Long-term dynamics of small-bodied and large-bodied cladocerans during the eutrophication of a shallow reservoir,with special attention for Chydorus sphaericus

Eutrophication in Tjeukemeer involved a gradualincrease in chlorophyll concentrations from ca. 30 mg m^-3in 1968–69 to 125 mg m^-3 in 1976.From 1976 onwards, chlorophyll concentrations remainedat a high level fluctuating between100–225 mg m^-3. Hillbricht-Illkowska (1977)hypothesized that small-bodied species will becomeincreasingly abundant and dominant over large-bodiedspecies with increasing eutrophication. We tested thishypothesis using observations from life historyexperiments on Chydorus sphaericus, combinedwith data from 25 years of field observations on thepopulation dynamics of cladocerans in Tjeukemeer.In life history experiments with C. sphaericus,the fitness measure r in treatments with naturallake seston and laboratory cultured green algae wassignificantly higher on lake seston from Tjeukemeer,containing a high proportion of detritus. Thissuggests that detrital particles are good quality foodfor C. sphaericus. Field observations during theperiod 1968–1976 showed that all three categories ofcladocerans: C. sphaericus,'other' small-bodiedcladocerans (predominantly Bosmina spp.) andlarge-bodied cladocerans (predominantly Daphniagaleata), increased in biomass with increasingchlorophyll concentration. However, of these threecladoceran categories only sphaericus showeda distinct and significant increase whereas the othertwo only showed a marginally significant increase.During the period 1977–1992, both 'other' small-bodiedcladocerans and C. sphaericus significantlydecreased in biomass with increasing chlorophyllconcentration, whereas the biomass of the large-bodiedcladocerans significantly increased with increasingchlorophyll content. These observations are not inagreement with the hypothesis that small-bodiedzooplankton become increasingly abundant withincreasing eutrophication. We suggest that theobserved trends are partially caused by a food effect,and partially caused by predation pressure. Daphnia shows a better response to the increase indetritus and filaments of Cyanobacteria thansmall-bodied cladocerans, but is more vulnerable tofish predation. Densities of 0+ zooplanktivorous fishshow strong annual fluctuations in Tjeukemeer, andbecause of hydrological conditions, 0+ fish abundancein this lake is probably negatively related tochlorophyll content.     

不同生境间龟鳖类体型的差异

体型是动物重要的形态特征,影响动物的生境利用。为揭示龟鳖类体型与生境之间的关系,通过文献收集331种龟鳖（龟鳖目Tesudines总物种数的98.8%）的最大背甲长及其生境信息,将生境分为海洋、淡水、岛屿性陆地和大陆性陆地4种类型,再将淡水生境分为大静水、大流水、小静水、小流水和所有水域5种亚类型,大陆性陆地生境分为高地、平地和荒漠3种亚类型,从而比较不同生境类型或亚类型之间龟鳖类体型的差异。广义线性混合模型分析结果显示：1）海龟体型最大,岛屿性陆龟次之,淡水龟鳖和大陆性陆龟体型最小,且后两者差异无统计学意义。2）淡水龟鳖类的体型在5种亚类型生境间存在差异,大静水和大流水水域的体型均显著大于小静水和小流水水域,而体型在大静水与大流水水域、小静水与小流水水域之间的差异均无统计学意义,表明淡水龟鳖类体型与水域面积有关,而与水域是静水或流水无关。广布所有水域的淡水龟鳖类体型趋于中间型,且与其他4种亚类型生境中的体型之间的差异均无统计学意义。3）大陆性陆龟的体型从高地到平地再到荒漠有逐渐变大的趋势,但差异无统计学意义。本研究揭示龟鳖类的保护对策需要考虑其体型和生境面积的相关性。     

Potential for climate effects on the size-structure of host–parasitoid indirect interaction networks

Communities of insect herbivores are thought to be structured mainly by indirect processes mediated by shared natural enemies, such as apparent competition. In host–parasitoid interaction networks, overlap in natural enemy communities between any pair of host species depends on the realized niches of parasitoids, which ultimately depend on the foraging decisions of individuals. Optimal foraging theory predicts that egg-limited parasitoid females should reject small hosts in favour of future opportunities to oviposit in larger hosts, while time-limited parasitoids are expected to optimize oviposition rate regardless of host size. The degree to which parasitoids are time- or egg-limited depends in part on weather conditions, as this determines the proportion of an individual''s lifespan that is available to foraging. Using a 10-year time series of monthly quantitative host–parasitoid webs, we present evidence for host-size-based electivity and sex allocation in the common secondary parasitoid Asaphes vulgaris. We argue that this electivity leads to body-size-dependent asymmetry in apparent competition among hosts and we discuss how changing weather patterns, as a result of climate change, may impact foraging behaviour and thereby the size-structure and dynamics of host–parasitoid indirect interaction networks.     

Evolving ecological networks and the emergence of biodiversity patterns across temperature gradients

In ectothermic organisms, it is hypothesized that metabolic rates mediate influences of temperature on the ecological and evolutionary processes governing biodiversity. However, it is unclear how and to what extent the influence of temperature on metabolism scales up to shape large-scale diversity patterns. In order to clarify the roles of temperature and metabolism, new theory is needed. Here, we establish such theory and model eco-evolutionary dynamics of trophic networks along a broad temperature gradient. In the model temperature can influence, via metabolism, resource supply, consumers' vital rates and mutation rate. Mutation causes heritable variation in consumer body size, which diversifies and governs consumer function in the ecological network. The model predicts diversity to increase with temperature if resource supply is temperature-dependent, whereas temperature-dependent consumer vital rates cause diversity to decrease with increasing temperature. When combining both thermal dependencies, a unimodal temperature-diversity pattern evolves, which is reinforced by temperature-dependent mutation rate. Studying coexistence criteria for two consumers showed that these outcomes are owing to temperature effects on mutual invasibility and facilitation. Our theory shows how and why metabolism can influence diversity, generates predictions useful for understanding biodiversity gradients and represents an extendable framework that could include factors such as colonization history and niche conservatism.