Rodent Exclosures Research Articles

Granivory from native rodents and competition from an exotic invader strongly and equally limit the establishment of native grasses.

Seed predation and resource competition are fundamental biotic filters that affect the assembly of plant communities, yet empirical studies rarely assess their importance relative to one another. Here, we used rodent exclosures and experimental seed additions to compare how rodent granivory and resource competition affected the net establishment of an exotic invader (Bromus tectorum) and two native bunchgrasses (Pseudoroegneria spicata and Elymus elymoides) in the Great Basin Desert, USA. Rodent granivory limited the establishment of both native grasses, but had no significant effect on B. tectorum. Competition from B. tectorum limited the establishment of both native grasses, but neither native grass imposed a significant competitive effect on B. tectorum. Interestingly, we found that rodent granivory and B. tectorum competition limited the establishment of native grasses to the same extent, suggesting that these biotic interactions may impose equally important barriers to the local establishment of P. spicata and E. elymoides. By evaluating the strength of multiple biotic interactions in simultaneous, coordinated experiments, we can understand their relative contributions to community-level patterns.

Deciphering the effects of disperser assemblages and seed mass on patterns of seed dispersal in a rodent community.

The sizes of both seed dispersers and seeds are traits that are likely to interact to influence seed fate in many synzoochoric plant species. Here, we examined whether members of a granivorous rodent community consisting of species of different body size vary in their effectiveness as seed dispersers, and how this relationship may be altered by seed size. We marked northern red oak (Quercus rubra) acorns with plastic tags and placed them in size-selective rodent exclosures. The exclosures allowed differential access of rodent groups based on different body size: (i) small (e.g. Peromyscus spp.); (ii) small and medium (e.g. Tamias striatus); and (iii) small, medium and large (e.g. Sciurus carolinensis) species of rodents. Acorn removal did not differ among exclosure types, but more seeds were missing when removed by small rodents, probably because of larderhoarding. The treatments did not influence the relative frequency of acorn consumption. However, small rodents cached considerably fewer and partially ate more acorns than the other 2 groups. The mean dispersal distance was the longest for cages with medium openings, intermediate for cages with large openings and the shortest for cages with small openings. Acorn mass positively affected the probability of caching and this relationship was unaffected by exclosure type. In conclusion, granivorous rodents of different body sizes strongly differed in their interactions with acorns, with small rodents acting primarily as acorn predators and medium and large species contributing significantly more to dispersal of red oaks.

Selective plant foraging and the top‐down suppression of native diversity in a restored prairie

Summary Clarifying what species are being consumed at what times can improve our understanding of how anthropogenic change affects food web dynamics, with implications for community assembly including restoration. This includes human‐based changes to plant communities via species introductions, which can interact with consumer feeding preferences to indirectly alter assembly outcomes including reduced restoration success if planted species are preferentially targeted. We used DNA barcoding of plant material in rodent scat, combined with field‐based feeding trials, rodent trapping, and rodent exclosures to test for dietary preferences and assembly impacts of native rodents on a restored and regionally rare tallgrass prairie of central North America. We examined whether native rodents preferred non‐targeted and mostly non‐native oldfield plants that are more locally abundant, thus protecting the rarer native planted species from consumption, or if rodents preferred native plants regardless of abundance. Our results supported the latter outcome. Barcoding revealed that native rodents consumed mostly non‐planted oldfield species for 10 months of the year (92% of the diet). The exceptions were August–September, when planted prairie species accounted for 87% of plants consumed and coincided with their peak seed production. Cafeteria trials suggested diet seasonality to be explained by food limitation – native prairie seeds were consumed year‐round when made available. Non‐targeted oldfield species thus appeared to be of sufficient quality to support rodent populations (peaking at 23 individuals ha−1 in summer), but without rescuing rarer prairie species from targeted granivory and resulting in reduced prairie diversity outside of experimental exclosures. Synthesis and applications. Our work illustrates how anthropogenic‐based changes to producer communities may affect feeding pathways in grassland food webs, potentially facilitating establishment by non‐target species via indirect consumer effects that can be difficult to detect. These findings suggest that rodents may reduce the restoration success of tallgrass prairie in our region, with some planted species likely to suffer future recruitment difficulties due to granivory. Managers may need to consider multiple trophic levels with restoration, not just the commonly planted producer communities but also the consumers and predators associated with them.

Staged invasions across disparate grasslands: effects of seed provenance, consumers and disturbance on productivity and species richness

Exotic plant invasions are thought to alter productivity and species richness, yet these patterns are typically correlative. Few studies have experimentally invaded sites and asked how addition of novel species influences ecosystem function and community structure and examined the role of competitors and/or consumers in mediating these patterns. We invaded disturbed and undisturbed subplots in and out of rodent exclosures with seeds of native or exotic species in grasslands in Montana, California and Germany. Seed addition enhanced aboveground biomass and species richness compared with no-seeds-added controls, with exotics having disproportionate effects on productivity compared with natives. Disturbance enhanced the effects of seed addition on productivity and species richness, whereas rodents reduced productivity, but only in Germany and California. Our results demonstrate that experimental introduction of novel species can alter ecosystem function and community structure, but that local filters such as competition and herbivory influence the magnitude of these impacts.

Biotic resistance: exclusion of native rodent consumers releases populations of a weak invader

Summary Biotic resistance is a commonly invoked hypothesis to explain why most exotic plant species naturalize at low abundance. Although numerous studies have documented negative impacts of native consumers on exotic plant performance, longer‐term multi‐generation studies are needed to understand how native consumer damage to exotics translates to their population‐level suppression over large landscapes. We used rodent exclosures and embedded seed‐addition experiments to evaluate the effects of rodent herbivory and granivory on populations of Tragopogon dubius, a Eurasian aster that typically occurs at low abundance across North America. Vegetation surveys encompassing 20 000 km2 of west‐central Montana grasslands established that T. dubius is regionally well distributed, occurring in 81% of 16 grasslands sampled. However, it consistently occurred at low local abundance, averaging < 0.5% cover in 1 m2 plots. In large rodent exclosures, T. dubius attained population densities that averaged over five times higher than in paired rodent‐access control plots at 11 grassland sites spread over 750 km2. This pattern was six times stronger in older compared with newer rodent exclosures, indicating enclosed populations were still increasing. Floral herbivory, which reduces or prevents reproduction in this biennial plant, decreased by 99% in rodent exclosures compared with control plots. Additionally, seedling establishment was nearly eight times higher in seed‐addition subplots within versus outside of rodent exclosures. Synthesis. Our findings illustrate how biotic resistance from granivory and florivory of native generalist rodents can provide an important ecosystem service by strongly limiting the local abundance of a widely distributed exotic weed.

Biotic resistance via granivory: establishment by invasive, naturalized, and native asters reflects generalist preference

Escape from specialist natural enemies is frequently invoked to explain exotic plant invasions, but little attention has been paid to how generalist consumers in the recipient range may influence invasion. We examined how seed preferences of the widespread generalist granivore Peromyscus maniculatus related to recruitment of the strongly invasive exotic Centaurea stoebe and several weakly invasive exotics and natives by conducting laboratory feeding trials and seed addition experiments in the field. Laboratory feeding trials showed that P. maniculatus avoided consuming seeds of C. stoebe relative to the 12 other species tested, even when seeds of alternative species were 53-94% smaller than those of C. stoebe. Seed addition experiments conducted in and out of rodent exclosures revealed that weakly invasive exotics experienced relatively greater release from seed predation than C. stoebe, although this was not the case for natives. Seed mass explained 81% of the variation in recruitment associated with rodent exclusion for natives and weak invaders, with larger-seeded species benefiting most from protection from granivores. However, recruitment of C. stoebe was unaffected by rodent exclusion, even though the regression model predicted seeds of correspondingly large mass should experience substantial predation. These combined laboratory and field results suggest that generalist granivores can be an important biological filter in plant communities and that species-specific seed attributes that determine seed predation may help to explain variation in native plant recruitment and the success of exotic species invasions.

Joshua tree (Yucca brevifolia) seeds are dispersed by seed-caching rodents

:Joshua tree (Yucca brevifolia) is a distinctive and charismatic plant of the Mojave Desert. Although floral biology and seed production of Joshua tree and other yuccas are well understood, the fate of Joshua tree seeds has never been studied. We tested the hypothesis that Joshua tree seeds are dispersed by seed-caching rodents. We radioactively labelled Joshua tree seeds and followed their fates at five source plants in Potosi Wash, Clark County, Nevada, USA. Rodents made a mean of 30.6 caches, usually within 30 m of the base of source plants. Caches contained a mean of 5.2 seeds buried 3–30 mm deep. A variety of rodent species appears to have prepared the caches. Three of the 836 Joshua tree seeds (0.4%) cached germinated the following spring. Seed germination using rodent exclosures was nearly 15%. More than 82% of seeds in open plots were removed by granivores, and neither microsite nor supplemental water significantly affected germination. Joshua tree produces seeds in indehiscent pods or capsules, which rodents dismantle to harvest seeds. Because there is no other known means of seed dispersal, it is possible that the Joshua tree–rodent seed dispersal interaction is an obligate mutualism for the plant.

Granivores, exclosures, and seed banks: harvester ants and rodents in sagebrush-steppe

Experiments were conducted to assess the influences of granivores on the seed bank in sagebrush-steppe. Rodent and/or ant exclosures were established at two distances from five harvester ant mounds. Monthly soil samples were collected from the exclosures, and from open access areas at two distances to ascertain changes in the seed bank. Seed dish experiments were conducted monthly to establish a maximum seed removal rate. Rodents were trapped monthly. More seeds were recovered from rodent exclosures than from ant exclosures. Seed banks changed temporally and spatially, with more seeds recovered in September and at 6 m from mounds. Rodents removed seeds more rapidly from seed dishes than did ants. Ants removed more seeds from dishes closest to the nests. Rodents may have a larger impact on the local seed bank than harvester ants. Ant activities, other than seed consumption, may have more influence on vegetation.

Effects of Selective Seed Predation by Rodents on Shortgrass Establishment

Our experiment tested whether selective seed predation by rodents affects the establishment of blue grama (Bouteloua gracilis) and buffalo grass (Buchloe dactyloides) at a disturbed site in the Colorado shortgrass steppe. We hypothesized that foraging rodent granivores respond positively to large seed sizes and to high seed densities, and we predicted that intensive foraging would constrain grass establishment. Capture‐recapture studies confirmed that heteromyid granivores, kangaroo rats (Dipodomys ordii) and hispid pocket mice (Perognathus hispidus), comprised 48% of the local rodent community. Seeds of blue grama and buffalo grass were shallowly planted at three densities in replicated plots within and outside rodent exclosures. The frequency of digging in seeded rows (seed predation) and the frequency of herbivory on seedling were calculated. We assessed grass establishment using seedling frequency and harvested aboveground biomass. Seed foraging was related positively to seed size, but foraging responses to seed densities were not confirmed. Herbivory on seedlings was not intensive and was similar between the two grasses. Seedling frequency and harvested biomass numbers for the species with the larger seeds, buffalo grass, were reduced more in rodent‐accessible plots than were values for blue grama. Rodent seed predation can constrain the establishment of large‐seeded species, but results should be interpreted to reflect site‐specific conditions and the characteristics of seeded species.

Factors Influencing Species Composition and Population Sizes in a Ground Beetle Community (Carabidae): Predation by Rodents

In this study, we examined the hypothesis that rodent predation significantly alters the ground beetle community in a shrub-steppe ecosystem in southwestern Wyoming, USA. Four resident rodent species (Peromyscus maniculatus, Onychomys leucogaster, Perognathus parvus and Spermophilus armatus) were shown to feed readily on ground beetles in captivity. We tested the impact of these rodent predators on field populations of ground beetles by excluding rodents from ten 15 x 15 m plots for 2 yr (1984-86). We found that total carbid abundances were 111% higher on rodent exclosure plots relative to control plots. Seven of eight carabid species (Amara farcta, A. quenseli, Calosoma luxatum, Carabus taedatus, Cyminidis cribricollis, Harpalus fraternus and H. innocuus) exhibited greater population sizes in rodent-free plots than in control plots, while only one species (Philophuga viridis) was less abundant in rodent exclosures. Rodent-free plots also contained significantly greater mean numbers of carabid species (per plot) than control plots. Total densities of insectivorous and omnivorous rodents in the study area during 1986 averaged about 10 rodents ha-1, which was slightly larger than in previous years but far smaller than densities during rodent cycle peaks. Hence, we assert that typical densities of rodents can significantly reduce the abundances of ground beetles in our shrub-steppe ecosystem, and may contribute to temporary, localized extinctions of rarer carabid species.

Factors Limiting Populations of Arid-Land Darkling Beetles (Coleoptera: Tenebrionidae): Predation by Rodents

The role of rodent predation as a limiting factor in the abundance of sixspecies of darkling beetles (Tenebrionidae) was examined in a sagebrush–steppe ecosystem (Kemmerer, Wyo.). Rodent predators were identified in feeding experiments using captive rodents and darkling beetles. Grasshopper mice ( Onychomys ) and deer mice ( Peromyscus ) successfully captured and consumed Eleodes extricatus Say. Ground squirrels ( Spermophilus ) and pocket mice ( Perognathus ) initially fed upon E. extricatus but developed an aversion to them after several exposures. Subsequent experiments demonstrated that the chemical aerosol produced by E. extricatus as an antipredator defense was effective in providing escape time during deer mouse attacks but not during grasshopper mouse attacks. A replicated field experiment, employing 10 rodent exclosures (15 by 15 m) and 10 control plots, showed that the removal of rodent predators resulted in population increases in five of the six species of darkling beetles ( Coniontis ovalis, E. extricatus, E. pimelioides, E. rileyi , and Helops difficilis ). The total population of beetles exhibited a 63% increase in abundance following rodent removal. Based on 1986 field density measurements of rodents (8/ha) and beetles (4,931/ha), an average daily consumption of fewer than two beetles per rodent was calculated to be sufficient to produce the observed differences in darkling beetle populations on the site. Therefore, it is concluded that rodent predation functioned as a significant limiting factor on darkling beetle populations in the sagebrush–steppe study area.