Browsing by Subject "density dependence"

Now showing 1 - 5 of 5
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    Arctic Peregrine Falcon Abundance on Cliffs Along the Colville River, Alaska, 1981-2002 and Covariate Input Files
    (2015-04-15) Bruggeman, Jason E.; Swem, Ted; Andersen, David E; Kennedy, Patricia L.; Nigro, Debora; brug0006@umn.edu; Bruggeman, Jason E.
    Arctic peregrine falcons (Falco peregrinus tundrius; hereafter Arctic peregrine) have a limited and northern breeding distribution, including the Colville River Special Area (CRSA) in the National Petroleum Reserve-Alaska, USA. We quantified influences of climate, topography, nest productivity, prey habitat, density dependence, and interspecific competition affecting Arctic peregrines in the CRSA by applying the Dail-Madsen model to estimate abundance and vital rates of adults on nesting cliffs from 1981 through 2002. Arctic peregrine abundance increased throughout the 1980s, which spanned the population's recovery from DDT-induced reproductive failure, until exhibiting a stationary trend in the 1990s. Apparent survival rate (i.e., emigration; death) was negatively correlated with number of adult Arctic peregrines on the cliff the previous year, suggesting effects of density-dependent population regulation. Apparent survival rate and arrival rate (i.e., immigration; recruitment) were higher during years with earlier snowmelt and milder winters, and apparent survival was positively correlated with nesting season maximum daily temperature. Arrival rate was positively correlated with average Arctic peregrine productivity along a cliff segment from the previous year and initial abundance was positively correlated with cliff height. Higher cliffs with documented higher productivity, and presumably indicative of higher quality habitat, are a priority for continued protection from potential nearby development and disturbance to minimize population-level impacts. Our work provides insight into factors affecting a population during and after recovery, and demonstrates how the Dail-Madsen model can be used for any unmarked population with multiple years of abundance data collected through repeated surveys.
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    Does relatedness matter? Phylogenetic density-dependent survival of seedlings in a tropical forest
    (Ecological Society of America, 2014) Lebrija-Trejos, Edwin; Wright, S. Joseph; Hernandez, Andres; Reich, Peter B
    A complex set of interactions among neighbors influences plant performance and community structure. Understanding their joint operation requires extensive information on species characteristics and individual performance. We evaluated first-year survival of 35 719 tropical forest seedlings of 222 species and 15 annual cohorts relative to the density of conspecific and heterospecific neighbors and the phylogenetic similarity of heterospecific neighbors. Neighbors were from two size classes, and size asymmetric interactions provided insight into likely mechanisms. Large heterospecific and conspecific neighbors reduced seedling survival equally, suggesting resource competition rather than host-specific enemies as a mechanism. In contrast, much stronger negative conspecific effects were associated with seedling neighbors capable of limited resource uptake, suggesting shared pests rather than competition as the mechanism. Survival improved, however, near phylogenetically similar heterospecific neighbors, suggesting habitat associations shared among closely related species affect spatial patterns of performance. Improved performance near phylogenetically similar neighbors is an emerging pattern in the handful of similar studies.
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    Factors Influencing Beaver (Castor Canadensis) Population Fluctuations, And Their Ecological Relationship With Salmonids
    (2019-08) Johnson-Bice, Sean
    Within the western Great Lakes (WGL) U.S. region (Michigan, Minnesota, Wisconsin), the ecological impacts that North American beavers (Castor canadensis) have on cold- water streams are generally considered to negatively affect salmonid populations where the two taxa interact. Here, we review the history of beaver-salmonid interactions within the WGL region, describe how this relationship and management actions have evolved over the past century, and review all published studies from the region that have evaluated beaver-salmonid interactions. Our review suggests the impact beavers have varies spatially and temporally, depending on a variety of local ecological characteristics. We found beaver activity is often deleterious to salmonids in low-gradient stream basins, but generally beneficial in high- gradient basins; and ample groundwater inputs can offset the potential negative effects of beavers by stabilizing the hydrologic and thermal regimes within streams. However, there was an obvious lack of empirical data and/or experimental controls within the reviewed studies, which we suggest emphasizes the need for more data-driven beaver-salmonid research in the WGL region. Resource managers are routinely faced with an ecological dilemma between maintaining natural environmental processes within cold-water ecosystems and conducting beaver control for the benefit of salmonids, and this dilemma is further complicated when the salmonids in question are a non-native species. We anticipate future beaver-salmonid research will lead to a greater understanding of this ecologically-complex relationship that may better inform managers when and where beaver control is necessary to achieve the desired management objectives. Understanding how wildlife populations respond to density-dependent (DD) and density- independent (DI) factors is critically important for wildlife management and research, as this knowledge can allow us to predict population responses to forcing mechanisms such as climate, predation, and exploitation. Recent advancements in statistical methods have allowed researchers to disentangle the relative influence each factor has on wildlife population dynamics, but this work is ongoing. Using a long-term dataset collected from 1975 to 2002, we sought to evaluate the relative influence DD and a suite of covariates (weather, harvest, habitat quality, and wolf [Canis lupus] predation) had on annual rates of change in the number of beaver (Castor canadensis) colonies among 15 populations in northern Minnesota, USA. We modeled changes in beaver colony densities using a discrete-time Gompertz model within a Bayesian inference framework, and compared model performance among three global models using Deviance Information Criterion (DIC) widely available information criterion (WAIC): a DI model without covariates; a DD model without covariates; and a DD model with covariates. Our results provide strong evidence for compensatory (negative) DD within beaver colony dynamics. We found no evidence that covariates related to harvest, wolf predation, or habitat quality significantly influenced beaver colony growth rates, but cold winters (lag-0), spring drought (lag-0), and fall drought conditions (lag-2) were correlated with greater colony growth rates. Despite strong evidence of the effect of environmental covariates on beaver colony dynamics, prediction of colony dynamics using these covariates showed only minimal improvements. We suggest the lack of improvement in prediction was the result of model over-fitting, indicating our significant covariate effects may not be biologically relevant. Our analysis demonstrates how reliance on information criterion values may lead to erroneous conclusions in time-series analyses, and using a hindcasting approach like the one we present here may help determine whether model results are biologically relevant or merely statistically significant. Our results highlight the importance of long-term monitoring programs for evaluating the efficacy of predictive ecological models. That beaver populations are primarily intrinsically regulated has important management implications depending on whether the objectives concern eradicating beavers from unwanted regions, mitigating conflicts, or facilitating rewilding or colonization efforts.
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    Soil microbes drive the classic plant diversity–productivity pattern
    (Ecological Society of America, 2011) Schnitzer, Stefan A; Klironomos, John N; HilleRisLambers, Janneke; Kinkel, Linda L; Reich, Peter B; Xiao, Kun; Rillig, Matthias C; Sikes, Benjamin A; Callaway, Ragan M; Mangan, Scott A; van Nes, Egbert H; Scheffer, Marten
    Ecosystem productivity commonly increases asymptotically with plant species diversity, and determining the mechanisms responsible for this well-known pattern is essential to predict potential changes in ecosystem productivity with ongoing species loss. Previous studies attributed the asymptotic diversity–productivity pattern to plant competition and differential resource use (e.g., niche complementarity). Using an analytical model and a series of experiments, we demonstrate theoretically and empirically that host-specific soil microbes can be major determinants of the diversity–productivity relationship in grasslands. In the presence of soil microbes, plant disease decreased with increasing diversity, and productivity increased nearly 500%, primarily because of the strong effect of density-dependent disease on productivity at low diversity. Correspondingly, disease was higher in plants grown in conspecific-trained soils than heterospecific-trained soils (demonstrating host-specificity), and productivity increased and host-specific disease decreased with increasing community diversity, suggesting that disease was the primary cause of reduced productivity in species-poor treatments. In sterilized, microbe-free soils, the increase in productivity with increasing plant species number was markedly lower than the increase measured in the presence of soil microbes, suggesting that niche complementarity was a weaker determinant of the diversity–productivity relationship. Our results demonstrate that soil microbes play an integral role as determinants of the diversity–productivity relationship.
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    Species with greater seed mass are more tolerant of conspecific neighbours: a key driver of early survival and future abundances in a tropical forest
    (Wiley, 2016) Lebrija‐Trejos, Edwin; Reich, Peter B; Hernández, Andres; Wright, S Joseph
    Multiple niche‐based processes including conspecific negative density dependence (CNDD) determine plant regeneration and community structure. We ask how interspecific and intraspecific density‐dependent interactions relate to plant life histories and associated functional traits. Using hierarchical models, we analysed how such interactions affected first‐year survival of seedling recruits of 175 species in a tropical forest, and how species abundances and functional traits are related to interspecific variation in density‐dependent effects. Conspecific seedling neighbour effects prevailed over the effects of larger conspecific and all heterospecific neighbours. Tolerance of seedling CNDD enhanced recruit survival and subsequent abundance, all of which were greater among larger seeded, slow‐growing and well‐defended species. Niche differentiation along the growth–survival trade‐off and tolerance of seedling CNDD strongly correlated with regeneration success, with manifest consequences for community structure. The ability of larger seeded species to better tolerate CNDD suggests a novel mechanism for CNDD to contribute to seed‐size variation and promote species coexistence through a tolerance–fecundity trade‐off.