1,721,231 research outputs found
The role of size inequality in self-thinning: A pattern-oriented simulation model for arid savannas
No full textThe self-thinning line is a very robust pattern, which can be obtained in modeling studies by a variety of different mechanistic assumptions. Our opinion is that we can only advance in our understanding of mechanisms leading to the self-thinning relationship if we demand that the model also reproduces several other characteristic features (patterns) of the self-thinning process such as the degree of size inequality and the average size. We use a pattern-oriented modeling approach to develop a model of self-thinning under size inequality in overcrowded, even-aged stands, which reproduces these three patterns simultaneously. Our approach is to first develop an initial model based on our current ecological knowledge and then to refine the model by modifying the initial model to derive the model that reproduces all patterns of interest. The initial model is as simple as possible while avoiding incidental, ecologically unjustified, assumptions. It is a further development of zone of influence-simulation models: each plant is described by two circles, one describing a minimum-domain-area and one describing the zone of influence. In the initial model, mortality is “death-by-contact” of minimum-domain-areas and growth is a function of inter-tree competition, i.e. overlapping zones of influence. Model parameterization is based on field data on Acacia reficiens in southern Africa. Simulations follow patches of initially small trees through time for up to 1000 years with five parameters, three describing growth and two describing inter-tree competition. A sensitivity analysis shows that all parameters of the initial model contribute significantly to the number and size of plants through time. The two competition parameters, which describe competitive asymmetry and the size of the zone of influence relative to canopy size, are both important for generating size inequality. Thus, both competitive asymmetry and spatial pattern contribute to size inequality, and their relative importance may vary greatly. The sensitivity analysis suggests that all processes included in the initial model are essential to the evolution of size inequality. However, size inequality under the initial model is below field values, meaning that additional, as yet unconsidered processes, contribute to size inequality. Our best-fit model additionally contains details on growth stochasticity. This study establishes the often-proposed direct link between mortality driven by local competition and self-thinning and highlights the importance of stochasticity in ecological processes
Topics in Evolutionary Ecology
Full text link46 pages, 1 article*Topics in Evolutionary Ecology* (Levin, Simon A.; Castillo-Chavez, Carlos) 46 page
Regime shifts in a social-ecological system
No full textEcological regime shifts are rarely purely ecological. Not only is the regime shift frequently triggered by human activity, but the responses of relevant actors to ecological dynamics are often crucial to the development and even existence of the regime shift. Here, we show that the dynamics of human behaviour in response to ecological changes can be crucial in determining the overall dynamics of the system. We find a social-ecological regime shift in a model of harvesters of a common-pool resource who avoid over-exploitation of the resource by social ostracism of non-complying harvesters. The regime shift, which can be triggered by several different drivers individually or also in combination, consists of a breakdown of the social norm, sudden collapse of co-operation and an over-exploitation of the resource. We use the approach of generalized modeling to study the robustness of the regime shift to uncertainty over the specific forms of model components such as the ostracism norm and the resource dynamics. Importantly, the regime shift in our model does not occur if the dynamics of harvester behaviour are not included in the model. Finally, we sketch some possible early warning signals for the social-ecological regime shifts we observe in the models. © 2013 Springer Science+Business Media Dordrecht
The Role of Long Incubation Periods in the Dynamics of Acquired Immunodeficiency Syndrome (AIDS). Part 1: Single Population Models
Full text link44 pages, 1 article*The Role of Long Incubation Periods in the Dynamics of Acquired Immunodeficiency Syndrome (AIDS). Part 1: Single Population Models* (Castillo-Chavez, Carlos; Cooke, Kenneth; Huang, Wenzhang; Levin, Simon A.) 44 page
The Role of Long Periods of Infectiousness in the Dynamics of Acquired Immunodeficiency Syndrome (AIDS)
Full text link14 pages, 1 article*The Role of Long Periods of Infectiousness in the Dynamics of Acquired Immunodeficiency Syndrome (AIDS)* (Castillo-Chavez, Carlos; Cooke, Kenneth; Huang, Wenzhang; Levin, Simon A.) 14 page
Vertically structured prokaryotic community can control the efficiency of the biological pump in the oceans
No full textVertically-structured prokaryotic community can control the efficiency of the biological pump in the oceans.
No full text
Epidemiological Models with Age Structure, Proportionate Mixing, and Cross-Immunity
Full text linkThis issue was undated. The date given is an estimate.44 pages, 1 article*Epidemiological Models with Age Structure, Proportionate Mixing, and Cross-Immunity* (Castillo-Chavez, Carlos; Hethcote, Herbert W.; Andreasen, Viggo; Levin, Simon A.; Liu, Wei-min) 44 page
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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