Resource productivity and availability impacts for food-chain length

Ecologists have focused on food-chain length (FCL) for the past eight decades as an index of food-web structure. Here, I review the hypotheses determining FCL with a focus on resource productivity and availability effects on FCL. First, I introduce the mainstream hypotheses to explain FCL variations: productivity, ecosystem size, and disturbance. For the existing productivity and productive space hypotheses, I stress the importance of using resource availability to estimate the productivity effect on FCL. Using a FCL dataset from 15 ponds, I tested the resource stoichiometry effect on FCL for ponds with between carbon:nitrogen ratio of primary producers and FCL. Moreover, I provide a perspective for studying resource availability and stoichiometry effects on FCL and of the alternative hypotheses of productive-space and ecosystem size. Finally, I suggest the future directions of the FCL study: a resource subsidy and climate change effects on FCL and food-web structure.     

Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams

We used standardized techniques to assemble eighteen food webs in streams. Our aim was to identify the determinants of food-web structure with particular reference to energy availability (related to land use), spatial heterogeneity and ecosystem size (both independent of land use). Forested streams displayed lower algal productivity and higher standing crops of organic matter than the grassland streams. The organic matter in the pine streams was probably of lower quality than that elsewhere. Measures of energy availability and spatial heterogeneity predicted species richness and connectance. A combination of energy availability, spatial heterogeneity and ecosystem size accounted for the representation of particular invertebrate feeding groups in the streams. Algal production and organic matter standing crop were important determinants of invertebrate biomass and overall food-web structure. Grassland sites showed a positive relationship between algal productivity and food chain length whereas forest sites displayed a positive relationship between ecosystem size and food chain length. Therefore, these results provide support for both Pimm's productivity hypothesis and Cohen and Newman's ecosystem size hypothesis.     

Resource availability and population size in cactophilic Drosophila

1. Four species of Drosophila, Drosophila nigrospiracula ( Patterson & Wheeler 1942 ) , Drosophila mettleri ( Heed 1977 ) , Drosophila pachea ( Patterson & Wheeler 1942 ) , and Drosophila mojavensis ( Patterson & Crow 1940 ) , are endemic to the Sonoran Desert of North America and breed in different species of necrotic columnar cacti. Differences in resource availability have been suggested to explain the interspecific variability in fly population biology, but resource availability for these species has not been quantitatively assessed thoroughly in either spatial or temporal terms. The resource availability was quantified quarterly at three sites for 3 years and population sizes for each Drosophila species were estimated.
2. Spatial and temporal availability of resources differed significantly among species of host cacti, with organpipe cactus ( Stenocereus thurberi ) being the least abundant and senita ( Lophocereus schottii ) the most abundant spatially.
3. Drosophila species differed significantly in population size. The largest population sizes were found for D. nigrospiracula and D. mojavensis and smallest for D. pachea . Populations of D. mettleri were intermediate to these.
4. Population size was greatest for fly species utilizing host species having the largest and longest lasting necroses.
5. Resource availability does not explain the reduction of fly populations in the summer. Necroses were most abundant when flies were absent.     

Proximate structural mechanisms for variation in food-chain length

Food-chain length is a central characteristic of ecological communities because of its strong influence on community structure and ecosystem function. While recent studies have started to better clarify the relationship between food-chain length and environmental gradients such as resource availability and ecosystem size, much less progress has been made in isolating the ultimate and proximate mechanisms that determine food-chain length. Progress has been slow, in part, because research has paid little attention to the proximate changes in food web structure that must link variation in food-chain length to the ultimate dynamic mechanism. Here we outline the structural mechanisms that determine variation in food-chain length. We explore the implications of these mechanisms for understanding how changes in food-web structure influence food-chain length using both an intraguild predation community model and data from natural ecosystems. The resulting framework provides the mechanisms for linking ultimate dynamic mechanisms to variation in food-chain length. It also suggests that simple linear food-chain models may make misleading predictions about patterns of variation in food-chain length because they are unable to incorporate important structural mechanisms that alter food-web dynamics and cause non-linear shifts in food-web structure. Intraguild predation models provide a more appropriate theoretical framework for understanding food-chain length in most natural ecosystems because they accommodate all of the proximate structural mechanisms identified here.     

Seabird-driven shifts in Arctic pond ecosystems

Migratory animals such as seabirds, salmon and whales can transport large quantities of nutrients across ecosystem boundaries, greatly enriching recipient food webs. As many of these animals biomagnify contaminants, they can also focus pollutants at toxic levels. Seabirds arguably represent the most significant biovectors of nutrients and contaminants from the ocean to the land, given their sheer numbers and global distribution. However, long-term census data on seabirds are rare. Using palaeolimnological proxies, we show that a colony of Arctic seabirds has experienced climate-induced population increases in recent decades. We then document increasing concentrations of contaminants, including polychlorinated biphenyls and cadmium, in pond sediments that are linked to biotransport by seabirds. Our findings suggest that climate-related shifts in global seabird populations will have the unexpected consequence of restructuring coastal ecosystems.     

Resource availability dominates and alters the relationship between species diversity and ecosystem productivity in experimental plant communities

Experimental evidence that plant species diversity has positive effects on biomass production appears to conflict with correlations of species diversity and standing biomass in natural communities. This may be due to the confounding effects of a third variable, resource availability, which has strong control over both diversity and productivity in natural systems and may conceal any positive effects of diversity on productivity. To test this hypothesis, I independently manipulated resource availability (soil fertility) and sown species diversity in a field experiment and measured their individual and interactive effects on productivity. Although fertility was a far stronger predictor of productivity than diversity, the effect of diversity on productivity significantly increased with fertility. Relative yield analyses indicated that plant mixtures of high fertility treatments significantly "overyielded," or were more productive than expected based on monoculture yields of component species. In contrast, plant mixtures of low fertility treatments had significantly lower-than-expected yields. The effect of diversity on productivity was also driven by sampling effects, where more species-rich mixtures were more likely to include particularly productive species. Unexpectedly, the strength of sampling effects was largely insensitive to fertility, although the particular species most responsible for sampling effects did change with fertility. These results suggest that positive effects of species diversity on ecosystem productivity in natural systems are likely to be masked by variation in environmental factors among habitats.     

Limnological studies of and primary production in temple pond ecosystems

Three temple ponds with permanent blooms of blue green algae were highly productive. They all showed high alkalinity, hardness, electrical conductivity and pH. Organic carbon and nitrogen were highest in Sarvatheertham pond—60 to 79.6 mg./l. C and 4.10 to 7.60 mg./l. N. In Tamaraikulam it was 16.5 to 20.3 mg. C/l. and 1.03 to 1.32 mg. N/l. In Sarvatheertham, the gross production ranged from 2.85 to 20.72 g. O₂/m.²/d. Self shading by blanket algae of blue greens reduced productivity in Sarvatheertham, where a persistent thermal and biochemical stratification was noted. Very high organic carbon and nitrogen contents were noted in Sarvatheertham pond. The dry weight of plankton in this pond ranged from 430 to 900 mg./l. Productivity computed from diurnal changes in alkalinity and dissolved oxygen also revealed a high rate in Ayyankulam, Tamaraikulam and Sarvatheertham in descending order. Very wide fluctuations in pH, both diurnally and depth-wise, were recorded in Sarvatheertham and to a lesser extent in the other two ponds. Photosynthetic efficiency was 4.03% in Ayyankulam, 2.09% in Tamaraikulam and 1.56% in Sarvatheertham. By the diurnal oxygen curve method, a gross primary production of 97.5 g. O₂/m.²/d was recorded in Ayyankulam.     

Hydrogeomorphology and river impoundment affect food-chain length of diverse Neotropical food webs

Food-chain length is a central characteristic of ecological communities that affects community structure and ecosystem function. What determines the length of food chains is not well resolved for most ecosystems. Herein, we examine environmental correlates of food-chain length based on the productivity hypothesis, compare food-chain lengths among aquatic ecosystem types and identify bi-directional effects of river impoundment on food-chain length in the Paraná River Basin of South America. Both temperature regime, a surrogate of productivity, and ecosystem type significantly affected food-chain length in independent analyses. However, when analyzed together, only ecosystem type explained significant variation in food-chain length. Food chains were longest in reservoirs, and shortest in high-gradient rivers. The proximate mechanism driving this pattern appears to be body-size ratios of primary consumers to apex predators, which differ among trophic pathways. Food chains based on phytoplankton production may have an additional size-structured link not present in food chains based on other basal sources such as detritus and algae. Hydrogeomorphology is the ultimate mechanism influencing food-chain length because it affects the relative importance of basal carbon sources supporting higher trophic levels, which through differences in the number of trophic links along the different size-structured pathways, appears to drive the observed patterns in food-chain length. We discuss a hypothesis of food-chain length that integrates energy flow and size-structure, facilitates inclusion of temporal dynamics and which is readily testable in both 'closed' and 'open' ecosystems.     

草基—鱼塘生态系统的能量转化与养分循环研究

应用模拟试验的方法,研究了“草基-鱼塘”系统中的能量转化与养分循环．结果 表明,该系统中饲草对太阳能的利用率为 ０． ８３％,鱼对饲料能的转化率为 ７． ３％．与以粮 食作为鱼饲料比较,单位面积草地的产鱼当量是粮食作物的１．６倍．鱼对饲料Ｎ、Ｐ、Ｋ的 转化率分别为１６．８％、３２．３％和２．０％．塘泥沉积的Ｎ、Ｐ分别占饲料的２３．４％和５６．１％; 猪对饲料 Ｎ、 Ｐ、 Ｋ的转化率分别为 ２０． ５％、 ３３． ７％和 ４． ６％,猪粪尿回收饲料 Ｎ为 ３６． ４％、 Ｐ为 ６３． ８％、 Ｋ为 ３９． ４％．猪-草-鱼结合的基塘系统其能量和养分转化效率均高于单一的 养鱼系统．     

Mass culture ofTilapia zillii [Cichlidae] in pond ecosystems

The culture ofTilapia zillii (Gervais) in 18 and 34 cm deep, 5.5×7.6 m earthen ponds at 3 initial stocking densities, 6, 9 and 12 pairs, revealed an optimum 9 pair initial density for the maximum production of fry after 104 days. However, biomass increase was significantly greater at the 6 pair stocking density and in deeper ponds. Pond depth had no significant effect on size and weight gain of parental fish. Nest and brooding tube construction was variously influenced by pond depth and stocking density. Partial shading of ponds, while not affecting the maximum water temperature, did apparently reduce fish biomass production, the average length and weight of fry, and the average depth of nests. An estimated 1 million fish could be produced in 104 days from 190 similar ponds with a water surface equivalent to 0.8 hectares. This would be equivalent to ca. 725 kg fish biomass production and would require ca. 1,900 kg of catfish pellets or their equivalent. Possible ways to increase rearing efficiency are discussed.     

Net ecosystem carbon exchange in two experimental grassland ecosystems

Increases in net primary production (NPP) may not necessarily result in increased C sequestration since an increase in uptake can be negated by concurrent increases in ecosystem C losses via respiratory processes. Continuous measurements of net ecosystem C exchange between the atmosphere and two experimental cheatgrass (Bromus tectorum L.) ecosystems in large dynamic flux chambers (EcoCELLs) showed net ecosystem C losses to the atmosphere in excess of 300 g C m^?2 over two growing cycles. Even a doubling of net ecosystem production (NEP) after N fertilization in the second growing season did not compensate for soil C losses incurred during the fallow period. Fertilization not only increased C uptake in biomass but also enhanced C losses through soil respiration from 287 to 469 g C m^?2, mainly through an increase in rhizosphere respiration. Fertilization decreased dissolved inorganic C losses through leaching of from 45 to 10 g C m^?2. Unfertilized cheatgrass added 215 g C m^?2 as root‐derived organic matter but the contribution of these inputs to long‐term C sequestration was limited as these deposits rapidly decomposed. Fertilization increased NEP but did not increase belowground C inputs most likely due to a concurrent increase in the production and decomposition of rhizodeposits. Decomposition of soil organic matter (SOM) was reduced by fertilizer additions. The results from our study show that, although annual grassland ecosystems can add considerable amounts of C to soils during the growing season, it is unlikely that they sequester large amounts of C because of high respiratory losses during dormancy periods. Although fertilization could increase NEP, fertilization might reduce soil C inputs as heterotrophic organisms favor root‐derived organic matter over native SOM.     

Effective size and the persistence of ecosystems

Summary Ecosystems distributed in space have an effective size, reflecting both their absolute size (extent) and their fine-scale physical structure (viscosity). In this paper, a general mathematical model of a predator-prey interaction is presented via the phase-plane graphs of Rosenzweig and MacArthur (1963) to show one reason why ecosystems of larger effective size should persist longer than smaller ones: oscillations of population densities tend to be displaced farther from extinction thresholds—even in spatially homogeneous systems. Experimental results obtained by Gause and Luckinbill with protozoa and Huffaker with mites are interpreted in this context.     

Food availability determines the response to pond desiccation in anuran tadpoles

Food availability and pond desiccation are two of the most studied factors that condition amphibian metamorphosis. It is well known that, when food is abundant, organisms undergo metamorphosis early and when they are relatively large. The capability of anurans to accelerate their developmental rate in response to desiccation is also common knowledge. These two variables must act together in nature, since we know that, as a pond dries, the per capita resources decrease. We conduct an experiment to evaluate the effects of desiccation and food availability separately and in combination in tadpoles of the painted frog (Discoglossus pictus). We demonstrate that food deprivation leads to slow growth rates, which delay metamorphosis and produce smaller size and weight. The capability to accelerate metamorphosis when facing a drying pond is also confirmed, but, nevertheless, with factor interaction (when the pool is drying and resources are scarce) the capacity to respond to desiccation is lost. In addition, slow drying rates are shown to be stressful situations, but not enough to provoke a shortening of the larval period; in fact, the larval period becomes longer. We also demonstrate that the interaction of these factors changes the allometric relationship of different parts of the hind limb, which has implications for the biomechanics of jumping. Due to low mortality rates and an adequate response to both environmental factors, we expect D. pictus to have a great invasive potential in its new Mediterranean distribution area, where lots of temporary and ephemeral ponds are present.     

Cross-scale ecological dynamics and microbial size spectra in marine ecosystems

Evaluating the component features of 'scaling' planktonic size spectra, commonly observed in marine ecosystems, is crucial for understanding the ecological and evolutionary processes from which they emerge. Here, we develop a theoretical framework that describes such spectra in terms of the size distributions of individual species, and test it against actual datasets of microbial size spectra from the Atlantic Ocean. We describe characteristics of size probability distributions of component species that are sufficient to support the observational evidence and infer that, when a power law describes the community size spectrum (thus suggesting critical self-organization of microbial ecosystem structure and function), a related power law links the total number of individuals of a given species to its mean size.     

Resource availability and its role in development of invasion processes

The state of biotic communities inhabiting different water bodies is analyzed aiming to determination of conditions facilitating development of biological invasions. Mass-balance and dynamic models are used to reveal the factors ensuring the invasion of zebra mussel, Dreissena polymorpha, in Naroch Lakes (Byelorussia), and smelt, Osmerus eperlanus, in Lake Syamozero (Karelia). The results obtained indicate that for invasions leading to trophic chain lengthening, the main factor of successful invasion is the availability of resources that are not utilized by aboriginal species. Highest chances for successful invasion belong to invading species feeding on those trophic groups which contribute to the most extent to forming of 'excess' (i.e., available for the invader) production. It is shown that establishment of an alien species in a native community appears to be possible only when total amount of such 'excess' production is higher then some threshold value. Under conditions of spatio-temporal environmental heterogeneity, the timing factor, which determines the exact moment of an alien species intrusion, becomes important as well.     

Tooth size and arcadal length correlates in man

Intra-arcadal mesiodistal and buccolingual tooth size correlations were evaluated in a sample of 125 caucasoids with ideal occlusion. Dental dimensions were corrected for arcade mength (as a measure of jaw size) by a series of regression analyses of each mesiodistal dimension on the sum of the mesiodistal dimensions within each arcade. Regression coefficients of tooth dimension on arcade length were calculated to gain an insight into the dimensional sensitivity of individual teeth to arcade length variation. The data presented here suggest a strong association between arcadal length (jaw size) dependence, and the dimensional stability of individual teeth. When corrected for arcade length, a definite pattern of tooth size correlation emerges: postcanine maxillary and mandibular teeth are negatively correlated to the anterior teeth and are positively correlated to one another. The hypothesis is developed that anterior and postcanine teeth should be viewed as two separate and negatively size-correlated units, beyond the boundaries of the four morphological tooth classes. Recognition of this basic dichotomous size arrangement within each jaw allows for a reassessment of some of the problems associated with hominid dental evolution.