An enemy of your enemy is your friend: Impact of predators on aggregation behavior of gammarids

Alpine grassland degradation intensifies the burrowing behavior of small mammals: evidence for a negative feedback loop

Globally, grassland degradation is an acute ecological problem. In alpine grassland on the Tibetan Plateau, increased densities of various small mammals in degraded grassland are assumed to intensify the degradation process and these mammals are subject to lethal control. However, whether the negative impact of small mammals is solely a result of population size or also a result of activity and behavior has not been tested. In this study, we use plateau pika as a model to compare population size, core area of colony, and the number of burrow entrances and latrines between lightly and severely degraded grassland. We test whether the alleged contribution of pika to grassland degradation is a result of increased population size or increased burrowing activities of individuals in response to lower food abundance. We found that grassland degradation resulted in lower plant species richness, plant height, and biomass. Furthermore, the overall population size of pika was not significantly affected by location in lightly and severely degraded grassland. However, pika core areas in severely grassland degradation were significantly larger and had significantly higher densities of burrows and latrines. Our study provides convincing evidence that habitat-induced changes in the behavior of small, burrowing mammals, such as pika, can exacerbate grassland degradation. This finding has significant implications for managing small mammals and restoring degraded grassland ecosystems.

Both Light Stimuli and Predation Risk Affect the Adult Behavior of a Stygobiont Crustacean

Stygobiont species show common, typical traits derived from their adaptation to subterranean life. Due to the general absence of light in cave environments, the majority of them are eyeless. Although the absence of eyes generally does not allow them to perceive luminous stimuli, some stygobionts still present phototaxis. Previous studies determined that different species of the eyeless amphipod crustaceans of the genus Niphargus are able to react to light; this has been interpreted as an adaptation to avoid dangerous surface habitats, even if recent studies suggest that this could also be an adaptation to exploit them when a situation is less dangerous (i.e., during the night). Niphargus thuringius is a stygobiont amphipod that can also be observed in spring environments despite possessing all the main morphological features of subterranean organisms, such as depigmentation and a lack of eyes. In the present study, we test how the species respond to light stimuli according to the light cycle and predation risk experienced during a conditioning period. We assessed the reactions to light stimuli of adult individuals of N. thuringius after 30 days of rearing in microcosms with different conditions of light occurrence (total darkness or a light/darkness daily cycle) and predation risk (without predators, with one predator, and with two predators). Both light stimuli during the test and rearing conditions affected the behavior of Niphargus thuringius. With light stimuli, individuals presented a strong photophobic response. Moreover, individuals reared in conditions of high predation risk preferred a more sheltered environment during behavioral tests than individuals reared in safe conditions. Our results add a new species to those of stygobiont amphipods known to display negative phototaxis, confirming that this pattern is widespread and conserved in the field. N. thuringius could be a good candidate model to perform further studies aiming to assess if differences occur between spring populations and populations present in deeper groundwater.

On the importance of concomitant conditions: Light and conspecific presence modulate prey response to predation cue

Assessment of individual costs of the anti-predator defence translating into changes in population parameters is meagre. This is because prey responses are likely to be modulated by additional factors, commonly present in the environment, but often neglected in experimental studies. To evaluate the effect of external factors on prey behaviour and physiology, we exposed amphipods Dikerogammarus villosus and Gammarus jazdzewskii to the predation cue of Perca fluviatilis in different densities and light conditions. Singletons of both species exposed to the predation cue in light modified their oxygen consumption (D. villosus: reduction, G. jadzewskii: increase) compared to their respiration in predator-free conditions. However , in the presence of conspecifics or in darkness, their respiration became insensitive to the predation cue. On the other hand, the swimming activity of prey was reduced in the presence of the predation cue irrespective of prey density and light conditions, but singletons were consistently more active than groups. Thus, external factors, such as conspecifics and darkness, constantly or periodically occurring in the field, may reduce the costs of predator non-consumptive effects compared to the costs measured under laboratory conditions (in light or absence of conspecifics). Moreover, we showed that behavioural and physiological parameters of prey may change differently in response to predation risk. Thus, conclusions drawn on the basis of single defence mechanisms and/or results obtained in artificial conditions, not reflecting the environmental complexity, strongly depend on the experimental design and endpoint selection and therefore should be treated with care.

Chronic predation risk affects prey escape abilities through behavioral and physiological changes

One of the options to reduce predation risk is reallocation of energy into locomotion system. The higher aerobic capacity, the more energy can be partitioned into an escape. Thus, increase in aerobic capacity can increase prey escape abilities. We investigated prey (freshwater crustaceans: Dikerogammarus villosus and Gammarus jazdzewskii) ability to improve their locomotor performance through an increase in aerobic capacity under chronic predation risk. We forced gammarids (pre-exposed to the predation cue or control) to long-distance swimming in the presence or absence of predation cues to obtain: control (pre-exposure and test in control conditions), acute (pre-exposure to control conditions, test in the presence of the predation cue) and chronic (pre-exposure and test in the presence of the cue) risk treatments. After this forced swimming effort, we measured various swimming parameters, glycogen content, and lactate concentration of the experimental amphipods. Exposure to predation cues made G. jazdzewskii reduce its speed after prolonged forced swimming in the presence of predation cues, whereas D. villosus never changed its swimming performance due to predation risk. In both species, post-effort lactate concentration was higher under the acute predation risk than in risk-free conditions. However, only D. villosus demonstrated lower lactate concentration when exposed to chronic vs. acute predation risk. Moreover, pre-exposure of both species to predation cues caused reduction in glycogen content. We showed that under chronic predation risk some prey species can modify their physiology to increase aerobic capacity and sustain high efficiency of escape performance. However, this phenomenon is species-specific and costly.

Does global warming intensify cost of antipredator reaction? A case study of freshwater amphipods

Global warming is a worldwide phenomenon affecting the functioning of diverse ecosystems, including fresh waters. Temperature increase affects physiology and behaviour of ectotherms due to growing energetic demands necessary to sustain increased metabolic rate. Anti-predator responses may resemble temperature-induced changes in organisms, suggesting synergism between these factors. To check how temperature shapes physiological and behavioural responses of ectotherms to predation risk, we exposed amphipods: Dikerogammarus villosus and Gammarus jazdzewskii to fish kairomones at 10, 17 or 24 °C. Animals were placed in tanks where temperature was gradually adjusted to the desired test temperature and acclimated under such conditions for 3 subsequent days. Then they were exposed to the predator cue (the Eurasian perch kairomone) for 35 min to test their acute responses. We measured metabolic rate (as respiration), antioxidant defence (CAT: catalase activity, TAS: total antioxidant status), oxidative molecules (TOS: total oxidative status), oxidative damage (TBARS: thiobarbituric acid reactive substances) and behaviour (locomotor activity). Amphipods increased respiration with raising temperature and when exposed to predation risk (all temperatures). Only G. jazdzewskii exhibited increased TOS when exposed to 24 °C or to predation risk at all temperatures. Antioxidant defence increased with raising temperature (CAT, TAS) and decreased under predation risk (CAT). Cellular damage increased in G. jazdzewskii under predation risk at 10 and 24 °C, but raised temperature itself did not generate any damage. Amphipods reduced locomotor activity at 24 °C. Thus, at elevated temperatures, amphipods minimized their cellular damage at the cost of increased antioxidant defence and lower locomotor activity (potentially disadvantageous under higher energetic demands). Under predation risk, the performance of antioxidant systems was reduced, probably due to energy allocation into anti-predatory mechanisms, leading to increased cellular damage at suboptimum temperatures. Thus, negative consequences of elevated temperature for organisms may be amplified by changes in behaviour (compromising food acquisition) and non-consumptive predator effects.

Substrate mediated predator–prey interactions between invasive crayfish and indigenous and non-native amphipods

The increasing number of taxa being translocated across the globe is leading to many non-native species encountering indigenous taxa as well as other non-native species. Environmental heterogeneity may strongly influence the spatial distribution, habitat use and refuge availability for these taxa. Using a series of 24-h mesocosm experiments we examined the predator–prey interactions between an invasive crayfish (Pacifastacus leniusculus) and four amphipod taxa, one indigenous (Gammarus pulex) and three non-native species (Crangonyx pseudogracilis, Dikerogammarus villosus and Gammarus tigrinus) to Great Britain. The potential mediating effect of physical habitat on predator–prey interactions was examined via the use of different substrate particle sizes; cobbles, gravels and, sand. Survivorship of amphipods in response to crayfish predation varied significantly with the highest rates recorded for the non-native species D. villosus, followed by G. tigrinus, and C. pseudogracilis, with the lowest survivorship recorded for the indigenous species G. pulex for all substrates except cobble. However, total biomass consumption of the indigenous G. pulex and the non-native D. villosus by P. leniusculus were similar suggesting that crayfish may have been satiated by larger D. villosus individuals. Substrate size had a significant influence on the predation success of P. leniusculus, with larger substrate clasts typically resulting in increased survivorship rates for all species except C. pseudogracilis, which displayed lower predation rates for sand substrates. The findings of this study highlight the risks that naïve indigenous taxa may face from new invasive species and the importance of characterising physical habitat (complexity and refugia potential) when considering the potential ecological effects of invaders on predation success.

Continuity of chronic predation risk determines changes in prey physiology

Prey reconfigure their physiology to avoid costs of prolonged predator pressure. However, these changes might not occur under periodic predation risk, with repeating acute phases. To test the effect of predation risk continuity on changes in prey physiology, we exposed amphipods: Dikerogammarus villosus and Gammarus jazdzewskii to periodic and constant predation cue. After one week, we measured: cellular defence systems: total antioxidant status (TAS), heat shock proteins (Hsp70); intracellular damage marker: lipid peroxidation (TBARS); condition index: glycogen concentration. Predator presence reduced TAS level in G. jazdzewskii independent of its continuity and in D. villosus after periodic exposure. Amphipods showed downregulation of Hsp70 when exposed to periodic (D. villosus) or constant (G. jazdzewskii) predation risk. Exposure to predators reduced TBARS level in D. villosus (irrespective of the continuity) and G. jazdzewskii (periodic exposure). Glycogen concentration in both species was not affected by predator presence. Thus, the continuity of the predator cue shaped prey physiology reconfiguration, optimizing costs of physiological adjustments under challenging conditions. Nevertheless, the lack of negative consequences of the prolonged exposure to the predator cue, whether constant or periodic, shows that amphipods can thrive under chronic predation risk, which is a constant part of the wild environment.

Experimental evidence for the adaptive response of aquatic invertebrates to chronic predation risk

As acute stress induced by predation risk can generate significant oxidative damage, prey organisms are forced to balance their defence reaction and the cost of activating the cellular defence system. Stress tolerance differs significantly among species; therefore predator pressure indirectly shapes the community structure. To test adaptation abilities of amphipod crustaceans (Dikerogammarus villosus and Gammarus jazdzewskii) we exposed them to acute (35 min.) and chronic (1 or 7 days) predation risk (the Eurasian perch). We measured respiration (related to metabolic rate), cellular defence systems (antioxidant enzyme (catalase) activity and heat shock protein (Hsp70) concentration), and the level of oxidative damage (thiobarbituric acid reactive substances (TBARS) concentration). Both amphipods increased their respiration rate in the presence of predation cues, irrespective of the duration of their pre-exposure to danger. This increase in D. villosus was initiated more quickly (immediately vs. after 10 min. of the test) and lasted for a longer time (20 vs. 10 min.) than in G. jazdzewskii. However, only G. jazdzewskii after a short exposure to predation risk exhibited an increase in its catalase activity, Hsp70 concentration and oxidative damage. No changes in these parameters were exhibited by D. villosus or after a chronic exposure of G. jazdzewskii to predation cues. Our results show that prey organisms are able to reconfigure their physiology to maintain increased metabolic rate under prolonged predator pressure and, at the same time, reduce oxidative damage as well as costs related to anti-oxidant defence.

Experimental evidence of chemical attraction in the mutualistic zebra mussel-killer shrimp system

Invasion facilitation, whereby one species has a positive effect on the establishment of another species, could help explain the rapid colonisation shown by some freshwater invasive species, but the underlying mechanisms remain unclear. We employed two-choice test arenas to test whether the presence of zebra mussel (Dreissena polymorpha) could facilitate the establishment of the killer shrimp (Dikerogammarus villosus). Killer shrimp preferred to settle on mats of zebra mussel, but this was unrelated to mat size, and was not different from attraction shown to artificial grass, suggesting that zebra mussel primarily provides substrate and refuge to the killer shrimp. Killer shrimp were strongly attracted to water scented by zebra mussel, but not to water scented by fish. Chemical attraction to the zebra mussel's scent did not differ between sympatric and allopatric populations of killer shrimp, suggesting that chemical attraction is not an acquired or learned trait. Our study shows, for the first time, chemical attraction between two highly invasive freshwater species, thereby providing a plausible mechanism for invasion facilitation. This has implications for managing the spread of killer shrimp, and perhaps other freshwater invasive species, because chemical attraction could significantly increase establishment success in mutualistic systems. Failure to consider invasion facilitation may underestimate the risk of establishment, and likely also the impact of some aquatic invaders.

The Braveheart amphipod: a review of responses of invasive Dikerogammarus villosus to predation signals

Predator pressure is a fundamental force driving changes at all levels of the community structure. It may protect native ecosystems from alien species. Therefore, resistance to diverse predators resulting from a universal anti-predator strategy seems crucial for invasion success. We present a comprehensive review of the responses of an invasive amphipod Dikerogammarus villosus to sympatric and allopatric predator signals. We summarize diverse aspects of the gammarid anti-predator strategy, including predator identification, morphological and behavioural adaptations, effectiveness of shelter use and resistance to indirect predator effects. The response of D. villosus is independent of predator species (including totally allopatric taxa), which assures the high flexibility of its predator recognition system. It has a harder exoskeleton and better capability of utilizing shelters compared to other gammarids, resulting in relatively high resistance to predators. Therefore, it can use predator kairomones as indirect food signals (sharing the diet with the predator) and follow the predator scent. This resistance may allow D. villosus to reduce the costs of its physiological responses to predators and sustain growth in their presence. This might facilitate invasion success by increasing its competitive advantage.