Density dependence
In
Positive density-dependence
Positive density-dependence, density-dependent facilitation, or the Allee effect describes a situation in which population growth is facilitated by increased population density.[citation needed]
Examples
dioecious (separate sex) obligatory parasites, mated female worms are required to complete a transmission cycle. At low parasite densities, the probability of a female worm encountering a male worm and forming a mating pair can become so low that reproduction is restricted due to single sex infections. At higher parasite densities, the probability of mating pairs forming and successful reproduction increases. This has been observed in the population dynamics of Schistosomes.[2]
Positive density-dependence processes occur in
Positive density-dependence processes may also occur in macroparasite infections that lead to immunosuppression. Onchocerca volvulus infection promotes immunosuppressive processes within the human host that suppress immunity against incoming infective L3 larvae. This suppression of anti-parasite immunity causes parasite establishment rates to increase with higher parasite burden.[4]
Negative density-dependence
Negative density-dependence, or density-dependent restriction, describes a situation in which population growth is curtailed by crowding, predators and competition.[citation needed]
In cell biology, it describes the reduction in cell division. When a cell population reaches a certain density, the amount of required growth factors and nutrients available to each cell becomes insufficient to allow continued cell growth.[citation needed]
This is also true for other organisms because an increased density means an increase in intraspecific competition. Greater competition means an individual has a decreased contribution to the next generation i.e. offspring. Density-dependent mortality can be overcompensating, undercompensating or exactly compensating.[citation needed]
There also exists density-independent inhibition, where other factors such as weather or environmental conditions and disturbances may affect a population's carrying capacity.[citation needed]
An example of a density-dependent variable is crowding and competition.
Examples
Density-dependent fecundity exists, where the birth rate falls as competition increases. In the context of gastrointestinal nematodes, the weight of female Ascaris lumbricoides and its rates of egg production decrease as host infection intensity increases. Thus, the per-capita contribution of each worm to transmission decreases as a function of infection intensity.[5]
In macroparasite life cycles
In reality, combinations of
Implications for parasite persistence and control
Negative density-dependent (restriction) processes contribute to the resilience of macroparasite populations. At high parasite populations, restriction processes tend to restrict population growth rates and contribute to the stability of these populations. Interventions that lead to a reduction in parasite populations will cause a relaxation of density-dependent restrictions, increasing per-capita rates of reproduction or survival, thereby contributing to population persistence and resilience.[7]
Contrariwise, positive density-dependent or facilitation processes make elimination of a parasite population more likely. Facilitation processes cause the reproductive success of the parasite to decrease with lower worm burden. Thus, control measures that reduce parasite burden will automatically reduce per-capita reproductive success and increase the likelihood of elimination when facilitation processes predominate.[8]
Extinction threshold
The extinction threshold refers to minimum parasite density level for the parasite to persist in a population. Interventions that reduce parasite density to a level below this threshold will ultimately lead to the extinction of that parasite in that population. Facilitation processes increase the extinction threshold, making it easier to achieve using parasite control interventions. Conversely, restriction processes complicates control measures by decreasing the extinction threshold.[8]
Implications for parasite distribution
Anderson and Gordon (1982) propose that the distribution of macroparasites in a host population is regulated by a combination of positive and negative density-dependent processes. In overdispersed distributions, a small proportion of hosts harbour most of the parasite population. Positive density-dependent processes contribute to
Consequently, interventions that lead to a reduction in parasite burden will tend to cause the parasite distribution to become overdispersed. For instance, time-series data for Onchocerciasis infection demonstrates that 10 years of vector control lead to reduced parasite burden with a more overdispersed distribution.[10]
See also
References
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