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Abstract

Low-density populations of gypsy moth, Lymantria ia dispar, were studied over a 10-yr period in Massachusetts. Increases in gypsy moth density were associated with declines in density of the white-fooled mouse, Peromyscus leucopus, a principal predator. Furthermore, changes in density of P. leucopus populations were positively correlated with the density of acorn crops, a dominant winter food sourer for these mice. To demonstrate these effects we used a novel bootstrap regression method that adjusts for spatial and temporal autocorrelation in the time series data. The findings are compatible with a dual equilibrium model of gypsy moth population dynamics, in which low densities are regulated by mice and high densities are regulated by other factors, notably a virus disease.
... Small mammals are ideal study subjects for understanding community dynamics; they directly impact community structure, interact with other trophic levels, and are sensitive to ecosystem changes (Elkinton 1996;Ostfeld et al., 1996). Understanding the dynamics and structure of the small mammal community can serve as a way of understanding how an ecosystem reacts to change (Ostfeld 1996). ...
... Small mammals typically have high populations and therefore play an important role in trophic webs. They may alter food resource abundances and are able to significantly change the population dynamics of their prey items (Elkinton 1996;Ostfeld et al., 1996). ...
Technical Report
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The study was implemented through the use of conventional rodent traps, Pit falls and mist nets. It was conducted in different survey areas 4 in MCFR and 2 in KFNP for a period of four months. Each survey area was systematically divided into three sampling sites i.e. Peripheral disturbed habitat, Forest edge and Forest interior. Overall a total of 1076 small mammals were captured comprising of 540 from KFNP and 536 from MCFR. Population structure analysis indicated that in both forests rodents and shrews exhibit a male biased population while bats exhibit a female biased population; with significantly higher numbers of adults compared to other age classes.
... The most abundant oak herbivore families are noctuids, tortricids, erebids, geometrids, and nolids, which start feeding on the new shoots in early April (Elkinton et al., 1996). The community of caterpillars also changes during the season, with tortricids the first to feed on the new shoots and geometrid species the last (Soria, 1988). ...
Article
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DNA barcoding identification needs a good characterization of intraspecific genetic divergence to establish the limits between species. Yet, the number of barcodes per species is many times low and geographically restricted. A poor coverage of the species distribution range may hamper identification, especially when undersampled areas host genetically distinct lineages. If so, the genetic distance between some query sequences and reference barcodes may exceed the maximum intraspecific threshold for unequivocal species assignation. Taking a group of Quercus herbivores (moths) in Europe as model system, we found that the number of DNA barcodes from southern Europe is proportionally very low in the Barcoding of Life Data Systems. This geographical bias complicates the identification of southern query sequences, due to their high intraspecific genetic distance with respect to barcodes from higher latitudes. Pairwise intraspecific genetic divergence increased along with spatial distance, but was higher when at least one of the sampling sites was in southern Europe. Accordingly, GMYC (General Mixed Yule Coalescent) single-threshold model retrieved clusters constituted exclusively by Iberian haplotypes, some of which could correspond to cryptic species. The number of putative species retrieved was more reliable than that of multiple-threshold GMYC but very similar to results from ABGD and jMOTU. Our results support GMYC as a key resource for species delimitation within poorly inventoried biogeographic regions in Europe, where historical factors (e.g., glaciations) have promoted genetic diversity and singularity. Future European DNA barcoding initiatives should be preferentially performed along latitudinal gradients , with special focus on southern peninsulas.
... The increase in the gypsy moth populations occurs at low population of mice Peromyscus leucopus (Rafinesque, 1818), which is in dependence on the fructifying of oak tree. In the case of low population, the reduction in population is mainly carried out by mice, while other factors affect high population (Elkinton et al. 1996). Other authors (Jones et al. 1998;Liebhold et al. 2000) also ascertained the connection among the gypsy moths, predators and the quantity of acorns. ...
Article
Seven outbreaks of the gypsy moth in Serbia for the last seven decades were researched in relation to the solar activity cycles and the solar flux at 2.8 GHz. Based on data analysis, three types of outbreaks were selected. A–type includes the second half of the solar cycle. This type of the outbreak doesn’t get into the next cycle (the values of the solar flux at 2.8 GHz decrease in the final phases). Besides the final years of the solar cycle, B–type also includes the initial years of the next cycle. C–type appears in the beginning of the solar activity cycle. At B-type and C-type the interruption of the outbreak occurs while the solar flux at 2.8 GHz value is increasing. The outbreaks of A-type were: 1970–1976 and 2004–2007; B-type: 1952–1956, 1962–1966, 1984–1987 and 1995–1998; C-type: 2009–2014. The obtained results suggest that the gypsy moth outbreaks are caused by corresponding range of energy coming from the Sun.
... During these cyclical infestation periods, defoliated trees and shrubs become vulnerable to attack by disease organisms and other insects. Moreover, this pest causes habitat alterations, including shifts in forest stand composition (Campbell and Sloan 1977;Gottschalk and MacFarlanes 1993;Fajvan and Wood 1996;Davidson et al. 1999), wildlife composition and distribution (Smith 1985;Elkinton et al. 1996) and alterations in nutrient cycling (Collins 1961;Grace 1986). Hence, the impacts of L. dispar can be devastating both ecologically and economically. ...
Article
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Several products are widely used to control gypsy moth (Lymantria dispar) populations. The main purpose of the present study is to evaluate the aerial application effects of B. thuringiensis and diflubenzuron on non-target arthropods in a Mediterranean holm-oak forest. The density of gypsy moth population at the time of application of treatments was the same in the three forests and the defoliation levels in the three holm-oak forests were between level 0 (no apparent defoliation) and level 1 (small defoliations of new leaf). Sampling litter, soil, ground-dwelling, and epiphytic arthropods showed that the holm-oak forests treated with diflubenzuron had a better quantitative faunal outcome than the Control, whereas the highest taxonomic diversity was found in the site treated with B. thuringiensis. These results could probably be explained by a temporary competitive decrease due to the huge number of gypsy moth caterpillars on plants, as well as the enormous accumulation of their excrements in the soil during a prolonged period of time.
... Recent work has suggested that competition with E. maimaiga may drive the baculovirus to low levels (Hajek et al., 2015), but because E. maimaiga prefers cool, moist weather (Hajek, 1999), and because climate-change models predict warm, dry conditions for the range of the gypsy moth, the competitive balance is likely to favour the baculovirus more strongly in the future. Meanwhile, gypsy moth dynamics at low densities are strongly affected by mortality due to generalist predators (Elkinton et al., 1996), leading to a low-density equilibrium that interacts in complex ways with the cycles driven by the baculovirus (Dwyer et al., 2004). ...
... Apodemus speciosus is just one of numerous species worldwide that experience seasonal changes in tannin intake due to their reliance on acorns. Many of these species' population dynamics or social structure are directly linked to acorn production (Azad, Wactor, & Jachowski, 2017;Elkinton et al., 1996;McShea, 2000;Wolff, 1996). However, the complexities of these relationships are often realized (Díaz & Alonso, 2003;Selås, 2016). ...
Article
1.The foraging ecology of mammalian herbivores is regulated in part by their ability to detoxify plant secondary metabolites (PSM). Ambient temperature has been shown to alter liver function in rodents and the toxicity of some PSMs, but little is known about the physiological and nutritional consequences of consuming PSMs at different ambient temperatures. Furthermore, the effect of ambient temperature on the response of mammals to the most ubiquitous class of PSM, tannins, is unknown. 2.We measured the effect of temperature and tannin intake on liver function, and the subsequent effect on the tannin tolerance of wild Japanese wood mice, Apodemus speciosus. The experiment involved acclimation to one of two ambient temperatures (10 °C or 20 °C) followed by acclimation to a diet of acorns (6.2 % tannin DW). Liver function was measured both before and after acclimation to acorns by measuring the clearance time of a hypnotic agent. Finally, the mice were fed only acorns in a 5‐day feeding experiment to assess their tolerance to tannin in the diet. 3.Acclimation to acorns had a significant effect on liver function, but the direction of this effect was dependent on ambient temperature. Acorn consumption improved the liver function of wood mice at 10 °C, but reduced liver function at 20 °C, revealing a complex relationship between ambient temperature and tannin intake on liver function. Furthermore, mice with better liver function, indicated by faster clearance of the hypnotic agent, exhibited higher protein digestibility on an acorn‐only diet, indicative of higher tannin tolerance. 4.These results suggest that environmental temperature plays a significant role in the tolerance of A. speciosus to tannins, providing new insight in to their seasonal feeding behavior and winter ecology. We contend that cold‐induced tannin tolerance may help to explain the population dynamics of mammalian herbivores with seasonal changes in the tannin content of their diet, and inform predictions about the response of these animals to a changing climate.
... S mall mammals influence distribution and habitat use of predators (Carey et al. 1992), regulate invertebrate populations (Buckner 1966, Carey and Johnson 1995, Elkinton et al. 1996, Carey and Harrington 2001, disperse fungal spores (Maser et al. 1978), and serve as indicators of sustainable forest management (Carey and Harrington 2001). Small mammals are also prey for protected species in the Pacific Northwest (PNW), most notably spotted owls (Strix occidentalis), Pacific fisher (Pekania pennant pacifica), and marten (Martes americana caurina and M. a. sierrae). ...
Article
We evaluated how forest type, vegetation structure in trapping webs, and proximate forest types influenced localized (~6.35 hectares) abundances for commonly captured small mammals in northern California, USA. We trapped from May to August of 2011–13 in 69 forest patches that represented: (1) clearcuts (3–5 years postharvest), (2) 10–20 year-old conifer plantations, (3) rotation-aged conifer stands, and (4) Watercourse and Lake Protection Zones. We captured 11 species; four in sufficient numbers for regression modeling. Our average abundance estimates for the study were 4.57 (standard error [SE] = 0.43), 0.32 (SE = 0.11), 0.90 (SE = 0.30), and 0.25 (SE = 0.09) individuals per web location (~0.75 hectares) for Peromyscus spp., Neotoma spp., California ground squirrels, and Allen’s chipmunks. We found that web-level ground cover (shrubs and grass), downed wood, and types of forests containing our trapping webs best described small mammal abundances, whereas proximate forest types were not important. Our results indicated that retaining localized structures in the form of understory shrub cover and downed wood positively influences small mammal abundance in intensively managed forests of northern California.
... This method of calculating percent parasitism incorporates the fact that parasitism rates can be obscured by predation rates because predation typically occurs on the pupae whether or not they were parasitized. This method aims to estimate the true underlying mortality rate of each source in the system (Buonaccorsi and Elkinton 1990;Elkinton et al. 1996;Royama 1981;Van Driesche 1983). To test for any relationship between the rates of Pimpla wasp parasitism and predation, we regressed the proportion parasitized by the proportion lost to predation and no trend was detected (Fig. S2); this further suggests that predators do not discriminate between parasitized and unparasitized pupae. ...
Article
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Ecological communities may be resistant to invasive species through a combination of top-down and bottom-up mechanisms, including predation, competition, parasitism, and disease. In particular, natural enemies that cross over from native species to use newly introduced non-native species as hosts can influence invasive species population dynamics and may slow down invasions. However, research on parasitism in biological invasions is lagging behind research on biological invasions in general. We used the model species winter moth (Operophtera brumata) to study the effect of recruitment of native parasitoids on an invasive population of winter moth in the northeastern United States. We deployed sentinel pupae over 4 years across this population’s range, identified recovered parasitoids, and measured the rate of parasitism by native sources across years, seasons, invasion history, and host densities. Native Pimpla wasps inflicted 98% of the parasitism detected, resulting in an annual average of 15–40% mortality on pupae not depredated. Pimpla were present across all years, seasons, and sites. Where winter moth has invaded, parasitism was greatest when winter moth pupal density was high (i.e., positive density-dependent mortality) suggesting that Pimpla is helping to regulate the population. The wasps were morphologically identified as Pimpla aequalis Provancher; however, using a multilocus genetic comparison approach, they were determined to comprise two cryptic species. Overall, this study shows that recruitment of these native wasps to the invasive winter moth population is likely playing a significant role in regulating population outbreaks and is aiding in biological control of winter moth.
... Population densities increase quickly over a few generations, sometimes so much so that they can defoliate trees with > 5000 egg masses/ha, although it is difficult to detect life stages of L. dispar at low densities over many years (Liebhold et al., 2000). Population dynamics of L. dispar is considered to be regulated by parasitoids (Bakhvalov et al., 2010), pathogens, such as Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) Elkinton, 1987a,1987b) and fungus Entomophaga maimaiga (Hajek, 1999;Zubrik et al., 2016), and/or small mammals (Elkinton et al., 1996;Liebhold et al., 1998). In the United States, cyclic dynamics of L. dispar populations were not observed until after the establishment of introduced parasitoids (Allstadt et al., 2013;Elkinton and Liebhold, 1990). ...
Article
The gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), is distributed throughout most of the northern hemisphere and known as one of the most significant insect pest both in its native and introduced regions. The aim of this study was to examine whether there are differences in the outbreak pattern of L. dispar, and these differences can be explained by different subspecies or divided populations across Japan for appropriate management of L. dispar. We reviewed the records of outbreaks of L. dispar in Japan using available literature and internet sources. We also examined phylogenetic relationships between L. dispar populations in Japan and other global populations to clarify current debated classification of the Japanese L. dispar. We then estimated the distribution of Japanese L. dispar with a species distribution model (Maxent). Phylogenetic mtDNA analysis revealed that Lymantria species consisted of four clusters: the first cluster contained three L. dispar subspecies from Europe and continental Asia (classified into L. dispar dispar), the populations of Honshu and western Hokkaido (L. dispar asiatica), and the central and eastern Hokkaido populations of L. dispar hokkaidoensis, while the second and third clusters comprised L. albescens and L. postalba, respectively. According to historical records, L. dispar outbreaks occur in approximately 11 year-cycles in Hokkaido, regardless of its classification at the subspecies level. Within northeastern Japan, L. dispar outbreaks occurred simultaneously from 2014 to 2015, whereas there were few reports of outbreaks in southwestern Japan. The distribution ranges of L. dispar were determined mainly by the maximum depth of snow cover (100–250 cm), forest patch area (0–0.6 km²), and elevation (600–1400 m); clearly divided at the species level but not at the subspecies or divided population level. On the other hand, the forest types are likely to contribute to the population dynamics and geographical distribution of L. dispar.
Preprint
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***THIS IS A PREPRINT: PUBLISHED VERSION ALREADY AVAILABLE*** DNA barcoding identification needs a good characterization of intra-specific genetic divergence to establish the limits between species. Yet, the number of barcodes per species is many times low and geographically restricted. A poor coverage of the species distribution range may hamper identification, especially when undersampled areas host genetically distinct lineages. If so, the genetic distance between some query sequences and reference barcodes may exceed the maximum intra-specific threshold for unequivocal species assignation. Taking a group of Quercus herbivores (moths) in Europe as model system, we found that the number of DNA barcodes from southern Europe is proportionally very low in the Barcoding of Life Data Systems (BOLD). This geographical bias complicates the identification of southern query sequences, due to their high intra-specific genetic distance with respect to barcodes from higher latitudes. Pairwise intra-specific genetic divergence increased along with spatial distance, but was higher when at least one of the sampling sites was in southern Europe. Accordingly, GMYC (General Mixed Yule Coalescent) single threshold model retrieved clusters constituted exclusively by Iberian haplotypes, some of which could correspond to cryptic species. The number of putative species retrieved was more reliable than that of multiple threshold GMYC but very similar to results from ABGD and jMOTU. Our results support GMYC as a key resource for species delimitation within poorly inventoried biogeographic regions in Europe, where historical factors (e.g. glaciations) have promoted genetic diversity and singularity. Future European DNA barcoding initiatives should be preferentially performed along latitudinal gradients, with special focus on southern peninsulas.
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We examined movements and behavior of female white-tailed deer (Odocoileus virginianus) relative to the acorn mast-fall from 1986 through 1989 in a mature deciduous forest in Front Royal, Virginia. Ten white-tailed deer with radiotransmitters increased their home range to incorporate acorn-producing areas during mast-fall. Consumption of acorns by deer con­ stituted ca. 50% of foraging time during peak mast-fall; average consumption rate was 0.75 acorns/min searching. Although the number of acorns eaten by deer was correlated with mast-fall, a prolonged time was spent searching for acorns after mast-fall. Deer consumed 70% ofmarked acorns placed out during mast-fall, while medium-sized animals (e.g., Tamias striatus, Sciurus niger, Sciurus carolinensis) consumed 61% of acorns placed out later in autumn. We hypothesize that high densities of deer may limit populations of more mast­ dependent species, particularly at low acorn-crop densities.
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Conducted an 8-yr study of flower and acorn production in white oak Quercus alba, red oak Q. rubra and black oak Q. velutina in E-central Missouri. Red oak and white oak showed a greater degree of mast-fruiting than did black oak. Within a species, individuals tended to produce large acorn crops in the same years, but each species differed in which years they produced large crops. The size of a given acorn crop was determined by both flower abundance and survival of flowers to fruit. The first principal component explained the largest amount of the variation in black oak (R2 = 0.55) and red oak (R2 = 0.89). In white oak, two principal components combined to explain 77% of the variation in acorn production. Weather variables associated with these principal components included spring temperature (positive effect) and summer drought (negative effect). Past acorn production had a major impact on the size of the current acorn crop, with each species showing a different pattern. In black oak, the current acorn crop was negatively correlated with the crop 3 yr prior but positively correlated with the crop 2 yr prior. In red oak, acorn crop size 1, 2, and 3 yr prior had negative correlations with current acorn crop, while acorn crop size 4 yr prior was positively correlated. In white oak, there were negative correlations between acorn crop size and crop size 1, 2, and 4 yr prior but a positive correlation with the acorn crop size 3 yr prior. Data are consistent with the hypothesis that mast-fruiting species must store resources during some years in order to produce a mast crop. The patterns of acorn production for black, red, and white oak are apparently not simply responses to weather events but are also a function of prior reproductive events. This suggests that masting is an evolved reproductive strategy. -from Authors
Article
(1) Experimental manipulations of densities of gypsy moths revealed a strong, positive spatially density-dependent reduction in population size, a response not evident in past studies of natural populations in North America. (2) Positive density-dependent mortality occurred during the early and mid larval stages and was primarily due to Compsilura concinnata, a polyphagous parasitoid. (3) The oviposition rate of Parasetigena silvestris, an oligophagous parasitoid of gypsy moths, was initially inversely density-dependent but became positively density-dependent during the late larval period. (4) Phobocampe disparis showed an inversely density-dependent response, and predation by small mammals on pupae deployed in the litter was lower in plots with higher numbers of pupae. (5) We conclude that if gypsy moth population densities fluctuate asynchronously on a spatial scale of a few hectares, the density-dependent responses of C. concinnata and P. silvestris could suppress the populations to a point where small mammal predation would be able to prevent population increase. This phenomenon may explain the apparent stability of gypsy moth populations on a region-wide basis for the many years between outbreaks.
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Yields of acorns by seven species of oak (Quercus spp.), indigenous to forests of the upper coastal plain, were investigated in relation to tree characteristics and climatological variables in Louisiana and east Texas. Three species were studied from 1950 through 1967, three species from 1950 through 1955, and one species from 1950 through 1954. The amount of seed produced was related to bole diameter and size of crown, and the expected yields for six species were calculated by size-classes of boles and crowns. Radial growth was not a reliable indicator of seed yield. Some trees were inherently poor producers. Few trees below the age of 20 years produced seed. One freeze in late March during the flowering period affected seed production. There was no apparent relationship between amount of rainfall and seed production. The moisture content of acorns and the mean number of acorns required to weigh 1 pound was determined for each species. Forest dwellers, both mammalian and avian, made ready use of acorns of all species. In normal production years, the seed was usually gone from ground quadrats about the time seedfall was complete in early February, but in bumper years, some seed remained for longer periods. An estimate was made of the pounds of acorns required for five game species for a given period of time. The number of oak trees required to fulfill these needs, by species and size-classes of boles and crowns, can be determined from the expected yield tables.
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The influence of supplemental food on the movement and demography of the white-footed mouse (Peromyscus leucopus) was studied; on two experimental and two control grids established within a 24-ha tract of deciduous forest. Preliminary trapping for 1 year prior to supplemental feeding indicated only minor differences between grids. After supplemental feeding, mice bred somewhat earlier in the spring on the experimental plot, but densities, survival, movement, reproductive intensity, and weights were not influenced by supplemental food. These parameters varied more between years than between the experimental and control plots within a year. We concluded that natural food supply was not limiting population densities at the time of supplemental feeding. However, increased food availability (mast crop) may have affected winter survival, thus causing the increased densities observed on all grids during the second year.
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White-footed mice (Peromyscus leucopus) were sampled during spring, summer and autumn on the Konza Prairie Research Natural Area, Kansas, from autumn 1981 to spring 1988. Abundance was greater in gallery forest than in wooded outcrop habitat, although temporal patterns of abundance were similar with highs in 1981-1982 and 1985-1986. Standard deviations (SD) of the common logarithm of abundance were ≤ 0.32 for each of four sites in spring, summer and autumn, except for one forest site in spring. Our SD values were similar to those reported for other populations of P. leucopus. Abundance of P. leucopus was related to its abundance in the previous season (spring abundance vs. that in previous autumn, summer abundance vs. that in previous spring and autumn abundance vs. that in previous summer), seed production by woody vegetation and precipitation, but the factors that had a major influence on abundance of P. leucopus varied among spring, summer and autumn populations. Ambient temperature was unrelated to abundance of P. leucopus during each of the 3 seasons studied.
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Comparative observations of the density and demography of two populations of the white-footed mouse (Peromyscus leucopus) were made in a floodplain forest and adjacent upland. There was little exchange of individuals (<3%) between populations. Fluctuations of density were greater in the floodplain because better recruitment in autumn produced higher densities than in the upland, and poor recruitment in winter and spring produced lower densities. In other respects - breeding season, adult survival and age structure - the populations were similar. Contrary to expectations, the floodplain population served as a small source of recruits for the upland rather than the reverse. Recruitment patterns and population fluctuations were probably related to food availability; the best survival of young occurred in autumn after the mast crop had fallen. Poor recruitment in the floodplain was related to extensive flooding. Neither population bred during the coldest months, December and January. General food habits of the two populations were similar, and the proportion of seeds in the diet and fat content of carcasses were high during the most successful recruitment period (autumn). Overall, the two populations were surprisingly similar. Greater fluctuations of density in the floodplain, while agreeing with theoretical predictions for an unstable habitat, occurred primarily because the floodplain was more productive in autumn rather than because of poor survival. Thus, floodplain forests should not be considered marginal habitat for these mice.
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(1) Wood mice were trapped on two similar areas at Wytham. On the experimental area of 1.1 ha wheat was provided at each of 45 trap points; the adjacent control area was 1.5 ha. (2) During the spring growth period the mean weights of males and females on the experimental area were up to 20% higher than those on the control. (3) In the spring of 1968 the decline in numbers on the control area was much greater than on the experimental plot. In the summer of 1968 and the late winter of 1968-69 the experimental area had twice the population of the control. (4) The supplementary food increased both immigration and survival compared to the control. (5) A short interruption in the food supply on the experimental area coincided with a sudden decrease in numbers and survival. (6) The increased density on the experimental area in summer 1968 was shown to be the result of the better survival of overwintered and juvenile mice. (7) In the autumn of 1968 the increase in numbers was unexpectedly later on the experimental area. (8) Factors other than food supply affect wood mouse populations, especially in summer.
Article
Deer mice (Peromyscus maniculatus rubidus) averaged 575 males in a sample of 3833 collected in the Douglas fir (Pseudotsuga menziesii) belt in western Ore-gon from 1952-1966. The percentage of mice captured in autumn and recaptured in spring was significantly greater in good (2.66 kg/ha or more) than in poor (0.45 kg/ha or less) Douglas fir and western hemlock (Tsuga heterophylla) seed years. However, the autumn-spring population estimates for good and poor seed years were not sitrnificantly different, but by the following autumn the good seed years had significantly greater populations. There was a significantly larger number of young female (and. by infer-ence, male) deer mice in the population during the good seed years. Individuals of both sexes were fecund each month of the year; however, yearly fecundity periods varied and the longest were 11 months. Males were most active sexually from February -through November and females from March through October. Males reached peak fecundity in May, 1-2 months earlier than the females. The average male and female fecundity per-centage from September-March, when tree seed was most abundant, was significantly greater for good seed years. Average yearlong fecundity for both sexes was not signifi-cantly larger in the good seed years, but for the females there was a significant difference. The percentage of pregnancies from September-March was significantly greater (eight times) in good years, but during April-August was only one-half of those for the poor, a significant difference. However, on a seed year basis, the average percentage of preg-nancies was 2.9% greater for good years but was not significantly different. Average litter size was not significantly affected by the size of the seed fall, being 4.6 for corpora lutea and 4.4 for both the embryos and placental scars. Smallest average embryo litters were found in winter and spring and larger ones were tallied in summer and autumn. Calculated average number of litters per year was 2.9 for good seed years and 2.5 for poor, a significant difference. The positive factors contributed variously to higher popu-lations during and/or following a good seed crop, at least by the following autumn.