The Mechanics and Rates of Organic Matter Decomposition

Table of Contents

Abstract

Despite their differences, living organisms find common ground when their time comes to an end. While life activities cease, the spirit of the afterlife continues in the form of decay – a critical mechanism by which an ecosystem’s nutrients and energy resources are recycled. Within nature’s ecosystems, the important task of decomposition is carried out by organisms such as scavengers and decomposers. As decomposers are intrinsically connected to the environment in which they carry out their processes, changes in many environmental factors can have a profound impact on the flow of nutrients, energy, and the rate of decomposition. This report investigates the adaptations and physical phenomena these creatures have developed to deal with a myriad of problems, and how these adaptations hold up to variance in abiotic conditions. Given this, we conclude by discussing the relevance of a warming climate in the modern era and the consequences for decomposition processes, especially the rate of decay.

Introduction: The Importance of Decomposition

Decomposition is the process by which resources found in the waste and remains of organisms are liberated and returned to an ecosystem. It facilitates the cycling of nutrients and energy which are essential for synthesizing organic matter in all trophic levels; nutrients are especially important since chemical elements are constrained in quantity for a given ecosystem. Organisms that contribute to decomposition therefore play a pivotal role in the healthy functioning of an ecosystem. Without them, the nutrient pool would eventually be exhausted.

Organic material follows several stages of decomposition and ecological succession across time. Whale falls within marine environments highlight this process, as seen in Figure 1. In the mobile scavenger stage immediately following the death of a whale, aggregations of large necrophages such as hagfish and sleeper sharks gather to feed on soft tissue (Smith et al., 2015). After the soft tissue is removed, smaller organisms such as crustaceans and polychaetes then colonize the whale bones and the surrounding nutrient-rich areas (Smith et al., 2015). Anaerobic bacteria eventually begin to decompose materials such as bone lipids, releasing sulphur and methane which attracts chemolithoautotrophs and methanotrophs (Smith et al., 2015). Finally, the organic material is completely depleted, leaving only mineral remains of the whale skeleton which can fossilize (Smith et al., 2015). These fossils may continue to support marine ecosystems thousands of years after the whale’s death. In fact, they have been observed to become the homes of suspension feeders such as sea anemones, which benefit from enhanced flow around the skeleton (Smith et al., 2015).

Figure 1: The stages of ecological succession observed on various decomposing whale falls. (Adapted from Smith et al., 2015)

Clearly, the decomposition of organic material is an intricate process, involving an extensive range of contributing organisms. Whether small or large, scavenger or decomposer, these organisms recycle an ecosystem’s resources, compelled by the need to obtain nutrients for survival and reproduction. As the availability of organic matter is limited, nutrient acquisition by these decomposing organisms is therefore a problem of both competition and optimization. This has led to many remarkable evolutionary adaptations which we will discuss in this paper. Before that, however, we begin from a top-down view by considering the physical environment and how it governs the ways in which decomposition proceeds. This will contextualize the functioning of individual organisms in the second section, as well as the third section of our paper on the ramifications of climate change for decomposition processes.

Physical Conditions

The decomposition of organic matter is dictated by the environmental conditions it is subject to. In warm and humid tropical rainforests, decomposition rates are often accelerated, evidenced by the fact that only 10% of rainforest nutrients are found in the soil, whereas 75% of nutrients are found in trees (Urry et al., 2021). This is because microbial activities increase under warmer conditions, regardless of the aerobic nature of the microbe (Pietikäinen et al., 2005). In contrast, decay patterns in dry, anaerobic, or less temperate regions show that the overall rate is slowed (Powers et al., 2009) due to diminishing bacterial and faunal activity. Acknowledging that physical conditions impact decomposition quite heavily, this section of the paper will investigate several interesting examples of this.

Ultraviolet Photodegradation

In many ecosystems, photodegradation via solar radiation plays an important role in the decay of organic matter. While photodegradation can be directly involved in the mechanical breakdown of the matter, it can also indirectly facilitate decomposition by assisting microbial activity.

Exposure to ultraviolet radiation is known to degrade materials due to its high energy. Multiple studies have investigated the effect of solar radiation on the rate of decay, and they indicate a significant impact on mass lost to decomposition (Baker & Allison, 2015). The mechanism by which photodegradation induces mass loss is photolysis. During photolysis, a chemically complex compound absorbs enough energy to be cleaved into simpler compounds, which can then be expelled more readily. In an alternate pathway, the energy absorbed may produce a reactive intermediate, triggering a secondary reaction that alters nearby compounds (Baker & Allison, 2015). This process will mechanically break down the matter, either releasing it into the environment or increasing its availability to microbial decomposers (Baker & Allison, 2015).

In the case of plant matter decay, it is usually cellulose that undergoes photolysis. In research conducted by Baker and Allison (2015), it was shown that cellulose content in UV treated decomposing plant matter was significantly reduced compared to the untreated sample. On the other hand, in comparison to cellulose, the photolytic effect on lignin mass was reduced. This can be observed in Figure 2, where lignin-rich (L+) samples do not decay as significantly as lignin poor samples (L-).

Figure 2: Mass loss over time on low (L-) and high lignin (L+) samples with UV treatment. UV+ indicates the presence of UV treatment, whereas UV- indicates UV-blocking. (Adapted from Baker & Allison, 2015)

Cellulose is the basic structural component of plant cell walls and is responsible for the plant’s mechanical characteristics. As photodegradation breaks down cellulose, it weakens the mechanical structure of the material. Loosening of the cell walls facilitates better extracellular enzyme exchange for decomposing microbes, thereby accelerating the rate of decay (Baker & Allison, 2015). Deterioration of organic matter additionally results in the increase of carbon lability, thus further improving the rate at which microbes digest matter (Pieristè et al., 2019). Although photodegradation cannot easily decompose lignin compared to cellulose, it does weaken the lignocellulose matrix, improving the accessibility of lignin and other shielded compounds for microbial attack (Gallo et al., 2006). This further increases carbon lability and decay rate.

Photofacilitation of decomposition has also been observed in similar research conducted in a temperate forest ecosystem (Pieristè et al., 2019). Even in such ecosystems where solar radiation is reduced due to greater tree coverage, UV radiation is important in accelerating decomposition (Pieristè et al., 2019).

Effect of Environmental Conditions on Insect-Driven Decomposition

As previously mentioned, there is an association between the process of decomposition and the physical conditions at the decomposing carcass or deadwood: accelerated decomposition is usually observed in more tropical environments. However, with closer examination, the physical conditions of the site of decomposition and the rate of decomposition are often linked by a third variable: the presence of insect abundance. Insects such as dung beetles and termites act as a “catalyst” to decomposition.

Multiple studies were conducted to show that high mean ambient temperature in forests is associated with higher insect richness (Seibold et al., 2021). The greater abundance of insects in especially tropical forests accelerates the decomposition of cadavers and assists with carbon release from deadwood. As shown by the data in Figure 3 below, the net effect of insects on the decomposition of deadwood is the greatest in tropical forests (3.18 Pg), which can explain the faster decomposition rate and greater annual carbon release.

Figure 3: Annual total carbon release and the effects of insects by forest type. (Adapted from Seibold et al., 2021)

The net effects of insects on the carbon release, and thus the decomposition of deadwood (measured in petagrams, Pg) is determined by several physical factors. Firstly, the wood-feeding termites contribute significantly to the deadwood decay process – however, they have very thin, soft cuticles that readily desiccate. Therefore, wood-feeding termites require highly moist and torrid regions to survive (Fei & Henderson, 2002), and thus concentrate the “acceleration” of decomposition rate in tropical forests. Secondly, insects are cold-blooded, which results in a rise in the rate of their metabolism when temperature increases. Under more tropical temperatures, insect movement increases, the amount of consumption improves, and larva development excels (Seibold et al., 2021). With greater population in insect colonies, and with greater mobility and life activities, the rate of decomposition of deadwood soars in tropical forest environments. Concluded by the two above examples, there is indeed a positive correlation between temperature and the abundance of insects in the decomposition of organic remains. The more tropical the temperature, the greater the insect mobility and abundance, and thus, the more carbon is released from the deadwood, which indicates a higher rate of decomposition (Figure 4). The carbon map in Figure 4 further agrees with this result, as seen with greater carbon release (and thus the rate of decomposition) in the tropical regions of the world. Also, Figure 3 shows that insects are responsible for almost one third of the total carbon released, which emphasizes the importance of insect abundance on the process of decomposition.

Figure 4: Global deadwood carbon map and the effect of insects on carbon release. Insects concentrate in equatorial regions, and so does annual deadwood carbon release. (Adapted from Seibold et al., 2021)

Ocean Currents

It is undoubtedly true that bacteria and microbes contribute greatly to the geochemical cycles of the environment. In marine ecosystems, an enormous source of organic matter exists in the form of dissolved organic matter (DOM), which is a mixture of thousands of unique compounds that microbes digest. Given this, aquatic microbes carry an important responsibility in the recycling and activation of dormant nutrients found in DOM. To quantify this, about 10GT of dissolved organic carbon is added by microbes to the ocean annually (Repeta & Boiteau, 2017).

The characteristics of ocean currents facilitate microbial activity in marine environments. On a basic level, these currents introduce mechanical stress to the fluid body, horizontally cycling and mixing nutrients. Since marine environments are quite nutrient-poor, this cycling improves nutrient access, while giving immotile bacteria a medium of movement (Repeta & Boiteau, 2017). Moreover, turbulence created by ocean currents benefits motile bacteria in a special way. A simulation conducted by Taylor et al. (2012) illustrates this phenomenon. In this simulation, it was found that motile species function best under moderately turbulent conditions. These conditions help create a short-lasting filament-like network of nutrients which can be exploited by the motile bacteria, since they rely on chemical gradients for finding food. This is known as chemotaxis.

Figure 5 illustrates a simulated nutrient filament network caused by turbulence and the bacterial formations in response. Such ephemeral formation of nutrient filaments induces a temporary nutrient gradient which intensifies the chemotactic capability of the bacteria, ultimately increasing microbial activity (Taylor & Stocker, 2012). This effect is not easily exploited by nonmotile bacteria. Overall, on both a macro and microscopic scale, ocean currents indirectly contribute to microbial decomposition, illustrating the interconnections between environment and organism. Evolutionary details of bacterial chemotaxis will be further explored in the next section of this paper.

Figure 5: Simulated effects of turbulence on stretching a nutrient patch into thin filaments (top row). Motile bacterial formations in response are depicted in the bottom row. (Adapted from Taylor et al., 2012)

Adaptations and Phenomena

As discussed in Section 1, food in the form of organic matter is limited for organisms involved in the decomposing process. Consequently, nutrient acquisition is challenging in terms of both competition and optimization – organisms must contend for their food while maintaining an energy budget. What follows is a survey of how various decomposers and scavengers have adapted to obtain nutrients and meet the challenges that this entails.

Bacterial foraging

Marine bacteria fulfill much of their ecosystem’s decomposing needs, with bacteria located near the ocean surface decomposing approximately 75-95% of autotroph organic matter (Das & Mangwani, 2015). As such, these microbes are primary drivers of chemical cycles and energy transfer in the world’s oceans. However, accomplishing this task is not so simple. From the perspective of a bacterium, the ocean is an extremely heterogeneous environment, with nutrient patches constantly shifting and dissipating due to turbulence (Taylor & Stocker, 2012). Thus, to obtain food in time, the ability to move and forage is favourable. Marine bacteria accomplish this through chemotaxis by scaling the chemical gradients surrounding all solid surfaces in the ocean (Figure 6). They have developed two specific adaptations in relation to this.

Figure 6: The chemical/nutrient gradient surrounding a sinking nutrient particle. In C, the black dots illustrate bacterial accumulations in the plume which have developed due to the bacteria’s chemotactic ability. The image in C was obtained using video microscopy of a model marine particle in a microfluidics experiment. (Adapted from Stocker et al., 2012)

First, in comparison to bacteria such as E. coli, marine bacteria are relatively fast swimmers. For example, E. coli swims at 15-30 μm/s while the chitin-decomposing V. alginolyticus swimsat 45-116 μm/s (Stocker & Seymour, 2012). This is likely due to differences in their flagellar motors which generate the propulsive forces needed for motion. Like other marine bacteria, the flagellar motor of V. alginolyticus is driven by a sodium gradient, in contrast to the proton gradient that E. coli uses (Stocker & Seymour, 2012). As sodium concentrations are typically higher in ocean environments, this allows V. alginolyticus to have ~4-6 times higher flagellar rotation rates, which scales with swimming speed (Stocker & Seymour, 2012). Not only does high swimming speed allow bacteria to better capture erratically moving nutrient particles, but it also enables more encounters with particles in an environment that is nutrient poor. Furthermore, speed functions importantly to attenuate the reorienting effects of rotational Brownian motion, because the journey to a nutrient particle will consist of fewer jostling events (Stocker & Seymour, 2012).

Second, to exploit nutrient niches quickly, marine bacteria also efficiently move up the chemical gradients that surround organic matter. This is quantified via chemotactic efficiency, where 100% efficiency indicates a perfect directional response by the bacterium. This means it moves exactly upwards along the chemical gradient with no random deviations (Stocker & Seymour, 2012). For marine bacteria, the chemotactic efficiency is close to 50%, whereas E. coli is around 5-15% efficient (Stocker & Seymour, 2012). In other words, marine bacteria are adapted to effectively turn and adjust the trajectory of their motion based on chemical gradients, therefore arriving at nutrient particles sooner. This ability is depicted below in Figure 7.

Figure 7: A marine bacterium, Pseudoalteromonas haloplanktis, chasing a phytoplankton cell. There is a nutrient/oxygen gradient surrounding the phytoplankton, which the bacterium accurately scales via sharp turning. (Adapted from Stocker et al., 2012)

The reason why E. coli is not as fast or efficient is likely because achieving the level of performance found in marine bacteria is energetically unnecessary. After all, E. coli tends to live in animal intestines, where the environment is several orders richer in nutrients than the ocean (Stocker & Seymour, 2012). In contrast, the highly dynamic and nutrient-poor environments in which marine bacteria live calls for new foraging strategies such as faster swimming and efficient turning. Fundamentally, these strategies represent how a bacterium optimizes according to its environment to meet nutrient and energy requirements. Interestingly, beyond the individual scope, marine bacteria have also been observed to participate in collective efforts – colonies are known to swarm, which facilitates translocation across water more efficiently (Be’er & Ariel, 2019). This type of collective action is another tactic used to optimize for energy requirements, and is explored next with black soldier fly larvae.

Collective Feeding in Black Soldier Flies

Like whale falls in the marine ecosystem, the death of terrestrial organisms is quickly succeeded by a host of scavengers and decomposers. With specialized olfactory systems, carrion flies are often part of the first few to find the carcass, laying several hundred eggs within wounds and natural orifices (Ives, 1991). These eggs hatch into larvae which colonize the carrion. Remarkably, despite their sheer numbers, the larvae are observed to work together, introducing emergent behaviours that increase the rate of feeding. This allows the larvae to often outcompete other scavengers in the hunt for nutrients.

Black soldier fly larvae, Hermetia illucens, provide an interesting case study of collective feeding. In general, larval group feeding faces two difficulties. Firstly, given that the surface area of the decomposing substrate is finite, the number of larvae that can feed at any time is limited (Shishkov et al., 2019). Secondly, larvae tend to take frequent breaks, which is not a problem in itself, but blocks non-resting larvae from eating (Shishkov et al., 2019). In a study by Shishkov et al. (2019), time-lapse videography and particle image velocimetry was used to analyze the feeding behaviour of black soldier fly larvae on an orange in a glass tank. Monitoring the larval aggregations, a “mixing region” appeared around the orange, where there was an inflow of larvae from the bottom layer and an outflow from the top layer.

As shown in Figure 8, the larval flow resembles that of a fountain; larvae crawl in from the bottom and are pumped out of the top layer with the mixing region spinning clockwise (Shishkov et al., 2019). When compared to a theoretical static system of larvae, it is clear that this cyclical method of feeding provides more larvae with the opportunity to eat, and diminishes inefficiencies caused by resting larvae since they are cycled away from the food source. In effect, this enables the aggregation to continuously eat, thus increasing the rate of eating which assists in out-competing other scavengers. Therefore, by organizing themselves to behave collectively as a ‘larval fountain,’ these small creatures have come up with an elegant solution to the difficulties they potentially face in nutrient acquisition.

Figure 8: An aggregation of black soldier fly larvae demonstrating collective feeding of an orange. C and D respectively illustrate top and bottom views of the larval mass. E and F respectively model the velocity fields of larva from a top and bottom view using particle image velocimetry. (Adapted from Shishkov et al., 2019)

It is interesting to note that the researchers also modelled the eating rates of collective feeding in black soldier larvae. For an intermediate-sized group, the eating rate dM/dt is modelled as (Shishkov et al., 2019):

\frac{dM}{dt} = \eta(N_{max}+Q\tau) \space\space\space \text{if} \space\space\space N_{max} \le N \le N'

Here, Q defines the flow rate of larvae, which the researchers observed is positively linearly related to the number of larvae in an intermediate-sized group (Shishkov et al., 2019). This can be observed below in Figure 9. The other variables are constants, and the inequality for N simply defines the size of an intermediate group. It is then apparent that by increasing the number of larvae, the flow rate will increase, thus also increasing the group’s eating rate. Therefore, this suggests that to a certain extent, larger numbers of black soldier larvae do not impair the group’s overall eating ability. Instead, this is actually favourable, which serves to highlight the effectiveness of the larval fountain.

Figure 9: Flow rate of black soldier fly larvae is approximately positively linearly related to the number of larvae present in a feeding aggregation. (Adapted from Shishkov et al., 2019)

Tree Decomposition

The decomposition of wood is an essential contributor to the global carbon cycle. Wood consumes atmospheric carbon (in the form of carbon dioxide) by photosynthesis to provide chemical energy to wood, and it gradually releases carbon back to the atmosphere during its own decomposing phase. As decomposing wood is often under the setting of a forest ecosystem, the decomposition of wood is heavily assisted with the help of natural factors such as biological substances, microorganisms and insects. During the process of decomposition, the deadwood undergoes several physical changes, including changes to its moisture content, toughness, and structure.

The course of the decomposition of wood can be summarized to the digestion of wood cell walls by enzymes. According to Kirk and Cowling (1984), the susceptibility of the wood cell walls to decay is heavily related to their accessibility by digestive enzymes and other metabolites, which are produced by wood-consuming fungi and microorganisms living in the digestive tract of insects. Wood cell walls also contain a mixture of lignin, cellulose, and hemicellulose polymers, all three of which are insoluble in water. The mixture exists in the cell wall in concentrated form, so the enzyme’s physical contact with the cell wall is only possible through diffusion. In addition, lignified tissues resist decomposition; therefore, the existence of lignin in the cell wall and its interaction with other polymers physically prevents enzyme’s access to cellulose and hemicellulose. It is also implied that the digestibility of polymers in wood is inversely related to the lignin content in the wood; the greater the lignin content in the wood, the less digestible the wood is, as seen in Figure 10.

Figure 10: Percentage of lignin content versus percentage of polysaccharides digestibility of wood in decomposition. (Adapted from Kirk & Cowling, 1984)

Nature has evolved to overcome biological problems such as lignin’s resistance to decomposition. One example of such mechanisms is the pulverization of wood particles to fine particles; below a certain wood particle size, the lignified mixture is partially disrupted, exposing the cellulose and hemicellulose to cellulase and hemicellulase (Kirk & Cowling, 1984). Therefore, the relationship between the size of the wood and polymer digestibility can be drawn.

To continue, during the process of decomposition, wood’s physical structure and content undergo significant changes. Firstly, liquid water is essential in the decomposition of wood as a medium for enzymes and metabolites to reach the cell wall. Through a study conducted by Richards (1954), it is mentioned that wood moisture increases with advanced stages of decay. This phenomenon can be attributed to fungal activity: some selected brown-rot fungi and subterranean termites have the ability to conduct liquid water from nearby moist soil into dry wood to facilitate decomposition, causing its moisture content to increase over the entire duration of decomposition (Kirk & Cowling, 1984). Secondly, noticeable distortion and shrinkage of the wood can be observed with advanced stages of decay (Richards, 1954). This phenomenon may be explained by the gradual cellulose and hemicellulose polymer digestion, which give structures to the wood. Thirdly, with brown fungi, the wood’s toughness and weight decrease over time at different rates. It is quite intuitive that with more advanced stages of decomposition, the weight (measured in relative density) and the strength (measured in toughness) of wood will be reduced. But moreover, through the study of Richards (1954), it was discovered that the toughness of the wood decreases rapidly during the early stages of decomposition in contrast to the weight of the wood. It is worth noting that wood in incipient decay can nearly maintain its relative density, and yet have less than 50 percent of the original toughness value (Richards, 1954). Given toughness as a function of percentage weight loss for various types of wood (Figure 11), it is clear that the toughness of every wood type trends downwards for increased weight loss. Thus, the mechanical properties of all types of wood in this study are extremely sensitive to decay. Kirk and Cowling (1984) suspected that the main reason for this phenomenon is the mass depolymerization of cellulose by brown-rot fungi in the early stages of decomposition.

Figure 11: Weight-loss in percent versus toughness in inch/lb of wood in decomposition. (Adapted from Richards, 1954)

Insect Body Adaptations

Throughout the stages of carrion decomposition, there are many diverse species that hope to consume some of the available nutrients for themselves. As decay is a rather time-sensitive affair, it surely makes sense that many necrophilous insects have developed adaptations that afford them some genre of competitive advantage in the feeding frenzy that is a decomposing carcass. In reality, Evans et al.(2020) demonstrate that the ecological strategy of decomposers is so closely related to the timeline of carrion decomposition that it is possible to classify physiological traits of insects in relation to the length of time it takes such species to appear at an animal carcass after its death.

Specialist necrophilous beetles and flies, i.e., those found predominantly at carrion sites and not commonly elsewhere, displayed a clear body size trend throughout a twelve-day timeline of rabbit carcass decay in the Evans et al. study (2020). Species arriving in the early stages of the process had much larger body and wing dimensions, which present a clear advantage of greater stamina and faster locomotion to the carrion site over smaller-bodied, smaller-winged species. Products of early stages of decay are more nutrient-rich and highly sought-after, hence the evolutionary pressure for insects to detect and reach dead animals as quickly as possible (Evans et al., 2020). They note that additionally, the larger size allows insects such as the blowfly to outcompete others for prime oviposition sites in the carcass. A comparison of specialist vs. generalist insect populations throughout the timeline of the study can be seen in Figure 12 below.

Figure 12: Relative proportion over time of specialist and generalist insect species on rabbit carrion. (Adapted from Evans et al. 2020)

Some interesting observations can be made from Figure 12, notably that flies show a much stronger time gradient between specialist and generalist species as compared to the time gradient shown by beetle species. Since generalist species are adapted to a wider variety of food sources, they tend not to possess any rapid discovery traits, and thus arrive at the scene of the carcass at a later time (Evans et al., 2020). Thus, it makes sense that they are physiologically smaller, and can survive off of fewer nutrients. Evans et al. (2020) offer up an explanation for why beetles follow this trend much more loosely, namely that generalist beetles still see an advantage to larger body size with regards to defence from predators, although they do have significantly smaller wings, as they have less need for dispersion across longer distances to find carrion.

In Figure 13, there is a distinct spike in both specialist flies and beetles at the very late stages of decay. When comparing with Figure 12, it can be seen that this spike corresponds with a spike in body size for beetles, but it does not result in a spike in body size for flies. The flies in question here are smaller flies that have adapted to feeding off of tough tissues in late-stage decomposition which possess little nutritional value to the generalist population; their small size means that they are not as dependent on large quantities of nutrients (Evans et al., 2020). And so, through a wide spectrum of necrophilous insects, the entirety of the carrion gets consumed.

Figure 13: Body size of specialist and generalist beetles and flies over time, compared to the mean. (Adapted from Evans et al., 2020)

Ant Activity

While Evans et al. (2020) classify all ants as generalist scavengers rather than carrion specialists, they are still an integral part of the carrion decomposition process. Their interaction with cadaver decomposition islands is unique in that ants participate in carrion decay both actively and passively, enacting many changes which influence the activity of other nearby decomposers (Eubanks et al., 2019).

Through a study of 67 ant species in the Myrmicinae subfamily, Eubanks et al. (2019) observed 14 species that directly participated in necrophagy, meaning that they consumed either the meat or fluids of decomposing carcasses. In fact, worker ants of all ant species in the study possess mandibles large enough to cleave sections of meat off of animal corpses for consumption by the rest of the colony. Eubanks et al. (2019) remark that this provides a mechanism for the opening of lacerations deeper into the animal carcass, which blowflies use to their advantage when depositing eggs, such that the maggots find themselves in a more nutrient-rich location when they hatch. As a by-product, ants are thus responsible for increasing the rate of decomposition when directly feeding on a carcass.

However, this is not always the case. Some carrion-feeding ant species such as the fire ant, Solenopsis invicta, are notorious predators of other insects and insect larvae. Eubanks et al.(2019) observed a significant decrease in blowfly and flesh fly larvae, as well as in necrophagous beetle populations, when S. invicta was present in high concentrations. The insect populations were not significantly affected when S. invicta concentration was low, as ants tend to work as a team to take down prey that outmatches them in size. Thus, the decomposition rate is only negatively affected when ant mounds are constructed in the near vicinity of carcass decomposition islands, allowing for ease of access by a large number of fire ants. In the extreme case observed in the Eubanks et al.study (2019), ants of the Solenopsis genus may construct a large mound that either partially or fully covers the animal carcass, obscuring it from other potential decomposers, most notably fly and beetle species. As maggots (fly larvae) are some of the most, if not the absolute most important organisms in the carrion decay process, the presence of Solenopsis colonies can thus dramatically decrease decomposition rates.

Dung Beetles Rely on a Variety of Environmental Adaptations to Find and Keep Food Sources

The scavenger and decomposer insects seen thus far have demonstrated various adaptations tailored for confronting competition. Fly larvae work collectively to consume organic matter before other scavengers arrive, and ants are observed to hide food. The world’s strongest insect, the dung beetle, adopts its own unique solution. After carefully manufacturing its ball of dung, these beetles would like to avoid other thieving beetles at the dung site. As such, they will roll their ball of dung radially outwards in the straightest path they can manage (Dacke et al., 2021). This makes sense, as this path is the quickest for escaping the congregation of beetles still competing for dung. However, the question remains: how do the dung beetles steer straight? In general, the answer is that dung beetles calibrate their direction in accordance with celestial cues, such as the sun, moon, or the Milky Way (Dacke et al., 2021). Indeed, as Dacke et al. (2021) demonstrated, blocking the dung beetle’s vision of the sky inhibits their steering abilities (Figure 14). This results in aimless paths which often lead back to their initial starting point.

Figure 14: A comparison of dung beetle navigation abilities under normal visual conditions (a) and when their dorsal visual fields are covered (c). (Adapted from Dacke et al., 2020)

At the beginning of their journey, dung beetles are observed to first climb on top of their ball of dung and perform a dance by rotating about its dorsal axis (Figure 15) (Dacke et al., 2021). It is believed that the beetle takes a snapshot of the sky during this dance. The snapshot serves as a reference image for the dung beetle, capturing positional information about celestial cues, as well as gradients of light intensity and polarization patterns surrounding the sun or moon (Dacke et al., 2021). If at any time the beetle finds that its current view does not align with the snapshot, then the beetle knows it has deviated from a straight path and adjustments will then be made (Dacke et al., 2021).

Figure 15: The dung beetle’s dance. Climbing on top of its ball of dung, it rotates horizontally before dismounting head-first and then rolling the dung in the chosen direction. (Adapted from Dacke et al., 2020)

The rotational aspect of the dung beetle’s dance is also potentially useful in comparing how different celestial cues move relative to the dung beetle itself (Dacke et al., 2021). This would enable the dung beetle to establish a hierarchy of cues in terms of navigational usefulness. In fact, these beetles do appear to follow a hierarchy, since they are found to operate according to different cues based on the environment. In forests where the overhead canopy obscures the sun’s position, beetles are found to primarily navigate according to polarization patterns instead, which are more visible (Dacke et al., 2021). At midday in the Kalahari Desert when the sun reaches its zenith and thus becomes unreliable for differentiating direction, the beetles will orient according to the wind (Dacke et al., 2021). This is quite a seamless adaptation since the Kalahari wind also blows strongest at midday. 

Besides the dung beetle’s unique approach to combat competition, the dung beetle is also well suited to forage for organic matter in the first place. Receptors in the antennae of dung beetles can respond to at least 24 different volatile organic compounds (VOCs) (Weithmann et al., 2020). Each stage of decomposition is characterized by the release of different VOCs, and thus dung beetles are tuned to find decomposing matter across a variety of stages (Weithmann et al., 2020). Clearly, dung beetles are seasoned decomposers, with evolutionary adaptations that enable them to forage effectively for food and guard against competitions.

Consequences of Climate Change

As has become undeniably clear in recent years, the climate of our planet is changing at an unprecedented pace. This drastic shift in environmental conditions has begun to wreak havoc on all aspects of life across the globe, and the effects are only set to worsen as the climate runs rampant with nothing to stop it. In Section 2, we discussed the important ways in which the environment dictates decomposition processes. Under this framework, it is therefore logically important to consider how this global predicament will affect decomposers and decomposition processes, and moreover, how these changes in decomposition might affect nutrient cycles, of which decomposers are an instrumental piece.

Microbial Activity

Consider first an example of leaf decay on forest floors, a process that is observed every year in Canada and which is observed everywhere on Earth where there are trees, ferns, bushes, or other leafy vegetation. In fact, leaf litter decomposition is so abundant that Prieto et al. (2019) consider it to be one of the planet’s largest carbon fluxes between the soil and the atmosphere. Evidently, knowing how this process may be affected as climates become ever warmer and ever drier is crucial to understanding the future of the planet.

Prieto et al. (2019) noted two biotic factors resulting from the warming and drying of dryland ecosystems which have a reasonably consequential effect on the rates of decomposition of leaf litter. One such factor stems from the change in the quality of the vegetation that is decomposing, namely its nutrient content. As plants have greater difficulty obtaining nutrients from drying soil, the nutrient content of the leaves decreases while the lignin content tends to remain the same. As lignin is tough, these changes in the chemical composition of leafy detritus tend to slow down the rate of decomposition by soil bacteria.

The second and potentially more interesting biotic consequence of climate change on leaf litter decay relates to the microbial and fungal community responsible for breaking down leaves. Warming and drying climates will predictably select for more heat- and drought-resistant bacteria. Over the period of five years, by simulating a 2.5-degree centigrade increase in mean annual temperature along with a 30% reduction in rainfall, Prieto et al. (2019) demonstrated that these microbial communities will have a distinctly lower capacity to decompose leaf litter as compared to the endemic microbial communities. Under the aforementioned conditions (consistent with current climate predictions for the next half-century), the decomposition of leaf litter from the Helianthemum squamatum shrub (Figure 16) was considerably affected. Specifically, when both rainfall reduction and increased mean temperature treatments were applied simultaneously, the rate of decomposition dropped by 29.8% when compared to leaf litter decomposition at ambient climate conditions (Figure 17), along with greater microbial nitrogen immobilization and thus a lower carbon to nitrogen ratio.

Figure 16: H. squamatum leaves and flower. (Adapted from “Helianthemum squamatum,” 2010)
Figure 17: % Mass loss over time of leaf litter; W shows warming and RR shows rainfall reduction.(Adapted from Prieto et al., 2019)

Fungal Activity

Unsurprisingly, fungi react to the warming of climates in a similar manner, as both Prieto et al. (2019) and Romero-Olivares et al. (2019) concluded. The Romero-Olivares et al. (2019) study focused on a more northern climate where rainfall is not predicted to significantly decrease, yet they observed a similar drying factor to the Prieto et al. (2019) study due to increased soil transpiration rates. By observing gene transcription rates in Neurospora discreta colonies over a period of 1500 generations, Romero-Olivares et al. (2019) noticed a distinct drop in resources allocated to genes involved in decomposition processes. When N. discreta is influenced by environmental stress (warming and drying), it must funnel many more resources into cell metabolism, maintenance, and propagation. In other words, the fungus exerts more energy to simply survive.

In fact, the ramifications of this reallocation of resources on the nutrient cycle are multifold. Primarily, nitrogen deposits into the soil are decreased, as are carbon deposits. Soil bacteria and plant life alike are thus faced with lower soil nutrient concentrations which in turn means a lower uptake of nutrients. As leafy matter from these plants falls to the ground, it will take longer to decompose due to lower quality leaf litter, as stated by Prieto et al. (2019). Notably, low quality leaf detritus in the Prieto et al. study (2019) from plants growing in reduced rainfall and warmed climates, combined with lower microbial decomposition activity previously examined in the simulated climate, lead to a whopping 39.9% decrease in total decomposition rate over a period of one and a half years (Figure 18). It is quite evident that the interaction between fungal and bacterial communities thus creates a positive feedback loop that exacerbates the detrimental effects of climate change on decomposition.

Figure 18: % Mass loss over time in low-nutrient leaf litter. W+RR shows simultaneous warming and drying. (Adapted from Prieto et al., 2019)

One note worth mentioning is the spike in abundance of Neurospora shortly after forest fires, growing as seen in Figure 19 below (Jacobson, 2004). As climate change is rapidly increasing the severity of forest fire systems due to drier vegetation and winds, the population of N. discreta will thus see more such spikes, as will similar Neurospora strains around the world. This increase in the number of fungi would have some effect in offsetting the overall decreased rate of decomposition due to reduced decomposition efficiency. However, the net rate of decomposition would still decrease, and a positive feedback cycle would persist. Evidently, this will not occur instantaneously, but rather over the span of many decades, matching the progress of global warming.

Figure 19: N. discreta fungus growing out of a burnt tree trunk. (Adapted from Jacobson, 2004)

Peatlands

While drylands seem to be responding to climate change through a drastic decrease in decomposition rate, one might find that some wetlands are behaving in an opposite way. The specific wetlands in question are known as northern peatlands, which have a characteristically slow rate of decay due to anoxic conditions and low temperatures (Aminitabrizi et al., 2022). Peat samples from Sphagnum magellanicum moss-dominated bogs were subjected by Aminitabrizi et al. (2022) to a 9-degree centigrade increase in temperature, to mimic end-of-century predictions for arctic climates. Proteobacteria and acidobacteria in the samples displayed a positive correlation between temperature and structures associated with bacterial signalling. This would imply that the bacteria are more actively interacting with the extracellular environment, which corresponds with the increased rate of organic matter to carbon dioxide conversion noticed in the study. In this case, since carbon dioxide is a greenhouse gas and peatlands are enormous carbon sinks, there exists a positive feedback system for increased decay rate. Ise et al. (2008) predict that in the next hundred years, this will lead to an exponential loss of soil organic carbon in peatlands, as shown in Figure 20 below.

Figure 20: Predicted soil organic carbon levels over the next two millennia at a four-degree centigrade increase in temperature. (Adapted from Ise et al., 2008)

Considering the above implications of climate change, in the future, it may become necessary to genetically engineer microorganisms and fungal species which are resilient to warming and droughts without losing significant ability to decompose. Such a process may be as simple as artificial selection of bacterial strains in a lab environment or as complex as the introduction of foreign DNA into fungal species to promote the expression of desiccant-resistant proteins. Indeed, it is necessary to consider biotic decomposition pathways when planning to curb the effects of global climate change.

Conclusion

The decomposition of organic matter is a highly important process that is interlinked with a wide array of ecological phenomena. It is a focal point for the functioning of nutrient cycles, upon which all organisms rely for survival. Thus, scavengers and decomposers bear a responsibility to ensure the healthy functioning of the world’s ecosystems. However, it must be acknowledged that the ways in which decomposition proceeds largely depend on the surrounding environment. From the most sun-stricken grasslands to the most well-canopied forests, variance in UV photodegradation of lignin dictates the rate and ease at which soil microorganisms can break down plant matter. In humid and warm tropical rainforests, insects can spend less energy regulating internal body conditions, and pour more energy into decomposition. Conversely, in drier, cooler biomes, insect abundance and activity is limited by the harsher ambient conditions. Even matters as seemingly trivial as ocean current turbulence determines how a decomposer carries out its task, helping to facilitate the chemotactic ability of motile bacteria by introducing nutrient gradients.

Given the controlling effect of the surrounding environment, it therefore makes sense that decomposing organisms must carefully adapt and optimize, resulting in a myriad of interesting evolutionary outcomes. For example, given the need to forage in a nutrient-poor environment, oceanic bacteria have developed flagella that are more powerful compared to their land counterparts, providing incredible speed and turning abilities for improving chemotaxis. Seeking to optimize the feeding process in order to outcompete other scavengers, maggots move collectively as a flowing larval fountain. This improves on efficiencies that naturally arise in large group feeding events, and increases the overall eating rate. Even fungi in forest ecosystems also demonstrate clever adaptations in response to their environmental conditions. In order to overcome lignin’s resistance to decomposition, the brown-rot fungi is capable of conducting water into trees, increasing the moisture content which promotes lignin decomposition. In the insect world, specialist necrophilous insects have developed adaptations in their body size according to their niche in the decomposing carcass. In an opposite fashion, specific ant species take advantage of large mandibles adapted to their generalist feeding strategy to feast on decomposing carcasses, as well as the insect larvae which inhabit them. Moreover, the remarkable pathfinding strategies of the dung beetle illustrate how organisms adapt to exploit their environmental resources.

Understanding the ecological importance of decomposition and the physical conditions that govern this process, the environmental impact of climate change can be better contextualized. It is evident that decomposition only functions as efficiently as it does due to years of adaptations to the selective pressures of the environment. With conditions now changing at a rapid pace, the diversity of organisms involved in the decomposition process will likely decrease substantially. With less diversity and increased energy expenditure on survival, the efficiency and thus the rate of decomposition is set to slow through a series of positive feedback loops. While it may not be the first thing that comes to mind when thinking about climate change, deterioration of nature’s ability to decompose will surely be consequential, given that the world’s ecosystems depend on it.

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