Theoretical Ecology最新文献

Climate change can affect reef fish both directly (e.g., mortality, growth, fecundity) and indirectly (e.g., habitat degradation). The extent to which the effects of rising water temperature could drive changes in fish populations and if and how these effects may interact with potential management interventions remain unclear. The objective of this study was to test various hypothesized mechanisms by which sea surface temperature (SST) could affect reef fish population dynamics and explore these effects in combination with fishing effort restrictions and spatial closures. To do this, we modeled hypothesized relationships between SST and two governing parameters of the fish populations: intrinsic growth rate (r) and carrying capacity (K). We coupled these temperature-dependent fish population models with a fisheries harvest model and explored interactions between thermal effects, fishing effort level, and spatial closures. Under small closure scenarios, we found that the thermal effects models predicted substantially lower fish population biomass and harvest compared to the baseline (constant r and K) model. Under large closure scenarios, the thermal effects models more closely resembled the baseline. Generally, incorporating spatial closures mitigated some of the detrimental thermal effects on fish biomass and allowed for increased harvest under certain fishing effort levels. Whether intrinsic growth or carrying capacity most affected fish population levels also depended on the fishing effort and the spatial closure area. Overall, we described how fishing effort and spatial closures can influence the relative importance of key processes and the extent to which rising water temperatures affect fish populations and harvest.

de Aguiar et al. have shown that basic patterns of species diversity found in nature can be described by a neutral model of speciation in which species emerge simply as a consequence of local mating and mate preference for genetic similarity. Their results have been cited as support for the neutral theory of biodiversity. However, because the mutation rates considered in their work are much larger than those experienced by living organisms, there is still some question as to whether speciation will occur in this type of model under realistic conditions. Here, I develop a variant of the neutral model that includes a realistic mechanism for organism dispersal. I explore speciation in the model for a class of mobile organisms (butterflies), and I find that speciation does occur under conditions consistent with butterfly populations, albeit on narrow landscapes. The model also appears to exhibit scaling behavior—specifically, if the model is “scaled up” by increasing the area of the landscape while holding its length to width ratio and population density constant, the number of species tends to an asymptotic value. The results suggest that it is possible to infer speciation patterns in large populations by simulating much smaller, computationally tractable populations.

In tropical forests, deciduous and evergreen leaf habits represent contrasting tree adaptations to precipitation seasonality. Both rainfall seasonality and interannual variation in rainfall are determinants of forest deciduousness, but their relative influence is not well understood. In this study, we evaluate the extent of deciduous-evergreen coexistence in tropical forests and develop a simple model of competition for water between leaf habits. Using this model, we formalize two mechanisms representing rainfall variability across time scales that may explain their stable coexistence: the temporal storage effect via interannual variability in rainfall vs. rainfall partitioning via evergreen access to dry-season rainfall. In our model, both mechanisms resulted in coexistence, but coexistence was more robust via resource partitioning. Empirically, remotely sensed deciduousness increased with precipitation seasonality, but effects of interannual rainfall variability on deciduousness were minor. We hypothesize that dry-season rainfall may prove a stronger influence on coexistence between leaf habits, and that changes in rainfall seasonality will have a greater impact on forest deciduousness than changes in the interannual variability of rainfall.

The critical community size (CCS) is the minimum closed population size in which a pathogen can persist indefinitely. Below this threshold, stochasticity eventually causes pathogen extinction. Here, we introduce a mechanism of behaviour-mediated persistence, by which the population response to the pathogen alters the CCS. We exemplify this with infection transmission and non-pharmaceutical interventions (NPIs) in a population where both individuals and government authorities restrict transmission more strongly when case numbers are higher. This results in a coupled social-ecological feedback between disease dynamics and population behaviour. In a parameter regime corresponding to a moderate population response, this feedback allows the pathogen to avoid extinction in epidemic troughs. The result is a very low CCS that allows long-term pathogen persistence. Hence, an incomplete population response represents a “sour spot” that not only ensures relatively high case incidence but also promotes long-term persistence of the pathogen by reducing the CCS. We illustrate this mechanism for parameters corresponding to severe coronavirus 2 (SARS-CoV-2). Given the worldwide prevalence of small, isolated populations, these results emphasize the need for incorporating behavioural feedback into CCS estimates. Regional elimination and global eradication programs for vaccine-preventable infections could also account for this effect.

Spatio-ecological heterogeneity has a significant impact on various ecosystem properties, such as biodiversity patterns, variability in ecosystem resources, and species distributions. Given this perspective, remote sensing has gained widespread recognition as a powerful tool for assessing the spatial heterogeneity of ecosystems by analyzing the variability among different pixel values in both space and, potentially, time. Several measures of spatial heterogeneity have been proposed, broadly categorized into abundance-related measures (e.g., Shannon’s H) and dispersion-related measures (e.g., Variance). A measure that integrates both abundance and distance information is the Rao’s quadratic entropy (Rao’s Q index), mainly used in ecology to measure plant diversity based on in-situ based functional traits. The question arises as to why one should use a complex measure that considers multiple dimensions and couples abundance and distance measurements instead of relying solely on simple dispersion-based measures of heterogeneity. This paper sheds light on the spatial version of the Rao’s Q index, based on moving windows for its calculation, with a particular emphasis on its mathematical and statistical properties. The main objective is to theoretically demonstrate the strength of Rao’s Q index in measuring heterogeneity, taking into account all its potential facets and applications, including (i) integrating multivariate data, (ii) applying differential weighting to pixels, and (iii) considering differential weighting of distances among pixel reflectance values in spectral space.

Natural ecological systems typically exhibit transient dynamics, driven by various ecological and environmental factors. Understanding the root causes of transient behaviour and the associated regime shifts is of central importance for the sustainable management of ecosystems. Here, we develop and analyse a process-based model to describe the interaction between plants and their herbivore predators and to elucidate the mechanisms underlying transient patterns in plant-herbivore systems. Our model involves key components including seed-reproduction rates, plant dispersal abilities, the germination probabilities of seeds surviving predation, local interactions among plants, seed-predation rates, and herbivore conversion efficiencies. The plant-herbivore system has exhibited short-term and long-term transient behaviour and strong dependence of plant demography and predation pressure on abrupt transient shifts and duration of transients. Our results have demonstrated that high seed-reproduction rates obstruct long transients and can lead to extinctions of plants with low dispersal abilities. Herbivore predators have also exhibited delayed response to abrupt increases in plant density, even if their seed-predation rate is high. Taken together, our findings suggest that transient patterns are predominantly driven by the ecological and environmental pressure that plants experience.

Treeline is a global ecological phenomenon in which tree populations decline, often abruptly, above a specific elevation or latitude. Temperature is thought to be a key determinant of treeline because it affects the rates at which trees establish, grow, produce seeds, and die. As climate change causes temperature increases, treelines have been observed to move in response—but there is considerable variability. In this study, we present a general mathematical model that provides possible explanations for both the general patterns observed in treelines and some of the variation. Avoiding system-specific details, our model assumes simply that all life processes are temperature-dependent. We incorporate the possibility of positive or negative feedback, in which the presence of trees either increases or decreases the temperature at their location. Our results indicate that this feedback and the relationship between temperature thresholds for growth, seed production, and seedling establishment are the key determinants of tree line form and movement. The model also shows that under many conditions bistability is predicted: treeline can equilibrate at two different elevations under the same conditions, depending on the system’s history. General, flexible models like ours are essential for generating a unifying theory of treeline form and dynamics across multiple ecosystems.

A cellular automata model was developed and parameterized to test the effectiveness of application of biological control insects to water hyacinth (Pontederia crassipes), which is an invasive floating plant species in many parts of the world and outcompetes many submersed native aquatic species in southern Florida. In the model, P. crassipes was allowed to compete with Nuttall’s waterweed (Elodea nuttallii). In the absence of biocontrol acting on the P. crassipes, E. nuttallii excluded P. crassipes at low concentrations of the limiting nutrient (nitrogen), and the reverse occurred at high nutrient concentrations. At intermediate values, alternative stable states could occur; either P. crassipes alone or a mixture of the two species. When the biocontrol agent, the weevil Neochetina eichhorniae, was applied in the model, there was initially a rapid reduction of the P. crassipes, however, over time a regular striped pattern of moving spatially alternating stripes of P. crassipes and E. nuttallii emerged. -This pattern of moving stripes emerged and persisted over thousands of days but could quickly transform into an irregular pattern at some apparently random time, when either external stochasticity (added adult weevils) or only the weak internal stochasticity of weevil movements occurred. The cause of the end of the long transient can be traced to a single slightly irregular pixel within the striped pattern. Model parameters were varied to study effects of plant growth rate, nutrient concentration and nutrient diffusion rate on the dynamics of the system.

Adults of site-dependent species require a discrete structure, e.g., a cavity, for breeding, which they are unable to construct and must locate and occupy. The environment provides only a limited number of such sites, which may vary in overall quality due to their environmental context. Heterogeneity of site quality can result in population equilibrium, often construed as source-sink dynamics. Rodenhouse et al. (Ecology 78:2025-2042, 1997) proposed a mechanism of site-dependent equilibrium that they claimed was more general than source-sink dynamics. After defining notions of source and sink, I use explicit dynamical models for a site-dependent population, based on the life history of golden eagles (Aquila chrysaetos), with two levels of site quality, to investigate the existence of population equilibria under several scenarios: source-source, source-sink, and source-floater. The life history traits I employ are not overly restrictive and serve the purpose only of providing models explicit enough to be treated analytically. I use a generalized notion of “golden eagle” since site dependency is often discussed in the literature on raptors, and I have exploited details from Hunt et al. (PLoS ONE 12:e0172232, 2017) for numerical simulations. The crucial features of the modeling, however, are those of site dependency. The modeling emphasizes that equilibrium results from the limited supply of source sites and that vital rates averaged across site qualities do not provide a compelling explanation of equilibria, contra Rodenhouse et al. Counterintuitively, equilibria are theoretically possible, even when both site qualities are intrinsically source sites.