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James H Brown - One of the best experts on this subject based on the ideXlab platform.
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redundant or complementary impact of a colonizing species on community structure and function
Oikos, 2010Co-Authors: Katherine M Thibault, S Morgan K Ernest, James H BrownAbstract:Recent studies suggest that species with similar functional traits will have similar effects on ecosystems, but evidence for redundancy of species impacts is limited. Here we use a long-term experiment to gain insight into functional relationships within a desert rodent community. Experimental removal of kangaroo rats, Dipodomys spp., coupled with the recent, serendipitous colonization of a single species of large pocket mouse Chaetodipus baileyi yielded treatments that differed in the diversity of large granivorous rodents present. We evaluated functional overlap of C. baileyi and the other resident large Granivores (i.e. the kangaroo rats) by comparing total energy use of granivorous rodents and total abundance and species richness of small Granivores across treatments before and after the arrival of C. baileyi. We found that C. baileyi almost completely compensated for the changes in these key ecosystem-level properties caused by kangaroo rat removal, but it differentially impacted the population dynamics of individual small granivorous rodent species. Thus, its effects were largely complementary, rather than redundant, to those of the missing kangaroo rats. Although short-term or single-measure analyses may suggest redundancy, our results support the longstanding dictum that niches of coexisting species are often similar but rarely, if ever, identical.
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the effects of vertebrate Granivores and folivores on plant community structure in the chihuahuan desert
Oikos, 1995Co-Authors: Daniel B. Thompson, Thomas J Valone, James H BrownAbstract:Guo, Q., Thompson, D.B., Valone, T.J. and Brown, J.H. 1995. The effects of vertebrate Granivores and folivores on plant community structure in the Chihuahuan Desert. - Oikos 73: 251-259. We examined the effects of vertebrate Granivores and folivores on winter and summer plant communities in the Chihuahuan Desert by selectively excluding different vertebrate combinations of birds, rodents, and large folivorous mammals from small experimental plots continuously since 1982. Few differences were observed across treatments during the first two yr of the experiment. The influence of vertebrates on winter and summer plant communities became apparent after three yr, and changes continued to occur after 11 yr of exclusion. Multivariate repeated-measures analyses indicated that removal of birds and both birds and rodents caused significant increases in the total density of winter annuals. The winter annual plant density was highest on bird removal plots and this increase was significant by 1985. Canonical Discriminant Analyses revealed that the exclusion of birds and both birds and rodents had the strongest influence on species composition of both winter and summer plant communities, although the winter plant community was more sensitive to vertebrate exclusion than the summer plant community. The exclusion of birds had the greatest effect on composition of summer plant communities followed in importance by the exclusion of rodents and mammalian folivores. Most of the plant species that responded significantly to the treatments were large-seeded winter annuals. Our results both complement earlier experimental work documenting the impacts of the granivorous rodents and folivorous mammals on desert plant communities and now demonstrate the important influence of avian Granivores.
Joel S. Brown - One of the best experts on this subject based on the ideXlab platform.
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foraging and community consequences of seed size for coexisting negev desert Granivores
Oikos, 2000Co-Authors: Jessica E Garb, Burt P. Kotler, Joel S. BrownAbstract:We examined the effects of seed size on patch use and diet selection for three co-existing Negev Desert Granivores: Allenby’s gerbil (Gerbillus allenbyi ), greater Egyptian sand gerbil (Gerbillus pyramidum), and crested lark (Galerida cristata). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving-up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (B1.4 mm in diameter cracked wheat), medium (2.0‐3.3 mm), and large (\3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2‐5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed-size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed-size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal’s patch use behavior all played major roles in determining gerbils’ and larks’ diet selectivities and GUDs.
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mechanisms of coexistence of optimal foragers as determinants of local abundances and distributions of desert Granivores
Journal of Mammalogy, 1999Co-Authors: Burt P. Kotler, Joel S. BrownAbstract:Foraging theory can be applied to the study of ecological communities. We developed and tested several local mechanisms of species coexistence based on foraging theory for a desert Granivore community in a heterogeneous landscape in the Negev Desert, Israel. The most common species were Allenby's gerbil ( Gerbillus allenbyi ), Wagner's gerbil ( Gerbillus dasyurus ), and crested lark ( Galerida cristata ). Allenby's gerbils were most abundant on sandy substrates; Wagner's gerbils were most abundant on cobble and loess substrates; crested larks occurred across all three. To test among predictions of the different mechanisms of coexistence, we used live-trapping and counts to census rodents and birds, recorded spoor in sand-tracking plots to determine habitat-specific activity patterns, and measured giving-up densities in depletable food patches to estimate relative foraging efficiencies on different substrates and within different habitats. Allenby's gerbil had an advantage in habitat selection by being the most efficient forager (lowest giving-up densities on seeds in feeding trays) in the sandy habitat. The crested lark was the tolerant forager. Its foraging abilities were affected little by escape substrate, foraging substrate, or food type. As a result, the crested lark had three advantages: it was the most efficient forager in the loess habitat, it was a better insectivore than the gerbils, and its diurnal habit allowed it first access to seed patches that renew in the afternoon in the sandy habitat when winds typically arise. In response to our measures of foraging aptitude, Wagner's gerbil revealed no advantage over the other species, and factors promoting its success eluded our proposed mechanisms of species coexistence. Foraging ecology and community ecology can be integrated to understand local species distributions, abundances, and community structure and organization. Uniform conceptual and methodological techniques can be applied across taxa and communities. We were able to relatively quickly test which mechanisms of coexistence apply. In the community we studied, there were two species whose presence we feel we understood and one species whose presence remained an enigma.
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the effects of water on patch use by two simpson desert Granivores corvus coronoides and pseudomys hermannsburgensis
Austral Ecology, 1998Co-Authors: Burt P. Kotler, Chris R Dickman, Joel S. BrownAbstract:Water loss while foraging may affect the overall value of food to desert animals. When water is scarce, foragers may alter activity and shun certain types of food due to elevated water loss. When water is abundant, foragers can exploit food patches more thoroughly and remain active over a broader range of ambient conditions. In short, food and water may be complementary resources. The presence of water raises the marginal value of food, particularly those foods low in water content. We tested for the complementarity of food and water to foragers at a sand dune site in the Simpson Desert of arid Australia. To do so, we quantified patch exploitation of foragers in the presence or absence of bowls filled with water. In order to quantify patch use, we provisioned feeding trays with granulated peanuts mixed into a sand substrate. In these trays we measured giving-up densities (GUD; the amount of food left in a tray after a foraging bout) of diurnal (mostly Australian ravens, Corvus coronoides) and nocturnal foragers (mostly sandy inland mouse, Pseudomys hermannsburgensis). The presence of water affected the GUD of ravens but not of rodents. For the ravens, GUD dropped about 50% in response to added water. For ravens, water and food are strongly complementary. In addition, ravens had lower GUD in the open than the bush microhabitat, and lower GUD at the bottom than the tops of sand dunes.
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foraging theory patch use and the structure of a negev desert Granivore community
Ecology, 1994Co-Authors: Joel S. Brown, Burt P. Kotler, William A MitchellAbstract:We tested five mechanisms of coexistence in a community of three common rodent species (two gerbils, Gerbillus allenbyi and G. pyramidum, and one jerboa, Jaculus jaculus) inhabiting sand dunes in the Negev Desert, Israel. The five mechanisms, based on foraging theory, considered various forms of habitat selection in time and space. From November 1986 until January 1988, we live-trapped to census rodent populations, counted rodent spoor in tracking plots to quantify activity, and measured the rodents' giving-up densities (GUDs: the amount of food remaining within a resource patch following exploi- tation by a forager) in seed trays to determine relative foraging efficiencies. The population sizes of the two gerbil species tended to fluctuate synchronously (unfortunately, we could not live-trap any jerboas). G. allenbyi biased its activity towards the stabilized sand habitat and towards the bush microhabitat. In contrast, G. pyramidum and J. jaculus biased their activity towards semistabilized sand habitats, and J. jaculus also biased its activity towards the open microhabitat. Despite divergent patterns of habitat use among species, G. allenbyi tended to be the most efficient forager and J. jaculus the least efficient forager regardless of microhabitat, sand habitat, or month. G. allenbyi's presence in the community may be assured by its higher foraging efficiency. J. jaculus's presence in the community may either result from its ability to travel greater distances and utilize rare, rich resource patches, or result from an herbivorous diet. G. pyramidum's presence with G. allenbyi in the com- munity appears to require the less stabilized sand habitats and the ability of G. pyramidum to dominate rich patches by interference or nightly temporal partitioning.
Andrew J Edelman - One of the best experts on this subject based on the ideXlab platform.
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positive interactions between desert Granivores localized facilitation of harvester ants by kangaroo rats
PLOS ONE, 2012Co-Authors: Andrew J EdelmanAbstract:Facilitation, when one species enhances the environment or performance of another species, can be highly localized in space. While facilitation in plant communities has been intensely studied, the role of facilitation in shaping animal communities is less well understood. In the Chihuahuan Desert, both kangaroo rats and harvester ants depend on the abundant seeds of annual plants. Kangaroo rats, however, are hypothesized to facilitate harvester ants through soil disturbance and selective seed predation rather than competing with them. I used a spatially explicit approach to examine whether a positive or negative interaction exists between banner-tailed kangaroo rat (Dipodomys spectabilis) mounds and rough harvester ant (Pogonomyrmex rugosus) colonies. The presence of a scale-dependent interaction between mounds and colonies was tested by comparing fitted spatial point process models with and without interspecific effects. Also, the effect of proximity to a mound on colony mortality and spatial patterns of surviving colonies was examined. The spatial pattern of kangaroo rat mounds and harvester ant colonies was consistent with a positive interspecific interaction at small scales (<10 m). Mortality risk of vulnerable, recently founded harvester ant colonies was lower when located close to a kangaroo rat mound and proximity to a mound partly predicted the spatial pattern of surviving colonies. My findings support localized facilitation of harvester ants by kangaroo rats, likely mediated through ecosystem engineering and foraging effects on plant cover and composition. The scale-dependent effect of kangaroo rats on abiotic and biotic factors appears to result in greater founding and survivorship of young colonies near mounds. These results suggest that soil disturbance and foraging by rodents can have subtle impacts on the distribution and demography of other species.
Moshe Shachak - One of the best experts on this subject based on the ideXlab platform.
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shrubs Granivores and annual plant community stability in an arid ecosystem
Oikos, 2004Co-Authors: Andrew Wilby, Moshe ShachakAbstract:Compensatory population dynamics among species stabilise aggregate community variables. Inter-specific competition is thought to be stabilising as it promotes asynchrony among populations. However, we know little about other inter-specific interactions, such as facilitation and granivory. Such interactions are also likely to influence population synchrony and community stability, especially in harsh environments where they are thought to have relatively strong effects in plant communities. We use a manipulative experiment to test the effects of Granivores (harvester ants) and nurse plants (dwarf shrubs) on annual plant community dynamics in the Negev desert, Israel. We present evidence for weak and inconsistent effects of harvester ants on plant abundance and on population and community stability. By contrast, we show that annual communities under shrubs were more species rich, had higher plant density and were temporally less variable than communities in the inter-shrub matrix. Species richness and plant abundance were also more resistant to drought in the shrub under-storey compared with the inter-shrub matrix, although population dynamics in both patch types were synchronised. Hence, we show that inter-specific interactions other than competition affect community stability, and that hypothesised mechanisms linking compensatory dynamics and community stability may not operate to the same extent in arid plant communities.
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harvester ant response to spatial and temporal heterogeneity in seed availability pattern in the process of granivory
Oecologia, 2000Co-Authors: Andrew Wilby, Moshe ShachakAbstract:The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.
Thomas O Crist - One of the best experts on this subject based on the ideXlab platform.
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Harvester Ants (Pogonomyrmex spp.): Their Community and Ecosystem Influences
Annual Review of Ecology Evolution and Systematics, 2000Co-Authors: James A Macmahon, John F. Mull, Thomas O CristAbstract:▪ Abstract We summarize the influences of harvester ants of the genus Pogonomyrmex on communities and ecosystems. Because of nest densities, the longevity of nests, and the amount of seed harvested and soil handled, harvester ants have significant direct and indirect effects on community structure and ecosystem functioning. Harvester ants change plant species composition and diversity near their nests. These changes result from differential seed predation by the ants, their actions as seed dispersers and competitors with other Granivores, and the favorable soil conditions they create through their digging. Their nest building creates islands of increased nutrient density. In some areas, the effects of their activities may be so pervasive that plant community structure is strongly influenced. Ant removal studies, which would reveal their total impact, have generally not been done. Granivore removals have been conducted in North America where ants are of lesser importance than small mammals, in contrast to ...
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the impact of fungi on soil seeds implications for plants and Granivores in a semiarid shrub steppe
Ecology, 1993Co-Authors: Thomas O Crist, Carl F FrieseAbstract:Fungi are ubiquitous in soil and may affect seed survivorship directly by decomposition or pathogenesis, or indirectly through the effects of fungi on seed preference of Granivores. We studied the role of fungi in the persistence of soil seeds in a shrub-steppe ecosystem using two experimental approaches. In the first, we examined survivorship of 900 seeds of five shrub-steppe species in nylon bags over a 1 0-mo period and determined viable seed losses to germination, decomposition, and pathogens. The proportion of seeds representing a viable seed bank decreased from 84.4% in the May pre-experimental con- ditions to 4.4% in March. Decomposition and attack by fungi were responsible for the greatest decrease from fall to winter (0% in August, 32.0% in November, 35.2% in March) while germination accounted for the greatest decrease from winter to spring (18.5% in August, 21.9% in November, 55.0% in March). Small seeds were more likely to be lost to decomposition and pathogens, whereas larger seeds had greater losses to germination. A subset of 80 experimental seeds was cultured for seed-infecting fungi. We found fungi in virtually all seeds; the most common fungal isolates were cosmopolitan soil fungi, such as species of Penicillium and Mucor. These species are known producers of extracellular enzymes and mycotoxins. Two sterile (nonfruiting) forms of fungi were also frequently isolated, as were two pathogenic species of Fusarium. In a second experiment we placed seeds in fungal cultures obtained from soil seeds and presented moldy seeds along with control seeds to a shrub-steppe Granivore, the western harvester ant (Pogonomyrmex oc- cidentalis). Ants harvested control seeds at nearly twice the rate of moldy seeds, suggesting that they avoided seeds infected with fungi. In this way, fungi may indirectly affect seed mortality due to predation by Granivores, in addition to the direct effects fungi can have on seed survivorship. We suggest that fungi may play an important role in soil seed dynamics, but one that is poorly understood.
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harvester ant foraging and shrub steppe seeds interactions of seed resources and seed use
Ecology, 1992Co-Authors: Thomas O Crist, James A MacmahonAbstract:Granivore—seed interactions involve a feedback between Granivore seed selectivity and seed availability. We examined this feedback to determine how seed preferences by the western harvester ant, Pogonomyrmex occidentalis, related to seed availability and, in turn, affected the soil seed pool. Preferences were estimated from natural diets as well as from experiments that controlled seed size, relative availability, and distance from ant nests. Seed availability to ants varied with season and over 2 yr. Colony activity and seed intake rates were correlated with seed availability. Seed preference by ants was correlated with the seasonal availability of preferred species, but not with unpreferred seeds. From the soil seed pool, ants preferentially harvested small, sound seeds. They removed 9—26% of the potentially viable seed pool each year, and as much as 100% of available preferred species. Seed densities were lower 2—7 m from nests, where foraging activity was concentrated, than 7—12 m from nests. In contr...
