Effects of rodent species, seed species, and predator cues on seed fate

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.

The ecological and evolutionary significance of effectiveness landscapes in mutualistic interactions

Mutualism effectiveness, the contribution of an interacting organism to its partner's fitness, is defined as the number of immediate outcomes of the interactions (quantity component) multiplied by the probability that an immediate outcome results in a new individual (quality component). These components form a two-dimensional effectiveness landscape with each species' location determined by its values of quantity (x-axis) and quality (y-axis). We propose that the evolutionary history of mutualistic interactions leaves a footprint that can be identified by three properties of the spatial structure of effectiveness values: dispersion of effectiveness values, relative contribution of each component to the effectiveness values and correlation between effectiveness components. We illustrate this approach using a large dataset on synzoochory, seed dispersal by seed-caching animals. The synzoochory landscape was clumped, with effectiveness determined primarily by the quality component, and with quantity and quality positively correlated. We suggest this type of landscape structure is common in generalised coevolved mutualisms, where multiple functionally equivalent, high-quality partners exert similarly strong selection. Presumably, only those organisms located in high-quality regions will impact the evolution of their partner. Exploring properties of effectiveness landscapes in other mutualisms will provide new insight into the evolutionary and ecological consequences of mutualisms.

Effect of native and non-native snake scents on foraging activity of native rodents in Florida

Rodents use direct and/or indirect cues of predators to assess predation risk. The responses to these cues are well studied with regard to mammalian predators, but less understood with regard to reptilian predators. These responses are of particular importance in tropical and subtropical regions where reptile diversity is high and the likelihood of establishment of invasive reptilian predators also is high. We hypothesized that rodents would respond to direct scent cues of snake predators and that rodents would show greater aversion to scents of native snake predators than non-native snake predators. To assess this, scents of three snake species, two native and one non-native, and a non-snake control odor were distributed in Sherman live traps using a randomized block design. A total of 69 rodents representing four species were captured. Responses varied by species reinforcing that some species utilize indirect cues to assess predation risk, whereas others use direct cues. Moreover, one species (Neotoma floridana) showed a preference for non-native Python scent, indicating a lack of the appropriate anti-predator behavior, suggesting that some native rodents are more at risk of attack from invasive snakes than other native rodents.

Seed ingestion and germination in rattlesnakes: overlooked agents of rescue and secondary dispersal

Seed dispersal is a key evolutionary process and a central theme in the population ecology of terrestrial plants. The primary producers of most land-based ecosystems are propagated by and maintained through various mechanisms of seed dispersal that involve both abiotic and biotic modes of transportation. By far the most common biotic seed transport mechanism is zoochory, whereby seeds, or fruits containing them, are dispersed through the activities of animals. Rodents are one group of mammals that commonly prey on seeds (granivores) and play a critical, often destructive, role in primary dispersal and the dynamics of plant communities. In North America, geomyid, heteromyid and some sciurid rodents have specialized cheek pouches for transporting seeds from plant source to larder, where they are often eliminated from the pool of plant propagules by consumption. These seed-laden rodents are commonly consumed by snakes as they forage, but unlike raptors, coyotes, bobcats, and other endothermic predators which eat rodents and are known or implicated to be secondary seed dispersers, the role of snakes in seed dispersal remains unexplored. Here, using museum-preserved specimens, we show that in nature three desert-dwelling rattlesnake species consumed heteromyids with seeds in their cheek pouches. By examining the entire gut we discovered, furthermore, that secondarily ingested seeds can germinate in rattlesnake colons. In terms of secondary dispersal, rattlesnakes are best described as diplochorous. Because seed rescue and secondary dispersal in snakes has yet to be investigated, and because numerous other snake species consume granivorous and frugivorous birds and mammals, our observations offer direction for further empirical studies of this unusual but potentially important channel for seed dispersal.

Risk of predation makes foragers less choosy about their food

Animals foraging in the wild have to balance speed of decision making and accuracy of assessment of a food item's quality. If resource quality is important for maximizing fitness, then the duration of decision making may be in conflict with other crucial and time consuming tasks, such as anti-predator behaviours or competition monitoring. Individuals facing the risk of predation and/or competition should adjust the duration of decision making and, as a consequence, their level of choosiness for resources. When exposed to predation, the forager could either maintain its level of choosiness for food items but accept a reduction in the amount of food items consumed or it could reduce its level of choosiness and accept all prey items encountered. Under competition risk, individuals are expected to reduce their level of choosiness as slow decision making exposes individuals to a higher risk of opportunity costs. To test these predictions, the level of choosiness of a seed-eating carabid beetle, Harpalus affinis, was examined under 4 different experimental conditions of risk: i) predation risk; ii) intraspecific competition; iii) interspecific competition; and, iv) control. All the risks were simulated using chemical cues from individual conspecifics or beetles of different species that are predatory or granivorous. Our results show that when foraging under the risk of predation, H. affinis individuals significantly reduce their level of choosiness for seeds. Reductions in level of choosiness for food items might serve as a sensible strategy to reduce both the total duration of a foraging task and the cognitive load of the food quality assessment. No significant differences were observed when individuals were exposed to competition cues. Competition, (i.e opportunity cost) may not be perceived as risk high enough to induce changes in the level of choosiness. Our results suggest that considering the amount of items consumed, alone, would be a misleading metric when assessing individual response to a risk of predation. Foraging studies should therefore also take in account the decision making process.

Dispersal by rodent caching increases seed survival in multiple ways in canopy-fire ecosystems

Seed‐caching rodents have long been seen as important actors in dispersal ecology. Here, we focus on the interactions with plants in a fire‐disturbance community, specifically Arctostaphylos species (Ericaceae) in California chaparral. Although mutualistic relationships between caching rodents and plants are well studied, little is known how this type of relationship functions in a disturbance‐driven system, and more specifically to systems shaped by fire disturbance. By burying seeds in the soil, rodents inadvertently improve the probability of seed surviving high temperatures produced by fire. We test two aspects of vertical dispersal, depth of seed and multiple seeds in caches as two important dimensions of rodent‐caching behavior. We used a laboratory experimental approach to test seed survival under different heating conditions and seed bank structures. Creating a synthetic soil seed bank and synthetic fire/heating in the laboratory allowed us to have control over surface heating, depth of seed in the soil, and seed cache size. We compared the viability of Arctostaphylos viscida seeds from different treatment groups determined by these factors and found that, as expected, seeds slightly deeper in the soil had substantial increased chances of survival during a heating event. A key result was that some seeds within a cache in shallow soil could survive fire even at a depth with a killing heat pulse compared to isolated seeds; temperature measurements indicated lower temperatures immediately below caches compared to the same depth in adjacent soil. These results suggest seed caching by rodents increases seed survival during fire events in two ways, that caches disrupt heat flow or that caches are buried below the heat pulse kill zone. The context of natural disturbance drives the significance of this mutualism and further expands theory regarding mutualisms into the domain of disturbance‐driven systems.

Spatial and temporal effects on seed dispersal and seed predation of Musa acuminata in southern Yunnan, China

Wild bananas are abundant in tropical areas and many ecologists have observed that the succession process is quicker following increased disturbance. This study was conducted to analyze animal-seed interactions and their effects on the seed fate of a wild banana species (Musa acuminata) in tropical southern Yunnan (China) through experiments considering spatial (site and habitat) and temporal (seasons) variation. The largest proportion of fruits (81%) was removed by frugivorous seed dispersers, especially by bats at nighttime. Only 13% of the fruits were removed by climbing seed predators (different species of rats). In the exclosure treatment, rodents accounted for a significantly higher total artificially exposed seed removal number than ants, but with spatial and temporal differences. The highest seed predation rate by rodents (70%) was found in forest with wild banana stands, corresponding with the highest rodent diversity (species numbers and abundance) among the habitat types. In contrast, the seed removal number by ants (57%) was highest in the open land habitats, but there was no close correlation with ant diversity. Seed removal numbers by ants were significantly higher in the dry compared to the rainy season, but rodent activity showed no differences between seasons. The overall results suggest that the largest proportion of seeds produced by wild banana are primarily dispersed by bats. Primary seed dispersal by bats at nighttime is essential for wild banana seeds to escape seed predation.