Raptors as secondary agents of dispersal of mycorrhizal fungal spores

Reférence: Anderson, Joseph, “Raptors As Secondary Dispersers of Mycorrhizal Fungal Spores” (2025). Electronic Theses and Dissertations. Paper 4588. https://dc.etsu.edu/etd/4588

Introduction

Joseph Anderson’s master’s thesis, entitled “Raptors As Secondary Dispersers of Mycorrhizal Fungal Spores,” submitted to East Tennessee State University in August 2025, explores a crucial and still poorly understood aspect of fungal ecology: the role of raptors as secondary dispersers of mycorrhizal fungal spores.. Fungi depend on abiotic and biotic mechanisms for the dispersal of their spores. While primary dispersal, where animals consume fruiting bodies (such as truffles) and deposit spores via their excrement, is well documented, secondary dispersal—involving a predator consuming a primary disperser that has ingested spores—is less understood, particularly for fungi.

Join the community on WhatsApp

Our community allows you to exchange and learn thanks to the many members who exchange daily on the platform.

Click here

Truffles, which produce their spores underground, are commonly consumed by small mammals that act as primary dispersers. However, the range of these small mammals is limited by their small territories and physical barriers. Raptors, as predators of small mammals and capable of traveling long distances, could play a key role in overcoming these dispersal limitations, offering unique advantages such as maintaining or restoring genetic connectivity over large spatial scales. However, the influence of specific characteristics of raptors and fungal spores on dispersal mechanisms remains largely unknown.

This study therefore sought to understand the mechanisms that influence the secondary dispersal capacity of raptors. It specifically examined how differences in the chemical composition of raptors’ digestive systems, gastrointestinal structures, and deposition methods (feces or regurgitated pellets) affect spore viability and their average retention time (ART). The interaction between the characteristics of raptors and those of fungi, such as spore size and cell wall thickness, was also a central component of the investigation.

Discover our Press Review

Are you new to the world of truffles and want to know the latest news from the estate?

Click here

The study addressed two main questions:

Does ingestion through the digestive tract affect spore viability, and does this vary depending on bird species, method of deposition (pellet, feces), and spore size?

Does spore size influence intestinal retention time and deposition method (feces or pellets), and does this vary depending on the species of raptor?

Based on the existing literature, the hypotheses were as follows:

Raptors of the order Accipitriformes (falcons and hawks), known for their highly acidic gastric pH capable of dissolving chitin (the material that makes up the cell walls of fungi), do not produce viable fungal spores.
A general positive correlation would be observed between body mass and intestinal retention time.
Spore size may play a role in TRM, influencing both the duration of retention in the intestine and the method of deposition.
The characteristics of spores, such as cell wall thickness, are believed to be crucial for their survival during digestion.

In addition to the experimental feeding trial, an analysis of regurgitated pellets from wild barn owls was conducted to confirm that secondary dispersal of fungi by raptors does indeed occur in nature.

Experimental Methodology

The study used six species of raptors (four from the order Strigiformes and two from the order Accipitriformes), selected for their differences in body mass, intestinal chemistry, and morphology. The Strigiformes species included the small tawny owl ( Megascops asio), barn owl (Tyto alba), barred owl (Strix varia), and great horned owl (Bubo virginianus). The Accipitriformes were represented by the broad-winged hawk (Buteo platypterus) and the red-tailed hawk (Buteo jamaicensis).

Three replicates of each species were used. The captive birds of prey were fed mice containing a mixture of spores from four species of ectomycorrhizal truffles, ranging in size from 7 to 70 µm. The truffle species were Rhizopogon (approximately 6.51 µm), Hymenogaster sp. (approximately 20.9 µm), Elaphomyces macrosporus (approximately 40 µm in diameter), and Leucangium (approximately 69.8 µm). The truffles were ground into powder and administered to the mice.

Feces and regurgitation pellets were collected over a period of three days. Cameras set at 5-minute intervals were used to record the exact time of each deposit, which made it possible to calculate the average retention time. The samples were then processed to quantify the spore load by microscopy and assess spore viability using vital staining (propidium iodide). The density of the pellets was also measured to examine its correlation with the method of spore deposition.

To supplement the feeding trial, pellets and feces were collected from roosting sites of wild barn owls in Washington County, Tennessee. These samples were analyzed to confirm the presence of mycorrhizal fungal spores, thus proving the occurrence of secondary dispersal in the wild.

Results

The study results highlighted significant differences in spore viability and dispersal patterns, influenced by the characteristics of raptors and spores.

Spore Viability

Impact of Intestinal Transit Time and Bird Order: Passage through the digestive tract generally resulted in a loss of spore viability, but this varied significantly between bird orders (F=47.12, p>.0001) and sample types (feces/pellets) (F=164, p>.0001), as well as between fungal species (F=42.12, p>.0001).
Differences between Hawks (Accipitriformes) and Owls (Strigiformes) : Spore viability was significantly lower in hawks than in owls for all four fungal species and both sample types (p < 0.04). In hawks, viability in pellets was almost zero, in stark contrast to owls (p < 0.0001). This is attributed to the high acidity of the stomach of hawks (pH ~1), capable of dissolving bone and chitin, compared to owls, whose stomachs are less acidic. Spores trapped in hair and bones during the formation of the pellet in the gizzard are exposed to this acidity for longer, explaining the absence of viable spores in their pellets.
Impact of Spore Size and Mechanical Forces: In owls, larger spores (such as Elaphomyces and Leucangium) underwent a more rapid loss of viability in pellets than in feces (p < 0.0001), despite thicker cell walls. This suggests that The mechanical processes involved in pellet formation can damage large spores due to their larger surface area.. Physically damaged spores were observed, including numerous broken endomycorrhizal fungal spores in wild barn owl pellets, which are generally larger than those used in the trial. Conversely, smaller spores with thinner cell walls were more vulnerable to chemical degradation.

Discover our Press Review

Are you new to the world of truffles and want to know the latest news from the estate?

Click here

Mean Residence Time (MRT) and Spore Deposition

Proportion of Spores in Clumps The proportion of spores deposited in pellets varied significantly between raptor species. A significant interaction between spore volume and raptor species (F = 4.46, p = 0.012) showed that Larger spores were more likely to be found in pellets in certain species.
Barn Owl and Ball of Fluff Density Barn owls had the highest proportion of spores in their pellets, with some containing more than 90% of the largest spores. This proportion correlated with the average pellet density of each species, with barn owls producing the densest pellets. The faster pellet formation time in barn owls may also contribute to this high proportion, as it gives spores less time to be carried into the intestine.
MRT according to Species and Method of Deposit The raptor species had a significant effect on the TRM (F = 10.498, p < 0.0002), with a significant interaction between species and deposit method (F = 36.46, p < 0.0001). However, No correlation was found between the body mass of the raptor and the overall TRM.
Differences in TRM between feces and pellets Significant differences in TRM between fecal deposits and pellets were observed only in the Great Horned Owl (p < 0.0001) and the Barred Owl (p < 0.026), with a clear trend in the Northern Hawk Owl (p < 0.07). In contrast, barn owls and hawks showed no significant differences in TRM between deposit methods. All species had similar TRMs for fecal deposits.
Indirect Influence of Spore Size on TRM: Although spore size did not directly affect overall TRM, its influence on the method of deposition (pellets vs. feces) indirectly impacted TRM in certain species.

• Special features of owls : Owls showed a “tail” in their TRM curves, with small amounts of spores detectable later in the trial, potentially explained by the presence of a functional caecum. They also displayed two “peaks” in deposits, likely related to their nocturnal activity and circadian rhythms.

Analysis of Wild Barn Owls

Confirmation of Secondary Dispersal: Analysis of 29 wild barn owl pellets confirmed the presence of fungal spores, demonstrating that secondary dispersal occurs in nature.
Wide Range of Spores: A significant amount of spores was observed, with a wide variation in the number of spores found.

Discussion and General Conclusions

The results of this thesis highlight the importance of specific traits of raptors in determining spore retention time and viability loss rates, as well as how these traits interact with fungal spore characteristics to influence their dispersal potential.

Impact of Gastric Acidity : Raptors with highly acidic stomachs (Accipitriformes) caused a total loss of viability in spores deposited via pellets, probably due to prolonged exposure to the acidic environment of the gizzard. In contrast, raptors with less acidic stomachs (Strigiformes) maintained higher spore viability in feces and pellets.
Role of Spore Size and Mechanical Forces : Spore size interacted with digestive processes. Smaller spores with thinner walls were more susceptible to chemical degradation, while larger spores were more vulnerable to mechanical damage during pellet formation, particularly in owls.
Influence of Density and Pellet Formation Time : The proportion of spores deposited in pellets relative to feces is largely species-specific. Denser pellets, such as those of barn owls, had a higher proportion of spores. The rapid formation time of pellets in barn owls may also play a role, reducing the time available for spores to be carried into the intestine.
Effects on the TRM and Dispersion Distance Although spore size did not directly influence overall TRM, its impact on the exit route (pellets or feces) shaped the length of time spores remained in the digestive system. Species-specific differences in TRM between pellets and feces could have implications for dispersal distance. A longer TRM, such as that observed in great horned owls, could promote long-distance dispersal, but potentially at the expense of spore viability.
Additional Environmental Factors Beyond digestive physiology, the ecological traits of raptors are crucial. Raptors whose diet is rich in small mammals, the main primary dispersers of fungi, are more likely to be effective secondary dispersers. Owl species consume a high proportion of small mammals (60 to 90% of their diet), while some hawks and falcons feed more on birds, reptiles, or fish, limiting their potential for fungal dispersal. The way raptors handle their prey, for example by regurgitating the gastrointestinal tract, could also allow spores to bypass digestion and re-enter the environment intact.

Limitations and Future Prospects

The study did not fully simulate secondary dispersal, as the spores were introduced directly into dead mice rather than passing through the digestive system of a living mouse. Although the intestinal pH of mice is similar to that of owls, suggesting that general trends would be maintained, a reduction in viability is possible after a first passage.
Due to the limited number of spore types used, the influence of traits such as spore ornamentation (spines or ridges) on viability and deposition method could not be statistically tested. Exploration of a wider range of fungal species would be necessary to study these effects.

Ecological Implications

The results of this thesis have significant implications for understanding fungal dispersal dynamics and predator ecology.
- Predators with prolonged digestion times, such as certain snakes where bones are dissolved in nearly a week, are likely to be “dead ends” for spore viability.
- Spore size is likely to influence the location of dispersal in animals that regurgitate.. Since pellets are often deposited near the resting places of birds of prey, such as agricultural structures for barn owls, spores deposited in these locations may not have a chance to colonize.
- Smaller spores appear to be more likely to be dispersed further, as they tend to be excreted in feces, a method that generally promotes wider distribution than pellets.
- Larger spores face greater challenges: they are more susceptible to mechanical damage in owl pellets (although they remain more viable in fecal deposits) and are completely dissolved in hawk pellets.
In conclusion, the ability of raptors to act as effective secondary dispersers of fungi depends on a multitude of interdependent traits. Raptors with less acidic stomachs, a diet rich in small mammals, and rapid pellet formation may provide more favorable conditions for the survival and dispersal of viable fungal propagules.. This study represents a major advance in understanding secondary dispersal networks and highlights the complex but potentially vital role that raptors play in maintaining connectivity and fungal biodiversity within ecosystems.