The Relationship between Temperature and Development in Globodera ellingtonae

A new cyst nematode species, Globodera ellingtonae, was recently described from populations in Oregon and Idaho. This nematode has been shown to reproduce on potato. Because of this nematode’s close relationship to the potato cyst nematodes, G. rostochiensis and G. pallida, an understanding of the risk of its potential spread, including prediction of potential geographical distribution, is required. To determine the development of G. ellingtonae under different temperatures, we conducted growth chamber experiments over a range of temperatures (10.0°C to 26.5°C) and tracked length of time to various developmental stages, including adult females bearing the next generation of eggs. Both the time to peak population densities of G. ellingtonae life stages and their duration in roots generally increased with decreasing temperature. Regression of growth rate to second-stage (J2) and third-stage (J3) juveniles on temperature yielded different base temperatures: 6.3°C and 4.4°C for J2 and J3, respectively. Setting a base temperature of 6°C allowed calculation of the degree-days (DD6) over which different life stages occurred. The largest population densities of J2 were found in roots between 50 and 200 DD6. Population densities of J3 peaked between 200 and 300 DD6. Adult males were detected in soil starting at 300 to 400 DD6 and remained detectable for approximately 500 DD6. By 784 to 884 DD6, half of the eggs in adult females contained vermiform juveniles. Given the similarity in temperature ranges for successful development between G. ellingtonae and G. rostochiensis, G. ellingtonae populations likely could survive in the same geographic range in which G. rostochiensis now occurs.     

Reproductive investment under fluctuating predation risk: Microtine rodents and small mustelids

Summary We studied the reproductive investment of microtine rodents (bank vole (Clethrionomys glareolus),Microtus epiroticus andMicrotus agrestis) in western Finland under predation risk from small mustelids. During 1984–1992, the yearly mean litter size of overwintered bank voles was smaller at high least weasel and stoat densities than at low densities (close to 3 versus 4–5). In addition, the annual mean litter size of young bank voles was negatively correlated to the least weasel density. In youngM. agrestis voles, the yearly late summer litter size was negatively associated with the autumn density of small mustelids. In the crash phase of the vole cycle (1989 and 1992), we removed small mustelids (mainly least weasels) from four unfenced areas in late April to late May and studied the reproduction of voles in four removal and comparable control areas (each 2–4 km²). Reduction of small mustelids significantly increased the proportion of pregnant bank vole females, but not that of pregnantMicrotus vole females. We conclude that predation risk apparently reduced reproductive investment of free-living bank vole females; these voles appear to trade their current parental investment against future survival and reproductive prospects. Accordingly, the presence of small mustelids (or their scent) may slow down the reproductive rate of voles. As antipredatory behaviours occurred on a large scale, our results add evidence to the hypothesis that crashes in multiannual vole cycles are driven by small mustelid predators.     

Density-dependent variation in body mass of voles

We studied inter-annual, spatial and sexual variation in the body mass of bank volesMyodes glareolus Schreber, 1780 and grey-sided volesMyodes rufocanus Sundevall, 1846 using live trappings from two grids on the southand north-facing slopes of a mountain valley in Southern Norway. Variation in spring density of the four populations was consistent with cyclic dynamics (n=7,s-values >0.5). Individuals caught on the south-facing slope were larger than those caught on the north-facing slope. Reproductively mature bank vole males were smaller than females, whereas reproductively mature grey-sided vole males were larger than females. Body mass was related to density in both species. In bank voles, we found a direct positive density dependence caused by a higher rate of survival at higher densities resulting from individual allocation of resources from reproduction to survival and growth. In grey-sided voles, we found a negative delayed density dependence resulting from grazing on preferred plants that determined the resource available for individual vole growth the following year.     

Density-dependent regulation of natural and laboratory rotifer populations

Density-dependent regulation of abundance is fundamentally important in the dynamics of most animal populations. Density effects, however, have rarely been quantified in natural populations, so population models typically have a large uncertainty in their predictions. We used models generated from time series analysis to explore the form and strength of density-dependence in several natural rotifer populations. Population growth rate (r) decreased linearly or non-linearly with increased population density, depending on the rotifer species. Density effects in natural populations reduced r to 0 at densities of 1–10 l^–1 for 8 of the 9 rotifer species investigated. The sensitivities of these species to density effects appeared normally distributed, with a mean r=0 density of 2.3 l^–1 and a standard deviation of 1.9. Brachionus rotundiformis was the outlier with 10–100× higher density tolerance. Density effects in laboratory rotifer populations reduced r to 0 at population densities of 10–100 ml^–1, which is 10⁴ higher than densities in natural populations. Density effects in laboratory populations are due to food limitation, autotoxicity or to their combined effects. Experiments with B. rotundiformis demonstrated the absence of autotoxicity at densities as high as 865 ml^–1, a much higher density than observed in natural populations. It is, therefore, likely that food limitation rather than autotoxicity plays a major role in regulating natural rotifer populations.     

Density-dependent growth and reproduction of the apple snail, Pomacea canaliculata: a density manipulation experiment in a paddy field

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of 15–20 mm shell height into 12 enclosures (pens) of 16 m² at one of five densities – 8, 16, 32, 64, or 128 snails per pen. The survival rate of released snails was 95% and was independent of snail density. The snail density had a significant effect on the growth and egg production of individual snails. This density dependence may have been caused by reduced food availability. The females at high density deposited fewer and smaller egg masses than those at low density, and consequently produced fewer eggs. The females at densities 8 and 16 deposited more than 3000 eggs per female, while the females at density 128 oviposited only 414 eggs. The total egg production per pen was, however, higher at higher snail density. The survival rates of juvenile snails were 21%–37% and were independent of adult density. The juvenile density was positively correlated with the total egg production per pen and hence was higher at higher adult density. However, the density of juveniles larger than 5 mm in shell height, i.e., juveniles that can survive an overwintering period, was not significantly different among density treatments. These results suggest that snail density after the overwintering period is independent of the density in the previous year. Thus, density dependence in growth and reproduction might regulate the population of P. canaliculata in paddies. Received: October 23, 1998 / Accepted: July 16, 1999     

Density dependence,boundedness, and attraction: detecting stability in stochastic systems

Summary By analogy with deterministic stability, the stability of stochastic ecological systems can be viewed as a tendency for population densities to avoid dynamic boundaries (i.e. boundedness) or to approach a dynamic attractor (i.e. attraction). At the population level, these two views generate predictions consistent with density dependence. I therefore devised two new statistical tests of attraction, the random-walk attraction test and the randomized attraction test; I then used them successfully, along with randomization techniques that detect boundedness and two autocorrelation methods, to test for density dependence in published sequences of population densities. The attraction tests identify the apparent attractor, the band of densities toward which density tends to shift between generations. Locating the apparent attractor can generate a prediction of the next direction of density change; for data from a dragonfly assemblage, about 80% of these predictions were correct. From the single-population tests, I also developed two multispecies tests of attraction (the multispecies random-walk and randomized attraction tests) and two multispecies tests of boundedness (the multispecies permutation and randomization tests). These detected attraction and boundedness in the dragonfly assemblage and attraction in a collection of laboratory fruitfly populations. An evaluation of the statistical power of the new density attraction tests indicates a strong dependence on the sequence length n and on the number of populations m: power increases with n and particularly with m. Nevertheless, detecting attraction becomes likely even in populations with strong linear density-dependence only with n>30 or for shorter sequences in multispecies assemblages.     

Effects of fish density on planktonic communities and water quality in a manipulated forest pond

We examined the effects of fish on lower trophic levels in a small pond in eastern Finland. The pond was divided into four sections with plastic curtains and stocked with crucian carp (Carassius carassius); two sections had low (4.4–5.5 g m^–3) and two high (10.4–13.7 g m^–3) densities of fish. In the summer of 1987, the pond was sampled weekly for phyto- and zooplankton until the fish were removed by rotenone in September after a three month experiment. Fish density as well as the extent of macrophyte cover had a considerable impact on planktonic communities and water quality. Mean zooplankton biomass was significantly lower and phytoplankton biomass higher, at high fish density. Water transparency was correlated negatively with chlorophyll-a at low fish density but turbidity appeared to reduce transparency at high fish density. The composition and dynamics of the plankton also differed at different fish densities. The mechanisms behind these effects, and the influences of habitat and fish behaviour on the results, are discussed.     

A bioenergetic study of a benthic nematode,Plectus palustris de man 1880, throughout its life cycle

Summary Respiration rates of the bacterivorous freshwater nematode Plectus palustris were measured during the whole life cycle of the species and for animals grown at two food densities. Covariance analysis showed that small, but significant differences exist in the elevation of the respiration rate—body weight regressions (R=aW ^b, in nl O₂/ind·h and g wet weight) for different food densities. At a food density of 6–9·10⁸ bacterial cells/ml the level of respiration is 14% lower compared to rates of animals cultured at a ten times higher food density. However, the allometric function, R-aW ^b, adequately describes the relationship of respiration and body weight only during the larval growth phase and for young females, while respiration rates of newly hatched larvae and mature females at maximal egg production have lower metabolic rates. Cumulated metabolic costs to attain a certain age, size and stage of development have been determined and are used in a subsequent paper (Schiemer et al., 1979) to calculate the energy budgets of the species.     

Regulation of Collagen Synthesis in Human Dermal Fibroblasts in Contracted Collagen Gels by Ascorbic Acid, Growth Factors, and Inhibitors of Lipid Peroxidation

Ascorbic acid has been shown to stimulate collagen synthesis in monolayer cultures of human dermal fibroblasts. In the present studies, we examined whether the presence of a collagen matrix influences this response of dermal fibroblasts to ascorbic acid. Fibroblasts and collagen were mixed and allowed to gel and contract for 6 days to form a matrix prior to determining the concentration and time dependence for ascorbic acid to affect collagen synthesis by fibroblasts within the matrix. Collagen synthesis was stimulated at levels at or above 10 μM ascorbic acid and was maximal after 2 days of treatment. This concentration and time dependence is similar to that of cells grown in monolayer cultures. The effects of transforming growth factor-β (TGF-β) and fibroblast growth factor (FGF) were also examined in this model. TGF-β increased and FGF inhibited collagen synthesis in the gels, as has been shown for cells in monolayer cultures. The effects of potential inhibitors of lipid peroxidation induced by ascorbic acid were also examined in these matrices and compared to previous results obtained in monolayer cultures. Propyl gallate, cobalt chloride, α,α-dipyridyl, and α-tocopherol inhibited the ascorbic acid-mediated stimulation of collagen synthesis while mannitol had no effect. Natural retinoids inhibited total protein synthesis without the specific effect on collagen synthesis that was seen in monolayer cultures. These results indicate that ascorbic acid stimulates collagen synthesis in fibroblasts grown in a collagen matrix in a manner similar to that found in monolayer cultures. In contracting collagen gels, however, the magnitude of the effect is less and retinoids do not specifically inhibit collagen synthesis.     

Comment arising from a paper by Wolda and Dennis: using and interpreting the results of tests for density dependence

We argue that tests for density dependence are useful in analyses of population dynamics and suggest guide lines for their use and interpretation of results which avoid many of the problems discussed by Wolda and Dennis (1993). Processes other than density dependence per se can cause statistical tests to indicate the presence of density dependence (Wolda and Dennis 1993 and unpublished simulations). Tests for density dependence cannot reveal the mechanism of regulation, but they do indicate the nature of long-term population dynamics. Tests for density dependence give misleading results if sampling is not at generation intervals; however, this problem is avoided if we only use tests on data collected in each generation (Holyoak 1993a). Similarly, species should be semelparous. Non-delayed density dependence should not be considered without looking for delayed density dependence, since the presence of delayed density dependence can lead to over-detection of non-delayed density dependence (Woiwod and Hanski 1992; Holyoak 1993b). The partial autocorrelation function and knowledge of life-history are more useful than tests for density dependence for indicating whether any density dependence is delayed or not (Royama 1992; Holyoak 1993b). Estimation error with a constant upper size limit causes tests for density dependence to overestimate the frequency of delayed density dependence; however we do not know whether estimation error is bounded in real populations. Work in progress suggests that 20–40 generations (depending on the nature of population dynamics) gives a moderate level of accuracy with tests for density dependence, and >40 generations are necessary for tests to be accurate in their assessment of the strength of density dependence. We conclude that tests are useful indicators of whether density dependence, or other feedback mechanisms are likely to be acting.     

At What Level of Heat Load Are Age-Related Impairments in the Ability to Dissipate Heat Evident in Females?

Studies have reported that older females have impaired heat loss responses during work in the heat compared to young females. However, it remains unclear at what level of heat stress these differences occur. Therefore, we examined whole-body heat loss [evaporative (H_E) and dry heat loss, via direct calorimetry] and changes in body heat storage (∆H_b, via direct and indirect calorimetry) in 10 young (23±4 years) and 10 older (58±5 years) females matched for body surface area and aerobic fitness (VO₂peak) during three 30-min exercise bouts performed at incremental rates of metabolic heat production of 250 (Ex1), 325 (Ex2) and 400 (Ex3) W in the heat (40°C, 15% relative humidity). Exercise bouts were separated by 15 min of recovery. Since dry heat gain was similar between young and older females during exercise (p=0.52) and recovery (p=0.42), differences in whole-body heat loss were solely due to H_E. Our results show that older females had a significantly lower H_E at the end of Ex2 (young: 383±34 W; older: 343±39 W, p=0.04) and Ex3 (young: 437±36 W; older: 389±29 W, p=0.008), however no difference was measured at the end of Ex1 (p=0.24). Also, the magnitude of difference in the maximal level of H_E achieved between the young and older females became greater with increasing heat loads (Ex1=10.2%, Ex2=11.6% and Ex3=12.4%). Furthermore, a significantly greater ∆H_b was measured for all heat loads for the older females (Ex1: 178±44 kJ; Ex2: 151±38 kJ; Ex3: 216±25 kJ, p=0.002) relative to the younger females (Ex1: 127±35 kJ; Ex2: 96±45 kJ; Ex3: 146±46 kJ). In contrast, no differences in H_E or ∆H_b were observed during recovery (p>0.05). We show that older habitually active females have an impaired capacity to dissipate heat compared to young females during exercise-induced heat loads of ≥325 W when performed in the heat.     

Competition between herbivourous fishes and urchins on Caribbean reefs

Summary When the common sea urchin Diadema antillarum was removed from a 50 m strip of reef in St. Thomas, US Virgin Islands, cover of upright algae and the grazing rates and densities of herbivorous parrotfish and surgeonfish increased significantly within 11–16 weeks when compared to immediately adjacent control areas. Sixteen months after removal, Diadema had recovered to 70% of original density, abundance of upright algae no longer differed between removal and control areas, and the abundance and grazing activity of herbivorous fish in the removal was approaching equivalence with control areas. On a patch reef in St. Croix that had been cleared of Diadema 10–11 years earlier (Ogden et al. 1973b), urchins had recovered to only 50–60% of original density. This reef still showed significantly higher rates of grazing by fish and a significantly greater density of parrotfish and surgeonfish than a nearby control reef where Diadema densities had not been altered. These results indicate that high Diadema densities (7–12/m² for this study) may suppress the densities of herbivorous fish on Caribbean reefs.     

Delayed density dependence and oscillatory population dynamics in overlapping-generation systems of a seed beetle Callosobruchus chinensis: matrix population model

Long-term experimental systems with overlapping generations using a seed beetle, Callosobruchus chinensis, were maintained by providing 5 g of azuki beans (Vigna angularis) in two different renewal intervals: either 7 days or 10 days. The 7-day-renewal system (system 1) showed oscillatory dynamics with a constant periodic cycle of ca. 7 weeks. More stable population dynamics were seen in the 10-day-interval system (system 2). Short-term experiments showed that survivorship of adults increased with higher adult density, and that the survival rate of adults up to the age of 7 days was much higher than up to 10 days of age. In addition, the per capita production of hatched eggs by females which had survived for 7 days increased with increasing density experienced by the females. Females aged 10 days rarely laid eggs which hatched. We constructed a matrix population model based on either 1 week for system 1 or 10 days for system 2. The model included five stages in system 1: the hatched egg, the final instar larva, the pupa, the young adult and the old adult. Four stages were incorporated in the model for system 2: the young instar larva, the pupa, the young adult, and the old adult. Logistic-difference equations were applied to formulate both overcompensatory density dependence in the hatched-egg production by adults and undercompensatory response in the larval development up to the pupa. The survivorship of young adults to the old stage and the per capita hatched-egg productivity of the old females followed a linear regression against the young adult density. Inside-bean processes were adjusted to be equivalent in the two models, irrespective of the resource renewal intervals. The model predicted that system 1 would oscillate for a long time but that system 2 would rapidly converge to the equilibrium point. Multiplicative effects of both the delayed density dependence through interstage restraint effects and the overcompensatory density dependence in hatched-egg production generated various dynamic patterns ranging from a quickly disappearing damped oscillation to stable limit cycles in system 1. The relationship between resource renewal cycles and delayed density dependence was discussed based on these simulations.     

Concentration and shear rate dependence of viscosity in random coil polysaccharide solutions

The concentration (c) and shear rate (γ) dependence of viscosity (η) has been studied for a wide range of random coil polysaccharide solutions, and the following striking generalities are observed:

1. 1. The transition from dilute to concentrated solution behaviour occurs at a critical concentration , when ‘zero shear’ specific viscosity (η_sp) ≈ 10. η_sp varies as c^1.4 for dilute solutions, and as c^3.3 for concentrated solutions.

2. 2. The shear rate dependence of viscosity, and frequency dependence of dynamic (oscillatory) viscosity are closely superimposable.

3. 3. Double logarithmic plots of against (where η₀ is ‘zero shear’ viscosity, and is the shear rate at which ) are essentially identical for all concentrated solutions studied, and thus the two parameters η₀ and completely define the viscosity at all shear rates of practical importance.

Departures from points 1 and 2, but not 3, are observed for concentrated solutions of locust bean gum, guar gum, and hyaluronate at low pH and high ionic strength and are attributed to specific intermolecular associations (‘hyperentanglements’) of longer timescale than non-specific physical entanglements.     

Inter and intra-species-related differences in the regulation of the cardiac autonomic system

The heart rate response to isoproterenol (HR-Iso), density and affinity (k_d) of β-adrenergic (β-AR) and muscarinic (M₂) receptors were compared among three rodents with different generation-life histories of confinement and of high altitude exposure. The European guinea pig (Cavia porcellus) (EGp), a laboratory animal that arrived in Europe after the Spanish Conquest of South America and the Peruvian guinea pig (C. porcellus) (PGp), a semi-wild animal that came from the altiplano to sea level at least 25 generations ago, were used for intra-species comparison. Wistar rats (WR) were used for inter-species comparison as representative of a typical sea level laboratory animal. The HR-Iso was lower in EGp than in the PGp. The PGp showed the highest β-AR density (P<0.0005) and the highest β-AR k_d values (P<0.0005) when compared to both EGp and WR groups (β-AR B_max (fmol mg⁻¹ prot), WR, 19±4; Egp, 34±10; PGp, 74±15. β-AR k_d (pM), WR, 24±10; Egp, 17±7; PGp, 39±14). In contrast, PGp showed lower M₂ receptor density values than the EGp (P<0.0005). The WR had the highest M₂ receptor densities (M₂ B_max (fmol mg⁻¹ prot), WR, 188±15; Egp, 147±9; PGp, 118±6 and M₂ k_d (pM), WR, 65±12; Egp, 67±6; PGp, 92±2). The inter and intra-species differences found may be related to their respective history of confinement rather than to their history of exposure to high altitude.     

Dynamics and stability of muscle activations during walking in healthy young and older adults

To facilitate stable walking, humans must generate appropriate motor patterns and effective corrective responses to perturbations. Yet most EMG analyses do not address the continuous nature of muscle activation dynamics over multiple strides. We compared muscle activation dynamics in young and older adults by defining a multivariate state space for muscle activity. Eighteen healthy older and 17 younger adults walked on a treadmill for 2 trials of 5 min each at each of 5 controlled speeds (80–120% of preferred). EMG linear envelopes of v. lateralis, b. femoris, gastrocnemius, and t. anterior of the left leg were obtained. Interstride variability, local dynamic stability (divergence exponents), and orbital stability (maximum Floquet multipliers; FM) were calculated. Both age groups exhibited similar preferred walking speeds (p=0.86). Amplitudes and variability of individual EMG linear envelopes increased with speed (p<0.01) in all muscles but gastrocnemius. Older adults also exhibited greater variability in b. femoris and t. anterior (p<0.004). When comparing continuous multivariate EMG dynamics, older adults demonstrated greater local and orbital instability of their EMG patterns (p<0.01). We also compared how muscle activation dynamics were manifested in kinematics. Local divergence exponents were strongly correlated between kinematics and EMG, independent of age and walking speed, while variability and max FM were not. These changes in EMG dynamics may be related to increased neuromotor noise associated with aging and may indicate subtle deterioration of gait function that could lead to future functional declines.     

Window automata analysis of population dynamics in the immune system

A formalism based on window automata is proposed as a method to analyse complex population dynamics. The method is applied to a model of the immune network (Weisbuch, G.et al., 1990.J. theor. Biol. 146, 483–499), and used to predict which attractor the system reaches after antigenic stimulation, as a function of the parameters. The attractors of the dynamics are interpreted in terms of immune conditions such as vaccination or tolerance. Scaling laws that define the regimes in the parameter space corresponding to the specific attractor reached under antigenic stimulation are derived.     

The effect of density and mutual interference by a Predator: A laboratory study of predation by the Nudibranch Coryphella rufibranchialis on the hydroid Tubularia larynx

The nudibranch Coryphella rufibranchialis (JOHNSTON) feeds on a variety of hydroids, including Tubularia larynx Ellis & Solander. Experiments in which density of prey and predators were altered showed that more prey were eaten as prey density increased. However, more prey were consumed at low predator densities, presumably because of mutual interference among nudibranchs at the higher predator densities. The number of prey consumed per nudibranch was maximal with low predator densities and a ratio of 25–50 polyps per predator. Coryphella seems to show an opportunistic feeding strategy involving solitary predators rapidly depleting hydroid colonies and moving on to new colonies.     

Laboratory culture of Chironomus tentans for use in toxicity testing: optimum initial egg-stocking densities

The midge Chironomus tentans Fabricius is a commonly used freshwater invertebrate in sediment toxicity tests. Rigorous laboratory culturing techniques are needed to provide organisms of uniform quality and known age for use in testing and for the continuation of the culture itself. This study was conducted to determine the effect of initial culture stocking density on: (1) post-hatch (larval) dry weight, body length and head-capsule width at 10 and 20 days; (2) time to emergence; (3) number and sex of emergent adults; (4) number of larvae and pupae at test termination (day 42 post hatch); and (5) adult dry weight. Three egg stocking densities were used 690 (1.1 eggs cm^–2), 1043 (1.7 eggs cm^–2) and 1463 (2.4 eggs cm^–2). Mean weight of larvae at 10 days in high density tanks (0.13 mg/organism) was significantly higher (P=0.003) than both the medium and low density tanks (0.10 and 0.09 mg/organism, respectively). No significant differences between the three stocking densities were observed for the body length or head-capsule width at either 10 or 20 days post-hatch. Although not statistically significant, larval dry weight decreased with increased stocking density at day 20. A significantly (P=0.02) greater number of females (173±28) emerged from the low stocking density compared to both the medium and high stocking densities (123±45 and 118±54, respectively). Peak adult emergence for the low and medium stocking densities occurred between days 22 and 25 post-hatch, whereas peak adult emergence occurred between days 30 and 33 for the high stocking density. Survival relative to the initial number of eggs stocked was significantly greater (P=0.007) in the low density treatment compared to that in either the medium or the high density treatments. Mean adult weight exhibited an inverse relationship with initial stocking densities. At test end, there was not a significant difference in the mean number of organisms surviving and emerging in the three density levels. The central tendency for number of organisms surviving for all three treatments was 504 organisms per tank (0.82 organisms cm^–2). The results of this experiment suggest that an optimal egg stocking density of 1.0 egg cm^–2 (600 eggs/tank) be used with the feeding rate identified. This would ensure uniform larvae at the appropriate developmental stage (2nd–3rd instar) needed for toxicological research/testing (e.g. 10 days post-hatch), as well as producing sufficient emergence of males and females for future culture establishment.     

Linking White-Tailed Deer Density,Nutrition, and Vegetation in a Stochastic Environment

Density-dependent behavior underpins white-tailed deer (Odocoileus virginianus) theory and management application in North America, but strength or frequency of the phenomenon has varied across the geographic range of the species. The modifying effect of stochastic environments and poor-quality habitats on density-dependent behavior has been recognized for ungulate populations around the world, including white-tailed deer populations in South Texas, USA. Despite the importance of understanding mechanisms influencing density dependence, researchers have concentrated on demographic and morphological implications of deer density. Researchers have not focused on linking vegetation dynamics, nutrition, and deer dynamics. We conducted a series of designed experiments during 2004–2012 to determine how strongly white-tailed deer density, vegetation composition, and deer nutrition (natural and supplemented) are linked in a semi-arid environment where the coefficient of variation of annual precipitation exceeds 30%. We replicated our study on 2 sites with thornshrub vegetation in Dimmit County, Texas. During late 2003, we constructed 6 81-ha enclosures surrounded by 2.4-m-tall woven wire fence on each study site. The experimental design included 2 nutrition treatments and 3 deer densities in a factorial array, with study sites as blocks. Abundance targets for low, medium, and high deer densities in enclosures were 10 deer (equivalent to 13 deer/km²), 25 deer (31 deer/km²), and 40 deer (50 deer/km²), respectively. Each study site had 2 enclosures with each deer density. We provided deer in 1 enclosure at each density with a high-quality pelleted supplement ad libitum, which we termed enhanced nutrition; deer in the other enclosure at each density had access to natural nutrition from the vegetation. We conducted camera surveys of deer in each enclosure twice per year and added or removed deer as needed to approximate the target densities. We maintained >50% of deer ear-tagged for individual recognition. We maintained adult sex ratios of 1:1–1:1.5 (males:females) and a mix of young and older deer in enclosures. We used reconstruction, validated by comparison to known number of adult males, to make annual estimates of density for each enclosure in analysis of treatment effects. We explored the effect of deer density on diet composition, diet quality, and intake rate of tractable female deer released into low- and high-density enclosures with natural nutrition on both study sites (4 total enclosures) between June 2009 and May 2011, 5 years after we established density treatments in enclosures. We used the bite count technique and followed 2–3 tractable deer/enclosure during foraging bouts across 4 seasons. Proportion of shrubs, forbs, mast, cacti, and subshrubs in deer diets did not differ (P > 0.57) between deer density treatments. Percent grass in deer diets was higher (P = 0.05) at high deer density but composed only 1.3 ± 0.3% (SE) of the diet. Digestible protein and metabolizable energy of diets were similar (P > 0.45) between deer density treatments. Likewise, bite rate, bite size, and dry matter intake did not vary (P > 0.45) with deer density. Unlike deer density, drought had dramatic (P ≤ 0.10) effects on foraging of tractable deer. During drought conditions, the proportion of shrubs and flowers increased in deer diets, whereas forbs declined. Digestible protein was 31%, 53%, and 54% greater (P = 0.06) during non-drought than drought during autumn, winter, and spring, respectively. We studied the effects of enhanced nutrition on the composition and quality of tractable female deer diets between April 2007 and February 2009, 3 years after we established density treatments in enclosures. We also estimated the proportion of supplemental feed in deer diets. We used the 2 low-density enclosures on each study site, 1 with enhanced nutrition and 1 with natural nutrition (4 total enclosures). We again used the bite count technique and 2–3 tractable deer living in each enclosure. We estimated proportion of pelleted feed in diets of tractable deer and non-tractable deer using ratios of stable isotopes of carbon. Averaged across seasons and nutrition treatments, shrubs composed a majority of the vegetation portion of deer diets (44%), followed by mast (26%) and forbs (15%). Enhanced nutrition influenced the proportion of mast, cacti, and flowers in the diet, but the nature and magnitude of the effect varied by season and year. The trend was for deer in natural-nutrition enclosures to eat more mast. We did not detect a statistical difference (P = 0.15) in the proportion of shrubs in diets between natural and enhanced nutrition, but deer with enhanced nutrition consumed 7–24% more shrubs in 5 of 8 seasons. Deer in enhanced-nutrition enclosures had greater (P = 0.03) digestible protein in their overall diet than deer in natural-nutrition enclosures. The effect of enhanced nutrition on metabolizable energy in overall diets varied by season and was greater (P < 0.04) for enhanced-nutrition deer during summer and autumn 2007 and winter 2008. In the enhanced-nutrition treatment, supplemental feed averaged 47–80% of the diet of tractable deer. Of non-tractable deer in all density treatments with enhanced nutrition, 97% (n = 128 deer) ate supplemental feed. For non-tractable deer averaged across density treatments, study sites, and years, percent supplemental feed in deer diets exceeded 70% for all sex and age groups. We determined if increasing deer density and enhanced nutrition resulted in a decline in preferred forbs and shrubs and an increase in plants less preferred by deer. We sampled all 12 enclosures via 20, 50-m permanent transects in each enclosure. Percent canopy cover of preferred forbs was similar (P = 0.13) among deer densities averaged across nutrition treatments and sampling years (low density: = 8%, SE range 6–10; medium density: 5%, 4–6; high density: 4%, 3–5; SE ranges are presented because SEs associated with backtransformed means are asymetrical). Averaged across deer densities, preferred forb canopy cover was similar between nutrition treatments in 2004; but by 2012 averaged 20 ± 17–23% in enhanced-nutrition enclosures compared to 10 ± 8–13% in natural-nutrition enclosures (P = 0.107). Percent canopy cover of other forbs, preferred shrubs, other shrubs, and grasses, as well as Shannon's index, evenness, and species richness were similar (P > 0.10) among deer densities, averaged across nutrition treatments and sampling years. We analyzed fawn:adult female ratios, growth rates of fawns and yearlings, and survival from 6 to 14 months of age and for adults >14 months of age. We assessed adult body mass and population growth rates (lambda apparent, λ_APP) to determine density and nutrition effects on deer populations in the research enclosures during 2004–2012. Fawn:adult female ratios declined (P = 0.04) from low-medium density to high density in natural-nutrition enclosures but were not affected (P = 0.48) by density in enhanced nutrition enclosures although, compared to natural nutrition, enhanced nutrition increased fawn:adult female ratios by 0.15 ± 0.12 fawns:adult female at low-medium density and 0.44 ± 0.17 fawns:adult female at high density. Growth rate of fawns was not affected by deer density under natural or enhanced nutrition (P > 0.17) but increased 0.03 ± 0.01 kg/day in enhanced-nutrition enclosures compared to natural nutrition (P < 0.01). Growth rate of yearlings was unaffected (P > 0.71) by deer density, but growth rate increased for males in some years at some density levels in enhanced-nutrition enclosures. Adult body mass declined in response to increasing deer density in natural-nutrition enclosures for both adult males (P < 0.01) and females (P = 0.10). Enhanced nutrition increased male body mass, but female mass did not increase compared to natural nutrition. Survival of adult males was unaffected by deer density in natural- (P = 0.59) or enhanced- (P = 0.94) nutrition enclosures. Survival of adult females was greatest in medium-density enclosures with natural nutrition but similar at low and high density (P = 0.04). Enhanced nutrition increased survival of females (P < 0.01) and marginally for males (P = 0.11). Survival of fawns 6–14 months old was unaffected (P > 0.35) by density in either natural- or enhanced-nutrition treatments but was greater (P = 0.04) under enhanced nutrition. Population growth rate declined (P = 0.06) with increasing density in natural-nutrition enclosures but not (P = 0.55) in enhanced nutrition. Enhanced nutrition increased λ_APP by 0.32. Under natural nutrition, we found only minor effects of deer density treatments on deer diet composition, nutritional intake, and plant communities. However, we found density-dependent effects on fawn:adult female ratios, adult body mass, and population growth rate. In a follow-up study, deer home ranges in our research enclosures declined with increasing deer density. We hypothesized that habitat quality varied among home ranges and contributed to density-dependent responses. Variable precipitation had a greater influence on deer diets, vegetation composition, and population parameters than did deer density. Also, resistance to herbivory and low forage quality of the thornshrub vegetation of our study sites likely constrained density-dependent behavior by deer. We posit that it is unlikely that, at our high-density (50 deer/km²) and perhaps even medium-density (31 deer/km²) levels, negative density dependence would occur without several wet years in close association. In the past century, this phenomenon has only happened once (1970s). Thus, density dependence would likely be difficult to detect in most years under natural nutrition in this region. Foraging by deer with enhanced nutrition did not result in a reduction in preferred plants in the vegetation community and had a protective effect on preferred forbs because ≤53% of deer diets consisted of vegetation. However, enhanced nutrition improved fitness of individual deer and deer populations, clearly demonstrating that nutrition is limiting for deer populations under natural conditions in western South Texas. © 2019 The Authors. Wildlife Monographs published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.