Density dependence

In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. This article will focus on density-dependence in the context of macroparasite life cycles. In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. This article will focus on density-dependence in the context of macroparasite life cycles. Positive density-dependence, density-dependent facilitation, or the Allee effect describes a situation in which population growth is facilitated by increased population density. For 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. Positive density-dependence processes occur in macroparasite life cycles that rely on vectors with a cibarial armature, such as Anopheles or Culex mosquitoes. For Wuchereria bancrofti, a filarial nematode, well-developed cibarial armatures in vectors can damage ingested microfilariae and impede the development of infective L3 larvae. At low microfilariae densities, most microfilariae can be ruptured by teeth, preventing successful development of infective L3 larvae. As more larvae are ingested, the ones that become entangled in the teeth may protect the remaining larvae, which are then left undamaged during ingestion. 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. Negative density-dependence, or density-dependent restriction, describes a situation in which population growth is curtailed by crowding, predators and competition. 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. 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. There also exists density-independent inhibition, where other factors such as weather or environmental conditions and disturbances may affect a population's carrying capacity.