundances of any other nonnative tunicate species, and no species were excluded from communities. In contrast, predation resulted in striking reductions in abundance and often exclusion of nonnative tunicates from experimental communities in Panama. If proved to be general, latitudinal differences in the biotic resistance of communities to nonnative species establishment may help explain emerging patterns of global invasions.
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http://dx.doi.org/10.1890/12-1382.1DOI Listing
June 2013

Disentangling the drivers of β diversity along latitudinal and elevational gradients.

Science 2011 Sep;333(6050):1755-8

Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada.

Understanding spatial variation in biodiversity along environmental gradients is a central theme in ecology. Differences in species compositional turnover among sites (β diversity) occurring along gradients are often used to infer variation in the processes structuring communities. Here, we show that sampling alone predicts changes in β diversity caused simply by changes in the sizes of species pools. For example, forest inventories sampled along latitudinal and elevational gradients show the well-documented pattern that β diversity is higher in the tropics and at low elevations. However, after correcting for variation in pooled species richness (γ diversity), these differences in β diversity disappear. Therefore, there is no need to invoke differences in the mechanisms of community assembly in temperate versus tropical systems to explain these global-scale patterns of β diversity.
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http://dx.doi.org/10.1126/science.1208584DOI Listing
September 2011

Reorienting systematic conservation assessment for effective conservation planning.

Conserv Biol 2011 Aug;25(4):688-96

Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA 95616, USA.

Systematic conservation assessment (an information-gathering and prioritization process used to select the spatial foci of conservation initiatives) is often considered vital to conservation-planning efforts, yet published assessments have rarely resulted in conservation action. Conservation assessments may lead more directly to effective conservation action if they are reoriented to inform conservation decisions. Toward this goal, we evaluated the relative priority for conservation of 7 sites proposed for the first forest reserves in the Union of the Comoros, an area with high levels of endemism and rapidly changing land uses in the western Indian Ocean. Through the analysis of 30 indicator variables measured at forest sites and nearby villages, we assessed 3 prioritization criteria at each site: conservation value, threat to loss of biological diversity from human activity, and feasibility of reserve establishment. Our results indicated 2 sites, Yiméré and Hassera-Ndrengé, were priorities for conservation action. Our approach also informed the development of an implementation strategy and enabled an evaluation of previously unexplored relations among prioritization criteria. Our experience suggests that steps taken to ensure the closer involvement of practitioners, include a broader range of social data, encourage stakeholder participation, and consider the feasibility of conservation action can improve the relevance of assessments for conservation planning, strengthen the scientific basis for conservation decisions, and result in a more realistic evaluation of conservation alternatives.
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http://dx.doi.org/10.1111/j.1523-1739.2011.01697.xDOI Listing
August 2011

Stronger predation in the tropics shapes species richness patterns in marine communities.

Ecology 2011 Apr;92(4):983-93

Department of Biology, Temple University, Philadelphia, Pennsylvania 19122, USA.

Species interactions are widely assumed to be stronger at lower latitudes, but surprisingly few experimental studies test this hypothesis, and none ties these processes to observed patterns of species richness across latitude. We report here the first experimental field test that predation is both stronger and has a disproportionate effect on species richness in the tropics relative to the temperate zone. We conducted predator-exclusion experiments on communities of sessile marine invertebrates in four regions, which span 32 degrees latitude, in the western Atlantic Ocean and Caribbean Sea. Over a three-month timescale, predation had no effect on species richness in the temperate zone. In the tropics, however, communities were from two to over ten times more species-rich in the absence of predators than when predators were present. While micro-and macro-predators likely compete for the limited prey resource in the tropics, micropredators alone were able to exert as much pressure on the invertebrate communities as the full predator community. This result highlights the extent to which exposure to even a subset of the predator guild can significantly impact species richness in the tropics. Patterns were consistent in analyses that included relative and total species abundances. Higher species richness in the absence of predators in the tropics was also observed when species occurrences were pooled across two larger spatial scales, site and region, demonstrating a consistent scaling relationship. These experimental results show that predation can both limit local species abundances and shape patterns of regional coexistence in the tropics. When preestablished diverse tropical communities were then exposed to predation for different durations, ranging from one to several days, species richness was always reduced. These findings confirmed that impacts of predation in the tropics are strong and consistent, even in more established communities. Our results offer empirical support for the long-held prediction that predation pressure is stronger at lower latitudes. Furthermore, we demonstrate the magnitude to which variation in predation pressure can contribute to the maintenance of tropical species diversity.
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http://dx.doi.org/10.1890/09-2379.1DOI Listing
April 2011

Latitudinal variation in local interactions and regional enrichment shape patterns of marine community diversity.

Ecology 2011 Jan;92(1):208-17

Department of Biology, Temple University, 1900 North 12th Street, Philadelphia, Pennsylvania 19122, USA.

While communities are shaped by both local interactions and enrichment from the regional species pool, we propose a hypothesis that the balance of these forces shifts with latitude, with regional enrichment dominating at high latitudes and local interactions dominating at low latitudes. To test this hypothesis, we conducted a latitudinal-scale experiment with marine epifaunal communities. In four regions of the North Atlantic Ocean and Caribbean Sea, we used mimics of ecosystem engineers to manipulate biogenic structural complexity. We iteratively evaluated diversity patterns of experimental communities up to one year after deployment. Additional data were also collected from one of our tropical sites 2.5 years after initial deployment. As hypothesized, we found a reciprocal latitudinal gradient in the effects of the structurally complex mimics and regional enrichment. In the tropics, local diversity was always higher in association with the mimics than in exposed areas that were more open to predation. This effect was consistent across two spatial scales and beyond the one-year timescale of the experiment. In temperate communities, no consistent effects of the mimics on diversity were observed. However, the proportion of species from the regional species pool that were present at the local scale increased from the tropics to the temperate zone, consistent with the hypothesis that higher-latitude communities may experience greater influence from the regional species pool than communities at low latitudes. This study represents the first large-scale experimental demonstration that suggests that the relative impact of local interactions and regional enrichment on community diversity may depend on latitude.
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http://dx.doi.org/10.1890/09-1841.1DOI Listing
January 2011

Navigating the multiple meanings of β diversity: a roadmap for the practicing ecologist.

Ecol Lett 2011 Jan 11;14(1):19-28. Epub 2010 Nov 11.

Institute of Information and Mathematical Sciences, Massey University, Albany Campus, Auckland, New Zealand.

A recent increase in studies of β diversity has yielded a confusing array of concepts, measures and methods. Here, we provide a roadmap of the most widely used and ecologically relevant approaches for analysis through a series of mission statements. We distinguish two types of β diversity: directional turnover along a gradient vs. non-directional variation. Different measures emphasize different properties of ecological data. Such properties include the degree of emphasis on presence/absence vs. relative abundance information and the inclusion vs. exclusion of joint absences. Judicious use of multiple measures in concert can uncover the underlying nature of patterns in β diversity for a given dataset. A case study of Indonesian coral assemblages shows the utility of a multi-faceted approach. We advocate careful consideration of relevant questions, matched by appropriate analyses. The rigorous application of null models will also help to reveal potential processes driving observed patterns in β diversity.
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http://dx.doi.org/10.1111/j.1461-0248.2010.01552.xDOI Listing
January 2011

Invasion in a heterogeneous world: resistance, coexistence or hostile takeover?

Ecol Lett 2007 Jan;10(1):77-94

Department of Environmental Science and Policy, University of California, Davis, CA 95616, USA.

We review and synthesize recent developments in the study of the invasion of communities in heterogeneous environments, considering both the invasibility of the community and impacts to the community. We consider both empirical and theoretical studies. For each of three major kinds of environmental heterogeneity (temporal, spatial and invader-driven), we find evidence that heterogeneity is critical to the invasibility of the community, the rate of spread, and the impacts on the community following invasion. We propose an environmental heterogeneity hypothesis of invasions, whereby heterogeneity both increases invasion success and reduces the impact to native species in the community, because it promotes invasion and coexistence mechanisms that are not possible in homogeneous environments. This hypothesis could help to explain recent findings that diversity is often increased as a result of biological invasions. It could also explain the scale dependence of the diversity-invasibility relationship. Despite the undoubted importance of heterogeneity to the invasion of communities, it has been studied remarkably little and new research is needed that simultaneously considers invasion, environmental heterogeneity and community characteristics. As a young field, there is an unrivalled opportunity for theoreticians and experimenters to work together to build a tractable theory informed by data.
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http://dx.doi.org/10.1111/j.1461-0248.2006.00987.xDOI Listing
January 2007

Facilitation drives local abundance and regional distribution of a rare plant in a harsh environment.

Authors:
Amy L Freestone

Ecology 2006 Nov;87(11):2728-35

Environmental Science and Policy, University of California, Davis, California 95616, USA.

The importance of facilitation to local community dynamics is becoming increasingly recognized. However, the predictability of positive interactions in stressful environments, the balance of competition and facilitation along environmental gradients, and the scaling of local positive interactions to regional distributions are aspects of facilitation that remain unresolved. I explored these questions in a habitat specialist, Delphinium uliginosum, and a moss, Didymodon tophaceus, both found in small serpentine wetlands. I tested three hypotheses: (1) moss facilitates germination, growth, and/or fecundity of D. uliginosum; (2) facilitation is stronger at the harsher ends of gradients in soil moisture, toxicity, and/or biomass; and (3) facilitation is reflected in positive associations at the levels of local abundance and regional occurrence. Although considerable competitive interactions occurred in later life stages, moss strongly facilitated D. uliginosum seedling emergence. There was no evidence that this facilitative effect weakened, or switched to competition, in benign environments. D. uliginosum was more locally abundant and more frequently present, across a large portion of its range, with than without moss, indicating a net facilitative effect in the face of competitive influences. Facilitated recruitment, possibly by seed retention, was found to be an important control on abundance and distribution in this rare species.
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http://dx.doi.org/10.1890/0012-9658(2006)87[2728:fdlaar]2.0.co;2DOI Listing
November 2006

Dispersal limitation and environmental heterogeneity shape scale-dependent diversity patterns in plant communities.

Ecology 2006 Oct;87(10):2425-32

Environmental Science and Policy, University of California, Davis, California 95616, USA.

Understanding the large-scale distribution of species diversity requires distinguishing two of the primary factors that cause compositional differences: dispersal limitation and environmental variation. In a community with a naturally discontinuous spatial structure, we asked (1) at what scale(s) nonrandom variation in species composition occurs and (2) at what scale(s) such variation is associated with spatial separation, indicative of dispersal limitation, and at what scale(s) variation is associated with environmental heterogeneity? We sampled 50 seeps (small wetlands) on five serpentine outcrops. Using a randomization model, we showed that additive beta diversity (a measure of community dissimilarity) was lower than random within seeps and higher than random among both seeps and outcrops. Using Mantel tests, we showed that plant community dissimilarity, in both the full seep assemblage as well as in a subset of seep endemics, at the two larger scales was associated with different forms of environmental heterogeneity and, at the largest scale, was also associated with geographic distance. We conclude that diversity in this system is shaped by multiple scales of heterogeneity and by dispersal limitation at the largest scale.
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http://dx.doi.org/10.1890/0012-9658(2006)87[2425:dlaehs]2.0.co;2DOI Listing
October 2006

Regional enrichment of local assemblages is robust to variation in local productivity, abiotic gradients, and heterogeneity.

Ecol Lett 2006 Feb;9(2):95-102

Theory predicts that the effects of regional richness on the richness of local communities may depend on the productivity, resource availability, and/or heterogeneity of local sites. Using the wetland plant communities of 50 independent streams as 'regions', we tested whether: (1) local richness in 1-m(2) quadrats and 50-m stream segments was positively related to regional richness, even after environmental influences were considered; and (2) the effect of regional richness would interact with the effects of biomass, soil moisture, and/or heterogeneity on local richness. In models that explained up to 88% of variation in local richness, we found that richness at both local scales was positively related to regional richness, and that regional richness did not interact with any of the environmental gradients that also shaped local richness. We conclude that species availability from the regional pool may consistently enrich local communities, even while other constraints on local richness operate.
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http://dx.doi.org/10.1111/j.1461-0248.2005.00849.xDOI Listing
February 2006
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