Functional traits determine formation of mutualism and predation interactions in seed-rodent dispersal system of a subtropical forest

Group effects of desert rodent communities on plant seed dispersal

Desert rodent communities spread plant seeds through the group effect of "selection complementation" and "fate complementation," which promotes the recovery of plant populations and the reconstruction of plant communities in desert areas.

Effects of Seed Size and Frequency on Seed Dispersal and Predation by Small Mammals

Frequency-dependent predation is common in predator-prey interactions. Size is an important characteristic of seeds and is crucial in the regeneration stage of plant seeds. However, the frequency dependence of animal predation on seed size has not been reported. In this study, we conducted a field experiment and used different sizes of Liaodong oak (Quercus wutaishanica) seeds to test the frequency dependence of intraspecific seed size selection in rodents. We used the number ratio of large to small seeds as the frequency. The results show that the rate of small seeds being eaten in situ was significantly higher than that of large seeds (p < 0.05). The rates of different-sized seeds being eaten after removal decreased with increasing frequencies, and there was no significant difference between frequencies except for 1:9 and 9:1. The rates of large seeds being scatter-hoarded were significantly higher than those of small seeds at different frequencies (p < 0.05). The eating distances after removal of large seeds were significantly longer than those of small seeds at the same frequencies (p < 0.05). Furthermore, the scatter-hoarding distances of large seeds were significantly longer than those of small seeds at three frequencies (1:9, 3:7, and 9:1) (p < 0.05). That is, rodents consumed more small seeds in situ, dispersed and scatter-hoarded more large seeds, and dispersed large seeds over longer distances. Rodents exhibited a negative frequency dependence for small seeds and a positive frequency dependence for large seeds on being eaten in situ. Moreover, rodents exhibited a negative frequency dependence for large seeds and a positive frequency dependence for small seeds on being eaten after removal and scatter-hoarding. These results reveal the frequency dependence of rodent selection on seed size and provide new insights into animal-mediated seed dispersal and the regeneration of plant populations.

So many choices, so little time: Food preference and movement vary with the landscape of fear

Spatial and temporal variation in perceived predation risk is an important determinant of movement and foraging activity of animals. Foraging in this landscape of fear, individuals need to decide where and when to move, and what resources to choose. Foraging theory predicts the outcome of these decisions based on energetic trade-offs, but complex interactions between perceived predation risk and preferences of foragers for certain functional traits of their resources are rarely considered. Here, we studied the interactive effects of perceived predation risk on food trait preferences and foraging behavior in bank voles (Myodes glareolus) in experimental landscapes. Individuals (n = 19) were subjected for periods of 24 h to two extreme, risk-uniform landscapes (either risky or safe), containing 25 discrete food patches, filled with seeds of four plant species in even amounts. Seeds varied in functional traits: size, nutrients, and shape. We evaluated whether and how risk modifies forager preference for functional traits. We also investigated whether perceived risk and distance from shelter affected giving-up density (GUD), time in patches, and number of patch visits. In safe landscapes, individuals increased time spent in patches, lowered GUD and visited distant patches more often compared to risky landscapes. Individuals preferred bigger seeds independent of risk, but in the safe treatment they preferred fat-rich over carb-rich seeds. Thus, higher densities of resource levels remained in risky landscapes, while in safe landscapes resource density was lower and less diverse due to selective foraging. Our results suggest that the interaction of perceived risk and dietary preference adds an additional layer to the cascading effects of a landscape of fear which affects biodiversity at resource level.

Food preference strategy of four sympatric rodents in a temperate forest in northeast China

Rodents are well known as both seed predators and dispersers of various plant species in forest ecosystems, and they play an important role in the regeneration of vegetation. Thus, the research on seed selection and vegetation regeneration by sympatric rodents is an interesting topic. To understand the characteristics of preferences of rodents for different seeds, a semi-natural enclosure experiment was performed with four rodent species (Apodemuspeninsulae, Apodemusagrarius, Tscherskiatriton, and Clethrionomysrufocanus) and the seeds of seven plant species (Pinuskoraiensis, Corylusmandshurica, Quercusmongolica, Juglansmandshurica, Armeniacasibirica, Prunussalicina, and Cerasustomentosa) to investigate the differentiation in niches and patterns of resource utilization of sympatric rodents. The results showed that all the rodents had consumed many seeds of Pi.koraiensis, Co.mandshurica, and Q.mongolica but differed significantly in how they selected the different seeds. The rate of utilization (R_i) of Pi.koraiensis, Co.mandshurica, and Q.mongolica exhibited the highest values. The E_i values indicated that the rodents tested exhibited differences in their priorities used to select the seeds from different plant species. All four species of rodents exhibited obvious preferences for certain seeds. Korean field mice preferentially consumed the seeds of Q.mongolica, Co.mandshurica, and Pi.koraiensis. Striped field mice favor the seeds of Co.mandshurica, Q.mongolica, P.koraiensis, and Nanking cherry. Greater long-tailed hamsters prefer to consume the seeds of Pi.koraiensis, Co.mandshurica, Q.mongolica, Pr.salicina, and Ce.tomentosa. Clethrionomysrufocanus likes to eat the seeds of Pi.koraiensis, Q.mongolica, Co.mandshurica, and Ce.tomentosa. The results supported our hypothesis that sympatric rodents overlap in food selection. However, each rodent species has a marked preference for food selection, and different rodent species differ in their food preferences. This reflects the role of distinct food niche differentiation in their coexistence.

High seed diversity and availability increase rodent community stability under human disturbance and climate variation

The relationship between diversity and stability is a focus in community ecology, but the relevant hypotheses have not been rigorously tested at trophic and network levels due to a lack of long-term data of species interactions. Here, by using seed tagging and infrared camera tracking methods, we qualified the seed-rodent interactions, and analyzed the associations of rodent community stability with species diversity, species abundance, and seed-rodent network complexity of 15 patches in a subtropical forest from 2013 to 2021. A total of 47,400 seeds were released, 1,467 rodents were marked, and 110 seed-rodent networks were reconstructed to estimate species richness, species abundance, and seed-rodent network metrics. We found, from younger to older stands, species richness and abundance (biomass) of seeds increased, while those of rodents decreased, leading to a seed-rodent network with higher nestedness, linkage density, and generality in older stands, but higher connectance in younger stands. With the increase of temperature and precipitation, seed abundance (biomass), rodent abundance, and the growth rate of rodent abundance increased significantly. We found rodent community stability (i.e., the inverse of rodent abundance variability) was significantly and positively associated with seed diversity, seed availability, linkage density and generality of seed-rodent networks, providing evidence of supporting the Bottom-Up Diversity-Stability Hypotheses and the Abundant Food Diversity-Stability Hypothesis. Our findings highlight the significant role of resource diversity and availability in promoting consumers' community stability at trophic and network levels, and the necessity of protecting biodiversity for increasing ecosystem stability under human disturbance and climate variation.

Forager-mediated cascading effects on food resource species diversity

Perceived predation risk varies in space and time. Foraging in this landscape of fear alters forager-resource interactions via cascading nonconsumptive effects. Estimating these indirect effects is difficult in natural systems. Here, we applied a novel measure to quantify the diversity at giving-up density that allows to test how spatial variation in perceived predation risk modifies the diversity of multispecies resources at local and regional spatial levels. Furthermore, we evaluated whether the nonconsumptive effects on resource species diversity can be explained by the preferences of foragers for specific functional traits and by the forager species richness. We exposed rodents of a natural community to artificial food patches, each containing an initial multispecies resource community of eight species (10 items each) mixed in sand. We sampled 35 landscapes, each containing seven patches in a spatial array, to disentangle effects at local (patch) and landscape levels. We used vegetation height as a proxy for perceived predation risk. After a period of three nights, we counted how many and which resource species were left in each patch to measure giving-up density and resource diversity at the local level (alpha diversity) and the regional level (gamma diversity and beta diversity). Furthermore, we used wildlife cameras to identify foragers and assess their species richness. With increasing vegetation height, i.e., decreasing perceived predation risk, giving-up density, and local alpha and regional gamma diversity decreased, and patches became less similar within a landscape (beta diversity increased). Foragers consumed more of the bigger and most caloric resources. The higher the forager species richness, the lower the giving-up density, and alpha and gamma diversity. Overall, spatial variation of perceived predation risk of foragers had measurable cascading effects on local and regional resource species biodiversity, independent of the forager species. Thus, nonconsumptive predation effects modify forager-resource interactions and might act as an equalizing mechanism for species coexistence.

Habitats Show More Impacts Than Host Species in Shaping Gut Microbiota of Sympatric Rodent Species in a Fragmented Forest

Gut microbiota play a significant role for animals to adapt to the changing environment. Host species and habitats are key drivers in shaping the diversity and composition of the microbiota, but the determinants of composition of the sympatric host gut microbiome remain poorly understood within an ecosystem. In this study, we examined the effects of habitats of different succession stages and host species on the diversity and composition of fecal gut microbiota in four sympatric rodent species (Apodemus draco, Leopoldamys edwardsi, Niviventer confucianus, and Niviventer fulvescens) in a subtropical forest. We found, as compared to the differences between species, habitat types showed a much larger effect on the gut microbiota of rodents. Alpha diversity of the microbial community of A. draco, N. fulvescens, and N. confucianus was highest in farmland, followed by primary forest and shrubland, and lowest in secondary forest. Beta diversity of the three rodent species showed significant different among habitats. The alpha diversity of gut microbiota of L. edwardsi was significantly higher than those of A. draco and N. confucianus, and its beta diversity showed significant difference from A. draco. Our results suggested that gut microbiota were important for animals in responding to diet changes in different habitats under human disturbances.

Seed traits and rodent community interact to determine seed fate: evidence from both enclosure and field experiments

Animal-mediated seed dispersal is an important ecological process in which a strong mutualism between animals and plants can arise. However, few studies have examined how a community of potential seed dispersers interacts with sympatric seed trees. We employed a series of experiments in the Qinling Mountains in both semi-natural enclosure and the field to assess the interactions among 3 sympatric rodent species and 3 Fagaceae tree seeds. Seed traits all showed similar tannin levels but markedly different physical traits and nutritional contents. We found that seeds with heavy weight, thick coat, and high nutritional contents were less likely to be eaten in situ but more often to be eaten after dispersal or hoarded by rodents. These results support both the handling time hypothesis and the high nutrition hypothesis. Surprisingly, we also found that rodents, maybe, preferred to consume seeds with low levels of crude fiber in situ, and to harvest and hoard those with high levels of crude fiber for later consumption. The sympatric rodent species, Cansumys canus, the largest rodent in our study, harvested and hoarded more Quercus variabilis seeds with high physical and nutritional traits, while Apodemus draco, the smallest rodent, harvested more Q. serrata seeds with low physical and nutritional traits, and Niviventer confucianus harvested and hoarded more Q. aliena seeds with medium physical and nutritional traits. Our study demonstrates that different seed traits play different roles in influencing the seed fate and the shaping of mutualism and predation interactions within a community of rodent species.

Palatability and profitability of co-occurring seeds alter indirect interactions among rodent-dispersed trees

Beyond direct species interactions, seed dispersal is potentially affected by indirect seed-seed interactions among co-occurring nut-bearing trees which are mediated by scatter-hoarding animals as shared seed dispersers. A relevant question in such systems is to what extent different functional traits related to food palatability and profitability affect the kinds of indirect interactions that occur among co-occurring seeds, and the consequences for seed dispersal. We used field experiments to track seed dispersal with individually tagged seeds in both monospecific and mixed seed communities. We measured indirect effects based on 3 seed-seed species pairs from the family Fagaceae with contrasting seed size, tannin level, and dormancy in a subtropical forest in Southwest China. When all else was equal, the presence of adjacently placed seeds with contrasting seed traits created different indirect effects measured through a variety of dispersal-related indicators. Apparent mutualism was reciprocal due to increasing seed dispersal in mixed seed patches with mixed differences in seed tannins and dormancy. However, differences in either seed size or dormancy in co-occurring adjacently placed seeds caused apparent competition with reduced seed removal or seed dispersal (distance) in at least one species. Our study supports the hypothesis that different functional traits related to food palatability and profitability in co-occurring seeds modify foraging decisions of scatter-hoarding animals, and subsequently cause indirect effects on seed dispersal among rodent-dispersed trees. We conclude that such indirect effects mediated by shared seed dispersers may act as an important determinant of seed dispersal for co-fruiting animal-dispersed trees in many natural forests.

Rodent abundance triggered switch between the relative mutualism and predation in a rodent-seed system of the subtropical island forest

Density-dependent non-monotonic species interactions are important in maintaining ecosystem stability and function, but empirical evidences are still rare. Rodents, as both seed dispersers and seed predators, have dual effects on plant regeneration and may result in non-monotonic rodent-plant interactions. According to the non-monotonic models, the relative positive or negative effects of rodents on seedling establishment can be measured based on the positive or negative association of seedling recruitment rate and rodent abundance. In this study, we investigated the fates of acorns of Quercus serrata by tracking tagged seeds on 21 fragmented subtropical islands in the Thousand Island Lake, China. We found that the proportion of germinated seeds of all released seeds showed a dome-shaped association with rodent abundance per seed. The proportion of removed seeds and cached seeds showed a saturated- and a weak dome-shaped association with rodent abundance per seed, respectively. Our results demonstrated a clear empirical evidence that rodent abundance per seed triggered a switch between the relative mutualism and predation in a rodent-seed system. Our study implied that the observed non-monotonic interactions between plants and animals may play a significant role in maintaining biodiversity and ecosystem function. We appeal for more investigations of the complex non-monotonic interactions in various ecosystems.

Interspecific synchrony of seed rain shapes rodent-mediated indirect seed-seed interactions of sympatric tree species in a subtropical forest

Animal‐mediated indirect interactions play a significant role in maintaining the biodiversity of plant communities. Less known is whether interspecific synchrony of seed rain can alter the indirect interactions of sympatric tree species. We assessed the seed dispersal success by tracking the fates of 21 600 tagged seeds from six paired sympatric tree species in both monospecific and mixed plots across 4 successive years in a subtropical forest. We found that apparent mutualism was associated with the interspecific synchrony of seed rain both seasonally and yearly, whereas apparent competition or apparent predation was associated with interspecific asynchrony of seed rain either seasonally or yearly. We did not find consistent associations of indirect interactions with seed traits. Our study suggests that the interspecific synchrony of seed rain plays a key role in the formation of animal‐mediated indirect interactions, which, in turn, may alter the seasonal or yearly seed rain schedules of sympatric tree species.

Fluctuation in seed abundance has contrasting effects on the fate of seeds from two rapidly germinating tree species in an Asian tropical forest

The seed predator satiation hypothesis states that high seed abundance can satiate seed predators or seed dispersers, thus promoting seed survival. However, for rapidly germinating seeds in tropical forests, high seed abundance may limit dispersal as the seeds usually remain under parent trees for long periods, which may lead to high mortality due to rodent predation or fungal infestations. By tracking 2 species of rapidly germinating seeds (Pittosporopsis kerrii, family Icacinaceae; Camellia kissi, family Theaceae), which depend on dispersal by scatter-hoarding rodents, we investigated the effects of seed abundance at the community level on predation and seed dispersal in the tropical forest of Xishuangbanna Prefecture, Southwest China. We found that high seed abundance at the community level was associated with delayed and reduced seed removal, decreased dispersal distance and increased pre-dispersal seed survival for both plant species. High seed abundance was also associated with reduced seed caching of C. kissi, but it showed little effect on seed caching of P. kerrii. However, post-dispersal seed survival for the 2 plant species followed the reverse pattern. High seed abundance in the community was associated with higher post-dispersal survival of P. kerrii seeds, but with lower post-dispersal survival of C. kissi seeds. Our results suggest that different plant species derive benefit from fluctuations in seed production in different ways.

Implications of lemuriform extinctions for the Malagasy flora

Madagascar's lemurs display a diverse array of feeding strategies with complex relationships to seed dispersal mechanisms in Malagasy plants. Although these relationships have been explored previously on a case-by-case basis, we present here the first comprehensive analysis of lemuriform feeding, to our knowledge, and its hypothesized effects on seed dispersal and the long-term survival of Malagasy plant lineages. We used a molecular phylogenetic framework to examine the mode and tempo of diet evolution, and to quantify the associated morphological space occupied by Madagascar's lemurs, both extinct and extant. Using statistical models and morphometric analyses, we demonstrate that the extinction of large-bodied lemurs resulted in a significant reduction in functional morphological space associated with seed dispersal ability. These reductions carry potentially far-reaching consequences for Malagasy ecosystems, and we highlight large-seeded Malagasy plants that appear to be without extant animal dispersers. We also identify living lemurs that are endangered yet occupy unique and essential dispersal niches defined by our morphometric analyses.

Seed trait-mediated selection by rodents affects mutualistic interactions and seedling recruitment of co-occurring tree species

As mutualists, seed dispersers may significantly affect mutualistic interactions and seedling recruitment of sympatric plants that share similar seed dispersers, but studies are rare. Here, we compared seed dispersal fitness in two co-occurring plant species (Armeniaca sibirica and Amygdalus davidiana) that inhabit warm temperate deciduous forest in northern China. We tested the hypothesis that seed trait-mediated selection by rodents may influence mutualistic interactions with rodents and then seedling establishment of co-occurring plant species. A. davidiana seeds are larger and harder (thick endocarps) than A. sibirica seeds, but they have similar levels of nutrients (crude fat, crude protein), caloric value and tannin. More A. sibirica seedlings are found in the field. Semi-natural enclosure tests indicated that the two seed species were both harvested by the same six rodent species, but that A. sibirica had mutualistic interactions (scatter hoarding) with four rodent species (Apodemus peninsulae, A. agrarius, Sciurotamias davidianus, Tamias sibiricus), and A. davidiana with only one (S. davidianus). Tagged seed dispersal experiments in the field indicated that more A. sibirica seeds were scatter-hoarded by rodents, and more A. sibirica seeds survived to the next spring and became seedlings. A. sibirica seeds derive more benefit from seed dispersal by rodents than A. davidiana seeds, particularly in years with limited seed dispersers, which well explained the higher seedling recruitment of A. sibirica compared with that of A. davidiana under natural conditions. Our results suggest that seed dispersers may play a significant role in seedling recruitment and indirect competition between co-occurring plant species.

Cacti supply limited nutrients to a desert rodent community

In the Sonoran Desert, cacti represent a potentially important source of nutrients and water for consumers. Columnar cacti, in particular, produce a large pulse of flowers and succulent fruit during hot summer months. The importance of cactus stems, flowers and fruit to the small mammal community has not been quantified. We exploited natural variation in the carbon isotope (δ(13)C) values of cacti (CAM) versus C3 plants to quantify the relative use of these resources by a diverse desert small mammal community. We also estimated trophic level by measuring nitrogen isotope (δ(15)N) values. We hypothesized that (H1) granivorous heteromyids (kangaroo rats, pocket mice) would exploit the summer pulse of seeds and pulp; (H2) folivorous and omnivorous cactus mice, wood rats, and ground squirrels would exploit cacti stems year-round and seeds when available; and (H3) kangaroo rats and pocket mice would shift from seeds to insects during hot dry months. We found that heteromyids made minimal use of seeds during the period of heavy seed rain. Of the cricetids, only the folivore Neotoma albigula made continuous but highly variable use of cacti resources (annual mean = 32%, range 0-81%), whereas the omnivore Peromyscus eremicus ignored cacti except during the summer, when it exploited seeds and/or fruit pulp (June-July mean = 39%, range 20-64%). We also found little evidence for a shift to greater consumption of insects by heteromyids during the hot dry months. Overall, use of cactus resources by the small mammal community is very limited and highly variable among species.

Mutualistic and predatory interactions are driven by rodent body size and seed traits in a rodent–seed system in warm-temperate forest in northern China

Context Mutualistic interactions between animals and plants shape the structure of plant–animal systems and, subsequently, affect plant-community structure and regeneration. Aims To assess the effects of plant and rodent functional traits on the formation of mutualistic and predatory interactions regarding seed dispersal and predation in a warm-temperate forest. Methods Seed scatter-hoarding and predation by six sympatric rodent species on seeds belonging to five sympatric tree species were tested under enclosure conditions. Key results Functional traits (i.e. rodent body size and seed traits) are important to mutualism/predation in this seed–rodent system. The rodent–seed network is highly nested: large-sized rodents have mutualistic or predatory interactions with both large- and small-sized seed species, but small-sized rodents interacted with small-sized seed species only. Large seeds or seeds with hard coats enhanced mutualism and reduced predation. Conclusion Body size of rodents and seed traits such as handling time and nutritional value are key factors in the formation of mutualistic and predatory interactions within seed–rodent systems. Implications To promote seedling establishment in degenerated forests, introducing or protecting large-sized scatter hoarders and reducing the density of pure seed eaters are needed.