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9 results for “competition indices”
High rates of nectar depletion in summer grasslands indicate competitive conditions for pollinators
<p>Competition among pollinators for floral resources is a phenomenon of both basic and applied importance. While competition is difficult to measure directly under field conditions, it can be inferred indirectly through the measurement of floral resource depletion. In this study, we conducted a pollinator exclusion experiment to calculate nectar depletion rates in summer across 16 grassland sites in the German regions of Franconia and Saxony-Anhalt. Overall depletion rates were estimated at 95% in Franconia and 79% in Saxony-Anhalt, indicating strong nectar limitation and likely competition among pollinators for nectar. Despite being ubiquitous in our study regions, honey bees were scarce at our sites at the time of nectar sampling. This demonstrates that wild pollinators alone are capable of massive nectar depletion, and the addition of managed honey bees to wild pollinator communities may intensify already competitive conditions. Nevertheless, the manifest diversity of the pollinator communities at our sites indicates that other factors, such as non-trophic constraints or temporal variation in nectar limitation, can mitigate competitive exclusion despite immediate conditions of acute nectar scarcity. </p>
High rates of nectar depletion in summer grasslands indicate competitive conditions for pollinators
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Data from: Quantifying neighbour effects on tree growth: are common "competition" indices biased?
<p>1. Interactions among neighbouring plants are key determinants of plant growth. To characterise the cumulative effect of all neighbours on the growth of a focal plant, neighbourhoods are often described by 'competition' indices. Common competition indices calculate the summed size of neighbour plants (focal-independent index) whilst others include the summed ratio of the neighbour size relative to focal plant size (focal-dependent). A frequently overlooked statistical artifact is that focal-dependent indices may lead to biased estimates of neighbourhood effects on plant growth when growth is size-dependent.</p> <p>2. Here, we conduct a literature search to determine the most common index types used to explain neighbour effects on tree growth. We then assess the ability of two common index types – focal-dependent and focal-independent – to correctly infer neighbourhood effects in (1) observations of tree growth in an experimental forest in south-east Tasmania, Australia, and (2) an artificially created dataset where tree growth is unrelated to the neighbourhood.</p> <p>3. Both indices detected the competitive neighbourhood effect on tree growth observed in our own dataset but differed in their conclusion regarding neighbour effects in the simulated data. Despite the simulated dataset being generated so there was no relationship between tree growth and their neighbourhood, the focal-dependent index detected strong, competitive neighbourhood effects when intrinsic growth was incorrectly related to tree size. In contrast, when we considered the focal-independent index as the neighbourhood metric, we correctly did not detect any neighbourhood effects in the simulated data regardless of how size-dependent growth was described.</p> <p>4. <em>Synthesis</em>. 'Competition' indices are a useful method to characterise the cumulative neighbourhood effect on plant growth, however, we demonstrate that indices which include the size of the focal plant in their calculation can be biased by an inherent relationship between tree growth and initial size. Whilst this bias typically overstates the strength of competition in determining focal tree growth, we show that it can be mitigated by correctly describing intrinsic growth. We discuss the limitations of both index types, provide recommendations for performing statistical modelling, and outline how to check for accurate neighbour inference.</p>
Narrow dietary niche with high overlap between snow leopards and Himalayan wolves indicates potential for resource competition in Shey Phoksundo National Park, Nepal
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Data from: Quantifying neighbour effects on tree growth: are common “competition” indices biased?
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Data from: Latitudinal variation in ecological opportunity and intraspecific competition indicates differences in niche variability and diet specialization of Arctic marine predators
Individual specialization (IS), where individuals within populations irrespective of age, sex, and body size are either specialized or generalized in terms of resource use, has implications on ecological niches and food web structure. Niche size and degree of IS of near-top trophic-level marine predators have been little studied in polar regions or with latitude. We quantified the large-scale latitudinal variation of population- and individual-level niche size and IS in ringed seals (Pusa hispida) and beluga whales (Delphinapterus leucas) using stable carbon and nitrogen isotope analysis on 379 paired ringed seal liver and muscle samples and 124 paired beluga skin and muscle samples from eight locations ranging from the low to high Arctic. We characterized both within- and between-individual variation in predator niche size at each location as well as accounting for spatial differences in the isotopic ranges of potential prey. Total isotopic niche width (TINW) for populations of ringed seals and beluga decreased with increasing latitude. Higher TINW values were associated with greater ecological opportunity (i.e., prey diversity) in the prey fish community which mainly consists of Capelin (Mallotus villosus) and Sand lance (Ammodytes sp.) at lower latitudes and Arctic cod (Boreogadus saida) at high latitudes. In beluga, their dietary consistency between tissues also known as the within-individual component (WIC) increased in a near 1:1 ratio with TINW (slope = 0.84), suggesting dietary generalization, whereas the slope (0.18) of WIC relative to TINW in ringed seals indicated a high degree of individual specialization in ringed seal populations with higher TINWs. Our findings highlight the differences in TINW and level of IS for ringed seals and beluga relative to latitude as a likely response to large-scale spatial variation in ecological opportunity, suggesting species-specific variation in dietary plasticity to spatial differences in prey resources and environmental conditions in a rapidly changing ecosystem.
Data from: Latitudinal variation in ecological opportunity and intraspecific competition indicates differences in niche variability and diet specialization of Arctic marine predators
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Data from: Competitive Indices in cereal and legume mixtures in a South Asian environment
The assessment of competitive performance of mixture components is important for maximizing benefits of intercropping systems. This field study tested binary mixtures of two cereals (sorghum and pearl millet) and three forage legumes (cowpea, cluster bean and soybean), along with their sole crops on competitive indices. Sorghum-cowpea binary mixture resulted in a lower green forage yield of sorghum and cowpea by 9% and 36% respectively, but overall biomass production was increased by 30% and 117% compared to their sole crop equivalents. Partial land equivalent ratios (LER) of all component crops were higher than 0.50, indicating better land use efficiency, except of soybean in binary mixtures with cowpea and cluster bean. However, the highest LER was of sorghum-cowpea (1.55), followed by sorghum-soybean (1.48) and pearl millet-soybean (1.48) binary mixtures. Pearl millet dominated sorghum and all legumes, while cowpea remained a superior competitor among legumes as per aggressivity value index. The highest crowding ratio was exhibited by pearl millet in binary mixture with cluster bean indicating its higher competitive ability incomparison to other mixture components. Observed yield loss data indicated that pearl millet was the most resistant crop to yield loss in all binary mixtures, while soybean had the highest yield reduction. In a short term, the highest area-time equivalent ratio for sorghum-cowpea binary mixture indicated the maximum advantage for this binary mixture compared to other binary mixtures.
Data from: Competitive Indices in cereal and legume mixtures in a South Asian environment
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OpenNeuro
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