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Density-dependent Processes (density-dependent + process)
Selected AbstractsThe effect of initial seed density on the structure of a desert annual plant communityJOURNAL OF ECOLOGY, Issue 3 2002Christopher J. Lortie Summary 1,Few experiments directly test the role of density dependence in natural plant communities. 2,We tested (i) whether different initial seed densities of the entire seed bank of an annual plant community affected performance (emergence, mean plant biomass and survival) and (ii) whether density-dependent processes were operating within the communities. We also tested whether life-stage, year and soil moisture (both on a topographic gradient and in an experimental manipulation) influenced the effects of seed density. 3,We considered two distinct phases: seed to emergent seedlings, and emergent seedlings to established plants. 4,The seed bank was collected from a semistabilized sand dune in the Negev Desert, Israel. This was added to sieved sand in plots at the same site at four different initial seed densities: 1/16×, 1/4×, 1× (natural seed density) and 2×. The experiment was repeated for three consecutive growing seasons. 5,Emergence of seedlings was significantly influenced by initial seed density in all 3 years, with higher initial seed densities having lower rates of emergence. 6,Mean final plant size was negatively density dependent and consistently unaffected by the initial seed density sown. 7,In general, there were no strong interactions of topographic position with initial seed density, processes within the vegetation were not density dependent and the experimental addition of water did not influence any of the performance measures tested. Density-dependent processes vary from year to year, while moisture effects do not. 8,We conclude that seedling emergence and some processes in the established plant community are density dependent, but the established plant community is also affected by other processes, such as resource limitation. [source] ORIGINAL ARTICLE: Implications of fisheries-induced evolution for stock rebuilding and recoveryEVOLUTIONARY APPLICATIONS (ELECTRONIC), Issue 3 2009Katja Enberg Abstract Worldwide depletion of fish stocks has led fisheries managers to become increasingly concerned about rebuilding and recovery planning. To succeed, factors affecting recovery dynamics need to be understood, including the role of fisheries-induced evolution. Here we investigate a stock's response to fishing followed by a harvest moratorium by analyzing an individual-based evolutionary model parameterized for Atlantic cod Gadus morhua from its northern range, representative of long-lived, late-maturing species. The model allows evolution of life-history processes including maturation, reproduction, and growth. It also incorporates environmental variability, phenotypic plasticity, and density-dependent feedbacks. Fisheries-induced evolution affects recovery in several ways. The first decades of recovery were dominated by demographic and density-dependent processes. Biomass rebuilding was only lightly influenced by fisheries-induced evolution, whereas other stock characteristics such as maturation age, spawning stock biomass, and recruitment were substantially affected, recovering to new demographic equilibria below their preharvest levels. This is because genetic traits took thousands of years to evolve back to preharvest levels, indicating that natural selection driving recovery of these traits is weaker than fisheries-induced selection was. Our results strengthen the case for proactive management of fisheries-induced evolution, as the restoration of genetic traits altered by fishing is slow and may even be impractical. [source] Relating juvenile spatial distribution to breeding patterns in anadromous salmonid populationsJOURNAL OF ANIMAL ECOLOGY, Issue 2 2010Anders Foldvik Summary 1. Spatial within-population heterogeneity in density probably affects competition intensity and may have a fundamental role in shaping population dynamics and carrying capacity. This may be particularly relevant for organisms where limitations on juvenile mobility cause maternal choice of breeding locations to influence the spatial distribution of younger life stages. 2. In this study, we mapped redd locations and the resulting densities of juveniles the following year along the entire reach (9·2 km) of a river holding natural populations of anadromous salmonids (Atlantic salmon and brown trout). These data were used to quantify the spatial scale over which breeding influences juvenile densities, and hence becomes important for density-dependent processes. 3. Although the observed cumulative distributions indicated a relatively uniform distribution of breeding along the river, autocorrelation analyses identified spatial patchiness of both breeding and resulting juveniles on a local scale. Furthermore, cross-correlations suggested a close spatial relationship between distribution of redds and juveniles. 4. Using spatially explicit hockey-stick stock,recruitment functions, we found juvenile salmonid density to be mostly influenced by the amount of breeding upstream of a given location. This influence decreased rapidly within the first 75,150 m. Thus, female choice with regard to breeding location gave rise to a heterogeneous distribution of offspring on a spatial scale of almost two orders of magnitude finer than that of the whole population (9·2 km). 5. The results are consistent with smaller scale experimental studies of salmonids, and emphasize the role for maternal choice of breeding locations in causing substantial spatial heterogeneity in juvenile densities within natural populations. Due to effects of density heterogeneity on overall levels of competition, this adds another layer of complexity to the dynamics of salmonid populations even in populations where breeding appears to be relatively uniformly distributed through space, and potentially also for a range of other organisms where juvenile dispersal is constrained. [source] Density dependence in a recovering osprey population: demographic and behavioural processesJOURNAL OF ANIMAL ECOLOGY, Issue 5 2008V. Bretagnolle Summary 1Understanding how density-dependent and independent processes influence demographic parameters, and hence regulate population size, is fundamental within population ecology. We investigated density dependence in growth rate and fecundity in a recovering population of a semicolonial raptor, the osprey Pandion haliaetus [Linnaeus, 1758], using 31 years of count and demographic data in Corsica. 2The study population increased from three pairs in 1974 to an average of 22 pairs in the late 1990s, with two distinct phases during the recovery (increase followed by stability) and contrasted trends in breeding parameters in each phase. 3We show density dependence in population growth rate in the second phase, indicating that the stabilized population was regulated. We also show density dependence in productivity (fledging success between years and hatching success within years). 4Using long-term data on behavioural interactions at nest sites, and on diet and fish provisioning rate, we evaluated two possible mechanisms of density dependence in productivity, food depletion and behavioural interference. 5As density increased, both provisioning rate and the size of prey increased, contrary to predictions of a food-depletion mechanism. In the time series, a reduction in fledging success coincided with an increase in the number of non-breeders. Hatching success decreased with increasing local density and frequency of interactions with conspecifics, suggesting that behavioural interference was influencing hatching success. 6Our study shows that, taking into account the role of non-breeders, in particular in species or populations where there are many floaters and where competition for nest sites is intense, can improve our understanding of density-dependent processes and help conservation actions. [source] The effect of initial seed density on the structure of a desert annual plant communityJOURNAL OF ECOLOGY, Issue 3 2002Christopher J. Lortie Summary 1,Few experiments directly test the role of density dependence in natural plant communities. 2,We tested (i) whether different initial seed densities of the entire seed bank of an annual plant community affected performance (emergence, mean plant biomass and survival) and (ii) whether density-dependent processes were operating within the communities. We also tested whether life-stage, year and soil moisture (both on a topographic gradient and in an experimental manipulation) influenced the effects of seed density. 3,We considered two distinct phases: seed to emergent seedlings, and emergent seedlings to established plants. 4,The seed bank was collected from a semistabilized sand dune in the Negev Desert, Israel. This was added to sieved sand in plots at the same site at four different initial seed densities: 1/16×, 1/4×, 1× (natural seed density) and 2×. The experiment was repeated for three consecutive growing seasons. 5,Emergence of seedlings was significantly influenced by initial seed density in all 3 years, with higher initial seed densities having lower rates of emergence. 6,Mean final plant size was negatively density dependent and consistently unaffected by the initial seed density sown. 7,In general, there were no strong interactions of topographic position with initial seed density, processes within the vegetation were not density dependent and the experimental addition of water did not influence any of the performance measures tested. Density-dependent processes vary from year to year, while moisture effects do not. 8,We conclude that seedling emergence and some processes in the established plant community are density dependent, but the established plant community is also affected by other processes, such as resource limitation. [source] Neighbourhood interactions and environmental factors influencing old-pasture succession in the Central PyreneesJOURNAL OF VEGETATION SCIENCE, Issue 1 2004í Dole Abstract. The shrub Buxus sempervirens and the trees Abies alba and Fagus sylvatica have recently recolonized old-pastures in the Central Pyrenees. We mapped all live and dead individuals (> 1.3 m tall) in a large forest plot in Ordesa Valley to examine the importance of density-dependent processes during recolonization. Biotic interactions were inferred from changes in horizontal structure and the influences of neighbours on tree survival. Buxus differentially influenced establishment and survival of tree species, thereby controlling future canopy composition and spatial structure. The rapidly invading Abies formed denser patches on elevated sites less occupied by Buxus, whereas Fagus preferentially established within shrubs. Abies reached densities which led to intense intraspecific competition and high mortality rates among saplings. Self-thinning in Abies led to smaller numbers of regularly spaced survivors, and greater relative dominance of Fagus. Disregarding intraspecific effects and abiotic environment, Abies survival was significantly lower under Buxus shrubs, which suggests that the spatial location and abundance of Abies was constrained by the location of Buxus. Fagus survival was not related to Buxus density, but remained significantly lower in denser Abies patches. The higher mortality of Fagus in denser Abies patches, and the resulting spatial segregation of the species, reflects asymmetric interspecific competition. Inhibition from Buxus shrubs and intraspecific competition prevent invading Abies from dominating and may therefore help in maintaining a mixed Abies-Fagus stand. [source] Single- or multistage regulation in complex life cycles: does it make a difference?OIKOS, Issue 2 2000Barbara Hellriegel Data on the different stages of complex life cycles are often rather unbalanced, especially those concerning the effects of density. How does this affect our understanding of a species' population dynamics? Two discrete three-stage models with overlapping generations and delayed maturation are constructed to address this question. They assume that survival or emigration in any life stage and/or reproduction can be density dependent. A typical pond-breeding amphibian species with a well-studied larval stage serves as an example. Numerical results show that the population dynamics resulting from density dependence at a single (e.g. the larval) stage can be decisively and unpredictably modified by density dependence in additional stages. Superposition of density-dependent processes could thus be one reason for the difficulties in identifying density dependence in the field. Moreover, in a simulated source-refuge system with habitat-specific density-dependent dispersal of juveniles density dependence in multiple stages can stabilize or destabilize the dynamics and produce misleading age structures. From an applied perspective this model shows that excluding multistage regulation prematurely clearly affects our ability to predict consequences of human impacts. [source] | ||||||||||