Small Mammalian Herbivores Research Articles

The Interplay of Biotic and Abiotic Factors in a Semiarid Chilean Mammal Assemblage: Results of a Long-Term Experiment

Since early 1989, we have conducted a large-scale ecological manipulation in a semiarid thorn scrub community in north-central Chile. We have excluded vertebrate predators (raptors and mammalian carnivores), and larger small mammal herbivores/ competitors (i.e., degus, Octodon degus) from replicated 0.56-ha plots, and monitored small mammal population and plant responses over more than ten years. Repeated tneasures ANOVAs on minimum number known alive (MNKA) estimates of small mammals for a six-year period (1990-1996) spanning an El Nino event in 1991-1992 showed strong responses of some species to predator exclusions (e.g., O. degus; Darwin's leaf-eared mouse, Phyllotis darwini; the chinchilla-rat, Abrocoma bennetti). However, responses varied in time with significant effects during pre-El Nino (1990-1992) and El Nino (1992-1994) periods (i.e., O. degus), or pre-El Nino and post-El Nino (1994-1996) periods (P. darwini, A. bennetti). Other species showed no responses to predator exclusions (e.g., olivaceous field mouse, Akodon olivaceus; long-haired field mouse, Abrothrix longipilis; long-tailed rice rat. Oligoryzomys longi-caudatus). Some effects of competitor (degu) exclusions were detected (e.g., A. bennetti during the El Nino and post-El Nino periods; O. longicaudatus during the El Nino). Top-down' factors (i.e., biotic interactions) appear to have greater effects on core species (i.e., P. darwini, O. degus) which persist in the thorn scrub. Other species (e.g., A. longipilis, O. longicaudatus) are transitory residents or opportunistic with lesser effects of biotic interactions, and their populations may be controlled by source-sink dynamics. All species had strong responses to the 1991-1992 El Nino indicating primary control by bottom-up factors.

Influence of Pocket Gophers on Meadow Voles in a Tallgrass Prairie

Creation of mounds by plains pocket gophers ( Geomys bursarius ) may affect the distribution of meadow voles ( Microtus pennsylvanicus ) and subsequent patterns of herbivory. An association between abundance of meadow voles and density of neighborhood mounds might be expected if voles use mounds as feeding sites (positive association), or if vole avoid mounds due to lack of cover (negative association). Mounds also may indirectly affect vegetative patterns, if mounds affect the distribution of herbivores. During two consecutive summers, we livetrapped voles and mapped mounds on three 80 m × 80 m plots. Regressions of vole captures at each trap station against the number of mounds within a range of radii of each trap were consistently negative across most spatial scales, but strongest correlation was found to be at 10 m. In all cases, bivariate scattergrams revealed a “factor-ceiling” distribution, with data points widely scattered below an upper limit. We also conducted an experiment to determine if seedlings had a greater or lesser mortality due to small-mammal herbivory when growing on mounds versus off mounds. A two-tailed paired t -test on the proportion of seedlings eaten after 13 days failed to show differences between on-mound and off-mound rates of herbivory ( P = 0.066); however, prior analyses showing voles avoiding areas with high rates of disturbance give weight to the hypothesis that seedlings growing off mounds experience higher rates of herbivory. Thus, through a second-order indirect effect, the presence of gopher mounds could lead to distinct changes in the plant community.

Response of Bushy-Tailed Woodrats (Neotoma cinerea) to Late Quaternary Climatic Change in the Colorado Plateau

Temperature profoundly influences the physiology and life history characteristics of organisms, particularly in terms of body size. Because so many critical parameters scale with body mass, long-term temperature fluctuations can have dramatic impacts. We examined the response of a small mammalian herbivore, the bushy-tailed woodrat (Neotoma cinerea), to temperature change from 20,000 yr B.P. to present, at five sites within the Colorado Plateau. Our investigations focused on the relationship between temperature, plant composition and abundance, and woodrat size. Body size was estimated by measuring fossil fecal pellets, a technique validated in earlier work. We found significant and highly covariable patterns in body mass over the five locations, suggesting that responses to temperature fluctuations during the late Quaternary have been very similar. Although woodrat mass and the occurrence of several plant species in the fossil recordweresignificantly correlated, in virtually all instances changes in woodrat size preceded changes in vegetational composition. These results may be due to the greater sensitivity of woodrats to temperature, or to the shorter generation times of woodrats as compared to most plants. An alternative hypothesis is that winter temperatures increased before summer ones. Woodrats are highly sensitive to warmer winters, whereas little response would be expected from forest/woodland plants growing at their lower limits. Our work suggests that woodrat size is a precise paleothermometer, yielding information about temperature variation over relatively short-term temporal and regional scales.

EFFECTS OF DEFOLIATION ON COMPETITIVE INTERACTIONS IN EUROPEAN WHITE BIRCH

Defoliation can affect plant competition in at least two ways. Both negative effects on the performance of defoliated plants and positive effects due to neighbor defoliation can potentially alter competitive relations between plants. Furthermore, if there is a trade-off between defense investment and growth or competitive ability, then selective herbivory may contribute to coexistence of defended and undefended plants. We tested these ideas in an experiment using two types of European white birch (Betula pendula). We compared growth (height) in two types that are known to differ with respect to palatability to small mammalian herbivores. The plants were grown from seeds in a greenhouse for three years and were potted either singly or in pairs. The pairs consisted either of two plants of the same type (pure pairs) or of one plant of each type (mixed pairs). In the second year, we defoliated randomly preselected plants twice. The experiment was concluded at mid-season in the third year. Both competition and defoliation (of the target plant) negatively affected growth. Competition reduced growth by 19–36%, the palatable type consistently being more affected than the unpalatable type. The negative effects of defoliation were about the same (9–13%) for both types. When grown singly, the palatable type grew taller (13%) than did the unpalatable type. The unpalatable type was the stronger competitor of the two types. As compared with plants grown singly, palatable plants grown in pure pairs were on average 24% shorter, while those grown in mixed pairs were 36% shorter. The corresponding data for the unpalatable type were 19% and 20%. The effects of neighbor defoliation differed between the types. The only significant effect of neighbor defoliation was that in palatable plants grown in mixed pairs neighbor defoliation compensated fully for the competitive effects, i.e., they were not significantly different from undefoliated plants grown singly. There were no significant effects of neighbor defoliation—neither on palatable plants grown in pure pairs, nor on unpalatable plants. Consequently, the effects of defoliation may not only depend on the presence of neighbors, but also on the composition of the neighborhood.

The effects of plant and tooth structure on intake and digestibility in two small mammalian herbivores.

We investigated the interaction between plant structure and animal tooth structure and how it can affect digestive performance in two small mammalian herbivores. Mastacomys fuscus, an exclusively herbivorous rodent, and Rattus lutreolus, a less herbivorous rodent, were fed diets varying in fibre structure and level (18%-45% neutral detergent fibre). On high-fibre whole wheat diets, R. lutreolus was not able to maintain body mass. M. fuscus was able to maintain body mass, and this was related to a much greater intake by M. fuscus on this diet. We attribute this result to more effective physical processing by M. fuscus as a consequence of a highly developed dentition. On a milled-wheat diet, intake by R. lutreolus increased significantly compared with the whole wheat diet, although intake was still not great enough to maintain body mass. On diets that were lower in fibre than the wheat diet, and with different fibre arrangements, R. lutreolus was able to achieve intakes great enough to maintain body mass. This suggests that, in addition to gut constraints, the physical form of the plant diet and dental structure can have a significant effect on digestive performance by animals in feeding trials.

Persistence, phytotoxicity, and management of arsenic, lead and mercury residues in old orchard soils of New York State

Lead arsenate was used for pest control in fruit orchards for many years in the United States, and its residues remain in most of these soils. Since arsenic and lead are toxic and only slightly mobile in soils, an analytical survey was conducted in 1993 to determine the concentrations of these elements persisting in soil samples from 13 older and newer orchards in New York State. Concentrations of arsenic and lead ranged, respectively, from 1.60 to 141 and from 1.48 to 720 (ppm, dry wt). Significant ( P < 0.01, n = 13) pairwise correlations were observed among concentrations of these elements: As vs. Pb r = 0.97), As vs. Hg (r = 0.79), and Pb vs. Hg (r = 0.83). Despite previous reports of phytotoxicity to fruit trees, elevated arsenic concentrations did not significantly reduce the dry weights of apple ( Malus domestica) seedlings grown in pasteurized soil from these orchards. The general environmental effects and best management strategies for arsenic and lead in orchard soils are discussed including phytotoxicity of arsenic and its uptake and accumulation in plants, ingestion by small mammal herbivores and their predators in orchards, and ways to minimize potential adverse effects on humans.

The Fence Effect Reconsidered

The fence effect consists of two related observations: (1) enclosed populations of small mammals grow to abnormally high densities; (2) they then overexploit resources, destroy their habitat, and decline precipitously. The fence effect is widely considered strong evidence for the critical role of emigration in regulating population size, and for ultimate limitation of populations of small-mammal herbivores by resource exhaustion. I reexamine published evidence for the fence effect and conclude that studies in small, artificial pens are unreliable in addressing the fence effect. Studies in large, natural enclosures are mixed in their support of the fence effect. Only two unreplicated studies show evidence for «abnormally» high densities of microtines inside enclosures, whereas several others do not

Effects of Resource Distribution on Animal-Plant Interactions

M.D. Hunter and P.W. Price, Introduction: Plants as a Variable Resource Base for Animals. M.C. Rossiter, The Impact of Resource Variation on Population Quality in Herbivorous Insects: A Critical Aspect of Population Dynamics. R.S. Ostfeld, Small Mammal Herbivores in a Patchy Environment: Individual Strategies and Population Responses. A.E. Weis and D.R. Campbell, Plant Genotype: A Variable Factor in Insect-Plant Interaction. B.J. Rathcke, Nectar Distributions, Pollinator Behavior, and Plant Reproductive Success. P.W. Price, Plant Resources as the Mechanistic Basis for Insect Herbivore Population Dynamics. J.C. Schultz, Factoring Natural Enemies into Plant Tissue Availability to Herbivores. T. Ohgushi, Resource Limitation on Insect Herbivore Populations. J.R. Karr, M. Dionne, and I. Schlosser, Bottom-Up versus Top-Down Regulation of Vertebrate Populations: Lessons from Birds and Fish. M.D. Hunter, Interactions Within Herbivore Communities Mediated by the Host Plant: The Keystone Herbivore Concept. D.W. Roubik, Loose Niches in Tropical Communities: Why Are There So Few Bees and So Many Trees? T.H. Fleming, How Do Fruit-and-Nectar Feeding Birds and Mammals Track Their Food Resources? T. Inoue and M. Kato, Inter-and Intraspecific Morphological Variation in Bumblebee Species, and Competition in Flower Utilization. J.M. Scriber and R.C. Lederhouse, The Thermal Environment as a Resource Dictating Patterns of Feeding Specialization of Insect Herbivores. Each chapter includes references. Index.

A Model of Dietary Fiber Utilization by Small Mammalian Herbivores, with Empirical Results for Neotoma

Allometric considerations have suggested that small herbivores are inefficient at or incapable of extracting energy from the microbial fermentation of structural carbohydrates. This notion is at odds with accumulating empirical evidence that demonstrates well-developed fiber digestion abilities for a number of small rodent genera. To examine the apparent inconsistency, we have constructed a model of plant fiber utilization tailored specifically for hindgut fermenters. Computer simulations provide estimates of fiber and overall dry-matter digestibilities as a function of body size, energy demand, and diet. Our calculations indicate that small mammals can obtain significant benefit from fiber fermentation, especially at moderate fiber levels. Comparisons with literature data are in general agreement, although fiber digestion abilities are still underestimated for the smallest animals. In an empirical test of the model, Neotoma obtained over 21% of their digestible energy solely from the microbial fermentation of plant fiber. We also observed an interesting pattern of allometric sorting predicted by the model. Smaller woodrats significantly reduced the fiber content of their diet, a behavior presumably reflecting energy limitations.

Evolution of Body Size Among Woodrats from Baja California, Mexico

Body size influences the physiology, bebaviour and community level interactions of organisms, but little is actually known about the factors that determine it. Close examination of evolution on islands may allow identification of pressures selecting on size. In this study, I collected and analysed body length data for 13 insular populations of a small mammalian herbivore (genus Neotoma). I found a significant tendency for insular woodrats to be larger than mainland forms, and this outcome is significantly related to the absence of mammalian predation pressure

Functional Response of a Small Mammalian Herbivore: The Disc Equation Revisited

Functional Response of a Small Mammalian Herbivore: The Disc Equation Revisited

Effects of feeding by mealybugs (Planococcus citri, homoptera: pseudococcidae) on the growth of Colliguaya odorifera seedlings

Mediterranean-type ecosystems are characterized by their capacity to recover after extended droughts and wildfires. Much of this resilience is attributable to underground storage tissue combined with the capacity for crown sprouting from root burls (Specht 1981; Kummerow and Mangan 1981). Some of the dominant shrubs, however, are poor crown sprouters or obligate seeders. Declining soil moisture typically limits the growing season to spring and early summer months. Only those seedlings which develop a deeply penetrating root system in the season following germination will survive the first summer. First-year shrub seedling survivorship in the southern California chaparral has been measured at 3-10% (Ellis 1983). Thus, seedlings must pass through a critical and very vulnerable establishment phase after fire. The effect of herbivores during this phase may be of great importance. Mills (1983) found that approximately 50% of seedlings of Ceanothus greggii, an obligate-seeding shrub in the southern California chaparral, were lost to small-mammal herbivores during the first growing season after fire. The effects of chewing insects and mammals are relatively easy to observe and quantify (Fuentes et al. 1981, 1983). Louda (1983) has cautioned, however, that assessing damage from estimates of the percent of missing leaf area may seriously underestimate the total herbivore effect and recommends an experimental approach which compares plants with herbivores to plants kept free of herbivores. The effects of sucking insects, although much more difficult to measure, can be significant. About 10% of the one-yearold Ceanothus greggii seedlings were killed by a psyllid infection in a post-burn area in the southern California chaparral (Mills, unpublished data). If seedlings are not killed, however, the relative damage to the plant is difficult to estimate. It would be valuable to correlate a particular level of infection by sap feeders with the growth reduction caused by these insects. The effects of phloem-feeding insects differ from those of chewing herbivores in several respects. Chewing insects or mammals often damage or remove the apical meristems and release axillary buds from apical dominance, causing the sprouting of new shoots which represent competitive sinks for photosynthate and increase the demand for water. Thus, the relative magnitude of the belowground sink may be decreased and relatively less photosynthate would reach the root system. Because phloem feeders do not stimulate axillary growth, the relative magnitude of the belowground sink should remain high and the immediate effect on root growth should be less severe. In fact, reallocation of assimilate from belowground to aboveground meristems has been observed due to feeding of chewing insects (Ryle and Powell 1975; Youngner 1972). On the other hand, increased root or shoot growth, or both, may result from feeding by chewing or sucking herbivores (Bardner and Fletcher 1974; Harris 1973, 1974; Howe et al. 1982; Owen 1980; Owen and Wiegert 1981). A second difference is that phloem feeders do not damage the existing photosynthetic structure but remove photosynthate. Thus all feeding effects would be manifest in decreased growth rates of aboveground and/or belowground plant organs. Regardless of the cause, however, reduction of the flow of carbohydrate to the roots of seedlings in summer-dry ecosystems might prevent establishment of the deep tap root system needed to survive the first summer. The purpose of this study is to assess the damage sustained by seedlings of a sclerophyllous shrub species from a Mediterranean ecosystem under three levels of infection by a phloem-feeding insect. The relative growth of root and shoot systems will be compared. The organisms involved were chosen for convenience. My goal was not to define a specific interaction, but to provide a general model which may be tested by more specific applications. The results should also facilitate estimates of damage to seedlings in the field.

Adaptation to oak and other fibrous, phenolic-rich foliage by a small mammal, Neotoma fuscipes.

Neotoma fuscipes, a small mammalian herbivore with apparently generalized food habits, was laboratory tested to determine its degree of dietary specialization. Woodrats from both oak woodland and coastal sage communities preferred Quercus agrifolia leaves (containing 40% phenolics and about 16% condensed tannin) over foliage from other dominant species. Approximately one-third of the oak phenolics and less than 10% of the oak condensed tannin remained in the feces. Their performance on pure oak leaves was comparable to that on a mixed diet of Quercus, Salvia, Eriogonum, and Rhus, with respect to weight maintenance, digestive efficiency and total amount ingested. Digestive efficiency was low on the oak diet (55%) relative to Salvia (77%), and to achieve similar weight levels, approximately twice as much oak as Salvia was ingested. Woodrats retained more nitrogen as oak consumption increased. Intake of oak and other foods increased with each experimental day. A sympatric species, N. lepida, was unable to maintain weight on oak leaves, although its digestive and polyphenolic-degrading capabilities, and nitrogen retention efficiency were equivalent to those of N. fuscipes. On a weight-adjusted basis, N. lepida ate about half as much oak per day as N. fuscipes. Oak intake may have been reduced by an inability to rapidly degrade fiber, which constitutes about 30% of the oak diet. In natural populations, N. fuscipes selectively feeds on evergreen sclerophyll vegetation high in fiber, tannins and related polyphenolics. Individuals ingest 2-3 plant types at a time, with a single species (oak when available) constituting most of the material consumed. Neotoma lepida diets are also dominated by a single species. The diversity of plant types eaten by different populations of N. lepida suggests that local dietary specializations may be developmentally acquired.

Factors Influencing Leaf Choice by Howler Monkeys: A Test of Some Hypotheses of Food Selection by Generalist Herbivores

Some theories on the foraging strategies of generalist herbivores suggest that the food choices of such animals are greatly influenced by the quality and antiquality components of potential foods. Many herbivores selectively feed on young rather than mature foliage. It has been hypothesized that young leaves are preferred because they have higher contents of protein and/or lower contents of toxic secondary compounds. These and several other hypotheses were tested with data from a long-term study of the feeding ecology of howler monkeys (Alouatta palliata), using a cost/benefit approach. Samples of young and mature leaves from the same species were analyzed for total protein, cell wall constituents, total phenolics, presence of condensed tannins, and total nonstructural carbohydrates. The results support the hypothesis that more than one factor determine howler leaf choices. The most important factors (relating to food content) appeared to be protein and fiber content, with perhaps some influence from secondary compounds. For howlers and other relatively small mammalian herbivores, the protein: fiber ratio may be a good predictor of leaf choices.