Can large herbivores enhance ecosystem carbon persistence? Kristensen et al. 2021
This article considers the overlooked role of grasslands and large herbivores in carbon storage. The principal question the authors pose is: what is the impact of large wild and domestic herbivores on the ability of ecosystems to absorb and store carbon over the long term? Their answer is that the activity of species like cattle, bison, boars, elephants, and rhinoceros, can significantly enhance ecosystem retention of carbon.
Questioning the assumption that fast-growing aboveground vegetation, especially trees, is the primary nature-based terrestrial sink for carbon, the authors argue for a whole-ecosystem carbon storage perspective. One problem with focusing on carbon storage in aboveground vegetation rather than that in the soil, they note, is that vegetation is more transient and vulnerable to disturbances, such as fire, while soil carbon tends to be stable, at least under natural and well-managed grasslands. Furthermore, the authors argue that the conventional focus on aboveground carbon storage has led to the “simplistic” generalization that large herbivores can be expected to damage vegetative ecosystems, and therefore have a negative impact on ecosystem carbon storage.
Yet this view misses the overall ecological impact of large herbivores, such as contributing to the soil through their wastes, and their bioturbation (churning of the soil by animals) activity. On the surface, large animals trample, forage, wallow and dig; just below the surface, the burrowing and digging of tunnels by soil-dwelling mammals like gophers, moles, voles, and shrews further loosens the soil; still deeper, there is the casting, burrowing and mining by macrofauna like insects, worms and dung beetles. Together, the multi-levelled bioturbation of these different species facilitates the vertical mixing of the organic material, putting it into contact with mineral soil particles for longer-term storage. Large grazing herbivores participate in vertical soil mixing (along with the smaller animals at lower levels in the soil), and therefore play an essential role in the long-term buildup of mineral-associated organic matter.
In addition to disturbing and mixing the soil, and enriching it through their body wastes, large herbivores clear pyrogenic (combustible) material on the ground and low shrubbery, thus increasing fire resistance. Their grazing also increases fine root growth and root exudation, which leads to increased microbial biomass. In turn, “microbial residues and plant exudates are effective substrates for persistent soil organic matter formation in the mineral-associated organic matter” [Kristensen 2021: 4].
In their conclusion, the authors emphasize the ecological value of natural grasslands, and the importance of preserving them:
Understanding the role that large herbivores may play in enhancing ecosystem carbon persistence, by reducing the flammability of aboveground carbon and shifting carbon storage from vulnerable pools towards more persistent soil pools at the biome scale, is crucial to balancing the ecosystem services provided by semi-open herbivore-rich systems against potential services from alternative land-uses, such as afforestation [Kristensen 2021: 9].
25 years after returning to Yellowstone, wolves have helped stabilize the ecosystem, Peterson 2020
Before the 1900s, wolves and other predators, such as bears and mountain lions, helped control the populations of herbivores in Yellowstone. However, the federal government exterminated these predators in a coordinated campaign. After the last wolf pack was killed, the elk numbers started increasing uncountably. The US Park Service subsequently attempted to control the elk population by shooting the animals or moving them out of the park.
When the park stopped killing elk in 1968, numbers shot up again from about 5,000 to close to 20,000. For the next several decades, elk cycled through population booms and collapses along with climate fluctuations; hard winters left the ground littered with hundreds of the carcasses of elk that had starved to death [Peterson 2020].
Wildlife officials, therefore, reintroduced wolves back to Yellowstone 25 years ago, which brought the elk population under control and ended their extreme population fluctuations due to climate variability. To study how the wolves maintained the balance, the scientists tracked the wolf packs and recorded details of elk kills by the wolves.
They found that the wolves killed cow elk during the years with normal amounts of rain and snow. During the dry years, when there is less vegetation and therefore less elk food, the wolves targeted bulls. The undernourished elk are generally easier to catch, so the wolves target bulls given their larger size. Sparing elk cows allows the elk to reproduce.
The wolves improve elk herd resilience by eliminating the weak and sick animals. Scientists believed the elk herds are now better prepared for climate change impact, such as the frequent droughts.
The result of reintroducing wolves to Yellowstone showed that wolves stabilize the elk population better than humans can. Now wolves may be reintroduced to other states which are home to a large number of elk.
Can large carnivores change streams via a trophic cascade? Beschta & Ripple 2020
After having been wiped out by the 1920s, wolves were reintroduced to Yellowstone National Park in 1995-1996. This study assessed the importance of large carnivores to wild ungulates’ behavior and density, with secondary effects on plant communities, rivers and channels, and beaver communities. Focusing on the West and East Forks of Blacktail Deer Creek, the authors summarized the population trends of wolves, elk, and beaver; sampled the heights, recruitment, and browsing intensity of Geyer willow (a common local tall willow); measured dimensions of the channel, and ascertained beaver dam heights.
After the reintroduction of wolves, the Rocky Mountain elk population decreased from 17,000 in 1994 to about 4,000 to 5,000 in recent years. Browsing intensity therefore greatly decreased, leading to taller riparian willow stems, which is an important food web support and physical habitat for both terrestrial and aquatic wildlife species. The willow canopy cover over the water surface has also increased rapidly over the last two decades, which holds a significant role in supporting the aquatic biota:
Canopy cover can reduce the amount of solar radiation reaching a stream, especially important during summertime periods when solar angles are high, day lengths are long, and flows are normally low, thereby mediating potential increases in water temperature. Furthermore, invertebrates in the canopies of near‐channel willows provide food for fish and seasonal leaf‐fall represents an important carbon base for aquatic invertebrates which, in turn, provide ‘reciprocal flows of invertebrate prey’ to adjacent terrestrial consumers [Beschta & Ripple 2020: 8].
Another benefit of protecting the riparian vegetation from herbivores is the improvement of streambank stability. During the period of wolf absence, intensive elk herbivory caused streambank erosion and channel incision (river cuts downward into its bed, deepening the active channel and may lead to dissected landscape), resulting in less frequent overbank flow. The channel incision lowered water tables and reduced subsurface moisture in flood plain vegetation during summer.
The return of wolves started the process of riparian vegetation restoration, which in turn supported stream-dependent species such as beavers. The reduction of elk herbivory increased food sources and materials for beavers to construct dams, while also fostering the narrower and shallower channels preferred by beavers. Thus, along with the recovery of vegetation and channels, beavers have returned in 2018, creating active dams to further rehabilitate the ecosystem.
If beaver populations continue to increase over time, the ecological effects of these ‘ecosystem engineers’ may well have a significant role in restoring riparian vegetation, floodplains, and channel dimensions for at least portions of northern range streams [Beschta & Ripple 2020: 9].
Pollination by bats enhances both quality and yield of a major cash crop in Mexico, Tremlett et al. 2019
“The majority of the world’s 350,000 species of flowering plants rely on animal pollinators for reproduction” [Tremlett 2019: 2]. Of the many vertebrates performing this function, including birds, rodents, and reptiles, bats are thought to be the primary pollinators for about 1,000 species of plants across the tropics.
The authors of this study conducted this research in the municipality of Techaluta de Montenegro, Jalisco, Mexico, where they held exclusion experiments (alternately excluding different pollinator species) on Stenocereus queretaroensis, a type of cactus with edible fruit, to determine the efficiency of different pollinators. The experimental treatments allowed the authors to distinguish between nocturnal and diurnal (active in the daytime) pollinators, and between invertebrate and vertebrate pollinators.
Pollination carried out by birds and diurnal insects resulted in low seed sets, significantly lighter fruit weights, and lower sucrose concentrations compared to pollination carried out by bats.
This was the first research study to assess the impact of bat pollination on not only the quality of a high socio-economically important crop but also the yield of the crop.
We found that in the absence of pollination by nectarivorous bats, yield and quality (i.e. fruit weight, as size determines market value) of S. queretaroensis decreased significantly by 35% and 46% respectively. Hence, nectarivorous bats contribute substantially to the economic welfare of the rural production region [Tremlett 2019: 6].
However, despite its economic value, the significance of pollination by bats is not valued and appreciated. It is important to recognize the ecosystem services provided by bats, which might be crucial to sustaining rural livelihoods and well-being.
Equids engineer desert water availability, Lundgren et al. 2014
Many large herbivores may have important roles in dryland ecosystems. Equids such as donkeys and horses, as well as elephants, have been reported to dig wells of a maximum depth of two meters, enhancing water availability for a variety of animals and plants. Noting that this subject has received limited research attention, the authors carried out a study for three summers at the Sonoran Desert of North America to survey changes in groundwater-fed streams and “equid well” water, and the associated effects on the ecosystem.
Effect on animals
They found that the equid wells “provided up to 74% of surface water by accessing the water table” at one of the four groundwater-fed streams they studied [Lundgren 2014: 1]. The wells were especially important at the intermittent stream (unsteady stream that occurs at irregular intervals), providing 100% of available surface water when all other water was lost.
The wells reduced the distance between neighboring water features significantly, thus reducing the distance that animals needed to travel to reach water. The water resources created by the equids also prevented some species from resorting to eating extra plant foods simply to extract its water content, as they are observed to do in the absence of available surface water. Using camera traps, the researchers observed 59 vertebrate species (limited to organisms weighing over 100g and excluding equids) at equid wells, 57 of which they recorded drinking. “Daily species richness was 64 and 51% higher on average at equid wells and background waters [other surface water, such as the streams], respectively, than at dry controls” [Lundgren 2014: 1].
Effects on vegetation
The presence of equid wells enhanced the growth of pioneer trees. The survey showed that the seeding density was higher in equid wells, which function as germination nurseries, than in the riverbank zone. Riverbanks were usually covered by herbs, which reduced the density of trees. Equid wells, on the other hand, provide a non-competitive environment for the small-seeded pioneer trees.
The feral donkeys that dug the equid wells are not native to this dryland ecosystem study site, and yet they proved to mitigate the effects of water reduction and high temperature on biodiversity and ecosystem function. Thus, the ecological roles once played by large native mammals that have since become extinct, can in some cases be filled by non-native substitutes (which are typically viewed as threats to conservation).
Microclimates mitigate against hot temperatures in dryland ecosystems: termite mounds as an example, Joseph et al. 2016
This paper presents an analysis of microclimatic temperature effects of termite mounds in Zimbabwe and South Africa that provide important climatic “refuges” for other local organisms. The research compared the vegetation growing on the mounds with that on control plots in the surrounding savannah with respect to temperature differences. They found that more tall woody vegetation grows on termite mounds, compared to surrounding areas, creating shade that cools the mounds.
The authors observed that: “tall trees, being more prevalent on mounds, provide increased leafy, large-volume canopy and subcanopy vegetation, which in turn furnish more shade relative to the savanna matrix” [Joseph 2016: 7]. They found a 2°C temperature difference on the termite mounds compared to the surrounding area when the surrounding temperature was 34°C; the difference rose to 4°C at 40°C. Thus, these mound microhabitats maintained an even greater ambient temperature difference the warmer the ambient environment became.
Data were collected on 44 large termite mounds, each paired with off-mound savannah plots, in October 2015 (which was one of the hottest months on record in these areas) during the dry season. The mounds were more than 2 meters tall or more than 10 meters in diameter, and they were compared with an equivalently sized circular plot in the surrounding habitat. For each termite mound and control plot, the variables measured included: temperature, humidity, number of trees taller than 4 meters, tree canopy size, and amount of shade.
The median mean shade on mounds was 21% compared to 3% on the control plots, while the median maximum shade was 70% on mounds and only 10% on the surrounding plots, while humidity did not differ significantly. Such microclimates are likely to be important refugia for wildlife as droughts, fire events and higher ambient temperatures become more prevalent due to climate change.
Migratory animals couple biodiversity and ecosystem functioning worldwide, Bauer & Hoye 2014
Billions of animals, including insects, mammals, fish, and birds, migrate through the planet every year, which uniquely influences the environment and the ecological communities along migration routes.
“The frequency of migrations and the immense number of individuals involved often mean that migrant inputs constitute “resource pulses,” defined as occasional, intense, brief episodes of increased resource availability that can profoundly alter demographic rates and abundances of interacting populations” [Bauer & Hoye 2014: 6]
Effect on nutrients, energy, and toxicants:
Migrants transport nutrients, energy, and other substances from one ecosystem to another, creating a net inflow of energy and nutrients into the destination ecosystem. For example, salmon increased the nitrogen and phosphorus in their spawning habitat by 190% and 390% when migrating from the ocean back to their natal lakes and streams. At the same time, migrants may also introduce and accumulate toxicants, such as heavy metals, to receiving communities.
Effect on propagule dispersal:
Migrants play an important role in dispersing propagules, such as seeds, suckers, or spores across the resident communities.
In light of the importance of dispersal for population structure, adaptive capabilities, and evolutionary trajectories in theoretical studies, such long-distance dispersal events may be highly important for the (re)colonization of unoccupied habitats, the recovery of lost populations, maintenance of gene flow, and gene mixing in metapopulations, even if they are relatively rare events [Bauer & Hoye 2014: 2].
Moreover, migrants could also disperse propagules within resident communities. For example, long-nosed bats are responsible for up to 100% of columnar cacti pollination when they migrate to western Mexico. It is important to note that the timing of migration is very important; the migrants can only serve as major pollinators when visiting the communities during peak flowering.
Effect on parasite dispersal:
Migrants may increase parasite dynamics by facilitating the long-distance dispersal of parasites (including zoonotic pathogens like Ebola that also affect humans) to resident species. A few key mechanisms are involved in migration-facilitated parasite dispersal. For example, migrating animals are likely exposed to a greater range of parasites than are resident species. Some migrant animals may have suppressed immune responses due to the high investment of energy into migration, increasing their susceptibility to infection. In addition, while migrating, animals tend to aggregate in larger groups, thus enhancing transmission rates, compared to other times of the year when they are stationary.
However, the role of migrants in transmitting parasites is complicated. Studies of monarch butterflies have shown that they have a shorter flying distance when infected with parasites, andinfected Bewick’s swans delay their departure and travel shorter distances. These findings suggest that migrants may reduce infection risk through infection-induced delays.
Effect of migratory herbivores (plant-eating species):
Migrants may alter the nutrient cycling, productivity, the biomass of edible plants, and ground cover of dead plant material. The grazing intensity of migrant herbivores is decoupled from the timing of plant growth so plants can grow when they are left, which substantially increases the primary productivity compared to an ecosystem with the equivalent number of resident herbivores.
The outcome of the interaction between migrants and residents differs depending on the food resources. During periods of plenty of food residents could share the excess resources with the migrants. However, during the dry season when food is scarcer, synergistic negative effects may be created.
Effects of migratory predators:
Migratory predators can positively influence the communities through prey population control. For example, birds and bats may control the insect population, which reduces damage to crops. Seasonal outmigration may also reduce pressure on prey in the places left behind by migrants, allowing those populations to regrow.
Effects of migratory prey:
Migratory prey could be an important resource for resident predators. Some predators even time their reproduction to coincide with migratory prey to increase their reproductive rate.
Migratory prey may also provide resident prey with a temporal refuge from predation. However, an abundant number of migrants may harm residents by boosting the abundance of resident predators, which then switch to resident prey after the migratory prey departs.
Many ecosystems have evolved to depend upon the activities of both resident and transitory migrating animals, and understanding these relationships is critical to preserving and restoring ecosystem complexity and resiliency.
Across the globe, migration is an increasingly threatened phenomenon as a consequence of habitat destruction, creation of barriers, over-exploitation, and climate change. The loss of migrants and migratory behavior also entails the loss of their ecosystem services—the manifold transport and trophic effects outlined above. Management strategies must therefore be designed to conserve not only migratory species but also their ecosystem functions. Yet, the conservation of migrants poses exceptional scientific and societal challenges, as events at each stage of the migratory cycle affect behavior and demographic rates and ecological interactions at other stages [Bauer & Hoye 2014: 9].
Let more big fish sink: Fisheries prevent blue carbon sequestration—half in unprofitable areas, Mariani et. al 2010
The ocean sequesters about 22% of global anthropogenic CO2 emissions. Marine vertebrates contribute to the ocean’s carbon sink capacity in various ways, such as by fertilizing coastal vegetated habitats, and (through the work of marine predators) protecting this vegetation from overgrazing. Additionally, fish sequester carbon in the deep sea when they sink to the bottom after their natural death, whereas fishing releases the carbon embodied in fish back into the atmosphere when the catch is processed and consumed. Large fish (tuna, mackerel, shark, and billfish) that die in the ocean particularly contribute to “blue carbon” because these species are more likely to sink than be eaten near the surface. Unlike the CO2 released by terrestrial animals after death, the embodied carbon in marine corpses remains in the deep ocean.
This study estimates the extent to which fisheries have obstructed blue carbon sequestration. Mariani et al. report that fishing prevented 21.8 ± 4.4 Mt C (million metric tons of carbon) between 1950 and 2014 from being sequestered in the deep ocean. Industrial fisheries (as opposed to smaller, artisanal fisheries) are responsible for 85% of this extraction.
The amount of blue carbon extracted from the ocean through the harvest of large fish increased by almost one order of magnitude in 65 years (from 0.13 Mt C in 1950 to 1.09 Mt C in 2015). Combining CO2 emissions from fishing fleet transport and that of the fish removal itself amounts to 20.4 MtCO2 emitted in 2014, which is equivalent to the annual emission of 4.5 million cars.
Moreover, the authors found that government subsidies are encouraging overfishing. Almost half of the blue carbon extracted from the world’s oceans comes from areas that would be economically unprofitable without subsidies.
Our findings thus show that government subsidies, through supporting large-scale exploitation of large-bodied fish that are economically unviable, exacerbate the depletion of a natural carbon sink [Mariani 2010: 2].
Limiting and managing all fisheries on the unprofitable areas of the oceans could reduce CO2 emissions, rebuild fish stocks, and promote carbon sequestration by increasing the populations of large-bodied fish and the eventual deadfall of their carcasses to the depths.
Iron defecation by sperm whales stimulates carbon export in the Southern Ocean, Lavery et. al 2010
Whales have been viewed as a source of CO2 because they respire tons of CO2 annually. However, their feces could possibly offset this impact, as they may be a great contributor to carbon export (removal from the atmosphere) to the depths of the ocean. Iron-rich whale feces stimulate the growth of phytoplankton, which leads to more CO2 drawn into the ocean through photosynthesis.
Lavery et al. conducted this study to find out whether the 12,000 sperm whales in the Southern Ocean are acting as a carbon sink. The authors wondered whether the whales help the ocean absorb more carbon from the atmosphere than the whales themselves release through respiration. They note that these animals consume prey outside of but defecate within the photic zone (the layer nearest to the ocean surface), raising nutrient availability in the layer of ocean where photosynthesis is possible. Whale feces are also in liquid form, which disperses and persists within this area.
Using existing data on whale populations, consumption patterns, and average rates of iron retention compared to what is expelled, the authors estimate that the South Ocean sperm whales contribute 36 tons of iron per year to the photic zone. After accounting for respiration rates, the authors conclude that whales do act as a net carbon sink by removing 2.4 X 105 metric tons of carbon from the atmosphere annually. Even under conservative scenarios (consumption of prey with lower iron concentrations), whales still help sequester more carbon than they respire.
These animals’ contribution to nutrient and carbon cycling in the ocean has previously been overlooked. Their feces not only enhance carbon sink in the ocean but also contribute to increasing numbers of prey. However, the reduction of sperm whales by commercial whaling has reduced krill populations and decreased allochthonous (originating externally) iron inputs to the Southern Ocean by 450 tons annually.
The reduction in sperm whale numbers owing to whaling has resulted in an extra 2 X 106 tonnes of carbon remaining in the atmosphere annually [Lavery 2010: 3].
In addition to sperm whales, there could be more organisms acting as carbon sinks in the ocean:
We have restricted our analysis to sperm whales; however, any organism that consumes prey outside the photic zone and defecates nutrient-rich waste that persists in the photic zone would stimulate new production and carbon export. Pygmy and dwarf sperm whales (Kogia spp.) and beaked whales (Family Ziphiidae) fulﬁll these criteria. The proportion of time baleen whales consume prey at depth is currently unknown, but ﬁn whales (Balaenoptera physalus) dive to at least 470 m while feeding. Seals and sealions often consume prey at depth, but whether the[ir] waste is liquid (and buoyant) requires further investigation. [Lavery 2010: 4]
Bauer, S. & B. J. Hoye, 2014. Migratory animals couple biodiversity and ecosystem functioning worldwide. Science 344(6179), https://www.science.org/doi/10.1126/science.1242552
Beschta, Robert and William Ripple, 2020, Can large carnivores change streams via a trophic cascade? Ecohydrology, https://onlinelibrary.wiley.com/doi/abs/10.1002/eco.2048.
Joseph, G. S., et al., 2016, Microclimates mitigate against hot temperatures in dryland ecosystems: termite mounds as an example, Ecosphere 7(11), https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.1509.
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Peterson, Christine, 25 years after returning to Yellowstone, wolves have helped stabilize the ecosystem, National Geographic (July 10, 2020), https://www.nationalgeographic.com/animals/article/yellowstone-wolves-reintroduction-helped-stabilize-ecosystem
Tremlett, Constance J., et al., 2019, Pollination by bats enhances both quality and yield of a major cash crop in Mexico, Journal of Applied Ecology, http://doi.org/0.1111/1365-2664.13545.