Ecological Entomology (2012), 37, 508–520 DOI: 10.1111/j.1365-2311.2012.01393.x Lethal and sublethal behavioural responses of saline water beetles to acute heat and osmotic stress SUSANA PALLAR ´ ES, PAULA ARRIBAS, VANESSA C ´ ESPEDES, ANDR ´ ES MILL ´ A N and J O S E F A V E L A S C O Departamento de Ecología e Hidrología, Universidad de Murcia, Murcia, Spain Abstract. 1. As species’ physiological breadth determines their potential to deal with environmental changes, and influences individuals’ survival and the persistence of populations, information about lethal and sublethal responses could be fundamental for conservation purposes. 2. We used a standard experimental approach to explore mortality and behavioural avoidance responses (i.e. flight and emersion from the water) to a combination of acute heat and osmotic stress on six species of saline water beetles (belonging to Enochrus, Nebrioporus, and Ochthebius genera). 3. Heat stress affected survival and behavioural responses in all of the species, whereas osmotic stress and the interaction between both stressors only showed significant effects for the Ochthebius genus. Behavioural and survival patterns were highly interrelated across the stress gradients. The Enochrus and Nebrioporus studied species showed maximum avoidance activity at 35–40 ◦ C, and a short (< 30 min) exposure to 45 ◦ C was lethal. Ochthebius species were the most heat tolerant and displayed increasing behavioural responses with increasing temperature. In the Nebrioporus and Ochthebius genera, the species occupying lotic, more environmentally stable habitats, showed greater mortality, and avoidance responses were higher or initiated at lower stress thresholds than lentic species. In contrast, both Enochrus species displayed a similar mortality, and the lentic species E. bicolor emerged and flew more than the lotic E. falcarius, in concordance with its higher dispersal capacity. 4. Avoidance responses could provide interesting information about species’ physiological amplitudes as a complement to lethal responses. The lotic species here studied showed narrower physiological amplitude (i.e. N. baeticus and O. glaber ) or lower dispersal ability (i.e. E. falcarius ) than their lentic relatives; both traits could result in a higher vulnerability of lotic species to thermal habitat changes. Key words. Behavioural avoidance responses, global change, habitat stability, heat stress, osmotic stress, physiological breadth, saline habitats, stress tolerance, water beetles. Introduction Understanding the ways in which organisms deal with and respond to environmental changes is of considerable importance in determining past and present processes affecting species (Chown, 2001). Species’ physiology defines the breadth of fundamental niches (Gaston, 2003) and so, has Correspondence: Susana Pallar´ es P´ arraga, Departamento de Ecol- ogía e Hidrología, Universidad de Murcia, Murcia 30100, Spain. E-mail: susana.pallares@um.es been identified as relevant when forecasting the effects of habitat modification on species and population viability (e.g. Tewksbury et al., 2008; Gaston et al., 2009; Helmuth, 2009), particularly in the current context of global warming and stressed biodiversity loss (Deutsch et al., 2008; Bozinovic et al., 2011). Recent studies have shown that laboratory- determined species’ physiological amplitudes are a good approximation to species fitness under natural changes in their habitats (Gaston & Spicer, 2001; Deutsch et al., 2008; Barnes et al., 2010). As a result, many studies examining the effects 508 © 2012 The Royal Entomological Society