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Article

Facilitation and Interference Between Native Fishes Influence Invasion Resistance

by
Jeffrey E. Hill
1,* and
Theresa P. Floyd
2
1
Tropical Aquaculture Laboratory, Program in Fisheries and Aquatic Sciences, School of Forest, Fisheries, and Geomatics Sciences, Institute of Food and Agricultural Sciences, University of Florida, Ruskin, FL 33570, USA
2
Department of Wildlife Ecology and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611, USA
*
Author to whom correspondence should be addressed.
Fishes 2025, 10(8), 398; https://doi.org/10.3390/fishes10080398
Submission received: 23 June 2025 / Revised: 30 July 2025 / Accepted: 31 July 2025 / Published: 8 August 2025
(This article belongs to the Section Biology and Ecology)
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Abstract

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.
Keywords:
aggressive competition; habitat complexity; invasion resistance; multiple predator effects; predation refuge
Key Contribution: We contribute novel insights into biotic resistance dynamics by showing how interactions between native species, such as Florida bass and eastern mosquitofish, influence the survival of small-bodied invasive fishes, highlighting the pivotal role of habitat complexity in mediating these interactions and emphasizing the need for further research to enhance our understanding of invasion dynamics in aquatic ecosystems.

1. Introduction

Species invasions have motivated attempts to understand how communities react to and potentially impede the establishment or spread of non-native species [1,2]. Predators or aggressive competitors may limit the success of invaders across a range of aquatic ecosystems (e.g., coastal marine: [3,4]; cold water lakes: [5,6]; subtropical freshwaters: [7]) and spatial scale [5,8]. What is less understood is the effect of species diversity on biotic resistance despite multiple interacting species in most aquatic habitats. Increased diversity is generally expected to increase invasion resistance [9,10,11]. Nevertheless, multiple predator effects (MPEs) may reduce or enhance prey risk with increased predator diversity [12,13]. Theoretically, results of interactions among such species are complex, exhibiting variable and context-specific outcomes [13,14,15], potentially enhancing or degrading biotic resistance.
Multiple strongly interacting species may have non-additive effects on the mortality of other species, whereby the effect of multiple species is greater than the sum of their individual effects due to facilitation (e.g., prey risk enhancement) [12,13]. For example, predators occupying different habitats may increase prey risk by reducing micro-habitats with lower predation pressure and heighten invasion resistance [16]. In one such case, a benthic predator refuging under rocks facilitated an open-water predator, increasing resistance to the spread of a non-native fish in a temperate river system [17]. However, the results of empirical studies are mixed [18], with common findings of simple additive effects due to substitutability of predators, risk reduction due to interference among predators, or intraguild predation [13,19]. For example, risk reduction and predator substitutability were observed for three non-native predatory fishes in African rivers [20]. Thus, diverse strongly interacting species may not enhance invasion resistance and might even reduce resistance to invaders [16]. Emergent properties of multiple species interactions are of particular interest in the study of biological invasions because they might enhance or impede the ability of resident species to resist invaders [13]. This knowledge gap hinders the incorporation of biotic resistance into predictive models of species invasions.
Peninsular Florida (south of the Suwannee River) in the USA has nearly 40 species of reproducing non-native freshwater fish [21]. Relatively few of the species are small-bodied (<150 mm maximum total length [TL]), and only a single small-bodied species is common and widespread [22]. This lack of small-bodied species is surprising given historic invasion pathways, including aquaculture and the aquarium trade, containing conservatively over 1000 small-bodied species, many with high trade volumes and presumably high propagule pressure [23]. In addition to abiotic factors, strong biotic resistance is thought to explain much of this pattern [7]. Small-bodied fish invaders face multiple predators and at least one aggressive competitor in the region, though most research has centered on the predatory Florida bass (Micropterus salmoides; hereafter bass) and the aggressive competitor/predator eastern mosquitofish (Gambusia holbrooki; hereafter mosquitofish). Both native species differ markedly in body size and predation mode, affecting the behavior and habitat use of other fishes and reducing invader survival. Both species are known to resist fish invasions in Florida [7,22], but their interactive effects on biotic resistance have not been investigated.
Heightened biotic resistance from facilitative interactions between mosquitofish and bass may partially explain the relative lack of success of small-bodied fish invaders in peninsular Florida [22]. Facilitation might occur if (1) the threat of open-water predators such as bass causes invaders to remain within structurally complex predation refuges and suffer mosquitofish attacks or (2) harassment by mosquitofish causes invaders to abandon predation refuges and suffer bass attacks. The alternative hypothesis is that bass also consumes mosquitofish and therefore might diminish the aggression of mosquitofish, weaken potential habitat exclusion effects, and reduce overall predation risk for small-bodied invaders [13]. Field studies show that small-bodied invaders did not co-occur with bass in ditches, streams, and wetlands, suggesting that bass exclude invaders from certain habitats [7]. We hypothesized that interactive effects would be strongly influenced by the vulnerability of mosquitofish to bass predation, a factor dependent on the structural complexity of the habitat as a predation refuge. We used a framework of MPEs [12] and tested for non-additive effects of the two species on a commonly introduced poeciliid invader using experimental tanks containing two levels of structurally complex habitat (strong and weak refuge) to understand potential habitat-mediated variation in mosquitofish and invader behavior and invader survival.

2. Materials and Methods

2.1. Study Region

Peninsular Florida has strong invasion pathways for non-native freshwater fish in aquaculture and the aquarium trade [24]. The warm climate and wide range of aquatic habitats provide a suitable environment for many non-native species [24]. West–central Florida (Tampa Bay region) is the heart of tropical ornamental fish aquaculture production in the USA (see maps showing the region, aquaculture facilities, and fish sampling locations in [25,26]). Cultured fish generally escape fish farms via effluent into heavily modified habitats such as roadside ditches or degraded streams [26]. These ditches flow into more natural habitats of streams or wetlands, which, in turn, flow into larger streams or coastal rivers. This low-lying, coastal region also has numerous stormwater ponds, borrow lakes, and natural lakes, which are typically connected to natural or constructed drainages (e.g., Hillsborough County alone has almost 2000 stormwater ponds and more than 200 natural lakes; [27]). High human population density and public accessibility of water systems in the region further suggest potentially strong propagule pressure through aquarium release into these habitats [28,29].

2.2. Study Species

Mosquitofish and bass are common, widespread native species that occupy many aquatic habitats in peninsular Florida [30,31]. The mosquitofish (Poeciliidae) is small-bodied (~60 mm TL) and bites and harasses small fish to death [32]. The interaction of mosquitofish with other small-bodied species is a combination of aggressive competition with adults and predation on adults and juveniles [22]. Bass (Centrarchidae) is large-bodied (>500 mm TL) and consumes prey whole [33]. Mosquitofish uses shallow waters and vegetated habitats as refuge from large-bodied predatory fishes [34]. Nevertheless, these two species occupy broad, partly overlapping habitat domains ranging from scour and culvert holes in ditches to pools and culvert holes in streams to vegetated and un-vegetated littoral zones of stormwater ponds, lakes, and rivers [7].
We used green swordtail (Xiphophorus hellerii, Poeciliidae) as non-native prey (hereafter swordtail). Swordtail is a small-bodied species (up to ~80 mm TL without the caudal sword) native to Mexico and Central America from the Yucatan Peninsula south to Honduras [35]. It is a common aquarium species cultured in Florida. The swordtails used in the experiment were the “red wag” variety, an aquarium color morph with red body and black fins. Swordtails of various color varieties have been collected in fish surveys in Florida [26], some other U.S. states [36], and at least 35 additional countries [37]. Localized populations, ranging from ephemeral to persistent, exist in west–central Florida [37] but are absent from much of the region in seemingly suitable habitat.

2.3. Experimental System and Design

The experiment was designed to mimic the introduction of swordtails into aquatic habitats already containing one or both native species. This is a common scenario whereby non-native fish escape from a fish farm or are released by aquarium hobbyists into a local waterbody [22]. Local ditches and small streams often have areas of dense vegetation adjacent to areas of deeper, more open waters, such as scour holes and pools. Likewise, local stormwater ponds, borrow lakes, and natural lakes frequently have dense beds of vegetation along the shoreline with adjacent open water. Therefore, experimental tanks were set up to mimic these types of habitats.
The experiment was completed in a recirculating tank system in a greenhouse at the University of Florida/IFAS Tropical Aquaculture Laboratory (TAL), Ruskin, Florida. Individual tanks were concrete (221 cm × 79 cm × 58 cm; 1.75 m2; 1000 L) with a water depth of 30 cm. Water parameters were dissolved oxygen (>8 mg/L), temperature = 25–30 °C, pH = 8.0, total ammonia nitrogen < 1.0 mg/L, nitrite < 0.02 mg/L, total alkalinity = 188 mg/L, and total hardness = 342 mg/L. All tanks had artificial vegetation (645 stems/m2) that covered 50% of the tank bottom [38], serving as a relatively weak refuge for mosquitofish and swordtails from bass predation. This was designed to mimic field conditions with habitat structural complexity, but where bass can readily access the habitat. In two treatments with bass (see below), a strong predation refuge was provided by the addition of a vertical barrier consisting of netting (3.8-cm mesh) fitted 8 cm outside the artificial vegetation. Bass were excluded from the vegetation by the barrier, but mosquitofish and swordtails could freely swim among habitats. The strong refuge was designed to mimic field conditions where shallow waters and structurally complex habitats provide a predation refuge for mosquitofish and other small-bodied fishes, and thick vegetation restricts the movement of large-bodied species [39].
The experimental design included tanks stocked with no predators (controls; n = 12 tanks), 100 mosquitofish (TL ± 1 SD = 33 ± 4 mm; 10 tanks), 1 bass (249 ± 23 mm; 10 tanks strong refuge and 12 tanks weak refuge), or both native species (8 tanks strong refuge and 10 tanks weak refuge) 4 days prior to the introduction of 20 swordtails (57 ± 7 mm, length without sword). Tanks were observed multiple times each day, but mortalities were not replaced. All fish were removed and counted 3 days after the introduction of swordtails. Bass were provided by the Florida Fish and Wildlife Conservation Commission following collection by boat electrofishing from a private lake in Apollo Beach, Florida. Mosquitofish and swordtails were collected using galvanized metal minnow traps from research ponds at the TAL. All fish were acclimated for one week in flow-through tank systems in a greenhouse at the TAL prior to use.
Numbers and sizes of native species were chosen to represent field conditions in west–central Florida. Bass in ditch and stream systems where swordtails would escape from fish farms occupy specific habitats that are somewhat deeper than average for the systems, such as scour and culvert holes or stream pools [7]. Many such locations are in the size and depth range of the tanks used in the present study. In these habitats, a single individual is most likely present (typically 150–300 mm TL; personal observations). Mosquitofish density varies widely across habitats in the region. Some of the densest populations occur in ditches near the effluent of fish farms where mosquitofish is the most abundant fish species [7,26], but densities of 50–100 or more individuals per m2 commonly occur in vegetated ditches and in the littoral zones of ponds and lakes [7,22]. Mosquitofish occur at lower densities in streams and ditches that are fast-flowing and shallow, and those subjected to frequent rapid flooding during wet periods.
During the experiment, mosquitofish and swordtails were offered a commercial ornamental fish feed once per day at a rate of approximately 3% of their initial combined body weight. This ensured that both species had adequate food resources and that mosquitofish were not motivated to attack swordtails by starvation.

2.4. Data Collection and Statistical Analysis

All statistical analyses were performed in SAS 9.4 (SAS Institute, Cary, NC, USA). Experimental data for strong and weak refuge treatments were analyzed separately, but mosquitofish treatments were used in both analyses because a barrier would make no difference.
We used the framework of multiple predator effects (MPEs) [12,40] to test for the interactive effects of the two native fishes, despite the mixed predator–aggressive competitor nature of mosquitofish, because both species exert lethal and non-lethal effects on small-bodied fish such as swordtails. The occurrence of non-additive effects on swordtail survival was tested by two-way analysis of variance (ANOVA) using a multiplicative predation model as the null hypothesis:
Prey surviving both predators = 1 − Pa − Pb + PaPb,
where Pa is the probability of consumption by predator A, and Pb is the probability of consumption by predator B; PaPb accounts for the fact that predators cannot consume the same prey individual twice [12,40]. Swordtail survival data were tested following log10-transformation [12,40]. A significant interaction term would indicate non-additive effects (prey risk enhancement or reduction). Mosquitofish survival across treatments was analyzed with ANOVA following an arcsine-square root transformation of the data.
Four tanks of each treatment group (n = 4 replicates each treatment) were randomly chosen for behavioral observations (i.e., habitat use and aggression) on day 2. Observations were made by the same 4 trained observers each time. Training sessions were completed with all observers prior to undertaking the experiment, including detailed instructions and sample observations. The inter-observer variation was typically <5% and all <10%. The observers waited 5 min for the fish to return to normal activity (this occurred usually within 1–2 min). Behavioral data were collected from 1130 to 1600 h. The use of habitat by mosquitofish and swordtails was quantified by scan samples of fish location—open water, edge (between barrier and open water or in open water but within 8 cm of vegetation), or vegetation. Mosquitofish within vegetation could not be reliably counted, so only those in the open water and edge were counted. Differences in the mean number of mosquitofish outside of the vegetation were tested with ANOVA. Conversely, swordtails were easy to observe due to bright coloration, and numbers could be counted in all three habitats (open water, edge, and vegetation). The proportion of swordtails in each habitat was then determined as the number of individuals in the habitat divided by the total number of swordtails remaining. Differences in mean proportion of swordtails inside the vegetation or in the edge habitat were detected across treatments using ANOVA following an arcsine-square root transformation of the data. Focal animal sampling was conducted in 30-minute bouts to determine the intensity of agonistic interactions between mosquitofish and swordtails. Attacks were defined as bites, chases, or other aggressive interactions, and attack data were analyzed using the Kruskal–Wallis rank sum test [22].
Surviving swordtails were assessed for caudal fin damage using a ranked, categorical scale with a score of 0 for no damage, 1 for slight to moderate damage (<50% of fin affected), and 2 for severe damage (>50% of fin affected) [22]. Fin damage data were analyzed using the Kruskal–Wallis rank sum test.

3. Results

Results (Figure 1) showed a shift in habitat use by swordtails in response to the addition of the native species. Swordtails shifted from a distribution throughout the habitats to open water in the mosquitofish treatment, back to higher use of vegetated habitat with the bass and barrier treatment, then to refuging in the edge habitat in the bass and mosquitofish treatment with the barrier. Swordtail mortality was negligible in the control and mosquitofish treatment, increased considerably with the addition of a bass, and was lowest for the combination of native species (Figure 1). The results (Figure 2) of the experiment with a weak predation refuge (i.e., no barrier) were the same for the control and mosquitofish treatment (Figure 1). The majority of swordtails shifted from open water back into vegetation or edge habitat in bass treatments, despite the usual presence of the bass and mosquitofish in the combination treatment in the vegetated habitat. Swordtail mortality was somewhat lower in the bass and mosquitofish treatment than in the bass treatment.
With a strong predation refuge for mosquitofish, swordtail survival was about 2.5 times lower with the combination of natives than with bass alone, indicating facilitation (LMB × MF: F1,34 = 13.81, p = 0.0007; Figure 3A). In contrast, swordtail survival in the combination treatment with a weak refuge was about two times higher than with the bass alone, indicating interference (LMB × MF: F1,40 = 4.47, p = 0.0408; Figure 3B). Swordtail mortality was negligible in controls and low in the presence of only mosquitofish (Figure 1).
Mean mosquitofish survival in the weak refuge treatment with bass was about 80% of survival in the mosquitofish treatment (F2,33 = 11.54, p = 0.0002; Figure 4). Mosquitofish survival in the strong refuge treatment with bass was not significantly different from that in the mosquitofish treatment or in the weak refuge treatment with bass.
Mosquitofish were strongly associated with vegetation, even in the absence of bass (mean number = 12.2 ± 0.4 observed in open waters, including edge habitat). Significantly fewer mosquitofish occurred in open waters in the weak refuge treatment (5.6 ± 0.9), and even fewer were in open waters in the strong refuge treatment with bass (2.9 ± 0.7; F2,9 = 32.37, p < 0.0001).
In controls, swordtails occupied vegetation (56 ± 12%) and open water randomly. The proportion of swordtails using open water (F4,14 = 10.01, p = 0.0005; Figure 5), edge habitat (F4,14 = 11.04, p = 0.0003), and vegetation (F4,14 = 6.33, p = 0.004) varied considerably in the presence of natives. Open water was most used in the presence of mosquitofish and least used in any treatment with bass. Swordtails used edge habitat most with the combination of bass and mosquitofish in the strong refuge treatment and least with bass in the weak refuge treatment. Vegetation was most used by swordtails in the presence of bass in the weak refuge treatment and least in the mosquitofish treatment.
Mosquitofish frequently harassed and chased swordtails. Attacks occurred mainly within vegetated habitat (>85%) and were significantly greater in number in the combination treatment with the strong refuge (χ2 = 16.97, df = 2, p = 0.0002; Figure 6). Attack numbers were not significantly different between treatments with mosquitofish alone or the combination of species within the weak refuge treatment. Observed attacks averaged about 1/min in the mosquitofish treatment but about 3/min in the combination. These values represent mean attacks per swordtail; thus, actual attack numbers for all swordtails in the tanks were considerably higher (i.e., ~observed attacks × remaining swordtails). Harassment and caudal fin damage resulted in direct evidence of swordtail mortality only in the mosquitofish treatment (six individuals); all other mortality was attributable to largemouth bass. No aggression by swordtails against mosquitofish was observed.
The pattern of caudal fin damage (Figure 7) varied significantly across treatments (χ2 = 32.98, df = 4, p < 0.0001; Figure 8). No caudal fin damage was observed in the absence of mosquitofish. Severe caudal damage to surviving swordtails was seen only in the mosquitofish treatment (3.5%), but damage in this treatment was significantly less overall than in the combination treatment with the strong refuge (29% with damage versus 75%). With both natives, bass consumed severely damaged swordtails prior to the end of the experiment.

4. Discussion

We showed that habitat plays a key role in the invasion resistance dynamics of two native fish species, resulting in powerful facilitation or moderate interference. Mosquitofish occupied strong refuge habitats and, via agonistic interactions, often excluded swordtails, increasing the predation effect of bass. In a relatively weak refuge habitat, bass suppressed the aggressive behavior of mosquitofish, thus reducing the effects of the combination of native species on swordtail survival. Nevertheless, swordtail survival was reduced in all treatments with bass by at least 60% relative to controls, suggesting strong resistance to invasion by this species across habitats. We conclude that under many field conditions in Florida, small-bodied invaders encounter opposing threats and are caught “between a rock and a hard place.” Namely, small-bodied fishes must remain in structurally complex habitats while facing intense mosquitofish harassment or leave this cover and expose themselves to bass, fundamentally supporting the hypothesis of facilitation [22]. Therefore, strongly interacting species, in this case predators and aggressive competitors, may act synergistically to reduce the probability of establishment by small-bodied, non-native freshwater fishes.
Theory predicts that biotic resistance to invaders increases with increased diversity [11,12,13]. Nevertheless, growing evidence shows that diversity per se is not as important in aquatic communities as the specific types of species interacting with invaders [6]. Greater resistance due to increasing diversity can result from additive effects of individual species or facilitative effects among species [4,12,13]. Additionally, increased diversity could reduce the mortality of invaders through interference [12,13]. Our results were non-additive and show that how species interact with each other and the invader, coupled with interactive effects with the habitat, determine the influence of increased diversity on biotic resistance. In our study, habitat complexity played a key role in the interactive outcome, with decreased resistance in simple habitats and increased resistance in and near complex habitats, all occurring with the same set of species. Therefore, in addition to species traits and behavior, habitat heterogeneity and complexity are important factors for studies of biotic resistance.
Structurally complex habitats may influence prey encounter rates and thus consumption rates by predators and lead to non-linear combined predator effects [41]. Habitat structural complexity reduces the access [39] and feeding efficiency [38] of large-bodied predators, frequently decreasing the proportion of fish in their diet [42]. Small fish often seek the protection of refuge habitats [43,44,45], although they may be affected by resource competition or aggression from other small fishes inside these refuges [46]. In our experiment, the safety offered by the complex habitat decreased the interaction between bass and mosquitofish, allowing the latter to aggressively interact with the invader. Swordtails were largely confined to the narrow edge habitat, using the barrier itself for cover. Any swordtail that moved into the vegetated habitat was repeatedly attacked by multiple mosquitofish. Mosquitofish continued to harass the swordtails along the edge, causing some swordtails to retreat past the barrier, where they were often attacked by bass.
Although our strongest result was facilitation decreasing invader survival, we identified a condition whereby interacting species can interfere with one another, resulting in risk reduction for an invader. Bass ate mosquitofish, somewhat reducing their density, and suppressed the latter species’ aggression. With only a weak refuge, mosquitofish responded to predation threat from bass by increased use of vegetated habitat, less aggression towards swordtails, and possibly other anti-predator behavior.
Mosquitofish had little effect on swordtail survival when alone. This is partly due to the short duration of the experiment, though some swordtails were heavily damaged by mosquitofish (Figure 5). The short duration and the use of adult swordtails caused the main effect of mosquitofish to be aggressive rather than predation (see [22] for longer interaction periods and predation on adults and juveniles). Swordtails decreased their encounter rate with mosquitofish and increased their survival by spending less time in vegetated habitat when bass was absent. Thompson et al. [22] also noted this behavior, though the benefit for swordtails was less in their experiments. They used smaller tanks and, although the proportion of the area covered by the artificial vegetation was similar, the oval shape of the tanks and the central placement of the vegetation resulted in relatively small open water areas surrounding the vegetation. Their open water was equivalent to our edge habitat, an area where mosquitofish attacks still commonly occurred. Thus, swordtails could not avoid mosquitofish interactions to as great an extent as in our tanks. Avoiding agonistic interactions with mosquitofish is an effective strategy in experimental tanks with adequate open water and no other predators, but it would be less effective under field conditions where predation threat is strong in open habitats.
Our results indicate that swordtails are highly vulnerable to predators. In addition to small body size, bright coloration, and lack of defensive morphology, swordtails do not seem to be able to organize their behavior, including use of habitat, to avoid heavy mortality unless a relatively predator-free habitat is available. In our experiments, swordtails generally reacted appropriately to mosquitofish alone, though some were still heavily damaged, but performed poorly when faced with bass predation, even with the barrier limiting access of bass to the vegetated habitat. Indeed, swordtails frequently crossed the barrier into open water, sometimes after bass had repeatedly chased them back into the vegetation. Swordtails also experience high mortality when confined in a vegetated habitat with mosquitofish [22]. Swordtails move frequently in the presence of mosquitofish, thereby increasing the potential for agonistic encounters. The most dramatic effects in our experiments were observed when both natives removed the potential for swordtails to avoid encounters. Another important impact of mosquitofish on other small fishes is consumption of larvae or neonates [32,47], an effect shown to be strong on swordtails in experimental systems.
Our data imply that biotic resistance to the establishment of non-native swordtails is strong unless there is a substantial refuge from predatory fishes. Field surveys at sites where swordtails have escaped from fish farms in Florida confirm that this species and numerous other small-bodied ornamental species quickly decline in abundance with increasing distance from the introduction source, and further that they do not persist in habitats with large-bodied predators [7]. Swordtails in Florida generally occur in small, localized populations, primarily in disturbed habitats such as roadside ditches, small streams, and stormwater ponds. Most persistent populations of swordtails are made up of drab, wild-type individuals despite the scarcity of this color morph in production or trade [48]. The extinction of the far more numerous colorful varieties and persistence of a morph mimicking a drab version of the wild-type suggests that higher levels of domestication render this species less invasive in the region, but that feralization operating within released stocks can lead to a morph capable of establishing within localized habitats [48].
Although the focus of the present study was on interactions among native Florida species as biotic resistance, this bass–mosquitofish system has been widely spread throughout the world. Florida bass and the closely related largemouth bass (Micropterus nigricans) are now present in at least 71 countries, and mosquitofish (collectively eastern mosquitofish and western mosquitofish Gambusia. affinis) in at least 93 countries [37]. Both species independently have been shown to negatively affect the abundance and alter habitat use of small-bodied native fishes (bass: [49,50,51]; mosquitofish: [32,52,53,54]. In regions where they are introduced, these two species may interact to greatly increase impacts on native fishes. Interactions among non-native predators may have strong effects on imperiled native species [55]. Additional research into emergent properties of the interaction among these and other species and their potential to negatively affect native biodiversity is warranted.
In addition to bass and mosquitofish, small-bodied fish invaders in Florida face predation threats from a wide range of predatory taxa, including native and non-native predatory fishes of varied size and feeding behavior, fish-eating reptiles, birds, and mammals, and predatory insects. Little is known about the interactions of such a diverse array of predators and their influence on the establishment success of non-native fishes. Field research has associated a suite of small- to medium-sized native fishes, such as bullhead catfishes (Ameiurus spp.) and sunfishes (Lepomis spp.), with collectively resisting the establishment of small-bodied non-native fishes [7]. More field and laboratory research investigating the effect of multiple predators and competitors across a range of spatial scales, habitat heterogeneity, and varied propagule pressure will provide a better understanding of the effects of species interactions on invader dynamics, as well as the general applicability of biotic resistance under field conditions. Incorporating species interactions and field patterns of co-occurrence or exclusion improves the predictive ability of invasion models. As an example, the use of conditional random fields (CRFs) and joint species distribution models (JSDMs) that predict species occurrence based on community structure, as well as habitat and other environmental factors, may be useful in identifying vulnerable locations and habitats for species invasions and potentially affected native species [56,57].

5. Conclusions

  • Habitat influenced invasion resistance by altering the interaction of mosquitofish and bass.
  • Facilitation of increasing predation effects was observed when mosquitofish aggression largely excluded swordtails from structurally complex habitat, increasing swordtail vulnerability to bass.
  • Interference reducing predation effects was observed at lower levels of structural complexity when bass consumed mosquitofish and suppressed their aggression effects on swordtails.
  • Despite interference, swordtails still encountered invasion resistance from bass.
  • Small-bodied fish invaders must remain in structurally complex habitat while facing intense mosquitofish harassment, or leave this cover and expose themselves to bass.

Author Contributions

Conceptualization, J.E.H.; methodology, J.E.H. and T.P.F.; data analysis, J.E.H.; resources, J.E.H.; data curation, J.E.H. and T.P.F.; writing—original draft preparation, J.E.H.; writing—review and editing, J.E.H. and T.P.F.; supervision, J.E.H.; project administration, J.E.H.; funding acquisition, J.E.H. All authors have read and agreed to the published version of the manuscript.

Funding

Funding was provided to J.H. by the Florida Fish and Wildlife Conservation Commission (#00059481), the Florida Department of Agriculture and Consumer Services (#00076778), and by the base budget of the UF/IFAS Tropical Aquaculture Laboratory (Craig Watson), and to T.F. by the UF Undergraduate Scholars Program.

Institutional Review Board Statement

The animal study protocol was approved by the UF/IFAS Animal Research Committee (approval code: #002-09RUS and date: 14 May 2009).

Data Availability Statement

Data are contained within the article.

Acknowledgments

We thank faculty, staff, and students of the UF/IFAS Tropical Aquaculture Laboratory (Craig Watson) and Bill Pouder (Florida Fish and Wildlife Conservation Commission) for assistance and four anonymous reviewers.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Diagram of a multiple predator experiment with two native species, bass and mosquitofish (MF), preying upon non-native swordtails. Artificial vegetation and a barrier (vertical dashed line) provided a strong predation refuge for swordtails and mosquitofish. Bass and swordtails (red color) represent single individuals, whereas mosquitofish (gray color) represent two individuals. The mean habitat use (open, edge, and vegetated habitats) and survival of each species at the end of the experiment are depicted by the general location and numbers within the diagram.
Figure 1. Diagram of a multiple predator experiment with two native species, bass and mosquitofish (MF), preying upon non-native swordtails. Artificial vegetation and a barrier (vertical dashed line) provided a strong predation refuge for swordtails and mosquitofish. Bass and swordtails (red color) represent single individuals, whereas mosquitofish (gray color) represent two individuals. The mean habitat use (open, edge, and vegetated habitats) and survival of each species at the end of the experiment are depicted by the general location and numbers within the diagram.
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Figure 2. Diagram of a multiple predator experiment with two native species, bass and mosquitofish (MF), preying upon non-native swordtails. Artificial vegetation provided a weak predation refuge for swordtails and mosquitofish. Bass and swordtails (red color) represent single individuals, whereas mosquitofish (gray color) represent two individuals. The mean habitat use (open, edge, and vegetated habitats) and survival of each species at the end of the experiment are depicted by the general location and numbers within the diagram.
Figure 2. Diagram of a multiple predator experiment with two native species, bass and mosquitofish (MF), preying upon non-native swordtails. Artificial vegetation provided a weak predation refuge for swordtails and mosquitofish. Bass and swordtails (red color) represent single individuals, whereas mosquitofish (gray color) represent two individuals. The mean habitat use (open, edge, and vegetated habitats) and survival of each species at the end of the experiment are depicted by the general location and numbers within the diagram.
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Figure 3. Mean swordtail survival (+1 SE) in a tank experiment with mosquitofish (MF), bass (BASS), and both natives in combination. Different letters (A, B, and C) indicate significantly different treatment means. (A). Strong predation refuge with barrier present in treatments with BASS. The result for the combination of BASS + MF showed decreased survival of swordtails relative to additive survival in the presence of BASS and MF singly, indicating predator facilitation. (B). Weak predation refuge with no barrier present in treatments with BASS. The result for the combination of BASS + MF showed increased survival of swordtails relative to additive survival in the presence of BASS and MF singly, indicating predator interference.
Figure 3. Mean swordtail survival (+1 SE) in a tank experiment with mosquitofish (MF), bass (BASS), and both natives in combination. Different letters (A, B, and C) indicate significantly different treatment means. (A). Strong predation refuge with barrier present in treatments with BASS. The result for the combination of BASS + MF showed decreased survival of swordtails relative to additive survival in the presence of BASS and MF singly, indicating predator facilitation. (B). Weak predation refuge with no barrier present in treatments with BASS. The result for the combination of BASS + MF showed increased survival of swordtails relative to additive survival in the presence of BASS and MF singly, indicating predator interference.
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Figure 4. Mean mosquitofish survival (+1 SE) in a tank experiment with mosquitofish (MF), bass and mosquitofish (BASS + MF), and bass, mosquitofish, and a barrier (BASS + MF + BARRIER). Different letters (A and B) indicate significantly different treatment means.
Figure 4. Mean mosquitofish survival (+1 SE) in a tank experiment with mosquitofish (MF), bass and mosquitofish (BASS + MF), and bass, mosquitofish, and a barrier (BASS + MF + BARRIER). Different letters (A and B) indicate significantly different treatment means.
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Figure 5. Mean proportion (+1 SE) of swordtails in vegetated (gray fill), edge (diagonal fill), and open water (light fill) habitat in the presence of mosquitofish (MF), bass (BASS), and both natives (BASS + MF), with barrier present or absent. Different letters (A, B, and C) indicate significantly different treatment means within habitats.
Figure 5. Mean proportion (+1 SE) of swordtails in vegetated (gray fill), edge (diagonal fill), and open water (light fill) habitat in the presence of mosquitofish (MF), bass (BASS), and both natives (BASS + MF), with barrier present or absent. Different letters (A, B, and C) indicate significantly different treatment means within habitats.
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Figure 6. Mean number of attacks (+1 SE) on swordtails during 30-minute observation periods in treatments with eastern mosquitofish (MF). Different letters (A and B) indicate significantly different treatment means.
Figure 6. Mean number of attacks (+1 SE) on swordtails during 30-minute observation periods in treatments with eastern mosquitofish (MF). Different letters (A and B) indicate significantly different treatment means.
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Figure 7. Damage to the caudal fin and caudal peduncle of swordtails due to mosquitofish attack. Photo credit: Jeffrey Hill, UF/IFAS Tropical Aquaculture Laboratory.
Figure 7. Damage to the caudal fin and caudal peduncle of swordtails due to mosquitofish attack. Photo credit: Jeffrey Hill, UF/IFAS Tropical Aquaculture Laboratory.
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Figure 8. Proportion of caudal fin scores for surviving swordtails exposed to mosquitofish (MF), bass (BASS), BASS + MF, BASS + BARRIER, or BASS + MF + BARRIER, for 3 days in a tank experiment. Caudal fin damage was assessed using a ranked, categorical scale with a score of 0 for no damage (gray fill), 1 for slight to moderate damage (<50% of fin affected; light fill), and 2 for severe damage (>50% of fin affected; diagonal fill).
Figure 8. Proportion of caudal fin scores for surviving swordtails exposed to mosquitofish (MF), bass (BASS), BASS + MF, BASS + BARRIER, or BASS + MF + BARRIER, for 3 days in a tank experiment. Caudal fin damage was assessed using a ranked, categorical scale with a score of 0 for no damage (gray fill), 1 for slight to moderate damage (<50% of fin affected; light fill), and 2 for severe damage (>50% of fin affected; diagonal fill).
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Hill, J.E.; Floyd, T.P. Facilitation and Interference Between Native Fishes Influence Invasion Resistance. Fishes 2025, 10, 398. https://doi.org/10.3390/fishes10080398

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Hill JE, Floyd TP. Facilitation and Interference Between Native Fishes Influence Invasion Resistance. Fishes. 2025; 10(8):398. https://doi.org/10.3390/fishes10080398

Chicago/Turabian Style

Hill, Jeffrey E., and Theresa P. Floyd. 2025. "Facilitation and Interference Between Native Fishes Influence Invasion Resistance" Fishes 10, no. 8: 398. https://doi.org/10.3390/fishes10080398

APA Style

Hill, J. E., & Floyd, T. P. (2025). Facilitation and Interference Between Native Fishes Influence Invasion Resistance. Fishes, 10(8), 398. https://doi.org/10.3390/fishes10080398

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