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Equilibrium Density (equilibrium + density)

Distribution by Scientific Domains

Life Sciences	100%

Selected Abstracts

Trade-offs and the evolution of life-histories during range expansion

ECOLOGY LETTERS, Issue 10 2010
Olivia J. Burton
Ecology Letters (2010) Abstract During range-advance, individuals on the expanding edge of the population face a unique selective environment. In this study, we use a three-trait trade-off model to explore the evolution of dispersal, reproduction and competitive ability during range expansion. We show that range expansion greatly affects the evolution of life-history traits due to differing selection pressures at the front of the range compared with those found in stationary and core populations. During range expansion, dispersal and reproduction are selected for on the expanding population front, whereas traits associated with fitness at equilibrium density (competitive ability) show dramatic declines. Additionally, we demonstrate that the presence of a competing species can considerably reduce the extent to which dispersal is selected upwards at an expanding front. These findings have important implications for understanding both the rate of spread of invasive species and the range-shifting dynamics of native species in response to climate change. [source]

The ecology of virulence

ECOLOGY LETTERS, Issue 10 2006
Curtis M. Lively
Abstract Theoretical work has shown that parasites should evolve intermediate levels of virulence. Less attention has been given to the ecology of virulence. Here I explore population-dynamic models of infection in an annual host. The infection does not kill the host; but it can decrease the number of offspring produced by the host, and the magnitude of this effect depends on host population size. Hence, ,virulence' is density dependent, and is defined here as the difference in birth rates between uninfected and infected hosts, divided by the birth rate of uninfected hosts. The results suggest that infection can be highly virulent at the host's equilibrium density, even though the parasite has no effect on the host's intrinsic birth rate. The results also suggest that parasites may help to stabilize host population dynamics. In general, the impact of infection may be underestimated in natural populations. [source]

Approaches for testing herbivore effects on plant population dynamics

JOURNAL OF APPLIED ECOLOGY, Issue 5 2006
STACEY L. HALPERN
Summary 1As plant invasions pose one of the greatest threats to biodiversity, it is critical to improve both our understanding of invasiveness and strategies for control. Much research into plant invasions and their management, including biological control, assumes strong demographic effects by natural enemies, including herbivores. However, the importance of natural enemies in the regulation of plant populations remains controversial: some ecologists contend that they rarely affect plant populations, and others that they can strongly limit plant population sizes. 2We briefly review the conflicting views and suggest that new approaches to gather and analyse data are needed before the effects of natural enemies on plant populations can be fully characterized. 3We outline experimental and analytical approaches that incorporate density dependence into population models and thus provide a more complete test of the long-term effects of natural enemies on plant populations. We also introduce new methods for obtaining stochastic estimates of equilibrium density, which will provide a key test of enemy effects on plant population size. 4Synthesis and applications. Designing effective strategies for invasive plant management requires information about the factors that limit plant population size. Together, the experiments and analyses we describe measure more clearly how natural enemies influence plant population dynamics. They will provide an important tool in evaluating the role of enemy release in plant invasions and for predicting the potential success of biological control. Such information should help to prioritize strategies that are most likely to control invasive plants effectively and will contribute to risk assessment when considering the release of non-native natural enemies as biological control agents. [source]

The stratification theory for plant coexistence promoted by one-sided competition

JOURNAL OF ECOLOGY, Issue 3 2009
Takashi Kohyama
Summary 1It is an essential feature of plants that leaves at higher levels have better access to light than those at lower levels. Thus, larger plants generally enjoy greater success in competing for light than smaller ones. We analyse the effect of such size-asymmetry, or one-sided competition, on the successful coexistence of plant species, using an analytically tractable model for stratified populations, in which a plant in the same layer exhibits the same crowding effect as any other, irrespective of species. 2A two-layer population that is reproductive in upper layer and juvenile in lower layer has a uniquely stable (plant-size-weighted) equilibrium density, as long as its fecundity is sufficient to compensate for its mortality rate. We also calculate a unique threshold lower-layer density of this layered population when there is no upper-layer plant. This threshold lower-layer density is larger than the weighted equilibrium density with upper layer, except for the case of perfect two-sided competition. 3A two-layer species can stably coexist with a one-layer, understorey species as a result of one-sided, but not two-sided competition. The coexistence condition is that the equilibrium density of the one-layer species lies between the threshold lower-layer density and the equilibrium density of the two-layer species. For an understorey species to coexist successfully with a two-layer species, any advantage in demographic performance, most prominently in a sufficiently high fecundity per plant must offset the disadvantage of living in dark conditions. 4Results from a model of multi-layer populations suggest that several species differing in terms of maximum layer and fecundity can coexist under conditions of one-sided competition. We demonstrate an example of the stable coexistence of eight species. The inter-specific trade-offs predicted by the model correspond to patterns observed in a rain forest. 5Synthesis. We propose a stratification theory that explains the generation and maintenance of the successful coexistence of plant species. Under the condition of one-sided competition, a canopy population that takes advantage of escaping from understorey competition shows an ability to invade an understorey with a density higher than its own equilibrium density, and which offers opportunities for an understorey population with high fecundity and/or shade tolerance to coexist. The predicted coexistence of species that share maximum canopy height is most pronounced for trees of tropical rain forests. [source]

Enemy-mediated apparent competition: empirical patterns and the evidence

OIKOS, Issue 2 2000
Enrique J. Chaneton
Apparent competition arises when two victim species negatively affect each other (,,,,) by enhancing the equilibrium density or changing the foraging behaviour of a shared natural enemy. Shared enemies can also mediate non-reciprocal (,,,0) indirect effects, i.e. indirect amensalism, whenever one prey species is not affected by the presence of alternative prey. We review 34 studies on terrestrial and freshwater systems to evaluate the extent to which apparent competition has been perceived as a reciprocal (,,,,) or non-reciprocal (,,,0) interaction. We found only three studies showing reciprocal effects between apparent competitors. Indirect amensalism was documented in 10 studies and could be inferred for 16 other cases (76% in total). The remaining five studies provided insufficient data to determine the form of indirect interaction. The apparent prevalence of non-reciprocal enemy-mediated interactions resembles that observed for resource-based interspecific competition. Amensal indirect effects via shared predation may result from differences in population size, nutritional value, susceptibility to attack, or asynchronous dynamics of alternative prey, or the predator's feeding preferences. Moreover, experimental protocols may confound the actual form of apparent competition through short-term observations, incomplete designs, or biased consideration of conspicuous interactions, leading to reciprocal effects being overlooked. We conclude that, at present, it is still difficult to determine the relative role of apparent competition vs indirect amensalism in natural food webs because most published studies have failed to document in full interactions via shared enemies. [source]

Carrying Capacity and Potential Production of Ungulates for Human Use in a Mexican Tropical Dry Forest

BIOTROPICA, Issue 4 2007
Salvador Mandujano
ABSTRACT Data are provided on the carrying capacity and potential production for sustainable human use of white-tailed deer (Odocoileus virginianus) and collared peccary (Pecari tajacu) in a protected tropical dry forest at Chamela on the Pacific coast of Mexico. In this paper, the carrying capacity was defined as the equilibrium density plus the number of animals removed by predators. The equilibrium point was estimated from the density dependent relationship between the finite population growth rate and the current density according to a logistic model. Annual density was estimated using the line transect method. Carrying capacity estimates were 16.5 to 17.2 deer/km2 and 9.3,9.5 peccaries/km2, representing a combined biomass of 841,874 kg/km2. A potential production for human use of 2.1 deer/km2 and 4.4 peccaries/km2 was estimated employing the model of Robinson and Redford (1991). The data suggest that, in the protected tropical dry forest of Chamela, the density and biomass of wild ungulates can maintain a similar or greater density and biomass than other Neotropical forests. To obtain an accurate estimation of the maximum sustainable yield (MSY), it is necessary to consider predation. From a management point of view, it is important to consider that carrying capacity varies as a function of the rainfall pattern. RESUMEN Se presentan datos acerca de la capacidad de carga y la producción potencial del venado cola blanca (Odocoileus virginianus) y pecarí de collar (Pecari tajacu) para aprovechamiento humano en un bosque tropical seco de Chamela en la costa Pacífica de México. En este trabajo se definió capacidad de carga como la densidad en el punto de equilibrio del crecimiento poblacional más el número de animales removidos por los depredadores. La densidad en equilibrio se estimó a partir de la relación de denso-dependencia entre la tasa finita de crecimiento poblacional y la densidad anual de acuerdo al modelo logístico. La densidad anual se estimó empleando el método de transecto de línea. La capacidad de carga se estimó en 16.5 a 17.2 venados/km2 y 9.3 a 9.5 pecaries/km2, y una biomasa combinada de 841 a 874 kg/km2. Empleando el modelo de Robinson y Redford (1991) se estimó una producción potencial para aprovechamiento humano de 2.1 venados/km2 y 4.4 pecaries/km2. Los datos indican que en bosque tropical seco protegido de Chamela la densidad y biomasa de los ungulados silvestres puede ser similar o mayor en comparación con otros bosques neotropicales. Para obtener una estimación precisa de la cosecha máxima sostenible es importante considerar el efecto de la depredación. Desde una perspectiva de manejo, se debe incorporar la variación en la capacidad de carga en función del patrón de lluvias. [source]