Defaunation
Defaunation is the global, local, or functional
It is estimated that more than 50 percent of all wildlife has been lost in the last 40 years.[10] In 2016, it was estimated that by 2020, 68% of the world's wildlife would be lost.[11] In South America, there is believed to be a 70 percent loss.[12] 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.[13][14]
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."[15]
Drivers
Overexploitation
Hunting and poaching may lead to local population declines or extinction in some species.[21] 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.[4][22]
One case study in Panama found an inverse relationship between poaching intensity and abundance for 9 of 11 mammal species studied.[23] In addition, preferred game species experienced greater declines and had higher spatial variation in abundance.[23]
Habitat destruction and fragmentation
Human population growth results in changes in
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 that which had minimal disturbance.[25] 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.[26] 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 which can remain unoccupied by local fauna. If this persists, species may become extinct in the area.[26]
A study on
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
Invasive species
Human influences, such as colonization and agriculture, have caused species to become distributed outside of their native ranges.[5] 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.[5][25][30]
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.[31]
Global patterns
Tropical regions are the most heavily impacted by defaunation.
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.[5][33] Access roads built for mining and logging operations fragment the forest landscape and allow hunters to move into forested areas which previously were untouched.[33] The bushmeat trade in Central Africa incentivizes the overexploitation of local fauna.[5] Indonesia has the most endangered animal species of any area in the world.[34] International trade in wild animals, as well as extensive logging, mining and agriculture operations, drive the decline and extinction of numerous species.[34]
Ecological impacts
Genetic loss
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 reduced[9][36]
- The sapling layers will be altered by hunting,[9]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[9][37]
One recent study analyzed seedling density and composition from two areas, Los Tuxtlas and Montes Azules. Los Tuxtlas, which is affected more by human activity, showed higher seedling density and a smaller average number of different species than in the other area. Results suggest that an absence of vertebrate dispersers can change the structure and diversity of forests.[38] As a result, a plant community that relies on animals for dispersal could potentially have an altered biodiversity, species dominance, survival, demography, and spatial and genetic structure.[39]
Poaching is likely to alter plant composition because the interactions between game and plant species varies in strength. Some game species interact strongly, weakly, or not at all with species. A change in plant
Effects on small-bodied seed dispersers and predators
As large-bodied vertebrates are increasingly lost from seed-dispersal networks, small-bodied seed dispersers (i.e. bats, birds, dung beetles) and seed predators (i.e. rodents) are affected. Defaunation leads to reduced species diversity.
The quality of the physical habitat may also suffer. Bird and bat species (many of who are small bodied seed dispersers) rely on
Defaunation has negative consequences for seed dispersal networks as well. In the western Amazon, birds and bats have separate diets and thus form separate guilds within the network.[42] It is hypothesized that large-bodied vertebrates, being generalists, connect separate guilds, creating a stable, resilient network. Defaunation results in a highly modular network in which specialized frugivores instead act as the connector hubs.[42]
Food webs
According to a 2022 study published in Science, terrestrial mammal food web links have declined by 53% over the past 130,000 years as a result of human population expansion and accompanying defaunation.[43]
Ecosystem services
Changes in predation dynamics, seed predation, seed dispersal, carrion removal, dung removal, vegetation trampling, and other ecosystem processes as a result of defaunation can affect ecosystem supporting and regulatory services, such as
Conservation
Efforts against defaunation include
Marine
Defaunation in the ocean has occurred later and less intensely than on land. A relatively small number of marine species have been driven to extinction. However, many species have undergone local, ecological, and commercial extinction.[46] Most large marine animal species still exist, such that the size distribution of global species assemblages has changed little since the Pleistocene, but individuals of each species are smaller on average, and overfishing has caused reductions in genetic diversity. Most extinctions and population declines to date have been driven by human overexploitation.[47]
Overfishing has reduced populations of oceanic
Consequences
Marine defaunation has a wide array of effects on ecosystem structure and function. The loss of animals can have both top-down (cascading) and bottom-up effects,
Two of the most important ecosystem services threatened by marine defaunation are the provision of food and coastal storm protection.[46]
See also
- Anthropocene
- Anthropocentrism
- Bushmeat
- Holocene extinction
- Human impact on the environment
- Human overpopulation
- Insect population decline
References
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- ^ Regional data from "How does the Living Planet Index vary by region?". Our World in Data. 13 October 2022. Archived from the original on 20 September 2023.
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- ^ Naik, Gautam (30 September 2014). "Wildlife Numbers Drop by Half Since 1970, Report Says". Wall Street Journal.
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- ^ a b 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.
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- ^ a b c 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.
- ^ 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 b 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.
- ^ a b Rybicki, J., "Species–area Relationships and Extinctions Caused by Habitat Loss and Fragmentation. Archived 2019-11-10 at the Wayback Machine" Ecology Letters 16 (2013): 27-38.
- ^ Saunders, D. A., Hobbs, R. J. and Margules, C. R., "Biological Consequences of Ecosystem Fragmentation: A Review." Conservation Biology 5 (1991): 18–32.
- ^ 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.
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- ^ Dirzo, R. and Miranda, A. "Altered Patterns of Herbivory and Diversity in the Forest Understory: A Case Study of the Possible Consequences of Contemporary Defaunation." In: Plant-Animal Interactions: Evolutionary ecology in tropical and temperate regions. P. W. Price, T. M. Lewinsohn, G. W. Fernandes & W. W. Benson (Eds.). Wiley and Sons Pub. New York pp: 273-287.
- ^ Beaune, David. "Seed Dispersal Strategies and the Threat of Defaunation in a Congo Forest." Biodiversity and Conservation 22.1 (2013): 225-38.
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- ^ Palumbi, S. R., Estes, J. A., Joyce, F. H., and Warner, R. R., "Marine defaunation: Animal loss in the global ocean." Science347 (2015): 12555641.
- ^ Dulvy, N. K., Pinnegar, J. K., and Reynolds, J. D. "Holocene extinctions in the sea." Pages 129-150 Turvey., S. T., editor. Holocene Extinctions. Oxford University Press, New York.
- ^ Einhorn, Catrin (January 27, 2021). "Shark Populations Are Crashing, With a 'Very Small Window' to Avert Disaster". The New York Times. Retrieved January 31, 2021.
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Further reading
- Benítez-López, A.; Alkemade, R.; Schipper, A. M.; Ingram, D. J.; Verweij, P. A.; Eikelboom, J. A. J.; Huijbregts, M. A. J. (April 14, 2017). "The impact of hunting on tropical mammal and bird populations" (PDF). S2CID 19603093.
- Finn, Catherine; Grattarola, Florencia; Pincheira-Donoso, Daniel (2023). "More losers than winners: investigating Anthropocene defaunation through the diversity of population trends". Biological Reviews. .
- Fricke, Evan C; Ordonez, Alejandro; Rogers, Haldre S; Svenning, Jens-Christian (2022). "The effects of defaunation on plants' capacity to track climate change". Science. 375 (6577): 210–214. S2CID 245933147.
- Hallmann, Caspar A.; Sorg, Martin; Jongejans, Eelke; Siepel, Henk; Hofland, Nick; Schwan, Heinz; Stenmans, Werner; Müller, Andreas; Sumser, Hubert; Hörren, Thomas; Goulson, Dave; de Kroon, Hans (October 18, 2017). "More than 75 percent decline over 27 years in total flying insect biomass in protected areas". PMID 29045418.
- Young, Hillary S.; McCauley, Douglas J.; Galetti, Mauro; .