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Shared Preference (shared + preference)
Selected AbstractsA probability-based analysis of temporal and spatial co-occurrence in grassland birdsJOURNAL OF BIOGEOGRAPHY, Issue 12 2006Joseph A. Veech Abstract Aim, To test for non-random co-occurrence in 36 species of grassland birds using a new metric and the C -score. The analysis used presence,absence data of birds distributed among 305 sites (or landscapes) over a period of 35 years. This analysis departs from traditional analyses of species co-occurrence in its use of temporal data and of individual species' probabilities of occurrence to derive analytically the expected co-occurrence between paired species. Location, Great Plains region, USA. Methods, Presence,absence data for the bird species were obtained from the North American Breeding Bird Survey. The analysis was restricted to species pairs whose geographic ranges overlapped. Each of 541 species pairs was classified as a positive, negative, or non-significant association depending on the mean difference between the observed and expected frequencies of co-occurrence over the 35-year time-span. Results, Of the 541 species pairs that were examined, 202 to 293 (37,54%) were positively associated, depending on which of two null models was used. However, only a few species pairs (<5%) were negatively associated. An additional 89 species pairs did not have overlapping ranges and hence represented de facto negative associations. The results from analyses based on C -scores generally agreed with the analyses based on the difference between observed and expected co-occurrence, although the latter analyses were slightly more powerful. Main conclusions, Grassland birds within the Great Plains region are primarily distributed among landscapes either independently or in conjunction with one another. Only a few species pairs exhibited repulsed or segregated distributions. This indicates that the shared preference for grassland habitat may be more important in producing coexistence than are negative species interactions in preventing it. The large number of non-significant associations may represent random associations and thereby indicate that the presence/absence of other grassland bird species may have little effect on whether a given focal species is also found within the landscape. In a broader context, the probability-based approach used in this study may be useful in future studies of species co-occurrence. [source] Seed mass and the competition/colonization trade-off: competitive interactions and spatial patterns in a guild of annual plantsJOURNAL OF ECOLOGY, Issue 1 2004Lindsay A. Turnbull Summary 1We used neighbourhood modelling to estimate individual-level competition coefficients for seven annuals growing in limestone grassland over 2 years. We calculated the relative strength of intra- and interspecific competition and related this to differences in seed size and plant size between targets and neighbours. 2Significant differences in the impact of neighbours on each target species were observed in half the models fitted, allowing us to reject a null hypothesis of competitive equivalence. 3In one year we found that as the seed size or plant size of neighbours increased relative to targets, so did their competitive effect. Although this is consistent with the competition/colonization trade-off model the competitive interactions were not sufficiently asymmetric to allow coexistence. In a second year we found only weak interspecific competition and no relationship with plant or seed size. 4We found no overall relationship between competition coefficients and the degree of segregation, contradicting the spatial segregation hypothesis for coexistence. However, segregation was linked to differences in plant traits: when targets were smaller than neighbours the degree of segregation increased with relative neighbour size. 5Most species were positively associated with each other due to a shared preference for otherwise unvegetated patches. The degree of association was negatively correlated with differences in plant and seed size, particularly when interspecific competition was weak. This might reflect (i) decreasing overlap in microhabitat use with increasing trait divergence or (ii) density-dependent mortality. 6Seed size is a key trait within this group of species, determining both competitive and colonizing ability. The presence of such a competition/colonization trade-off undoubtedly stabilizes community dynamics although other mechanisms may also be at work. [source] Dominant Party Strategy and DemocratizationAMERICAN JOURNAL OF POLITICAL SCIENCE, Issue 1 2008Kenneth F. Greene How do incumbent parties strategize against challengers when a new partisan cleavage cuts across the incumbent's electoral coalition? This article argues that a two-dimensional extension of Riker's anticoordination thesis conflicts with Downsian extensions. It shows that when voters coordinate on a single challenger based on their shared preference on a cross-cutting cleavage, a vote-maximizing incumbent party should move away from the challenger on the primary dimension of competition, even at the risk of abandoning the center. The article develops this hypothesis with reference to dominant parties in competitive authoritarian regimes where challenger parties constantly attempt "heresthetical" moves by mobilizing regime issues into the partisan debate, and it tests the predictions with an original sample survey of national leaders of Mexico's Revolutionary Institutional Party (PRI). It also spells out the implications of the findings for dominant party survivability in democratic transitions and, more broadly, for incumbents' spatial strategies in the face of new partisan cleavages. [source] Habitat selection of freshwater-dependent cetaceans and the potential effects of declining freshwater flows and sea-level rise in waterways of the Sundarbans mangrove forest, BangladeshAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 2 2009B.D. Smith Abstract 1.Generalized additive models of sighting data for cetaceans collected during two surveys of waterways in the Sundarbans mangrove forest of Bangladesh indicated that Ganges River dolphin Platanista gangetica gangetica distribution was conditionally dependent (P<0.05) on low salinity, high turbidity, and moderate depth during both low and high freshwater flow; and Irrawaddy dolphin Orcaella brevirostris distribution was conditionally dependent (P<0.05) on low salinity during high freshwater flow, high and moderate depths during low and high freshwater flow, respectively; low and high-low extremes of turbidity during low and high freshwater flow, respectively; and high temperature and increasing numbers of large,small channel confluences during low freshwater flow. 2.According to sighting data collected over a 3-year period by the captains of three nature tourism vessels, there were significant differences between the actual and expected frequencies of Ganges River dolphin sightings and individuals according to various channel types (chi-square=64.22, P<0.0001 and chi-square=134.14, P<0.0001, respectively, df=6) and of Irrawaddy dolphin sightings and individuals (chi-square=15.28, P=0.0182, and chi-square=29.42, P<0.0001, respectively, df=6), with shared preferences for wide sinuous channels with at least two small confluences or one large confluence. 3.The dependency exhibited by both species for environmental characteristics associated with abundant freshwater flow, including low salinity and the availability of confluences, make them particularly vulnerable to habitat loss due to upstream water abstraction and sea-level rise. 4.Although the results of this study may not affect plans for construction in India of large-scale, inter-basin water transfer projects that will result in further declines in freshwater flows, or decisions within the international community about CO2 emissions affecting global sea levels, they can be used to prioritize locations where protective measures could be employed to benefit the long-term conservation of both species. Copyright © 2008 John Wiley & Sons, Ltd. [source] | |||||||||||||