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Disturbance legacies and nutrient limitation influence interactions between grazers and algae in high elevation streams

机译：干扰遗留物和养分限制影响高海拔河流中放牧者与藻类之间的相互作用

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Debate about control of interaction strength among species is fueled by variation in environmental contexts affecting food webs. We used extensive surveys and two field experiments to test the individual and interactive influences of variation in the assemblages and associated traits of grazers as shaped by the legacy of disturbance, nutrient limitation and the presence of top predators on the accrual of basal resources. We quantified hydrologic variation and streambed movement to describe the legacy of disturbance and sampled biota of 20 streams over five years in a high‐elevation catchment in Colorado, USA. Grazer assemblages switched from caddisfly‐dominated to mayfly‐dominated as disturbance increased. We manipulated the composition of grazer assemblages and the availability of nutrients (N and P) within flow‐through mesocosms assembled adjacent to 10 streams, and also deployed larger in‐stream channels manipulating the presence of top predators (brook trout) in five streams varying in disturbance regimes. In both experiments we compared the rate of accrual of benthic algae and the strength of grazer‐algal interactions among treatments. We observed no indirect effects of top predators on grazer mobility, grazer consumption of algae, or accrual of algal biomass (no trophic cascades). However, in both experiments accrual rates of algae yielded a unimodal pattern and grazer impacts on algae decreased with increasing disturbance, but only at ambient (limiting) nutrient conditions. When nutrients were amended in the mesocosm experiment, algal accrual was uniformly high and grazer impacts on algae were consistently low. Reduced algae accrual at high disturbance levels may be explained by direct effects of environmental harshness on algae, and at low disturbance by indirect effects on grazer traits (behaviors) rather than on grazer density. In more benign streams per capita and per unit biomass grazer impacts on algae were high and drift dispersal was low, both behaviors that reduced accrual of algae. We conclude that nutrient limitation and indirect effects of disturbance on accrual of algae mediated by grazer traits can be stronger than indirect effects of predators on algae, providing a new contribution to the debate about the influence of environmental context on the strength of food web interactions.
机译：有关影响食物网的环境环境的变化加剧了关于物种间相互作用强度控制的争论。我们使用了广泛的调查和两个野外试验来测试放牧者的组合和相关性状的变异对个体的影响和互动影响，这些变异是由干扰，养分限制和顶级掠食者的存在对基础资源积累产生的。我们对水文变化和河床运动进行了量化，以描述美国科罗拉多州一个高海拔流域五年中20条河流的扰动和采样生物群的遗产。随着扰动的增加，Grazer组件从以虫蝇为主的组合转变为以虫蝇为主的组合。我们操纵了掠食者组合的组成以及在与10条溪流相邻的直通大肠内的营养物质（N和P）的利用率，还部署了较大的溪流通道，以控制5条溪流中顶级捕食者（溪鳟）的存在在骚乱制度下。在这两个实验中，我们比较了底栖藻类的增生率和处理之间食草藻与藻类相互作用的强度。我们没有观察到顶级掠食者对放牧者的活动性，放牧者的藻类消耗或藻类生物量的累积（无营养级联）没有间接影响。但是，在两个实验中，藻类的累积率均产生单峰模式，并且放牧者对藻类的影响会随着干扰的增加而降低，但仅在环境（有限）营养条件下才会发生。在介观实验中对营养物进行修正时，藻类的累积量始终很高，而放牧者对藻类的影响始终很低。高干扰水平下的藻类累积减少可能是由于环境恶劣对藻类的直接影响，而在低干扰下是由于对放牧特性（行为）而不是放牧者密度的间接影响。在更多的良性河流中，人均和单位生物量放牧者对藻类的影响较高，而漂移的扩散较低，这两种行为都会减少藻类的积累。我们得出的结论是，掠食者性状介导的养分限制和干扰对藻类积累的间接影响可能强于捕食者对藻类的间接影响，这为有关环境对食物网相互作用强度影响的辩论提供了新的贡献。

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