Despite their relevance for risk assessment, the interactive effects of pesticide and predation cues are poorly understood because the underlying behavioral and physiological mechanisms are largely unknown. To explore these mechanisms, we reared larvae of the damselfly Coenagrion puella at three different predation risk levels and a range of environmentally realistic concentrations of three pesticides used worldwide (atrazine, carbaryl, and endosulfan). We compared key development responses (growth rate, developmental time, and final size) against food ingestion, assimilation, and conversion efficiency, and acetylcholinesterase (AChE) activity. Predation risk impaired all endpoints, including AChE activity, while the effects of pesticide stress were smaller for atrazine and endosulfan and absent for carbaryl. The effects of both stressors and their interaction on life history were mostly indirect through resource acquisition and energy allocation. Compensatory physiological mechanisms to pesticide stress (atrazine and endosulfan) were present in larvae reared in the absence of predation stress but were offset under predation stress. As a result, smaller size (atrazine and endosulfan) and lower growth rate (endosulfan) from pesticide stress were only found in the highest predation risk treatment. Our results provide insight as to the conditions under which interactions between stressors are likely to occur: damselfly populations at high density and living in fish ponds will be more affected by pesticides than populations at low densities in fishless ponds. By identifying variables that may shape the interaction between predation stress and other stressors such as pesticides, our mechanistic approach may help to bridge the gap between laboratory and field studies.

The River Continuum Concept (RCC) explains the structural and functional characteristics of stream communities focusing on the gradually changing physical components from headwaters to downstream habitats of streams and rivers. The global value of the RCC is still uncertain, possibly because physical factors (e.g., altitude and then temperature, stream order, channel width) can vary differently in longitudinal river axes across the world. Moreover, RCC predictions in relation to different physical factors have not been tested adequately in different biomes, especially biomes outside of temperate North America. Here, we report on the functional structure of macroinvertebrate communities in neotropical streams from Bolivia along a broad altitudinal gradient (from 1120 to 4300 m a.s.l.), aiming to understand how altitude can affect the longitudinal changes in functional feeding groups (FFG) and richness predicted by the RCC. The RCC predictions for functional structure were not completely matched when analyzing FFGs in relation to an index of longitudinal stream gradient. However, after removing the effect of altitude by using residuals from regressions between FFGs and altitude, FFG patterns matched RCC predictions more closely. We detected significant relationships between altitude and the relative abundance of collector-gatherers, shredders and scrapers which may be related to changes in temperature, UV radiation and canopy cover along the altitudinal gradient. Our results indicate that altitude combined with position along the longitudinal gradient is an important factor governing the FFG structure of macroinvertebrate communities in neotropical streams.

1 Mass at emergence is a life-history trait strongly linked to adult fitness. Therefore, when faced with transient food shortage in the larval stage, mass-correcting mechanisms are common. 2 These correcting mechanisms may carry costs with them. On one hand, these costs may be overestimated because they can be confounded with the direct effects of the transient food shortage itself. On the other hand, costs may be underestimated by ignoring physiological costs. Another largely neglected topic is that correcting mechanisms and costs may critically depend upon other stressors that often co-occur. 3 Here, we identify the mass-correcting mechanisms and their associated costs at emergence in the damselfly Coenagrion puella, after being stressed by a transient period of starvation and a subsequent exposure to pesticide stress during the larval stage. We introduce path analysis to disentangle direct costs of starvation and the mass-correcting mechanisms in terms of immune response. 4 As predicted, we found no differences in mass at emergence. Starvation directly resulted in a costly delayed emergence and a decreased immune response at emergence. Mass-correcting mechanisms included a prolonged post-starvation period, reduced mass loss at emergence and compensatory growth, although the latter only in females under pesticide stress. 5 The mass-correcting mechanisms were associated with beneficial effects on investment in immune response, but only in the absence of pesticide stress. Under pesticide stress, these beneficial effects were mostly undone or overruled, resulting in negative effects of the mass-correcting mechanisms in terms of immune response. 6 Our results stress the importance of and introduce a statistical way of disentangling direct costs of starvation and the mass-correcting mechanisms themselves, and the importance of including physiological endpoints in this kind of studies.