Intraspecific variation in the consumption of exotic prey – a mechanism that increases biotic resistance against invasive species?

1. Hyper-successful exotic species can both displace the native prey that formerly made up a native predator's diet and represent an abundant potential prey resource for native predators. Little is known about how this drastic change affects native predators, or their short- and long term potential to regulate the exotic species.
2. We compared zebra mussel consumption by pumpkinseed sunfish ( Lepomis gibbosus ), redbreast sunfish ( Lepomis auritus ) and rock bass ( Ambloplites rupestris ) from populations that were either previously exposed to zebra mussels or naive to them.
3. Fish from populations with longer exposure to zebra mussels consumed many more zebra mussels than fish from populations with shorter or no previous exposure to zebra mussels.
4. Our experiment does not allow us to identify the mechanisms that underlie the patterns we found, but we discuss several plausible scenarios and their ecological implications.
5. Predator adaptation to exotic prey may be an important but overlooked factor in invasion biology. The initial response to exotic prey by a native predator may be a poor estimate of its ability to present biotic resistance to the invasion over the long term.     

Identification of 2-phenylethanol with a rose-like odor from anal sac secretions of the small Indian mongoose (Herpestes auropunctatus)

The small Indian mongoose (Herpestes auropunctatus) is an invasive species in Okinawa and Amami-Oshima, Japan. Major strategies for their eradication have been the use of baited traps, which suffer from decreasing efficiency with declining populations and the bycatch of native animals. To address these concerns, mongoose-specific lures are required. In this study, we aimed to identify species- and/or sex-specific compounds from anal sac secretions of small Indian mongooses. Volatile compounds emitted from male and female mongoose anal sac secretions were analyzed by thermal desorption-gas chromatography-mass spectrometry. In addition to several fatty acids, 2-phenylethanol was identified as a minor compound, which is uncommon in mammalian secretions but a dominant odorant in roses. Female samples emitted higher levels of 2-phenylethanol than male samples did. These findings indicate that 2-phenylethanol is a female-specific volatile compound of anal sac secretions in small Indian mongooses, and it may be useful as an ingredient of mongoose-specific scent lures.     

Can Auditory Playback Condition Predator Mobbing in Captive-reared <Emphasis Type="Italic">Saguinus oedipus</Emphasis>?

Captive reintroductions often suffer high mortality, with predation as one source. Many species learn about predators; thus training captive-born animals to recognize predators may increase survivorship. We adapted variants of methods developed for birds to attempt to condition monkeys to mob a predator. Captive-reared cotton-top tamarins (Saguinus oedipus) did not differentiate between a snake and a rat, confirming previous research that naïve cotton-top tamarins do not fear snakes. Tamarins then viewed a living snake during playback of mobbing calls. In posttests, tamarins did not mob the snake. We then eliminated the pretest to avoid potential habituation and placed a stillborn tamarin with the snake in an attempt to heighten arousal. In posttests, tamarins again did not mob the snake, although they did increase their rates of calls, indicating mild arousal. Overall, none of 8 groups of cotton-top tamarins learned to mob a predator. Potential reasons for failure include conditioning multiple subjects at once and the lack of an experienced demonstrator. Observing a demonstrator was not necessary for birds to acquire mobbing, but may be necessary for tamarins.     

Rubidium labelling demonstrates movement of predators from native vegetation to cotton

We employed rubidium labelling to track the movement of arthropod natural enemies from a shelterbelt into an adjacent cotton field. Findings demonstrate that Dicranolaius bellulus (Coleoptera: Melyridae) and Oxyopes spp. (Araneae: Oxyopidae) move from the shelterbelt into the crop interior. Capture of rubidium-marked arthropod-predators within the cotton field provides the first hard evidence that semi-natural perennial habitats such as shelterbelts on cotton farms in Australia act as a resource for arthropod predators in adjacent crops.     

Why are some weta (Orthoptera: Stenopelmatidae) vulnerable yet others are common?

The large (4 g) to very large (40 g) stenopelmatid orthopterans of New Zealand are known collectively as weta. A consideration of 20 species of Hemideina, Deinacrida and tusked weta reveals that at one end of a vulnerability gradient are those species which thrive in the presence of key predators (rats), while at the other end are species that have become extinct on the mainland but still survive on predator-free island refuges. Habitat modification does not appear to be a factor in these extinctions. This paper reviews the lifestyles and some important biotic parameters that seem to determine their relative vulnerability along this gradient. When predators are present, the factors leading to extinction are large body size, use of temporary refuges, protective quality of the refuges, time spent on the ground and the effectiveness of their defensive behaviour.     

Antipredator tactics: a kin-selection benefit for defensive spines in coral catfish?

Morphological features that impair a predator's ability to consume a prey item may benefit individual prey; but what of features that prolong prey-handling but do not enhance prey survival? For example, a striped eel catfish Plotosus lineatus will be fatally envenomated if struck by its specialist predator, the greater sea snake Hydrophis major. Nonetheless, the catfish typically erects long, toxic pectoral and dorsal spines that increase prey-handling times for the snake by around eightfold. Because the catfish travel in swarms of closely-related individuals, the delay enforced by spines may enable the victim's swarm-mates to disperse before the snake is able to search for another meal. In keeping with that hypothesis, defensive spines tend to be longer in catfish from regions where the greater sea snake occurs, than from areas where the snake does not occur.     

Responses of tadpoles to hybrid predator odours: strong maternal signatures and the potential risk/response mismatch

Previous studies have established that when a prey animal knows the identity of a particular predator, it can use this knowledge to make an ‘educated guess'' about similar novel predators. Such generalization of predator recognition may be particularly beneficial when prey are exposed to introduced and invasive species of predators or hybrids. Here, we examined generalization of predator recognition for woodfrog tadpoles exposed to novel trout predators. Tadpoles conditioned to recognize tiger trout, a hybrid derived from brown trout and brook trout, showed generalization of recognition of several unknown trout odours. Interestingly, the tadpoles showed stronger responses to odours of brown trout than brook trout. In a second experiment, we found that tadpoles trained to recognize brown trout showed stronger responses to tiger trout than those tadpoles trained to recognize brook trout. Given that tiger trout always have a brown trout mother and a brook trout father, these results suggest a strong maternal signature in trout odours. Tadpoles that were trained to recognize both brown trout and brook trout showed stronger response to novel tiger trout than those trained to recognize only brown trout or only brook trout. This is consistent with a peak shift in recognition, whereby cues that are intermediate between two known cues evoke stronger responses than either known cue. Given that our woodfrog tadpoles have no evolutionary or individual experience with trout, they have no way of knowing whether or not brook trout, brown trout or tiger trout are more dangerous. The differential intensity of responses that we observed to hybrid trout cues and each of the parental species indicates that there is a likely mismatch between risk and anti-predator response intensity. Future work needs to address the critical role of prey naivety on responses to invasive and introduced hybrid predators.     

Habitat structural complexity mediates the foraging success of multiple predator species

We investigated the role of freshwater macrophytes as refuge by testing the hypothesis that predators capture fewer prey in more dense and structurally complex habitats. We also tested the hypothesis that habitat structure not only affects the prey-capture success of a single predator in isolation, but also the effectiveness of two predators combined, particularly if it mediates interactions between the predators. We conducted a fully crossed four-factorial laboratory experiment using artificial plants to determine the separate quantitative (density) and qualitative (shape) components of macrophyte structure on the prey-capture success of a predatory damselfly, Ischnura heterosticta tasmanica, and the southern pygmy perch, Nannoperca australis. Contrary to our expectations, macrophyte density had no effect on the prey-capture success of either predator, but both predators were significantly less effective in the structurally complex Myriophyllum analogue than in the structurally simpler Triglochin and Eleocharis analogues. Furthermore, the greater structural complexity of Myriophyllum amplified the impact of the negative interaction between the predators on prey numbers; the habitat use by damselfly larvae in response to the presence of southern pygmy perch meant they captured less prey in Myriophyllum. These results demonstrate habitat structure can influence multiple predator effects, and support the mechanism of increased prey refuge in more structurally complex macrophytes.     

Predictors of clutch predation of a globally significant avifauna in New Zealand's braided river ecosystems

Predation by introduced mammals is decimating New Zealand's indigenous fauna. Understanding factors that influence this process allows resources for predator control to be applied with maximum effect. This study examines how predation of a secondary prey species (a relatively common but declining native plover, the banded dotterel Charadrius bicinctus ) varied with reductions in abundance of a major prey source (rabbits), kill-trapping of predators, nest density and habitat complexity. Banded dotterels mostly nest in open braided riverbeds alongside a number of endemic threatened species. We measured the fate of 753 dotterel clutches exposed to predation by cats, ferrets and hedgehogs. We found key times and places of high predation risk. Immediately after widespread reduction in rabbit populations by rabbit haemorrhagic disease (RHD), clutch predation rates were almost as high (mean, 50%) as those recorded during past rabbit poisoning programmes (mean, 57%). Both rates were significantly higher than the mean predation rate of 22% without rabbit control, suggesting a shift in predator diet immediately after rabbit population declines. Unlike after rabbit poisoning, clutch predation rate remained high in the years after RHD. Other patterns observed included higher clutch predation rate where nest density was lower, suggesting that predation can potentially cause local extinction. Clutch predation was also higher along riverbed margins where vegetation was dense. There was equivocal evidence for an effect of predator kill-trapping on clutch predation rate. Management strategies that could potentially reduce clutch predation risks include focusing predator mitigation measures during periods of rabbit decline, maintaining them for more than one breeding season if the rabbit declines are widespread (e.g. RHD epidemics), and applying greater effort at sites with relatively low nest density and along riverbed margins where predator use is more frequent.     

A biomechanical constraint on body mass in terrestrial mammalian predators

Observations on extant mammals suggest that large body mass is selectively advantageous for a terrestrial predator on large herbivores. Yet, throughout the Cenozoic, some lineages of terrestrial mammalian predators attained greater maximal body masses than others. In order to explain this evolutionary pattern, the following biomechanical constraint on body mass is hypothesized. The stress, set up in the humerus by the bending moment of the peak ground reaction force at maximal running speed, increased with increasing body mass within a given lineage of terrestrial mammalian predators, resulting in a decreasing safety factor for the bone, until a predator could no longer attain the maximal running speed of its smaller relatives. The selective disadvantage of reduced maximal running speed prevented further increase of body mass within the lineage. This hypothesis is tested by examining the scaling of humeral dimensions and estimating maximal body masses in several lineages of terrestrial mammalian predators. Among lineages with otherwise similar postcranial skeletons, those with the more robust humeri at a given body mass attained the greater maximal body masses. Lineages with the longer deltoid ridges/deltopectoral crests of the humeri and/or the more distally located deltoid scars (suggesting the more distal insertions of the humeral flexors) at a given body mass also attained the greater maximal body masses. These results support the existence of the proposed biomechanical constraint, although paleoecological data suggest that some lineages of terrestrial mammalian predators failed to reach the limits, imposed by this constraint, because of the small size of available prey.