Disease ecology is a sub-discipline of ecology concerned with the mechanisms, patterns, and effects of host-pathogen interactions, particularly those of infectious diseases. For example, it examines how parasites spread through and influence wildlife populations and communities. By studying the flow of diseases within the natural environment, scientists seek to better understand how changes within our environment can shape how pathogens, and other diseases, travel. Therefore, diseases ecology seeks to understand the links between ecological interactions and disease evolution. New emerging and re-emerging infectious diseases (infecting both wildlife and humans) are increasing at unprecedented rates which can have lasting impacts on public health, ecosystem health, and biodiversity.
History Over more than a century, disease ecology has evolved drastically. This field comes from the integration of epidemiology, zoology, and public health. The development of disease ecology over time has demonstrated the constantly changing scientific perspective about how pathogens, hosts, and environmental shape patterns of disease. The exact term of "disease ecology" was rather not as popular until the late 20th century, however, its key principles were determined early on through the idea that connected organisms to their environment.
Early foundations: 1800s-1940s
The earliest that the field of disease ecology dates back to is around the time of the germ theory (19th century). This theory established microorganisms as pathogens for infection; however, this was quickly realized that environmental conditions were the main factors that influenced disease patterns. For instance, studies done on malaria showed that the transmission of this disease relied on a parasite called Plasmodium and the ecology of mosquitoes called Anopheles as well as their habitats for breeding. Ronald Ross's mathematical models were one of the first of many attempts to quantify ecological interactions within disease systems. This work laid the foundation for future ecological frameworks. Researchers in the early 20th century studying wildlife diseases saw pathogens as a factor of a bigger ecological community. These diseases consisted of things such as rinderpest in Africa and rabies in carnivores. Disease studies, however, remained for the most part separate from the mainstream of ecology until the mid-20th century.
Development of disease ecology: 1950s-1980s The shift towards more integrated ideas started in the mid-20th century, when ecologists such as Charls Elton recognized diseases as an ecological force that influences the dynamics of populations, especially after major events such as in the beginning of myxomatosis in European rabbit populations. During the same period, public health scientists that were studying vector-borne diseases began to incorporate ecological methods that examined how land use, climate, and interactions between species influenced transmission cycles. The field of disease ecology further matured through the mathematical modeling work of Anderson and May. Their research integrates host density, reproduction of parasites, immunity, and regulation of populations. These models allowed for a unifying theoretical foundation for disease ecology and continue to influence modern fields such as epidemiology, conservation biology, and public health.
Modern disease ecology and emerging diseases: 1990s-present In the 1900s and 2000s, outbreaks such as HIV, SARS, Ebola, West Nile virus, and COVID-19 emphasized ecological drivers underlying infectious diseases. Researchers saw that spillover events were strongly influenced by the ecology of wildlife, biodiversity loss, human encroachment, and environmental change. The field of disease ecology became more interdisciplinary, integrating conservation science, global health, climate research, and veterinary medicine. During the same time period, new ecological frameworks suggested that reductions in biodiversity could possibly increase the risk of disease as they favor highly competent reservoir hosts. Improvements and advancements in molecular tools, satellite imaging, and climate modeling, helped to expand the field. This allowed researchers to map disease reservoirs, track the evolution of pathogens, and also predict the risk of outbreaks with precision. In today's modern world, disease ecology is important in order to understand things such as zoonotic spillover, wildlife conservation, and global health. The field's historic progression shows us that diseases are not just a medical phenomenon, rather an ecological process that is influenced by interactions among hosts, pathogens, and the constantly changing environment.
Factors affecting spread of diseases Parasitic infections, along with certain transmitted diseases, are present in wildlife which can have severe health effects on particular individuals and populations. Constant host-parasite interactions make disease ecology critical in conservation ecology.
Ecological factors Ecological factors that can determine the persistence and the spread of diseases are population size, density, and composition. Host population size is important in the context of host-parasite interactions since the spread of diseases needs a host population large enough to sustain parasitic interactions. The health of the overall population (and the size of the weakened population members) will also influence the way that parasites and diseases will transmit among members. Additionally, competition and predation dynamics in the ecosystem can influence the density of potential hosts which can either propagate or limit the spread of diseases.
Predator-prey interactions In some cases when a parasite has weakened an animal it will become easier prey for a predator species. Occasionally predators will prefer feeding on the sick or infected prey even though they carry a parasite because of the opportunity weak prey present. Without the presence of a predator species the prey species would likely exceed manageable numbers therefore leading to the rapid spread of pathogens throughout the prey population. Available host numbers increased when the infected individuals are not removed due to low predation. However, there are some situations where predator feeding can disturb a pathogen that previously was dormant leading to an epidemic that otherwise would not have occurred. Some parasites are able to survive when their host species is consumed leading to the parasite being distributed in the waste of the predator which can continue the spread of disease.
Parasitism Parasitism in disease ecology is important because it can shape the way many habitats function because they are disease carriers. These diseases can alter the timing of events, biogeochemical cycles, and even the flow of energy in a habitat. Parasites are able to limit population growth and reproduction of species which may lead to a shift in the balance of an ecosystem. Other ways parasites impact systems are through nutrient cycles. Parasites are able to create imbalances of the elements in a system through the relationship they have with a host and the host's diet.
Biological factors Biological factors that can determine the persistence of diseases include parameters pertaining at the level of the individual within the population (one single organism). Sex differences are found to be prevalent in disease transmission. For example, male American minks are larger and travel wider distances, making them more prone to come into contact with parasites and diseases. The host species age may additionally affect the rate in which diseases are transmitted. Younger members of populations have yet to acquire herd immunity and are therefore more susceptible to parasitic infections.
Anthropogenic factors Anthropogenic factors of disease spread can be through the introduction or translocation of wildlife for conservation purposes by humans. Additionally, human activity is changing the way in which diseases move through the natural environment.
In relation to anthropogenic factors Humans are strongly impacting how diseases spread by creating what is known as "novel species associations". Globalization, mainly through world travel and trade, has created a system in which pathogens, and other species, are more in contact with one another than before. Ecological disruption, including habitat fragmentation and road construction, degrade natural landscapes and have been studied as drivers of recent emergence and re-emergence of infectious diseases worldwide. Scientists have speculated that habitat destruction and biodiversity loss are some of the main reasons influencing the rapid spread of non-human, disease carrying vectors. The loss of predators, that mitigate the ability for pathogen transmission, can increase the rate of disease transmission. Human anthropogenic induced climate change is becoming problematic, as parasites and their associated diseases, can move to higher latitudes with increasing global temperatures. New diseases can therefore infect populations that were previously never in contact with certain pathogens.
Urbanization and biodiversity loss
Urbanization is considered one of the main land-use changes, defined as the growth in the area and number of people inhabiting cities and creates artificial landscapes of built-up structures for human use. With over 65% of the global human population living in cities by 2025, ecological impacts of urbanization focuses mainly on biodiversity loss defined as the decline in species richness. With empirical evidence, scientists are understanding that biodiversity loss is associated with increased disease transmission and worsening of disease severity for humans, wildlife, and certain plant species. As biodiversity is lost worldwide, it is oftentimes the larger, slower reproducing animal species that will go extinct first. This leaves smaller, more adaptable, fast reproducing species abundant. Research has shown that these smaller species are more likely the ones to carry and transmit pathogens (key examples include bats, rats, and mice).
Invasive species Globalization, especially world trade and travel, has facilitated the spread of non-native species worldwide. Newly introduced invasive species have the ability to alter ecological dynamics through local and regional extinction of native species. This can promote changes to the ecosystem including the shift in abundance and richness of native species. New invasive species, and the diseases they potentially carry, can escape into the environment and alter the existing natural ecosystems and the ecosystem services that people are dependent upon, including water quality and nutrient availability.
Habitat fragmentation
Encroachment on natural ecosystems and wildlife with rapid urbanization exposes humans to a wide variety of disease carrying animals. Habitat fragmentation leads to increased edge effects and increases the contact between different communities, vectors, and pathogens which can increase disease transmission. It is argued that between 2013 and 2015, the Ebola virus disease (EDB) outbreak in West Africa began due to deforestation and habitat degradation. In this case, frugivorous and insectivorous bat species had less forest serving as a barrier between them and dense human settlements. Transmission of the Ebola virus is believed to have occurred through direct contact with bat species carrying the pathogen and humans, encroaching on natural ecosystems.
Climate change