1

Complex Interactions between Native and Invasive Fish:

The Simultaneous Effects of Multiple Negative Interactions

Michael D. Mills, Russell B. Rader, and Mark C. Belk

Department of Integrative Biology, Brigham Young University

Department of Integrative Biology, 401 WIDB, Brigham Young University, Provo, UT 84602

E-mail: russell_rader@byu.edu Fax: 801-422-0090

2

Abstract

We suggest that the ultimate outcome of interactions between native species and invasive species

(extinction or coexistence) depends on the number of simultaneous negative interactions

(competition and predation), which depends on relative body sizes of the species. Multiple

simultaneous interactions may constrain the ability of native species to trade fitness components

(i.e. reduced growth for reduced risk of predation) causing a spiral to extinction. We found

evidence for five types of interactions between the adults and juveniles of introduced western

mosquitofish (Gambusia affinis) and the juveniles of native least chub (Iotichthys phlegethontis).

We added 10 large (23-28 mm) and 7 small (9-13 mm) young-of-the-year (YOY) least chub to

replicate enclosures with zero, low, and high densities of mosquitofish in a desert spring

ecosystem. Treatments with mosquitofish reduced the average survival of least chub by one-

third. No small YOY least chub survived in enclosures with high mosquitofish densities. We

also performed 2 laboratory experiments to determine mortality to predation, aggressiveness, and

habitat selection of least chub in the presence of mosquitofish. Mean mortality of least chub due

to predation by large mosquitofish was 69.7 % over a 3-hour trial. Least chub were less

aggressive, selected protected habitats (Chara sp.), and were more stationary in the presence of

mosquitofish where the dominance hierarchy was: large mosquitofish >> large least chub ≈ small

mosquitofish >> small least chub. Least chub juveniles appear to be figuratively caught in a

vice. Rapid growth to a size refuge could reduce the risk of predation, but the simultaneous

effects of competition decreased least chub growth and prolonged the period when juveniles

were vulnerable to mosquitofish predation.

3

Key words: Biological invasions, competition, predation, body size, scale, refugia, Gambusia

affinis, Iotichthys phlegethontis

4

Introduction

Invasive species often cause rapid population declines and the extinction of native

species. Our ability to minimize the effects of invasive species depends on understanding the

mechanisms underlying their interactions with native populations. Many invasive species are

successful because they are released from natural controls (e.g. competitors, predators, parasites)

that regulate population growth within their native range (Brown 1989; Simberloff 1989).

Invasive species also tend to be superior competitors with a high reproductive potential and

predation strategies that were absent during the evolution of isolated native populations (e.g.

Savidge 1987; Ogutu-Ohwayo 1993; Townsend 1996; Sakai 2001). Competition and predation

between introduced and native species, and the introduction of novel parasites can have obvious

direct negative effects (Ashton and Mitchell 1989; Brown 1989). However, efforts to preserve

declining native species can also be frustrated by the complexity of combined direct and indirect

biotic interactions.

Recent research has revealed complex interactions between invasive and native species

because of size-structured competition and predation (Kotler and Holt 1989; Wissinger 1992;

Belk et al. 2001). For example, large invasive predators that feed on large prey can consume the

adults of native populations while providing young native fish with refuge zones from

intermediate-sized predators, thereby increasing recruitment and survival of the entire population

(e.g. Mittelbach and Chesson 1987; Winkelman and Aho 1993). The overall effects of invasive

species on native populations can be difficult to predict.

Individuals can trade-off multiple components of fitness and continue to persist if

confronted by a single threat (Soderback 1995; Kiesecker et al. 2001; Bertolino and Genovesi

5

2003). Prey may respond to competition, predation, or parasitism by altering their morphology,

behavior, or life history to minimize negative effects (reviewed by Tollrian and Harvell 1999).

For example, small fish of many species typically inhabit vegetated or otherwise structured

habitats with poor food resources as a refuge from predation (e.g. Mittelbach 1988). Although

anti-predator responses often result in increased costs to other fitness components (e.g., growth

rate or fecundity), the prey population can persist because decreased growth is balanced by

decreased mortality. Similarly, increased risk of mortality can be traded for increased growth

when confronted with competition from a new species. For example, competing individuals may

forage in habitats that increase vulnerability to predators, but are energetically more profitable

than safer habitats (Wellborn 2002). Overall, native populations can often successfully respond

to single threats from invasive species and continue to persist (Tollrian and Harvell 1999;

Gurevitch et al. 2000).

Given the complexity of size-structured interactions, invasive species may affect native

species via multiple interactions. Native species may be unable to respond to multiple threats,

especially when the intensity of interactions is size-dependent (e.g., Wissinger 1992).

Competition and predation can exert simultaneous pressure on both large and small size classes

reducing the effectiveness of trade-offs. Trade-offs that ameliorate the effects of predation on

smaller size classes (e.g. habitat shifts) may intensify the harmful effects of competition.

Similarly, trade-offs that ameliorate the effects of competition between small size classes (e.g.

shifts in foraging time or place) may exacerbate the harmful effects of predation (reviewed by

Gurevitch et al., 2000). For example, when small fish shift to refuge habitats to avoid predation,

they may be confronted by intense competition from other species, including refuging size

classes of invasive species (Mittelbach 1986; Mittelbach and Chesson 1987). We propose that

6

multiple interactions can constrain the ability of native species to trade fitness components

causing a spiral to extinction.

We suggest that the coexistence of native species with invasive species depends on the

number and strength of simultaneous negative interactions. We emphasize the need to

understand the complex effects of multiple interactions when attempting to minimize the impact

of invasive species. Identifying the relative strength of multiple negative interactions may help

resource managers predict which species are most at risk, and may indicate effective

management actions for conservation of native species.

We conducted one field experiment and two laboratory experiments to examine multiple

interactions between small young-of-the-year (YOY) least chub (Iotichthys phlegethontis), native

to the Bonneville Basin of Utah, and introduced western mosquitofish (Gambusia affinis). We

tested three hypotheses: 1) large mosquitofish can prey on small least chub, 2) competition

(interference and/or exploitative) between mosquitofish and least chub will reduce least chub

growth, and 3) mosquitofish will force small least chub into refuge habitats (macrophytes), thus

increasing their exposure to invertebrate predators.

Conceptual Background

To evaluate the full range of interactions between two species it is necessary to consider

the size range of each species. Table 1 shows potential interactions for both juvenile and adult

life stages between native and invasive species with similar adult body sizes. This table was

made assuming that invasive species are more aggressive predators and competitors than native

7

species, and that they can reach a high population density because of a high reproductive

potential (Sakai 2001).

We have identified 8 interactions between similar sized invasive and native species that

could have detrimental effects on native populations (Table 1). We decided to focus our research

on the YOY of native species because 5 of these interactions are specific to that size class, thus

suggesting the greatest potential for examining the effects of simultaneous multiple negative

interactions. Although the occurrence of each interaction is contingent upon various traits of

invasive and native species (i.e. competition may depend on whether native fish occupy the same

refugia as invasive fish), this high number of potential negative interactions suggests that enough

interactions may often be realized that trade-offs among fitness components are not possible,

thus resulting in extinction.

Historical Background

Fifty years ago, least chub were widely distributed in the Bonneville Basin in a variety of

habitats including rivers (Provo River), lakes (Utah Lake), small streams, ponds, and marshes

scattered throughout central and southern Utah (Sigler and Miller 1987). Currently the

distribution of least chub is limited to ten isolated spring-fed pools in the deserts of central Utah

(Perkins et al. 1998). Five of these ten springs are infested with abundant mosquitofish

populations. Causes of declining least chub populations are not well documented, but habitat

loss and interactions with mosquitofish are the primary suspects (Holden et al. 1974; Hickman

1989; Crist and Holden 1990; Perkins et al. 1998).

8

Mosquitofish are members of the Poeciliidae family and give birth to live young.

Although they are native to the southeastern United States, their range has been expanded

throughout North America for mosquito control. Increases in mosquitofish have been correlated

with a decline in native species, but the mechanisms are rarely known (e.g. Galat and Robertson

1992; Rincon et al. 2002). However, mosquitofish are aggressive predators that feed on a variety

of aquatic organisms including their own young and the young of other fish (Meffe 1985; Sigler

and Miller 1996; Barrier and Hicks 1994; Gamradt and Kats 1996; Goodsell and Kats 1999;

Komak and Crossland 2000). Mosquitofish are also known to be competitively superior to some

native fish (Lydeard and Belk 1993; Crivelli 1995).

Our field research was conducted at Walter Spring (113.402° West, 31.871° North; Juab

County, Utah, USA), an impounded, spring-fed pond managed by the U.S. Fish and Wildlife

Service at the Fish Springs National Wildlife Refuge. The Fish Springs National Wildlife

Refuge is a large spring complex, consisting of 15 to 20 springheads and associated marshes

connected by surface and groundwater flows spread over approximately 40 km2. Walter Spring

is an averaged-sized spring for the refuge with a surface area of 320 m2 and maximum depth of

3.0 m. Variations in flow and depth are small on both a seasonal and annual basis. Although

temperature at the inflow is constant (18 C), daily and seasonal variations increase away from the

wellhead.

In 1996, mosquitofish were removed (via draining and Rotenone) and least chub were

introduced to Walter Spring as part of restoration efforts. Least chub rapidly increased during

1997 through 2000 reaching thousands of individuals. Until recently, state surveys (minnow

traps) found numerous juveniles indicating healthy recruitment of least chub in Walter Spring

(Wilson unpublished reports). However, mosquitofish managed to re-invade this spring in 1998,

9

and two surveys (July 2002 and May 2003) using 100 minnow traps at various depths for 24

hours found a total of 7 least chub smaller than 30 mm.

Methods

Competition and Predation Experiment

We placed 18 enclosures (1 m2 by 1.5 m in height) consisting of a polyvinyl chloride pipe frame

(PVC) and dark nylon netting (2 mm mesh) in Walter Spring at depths ranging from 29 to 66 cm

in areas where both species of fish were known to occur. Each enclosure was embedded at least

20 cm into the soft sediment and extended at least 20 cm out of the water. We used poles to

gently probe through the vegetation to remove fish before the area was enclosed. Any remaining

fish were regularly removed with a hand-held net for one week before the start of our

experiment. Every effort was made to minimize disturbance to the enclosed area, including the

dominant plants, Potamogeton spp., Chara sp., and metaphyton, which is suspended tufts of

filamentous green algae (Goldsborough and Robinson 1996).

We used a target-neighbor design to determine potential harmful effects of mosquitofish

on least chub (Goldberg and Werner 1983). Each enclosure was randomly assigned one of three

initial mosquitofish densities: no mosquitofish (control), 6 mosquitofish, or 100 mosquitofish.

There were six replicates of both treatments and the control. Treatment densities were based on

estimates of high and low mosquitofish densities at Walter Spring. Although males and females

were included in each treatment, large female mosquitofish were most common (> 30 cm). Two

distinct cohorts of YOY least chub were identified: larger, early-spawned (23-28 mm SL) and

smaller, late-spawned (9-13 mm SL). Based on these size differences, we added either 7 or 10

10

YOY least chub to each enclosure. Four replicates out of the six for each treatment received 10

small YOY least chub, whereas two replicates of each treatment received 7 large YOY least

chub.

We measured the initial length of each least chub at the start of the experiment using a

digital camera raised to 30 cm over the water in a white bucket with a plastic ruler fixed to the

bottom. We used the digital image of each fish to measure standard length (SL) to the nearest

millimeter using image analysis software (SigmaScan Pro 5.0, SPSS Inc.). Water depth was

always 2.5 cm, and only images where the fish was resting directly on the tape were used to

measure length. We used a length-mass regression (Wet Mass = 0.0029 e 0.1763 [Standard Length]; R2

= 0.95) to estimate the initial mass of least chub (to the nearest 0.001 g) in each enclosure based

on fifty individuals varying in size from 7 to 30 mm.

After eight weeks (between July and September, 2001) we removed and counted all fish

from each enclosure. We measured the final length and wet mass of all least chub to estimate

their growth in body length and mass. We calculated mean percent survivorship by dividing the

final number of least chub retrieved by the initial number placed in each enclosure. We used a

removal-sampling scheme to determine the number of surviving individuals. The same person

would sweep the entire interior of an enclosure with a hand-held net (45 cm X 40 cm; mesh size

= 2 mm) for five 1-minute periods, with a one-minute break between periods. This procedure

resulted in all the vegetation being removed and searched within the first 2 sampling periods.

Typically, no additional fish were collected after 3 sampling periods, and no fish were ever

collected on the fifth sampling period.

Mosquitofish reproduce multiple times during the summer (Courtenay and Meffe 1989),

so we expected YOY mosquitofish abundance in enclosures to increase during the course of the

11

experiment. Consequently, the number of large predaceous mosquitofish assigned to treatments

did not change, but the number of small mosquitofish that cannot prey on least chub increased,

especially in the low-density treatment (Table 2). Therefore, differences in levels of predation

were maintained between treatments throughout the study, whereas levels of competition

converged between the treatments containing mosquitiofish as the study progressed. This

difference between predation and competition treatments necessitated a difference in the

statistical analysis of survival and growth (see below).

Predation and Habitat Use Experiment

We used a 35-liter aquarium to determine the direct and indirect effects of mosquitofish

predation on least chub. We examined the potential effects of refuge habitat and invertebrate

predation by adding the most abundant types of vegetation (metaphyton, Chara sp., and

Potamogeton spp.) and invertebrate predators (Aeshna sp.: Aeshnidae, Notonecta unifasciata:

Notonectidae, and Belostoma flumineum: Belostomatidae) found at Walter Spring. The

experiment was a fully crossed factorial design manipulating two treatment factors: mosquitofish

presence (absent, or 3 large (> 30 mm) female mosquitofish present), and invertebrate predator

presence (absent, or 3 dragonfly nymphs, 3 adult backswimmers, and 3 giant waterbugs present).

Four small YOY least chub (11 to 14 mm SL) were present in each of the four treatments. Equal

amounts of each vegetation type were used in each trial, and each treatment was replicated four

times. We conducted the treatments in a randomly selected order, and conducted two trials per

day. All fish were used in only one trial and were haphazardly netted from 190-liter aquaria

containing separate populations of least chub and mosquitofish. Invertebrates were haphazardly

12

selected from a 35-liter aquarium containing 8 to 10 individuals of each species. A few

invertebrates could have been used in more than one trial because they were returned to the same

aquarium between trials. The response variables were percent survival of least chub and the

percent of time least chub spent in each habitat type. The percent time spent in habitat types was

determined using a standard scan sample procedure (e.g. Altmann 1974).

Mosquitofish and invertebrate predators were starved for 48 hours and allowed to

acclimate 1-hour before each trial. Least chub were placed in the aquarium at the same time as

mosquitofish and/or invertebrate predators, but they were enclosed in a glass container during the

acclimation period. Following acclimation, least chub were released and an observer recorded

all aggressive interactions and habitat use for each least chub every 15 minutes for 3 hours. All

trials were run between 0800 and 1600 hrs. In order to minimize bias associated with human

intrusion, we made observations through a port (30 cm X 30 cm) behind a blind, and covered the

aquarium on three sides with opaque paper.

Interference Competition Experiment

We compared the activity and habitat use of two size classes of least chub with and without two

size classes of mosquitofish to determine their behavioral interactions. The behavior of each

group of four least chub (two 20 mm– 30 mm SL and two 30 mm – 40 mm SL) was recorded

with no other fish present and with four mosquitofish present (two 20 mm - 35 mm SL and two

40 mm – 45 mm SL). We recorded the mean percentage of time least chub spent actively

swimming versus stationary, and the mean percentage of time both species spent in the open

water versus cover every two minutes throughout each 20-minute trial. Additionally, we

13

recorded every instance of aggressive interactions by noting the species and size of the initiator

and the recipient, and which individual dominated. We identified the initiator of an interaction

as the fish that approached rapidly and directly at another fish and the victor as the fish that

retained the occupied space following the confrontation (see McDonald et al. 1968). We did not

differentiate between size classes of least chub in terms of activity or habitat use. However, we

did note the size of each individual during aggressive interactions. We completed four replicates

of each treatment (least chub alone, and least chub with mosquitofish). This is a test of

interference competition because the smallest least chub were too large to be consumed by

mosquitofish.

We used a 35-liter aquarium containing two habitat types: open water and cover as

simulated vegetation consisting of green rope (5 mm diameter) attached to the bottom. We

covered three sides of the aquarium with opaque paper, and made observations from behind a

blind. Both species were allowed a 20-minute acclimation period before being allowed to feed

and interact for 20 minutes. All fish were used in only a single trial consisting of both treatments

in a paired design. That is, the same least chub used in the control with no mosquitofish were

also used in the treatment with mosquitofish present, which immediately followed the control.

Statistical Analyses

We used 2-way completely crossed analysis of variance (ANOVA) to determine the

effects of mosquitofish density (none, low, and high) and least chub size (23-28 mm SL and 9-13

mm SL) on the mean percentage of least chub that survived in each treatment of the predation

and competition field experiment. This analysis included a mosquitofish density by least chub

size interaction. Because the design was unbalanced we used Type III sums of squares for

14

interpretation (SAS 1997). We used the same design to analyze the effects of mosquitofish

density and least chub size on the growth of least chub, however none of the small least chub

survived in the high mosquitofish density treatment resulting in a missing cell. To solve this

problem and the problem of convergence between competition treatments described above, we

collapsed the low and high mosquitofish density levels and created a 2X2 design with two levels

of least chub size crossed with the presence and absence of mosquitofish. This design also

included the interaction between mosquitofish presence/absence and least chub size. Growth

was measured as the change in least chub length and mass. We used multivariate analysis of

variance (MANOVA, SAS 1997) with both change in mass and change in length as dependent

variables, and then used separate univariate ANOVA’s to analyze the effects of YOY least chub

size and the presence of mosquitofish on each dependent variable.

For the predation part of the laboratory experiment, we used a 2-way ANOVA to

determine the effects of predators on the short-term survival of small least chub in a 2X2

factorial design crossing the presence/absence of mosquitofish and invertebrate predators. This

analysis included the potential effects of the interaction between mosquitofish and invertebrate

predators on least chub. The response variable was the arcsine-square root transform of the

proportion of least chub surviving after each trial.

Determining the effects of predators on small least chub habitat choice was more

complicated because least chub could occupy only one of four habitat types (open water, Chara

sp. Potamogeton sp., and metaphyton) during each scan sample. This resulted in four response

variables for each trial as the percent time spent in each habitat. However, these are not

independent measurements because these fish could only be located in one of the four habitats

during each scan. To resolve this problem we followed the procedure outlined by Manly (1993),

15

and subtracted one response variable from the next for each trial to create differences between

successive response variables. The mean of these three differences was then used as response

variables in a multivariate analysis of variance (MANOVA) to determine the effect of predators

on least chub habitat use. Presence/absence of predators were main effects, and the interaction

between mosquitofish and invertebrate predators was included.

We used a paired t-test to determine the effect of mosquitofish on the mean percent time

least chub spent actively swimming versus stationary, and the mean percent time they spent in

open versus structured habitats in the laboratory interference competition experiment. Because

there were only two mutually exclusive choices for each response variable (active or inactive and

in or out of cover) these measurements were not independent. Thus, we used percent time of one

of the choices for each response variable, time spent stationary and time spent in the open

habitat, as dependent variables in the analysis (e.g. Manly 1993).

Results

Field Competition and Predation Experiment

The presence of mosquitofish reduced both the survivorship and growth of YOY least

chub and the magnitude of effects differed between the small and large cohorts. On average,

survival of both cohorts of least chub was reduced by about one-third (F2,12 = 4.44, P = 0.036)

when mosquitofish were present (Fig. 1). Survival of large YOY least chub was greater than the

survival of small least chub (F1,12 = 14.56, P = 0.003). None of the small YOY least chub

16

survived in any of the enclosures with high mosquitofish densities. The interaction between

mosquitofish presence and least chub size was not significant (F2,12 = 0.89, P = 0.436).

Young-of-year least chub growth (length and mass combined) was lower in the presence

of mosquitofish, (Wilk’s λ = 0.46, F2,8 = 4.69, P = 0.045; Fig. 2), growth differed depending on

the size of least chub (Wilk’s λ = 0.041, F2,8 = 94.14, P < 0.0001; Fig 2), and the least chub by

mosquitofish interaction was significant (Wilk’s λ = 0.44, F2,8 = 5.19, P < 0.036). Growth in

mass of large least chub was 6-fold less with mosquitofish present than in the control (F1,9 =

7.27, P = 0.024), whereas growth in small least chub was only 1.3-fold less than the control

(chub size x mosquitofish interaction: F1,9 = 5.36. P = 0.046, Fig. 2a). Growth in length was

greater for small compared to large least chub (F1,9 = 40.86, P = 0.0001; Fig. 2b), but neither the

presence of mosquitofish (F1,9 = 2.13, P = 0.178) nor the interaction between the mosquitofish

and least chub size (F1,9 = 0.61, P = 0.455) was significant. Even though growth in length of

large least chub was 7-fold lower in the presence versus the absence of mosquitofish (Fig. 2b),

high variance among replicates contributed to a lack of statistical significance.

Predation and Habitat Use Experiment

Mosquitofish readily consumed YOY least chub while invertebrates made only four

predatory attempts. Survival of least chub was lower in the presence of mosquitofish (F1,12 =

106.8, P = 0.0001), but neither the presence of invertebrate predators (F1,12 = 0.11, P = 1.00) nor

the interaction was significant (F1,12 = 0.11, P = 1.00). The combined mean survival of small,

YOY least chub in treatments with mosquitofish was 31.3 % (± S.E. = 6.25). Conversely, 100 %

of the small least chub survived in treatments without mosquitofish.

17

Mosquitofish presence affected habitat use of small least chub (Wilk’s λ1,0.5,4 = 0.225,

F3,10 = 11.47, P = 0.0014). Least chub spent the majority of their time in open habitat with

mosquitofish absent, but in Potamogeton spp. with mosquitofish present (Fig. 3). Habitat use of

small least chub was unaffected by the presence of invertebrate predators (Wilk’s λ1,0.5,4 = 0.82,

F3,10 = 0.74, P = 0.55) or, the interaction between invertebrate predators and mosquitofish

(Wilk’s λ1,0.5,4 = 0.73, F3,10 = 1.22, P = 0.35). Least chub rarely used metaphyton and only

occasionally used Chara sp. in treatments with or without mosquitofish (Fig. 3).

Interference Competition Experiment

Least chub spent more time stationary (t3 = -2.71, P = 0.07) and were more likely to

occupy cover habitat (t3 = 2.58, P = 0.08) in the presence of mosquitofish. Least chub spent an

average of 53 min (± S.E. = 8.8) in cover when mosquitofish were present and 27 min (± S.E. =

3.9) when they were absent. Similarly, they spent, on average, 7.3 min (± S.E. = 2.2) versus 0.75

min (± S.E. = 0.5) stationary when mosquitofish were present or absent, respectively.

Large mosquitofish were the dominant aggressor in all interactions. Large least chub and

small mosquitofish were approximately equal, whereas small least chub occupied the lowest

position in the dominance hierarchy. Although a total of 34 aggressive interactions were

recorded during all trials with mosquitofish, no aggressive interactions were observed between

least chub when mosquitofish were absent, and only two aggressive interactions occurred

between least chub when mosquitofish were present. Of the 34 interactions in treatments

containing mosquitofish, 22 (65 %) were initiated by mosquitofish, with 13 being directed

towards least chub. Mosquitofish (small or large) were successful, (i.e. they did not retreat and

18

retained the occupied space following a confrontation), in all interactions they initiated against

least chub. Large least chub were successful in all eight confrontations initiated against

mosquitofish. Small least chub initiated two aggressive interactions with mosquitofish (both

large) and were not successful in either attempt.

Discussion

Least chub and western mosquitofish provide an excellent example of the possible

interactions between invasive and native species with similar adult body sizes. We found

evidence for all five of the potential negative interactions between invasive species and native

juveniles (Table 1), although the evidence was stronger for interactions that are easier to

demonstrate (predation and interference competition). Together these interactions are complex

and depend on the life stage of both species. Large mosquitofish can eat smaller YOY least chub

and are aggressively dominant over all least chub size classes. Plus, smaller mosquitofish are

equal in dominance with the largest adult least chub. While predation probably accounted for

most of the mortality of small YOY least chub in our field enclosures, the effects of competition

were also apparent. Growth of surviving large YOY least chub was reduced by an average of

107% in treatments with mosquitofish compared to the control without mosquitofish. Our

laboratory experiments showed that YOY least chub responded to mosquitofish aggression by

remaining stationary for long periods, which would reduce their foraging opportunities (e.g.

Werner and Anholt 1996). It was not possible to separate the effects of interference competition

(increased time in plant refugia and remaining stationary) from exploitation competition,

especially because we did not measure differences in resource levels between enclosures with

19

and without mosquitofish. However, the reduced growth of large least chub in the presence of

mosquitofish indicates that one or both forms of competition had a strong effect.

When mosquitofish are present, perhaps least chub spend long periods in refugia (e.g.

dense vegetation) where food resources are reduced compared to open water habitats. This

hypothesis was supported by the observation that least chub in our aquarium experiments seldom

left refuge habitat (Potamogeton sp.) in the presence of mosquitofish, but seldom used it when

mosquitofish were absent. This shift in habitat use in the presence of large mosquitofish may

cause YOY least chub to compete with YOY mosquitofish that take refuge in the same habitat.

Future research should compare food availability and YOY least chub growth in and out of

refuge habitat with and without YOY mosquitofish.

This shift in habitat use of least chub in the presence of mosquitofish led us to

hypothesize that increased utilization of submersed aquatic vegetation (refugia) may increase

least chub exposure to invertebrate predators. However, there was no evidence that invertebrate

predators could efficiently attack and consume least chub in or out of aquatic vegetation under

laboratory conditions. The invertebrates were inactive and usually remained stationary in almost

all trials. Although evidence of an effect of invertebrates on least chub in the laboratory would

provide convincing evidence of the importance of this interaction, the absence of a response is

not sufficient to abandon the hypothesis that mosquitofish, in the field, could indirectly effect

least chub survival by increasing their exposure to invertebrate predators. It is possible that

invertebrate predators could actively forage on small fish at night, but remain inactive during the

day, especially if larger fish are present. Future studies could test this hypothesis by repeating

our laboratory experiments during the day and the night.

20

The overall effects of mosquitofish (predation and competition) depended on least chub

size. The effects of predation were most pronounced on small YOY least chub, whereas reduced

growth was evident for large YOY least chub. These data indicate that YOY least chub pass

through a period in which their small size makes them vulnerable to predation by mosquitofish.

This size window most likely starts at emergence and continues until they grow large enough in

depth that they exceed the gape-width of large mosquitofish. Further research is needed to

determine the exact sizes that define this period of vulnerability to predation. However, once

least chub reach a size refuge from predation they continue to be confronted by mosquitofish

aggression and interference competition. Although the threat of predation may decline with

growth in least chub, the effects of aggression might continue throughout its life cycle because

large mosquitofish can aggressively dominate all size classes of least chub. This aggression may

restrict habitat utilization and food availability throughout the life cycle of least chub.

Although we could not independently test for all single effects (e.g. we could not create

conditions where mosquitofish behaved as a predator but not a competitor), this study does

support our original hypothesis that the ability of native species to trade fitness components and

adjust to invasive species is limited when multiple negative interactions simultaneously affect

multiple size classes. Least chub juveniles appear to be figuratively caught in a vice between

predation and competition. Rapid growth and a larger juvenile body size could reduce the risk of

predation, but this requires energy that is probably not available because of the simultaneous

effects of competition. Instead, competition reduces growth and prolongs the period when

juvenile least chub are vulnerable to mosquitofish predation. A spatial or temporal refuge from

negative interactions would allow the time and/or energy needed for development of

characteristics (e.g. rapid growth) that would increase the survival of native species (e.g.

21

Chapman et al., 2003). However, this is often not possible, especially when native and invasive

species are similar in size and thus, view the world on similar spatial and temporal scales. The

result is often a rapid decline to extinction for native species. Least chub have shown such a

decline, and our results suggest mosquitofish are one of the primary causes.

The number of negative interactions between native species in the absence of invasive

species will vary among communities (e.g. Schoener 1986), and the effects of invasive species

(none, positive, and negative) on all species in a community can be difficult to predict (e.g.

Mittelbach and Chesson 1987). However, we suggest that the total number of negative

interactions that native species experience will increase with the colonization of invasive species

when native and invasive species are similar in body size. Because species perceive the world at

the same spatial and temporal scales if they are similar in size and mobility (MacArthur and

Levins 1964, Levins 1992, Addicott et al. 1987), the number of negative interactions between

invasive and native species should increase with similarity in size and mobility. Other studies

provide evidence that native species failed to coexist with invasive species of similar size and

mobility (see Fuller et al. 1999). For example, invasive mosquitofish have had their greatest

impact on similar-sized native fish of various species while having little impact on larger native

taxa (Myers 1967, Courtenay and Meffe 1989). Certainly, invasive species that are larger or

more mobile than native species can cause a reduction in native species through predatory

interactions (e.g. Elton 1958). But this is a single interaction and native species may trade

various fitness components to reduce the effects of predation and promote coexistence. For

example, if least chub were ten times smaller, then they might exploit smaller prey and habitats

unavailable to mosquitofish, and they might be able to achieve a higher reproductive potential.

Thus, although they would be vulnerable to predation by mosquitofish throughout their life, their

22

small size should allow them to find refuge from predation or competition in habitat unavailable

to the larger invasive species, which should allow coexistence. Multiple interactions are most

likely between similar-sized species. When native and invasive species view the world on

different scales, the number of simultaneous negative interactions decrease and the availability of

refugia increase, thus promoting coexistence and the time needed to make necessary

evolutionary adaptations.

Management efforts should focus on methods that reduce the number of negative

interactions between invasive and native species so that impacted life stages acquire sufficient

time to adjust to the pressures imposed by invasive species. When native juveniles are faced

with predation and competition an obvious management action would be to reduce the number of

invasive adults. Eradication of invasive species is a common management goal, but it is often an

overly daunting task, and in the case of some invasive species (i.e. mosquitofish), it is nearly

impossible. By focusing on efforts that reduce the number of negative interactions, management

goals may shift to practices that promote coexistence between native and invasive species. In the

case of least chub and mosquitofish, removal of large mosquitofish in the spring when least chub

are small and vulnerable could increase the potential of coexistence. Plus, habitat manipulations

that reduce the optimal habitat for mosquitofish, warm shallow water, could decrease

mosquitofish growth and the rate at which their populations increase in the spring and summer.

Although least chub and mosquitofish view the world on similar scales, they have different

fundamental niches because they have different evolutionary histories. Western mosquitofish

thrive in warm water because they evolved in subtropical environments (Courtenay and

Meffe1989), whereas least chub historically occupied a variety of habitats with both cool and

warm temperatures (Sigler and Miller 1987). Habitat manipulations that decrease warm waters

23

and increase cool temperatures, and the removal of large mosquitofish may provide sufficient

time for least chub to grow and reach a size refuge before predation and competition from

mosquitofish can diminish recruitment.

Acknowledgements

We gratefully acknowledge funding support from the Utah Division of Wildlife Resources and

logistical support from Jay Banta and the staff at the Fish Springs National Wildlife Refuge. We

thank Benjamin Shettell, Adrian Bell, and Amy Schweitzer for assistance in setting up our field

experiment. This study was conducted under the approval of the Institutional Animal Care and

Use Committee at Brigham Young University. Permits to collect the fish used in this study were

provided by the Utah Division of Wildlife Resources.

Literature Cited

Ashton PJ, Mitchell DS (1989) Aquatic plants: patterns and modes of invasion, attributes of

invading species and assessment of control programmes. In: Drake JA, Mooney HA, Di

Castri F, Groves RH, Kruger FJ, Rejmánek M, and Williamson M (eds) Biological

invasions: a global perspective. John Wiley & Sons, Chichester, pp 111-154

Altmann, J 1974. Observational study of behaviour: sampling methods. Behaviour 49:227-267

Barrier RFG, Hicks BJ (1994) Behavioral interactions between black mudfish (Neochanna

diversus) and mosquitofish (Gambusia affinis). Ecol Freshwat Fish 3:93-99

24

Belk MC, Whitney MJ, Schaalje GB (2001) Complex effects of predators: determining

vulnerability of the endangered June sucker to an introduced predator. Anim Conserv

4:251-256

Bertolino S, Genovesi P (2003) Spread and attempted eradication of the grey squirrel (Sciurus

carolinensis) in Italy, and consequences for the red squirrel (Sciurus vulgaris) in Eurasia.

Biol Conserv 109:351-358

Brown JH (1989) Patterns, modes, and extents of invasions by vertebrates. In: Drake JA,

Mooney HA, Di Castri F, Groves RH, Kruger FJ, Rejmánek M, and Williamson M (eds)

Biological Invasions: A Global Perspective. John Wiley & Sons, Chichester, pp 85-110

Chapman LJ, Chapman CA, Schofield PJ, Olowo JP, Kaufman L, Seehausen O, Ogutu-Ohwayo

R (2003) Fish faunal resurgence in Lake Nabnugabo, East Africa. Conserv Biol 17:500-

511

Courtenay, WR and GK Meffe. (1989) Small Fishes in Strange Places: A Review of Introduced

Poeciliids. In: GK Meffe and FF Snelson, Jr. (eds) Ecology and Evolution of Livebearing

Fishes (Poeciliidae). Prentice Hall, Englewood Cliffs, New Jersey, USA, pp 319-331

Crist L, Holden PB (1990) Aquatic biology study of a spring complex in Snake Valley, Utah

final summary report Pr-36-1. Bio/West Inc, Logan

25

Crivelli AJ (1995) Are fish introductions a threat to endemic freshwater fishes in the Northern

Mediterranean region. Biol Conserv 72:311-319

Elton, CS (1958) The ecology of invasions by animals and plants. University of Chicago Press,

Chicago

Galat DL, Robertson B (1992) Response of endangered Poeciliopsis-Occidentalis sonoriensis in

the Rio-Yaqui drainage, Arizona, to introduced Gambusia affinis. Environ Biol Fishes

33:249-264

Gamradt SC, Kats LB (1996) Effect of introduced crayfish and mosquitofish on California

newts. Conserv Biol 10:1155-1162

Goldberg DE, Werner PA (1983) Equivalence of competitors in plant communities: a null

hypothesis and a field experimental approach. Am J Bot 70:1098-1104

Goldsborough LG, Robinson GC (1996) Pattern in wetlands. In: RJ Stevenson, ML Bothwell,

RL Lowe (eds). Algal Ecology: Freshwater Benthic Ecosystems. Academic Press, New

York, New York, USA, pp 77-117

Goodsell JA, Kats LB (1999) Effect of introduced mosquitofish on Pacific treefrogs and the role

of alternative prey. Conserv Biol 13:921-924

26

Gurevitch J, Morrison JA, Hedges LV (2000) The interaction between competition and

predation: a meta-analysis of field experiments. Am Nat 155:435

Holden P, White W, Sommerville G, Duff D, Gervais R, Gloss S. (1974) Threatened fishes of

Utah. Utah Acad Sci Arts Let 2:46-65

Hickman TJ (1989) Status report of the least chub (Iotichthys phlegethontis) prepared for the

U.S. Fish and Wildlife Service. Western Ecosystems, St. George

Kiesecker JM, Blaustein AR, Miller CL (2001) Potential mechanisms underlying the

displacement of native red-legged frogs by introduced bullfrogs. Ecology 82:1964-1970

Komak S, Crossland MR (2000) An Assessment of the introduced mosquitofish (Gambusia

affinis holbrooki) as a predator of eggs, hatchlings, and tadpoles of native and non-native

anurans. Wildl Res 27:185-189

Kotler BP, Holt RD (1989) Predation and competition: the interaction of two types of species

interactions. Oikos 54:256-260

Lydeard C, Belk MC (1993) Management of indigenous species impacted by introduced

mosquitofish: an experimental approach. Southwest Nat 38:370-373

27

Manly, BFJ (1993). Comments on the design and analysis of multiple-choice feeding-preference

experiments. Oecologia 93:149-152

McDonald AL, Heimstra NW, Damkot DK (1968) Social modification of agonistic behaviour in

fish. Anim Behav 16:437-441

Meffe GK (1985) Predation and species replacement in American Southwestern fishes: a case

study. Southwest Nat. 30:173-187

Mittelbach GG (1986) Predator-mediated habitat use: some consequences for species

interactions. Environ Biol Fishes 16:159-169

Mittelbach GG (1988) Competition among refuging sunfishes and effects of fish density on

littoral zone invertebrates. Ecology 69: 614-623

Mittelbach GG, Chesson PL (1987) Predation risk: indirect effects on fish populations. In: Sih A,

Kerfoot CW (eds) Predation: direct and indirect impacts on aquatic communities.

University Press of New England, Hanover, pp 315-332

Myers, GS (1967) Gambusia, the fish destroyer. Australian Zoologist 13:102

28

Ogutu-Ohwayo R, (1993) The effects of predation by Nile perch, Lates niloticus L., on the fish

of lake Nabugabo, with suggestions for conservation of endangered endemic cichlids.

Conserv Biol 7:701-711

Perkins MJ, Lentsch LD, Mizzi J (1998) Conservation Agreement and Strategy for Least Chub

(Iotichthys phlegethontis). Utah Division of Wildlife Resources, Salt Lake

Rincon PA, Correas AM, Morcillo F, Risueno P, Lobon-Cervia J (2002) Interaction between the

introduced Western mosquitofish and two autochthonous Spanish toothcarps. J Fish Biol.

61:1560-1585

Sakai AK (2001) The population biology of invasive species. Annu Rev Ecol Syst 32:305-332

SAS, I. (1997). SAS/STAT software: changes and enhancements through release 6.12. Cary,

North Carolina, USA, SAS Institute.

Savidge JA (1987) Extinction of an island avifauna by an introduced snake. Ecology 68:660-668

Schoener TW (1986) Overview: kinds of ecological communities--ecology becomes pluralistic.

In: Diamond J and TJ Case (eds) Community Ecology. Harper and Row, New York, pp

467-479

29

Sigler WF, Sigler JW (1987) Fishes of the Great Basin, a natural history. University of Nevada

Press, Sigler WF, Miller RR (1996) Fishes of Utah. University of Utah Press, Salt Lake

Simberloff D (1989) Which insect introductions succeed and which fail. In: Drake JA, Mooney

HA, Di Castri F, Groves RH, Kruger FJ, Rejmánek M, and Williamson M (eds)

Biological invasions: a global perspective. John Wiley & Sons, Chichester, pp 61-76

Soderback B (1995) Replacement of the native crayfish Astacus astacus by the introduced

species Pacifastacus leniusculus in a Swedish lake- possible causes and mechanisms.

Freshwat Biol 33:291-304

Tollrian R, Harvell CD (1999) The ecology and evolution of inducible defenses. Princeton

University Press, Princeton

Townsend CR (1996) Invasion biology and ecological impacts of brown trout Salmo trutta in

New Zealand. Biol Conserv 78:13-22

Wellborn GA (2002) Trade-off between competitive ability and antipredator adaptation in a

freshwater amphipod species complex. Ecology 83:129-136

Werner EE, Anholt BR (1996) Predator-induced behavioral indirect effects: consequences to

competitive interactions in anuran larvae. Ecology 77:157-169

30

Winkelman DL, Aho JM (1993) Direct and indirect effects of predation on mosquitofish

behavior and survival. Oecologia 96:300-303

Wissinger SA (1992) Niche overlap and the potential for competition and intraguild predation

between size-structured populations. Ecology 73:1431-1444.

31

Table 1. Potential negative interactions between native and invasive species with similar adult

body sizes. A minus sign (-) indicates interactions that could have a negative impact on

native species, while a zero (0) indicates an improbable interaction.

Invasive Adults Invasive Juveniles Mechanism of Interaction Native

Juveniles Native Adults

Native Juveniles

Native Adults

Predation - 0 0 0 Interference Competition - - - 0 Exploitation Competition - - - -

32

Table 2. Mean number of mosquitofish and standard deviations (in parentheses) in each

mosquitofish density treatment at the conclusion of the field experiment.

Treatment Initial Size of Least

Chub

Mean Number Large

Mosquitofish (SL >25mm)

Mean Number Small

Mosquitofish (SL <25mm)

Mean Total Number of Mosquitofish

Least Chub Alone Large 2 (0) 4.5 (0.7) 6.5 (0.7) Small 1 (2) 6.25 (6.7) 7.25 (5.7)

Low Mosquitofish Large 15 (9.9) 141 (188.1) 156 (197.9) Small 8.5 (6.8) 191 (170.9) 199.5 (176.8)

High Mosquitofish Large 107 (31.1) 46.5 (23.3) 153.5 (7.8) Small 116.5 (18.9) 46.5 (15.7) 163 (34.5)

33

Figure Legends

Figure 1. Mean survivorship of small (9-13 mm standard length (SL)) and large (23-28 mm SL)

young-of-the-year (YOY) least chub (Iotichthys phlegethontis) cohorts in enclosures with

different mosquitofish (Gambusia affinis) densities. Bars represent one standard error. Data

points are covering some standard error bars.

Figure 2. Mean growth as the increase in (A) wet mass and (B) length of small (9-13 mm SL)

and large (23-28 mm SL) YOY least chub cohorts in enclosures with mosquitofish present or

absent. Bars represent one standard error.

Figure 3. Mean percent time that small (11 to 14 mm SL) YOY least chub spent in four habitat

types (open water, Potamogeton sp., Chara sp., and metaphyton). Bars represent one standard

error.

34

Mosquitofish density

None Low High

Perc

ent s

urvi

val

0

20

40

60

80

100Large least chubSmall least chub

35

Treatment

Incr

ease

in le

ngth

(mm

)

-4

-2

0

2

4

6

8

10

12

14

Incr

ease

in w

et m

ass

(g)

0.0

0.1

0.2

0.3

0.4

0.5

Large least chubSmall least chub

Mosquitofish absent Mosquitofish present

A)

B)

36

Habitat type

Perc

ent t

ime

(min

)

0

20

40

60

80

100 No predatorsInvertebrate predatorsMosquitofish predatorsInvertebrate and mosquitofish predators

Open Chara Metaphytonsp.Potamogeton spp.