Proportion Of Reproductive Females Research Articles

Reproduction and production in a social context: Group size, reproductive skew and increasing returns.

Evolutionary success requires both production (acquisition of food, protection and warmth) and reproduction. We suggest that both may increase disproportionately as group size grows, reflecting 'increasing returns' or 'group augmentation benefits', raising fitness in groups that cooperate in production and limit reproduction to one or a few high fertility females supported by non-reproductives, with high reproductive skew. In our optimisation theory both Allee effects (when individual fitness increases with group size or density) and reproductive skew arise when increasing returns determine optimal group size and proportion of reproductive females. Depending on which of food or maternal time is more important for reproduction, evolutionary trajectories of lineages may (1) reach a boundary constraint where only one female reproduces in a period (as with African wild dogs) or (2) reach a boundary where all females reproduce during their lifetimes but only during an early life stage (human menopause) or a late life stage (birds with non-dispersing helpers), where stage length optimises the proportion of females that is reproductive at any time or (3) reach the intersection of these boundary constraints where a single reproductive female is fully specialised in reproduction (as with eusocial insects). We end with some testable hypotheses.

Patterns of Survival of a Communally Overwintering Rattlesnake Using an Artificial Hibernaculum

Avoiding thermal stress by using hibernacula is fundamental to survival of snakes in temperate environments. Snakes may overwinter alone or aggregate in communal hibernacula to avoid temperature extremes. Limited information is available regarding the overwintering ecology of rattlesnakes, and basic demographic information is necessary for understanding population dynamics, habitat requirements, and management of rattlesnakes. Even less is known about the demographics of species and populations utilizing artificial hibernacula, which may become increasingly important as habitat is lost and/or becomes more fragmented. We used seven years of mark–recapture data of the Western Rattlesnake (Crotalus viridis) at an artificial hibernaculum to estimate annual apparent survival, encounter rates, and transition rates between life stages (mature vs. immature) to investigate how these rates vary over time and in response to drought conditions. Further, we described trends in body condition and age structure, as well as trends in the proportion of reproductive females. Apparent survival rates were consistent and high across the study period for all snakes (including immature and mature females and all males; 0.66). The probability of an immature female surviving and transitioning to a mature adult was high and consistent across the study period (0.73). The stable survival and transition rates observed in this study may indicate other demographic rates, such as reproductive output, may be important drivers of population dynamics. This study contributes basic knowledge about population dynamics of communally overwintering rattlesnakes and suggests survival rates were relatively stable for snakes regularly utilizing this artificial hibernaculum.

Are protected areas good for the human species? Effects of protected areas on rural depopulation in Spain

Protected areas (PAs) seek to conserve valuable genes, species and ecosystems by applying a legal regime that restricts some socioeconomic activities and also offers opportunities for new ones. As a result, PAs have been claimed by some authors to boost socioeconomic conditions in rural areas mainly through tourism activities. However, others have claimed that PAs contribute to rural depopulation through the worsening of living conditions of local residents because of restrictions resulting from protection regulations. Here, we applied a multiple-paired Before-After-Control-Impact (BACI) research design on a census on protected rural municipalities (cases; N = 52) versus unprotected rural municipalities (controls; N = 55) in Spain to ascertain whether PAs had positive or negative effects on rural populations using three indicators on depopulation with official municipal data from 1996 until 2019: Compound annual growth rate (CAGR); Proportion of reproductive individuals (REP); and Proportion of reproductive females (WREP). We controlled for some confounders such as biophysical characteristics and regional regulations by carefully selecting our sample of municipalities spatially. Our results show that depopulation figures were worse in cases than in controls, with some exceptions whose characteristics should be further explored. Municipalities in Sites of Community Importance (SCIs) performed best against rural depopulation and generally better than their controls, whereas municipalities in Biosphere Reserves and Special Protection Areas (SPAs) showed mostly worse figures. Our findings suggest that, while necessary and important for biodiversity, multiple-use PAs generally entailed negative consequences for Spanish rural populations that need to be offset by State's intervention.

Usage of buildings in the life cycle of two endangered Rhinolophus species in the Mediterranean region: implications for roost protection

Synanthropic roosting may allow cave-dwelling bats to cope with habitat fragmentation provided that suitable buildings are sustainably protected. This study on Asinara Island, Sardinia, focuses on roost requirements in synanthropy at different life stages of two endangered bat species, Rhinolophus hipposideros and Rhinolophus ferrumequinum. We rated the roost potential of 532 buildings and compared it with actual roost usage. Microclimate was compared across different roost types and between species, and bat composition and behaviour in nurseries of R. hipposideros were related to roost structure and microclimate. The two species occupied 25% of structures rated as “high potential”, versus 5% and 0% rated as “intermediate” and “low potential”. Concerning microclimate, R. hipposideros preferred warmer and drier day roosts, with higher temperature fluctuations during summer, while winter, and night, roost microclimate was comparable between species. In larger, warmer, and drier nurseries, colony size and proportion of reproductive females were higher and parturition started earlier. Before parturition, roost temperatures were inversely correlated to clustering, supporting its thermoregulatory function. Mothers spent in total 50% of the night inside the nursery caring the pup. Roost microclimate, size, and location close to foraging areas may thus promote breeding success. We conclude that a structure-based rating of roost potential supplemented by species-specific microclimatic requirements constitutes a promising predictor of roost usage. Moreover, adequate buildings support the complete life cycle of R. hipposideros in the absence of suitable caves. Buildings thus deserve increased protection measures in fragmented Mediterranean landscapes to ensure sustainable bat conservation.

What are the main drivers of survival and recruitment in tropical forest marsupials? A 16-year study

Abstract Tropical forest marsupials exhibit large interannual variation in population sizes, with direct negative density dependence capturing the essential features of their dynamics. However, the demographic mechanisms underlying population growth rate and driving both survival and reproduction are still unclear. We used a 16-year capture-mark-recapture data set for five tropical forest marsupials to test for seasonal and interannual density dependence in survival and recruitment. Hypotheses regarding the effects of exogenous (rainfall and minimum temperature) factors on survival, recruitment, and reproductive parameters (fecundity, litter size, and proportion of reproductive females) were also tested. Population size negatively affected survival in three of five species. High population sizes in a given year reduced survival rates in the following year, with strong detrimental effects on males. Recruitment and proportion of reproductive females were highly dependent on weather variables, and were not affected by previous population sizes (except for Metachirus nudicaudatus). Fecundity (number of female offspring/female) was related negatively to population size only in the black-eared opossum (Didelphis aurita), while litter size was a relatively conservative parameter, largely independent of external conditions. Our analyses indicate that density-dependent survival is the mechanism that regulates population size of tropical forest marsupials, either through a reduction in survival or an increase in emigration rates. This general regulatory mechanism may be common to other marsupials in the Atlantic Forest and other tropical forests. Marsupiais de florestas tropicais exibem grande variação interanual nos tamanhos populacionais, com dependência negativa e direta da densidade capturando a essência de sua dinâmica populacional. No entanto, os mecanismos demográficos subjacentes à taxa de crescimento populacional e determinantes da sobrevivência e reprodução ainda são incertos. Nós usamos 16 anos de dados de captura-marcação-recaptura de cinco espécies de marsupiais de florestas tropicais para avaliar a dependência de densidade sazonal e interanual na sobrevivência e recrutamento. Hipóteses sobre efeitos de fatores exógenos (pluviosidade e temperatura mínima) na sobrevivência, recrutamento e parâmetros reprodutivos (fecundidade, tamanho da ninhada e proporção de fêmeas reprodutivas) também foram testadas. O tamanho da população afetou negativamente a sobrevivência em três das cinco espécies. O tamanho populacional elevado em um ano reduziu as taxas de sobrevivência no ano seguinte, com efeitos mais negativos nos machos. Recrutamento e proporção de fêmeas reprodutivas foram dependentes das variáveis climáticas e não foram afetados pelos tamanhos populacionais anteriores (com exceção de Metachirus nudicaudatus). A fecundidade (número de filhotes fêmea/fêmea) foi relacionada negativamente ao tamanho da população do gambá-de-orelha-preta (Didelphis aurita), enquanto o tamanho da ninhada foi um parâmetro relativamente conservador e independente das condições externas. Nossas análises indicam que a sobrevivência dependente da densidade regula as populações de marsupiais em florestas tropicais, seja através da redução na sobrevivência ou no aumento da emigração. Esse mecanismo regulatório pode ser comum a outros marsupiais na Mata Atlântica e outras florestas tropicais.

Living in a tiny world: reproductive biology and population ecology of the Neotropical miniature frog Euparkerella aff. brasiliensis (Terraranae, Strabomantidae)

Abstract Miniaturisation is an important evolutionary trend for amphibians and has occurred several times in independent anuran lineages. Most miniaturised frogs live in the leaf litter of tropical forests and have terrestrial reproductive modes. They are expected to have reduced fecundity in number of eggs than larger-bodied related species, but little is known about reproductive cycles and proportion of reproductive females. Lower vagility is also assumed, however, as they are difficult to observe, there is little empirical evidence about their dispersal. We studied the reproductive biology (sex ratio, sexual size dimorphism and fecundity parameters) and population ecology (growth, dispersal, and phenology) of the miniature Guanabara Frog Euparkerella aff. brasiliensis (⩽ 20 mm). We collected and analysed 75 specimens of E. aff. brasiliensis, of which 27 were adult females with vitellogenic oocytes in their ovaries. Fecundity was low for number of eggs (average number = 9.7), but only one adult female had no vitellogenic oocytes. Sex ratio was relatively balanced among sexes, being female-biased (0.92) for all individuals and male-biased (1.17) for adults. Juveniles and ovigerous females were observed throughout the year during the two years of mark-recapture study, which indicates continuous breeding. We captured 121 individuals, of which 12% were recaptured in their original collection sites, suggesting low vagility. We discuss our findings considering the current knowledge about the ecology of miniaturised frogs and other Terraranae and suggest future directions for ecological studies and conservation planning.

Optimizing cold storage of the ectoparasitic mite Pyemotes zhonghuajia (Acari: Pyemotidae), an efficient biological control agent of stem borers.

Pyemotes zhonghuajia Yu, Zhang and He is a newly discovered native ectoparasitic mite that efficiently controls stem borers in China. To provide a steady and sufficient supply, extend adult lifespan and synchronize field augmentative releases of P. zhonghuajia, we determined the optimal cold storage temperature and duration by storing 1-day-old mated females at 8, 10 and 12°C for 10-90days with a 10-day interval in the laboratory. We then recorded mite survival during storage and monitored the post-storage reproductive performance of mites at a control temperature of 25°C. We found that all mites survived at 10 and 12°C for different durations, but mortality occurred when mites were stored at 8°C for ≥ 30days with more than 70% of mites dead when the storage duration prolonged up to 50days. The proportion of reproductive females was higher at 10°C but decreased with the prolonged storage duration at all test temperatures. Storage temperature had no significant effect on the pre-reproductive period and offspring sex ratio, whereas prolonged storage induced longer pre-reproductive period and lower proportion of female offspring. The reproductive period increased with increasing storage temperature and with prolonged storage up to 50 and 60days; however, the longer reproductive period did not directly translate into greater reproductive output. We found that compared with the control, mites stored at 10°C for up to 30days did not significantly reduce their survival, proportion of reproductive success and number and sex ratio of offspring, suggesting that 10°C and ≤ 30days were the optimal cold storage temperature and duration, respectively, for post-mass production storage before the field augmentative release of P. zhonghuajia.

Environmental causes and reproductive correlates of mercury contamination in European pond turtles (Emys orbicularis)

Assessing Hg contamination in aquatic ecosystems is difficult because wetlands are part of large and complex networks, and potential sources of Hg contamination are highly diverse. To investigate environmental determinants of Hg contamination, we studied one of the largest continental French wetlands structured as a dense network of artificial ponds. Such context allows to investigate the influence of pond characteristics on Hg contamination in an area relatively disconnected from direct sources of pollution. We relied on a bioindicator organism, the European pond turtle (Emys orbicularis) to assess Hg contamination in a relatively large number of sites (N > 255 turtles from 15 ponds sampled in 2016 and 2017). Non-invasive sampling in the claws of turtles show that Hg concentrations were not related to their sex or size, but we found an effect of age (1.62 ± 0.20 in juveniles and 2.21 ± 0.06 µg g−1 dw in adults), suggesting that turtles do bioaccumulate Hg through their life. Turtle Hg was different between ponds, and we found that pond age and pond usage (draining events linked to pond maintenance) were the main environmental determinants of Hg concentrations in turtles. Finally, and more importantly, our dataset allowed us to highlight potential negative effect of Hg concentrations on the proportion of reproductive females, suggesting an influence of Hg on reproductive mechanisms in this species. This result indicates that Hg contamination, even in absence of direct and strong sources of pollution, may have a critical impact on reproduction and thus the persistence of a long-lived vertebrate.

Contrasting effects of climate change on the timing of reproduction and reproductive success of a temperate insectivorous bat

AbstractWe used long‐term datasets to analyse (1) the patterns of covariation between basic climatic variables (temperature and rainfall) and the timing of reproduction and reproductive success; and (2) long‐term trends in both reproductive parameters of a maternity colony of Daubenton's bats Myotis daubentonii in South Bohemia, Czech Republic. The mean April temperature was the best predictor of the timing of reproduction. The higher the April temperature, the earlier the first neonates appeared. The mean date of first parturition was June 4, but it advanced significantly by c. 11 days between 1970 and 2012. Similarly, the mean April temperature increased over the study period by c. 2.7°C. Between 1999 and 2012, the mean reproductive success (proportion of reproductive females) was 74%, but varied between 33% (2009) and 93% (2006). It was negatively related to May–July precipitation. Thus, reproductive success was lower in years with increased rainfall. Given the published evidence that advancement in parturition is positively related to survival of juvenile bats rising spring temperatures may have a beneficial influence on the population dynamics of Daubenton's bats. However, increased incidence of climatic extremes, such as excessive summer rainfall, may largely buffer this effect. Consequently, populations of temperate insectivorous bats may experience increasing environmental stress under continuing climate change.

Effects of bisphenol A on Gammarus fossarum and Lumbriculus variegatus in artificial indoor streams

The impact of bisphenol A (BPA) on Gammarus fossarum and Lumbriculus variegatus was studied in four artificial indoor streams (0, 5, 50 and 500 µg L−1 BPA, nominal) over 103 days in a pulse–dose exposure scenario (weekly BPA application). For G. fossarum populations at day 103, the proportions of juveniles and of breeding females from the highest BPA treatment were in tendency reduced. For individually exposed gammarid pairs an EC10 of 17 µg L−1 BPA (nominal) for the proportion of reproductive females in the fourth brood was determined. During the first three broods, the largest brood size occurred at the highest BPA concentration, whereas in the fourth brood it decreased concentration-dependently (fourth brood EC10 = 5 µg L−1 BPA, nominal). Effects on L. variegatus were a reduced population growth (103 d-EC10 of 2 µg L−1 BPA, nominal) and an increase in dry weight and the number of segments in large, complete worms.

Population regulation and demography in a harvested freshwater crayfish from Madagascar

Despite advances in statistical techniques for investigating population dynamics based on mark–recapture data, the majority of our understanding about demography and regulation comes from relatively few taxa. Most proposed generalisations about the association between demography and variation in population size are based on data from vertebrates, there are few sufficiently detailed invertebrate studies to examine whether these generalisations are widely supported. The population biology of freshwater invertebrates is especially poorly known. We present a large‐scale mark–recapture study of an endemic freshwater crayfish from Madagascar (Astacoides granulimanus). Variation in density, caused by difference in fishing pressure due to local taboos, allowed us to investigate density‐dependent regulation. We found evidence of density dependence in fecundity operating through the proportion of reproductive females by size but no significant evidence of density dependence in growth. Using a prospective analysis based on the elasticities from a size‐structured matrix model, we found that both recruitment rates and survival rates of large individuals were strongly associated with deterministic population growth – a result that differs from generalisations drawn from vertebrate studies. A central assumption in mark–recapture studies is that handling does not affect mortality. By treating the number of times an individual was captured as an individual covariate, easily done using the freeware program MARK, we were able to test for, and take account of, handling‐induced mortality. Our results show interesting similarities, and important differences, to generalisations based on vertebrate studies and emphasise the importance of population studies on poorly known taxa.

Comparison of the biology, dynamics, and secondary production of Talorchestia brito (Amphipoda, Talitridae) in Atlantic (Portugal) and Mediterranean (Tunisia) populations

The biology, population dynamics, and production of Talorchestia brito were studied at two sandy beaches located on the Atlantic (Portugal) and on the Mediterranean (Tunisia) coasts, respectively. The seasonal variation in abundance and the overall densities were similar in both populations. Reproduction occurred from February to September in the Atlantic, and from March to early November in the Mediterranean. The sex ratio was male biased in the Atlantic, and female biased in the Mediterranean. Based on data from the Atlantic population, both abundance and the proportion of reproductive females were positively correlated with temperature, while the proportion of juveniles in the population was positively correlated with temperature and sediment moisture. On average, individuals from the Atlantic were larger than the ones from the Mediterranean. Life span was estimated at six to nine months in the Atlantic, and five to eight months in the Mediterranean. Talorchestia brito was shown to be a semiannual species, with iteroparous females producing two broods per year, and exhibited a bivoltine life cycle. The minimum age required for males' and females' sexual differentiation and for female sexual maturation was shorter in the Mediterranean. Growth production ( P) was estimated at 0.19 g m −2 y −1 ash free dry weight (AFDW; 4.3 kJ m −2 y −1) in the Atlantic population, and 0.217 g m −2 y −1 AFDW (4.9 kJ m −2 y −1) in the Mediterranean one. Elimination production ( E) was estimated at 0.35 g m −2 y −1 AFDW (7.9 kJ m −2 y −1) in the Atlantic, and 0.28 g m −2 y −1 AFDW (6.3 kJ m −2 y −1) in the Mediterranean. The average annual biomass ( B ) (standing stock) was estimated at 0.032 g m −2 in the Atlantic beach, and 0.029 g m −2 in the Mediterranean one, resulting, respectively, in P/ B ratios of 5.9 and 7.5 and E/ B ratios of 10.8 and 9.6. Like other talitrids, T. brito exhibited geographic variation in morphometrical characteristics, sex ratio, growth rates, life span, and reproduction period, with the Atlantic population presenting a slower life history.

Variation in Growth and Demography of a Knob‐scaled Lizard (Xenosaurus newmanorum: Xenosauridae) from a Seasonal Tropical Environment in México

ABSTRACTWe studied growth and demography in a population of Xenosaurus newmanorum a lizard that lives in rock crevices in the tropical cloud forests of Mexico. Growth rates varied with season (faster in the wet season) and year (faster in 1995 and 1997, slower in 1996 and 1999). Males and females did not differ in growth rate. Proportion of reproductive females varied from year to year, with 1996 having the lowest proportion of reproductive females (32.5%) compared to the other years (all >75%). The age/size structure changed slightly among years, but did not differ between the wet and dry seasons. On average, the sex ratio was female‐biased; however, the proportion of males in the population was greater in the dry season than in the wet season. Our results suggest that, like other lizards, growth and demographic characteristics of this population fluctuate with the proximate environment on both a seasonal and annual timescale.RESUMENEstudiamos los patrones de crecimiento y la demografia de una población de Xenosaurus newmanorum, una especie de lagartija que vive en las grietas de las rocas de los bosques de niebla de México. La tasa de crecimiento corporal varió en función de la época del año (más rápida en la época húmeda), y en función del año (más rápida en 1995 y 1997, y más lenta en 1996 y 1999). No hubo diferencias significativas en las tasas de crecimiento corporal de machos y hembras. La proporción de hembras reproductivas varió de ańo a ańo, 1996 fue el ańo con menor proporción de hembras reproductivas (32.5%) comparado con el resto de los ańos (todos ellos > 75 %). La estructura de las clases de edad de esta población fluctuó levemente entre los años, pero no hubo diferencias entre las estaciones húmedas y secas. En promedio la proporción de sexos estuvo sesgada hacia las hembras; aunque la proporción de machos en la población fue mayor en la época seca que en la época húmeda. Nuestros resultados sugieren que, al igual que otras lagartijas, las características demográficas y de crecimiento corporal de esta población pueden fluctuar con base a factores próximos a escalas estacionales y anuales.

Senescence and age‐related reproduction of female Columbian ground squirrels

Summary Three hypotheses have been proposed to explain age‐structured patterns of reproductive investment and somatic investment: residual reproductive value, senescence and evolutionary restraint. We evaluated these hypotheses for female Columbian ground squirrels (Spermophilus columbianus) by examining age‐related patterns of somatic and reproductive investment. Females were designated as successful (those that weaned litters) and unsuccessful (those that did not wean litters). Somatic investment varied among both successful and unsuccessful females of different ages, with yearlings having the highest investment. Considering all females, reproductive investment varied among age classes with yearlings and the oldest (6–9‐year‐olds) having the lowest investments. However, when only successful females were considered, reproductive investment was lowest in the yearlings and not significantly different among older females. The highest proportion of successful females occurred in the middle adult age classes, while yearlings and the oldest females displayed the lowest proportion of successful females. During the breeding season, somatic investments of successful and unsuccessful females differed significantly only in the yearling age class, with unsuccessful females having the highest investment. Evolutionary restraint or constraint explained patterns of reproduction in the yearling age class, where both reproductive investment and proportion of reproductive females were low. There was evidence for senescence of reproduction by some of the oldest females.

Seasonal Variation in Reproductive Traits of the Oviparous Water Snake, Natrix maura, in the Ebro Delta of Northeastern Spain

Seasonal Variation in Reproductive Traits of the Oviparous Water Snake, Natrix maura, in the Ebro Delta of Northeastern Spain

The effects of supplemental perch sites on avian predation and demography of vole populations

We erected supplemental perches to evaluate their effectiveness in attracting perching raptors and to determine if an increase in raptor visitation could affect vole demography. Our model experimental system consisted of six 0.2-ha enclosures containing gray-tailed vole, Microtus canicaudus. The availability of supplemental perch sites had no effect on northern harriers, Circus cyaneus, but resulted in an 11-fold increase in visitation by American kestrels, Falco sparverius. This increased predation pressure did not affect vole population size, growth rate, or adult survival. However, juvenile recruitment and the proportion of reproductive females were lower in treatment sites than in control sites. Male voles had smaller home ranges and were less active than expected, which may have been an avoidance response to predation risk. High densities of voles, adequate cover in treatment sites, and good foraging areas with less cover in surrounding habitat may have negated the effects of the availability of supplemental perch sites. We conclude that adding perches can increase raptor visitation substantially but may have little effect on high-density vole populations.

Reproductive life history variation among colour morphs of the pygmy grasshopper Tetrix subulata

Individuals of pygmy grasshoppers (Tetrix subulata[L.] Orthoptera: Tetrigidae) exhibit genetically coded discontinuous variation in colour pattern. To determine whether reproductive performance is likely to be affected by colour pattern, this study investigated variation in body size and reproductive life-history characteristics among individuals belonging to five different colour morphs. The proportion of reproductive females (i.e. females with eggs) declined significantly as the season progressed (from 100% in mid-May to 40% in mid-June), but no such seasonal trend was apparent for body size, clutch size or egg size. Colour morphs differed significantly in body size, and these size differences accounted for most of the variation in clutch size and egg size. Colour morphs also differed in the regression of egg size on clutch size, suggesting that trade-offs between number and size of offspring might vary among morphs. Finally, I found a negative relationship across colour morphs between the proportion of females with eggs and average clutch size. This suggests that individuals belonging to certain colour morphs produce a relatively large number of clutches per unit time, at the expense of fewer offspring in each clutch, compared to other morphs. Collectively, my results indicate that different colour morphs of T. subulata may have different reproductive strategies. These differences may reflect variation in thermoregulatory capacity or differences in probability of survival induced by visual predators.

Reproductive life history variation among colour morphs of the pygmy grasshopper Tetrix subulata

Individuals of pygmy grasshoppers (Tetrix subulata[L.] Orthoptera: Tetrigidae) exhibit genetically coded discontinuous variation in colour pattern. To determine whether reproductive performance is likely to be affected by colour pattern, this study investigated variation in body size and reproductive life-history characteristics among individuals belonging to five different colour morphs. The proportion of reproductive females (i.e. females with eggs) declined significantly as the season progressed (from 100% in mid-May to 40% in mid-June), but no such seasonal trend was apparent for body size, clutch size or egg size. Colour morphs differed significantly in body size, and these size differences accounted for most of the variation in clutch size and egg size. Colour morphs also differed in the regression of egg size on clutch size, suggesting that trade-offs between number and size of offspring might vary among morphs. Finally, I found a negative relationship across colour morphs between the proportion of females with eggs and average clutch size. This suggests that individuals belonging to certain colour morphs produce a relatively large number of clutches per unit time, at the expense of fewer offspring in each clutch, compared to other morphs. Collectively, my results indicate that different colour morphs of T. subulata may have different reproductive strategies. These differences may reflect variation in thermoregulatory capacity or differences in probability of survival induced by visual predators.

Influence of Resource Removal on Demography of Attwater's Pocket Gopher

Nutrient balancing is a foraging strategy whereby herbivores ingest a mixture of dietary items to ensure an adequate intake of nutrients to maintain individual health and reproduction. Recent findings indicate that terrestrial herbivorous rodents select dietary items from both dicotyledonous (dicots) and monocotyledonous (monocots) plants to obtain requisite nutrients, which may, in turn, influence demographic processes within the population. We investigated how availability of a major food source, dicots, affected population dynamics of Geomys attwateri , a fossorial herbivore. We removed dicots from the habitat in four plots and compared body mass, density, reproduction, recruitment, age-class, sex ratio, length of residency, and spatial distribution of pocket gophers in these areas to pocket gophers in areas with dicots. Removal of dicots reduced the proportion of reproductive females and average length of time in residency within a plot for both sexes. These effects, in turn, resulted in reduced density of pocket gophers in areas having lowered dicot availability. Spatial arrangement of pocket gophers within plots was not affected by removal of dicots but differed by season. Our experiment demonstrates the mechanism for reduction in density of pocket gophers and shows a link between foraging strategy, intake of dietary items, and demographic processes.

Influences of Patch Size and Microhabitat on the Demography of Two Old-Field Rodents

We used capture/recapture methods to test the responses of two small mammal species (Peromyscus leucopus and Microtus pennsylvanicus) to small- (microhabitat) and large- (patch) scale habitat variation. Analyses examined the responses of individuals to microhabitat variation among trap stations as well as differences in the density and persistence time of adults and juveniles, and the proportion of reproductive females on experimentally created patches of three sizes (0.0625, 0.25, and 1.0 ha). With the exception of transient P. leucopus, all groups shared significant correlations with microhabitat at the scale of trap stations. By contrast, only juvenile P. leucopus exhibited a response to patch-size (i.e., higher densities on small relative to larger patches). Microhabitat differences among patches also accounted for variation in M. pennsylvanicus densities (but not P. leucopus) in analyses of covariance. Our results suggest that both individuals and populations of M. pennsylvanicus responded to habitat variation at the microhabitat scale, while P. leucopus appeared to respond to both microhabitat and patch scale habitat variation. We note that species characteristics (particularly relative dispersal ability) may prove critical in predicting the scale of habitat responses. We conclude by noting that current theory that assumes uniform responses of population to homogeneous patches is too simplistic to be of much predictive value in field tests.