Density dependence

In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population.[1] 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 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.[2]

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.[3]

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. 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.

An example of a density-dependent variable is crowding and competition.


Generalized fecundity graph
Density-dependent fecundity

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]

Blackfly life expectancy
Parasite-induced vector mortality

Parasite-induced vector mortality is a form of negative density-dependence. The Onchocerciasis life cycle involves transmission via a black fly 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 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.[6]

In macroparasite life cycles

Density-dependence in filariasis
Density-dependence processes (red) in filariasis life cycle

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.[7]

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 mosquitoes, which lack a well-developed cibarial armature, restriction processes predominate. Thus, the number of L3 larvae per mosquito declines as the number of ingested microfilariae increases. Conversely, in Aedes and Anopheles 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 increases.[3]

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 overdispersion 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.[9]

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]


  1. ^ Hixon, M (2009), "Density Dependence and Independence", Encyclopedia of Life Sciences, Chichester: John Wiley & Sons Ltd, doi:10.1002/9780470015902.a0021219, ISBN 0470016175
  2. ^ May, R.M. (1977). "Togetherness among Schistosomes: its effects on the dynamics of the infection". Mathematical Biosciences. 35 (3–4): 301–343. doi:10.1016/0025-5564(77)90030-X.
  3. ^ a b Snow, L.C. (2006). "Transmission dynamics of lymphatic filariasis: vector-specific density dependence in the development of Wuchereria bancrofti infective larvae in mosquitoes". Medical and Veterinary Entomology. 20 (3): 261–272. doi:10.1111/j.1365-2915.2006.00629.x. PMID 17044876.
  4. ^ Duerr, H.P. (2003). "Density-dependent parasite establishment suggests infection-associated immunosuppression as an important mechanism for parasite density regulation in onchocerciasis". Transactions of the Royal Society of Tropical Medicine and Hygiene. 97 (2): 242–250. doi:10.1016/S0035-9203(03)90132-5. PMID 14584385.
  5. ^ Walker, M (2009). "Density-dependent effects on the weight of female Ascaris lumbricoides infections of humans and its impact on patterns of egg production". Parasites & Vectors. 2 (1): 11. doi:10.1186/1756-3305-2-11. PMC 2672930. PMID 19208229.
  6. ^ Basanez, M.G. (1996). "Density-dependent processes in the onchocerciasis: relationship between microfilarial intake and mortality of the simuliid vector". Parasitology. Cambridge University Press. 113 (4): 331–355. doi:10.1017/S003118200006649X.
  7. ^ a b Churcher, T.S. (2006). "Density dependence and the control of helminth parasites". Journal of Animal Ecology. 75 (6): 1313–1320. doi:10.1111/j.1365-2656.2006.01154.x. PMID 17032363.
  8. ^ a b Duerr, H.P. (2005). "Determinants of the eradicability of filarial infections: a conceptual approach". Trends in Parasitology. 21 (2): 88–96. doi:10.1016/ PMID 15664532.
  9. ^ Anderson, R.M. (1982). "Processes influencing the distribution of parasite numbers within host populations with special emphasis on parasite-induced host mortalities". Parasitology. 85 (2): 373–398. doi:10.1017/S0031182000055347. PMID 7145478.
  10. ^ Plaisier, A.P. (1996). Modelling Onchocerciasis Transmission and Control. Rotterdam: Erasmus University. ISBN 90-72245-68-7.

External links

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.

Arrhenius equation

In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 1884 that van 't Hoff equation for the temperature dependence of equilibrium constants suggests such a formula for the rates of both forward and reverse reactions. This equation has a vast and important application in determining rate of chemical reactions and for calculation of energy of activation. Arrhenius provided a physical justification and interpretation for the formula. Currently, it is best seen as an empirical relationship. It can be used to model the temperature variation of diffusion coefficients, population of crystal vacancies, creep rates, and many other thermally-induced processes/reactions. The Eyring equation, developed in 1935, also expresses the relationship between rate and energy.

Capricorn silvereye

The Capricorn silvereye (Zosterops lateralis chlorocephalus), also known as the Capricorn white-eye or green-headed white-eye, is a small greenish bird in the Zosteropidae or white-eye family. It is a subspecies of the silvereye that occurs on islands off the coast of Queensland in north-eastern Australia, and which is sometimes considered to be a full species.

Common goldeneye

The common goldeneye (Bucephala clangula) is a medium-sized sea duck of the genus Bucephala, the goldeneyes. Its closest relative is the similar Barrow's goldeneye. The genus name is derived from the Ancient Greek boukephalos ("bullheaded", from bous, "bull " and kephale, "head"), a reference to the bulbous head shape of the bufflehead. The species name is derived from the Latin clangere ("to resound").Common goldeneyes are aggressive and territorial ducks, and have elaborate courtship displays.

Delayed density dependence

In population ecology delayed density dependence describes a situation where population growth is controlled by negative feedback operating with a time lag.

Extrinsic mortality

Extrinsic mortality is the sum of the effects of external factors, such as sunlight and pollutants that contribute to senescence and eventually death. This is opposed to intrinsic mortality, which is the sum of the effects of internal factors, such as mutation due to DNA replication errors. Extrinsic mortality plays a significant role in evolutionary theories of aging, as well as the discussion of health barriers across socioeconomic borders.

Huai Kha Khaeng Wildlife Sanctuary

The Huai Kha Khaeng Wildlife Sanctuary (Thai: เขตรักษาพันธุ์สัตว์ป่าห้วยขาแข้ง, pronounced [kʰèːt.rák.sǎː.pʰān.sàt.pàː.hûa̯j.kʰǎː.kʰɛ̂ŋ]) is in Uthai Thani and Tak Provinces, Thailand. The park was established in 1974, and is part of the largest intact seasonal tropical forest complex in Mainland Southeast Asia. It, coupled with the Thungyai Naresuan Wildlife Sanctuary was declared a World Heritage Site by the United Nations in 1991. Together, the two sanctuaries occupy 622,200 hectares. As of 2014 it still contained viable populations of large mammals, including gibbons, bears, elephants and Indochinese tigers, although like all other sites in mainland Southeast Asia, some species (e.g., rhinoceroses) have disappeared or have experienced severe declines.

Janzen–Connell hypothesis

The Janzen–Connell hypothesis is a widely accepted explanation for the maintenance of tree species biodiversity in tropical rainforests. It was published independently in the early 1970s by Daniel Janzen and Joseph Connell. According to their hypothesis, host-specific herbivores, pathogens, or other natural enemies (often referred to as predators) make the areas near a parent tree (the seed producing tree) inhospitable for the survival of seedlings. These natural enemies are referred to as 'distance-responsive predators' if they kill seeds or seedlings near the parent tree, or 'density-dependent predators' if they kill seeds or seedlings where they are most abundant (which is typically near the parent tree). Such predators can prevent any one species from dominating the landscape, because if that species is too common, there will be few safe places for its seedlings to survive. However, because the predators are host-specific (also called specialists), they will not harm other tree species. As a result, if a species becomes very rare, then more predator-free areas will become available, giving that species' seedlings a competitive advantage. This negative feedback allows the tree species to coexist, and can be classified as a stabilizing mechanism.

The Janzen-Connell hypothesis has been called a special case of keystone predation, predator partitioning or the pest pressure hypothesis. The pest pressure hypothesis states that plant diversity is maintained by specialist natural enemies. The Janzen-Connell hypothesis expands on this, by claiming that the natural enemies are not only specialists, but also are distance-responsive or density-responsive.This mechanism has been proposed as promoting diversity of forests as it promotes survival of a number of different plant species within one localized region. While previously thought to explain the high diversity of tropical forests in particular, subsequent research has demonstrated the applicability of the Janzen–Connell hypothesis in temperate settings as well. The Black Cherry is one such example of a temperate forest species whose growth patterns can still be explained by the Janzen–Connell hypothesis.

Maximum sustainable yield

In population ecology and economics, maximum sustainable yield or MSY is theoretically, the largest yield (or catch) that can be taken from a species' stock over an indefinite period. Fundamental to the notion of sustainable harvest, the concept of MSY aims to maintain the population size at the point of maximum growth rate by harvesting the individuals that would normally be added to the population, allowing the population to continue to be productive indefinitely. Under the assumption of logistic growth, resource limitation does not constrain individuals' reproductive rates when populations are small, but because there are few individuals, the overall yield is small. At intermediate population densities, also represented by half the carrying capacity, individuals are able to breed to their maximum rate. At this point, called the maximum sustainable yield, there is a surplus of individuals that can be harvested because growth of the population is at its maximum point due to the large number of reproducing individuals. Above this point, density dependent factors increasingly limit breeding until the population reaches carrying capacity. At this point, there are no surplus individuals to be harvested and yield drops to zero. The maximum sustainable yield is usually higher than the optimum sustainable yield and maximum economic yield.

MSY is extensively used for fisheries management. Unlike the logistic (Schaefer) model, MSY has been refined in most modern fisheries models and occurs at around 30% of the unexploited population size. This fraction differs among populations depending on the life history of the species and the age-specific selectivity of the fishing method.

However, the approach has been widely criticized as ignoring several key factors involved in fisheries management and has led to the devastating collapse of many fisheries. As a simple calculation, it ignores the size and age of the animal being taken, its reproductive status, and it focuses solely on the species in question, ignoring the damage to the ecosystem caused by the designated level of exploitation and the issue of bycatch. Among conservation biologists it is widely regarded as dangerous and misused.

Nicholson–Bailey model

The Nicholson–Bailey model was developed in the 1930s to describe the population dynamics of a coupled host-parasitoid system.a It is named after Alexander John Nicholson and Victor Albert Bailey. Host-parasite and prey-predator systems can also be represented with the Nicholson-Bailey model. The model is closely related to the Lotka–Volterra model, which describes the dynamics of antagonistic populations (preys and predators) using differential equations.

The model uses (discrete time) difference equations to describe the population growth of host-parasite populations. The model assumes that parasitoids search for hosts at random, and that both parasitoids and hosts are assumed to be distributed in a non-contiguous ("clumped") fashion in the environment. In its original form, the model does not allow for stable coexistence. Subsequent refinements of the model, notably adding density dependence on several terms, allowed this coexistence to happen.

Organizational ecology

Organizational ecology (also organizational demography and the population ecology of organizations) is a theoretical and empirical approach in the social sciences that is considered a sub-field of organizational studies. Organizational ecology utilizes insights from biology, economics, and sociology, and employs statistical analysis to try to understand the conditions under which organizations emerge, grow, and die.

The ecology of organizations is divided into three levels, the community, the population, and the organization. The community level is the functionally integrated system of interacting populations. The population level is the set of organizations engaged in similar activities. The organization level focuses on the individual organizations (some research further divides organizations into individual member and sub-unit levels).

What is generally referred to as organizational ecology in research is more accurately population ecology, focusing on the second level.

Population dynamics

Population dynamics is the branch of life sciences that studies the size and age composition of populations as dynamical systems, and the biological and environmental processes driving them (such as birth and death rates, and by immigration and emigration). Example scenarios are ageing populations, population growth, or population decline.

Population growth

In biology or human geography, population growth is the increase in the number of individuals in a population.

Many of the world's countries, including many in Sub-Saharan Africa, the Middle East, South Asia and South East Asia, have seen a sharp rise in population since the end of the Cold War. The fear is that high population numbers are putting further strain on natural resources, food supplies, fuel supplies, employment, housing, etc. in some of the less fortunate countries. For example, the population of Chad has ultimately grown from 6,279,921 in 1993 to 10,329,208 in 2009, further straining its resources. Niger, Pakistan, Nigeria, Egypt, Ethiopia, and the DRC are witnessing a similar growth in population.

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 7.616 billion in 2018. It is expected to keep growing, and estimates have put the total population at 8.6 billion by mid-2030, 9.8 billion by mid-2050 and 11.2 billion by 2100.

Ray Beverton

Raymond (Ray) John Heaphy Beverton, CBE, FRS (29 August 1922 – 23 July 1995) was an important founder of fisheries science. He is best known for the book On the Dynamics of Exploited Fish Populations (1957) which he wrote with Sidney Holt. The book is a cornerstone of modern fisheries science and remains much used today. Beverton's life and achievements are described in detail in several obituaries written by prominent figures in fisheries science.

Russell Lande

Russell Scott Lande (born 1951) is an American evolutionary biologist and ecologist, and an International Chair Professor at Centre for Biodiversity Dynamics at the Norwegian University of Science and Technology (NTNU). He is a fellow of the Royal Society and a member of the United States National Academy of Sciences.

Sherry Yennello

Sherry J. Yennello, Ph.D. is an American nuclear chemist and an Elected Fellow of the American Association for the Advancement of Science. She is a Regents Professor and the holder of the Cyclotron Institute Bright Chair in Nuclear Science, who currently serves as the Director of the Cyclotron Institute at Texas A&M University. She is also a Fellow of the American Chemical Society and the American Physical Society. She has authored as well as co-authored more than 530 peer reviewed journal articles and has conducted many invited talks, presentations and seminars at several prestigious academic conferences and scholarly lectures.

Sidney Holt

Sidney J. Holt (born 28 February 1926) is a founder of fisheries science. He is best known for the book On the Dynamics of Exploited Fish Populations which he published with Ray Beverton in 1957. The book is a cornerstone of modern fisheries science and remains much used today. Holt served with the FAO in 1953 and with other UN agencies for another 25 years. After his retirement in 1979, Holt has remained active in work related to the International Whaling Commission and conservation of whales in general, also publishing his views about whaling and fisheries management in academic journals.

The Boston News-Letter

The Boston News-Letter, first published on April 24, 1704, is regarded as the first continuously published newspaper in British North America. It was heavily subsidized by the British government, with a limited circulation. All copies were approved by the governor. The colonies’ first newspaper was Publick Occurrences Both Forreign and Domestick, which published its first and only issue on September 25, 1690. In 1718, the Weekly Jamaica Courant followed in Kingston. In 1726 the Boston Gazette began publishing with Bartholomew Green, Jr., as printer.

Threshold host density

Threshold host density (NT), in the context of wildlife disease ecology, refers to the concentration of a population of a particular organism as it relates to disease. Specifically, the threshold host density (NT) of a species refers to the minimum concentration of individuals necessary to sustain a given disease within a population.Threshold host density (NT) only applies to density dependent diseases, where there is an "aggregation of risk" to the host in either high host density or low host density patches. When low host density causes an increase in incidence of parasitism or disease, this is known as inverse host density dependence, whereas when incidence of parasitism or disease is elevated in high host density conditions, it is known as direct host density dependence.

Host density independent diseases show no correlation between the concentration of a given host population and the incidence of a particular disease. Some examples of host density independent diseases are sexually transmitted diseases in both humans and other animals. This is due to the constant incidence of interaction observed in sexually transmitted diseases—even if there are only 20 individuals left of a given population, survival of the species requires sexual contact, and continued spread of the disease.

Density dependent diseases are significantly less likely to cause extinction of a population, as the natural course of disease will bring down the density, and thus the propinquity of individuals in the population. In other words, less individuals—as caused by disease—means lower infection rates and a population equilibrium.

Food webs
Example webs
Ecology: Modelling ecosystems: Other components


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