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Optimal central-place foraging by beavers: Tree-size selection in relation to defensive chemicals of quaking aspen
Abstract
At a newly occupied pond, beavers preferentially felled aspen smaller than 7.5 cm in diameter and selected against larger size classes. After one year of cutting, 10% of the aspen had been cut and 14% of the living aspen exhibited the juvenile growth form. A phenolic compound which may act as a deterrent to beavers was found in low concentrations in aspen bark, and there was no significant regression of relative concentration of this compound on tree diameter. At a pond which had been intermittently occupied by beavers for over 20 years, beavers selected against aspen smaller than 4.5 cm in diameter, and selected in favor of aspen larger than 19.5 cm in diameter. After more than 28 years of cutting at this site, 51% of the aspen had been cut and 49% of the living aspen were juvenileform. The phenolic compound was found in significantly higher concentrations in aspen bark than at the newly occupied site, and there was a significant negative regression of relative concentration on tree diameter. The results of this study show that responses to browsing by trees place constraints on the predictive value of standard energy-based optimal foraging models, and limitations on the use of such models. Future models should attempt to account for inducible responses of plants to damage and increases in concentrations of secondary metabolites through time. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/47775/1/442_2004_Article_BF00379963.pdf
... Beaver activity affects not only the proportion of species but also the structure of waterbank vegetation (Jones et al., 2009;Mahoney & Stella, 2020). Different diameter classes are often utilized at different ratios, and diameter selectivity may differ among taxa (Basey et al., 1988;Haarberg & Rosell, 2006;Jackowiak et al., 2020). Thus, the utilization of certain diameter classes of a given taxon may also be relevant to biological invasion, if the diameterclass distribution of native and invasive species is not the same. ...
... Certain tree species produce certain metabolites in different proportions at different plant ages (that is, in trees with different diameters) (Wam et al., 2017), which may influence the beaver's diameter selectivity and its differences within the taxa (Basey et al., 1988). ...
Herbivore species can either hinder or accelerate the invasion of woody species through selective utilization. Therefore, an exploration of foraging decisions can contribute to the understanding and forecasting of woody plant invasions. Despite the large distribution range and rapidly growing abundance of beaver species across the Northern Hemisphere, only a few studies focus on the interaction between beavers and invasive woody plants.
We collected data on the woody plant supply and utilization at 20 study sites in Hungary, at two fixed distances from the water. The following parameters were registered: taxon, trunk diameter, type of utilization, and carving depth. Altogether 5401 units (trunks and thick branches) were identified individually. We developed a statistical protocol that uses a dual approach, combining whole‐database and transect‐level analyses to examine foraging strategy.
Taxon, diameter, and distance from water all had a significant effect on foraging decisions. The order of preference for the four most abundant taxa was Populus spp. (softwood), Salix spp. (softwood), Fraxinus pennsylvanica (invasive hardwood), and Acer negundo (invasive hardwood). The diameter influenced the type of utilization, as units with greater diameter were rather carved or debarked than felled. According to the central‐place foraging strategy, the intensity of the foraging decreased with the distance from the water, while both the taxon and diameter selectivity increased. This suggests stronger modification of the woody vegetation directly along the waterbank, together with a weaker impact further from the water.
In contrast to invasive trees, for which utilization occurred almost exclusively in the smallest diameter class, even the largest softwood trees were utilized by means of carving and debarking. This may lead to the gradual loss of softwoods or the transformation of them into shrubby forms. After the return of the beaver, mature stages of softwood stands and thus the structural heterogeneity of floodplain woody vegetation could be supported by the maintenance of sufficiently large active floodplains.
The beaver accelerates the shift of the canopy layer's species composition toward invasive hardwood species, supporting the enemy release hypothesis. However, the long‐term impact will also depend on how plants respond to different types of utilization and on their ability to regenerate, which are still unexplored issues in this environment. Our results should be integrated with knowledge about factors influencing the competitiveness of the studied native and invasive woody species to support floodplain conservation and reconstruction.
... Cafeteria experiments performed by Basey et al. [52,53] showed that aspen resprout growth following beaver felling of trees was highly avoided by beavers relative to controls. They also found that an unidentified phenolic compound was about 18 times greater in the avoided resprout growth, which strongly suggests that the induction of phytochemical defenses deterred subsequent beaver herbivory. ...
... Previous research has shown that herbivory induces defensive (i.e., tannins and phenolic glycosides) and nutritional (i.e., carbon and nitrogen) chemistry in woody plants [12,52,53,57,58]. With narrowleaf cottonwood, Body et al. [59] found that the presence of the gall-forming aphid Pemphigus betae Doane, 1900, triggered the induction of 15 different phytohormones belonging to 5 different classes. ...
The North American beaver (Castor canadensis Kuhl) and cottonwoods (Populus spp.) are foundation species, the interactions of which define a much larger community and affect a threatened riparian habitat type. Few studies have tested the effect of these interactions on plant chemistry and a diverse arthropod community. We experimentally examined the impact of beaver foraging on riparian communities by first investigating beaver food preferences for one cottonwood species, Fremont cottonwood (P. fremontii S. Watson), compared to other locally available woody species. We next examined the impact of beaver foraging on twig chemistry and arthropod communities in paired samples of felled and unfelled cottonwood species in northern Arizona (P. fremontii) and southwestern Colorado (narrowleaf cottonwood, P. angustifolia James, and Eastern cottonwood, P. deltoides W. Bartram ex Marshall). Four major patterns emerged: (1) In a cafeteria experiment, beavers chose P. fremontii six times more often than other woody native and exotic species. (2) With two cottonwood species, we found that the nitrogen and salicortin concentrations were up to 45% greater and lignin concentration 14% lower in the juvenile resprout growth of felled trees than the juvenile growth on unfelled trees (six of seven analyses were significant for P. fremontii and four of six were significant for P. angustifolia). (3) With two cottonwood species, arthropod community composition on juvenile branches differed significantly between felled and unfelled trees, with up to 38% greater species richness, 114% greater relative abundance and 1282% greater species diversity on felled trees (six of seven analyses with P. fremontii and four of six analyses with P. angustifolia were significant). The above findings indicate that the highest arthropod diversity is achieved in the heterogenous stands of mixed felled and unfelled trees than in stands of cottonwoods, where beavers are not present. These results also indicate that beaver herbivory changes the chemical composition in 10 out of 13 chemical traits in the juvenile growth of two of the three cottonwood species to potentially allow better defense against future beaver herbivory. (4) With P. deltoides, only one of five analyses in chemistry was significant, and none of the four arthropod community analyses were significant, suggesting that this species and its arthropod community responds differently to beaver. Potential reasons for these differences are unknown. Overall, our findings suggest that in addition to their impact on riparian vegetation, other mammals, birds, and aquatic organisms, beavers also may define the arthropod communities of two of three foundation tree species in these riparian ecosystems.
... Transects: WBT-waterbank transect, OT-outer transect the fact that the time and work required for cutting down, processing, and mobilizing a tree increases with trunk diameter (Fryxell and Doucet 1991;Gallant et al. 2004). Avoidance of the thinnest units may be explained by the higher concentration of certain chemical substances with anti-herbivore properties in the younger units and shoots (Basey et al. 1988). In line with the results of our previous research conducted in active floodplains of larger rivers (Juhász et al. 2022), we found that the utilized trunk diameter range of the preferred Salix and Populus species was wider than that of other species. ...
Along small watercourses, the growth and renewal of native willows and poplars (Sali-caceae species) are hindered by the effects of past and recent man-made landscape alteration and climate change, while the selective foraging of the beaver (Castor fiber) is also becoming an increasingly important driver. Knowledge about foraging decisions can refine predictions about vegetational processes and help to develop better nature conservation and forest management strategies. We surveyed the woody plant supply (13,304 units) and its utilization by the beaver at 11 study sites along Central European small watercourses, at two fixed distances from the water. We collected information about the taxon, trunk diameter , and type of utilization (cutting, carving, debarking) of each unit. We built (generalized) linear mixed models aimed at answering questions regarding taxon and diameter preference , their interrelatedness, and their importance in foraging decisions. All of the factors examined had a significant effect on foraging decisions. Utilization was mostly explained by the taxon, with Salicaceae species being generally preferred and utilized in all diameter classes with a high ratio. Several further genera were frequently utilized (mainly Cornus and Ulmus), while others were almost completely avoided (including invasive Amorpha and Robinia). The beavers preferred units with a diameter of 5-9 cm. The type of utilization depended primarily on diameter class. Because native softwoods are the most affected by beaver impact, regardless of trunk diameter, their survival and regrowth should be consciously supported by increasing the water table and improving hydrological conditions.
... Beavers are based out of a central location, the majority of foraging activities take place in the 50 m buffer around water bodies, and they transport food not immediately consumed back to the same location (Stoffyn-Egli & Willison, 2011). Thus, they are centralplace foragers, which limits their dispersion into new areas and may complicate analysis of habitat selection (McGinley & Whitham, 1985;Basey et al., 1988;Basey & Jenkins, 1995;Raffel et al., 2009;Hood, 2020). Additionally, habitat selection by beavers in one study usually cannot be directly applied to another area, as important habitat variables may vary depending on spatiotemporal scale and ecological setting Touihri et al., 2018;Hood, 2020). ...
Warming of the Arctic is leading to permafrost degradation, increased vegetation cover, earlier breakup of ice on rivers and lakes, and the arrival and northward expansion of new wildlife species. Beaver expansion into the Arctic has been attributed to shrubification and observed to impact the hydrology, permafrost, wildlife, and people of these regions. The objectives of this research were to 1) characterize changing beaver distribution and associated habitat characteristics in Nunavik, 2) document Inuit knowledge about beaver expansion and the impact on Inuit food security, and 3) identify adaptation strategies to minimize these impacts, while co-producing this knowledge through regional and local research partnerships and collaboration. A mixed methods and knowledge co-production research approach, which included Inuit knowledge interviews, helicopter survey of beaver lodges, dams, and food caches, community questionnaires, and habitat selection analysis, indicated that communities prioritized beaver management because of their concerns regarding the impact of beaver dams on Arctic char and associated impacts on food security. The earliest observations of beavers in the Ungava region of Nunavik occurred near Kangiqsualujjuaq in the late 1950s and near Kuujjuaq in the 1970s, with more recent observations confirming beaver presence much farther north near Aupaluk and Kangirsuk. A habitat selection analysis underlined the importance of dominant water body type as a predictor of beaver presence at both a landscape and local scale of analysis, with beaver sign most often observed along streams, then rivers, then small lakes and less commonly on large lakes. The findings demonstrate how species expansion can be better monitored by integrating western science and Inuit knowledge. Inuit observations can detect beaver impacts on other species, are sensitive to small changes, and can capture transient events, such as sightings of beavers unsuccessfully attempting to colonize a new habitat. Helicopter surveys cover larger areas than Inuit may be able to travel by land and provides systematic information on presence and absence at one point in time. Increased awareness of the distribution of beavers, associated habitat variables, and possible future colonization routes achieved through knowledge co-production can help Inuit policy makers mitigate and adapt to changing wildlife distributions and hydrological regimes.
... On the other hand, due to their short-term exposure (e.g., saplings), relatively low biomass, and low density in some stands, trees with smaller diameters were sometimes "avoided", probably due to the greater energy expenditure required to obtain them in such stands, and larger diameters were preferred, as in the case of oak or hazel. In addition, woody plant shoots that have been repeatedly felled may contain relatively high amounts of toxic secondary substances [48][49][50]. Finally, beavers have no natural predators at these sites, which generally affects beaver foraging [51,52], hence they can afford to expend more energy and time felling larger woody plants to obtain the greater biomass available from the crowns of mature woody plants. ...
Native ecosystems have been transformed by humans into cultural landscapes, resulting in the disruption of natural interactions, with some species unable to adapt and disappearing from such landscapes. Other species were able to adapt their behavior to current environmental conditions. In some places, forest management has gradually transformed native diversified forests into stands converted for the greatest profit in the wood matter, thereby affecting the food availability for herbivores, among them the Eurasian beaver (Castor fiber Linnaeus, 1758). This study summarizes knowledge on non-vegetation diet selection by beavers in commercial and natural forests in the Czech Republic. A sample size of 25,723 woody plant specimens checked in 288 forest stands showed that beavers prefer willow, poplar, or hazel, but where these are less available, commercial tree species such as oak may dominate the diet and cause economic losses in forestry. Significant differences were also observed in a preference for different trunk diameters and distances from water in different types of stands. In terms of stand types, commercial monocultures are the most felled, while the probability of felling decreases with the age of stands. Based on these results and discussion, it is suggested that commercial forest stands including economically valuable woody plant species (e.g., oak, ash) could be protected through appropriate management measures, such as increasing the proportion of deciduous softwood stands along the riverbanks, which would distract the beavers from commercial woody plant species.
... While the propagating effects of beaver activity can be beneficial, careful consideration must be given to whether the history of a given watershed is naturally "tuned" to beaver ecology. For example, in areas where beaver has been introduced, trees often lack defensive mechanisms and reproductive strategies that occur in forests that are regularly subject to beaver activity because they do not share a common evolutionary history (Basey et al., 1988;Martínez Pastur et al., 2006;Anderson et al., 2009). Introduced beaver may be particularly devastating for systems invaded by other non-native species, as novel disturbance has the potential to drive feedback loops that favor survivorship of other invaders, promote new introductions or spread, and overwhelm any biotic resistance that might be perpetrated by native species (i.e., invasional meltdown: Simberloff and Von Holle, 1999;Crooks, 2002;Maret et al., 2006;Braga et al., 2018). ...
We discuss the problems associated with beaver disturbance and its effects on conserving the region's native fauna and flora. We refute arguments underlying the claim that beaver is native to the region, and review paleontological, zooarchaeological, and historical survey data from renowned field biologists and naturalists over the past ~160 years to show that no evidence exists that beaver arrived by any means other than deliberate human introduction.
Remobilization of carbon storage compounds in trees is crucial for the resilience to disturbances, stress, and the requirements of their perennial lifestyle, all of which can impact photosynthetic carbon gain. Trees contain abundant non-structural carbohydrates (NSC) in the form of starch and sugars for long term carbon storage, yet questions remain about the ability of trees to remobilize non-conventional carbon compounds under stress. Aspens, like other members of the genus Populus, have abundant specialized metabolites called salicinoid phenolic glycosides, which contain a core glucose moiety. In this study, we hypothesized that the glucose-containing salicinoids could be remobilized as an additional carbon source during severe carbon limitation. We made use of genetically modified hybrid aspen (Populus tremula x P. alba) with minimal salicinoid content and compared these to control plants with high salicinoid content during resprouting (suckering) in dark (carbon limited) conditions. As salicinoids are abundant anti-herbivore compounds, identification of such a secondary function for salicinoids may provide insight to the evolutionary pressures that drive their accumulation. Our results show that salicinoid biosynthesis is maintained during carbon limitation and suggests that salicinoids are not remobilized as a carbon source for regenerating shoot tissue. However, we found that salicinoid-producing aspens had reduced resprouting capacity per available root biomass when compared to salicinoid-deficient aspens. Therefore, our work shows that the constitutive salicinoid production in aspens can reduce the capacity for resprouting and survival in carbon limited conditions.
Salicinoids are salicyl alcohol-containing phenolic glycosides with strong anti-herbivore effects found only in poplars and willows. Their biosynthesis is poorly understood but recently a UDP-dependent glycosyltransferase, UGT71L1, was shown to be required for salicinoid biosynthesis in poplar tissue cultures. UGT71L1 specifically glycosylates salicyl benzoate, a proposed salicinoid intermediate. Here we analyzed transgenic CRISPR/Cas9-generated UGT71L1 knock-out plants. Metabolomic analyses revealed substantial reductions in the major salicinoids, confirming the central role of the enzyme in salicinoid biosynthesis. Correspondingly, UGT71L1 knock-outs were preferred to wild-type by white tussock moth (Orgyia leucostigma) larvae in bioassays. Greenhouse-grown knock-out plants showed substantial growth alterations, with decreased internode length and smaller serrated leaves. Reinserting a functional UGT71L1 gene in a transgenic rescue experiment demonstrated that these effects were due only to the loss of UGT71L1. The knock-outs contained elevated salicylate (SA) and jasmonate (JA) concentrations, and also had enhanced expression of SA- and JA-related genes. SA is predicted to be released by UGT71L1 disruption, if salicyl salicylate is a pathway intermediate and UGT71L1 substrate. This idea was supported by showing that salicyl salicylate can be glucosylated by recombinant UGT71L1, providing a potential link of salicinoid metabolism to SA and growth impacts. Connecting this pathway with growth could imply that salicinoids are under additional evolutionary constraints beyond selective pressure by herbivores.
Beavers (Castor fiber, Castor canadensis) are one of the most influential mammalian ecosystem engineers, heavily modifying river corridor hydrology, geomorphology, nutrient cycling, and ecosystems. As an agent of disturbance, they achieve this first and foremost through dam construction, which impounds flow and increases the extent of open water, and from which all other landscape and ecosystem impacts follow. After a long period of local and regional eradication, beaver populations have been recovering and expanding throughout Europe and North America, as well as an introduced species in South America, prompting a need to comprehensively review the current state of knowledge on how beavers influence the structure and function of river corridors. Here, we synthesize the overall impacts on hydrology, geomorphology, biogeochemistry, and aquatic and terrestrial ecosystems. Our key findings are that a complex of beaver dams can increase surface and subsurface water storage, modify the reach scale partitioning of water budgets, allow site specific flood attenuation, alter low flow hydrology, increase evaporation, increase water and nutrient residence times, increase geomorphic heterogeneity, delay sediment transport, increase carbon, nutrient and sediment storage, expand the extent of anaerobic conditions and interfaces, increase the downstream export of dissolved organic carbon and ammonium, decrease the downstream export of nitrate, increase lotic to lentic habitat transitions and aquatic primary production, induce ‘reverse’ succession in riparian vegetation assemblages, and increase habitat complexity and biodiversity on reach scales. We then examine the key feedbacks and overlaps between these changes caused by beavers, where the decrease in longitudinal hydrologic connectivity create ponds and wetlands, transitions between lentic to lotic ecosystems, increase vertical hydraulic exchange gradients, and biogeochemical cycling per unit stream length, while increased lateral connectivity will determine the extent of open water area and wetland and littoral zone habitats, and induce changes in aquatic and terrestrial ecosystem assemblages. However, the extent of these impacts depends firstly on the hydro-geomorphic landscape context, which determines the extent of floodplain inundation, a key driver of subsequent changes to hydrologic, geomorphic, biogeochemical, and ecosystem dynamics. Secondly, it depends on the length of time beavers can sustain disturbance at a given site, which is constrained by top down (e.g. predation) and bottom up (e.g. competition) feedbacks, and ultimately determines the pathways of river corridor landscape and ecosystem succession following beaver abandonment. This outsized influence of beavers on river corridor processes and feedbacks is also fundamentally distinct from what occurs in their absence. Current river management and restoration practices are therefore open to re-examination in order to account for the impacts of beavers, both positive and negative, such that they can potentially accommodate and enhance the ecosystem engineering services they provide. It is hoped that our synthesis and holistic framework for evaluating beaver impacts can be used in this endeavor by river scientists and managers into the future as beaver populations continue to expand in both numbers and range.
Beaver are important agents of plant disturbance in wetland and riparian environments. They have two roles as a plant disturbance agent. They have earned the title of ecosystem engineers as one of their more remarkable aspects is their ability to alter physical ecosystem processes through building dam and canal networks. Beaver also are herbivores, capable of cutting woody vegetation at scales rivaled only by humans. This chapter includes information on beaver biology and ecology, the hydrologic and geomorphic changes incurred in wetlands and riparian systems by dam-building beavers, and how plants and plant communities respond to herbivory and habitat modifications made by beaver. Also described are beaver impacts to plant communities in wetlands and riparian areas disturbed by other agents. The chapter concludes by identifying pressing research questions.
A model of food plant selection by a generalist herbivore was developed. The model was designed to predict the species composition of the diet of an herbivore based upon the joint probabilities of whether or not an individual of a plant species satisfied two threshold values: some nutritional minimum and a size limit (both minimum and maximum), and the probability that it was encountered while foraging. The model was tested using moose (Alecs alces) at Isle Royale National Park, Michigan, USA. Initially the threshold values for food selection were determined empirically from the moose's observed behavior, but these empirical values were later shown to be based upon time-energy considerations. Although the model satisfied some of the criteria of optimal foraging contingency models, it appeared that the perfect knowledge assumption was not met. Rather, moose appeared to utilize a strategy of risk aversion.
Isoprenoids and phenolics, major metabolites of important browse species, are reviewed in regard to concentrations, distribution within tissues, and between species. Seasonal variation of specific substances and changes with age of the plant are also considered. The distribution of substances may affect food selection and feeding behavior of animals. Wild mammalian herbivores tend to avoid plant parts rich in these substances, in spite of high nutritional content of the plant tissue. Possible mechanisms for defense by plants against depredation by mammalian herbivores are discussed within the framework of the plants' biochemistry.
Orians and Pearson's model for central place foragers which carry a single prey at a time was tested experimentally using two free-living pairs of red-backed shrikes feeding their nestlings. Mealworms of 2 sizes (6 and 13 segments) impaled on a sloe bush were presented to the birds. Handling time for the large worms was manipulated so that small worms on average yielded 8-13 times more energy per unit time of handling. For the experimental design where prey were encountered in pairs the model predicts a switching from selective foraging on small prey to selective foraging on large prey and unselective foraging at the switching distance. Mealworms were presented at three distances from the nests where the model prescribed: 1) selective foraging on small prey; 2) selective foraging on large prey close to the switching distances; 3) selective foraging on large prey. Three of 4 birds selected small worms at the short travelling distances as the model predicts. All birds delivered prey from the intermediate distance presentation in a fashion not deviating from chance, although the model in this case predicted selective foraging on large prey. All birds selected large prey at the long travelling distances as predicted.-from Author
The relationship between sizes of trees cut by beavers and distances from the borders of their ponds was examined at three sites in central Massachusetts. For most tree genera, the beavers cut a smaller range of sizes far from shore than close to shore, and relatively more small trees and fewer large trees at greater distances. The second of these results differs from the pattern of preference found in other studies in which predators were much larger than their prey, unlike this case of beavers feeding on trees. Both kinds of results are consistent with an optimal foraging model of size-distance relations in which pursuit or provisioning time depends on size of prey as well as distance (Schoener 1979).
Evidence from experimental feeding trials and chemical analysis showed that, when severe winter browsing of adult-form Alaska feltleaf willow (Salix alaxensis) causes it to revert to juvenile-form stump sprouts, the twigs are unpalatable and of lower nutritional quality to the hares than are twigs of adult-form S. alaxensis plants. Further feeding choice trials demonstrated that the palatability of winter-dormant S. alaxensis to snowshoe hares is a function of the age of the sprout and its chemistry rather than the accessibility (height) or morphology of the twigs. The low nutritional quality of juvenile sprouts is partially caused by decreased dry matter and nitrogen digestibility, which may be related to increased lignin and phenolic content of the twigs. This browsing-induced resistance to future winter browsing by hares relaxes within 3 yr and within the height range of twigs normally available to snowshoe hares in winter. These results are compatible with hypothesis that the inducation and relaxation of resistance to browsing in the preferred winter food of snowshoe hares is a cause of the well-known 10-yr snowshoe hare cycle.
Foraging data collected for beaver (Castor canadensis) at Isle Royale National Park, Michigan, during the summer of 1973 provided basic natural history information pertaining to diet, food preferences, rates of consumption and activity cycle. Beaver foraging was consistent with a linear programming model of herbivore optimal foraging. The model was used to predict beaver diet, the maximum distance a beaver foraged from its pond, and the manner in which the minimum and maximum diameters of beaver-cut woody vegetation changed with distance from the pond.