The hunt is on, the heat is up - linking hunting and climate change
(See here for a systematic map of the available evidence in poster format.)
Hunting is a major cause of wildlife declines, but the dangers are not limited to endangered species. The removal of one species from an ecosystem can have knock on effects – trophic cascades, or indirect effects between nonadjacent trophic levels such as carnivores and plants, is a well-known example. For instance, the loss of one top predator like wolves may cause huge increases in the populations of prey species such as deer. Overbrowsing by deer can then severely reduce plant biomass and lead to significant changes in the types of plants left. Evidence is emerging that hunting vertebrates could affect ecosystem services (the benefits we get from nature) such as carbon storage and sequestration, potentially contributing to climate change. Exploring these links between hunting and climate change is an important new area of research.
Carbon storage and sequestration (AKA carbon services) refer to the capacity of an ecosystem to hold quantities of carbon and to absorb it from the atmosphere. This reduces atmospheric carbon dioxide (CO2) concentrations, decreasing the amount of heat retained by Earth’s atmosphere with vital implications for climate change. Major stores of carbon include soil, tropical forests, and the oceans. Carbon services can be affected by many intermediary ecosystem processes which may be mediated by fauna. For instance, an increase in herbivory decreases the amount of aboveground plant biomass, which decreases the carbon storage capacity of an ecosystem.
There are multiple ways in which the hunting may affect carbon services (some more substantiated than others). The specific effects depend on the type of animal that has been removed. For instance, the removal of animals such as lemurs, which eat fruit and disperse the seeds in their faeces, can result in a decreased dispersal of large-seeded seed species (1). This can alter vegetation composition of forests by decreasing the diversity and proportion of large-seeded plant species, including dense and large trees which are linked to greater capacity for carbon storage and sequestration (2). As previously mentioned, the loss of carnivores can cause populations explosions in prey species such as deer and wild boar. This can increase seed and seedling predation and herbivory, both of which reduce tree recruitment and plant biomass, thereby decreasing capacity for carbon storage (3). An increase in wild boar may also lead to an increase in grubbing, the disturbance of vegetation and top soil layer which can increase the amount of CO2 released from soil (4).
Megafauna like mammoths are particularly known for their ability to affect entire landscapes. It has been theorised that mammoth extinction in the arctic tundra has led to increased permafrost thaw and carbon emissions. Megafauna disturb the snow cover, which decreases its insulating effect and reduces moss layer depth, lowering the temperature of the permafrost (6). A lack of megafauna may also increase leaf litter or fire load as less vegetation will be consumed, causing an increase in fire frequency and intensity when lightning strikes (7, 8). More fires will decrease plant biomass and therefore carbon storage capacity, and increase carbon emissions. The removal of bats and other pollinators can also cause decrease plant biomass through a lack of pollination (5).
It is also possible that hunting may actually benefit carbon services. While the loss of carnivores can have negative effects due to increases in the populations of prey species, directly hunting herbivores can have the opposite effect. In the absence of herbivores carbon services can increase due to increased tree recruitment and decreased grubbing (4, 9).
Whether hunting may be exacerbating climate change depends on a myriad of factors, the species hunted, current population levels, and the ecosystem in which the hunt takes place chief among them. There is a need for empirical evidence to directly link hunting to changes in carbon services, rather than a reliance on intermediary ecosystem processes. Future research should also focus on lesser-studied ecosystems such as savannas, temperate forests, and the arctic tundra, and in regions such as Australia, North America, and North Africa.
Kurten EL., Wright SJ. & Carson WP. (2015) Hunting alters seedling functional trait composition in a Neotropical forest. Ecology, 96(7): 1923–1932.
Osuri AM., Ratnam J., Varma V., Alvarez-Loayza P., Astaiza JH., Bradford M., Fletcher C., Ndoundou-Hockemba M., Jansen PA., Kenfack D., Marshall AR., Ramesh BR., Rovero F. & Sankaran M. (2016) Contrasting effects of defaunation on aboveground carbon storage across the global tropics. Nature Communications, 7:11351.
Poulsen JR., Clark CJ. & Palmer TM. (2013) Ecological erosion of an Afrotropical forest and potential consequences for tree recruitment and forest biomass. Biological Conservation, 163: 122-130.
Risch AC., Wirthner S., Busse MD., Page-Dumroese DS. & Schuetz M. (2010) Grubbing by wild boars (Sus scrofa L.) and its impact on hardwood forest soil carbon dioxide emissions in Switzerland. Oecologia, 164(3): 773-84.
Anderson SH., Kelly D., Ladley JJ., Molloy S. & Terry J. (2011) Cascading effects of bird functional extinction reduce pollination and plant density. Science, 331(6020): 1068-1071.
Van der Wal R. & Brooker RW. (2004) Mosses mediate grazer impacts on grass abundance in arctic ecosystems. Functional Ecology, 18(1): 77-86.
Holdo RM., Sinclair ARE., Dobson AP., Metzger KL., Bolker BM. Ritchie ME. & Holt RD. (2009) A disease-mediated trophic cascade in the serengeti and its implications for ecosystem C. PLoS Biol, 7(9): e1000210
Burchard I. (1997) Anthropogenic impact on the climate since man began to hunt. Palaeoclimatology, Palaeoecology, 1(19): 1-14.
Asner GP., Levick SR., Kennedy-Bowdoin T., Knapp DE., Emerson R., Jacobson J., Colgan MS. & Martin RE. (2009) Large-scale impacts of herbivores on the structural diversity of African savannas. PNAS, 106(12): 4947–4952.