Density-dependent mortality

Finally, density-dependent mortality from various enemies may be enhanced by host plant variation. Again, focusing feeding activities on a restricted set of suitable tissues should also focus the activities and abundance of pathogens, parasitoids, and predators. Sessile insects, such as gall-forming aphids (55,62),... 

The daily C fluxes (pg C L-1 d-1 ) originate from an ecosystem model by Ruardij et al. (2005) and are averaged values over a period of 36 days. The standard run represents the situation as was observed during the mesocosm experiment, with both P. globosa single cells and colonies present. Viral lysis of phytoplankton is specific for P. globosa. Viral lysis of bacteria is a second order density-dependent mortality. Respiration and high refractory DOC were modeled but are not included in the table... 

FIGURE 6.1 Overview of the scheduling of state change for an Asellus individual in the Metapopulation model for Assessing Spatial and Temporal Effects of Pesticides (M ASTEP). In boxes the different events in its life history are shown, and in italics the origin of the time delay after which the event takes place. Arrows without text point to events that take place immediately (time delay of 0). The main loops are the ones occurring with a 1-day time delay checking for density-dependent mortality, and the movement loop. Pesticide application was scheduled as a separate event. 

The representation of the processes reproduction, mortality, and movement or dispersal included stochasticity. As Figure 6.1 shows, the timing of most events was stochastic. In addition, some vital rates were represented as probabilities, for example, density-dependent mortality and the number of offspring. Stochasticity was included in order to incorporate individual variability in a natural way, and to avoid artifacts due to unrealistic synchronization (e.g., all offspring appearing at the same day). 

The density-dependent mortality rate was assumed to be linearly related to actual local density. Density-dependent reproduction was incorporated by decreasing the number of offspring with average experienced density for each individual. 

Kojola, I., Helle, T. (1993). Regional differences in density dependent mortality and reproduction in Finnish reindeer. Rangifer, 13, 33-38. 

Assumption 5 states that the endpoints measured in the toxicity tests on which the SSD is based must be ecologically relevant. Mortality is the most frequently studied endpoint in laboratory tests. In chronic tests, endpoints such as reproduction and inhibition of growth are also studied. Forbes et al. (2001a) argued that individual-level endpoints like survival, fecundity, and growth may not reflect effects at the population level (Assumption 5). They recommended that additional consideration be given to the relative frequency of different life cycle types, to the proportion of sensitive and insensitive taxonomic groups in communities, and to the role of density-dependent influences on population dynamics (see also Forbes and Calow 2002). 

In this chapter we present an individual-based population model (Metapopulation model for Assessing Spatial and Temporal Effects of Pesticides [MASTEP]). M ASTEP describes the effects on, and recovery of, populations of the water louse Asellus aqua-ticus following exposure to a fast-acting, nonpersistent insecticide caused by spray drift for pond, ditch, and stream scenarios. The model used the spatial and temporal distribution of the exposure in different treatment conditions as an input parameter. A dose-response relation derived from a hypothetical mesocosm study was used to link the exposure with the effects. The modeled landscape was represented as a lattice of 1 x 1 m cells. The model included processes of mortality of A. aquaticus, life history, random walk between cells, density-dependent population regulation, and in the case of the stream scenario, medium-distance drift of A. aquaticus due to flow. All parameter estimates were based on the results of a thorough review of published information on the ecology of A. aquaticus and expert judgment. 

Some of the parameters of the model, such as mortality, age of breeding, and number of young, have been accurately reported in the literature. The situation was completely different for parameters of movement and density-dependent processes. We know of only 1 experiment studying Asellus movement, and have derived the movement parameters for the model from this experiment, which was performed in an artificial environment without food and shelter. 

The explanation for the results was related to the assumptions built into the model relating to density dependence. Since density in ABMs was a local factor, the impact of density dependence was affected by the local history of events. In this case, if agricultural mortalities resulted in a reduction in population size before pesticide application, then those killed by the pesticide would have a large impact... 

Vonesh JR, De la Cruz O. 2002. Complex life cycles and density dependence assessing the contribution of egg mortality to amphibian declines. Oecologia 133 325-333. 

These equations contain two functional relationships which take into account for saturation effects with increasing prey density R. The first term g R) describes the density dependence of the prey growth due to intraspecific competition. The second term f R), the so-called functional response, models the saturation of the food uptake with the density of the prey. The parameter x describes the efficiency of energy conversion and b is the predator mortality. 

In general, it can be stated that the population density of an animal depends on the balance between the rate of recruitment and the rate of mortality. In the context of ecotoxicology, the influence of pollutants upon either of these factors is of fundamental interest and importance. When a population is at or near its carrying capacity, these two factors are in balance, and the critical question about the effects of pollutants is whether they can adversely affect this balance and bring a population decline. 

---