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Resource Abundance (resource + abundance)
Selected AbstractsExamining Factors that Impact Mississippi Counties' Unreserved Fund Balance during Relative Resource Abundance and Relative Resource ScarcityPUBLIC BUDGETING AND FINANCE, Issue 4 2009LA SHONDA M. STEWART Although using some of the same organizational and financial factors examined by prior researchers to build and test models that explain factors influencing change in the general fund unreserved balance for smaller, rural, and less affluent counties in Mississippi, the rationale of this study is to build additional support that applying the recommended 5,15 percent savings benchmark across all jurisdictions is not a sufficient guide. Overall, Mississippi counties maintain unreserved fund balances ranging from a negative balance to over one hundred percent of their current expenditures. Counties also increase reserves during times of relative resource abundance and decrease them during relative resource scarcity. Moreover, they tend to address short-term needs and resident demands when revenues are plentiful. During relative resource scarcity, however, they are more cost-conscious and focus on maintaining rather than expanding current expenditures. This research shows that counties using the Beat system, a political form of government, are more likely to behave more frugally than counties using a Unit system, an administrative form of government. [source] Spillover edge effects: the dispersal of agriculturally subsidized insect natural enemies into adjacent natural habitatsECOLOGY LETTERS, Issue 5 2006Tatyana A. Rand Abstract The cross-edge spillover of subsidized predators from anthropogenic to natural habitats is an important process affecting wildlife, especially bird, populations in fragmented landscapes. However, the importance of the spillover of insect natural enemies from agricultural to natural habitats is unknown, despite the abundance of studies examining movement in the opposite direction. Here, we synthesize studies from various ecological sub-disciplines to suggest that spillover of agriculturally subsidized insect natural enemies may be an important process affecting prey populations in natural habitat fragments. This contention is based on (1) the ubiquity of agricultural,natural edges in human dominated landscapes; (2) the substantial literature illustrating that crop and natural habitats share important insect predators; and (3) the clear importance of the landscape matrix, specifically distance to ecological edges, in influencing predator impacts in agroecosystems. Further support emerges from theory on the importance of cross-boundary subsidies for within site consumer,resource dynamics. In particular, high productivity and temporally variable resource abundance in agricultural systems are predicted to result in strong spillover effects. More empirical work examining the prevalence and significance of such natural enemy spillover will be critical to a broader understanding of fragmentation impacts on insect predator,prey interactions. [source] Interaction of an insecticide with larval density in pond-breeding salamanders (Ambystoma)FRESHWATER BIOLOGY, Issue 4 2005BRIAN S. METTS Summary 1. Amphibian populations residing in or near agricultural areas are often susceptible to pesticide contamination. Recent evidence suggests that the effects of pesticides on amphibians often exceed those estimated in laboratory toxicity tests because other environmental factors (e.g. predators, resource abundance) can influence pesticide toxicity. 2. To examine the effects of an insecticide (carbaryl) on two species of Ambystoma salamanders experiencing the natural stress of competition, we manipulated chemical concentration (control, 3.5 and 7.0 mg L,1) and larval density (low and high). We determined the effect of treatments on snout-vent length (SVL), growth rate, lipid reserves, time to metamorphosis, per cent survival and per cent metamorphosis. 3. Carbaryl negatively affected all response variables of Ambystoma maculatum significantly, and significantly reduced survival and metamorphosis of A. opacum. Increased density significantly influenced SVL, lipid reserves, growth rate and metamorphosis of A. maculatum. 4. The effects of carbaryl and increased density on per cent metamorphosis were nearly additive, but were generally less than additive on other variables. 5. The negative effects of chemical contamination on salamanders were likely because of pesticide-induced reductions of food resources, as zooplankton abundance decreased by as much as 97% following carbaryl application. 6. Our study demonstrates the importance of the interactive effects that chemical contamination and natural environmental factors have on salamanders. [source] Tawny Owls Strix aluco with reliable food supply produce male-biased broodsIBIS, Issue 1 2007KASI B. DESFOR Tawny Owls Strix aluco have been reported to skew the sex ratio of their offspring towards males when facing food shortage during the nestling period (and vice versa), because female fitness is more compromised by food shortage during development than male fitness. To test the generality of these results we used a DNA marker technique to determine the sex ratio in broods of Tawny Owls in Danish deciduous woodland during two years of ample food supply (rodent population outbreak) and two years of poor food supply. Of 268 nestlings, 59% were males (95% CI: 53,65%). This proportion was higher than previously reported for the species (49% in Northumberland, UK, and 52% in Hungary), but consistent with Fisherian sex allocation, which predicts a male bias of c. 57% based on inferred differences in energy requirements of male and female chicks. Contrary to previous results, brood sex ratios were not correlated with the resource abundance during the breeding seasons, despite considerable variation in breeding frequency, brood size or hatching date across years. Brood sex ratios were unaffected by brood reduction prior to DNA sampling, and nestling mortality rates after DNA sampling were not related to gender. The inconsistency between the sex ratio allocation patterns in our study and previous investigations suggests that adaptive sex allocation strategies differ across populations. These differences may relate to reproductive constraints in our population, where reproductive decisions seem primarily to concern whether to lay eggs at all, rather than adjust the sex ratio to differences in starvation risk of nestlings. [source] Shorebird predation of horseshoe crab eggs in Delaware Bay: species contrasts and availability constraintsJOURNAL OF ANIMAL ECOLOGY, Issue 3 2007S. GILLINGS Summary 1Functional responses , the relationship between resource intake rate and resource abundance , are widely used in explaining predator,prey interactions yet many studies indicate that resource availability is crucial in dictating intake rates. 2For time-stressed migrant birds refuelling at passage sites, correct decisions concerning patch use are crucial as they determine fattening rates and an individual's future survival and reproduction. Measuring availability alongside abundance is essential if spatial and temporal patterns of foraging are to be explained. 3A suite of shorebird species stage in Delaware Bay where they consume horseshoe crab Limulus polyphemus eggs. Several factors including spawning activity and weather give rise to marked spatial and temporal variation in the abundance and availability of eggs. We undertook field experiments to determine and contrast the intake rates of shorebird species pecking for surface and probing for buried eggs. 4Whether eggs were presented on the sand surface or buried, we demonstrate strong aggregative responses and rapid depletion (up to 80%). Depletion was greater at deeper depths when more eggs were present. No consistent give-up densities were found. Type II functional responses were found for surface eggs and buried eggs, with peck success twice as high in the former. Maximum intake rates of surface eggs were up to 83% higher than those of buried eggs. 5Caution is needed when applying functional responses predicted on the basis of morphology. Our expectation of a positive relationship between body size and intake rate was not fully supported. The smallest species, semipalmated sandpiper, had the lowest intake rate but the largest species, red knot, achieved only the same intake rate as the mid-sized dunlin. 6These functional responses indicate that probing is rarely more profitable than pecking. Currently, few beaches provide egg densities sufficient for efficient probing. Areas where eggs are deposited on the sand surface are critical for successful foraging and ongoing migration. This may be especially true for red knot, which have higher energetic demands owing to their larger body size yet appear to have depressed intake rates because they consume smaller prey than their body size should permit. [source] Recruitment pulses induce cannibalistic giants in Arctic charJOURNAL OF ANIMAL ECOLOGY, Issue 2 2006PÄR BYSTRÖM Summary 1Recent theoretical studies on the population dynamic consequences of cannibalism have focused on mechanisms behind the emergence of large cannibals (giants) in size-structured populations. Theoretically, giants emerge when a strong recruiting cohort imposes competition induced mortality on stunted adults, but also provides a profitable resource for a few adults that accelerate in growth and reach giant sizes. 2Here the effects of a recruitment pulse on the individual and population level in an allopatric Arctic char population have been studied over a 5-year period and these results were contrasted with theoretical model predictions for the conditions necessary for the emergence of cannibalistic giants. 3The recruitment pulse had negative effects on invertebrate resource abundance, and the decrease in body condition and increase in mortality of adult char suggested that strong intercohort competition took place. 4The frequency of cannibalism increased and a few char accelerated in growth and reached ,giant' sizes. 5The main discrepancy between model predictions and field data was the apparently small effect the recruited cohort had on resources and adult char performance during their first summer. Instead, the effects became pronounced when the cohort was 1 year old. This mismatch between model predictions and field observations was suggested to be due to the low per capita fecundity in char and the restricted nearshore habitat use in young-of-the-year (YOY) char. 6This study provides empirical evidence that the emergence of giants is associated with the breakthrough of a strong recruiting cohort and also suggests that the claimed stable char populations with large cannibals may instead be populations with dynamic size structure that results in intermittent breakthroughs of recruitment pulses, providing the conditions necessary for char to enter the cannibalistic niche. 7The data suggest that increased recruit survival through restricted habitat use may destabilize dynamics and cause the emergence of giants. However, they also suggest that this does not necessarily develop into populations with bi-modal size structure in populations with low per capita fecundity and size- and density-dependent habitat use of recruiting cohorts. [source] Diversity,stability relationships in multitrophic systems: an empirical explorationJOURNAL OF ANIMAL ECOLOGY, Issue 5 2003Priyanga Amarasekare Summary 1The relationship between diversity and stability is crucial in understanding the dynamics of multitrophic interactions. There are two basic hypotheses about the causal link between diversity and stability. The first is that fluctuations in resource abundance allow consumer coexistence, thus increasing diversity at the consumer trophic level (resource variability hypothesis). The second is that interactions between coexisting consumer species reduce consumer efficiency and dampen population fluctuations, thus increasing consumer,resource stability (consumer efficiency hypothesis). 2The two hypotheses lead to three comparative predictions: (i) fluctuations should be greater (resource variability) or smaller (consumer efficiency) in resource populations with coexisting consumer species, compared to those invaded only by the consumer species superior at resource exploitation; (ii) average resource abundance should be greater (resource variability) or smaller (consumer efficiency) in resource populations with greater fluctuations; and (iii) removal of the consumer species inferior at resource exploitation should increase or not affect resource population fluctuations (resource variability), or always increase them (consumer efficiency). 3I tested these predictions with data from a host,multiparasitoid community: the harlequin bug (Murgantia histrionica) and two specialist parasitoids (Trissolcus murgantiae and Ooencyrtus johnsonii) that attack the bug's eggs. 4Local host populations with coexisting parasitoids exhibited smaller fluctuations and greater average abundance compared to those with just Trissolcus, the species superior at host exploitation. Local populations that lost Ooencyrtus, the species inferior at host exploitation, exhibited an increase in host population fluctuations compared to those that did not. 5The results contradict the expectations of the resource variability hypothesis, suggesting that host population fluctuations are unlikely to be driving parasitoid coexistence. They are consistent with the consumer efficiency hypothesis, that interactions between coexisting parasitoid species dampens host population fluctuations. I discuss the implications of these results as well as possible caveats. [source] Consumer,resource interactions and cyclic population dynamics of Tanytarsus gracilentus (Diptera: Chironomidae)JOURNAL OF ANIMAL ECOLOGY, Issue 5 2002Árni Einarsson Summary 1Tanytarsus gracilentus population dynamics in Lake Myvatn show a tendency to cycle, with three oscillations occurring between 1977 and 1999 having periods of roughly 7 years. The population abundance fluctuated over four orders of magnitude. 2A partial autocorrelation function (PACF) accounting for measurement error revealed a strong positive lag-1 autocorrelation and a moderate negative lag-2 partial autocorrelation. This suggests that the dynamics can be explained by a simple second-order autoregressive process. 3We tested the alternative hypotheses that the cyclic dynamics of T. gracilentus were driven by consumer,resource interactions in which T. gracilentus is the consumer, or predator,prey interactions in which T. gracilentus is the prey. We analysed autoregressive models including both consumer,resource interactions and predator,prey interactions. 4Wing length of T. gracilentus was used as a surrogate for resource abundance and/or quality, because body size is known to fluctuate with resource abundance and quality in dipterans. Furthermore, the wing lengths of Micropsectra lindrothi , a species ecologically similar to T. gracilentus , fluctuated synchronously with T. gracilentus wing lengths, thereby indicating that the shared resources of these two species were indeed cycling. Wing lengths of other chironomid species were not synchronized. 5The predators of T. gracilentus included midges in the genera Procladius and Macropelopia , and the fish Gasterosteus aculeatus (three-spined stickleback). 6The autoregressive models supported the hypothesis that T. gracilentus dynamics were driven by consumer,resource interactions, and rejected the hypothesis that the dynamics were driven by predator,prey interactions. 7The models also revealed the consequences of consumer,resource interactions for the magnitude of fluctuations in T. gracilentus abundance. Consumer,resource interactions amplified the exogenous variability affecting T. gracilentus per capita population growth rates (e.g. temperature, rainfall, etc.), leading to variability in abundance more than two orders of magnitude greater than the exogenous variability. [source] Brood size and environmental conditions sex-specifically affect nestling immune response in the European starling Sturnus vulgarisJOURNAL OF AVIAN BIOLOGY, Issue 6 2005Eunice H. Chin In sexually size-dimorphic species, the larger sex can be more sensitive to stressful environmental conditions, often resulting in reduced growth and elevated mortality rates. Development of the immune system is regarded as highly resource dependent, and recent data suggest that nestling passerines experience a possible resource-based trade-off between growth and immunity. Given the hypothesized importance of maximizing growth for the larger sex, the corresponding immune system may also exhibit similar sensitivity to limited resources. To better understand how natural variation in brood size and resources might differentially affect growth and immune function in nestlings of a sexually size-dimorphic species, we examined the relationship between brood size and inter-sexual differences in cell-mediated immunity (CMI) and survival in European starling Sturnus vulgaris nestlings where males are larger in both mass and structural size. We hypothesized that male CMI response should be negatively impacted by increasing sibling competition (brood size), especially during periods of low resource availability. In a year of reduced parental provisioning rates and reduced chick growth rates, male offspring exhibited the predicted negative relationship, whereas female CMI response was unaffected. However, in a year of improved provisioning rates and chick growth, neither sex exhibited a negative relationship between immune response and brood size. Thus, natural variation in brood size can affect sex-specific immunity differently in offspring of a sexually size-dimorphic passerine. However, this relationship appears resource-dependent, suggesting that the hypothesized resource-based trade-off may be compensated for in years of adequate resource abundance. [source] Do Meyer's Parrots Poicephalus meyeri benefit pollination and seed dispersal of plants in the Okavango Delta, Botswana?AFRICAN JOURNAL OF ECOLOGY, Issue 3 2010Rutledge S. Boyes Abstract Recent studies of new world parrots repeatedly document, with few exceptions, that parrots are wasteful and destructive predispersal seed predators that are unlikely to contribute towards pollination and seed dispersal. Few detailed studies, however, have assessed the contribution of African parrots to forest ecology by quantifying the potential net benefit of seed and flower predation by parrots for most tree species in their diet. Due to the incidence of pollen on the heads of Meyer's Parrots when feeding on Leguminosae flowers and the dispersal of viable seeds to the ground during seed predation, we compared destruction rates, when feeding on pods, fruits and flowers, with dispersal rates of viable seeds to the ground and frequency of head contact with reproductive apparatus to estimate net benefit from Meyer's Parrot feeding activity. Meyer's Parrots were not implicated in endo- or epizoochory, but they dropped uneaten fruit pulp and seeds to the ground during feeding bouts, thus providing ripe, undamaged seeds to secondary seed dispersers. This link with forest recruitment was weak, as all tree species utilized by Meyer's Parrots either had more significant primary dispersal agents or were primarily wind-dispersed. In most cases, the negative effect of seed predation outweighed any positive effects in terms of dispersal, whereby almost three times more seeds were consumed or destroyed than were dispersed to the ground. Significantly, only Sclerocarya birrea caffra recorded marginal net dispersal benefit from utilization by Meyer's Parrots. Due to low relative resource abundance and high destruction rate, feeding activity on Berchemia discolor may be significant enough to influence its spatial distribution and abundance. Utilization of flowers of Kigelia africana and Adansonia digitata by parrots likely had a significant negative impact on pollination. Feeding on Acacia nigrescens flowers, however, was potentially advantageous to their pollination. We conclude that Poicephalus parrots are net consumers of ripe, undamaged seeds and flowers, thus having an overall negative impact on forest recruitment in subtropical Africa. Résumé Les études récentes des perroquets du nouveau monde disent toujours, à quelques exceptions près, que les perroquets sont des prédateurs qui gaspillent et détruisent les semences avant leur dispersion et qu'ils sont donc peu susceptibles de contribuer à la fertilisation et à la dispersion des graines. Cependant, peu d'études détaillées ont évalué la contribution des perroquets africains à l'écologie des forêts en quantifiant le bénéfice net potentiel de la consommation des graines et des fleurs pour la plupart des espèces d'arbres qui entrent dans leur régime alimentaire. Sachant que la tête des perroquets de Meyer se charge de pollen lorsqu'ils se nourrissent de fleurs de légumineuses et qu'ils dispersent des semences viables sur le sol lorsqu'ils en consomment, nous comparons le taux de destruction, lorsqu'ils mangent des gousses, des fruits et des fleurs, avec le taux de dispersion de semences viables sur le sol et la fréquence des contacts de leur tête avec l'appareil reproducteur des plantes, pour estimer le bénéfice net de l'alimentation du perroquet de Meyer. Ces perroquets n'étaient pas impliqués dans l'endo- ou l'épizoochorie, mais lorsqu'ils mangeaient, ils laissaient tomber de la pulpe et des graines de fruits non consommées, fournissant ainsi des semences intactes à des disperseurs secondaires. Ce rapport avec le recrutement des forêts était faible dans la mesure où toutes les semences des espèces d'arbres fréquentées par les perroquets de Meyer soit avaient des agents de dispersion primaires plus importants, soit étaient principalement dispersées par le vent. Dans la plupart des cas, l'effet négatif de la consommation des semences dépassait tout effet positif en termes de dispersion, et presque trois fois plus de graines étaient mangées ou détruites que dispersées sur le sol. Il est intéressant de noter que seul Sclerocarya birrea caffra présentait un bénéfice net de dispersion marginal dûà la fréquentation des perroquets de Meyer. En raison de la faible abondance relative de ressources et du taux de destruction élevé, la consommation de Berchemia discolor pourrait être assez significative pour influencer sa distribution spatiale et son abondance. La consommation des fleurs de Kigelia africana et d'Adansonia digitata par les perroquets avait probablement un impact négatif sur leur pollinisation. Mais la consommation des fleurs d'Acacia nigrescensétait peut-être avantageuse pour la pollinisation. Nous concluons que les perroquets Poicephalus sont des consommateurs nets de graines et de fleurs mûres et intactes, et qu'ils ont donc un impact global négatif sur le recrutement des forêts en Afrique subtropicale. [source] LEARNING BY DOING IN THE PRESENCE OF AN OPEN ACCESS RENEWABLE RESOURCE: IS GROWTH SUSTAINABLE?NATURAL RESOURCE MODELING, Issue 1 2005CAROL McAUSLAND ABSTRACT. We examine the relationship between growth, resource abundance and trade when the natural resource is renewable and open access and there is inter-industry learning by doing. We find growth is not sustainable in the closed economy and can be sustained in the open economy only so long as the labor forced engaged in resource extraction shrinks over time. Comparisons of steady state welfare in autarky and free trade reveal that for very high or low world prices of the resource-based good, it is possible for the economy to gain from trade. However if the price is intermediate, it may instead lose. [source] Seasonal differences in population-, ensemble- and community-level responses of bats to landscape structure in AmazoniaOIKOS, Issue 10 2010Brian T. Klingbeil The amount (composition) and spatial arrangement (configuration) of forest patches in fragmented landscapes influence the accessibility, as well as the abundance and diversity of resources available to bats. Moreover, tropical fruit and insect abundance differ seasonally in response to changes in precipitation, and many bats in the family Phyllostomidae employ seasonal reproductive strategies. Because reproductive activities involve constraints on time and energy as well as increased nutritional demands, foraging behavior and home range size may differ between wet and dry seasons. Nonetheless, seasonal variation in response to landscape structure by bats has not been examined previously. Consequently, population-, ensemble- and assemblage-level responses of phyllostomids to landscape composition and configuration were quantified separately during the wet and dry season at three circular focal scales (1, 3 and 5 km radii) for 14 sites in fragmented lowland Amazon forest. Responses to landscape characteristics were scale-dependent, species-specific, and seasonal. Abundances of frugivores responded to landscape composition in the dry season and to landscape configuration in the wet season. Conversely, abundances of animalivores responded to landscape configuration in the dry season and to landscape composition in the wet season. Divergent responses to landscape structure between seasons suggest that variation in resource abundance and diversity play a significant role in structuring population-, ensemble- and assemblage-level patterns. As such, considerations of the effects of dietary flexibility and reproductive constraints on foraging strategies and habitat use may be important when designing management plans that successfully promote long-term persistence of biodiversity in fragmented landscapes. [source] Resource abundance vs. resource dependence in cross-country growth regressionsOPEC ENERGY REVIEW, Issue 2 2010Annika Kropf Having analysed the macroeconomic performance of large oil exporters, I found that, in many cases, rents from natural resources have been successfully used to enhance economic growth. Nevertheless, adherents of the ,resource curse' seem to have found ample evidence suggesting that resource-abundant countries grow slower than resource-poor countries. A review of empirical research on the ,resource curse' reveals that the variables used were usually proxies for resource dependence. These variables introduce a bias, making less developed economies per se more resource ,abundant' than developed economies. As a consequence, a new variable, not containing any information on a country's stage of development, was introduced. Comparing the variables on resource dependence and resource abundance in a model by Sachs and Warner, resource abundance was not significant. In a new model, resource abundance was even positively correlated with growth. [source] Examining Factors that Impact Mississippi Counties' Unreserved Fund Balance during Relative Resource Abundance and Relative Resource ScarcityPUBLIC BUDGETING AND FINANCE, Issue 4 2009LA SHONDA M. STEWART Although using some of the same organizational and financial factors examined by prior researchers to build and test models that explain factors influencing change in the general fund unreserved balance for smaller, rural, and less affluent counties in Mississippi, the rationale of this study is to build additional support that applying the recommended 5,15 percent savings benchmark across all jurisdictions is not a sufficient guide. Overall, Mississippi counties maintain unreserved fund balances ranging from a negative balance to over one hundred percent of their current expenditures. Counties also increase reserves during times of relative resource abundance and decrease them during relative resource scarcity. Moreover, they tend to address short-term needs and resident demands when revenues are plentiful. During relative resource scarcity, however, they are more cost-conscious and focus on maintaining rather than expanding current expenditures. This research shows that counties using the Beat system, a political form of government, are more likely to behave more frugally than counties using a Unit system, an administrative form of government. [source] The curse of natural resources in the transition economies,THE ECONOMICS OF TRANSITION, Issue 3 2004Tobias Kronenberg Abstract The curse of natural resources is a well-documented phenomenon for developing countries. Economies that are richly endowed with natural resources tend to grow slowly. Among the transition economies of the former ,Eastern Bloc', a similar pattern can be observed. This paper shows that a large part of the variation in growth rates among the transition economies can be attributed to the curse of natural resources. After controlling for numerous other factors, there is still a strong negative correlation between natural resource abundance and economic growth. Among the transition economies the prime reasons for the curse of natural resources were corruption and a neglect of basic education. In order to overcome the curse of natural resources and move to a sustainable path of development, the resource abundant transition countries should fight corruption and ensure that their resource revenues are invested in human capital or the preservation of natural capital. [source] Seasonal variation in the feeding ecology of the grey-cheeked mangabey (Lophocebus albigena) in CameroonAMERICAN JOURNAL OF PRIMATOLOGY, Issue 2 2001John R. Poulsen Abstract Seasonal fluctuations in resource abundance often cause primates to change their feeding behavior and ecology. The objective of this study was to examine the response of a largely frugivorous monkey, the grey-cheeked mangabey (Lophocebus albigena), to seasonal variations in fruit abundance. We used 15-min scan sampling to quantify feeding, activity, and habitat use by monkeys between February and December 1998 in the Dja Reserve, Cameroon. L. albigena were found to have omnivorous feeding habits, consuming the fruits, seeds, leaves, and flowers of 132 plant species. Although monkeys fed from many plant species, only five plant species accounted for 45% of all feeding records. The number of feeding observations on a plant species was significantly correlated with its fruit production. L. albigena responded to fruit-lean periods by shifting from a diet dominated by fruit to one dominated by seeds, flowers, and young leaves. This diet shift coincided with greater use of swamp habitat and higher dietary diversity. L. albigena spent the greatest percentage of scan samples feeding and traveling, but activities varied significantly over the day. Individuals spent a significantly higher percentage of scan samples feeding during the fruit-rich season than in the fruit-lean season. Comparing our results to those of studies in Gabon and Uganda, we found that L. albigena differ across regions in the number of plant species they consume and time spent feeding. These differences may be a result of variations in tree diversity or the strength of seasonal fluctuations in resource abundance among sites. Am. J. Primatol. 54:91,105, 2001. © 2001 Wiley-Liss, Inc. [source] Influence of temporal fluctuations in seed abundance on the diet of harvester ants (Pogonomyrmex spp.) in the central Monte desert, ArgentinaAUSTRAL ECOLOGY, Issue 8 2009GABRIELA I. PIRK Abstract Harvester ants usually go through temporal fluctuations in environmental seed abundance and composition which could influence their behaviour and ecology. The aim of this study was to evaluate how these fluctuations influence the diet of Pogonomyrmex rastratus, P. pronotalis and P. inermis (Hymenoptera, Formicidae) in the central Monte desert during three consecutive growing seasons. Although seeds were the main item in the diet, these ants turned more generalist when seed abundance of the most consumed species (grasses Aristida spp., Trichloris crinita, Pappophorum spp., Digitaria californica and Stipa ichu) was low. Accordingly, diversity of items in the diet decreased with seed abundance in a logarithmical fashion, showing higher foraging efficiency for seeds at higher seed abundance. Seed diversity, however, was not related to seed abundance as ants always included several species in their diet, with alternating prevalence. The proportion of the most consumed species increased logarithmically in the diet of P. rastratus and P. pronotalis along with their abundance in the environment probably as a consequence of diet switching (from forb and shrub seeds to grass seeds) and by an increase in foraging efficiency at higher seed densities. In contrast, foraging activity of P. inermis was very low at low seed abundance and its diet included only the five grasses. Among the most consumed species, proportion in the diet was not associated with relative abundance in the environment. Aristida spp., Pappophorum spp. and D. californica were overall highly selected. However, the flexibility in the diet of P. pronotalis and P. rastratus and the low foraging activity of P. inermis during periods of low resource abundance could attenuate potential top-down effects in the central Monte desert. This study shows that bottom-up effects are important in ant-seed interactions and should be considered when predicting and evaluating ants' effects on seed resources. [source] Phyllostomid Bat Community Structure and Abundance in Two Contrasting Tropical Dry Forests,BIOTROPICA, Issue 4 2005Kathryn E. Stoner ABSTRACT Although tropical wet forests are generally more diverse than dry forests for many faunal groups, few studies have compared bat diversity among dry forests. I compared ground level phyllostomid bat community structure between two tropical dry forests with different precipitation regimes. Parque National Palo Verde in northwestern Costa Rica represents one of the wettest tropical dry forests (rainfall 1.5 m/yr), whereas the Chamela-Cuixmala Biosphere Reserve on the Pacific coast of central Mexico represents one of the driest (750 mm/yr). Mist net sampling was conducted at the two study sites to compare changes in ground level phyllostomid bat community structure between regions and seasons. Palo Verde was more diverse than Chamela and phyllostomid species showed low similarity between sites (Classic Jaccard = 0.263). The distinct phyllostomid communities observed at these two dry forest sites demonstrates that variants of tropical dry forest can be sufficiently different in structure and composition to affect phyllostomid communities. At both dry forest sites, abundance of the two most common foraging guilds (frugivores and nectarivores) differed between seasons, with greatest numbers of individuals captured coinciding with highest chiropterophilic resource abundance. RESUMEN A pesar de que los bosques tropicales húmedos, en general, son más diversos que los bosques tropicales secos para muchos grupos de fauna, pocos estudios han comparado la diversidad de murciélogos en los bosques tropicales secos. El presente estudio compara la estructura de la comunidad de los murciélagos filostómidos a nivel del suelo entre dos tipos de bosque tropical seco con diferentes regimenes de precipitación. El parque Nacional Palo Verde esta localizado en el Noroeste de Costa Rica y representa uno de los bosques tropicales secos mas húmedos (con una precipitación de 1.5 m/año), mientras que la Reserva de la Biosfera Chamela-Cuixmala esta localizada en la costa oeste del pacífico de México y representa uno de los bosques más secos (750 mm/año). Se realizó un muestreo con redes de niebla en los dos sitios para comparar los cambios en la estructura de la comunidad de murciélagos filostómidos a nivel de suelo. Palo Verde fue más diverso que Chamela y se encontró la simultud de las especies filostomidos entre los dos sitio fue bajo (Classic Jaccard = 0.263). Las comunidades distintas de filostomidos observado en estos dos sitios de bosque seco demuestra que las variantes en el bosque tropical seco pueden ser suficientemente diferentes en estructura y composición para poder afectar la comunidad de filostomidos. En ambos bosques secos la abundancia de lo dos gremios tróficos más comunes (frugívoros y nectarívoros) fue diferente en las estaciones, con un mayor número de individuos capturados coincidiendo con una mayor abundancia de recursos quiropterofílicos. [source] | |||||||||||||||||||||||||