The decline of sensitive species in agricultural streams is mainly attributed to pesticide contamination, even below the regulatory acceptable concentrations. Very low toxic pressure may thus determine ecological and evolutionary processes responsible for the current loss in biodiversity, and leading to adaption at community and individual level. In order to improve the current risk assessment, this dissertation aims to analyze factors that may shape the response of organisms to chemicals in the field.

To analyze the effect of long-term exposure to low pesticide concentrations in natural populations, a field investigation was conducted (Chapter 2). We observed that populations from contaminated streams were up to 2.5-fold more tolerant to clothianidin. However, populations showing increased insecticide tolerance were characterized by reducedsurvival, per capita growth and mating when cultured under pesticide free conditions.

Given that multi-stress conditions may occur more often under global change scenarios, the adaptation to one stressor might shape the response to another stressor (Chapter 3). We observed that agricultural populations are on average 2-fold more tolerant to insecticide clothianidin as compared to reference populations. After experimental pre-exposure to very low concentration (LC50/ 1000), only reference populations showed increased pesticide tolerance. Under multiple stress of pesticides and elevated temperature, both reference and agricultural populations showed a similar tolerance to the combined stress of pesticides and warming due to stronger synergistic effects in adapted populations. However, agricultural populations were more sensitive to elevated temperature alone due to the hypothesized fitness cost of genetic adaptation to pesticides and as a result, pesticide adaptation loses its advantage.

Although pesticide tolerance enables the survival of tolerant species incontaminated streams, long-term exposure to pesticides may alter theirgenetic structure (Chapter 4). G. pulex collected from 38 small streams showed that pesticide exposure increased the pesticide tolerance, reduced the genetic diversity, resulted in an adopted genetic composition and compromised individual fitness in locally adapted populations. Specifically, an increased frequency of “high contamination alleles” and a decrease of “low contamination alleles” was observed with increasing contamination. Furthermore, the individual per capita growth decreased with increasing trade-offs of genetic adaptation. Nevertheless, G. pulex contributed an average of 44% of macroinvertebrate abundance and benefited from reduced interspecific competition with vulnerable species in contaminated streams.

Condering global change scenario and persistent stress leading toadaptation, the question arises: How can the combined effects of theseapparently contradictory processes be predicted (Chapter 5)? We show that pesticide adapted G. pulex from agricultural streams were more tolerant to pesticides (clothianidin, prochloraz) as compared to nonadapted populations. However, joint exposure to both pesticides and temperature stress resulted in acute synergistic interactions, and the combined effects were stronger in adapted populations. We hypothesize that the pesticide adaptation reduces general stress capacity of individuals and trade-off process increases sensitivity to the combined stress. The general stress exerted by each of the individual factors was quantified using the Stress Addition Model (SAM). These studies showed that pesticide pollution triggers adaptation from sub-organismal to community level. Unraveling these processes explains effects from genes to ecosystem level.