Defaunation is the global, local, or functional extinction of animal populations or species from ecological communities. The growth of the human population, combined with advances in harvesting technologies, has led to more intense and efficient exploitation of the environment. This has resulted in the depletion of large vertebrates from ecological communities, creating what has been termed "empty forest". Defaunation differs from extinction; it includes both the disappearance of species and declines in abundance. Defaunation effects were first implied at the Symposium of Plant-Animal Interactions at the University of Campinas, Brazil in 1988 in the context of Neotropical forests. Since then, the term has gained broader usage in conservation biology as a global phenomenon.Dirzo, R. and Galetti, M. "Ecological and Evolutionary Consequences of Living in a Defaunated World." Biological Conservation 163 (2013): 1-6.
It is estimated that more than 50 percent of all wildlife has been lost in the last 40 years. In 2016, it was estimated that by 2020, 68% of the world's wildlife would be lost. In South America, there is believed to be a 70 percent loss.Ceballos, G.; Ehrlich, A. H.; Ehrlich, P. R. (2015). The Annihilation of Nature: Human Extinction of Birds and Mammals. Baltimore, Maryland: Johns Hopkins University Press. pp. 135 – via Open Edition. A 2021 study found that only around 3% of the planet's terrestrial surface is ecologically and faunally intact, with healthy populations of native animal species and little to no human footprint.
In November 2017, over 15,000 scientists around the world issued a second warning to humanity, which, among other things, urged for the development and implementation of policies to halt "defaunation, the poaching crisis, and the exploitation and trade of threatened species."
Drivers
Overexploitation
The intensive hunting and harvesting of animals threaten endangered vertebrate species across the world. Game vertebrates are considered valuable products of tropical forests and savannas. In Brazilian Amazonia, 23 million vertebrates are killed every year; large-bodied primates, tapirs, white-lipped peccaries, giant armadillos, and tortoises are some of the animals most sensitive to harvest. Overhunting can reduce the local population of such species by more than half, as well as reducing population density. Populations located nearer to villages are significantly more at risk of depletion.Peres, Carlos A., and Hilton S. Nascimento. "Impact of Game Hunting by the Kayapo´ of South-eastern Amazonia: Implications for Wildlife Conservation in Tropical Forest Indigenous Reserves." Biodiversity and Conservation 15.8 (2006): 2627-653. Abundance of local game species declines as density of local settlements, such as villages, increases.Altrichter, M., and Boaglio, G., "Distribution and Relative Abundance of Peccaries in the Argentine Chaco: Associations with Human Factors." Biological Conservation 116.2 (2004): 217-25.
Hunting and poaching may lead to local population declines or extinction in some species.Redford, K. H. (1992). The empty forest. BioScience 42(6): 412–422. Most affected species undergo pressure from multiple sources but the scientific community is still unsure of the complexity of these interactions and their feedback loops.Sreekar, R., Huang, G., Zhao, J., Pasion, B.O. et al. "The use of species–area relationships to partition the effects of hunting and deforestation on bird extirpations in a fragmented landscape" Diversity and Distributions, Vol. 21. No. 4 (2015). pp. 441-450. .
One case study in Panama found an inverse relationship between poaching intensity and abundance for 9 of 11 mammal species studied. In addition, preferred game species experienced greater declines and had higher spatial variation in abundance.Wright, S. J., Zeballos, H., Domínguez, I., Gallardo, M. M., Moreno, M. C. and Ibáñez, R. "Poachers Alter Mammal Abundance, Seed Dispersal, and Seed Predation in a Neotropical Forest." Conservation Biology 14.1 (2000): 227-239.
Habitat destruction and fragmentation
Human population growth results in changes in land use, which can cause natural habitats to become fragmented, altered, or destroyed. Large mammals are often more vulnerable to extinction than smaller animals because they require larger home ranges and thus are more prone to suffer the effects of deforestation. Large species such as elephants, rhinoceroses, large primates, tapirs and peccaries are the first animals to disappear in fragmented rainforests.Kinnaird, M. F., Sanderson, E. W., O'Brien, T. G., Wibisono, H. T. and Woolmer, G., "Deforestation Trends in a Tropical Landscape and Implications for Endangered Large Mammals." Conservation Biology (2003) 17: 245–257.
A case study from Amazonian Ecuador analyzed two oil-road management approaches and their effects on the surrounding wildlife communities. The free-access road had forests that were cleared and fragmented and the other had enforced access control. Fewer species were found along the first road with density estimates being almost 80% lower than at the second site which had minimal disturbance.Suárez, E., Morales, M., Cueva, R., Utreras Bucheli, V., Zapata-Ríos, G., Toral, E., Torres, J., Prado, W. and Vargas Olalla, J., "Oil Industry, Wild Meat Trade and Roads: Indirect Effects of Oil Extraction Activities in a Protected Area in North-Eastern Ecuador." Animal Conservation 12 (2009): 364–373. This finding suggests that disturbances affected the local animals' willingness and ability to travel between patches.
Fragmentation lowers populations while increasing extinction risk when the remaining habitat size is small. When there is more unfragmented land, there is more habitat for more diverse species. A larger land patch also means it can accommodate more species with larger home ranges. However, when patch size decreases, there is an increase in the number of isolated fragments that can remain unoccupied by local fauna. If this persists, species may become extinct in the area.Rybicki, J., "Species–area Relationships and Extinctions Caused by Habitat Loss and Fragmentation. " Ecology Letters 16 (2013): 27-38.
A study on deforestation in the Amazon looked at two patterns of habitat fragmentation: "fish-bone" in smaller properties and another unnamed large property pattern. The large property pattern contained fewer fragments than the smaller fishbone pattern. The results suggested that higher levels of fragmentation within the fish-bone pattern led to the loss of species and decreased diversity of large vertebrates.Saunders, D. A., Hobbs, R. J. and Margules, C. R., "Biological Consequences of Ecosystem Fragmentation: A Review." Conservation Biology 5 (1991): 18–32. Human impacts, such as the fragmentation of forests, may cause large areas to lose the ability to maintain biodiversity and ecosystem function due to loss of key ecological processes.Jorge, M. L. S. P., Galetti, M., Ribeiro, M. C., Ferraz, K.M.P.M.B. "Mammal Defaunation as Surrogate of Trophic Cascades in A Biodiversity Hotspot." Biological Conservation 163 (2013): 49–57. This can consequently cause changes within environments and skew evolutionary processes.
In North America, wild bird populations have declined by 29%, or around three billion, since 1970, largely as the result of anthropogenic causes such as habitat loss for human use, the primary driver of the decline, along with widespread use of neonicotinoid insecticides and the proliferation of domesticated cats allowed to roam outdoors.
Invasive species
Human influences, such as colonization and agriculture, have caused species to become distributed outside of their native ranges. Fragmentation also has cascading effects on native species, beyond reducing habitat and resource availability; it leaves areas vulnerable to non-native invasions. Invasive species can out-compete or directly prey upon native species, as well as alter the habitat so that native species can no longer survive.
In extinct animal species for which the cause of extinction is known, over 50% were affected by invasive species. For 20% of extinct animal species, invasive species are the only cited cause of extinction. Invasive species are the second-most important cause of extinction for mammals.
Global patterns
Tropical regions are the most heavily impacted by defaunation. These regions, which include the Brazilian Amazon, the Congo Basin of Central Africa, and Indonesia, experience the greatest rates of overexploitation and habitat degradation. However, specific causes are varied, and areas with one endangered group (such as birds) do not necessarily also have other endangered groups (such as mammals, insects, or amphibians).
Deforestation of the Brazilian Amazon leads to habitat fragmentation and overexploitation. Hunting pressure in the Amazon rainforest has increased as traditional hunting techniques have been replaced by modern weapons such as shotguns. Access roads built for mining and logging operations fragment the forest landscape and allow hunters to move into forested areas which previously were untouched. The bushmeat trade in Central Africa incentivizes the overexploitation of local fauna. Indonesia has the most endangered animal species of any area in the world. International trade in wild animals, as well as extensive logging, mining and agriculture operations, drive the decline and extinction of numerous species.
Ecological impacts
Genetic loss Inbreeding and genetic diversity loss often occur with endangered species populations because they have small and/or declining populations. Loss of genetic diversity lowers the ability of a population to deal with change in their environment and can make individuals within the community homogeneous. If this occurs, these animals are more susceptible to disease and other occurrences that may target a specific genome. Without genetic diversity, one disease could eradicate an entire species. Inbreeding lowers reproduction and survival rates. It is suggested that these genetic factors contribute to the extinction risk in threatened/endangered species.Frankham, R., "Genetics and Conservation Biology." C. R. Biologies 326 (2003): S22-S29.
Seed dispersal
Effects on plants and forest structure The consequences of defaunation can be expected to affect the plant community. There are three non-mutually exclusive conclusions as to the consequences on tropical forest plant communities:
If seed dispersal agents are targeted by hunters, the effectiveness and amount of dispersal for those plant species will be reducedFedriani JM, D Ayllón, T Wiegand, and V. Grimm. 2020. Intertwined effects of defaunation, increased tree mortality, and density compensation on seed dispersal. Ecography 43: 1352-1363. The species composition of the seedling and sapling layers will be altered by hunting, and Selective hunting of medium/large-sized animals instead of small-sized animals will lead to different seed predation patterns, with an emphasis on smaller seeds