HOME

TheInfoList



OR:

In
population ecology Population ecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment, such as birth and death rates, and by immigration and emigration. The discipline is import ...
, density-dependent processes occur when
population growth rate Population growth is the increase in the number of people in a population or dispersed group. Actual global human population growth amounts to around 83 million annually, or 1.1% per year. The global population has grown from 1 billion in 1800 to ...
s are regulated by the density of a
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
. This article will focus on density-dependence in the context of macroparasite life cycles.


Positive density-dependence

Positive density-dependence, density-dependent facilitation, or the
Allee effect The Allee effect is a phenomenon in biology characterized by a correlation between population size or density and the mean individual fitness (often measured as ''per capita'' population growth rate) of a population or species. History and backgro ...
describes a situation in which population growth is facilitated by increased population density. 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_ 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_Schistosoma">''Schistosomes''. Positive_density-dependence_processes_occur_in__macroparasite_life_cycles_that_rely_on_Vector_(epidemiology).html" ;"title="Schistosoma.html" ;"title="Dioecy.html" ;"title="


Examples

For Dioecy">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 Schistosoma">''Schistosomes''. Positive density-dependence processes occur in macroparasite life cycles that rely on Vector (epidemiology)">vectors with a cibarial armature, such as ''
Anopheles ''Anopheles'' () is a genus of mosquito first described and named by J. W. Meigen in 1818. About 460 species are recognised; while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus ''Plasmodium'', which ...
'' or ''
Culex ''Culex'' is a genus of mosquitoes, several species of which serve as vectors of one or more important diseases of birds, humans, and other animals. The diseases they vector include arbovirus infections such as West Nile virus, Japanese encep ...
'' mosquitoes. For ''Wuchereria bancrofti'', a filarial nematode, well-developed cibarial armatures in vectors can damage ingested microfilariae and impede the development of infective L3
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...
e. 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 Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...
. '' 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

Negative density-dependence, or density-dependent restriction, describes a situation in which population growth is curtailed by crowding, predators and competition.{{{cn, date=March 2022 In cell biology, it describes the reduction in
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
division. When a cell
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
reaches a certain density, the amount of required
growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regul ...
s and nutrients available to each cell becomes insufficient to allow continued cell growth. date=March 2022 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 disturbance (ecology), disturbances may affect a population's carrying capacity.{{cn, date=March 2022 An example of a density-dependent variable is crowding and competition.


Examples

Density-dependent
fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...
exists, where the birth rate falls as competition increases. In the context of gastrointestinal nematodes, the weight of female ''
Ascaris lumbricoides ''Ascaris lumbricoides'' is a large parasitic worm that causes ascariasis in humans. A roundworm of genus '' Ascaris'', it is the most common parasitic worm in humans. An estimated one-sixth of the human population is at some point infected by ...
'' 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. Parasite-induced vector mortality is a form of negative density-dependence. The '' Onchocerciasis'' life cycle involves transmission via a
black fly A black fly or blackfly (sometimes called a buffalo gnat, turkey gnat, or white socks) is any member of the family Simuliidae of the Culicomorpha infraorder. It is related to the Ceratopogonidae, Chironomidae, and Thaumaleidae. Over 2,200 spec ...
vector. In this life-cycle, the life expectancy of the black fly vector decreases as the worm load ingested by the vector increases. Because '' O. volvulus''
microfilariae ::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown, ...
require at least seven days to mature into infective L3 larvae in the black fly, the worm load is restricted to levels that allow the black fly to survive for long enough to pass infective L3 larvae onto humans.


In macroparasite life cycles

In macroparasite life cycles, density-dependent processes can influence parasite fecundity, survival, and establishment. Density-dependent processes can act across multiple points of the macroparasite life cycle. For filarial worms, density-dependent processes can act at the host/vector interface or within the host/vector life-cycle stages. At the host/vector interface, density-dependence may influence the input of L3 larvae into the host's skin and the ingestion of microfilariae by the vector. Within the life-cycle stages taking place in the vector, density-dependence may influence the development of L3 larvae in vectors and vector life expectancy. Within the life-cycle stages taking place in the host, density-dependence may influence the development of microfilariae and host life expectancy.{{cite journal , title=Density dependence and the control of helminth parasites , last=Churcher , first=T.S. , journal=Journal of Animal Ecology , year=2006 , volume=75 , issue=6 , pages=1313–1320 , doi=10.1111/j.1365-2656.2006.01154.x, pmid=17032363 In reality, combinations of negative (restriction) and positive (facilitation) density-dependent processes occur in the life cycles of parasites. However, the extent to which one process predominates over the other vary widely according to the parasite, vector, and host involved. This is illustrated by the '' W. bancrofti'' life cycle. In ''
Culex ''Culex'' is a genus of mosquitoes, several species of which serve as vectors of one or more important diseases of birds, humans, and other animals. The diseases they vector include arbovirus infections such as West Nile virus, Japanese encep ...
'' mosquitoes, which lack a well-developed cibarial armature,
restriction Restriction, restrict or restrictor may refer to: Science and technology * restrict, a keyword in the C programming language used in pointer declarations * Restriction enzyme, a type of enzyme that cleaves genetic material Mathematics and logi ...
processes predominate. Thus, the number of L3 larvae per mosquito declines as the number of ingested microfilariae increases. Conversely, in ''
Aedes ''Aedes'' is a genus of mosquitoes originally found in tropical and subtropical zones, but now found on all continents except perhaps Antarctica. Some species have been spread by human activity: '' Aedes albopictus'', a particularly invasive sp ...
'' and ''
Anopheles ''Anopheles'' () is a genus of mosquito first described and named by J. W. Meigen in 1818. About 460 species are recognised; while over 100 can transmit human malaria, only 30–40 commonly transmit parasites of the genus ''Plasmodium'', which ...
'' mosquitoes, which have well-developed cibarial armatures, facilitation processes predominate. Consequently, the number of L3 larvae per mosquito increases as the number of ingested
microfilariae ::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown, ...
increases.


Implications for parasite persistence and control

Negative density-dependent (restriction) processes contribute to the resilience of macroparasite populations. At high parasite populations,
restriction Restriction, restrict or restrictor may refer to: Science and technology * restrict, a keyword in the C programming language used in pointer declarations * Restriction enzyme, a type of enzyme that cleaves genetic material Mathematics and logi ...
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. 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.{{cite journal , title=Determinants of the eradicability of filarial infections: a conceptual approach , last=Duerr , first=H.P. , journal=Trends in Parasitology , year=2005 , volume=21 , issue=2 , pages=88–96 , doi=10.1016/j.pt.2004.11.011, pmid=15664532


Extinction threshold

The
extinction threshold Extinction threshold is a term used in conservation biology to explain the point at which a species, population or metapopulation, experiences an abrupt change in density or number because of an important parameter, such as habitat loss. It is at t ...
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 Restriction, restrict or restrictor may refer to: Science and technology * restrict, a keyword in the C programming language used in pointer declarations * Restriction enzyme, a type of enzyme that cleaves genetic material Mathematics and logi ...
processes complicates control measures by decreasing the extinction threshold.


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
overdispersion In statistics, overdispersion is the presence of greater variability (statistical dispersion) in a data set than would be expected based on a given statistical model. A common task in applied statistics is choosing a parametric model to fit a ...
of parasite populations, whereas negative density-dependent processes contribute to underdispersion of parasite populations. As mean parasite burden increases, negative density-dependent processes become more prominent and the distribution of the parasite population tends to become less overdispersed. 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.{{cite book , last=Plaisier, first=A.P., title=Modelling Onchocerciasis Transmission and Control, year=1996, publisher=Erasmus University, location=Rotterdam, isbn=90-72245-68-7


See also

* Plant density


References

{{Reflist, 30em


External links


Density dependence


{{modelling ecosystems, expanded=other * Cellular processes Epidemiology ategory:Evolutionary biology concepts Population dynamics