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Understanding the dynamics of species invasions in aquatic ecosystems is crucial for conservation and management efforts. We investigated the influence of species interactions and habitat complexity on biotic resistance to invasion by small-bodied freshwater fishes in peninsular Florida. Specifically, we focused on the interactions between two native species, Florida bass (Micropterus salmoides) and eastern mosquitofish (Gambusia holbrooki), and a common invader, the green swordtail (Xiphophorus hellerii). Our experiments included tanks with varying levels of structural complexity to mimic different habitat types. The presence of both native species significantly reduced swordtail survival, but the effect varied depending on habitat complexity. In habitats with strong predation refuge, mosquitofish facilitated bass predation on swordtails, whereas in habitats with weak predation refuge, bass suppressed mosquitofish aggression, leading to interference. Mosquitofish predominantly occupied vegetated areas and aggressively interacted with swordtails, significantly reducing invader survival. Our findings highlight the importance of considering species interactions and habitat complexity in predicting biotic resistance to invasions. We conclude that diverse interactions among native species can either enhance or impede invasion resistance, with implications for conservation and management strategies. Further research is needed to understand the broader impacts of multiple predators and competitors on invader dynamics in aquatic ecosystems.

In juvenile and adult animals, including mammals, birds, fishes and a crustacean, competition for food becomes increasingly aggressive as its spatial concentration increases. This ecological relationship has not been investigated in infant animals, although it is thought that broods of precocial chicks of some avian species compete aggressively for food or status in a brood hierarchy. When pairs of common quail broodmates were offered the same amount of ground corn in four spatial concentrations between ages 15 and 59 days, aggression increased progressively with concentration, culminating in an overall 16-fold increase when corn was in a single clump. These results suggest that aggressive defence may increase with spatial concentration of food in precocial chicks generally, and raise the possibility that a similar pattern could occur in some altricial chicks during the transition to independence and in infants of other animal species.

Florida is a hotspot for nonindigenous fishes with over 30 species established, although few of these are small-bodied species. One hypothesis for this pattern is that biotic resistance of native species is reducing the success of small-bodied, introduced fishes. The eastern mosquitofish Gambusia holbrooki is common in many freshwater habitats in Florida and although small-bodied (<50 mm), it is a predator and aggressive competitor. We conducted four mesocosm experiments to examine the potential for biotic resistance by eastern mosquitofish to two small-bodied nonindigenous fishes, variable platyfish ( Xiphophorus variatus ) and swordtail ( X. hellerii ). Experiments tested: (1) effect of eastern mosquitofish density on adult survival, (2) effect of eastern mosquitofish on a stage-structured population, (3) role of habitat structural complexity on nonindigenous adult survival, and (4) behavioral effects of eastern mosquitofish presence and habitat complexity. Eastern mosquitofish attacked and killed non-native poeciliids with especially strong effects on juveniles of both species. Higher eastern mosquitofish density resulted in greater effects. Predation on swordtails increased with increasing habitat complexity. Eastern mosquitofish also actively drove swordtails from cover, which could expose non-native fish to other predators under field conditions. Our results suggest that eastern mosquitofish may limit invasion success.

This study tests the hypothesis that prices in new markets change in a pattern similar to and related to the product life cycle. Three stages of price decline have been identified in electronic product markets and can be associated with the introduction, take‐off and growth, and maturity stages of the product life cycle. The study integrates the methodologies of the product life cycle concept and experience curve theory and analyses price and product behaviour for radios, monochrome (black and white) televisions, colour televisions and video cassette recorders. The empirical evidence supports the hypothesis and suggests, as a general rule, that a better understanding of a product's life cycle and the factors that affect it can aid in the development of price forecasts. In fact, the marriage of experience curve theory and the product life cycle concept provides a methodology for quantifying the impact of influential factors and gives the analyst the ability to bound the uncertainty around price forecasts, ultimately leading to better strategic decisions, today, for tomorrow's markets.

In fish, interspecific interactions between nonnative and other sympatric species are considered determinants in shaping species assemblages. Such interactions can also arise between nonnative fish species only, including salmonids such as the brown trout (Salmo trutta, Linnaeus, 1758) and the rainbow trout (Oncorhynchus mykiss, Walbaum, 1792), returning contrasting outcomes. The present manipulative experiment was aimed at exploring the effect of interspecific competition on the body growth and the oxidative status of parr (2 + -year-old individuals) of the brown trout and the rainbow trout. Allopatric (intraspecific competition) and sympatric (interspecific competition) populations of these species were experimentally recreated in two wild streams. At the end of a 2-month-long experiment, changes in specific growth rate (SGR), oxidative status (i.e., levels of reactive oxygen species and activity of antioxidant enzymes such as superoxide dismutase — SOD, catalase — CAT and glutathione peroxidase — GPx) and oxidative damage (i.e., lipid peroxidation) were investigated in brown and rainbow trout individuals maintained in allopatric or sympatric populations. Sympatric interactions between rainbow and brown trout parr resulted in a significant decrease in SGR of brown trout individuals only. Moreover, an overall modulation of the oxidative status, in terms of an increase in ROS levels coupled with the activation of SOD and CAT activity, occurred in brown trout individuals under sympatric conditions. These findings might suggest that, under sympatric conditions, parr of the rainbow trout are more competitive than brown trout for food acquisition. However, this competition affected the antioxidant defenses of the brown trout only, probably because of reduced ingestion of dietary antioxidants or increased physical activity and aggressive behavior. Thus, interspecific interactions can induce physiological and phenotypic effects on parr of nonnative salmonids, with potential consequences on the establishment of populations of these species in freshwater ecosystems.

Population expansion of the black‐winged kite Elanus caeruleus has been a global trend in recent decades. Since first recorded in Taiwan in 2001, they rapidly occupied plains areas of this subtropical island which has led to interspecific competition with the wintering Eurasian kestrel Falco tinnunculus. In this study, we analyzed historical surveys to estimate the population trends of both species in Taiwan. We then used Pianka's index to quantify the degree of niche overlap based on diet composition and foraging peaks. Finally, we designed an experiment to test the aggressiveness of the two raptors when facing conspecific or heterospecific intruders. Airport avifauna survey (2002–2017) data from eight different airports and eBird data (2000–2019) from 21 hotspot grids both showed population turnover between the two raptors, with a decline in the wintering Eurasian kestrel and an expansion of the black‐winged kite. Rodents were the largest share of prey for both species, indicating that the two raptors have a high degree of niche overlap (an overlap index of 0.74 by prey frequency, 0.97 by prey biomass and 0.89 by foraging time). The black‐winged kite exhibited higher boldness and aggressiveness toward intruders, with its aggressiveness toward kestrel specimens (2.9 ± 2.4 strikes) significantly higher than that toward kite specimens (1.1 ± 1.8 strikes). Attacks by the black‐winged kite may have prompted the Eurasian kestrel to choose other wintering grounds. As a result, the number of wintering kestrels decreased, leading to opposing population trends of the two species.

Body size often varies among insular populations relative to continental conspecifics – the ‘island rule’ – and functional, context‐dependent morphological differences tend to track this body size variation on islands. Two hypotheses are often proposed as potential drivers of insular population differences in morphology: one relating to diet and the other involving intraspecific competition and aggression. We directly tested whether differences in morphology and maximum bite capacity were explained by interisland changes in hardness of both available and consumed prey, and levels of lizard‐to‐lizard aggression among small‐island populations. Our study included 11 islands in the Greek Cyclades and made use of a gradient in island area spanning five orders of magnitude. We focused on the widespread lizard Podarcis erhardii. We found that on smaller islands, P. erhardii body size was larger, head height was larger relative to body size, and maximum bite capacity became proportionally stronger. This pattern in morphology and performance was not related to differences in diet, but was highly correlated with proxies of intraspecific aggression – bite scars and missing toes. Our findings suggest that critical functional traits such as body size and bite force in P. erhardii follow the predictions of the island rule and are changing in response to changes in the competitive landscape across islands of different sizes.

Biological invasions are a contemporary global threat because invasive species can have substantial negative economic and ecological impacts. Invasive species can outcompete native species through two main mechanisms: interference competition (direct, negative interactions like aggression) and/or exploitative competition (indirect, negative interactions resulting from species using the same, limited resources like food). The invasive Italian wall lizard (Podarcis siculus) was introduced into Lisbon, Portugal, 20 years ago, and is believed to be locally displacing the native green Iberian wall lizard (Podarcis virescens). We experimentally tested for competition between these two lizard species by establishing heterospecific (one pair of each species) and conspecific (two pairs of the same species; control) treatments in enclosures containing a high- and a low-quality refuge. Lizards were fed from food dishes every other day. We tested if species showed interference (aggressive behaviour, stealing food and shelter exclusion) or exploitative competition (tolerance between species but differences in food consumption efficiency). We found evidence for exploitative competition: the invasive species arrived first at food stations, consumed more food and gained more weight than the native species. We suggest that exploitative competition may, in part, explain the observed displacement of P. virescens from contact areas with the invasive P. siculus. Deciphering the competitive mechanisms between invasive and native species is vital for understanding the invasion process.

Fish social behavior can be affected by artificial environments, particularly by factors that act upon species that show aggressive behavior to set social rank hierarchy. Although aggressive interactions are part of the natural behavior in fish, if constant and intense, such interactions can cause severe body injuries, increase energy expenditure, and lead the animals to suffer from social stress. The immediate consequence of these factors is a reduced welfare in social fish species. In this paper, we consider the factors that impact on the social behavior and welfare of Nile tilapia, an African cichlid fish widely used both in fish farms and in research; this species is frequently used as a model for physiology and behavior research. This is a polygynous species whose males interact aggressively, establishing a territorial based hierarchy, where a dominant male and several subordinate males arise. When social stability is shrunk, the negative effects of prolonged fighting emerge. In this paper, we summarized how some of the common practices in aquaculture, such as classifying individuals by matching their sizes, water renewal, stock density, and environment lighting affect Nile tilapia social aggressive interactions and, in turn, impact on its welfare. We also discuss some ways to decrease the effects of aggressive interactions in Nile tilapia, such as environment color and body tactile stimulation.

Invasive species cause substantial changes to the biodiversity of freshwater systems. The African Nile tilapia (Oreochromis niloticus) is now widely distributed in tropical freshwaters globally. Despite indications that feral populations can influence native species through competitive effects, direct evidence of competition between Nile tilapia and native species is rare. Moreover, it is not clear if environmental variables such as temperature and oxygen concentration modulate competition. Here, interactions between Nile tilapia and the native Mayan cichlid (Mayaheros urophthalmus) were studied in experimental mesocosms in south-eastern Mexico. We found that Nile tilapia was the more active and aggressive of the two species, and their movement was only weakly influenced by temperature and oxygen concentration. By contrast, movement of the Mayan cichlid was strongly predicted by the movement and aggression of Nile tilapia, and the Mayan cichlid showed a steep decline in behaviours with increased water temperature and reduced oxygen. Our results suggest that broad environmental tolerance of the intrinsically aggressive Nile tilapia provides it with an advantage over native species. Collectively these traits may help to exacerbate its invasive success as those environmental conditions become more commonplace in a changing world.