Ecosystems DOI: 10.1007/s10021-015-9865-x 2015 Springer Science+Business Media New York

Invasive Cane Toads’ Predatory Impact on Dung Beetles is Mediated by Reservoir Type at Artificial Water Points

Benjamin Feit,1,2* Tim Dempster,3 Heloise Gibb,4 and Mike Letnic2

1Hawkesbury Institute for the Environment, University of Western Sydney, Richmond, New South Wales, Australia; 2School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, Australia; 3Department of Zoology, University of Melbourne, Melbourne, Victoria, Australia; 4Department of Zoology, La Trobe University, Melbourne, Victoria, Australia

ABSTRACT Disruption to ecosystem functioning associated populations, toad diets, and dung removal rates at with biological invasions can have dramatic effects AWP fitted with dams or tanks as reservoirs. In on the production and biodiversity values of comparison to dams, tanks provide toads with little ecosystems. In semi-arid rangelands of Australia, access to water. Population densities of toads were invasive cane toads (Rhinella marina) prey on dung five times higher at dams than tanks. Conversely, beetles that were themselves introduced to pro- population densities of dung beetles were 12 times mote nutrient cycling and reduce parasite burdens lower at dams than tanks. Mass loss of ex- of livestock. Cane toads’ colonization of rangelands perimental dung pats after 2 days was 13% greater has been facilitated by artificial water points (AWP) at tanks than dams. Our study provides evidence which provide cattle with drinking water. Most that consumption of detritivores by an introduced AWP in northern Australia comprise bores that predator can retard dung decomposition in a pump water into earthen reservoirs (dams). Dams rangeland ecosystem. Restricting toads’ access to typically support large toad populations because water at AWP should benefit livestock production they allow toads’ access to water without which by reducing both toad populations and toads’ they could not survive. Here, we ask if restricting predatory impact on dung beetles. toads’ access to water at AWP can reduce toad populations, toads’ predatory impact on dung Key words: cane toad; invasive species; pest spe- beetles, and the rate of dung decomposition by cies management; rangeland; water development; dung beetles. We contrasted toad and dung beetle dung decomposition.

INTRODUCTION Received 7 December 2014; accepted 10 February 2015 Electronic supplementary material: The online version of this article After their introduction to new ecosystems, the (doi:10.1007/s10021-015-9865-x) contains supplementary material, success of invasive plant and animal species is often which is available to authorized users. mediated by anthropogenic habitat change (Byers Author contributions BF, TD, HG and ML designed the research; BF collected and analyzed data; BF, TD, HG and ML wrote the manuscript. 2002; Didham and others 2007). In many situa- *Corresponding author; e-mail: [email protected] tions, invasive species only become established in B. Feit and others habitats where dominant native species have been larvae of flies and nematodes, dung beetles increase displaced as a result of human actions or where pasture availability for livestock and disrupt the life humans inadvertently provide essential resources cycles of cattle parasites (Bornemissza 1960;Fay such as food, water, or shelter (Vitousek 1997; and Macqueen 1990). By breaking down dung, Didham and others 2007; Letnic and others 2014). dung beetles also promote nutrient cycling in Once populations of invasive species are estab- rangeland ecosystems (Nichols and others 2008). lished, the novel interactions they have with other Although no figures exist for Australia, in the USA, species can transform ecosystems by modifying the combined value of nutrient turnover and sup- species assemblages and disrupting nutrient fluxes pression of parasites by dung beetles has been (Mack and D’Antonio 1998; Crooks 2002; Croll and valued at $380 million y-1 (Losey and Vaughan others 2005). The disruption to ecosystem dy- 2006). namics and loss of key ecosystem services that Recently, the invasive cane toad Rhinella marina frequently accompanies the establishment of in- L. has colonized semi-arid rangelands in northern vasive species can have adverse impacts on human Australia. In Australia, cane toads occur at very economic activities (Pimentel and others 2005). high densities (Freeland 1986; Florance and others Agricultural ecosystems and rangelands are at 2011), and consequently can exert strong sup- particular risk of impact from invasive species, as pressive effects on their prey (Greenlees and others they provide altered habitats that not only often 2006). Among a wide range of primarily inverte- facilitate invaders (Russell and others 2011) but brate prey species, cane toads show a predilection also rely on a range of ecosystem-derived services for dung beetles, an abundant food source in pas- to sustain production. toral ecosystems (Zug and Zug 1979; Gonza´lez- One of the most extensive uses of terrestrial Bernal and others 2012) (Figure 1C). ecosystems by people is the use of semi-arid and The cane toad invasion of semi-arid regions has arid landscapes as rangelands for livestock pro- been facilitated by the proliferation of AWP. In duction (Asner and others 2004). In rangeland northern Australia, most AWP consist of a bore- ecosystems, direct human modification of pastures hole from which water is pumped to an earthen is minimal; however, pastoral-related activities in- reservoir (commonly referred to as a dam) via a cluding the introduction of livestock, predator diesel pump or windmill (Figure 1A). Water is then control, fencing, and provision of water can have supplied via gravity to a trough from which live- profound effects on the composition and function stock drink (Figure 1B). Because cane toads, which of ecosystems (James and others 1999; Anderson need access to water to survive in the hot and dry and Hoffman 2007; Letnic and others 2009). To landscapes of semi-arid Australia, can readily re- ensure that livestock in the arid rangelands of hydrate in the water stored in dams, dams function Australia have access to water, pastoralists have as dry-season refuges for cane toads in otherwise established artificial water points (AWP) where naturally waterless landscapes (Florance and others water is typically captured using dams or bores and 2011; Letnic and others 2014). Consequently, supplied to livestock, which focus their activity during periods of dry conditions, cane toad around AWP (James and others 1999). Another populations are largely restricted to areas near human modification of Australia’s rangeland dams (Letnic and others 2014). ecosystems to enhance sustainability for livestock In semi-arid rangelands, the distribution of live- production has been the introduction of 23 species stock dung is concentrated around artificial water of dung beetles from Africa, Asia, and Europe sources, where the animals congregate during (Edwards 2009; Duncan and others 2009). daytime (James and others 1999), increasing the Dung beetles provide valuable ecosystem services attractiveness for dung beetles. The overlap in the for the pastoral industry. Prior to the introduction distribution of dung beetles and cane toads, fa- of dung beetles to Australian rangelands, large ex- cilitated by the presence of surface water, could panses of land were ‘‘locked-up’’ by dung which exacerbate the predatory impact of cane toads on hampered pasture growth and the utilization of dung beetles and diminish the valuable ecosystem pastures by cattle because cattle avoid grazing in services that dung beetles provide to the agricul- areas covered with dung (Dohi and others 1991). In tural industry. Indeed, recent research has shown addition, parasites such as buffalo flies Haematobia that predation of dung beetles by cane toads can irritans exigua L. and nematodes that require cattle lead to a reduction in the rate of dung decompo- dung to complete their life cycles had substantial sition (Gonza´lez-Bernal and Greenlees 2013). adverse effects on cattle health. By burying dung As a strategy to reduce the effects of cane toads and in the process causing damage to eggs and on dung beetles, landholders could potentially Introduced Predator Disrupts Dung Decomposition

Figure 1. A Traditionally used open earthen dams at AWP provide a large body of standing water readily accessible for cane toads (Rhinella marina) for rehydration and reproduction. B Earthen dams and tanks use gravity to feed water into a trough from which livestock drink. Toads cannot normally access the water in troughs. C Cane toad foraging for dung beetles at a dung pat. D Closed tanks as an alternative type of reservoir at AWP do not enable cane toads to access standing water.

suppress populations of toads over large areas by be lower at tanks than dams; (3) the rate of dung changing the type of reservoir used at AWP, removal should be lower at dams than tanks owing specifically by using tanks made of plastic or steel as to lower population densities of dung beetle at an alternative to earthen dams (Letnic and others dams. 2015) (Figure 1D). At tanks, cane toads cannot readily access the water stored in the reservoir. METHODS However, as cane toads are capable of absorbing moisture from the soil for rehydration (Zug and Study Area and Time Zug 1979), toads can still survive in small numbers The study area was located in the northern margin at tanks where they can rehydrate from water of the Tanami Desert in the Northern Territory, emanating from leaks in pipeline joints and in Australia, on three cattle stations, Dungowan temporary pools of water created when tanks (1642¢S and 13216¢E), Camfield (1720¢S and overflow (Letnic and others 2014). 13117¢E), and Riveren (1754¢S and 13013¢E). Here, we tested whether water management The area experiences an annual mean rainfall of strategy in semi-arid rangelands in Australia me- 660.8 mm (1974–2013) of which 96% occurs in diates the impact of invasive cane toads on dung the wet season (November to April) and 4% in the beetle populations and the ecosystem services they dry season (May to October; Australian Bureau of provide. We did this by comparing population Meteorology). Population densities of dung beetle densities of cane toads and dung beetles, cane toad in our study area follow a seasonal pattern with predation rates, and dung removal rates in the vi- high densities during the wet season and low cinity of dams and tanks. Applying prior knowledge densities during the dry season (Davis 1996). Sur- of the biology of cane toads and dung beetles, we veys for the population densities of cane toads and tested the following predictions: (1) Tanks should dung beetles were conducted during high dung support fewer toads than earthen dams because the beetle densities at the end of two wet seasons, in water available to toads is more limited at tanks April–May 2012 and April 2013. Experiments on than dams; (2) the population density of dung dung removal rates at dams and tanks were con- beetles should be greater at tanks than dams be- ducted during a period of low dung beetle densities cause the magnitude of predation by toads should at the end of the dry season in September 2014. B. Feit and others

The three stations have installed a mix of earthen bulbs. To investigate the consumption of dung dams and tanks made of plastic or steel at cattle beetles by toads, we conducted stomach content watering points. Traditionally, earthen dams have analyses on a random sample of 138 adult cane been the preferred reservoir for the storage of bore toads (on average 16.0 ± 5.3 toads at each dam and water in the region. However, because a substantial 7.3 ± 3.5 toads at each tank). The toads were eu- amount of water in earthen reservoirs is lost thanized and dissected immediately upon collec- through evaporation and seepage (Baillie 2008), tion. Dung beetles found in toads stomachs were landholders of both stations have recently installed enumerated and identified to species level. tanks at watering points to reduce water losses and Because multiple AWP are located within single ultimately the fuel costs required to supply reser- paddocks and cattle are free to roam between AWP, voirs with water. it was not possible to estimate resource availability for dung beetles based on stocking densities at each Population Densities of Dung Beetles and AWP. However, as cattle dung is a potentially Cane Toads and Dung Availability limiting resource for dung beetles, we indexed the availability of fresh cattle dung. At each AWP, we Dung beetle species occurring in our study area counted the number of fresh dung pats on two belonged to four species, F., Onthophagus gazella 2m9 100m transects (n = 2 per AWP) radiating in Onitis alexis K., Onitis viridulus B., and Euniticellus opposite directions from the AWP. The index of intermedius R. (Edwards 2009). Three of these spe- dung density per 100 m2 was calculated from the cies, O. gazella, O. onitis, and O. viridulus, were flying sum of dung pats on both transects. exclusively during dusk and dawn, whereas E. in- termedius was observed approaching dung pats Predation Pressure on Dung Beetles during the day (approximately 1–2 h before dusk, pers. obs.). However, beetles of each species are We estimated the total predatory impact of cane actively moving within dung pats throughout the toads on dung beetles at each AWP by adapting the night, and thus, their activity patterns closely re- index of estimated species impact of Wootton sembled the activity patterns of the nocturnal cane (1997): toads (Zug and Zug 1979), leaving them vulnerable q/ð1=dÞbkt; to predation. We therefore conducted nocturnal surveys for where parameter q is the average number of dung dung availability and population densities of dung beetles ingested by the dissected cane toads, pa- beetles and cane toads at a total of 13 AWP (five rameter / is an indicator of the proportion of cane dams and eight tanks) in areas stocked with cattle toads foraging at any given time, parameter d is the Bos indicus L. Each survey was conducted ap- number of dung beetles per 100 m2 calculated as proximately 1.5 h after sunset. the product of the average number of dung beetles We determined the population density of dung per sample and the number of fresh dung pats per beetles by using a 200 ml container to take samples 100 m2 with an average dung pat volume of 3 l, from randomly selected fresh dung pats (that is, parameter b is the number of cane toads per surface not dried up) in a 50-m radius around each 100 m2, and parameter k indicates the time over AWP. The number of dung pats selected was de- which dung beetles are vulnerable to predation and pendent on dung availability but was set at a parameter t incorporates the elapsed time. We de- maximum of ten dung pats per AWP. We sampled a termined /, the fraction of cane toads foraging at total of 110 dung pats (on average 9.2 ± 1.6 at each any given time, as the fraction of cane toads en- dam and 8.0 ± 2.2 at each tank) and counted a countered at distances greater than 2 m from a total of 3034 individual dung beetles. Specimens of water source, indicating that they were not rehy- adult dung beetles found in the samples were drating but likely foraging for prey. We can assume identified to species under a microscope following that k, the time over which dung beetles are vul- the key provided by Tyndale-Biscoe (1990). nerable to predation, is a function of the time the We determined the population density of cane beetles spend actively moving on and within dung toads at each dam and tank as individuals per pats, where they are visible to cane toads. As spe- 100 m2 calculated from the sum of cane toads en- cies composition in dung pats did not differ be- countered on surveys along 2 m 9 100 m transects tween sites (see ‘‘Results’’ section) and thus (n = 4 per AWP) radiating in each of the four car- potential behavioral differences between species dinal directions from the dam or tank, respectively, would not result in differing k between sites, using handheld 12 V spotlights with 25 W halogen we defined k as a constant in our calculations. Introduced Predator Disrupts Dung Decomposition

Parameter t, the elapsed time, is redundant in our ings, we classified each AWP as either being sand or calculations as each of the surveys was conducted clay. As the activity of insects is correlated with at approximately the same time of day. ambient temperature (Mellanby 1939), we ex- pected dung beetles to be less active at lower tem- Dung Decomposition Rates peratures. Ambient temperature at the time of each survey was recorded using a Kestrel 3000 Pocket To investigate consequences for ecosystem services Weather Meter (Nielsen-Kellerman, Boothwyn, that might arise from the differing levels of dung PA, USA). beetle population density at dams and tanks, we compared the rate of removal of fresh cattle dung at Dung Decomposition Rates dams and tanks. We did this by contrasting mass loss as a proportion of the initial mass of artificial For the analysis of variables that could affect de- dung pats at a total of six AWP (three dams and composition rates of dung pats, we selected reser- three tanks). To construct experimental dung pats, voir type (tank or dam), dung beetle density, dung we collected 8 kg of freshly dropped dung in the availability, average ambient temperature, and vicinity of each AWP that had not been colonized average relative humidity at 3 pm during the du- by dung beetles in the late afternoon. We molded ration of the experiment (Table 1). Ambient tem- the dung into a circular 675-ml plastic container perature and relative humidity data were obtained that was 15 cm in diameter. The experimental from the Australian Bureau of Meteorology. dung pats produced weighed approximately 800 g. Because of the higher population densities of We then randomly placed ten experimental dung dung beetles at tanks in comparison to dams (see pats within enclosures at each AWP that prevented Results), we expected dung decomposition rates to cattle from interfering but allowed both dung be lower at dams. The magnitude of dung decom- beetles and cane toads unrestricted access for the position should scale with the number of dung duration of the experiment. The original mass of beetles; thus, we expected the number of dung each dung pat was recorded, and dung pats were beetles found in the dung pats to be positively re- left undisturbed for 48 h. At the end of the ex- lated to dung decomposition. Ambient temperature periment, the number of dung beetles present in and relative humidity are both environmental each dung pat and the final mass of the dung pat factors that have an effect on evaporation. As our were recorded after all insects had been removed. experimental procedure did not allow the mea- surement of dry mass loss, we included these pre- dictors in our models to investigate their potential Predictor Variables impacts on mass loss due to evaporation. Because Population Density of Dung Beetles all AWP used in this experiment were located on sand, type of soil was not included as predictor. We selected a set of environmental variables as predictors that were considered likely to influence Data Analysis the population density of dung beetles. These variables included reservoir type (tank or dam), Because dung beetle count data were overdis- population density of toads, dung availability, soil persed, we used generalized linear mixed models type (sand or clay), and ambient temperature (GLMM) with a negative binomial distribution and (Table 1). a log-link function to compare population densities Because population densities of cane toads were of dung beetles between dams and tanks. For the higher at dams than tanks (see ‘‘Results’’ section), comparison of population densities of cane toads we expected the population density of dung beetles and consumption of dung beetles and other prey by to be lower at dams. As the magnitude of the im- cane toads between dams and tanks, we used pact of cane toads should scale with the population GLMM with a Poisson distribution and a log-link density of cane toads, we expected that population function. Type of reservoir was entered as predictor densities of toads and dung beetles would be variable, and AWP identity and the year in which negatively correlated. Previous studies have shown the survey was conducted were included as ran- that the population density of dung beetles could dom factors in each model. Differences in dung also be positively correlated with the availability of decomposition rates, dung availability and preda- dung (Koike and others 2014). Dung beetle densi- tory impact of cane toads on dung beetles between ties could be further affected by soil hardness, with dams and tanks were analyzed using linear mixed higher densities on sand than on harder clay soils models (LMM) with type of reservoir as predictor (Davis 1996). Based on the soil in their surround- and AWP identity and year of survey as random B. Feit and others

Table 1. Predictor Variables Used in the Models and Their Predicted Correlation with the Population Densities of Dung Beetles and Dung Removal Rates

Predictor Type Categories Correlation Cause of correlation

Population density of dung beetles Type of reservoir Categorical Closed (tank) Positive Low cane toad densities Open (dam) Negative High cane toad densities Type of soil Categorical Sand Positive Soft soil Clay Negative Hard soil Cane toad density Continuous Negative Predation Dung availability Continuous Positive Resource availability Ambient temperature Continuous Positive Higher activity Dung removal rates Type of reservoir Categorical Closed (tank) Positive High dung beetle densities Open (dam) Negative Low dung beetle densities Dung beetle density Continuous Positive Dung removal by beetles Dung availability Continuous Positive Resource availability Ambient temperature Continuous Positive Increased evaporation Relative humidity Continuous Negative Decreased evaporation

Models predicting the population density of dung beetles include the random factors: AWP identity and year of survey; models predicting dung removal rates include the random factor: AWP identity.

factors. Additionally, differences in dung beetle AWP identity and year of survey as random factors species composition in both dung pats and toad in each model. We used model selection based on stomachs were analyzed using chi-squared (v2) Akaike’s Information Criteria corrected for small tests. sample sizes (AICc), where a model with a lower We used GLMM with a Poisson distribution and AICc is preferred to one with a higher AICc a log-link function to assess which of the predictor (Burnham and Anderson 2002). The level of em- variables were most strongly associated with the pirical support for each model was determined by population density of dung beetles. The response calculating the relative values (DAICc) of each variable was the number of dung beetles in each model in relation to the best model. Given DAICc dung sample. The predictor variables reservoir type for each model, the relative likelihood of a model (tank or dam), population density of toads, dung was calculated using Akaike weights (AICw) availability, soil type (sand or clay), and ambient (Burnham and Anderson 2002) with the weight of temperature were fixed factors. any particular model depending on the entire set of We used LMM to assess which of the predictor candidate models, varying from 0 (no support) to 1 variables were most strongly associated with dung (complete support). We used AICw to estimate pa- decomposition rates. The response variable was the rameters and variances weighted by the relative percentage of mass loss of each dung pat after 48 h. strength of each model following Burnham and The predictor variables reservoir type (tank or Anderson (2002). dam), population densities of dung beetles and Because several models in the candidate set to toads, dung availability, soil type (sand or clay), explain dung decomposition rates had similar ambient temperature, and relative humidity were support, we used model averaging to overcome fixed factors. model uncertainty (Burnham and Anderson 2002). To analyze the relationship between predictor We calculated the average parameter estimates variables and population densities of dung beetles from four models with a DAICc of less than 2 as the and predictor variables and dung decomposition average of all coefficients, substituting zeros for rates, we first constructed global models including models where the particular variable was excluded. all predictor variables and subsequently selected a To account for the spatial distribution of AWP set of biologically sensible candidate models con- within the study area, we assessed the residuals of sisting of single and combinations of multiple pre- the best models in both analyses for spatial auto- dictor variables in an a priori approach. To account correlation. All statistical analyses were conducted for the nested structure of the data, we included in R (Version 3.0.3). Introduced Predator Disrupts Dung Decomposition

RESULTS which bury the dung where it was dropped (Doube 1991). Population Densities of Dung Beetles and The average number of fresh dung pats per 100 m2 Cane Toads and Dung Availability did not differ between dams (5.2 ± 0.15) and tanks The number of cane toads per 100 m2 was 6.4 times (4.8 ± 0.14; LMM, F = 0.46, df = 1, P = 0.5). higher at dams (0.9 ± 0.25) than tanks (0.14 ± 0.03; GLMM, F1,11 = 15.29, P < 0.01; Figure 2A). Stomach Contents Juvenile (n = 5, snout-vent length less than 90 mm; Zug and Zug 1979) and metamorph cane A higher percentage of toads collected from tanks toads (not counted during the surveys due to their (n = 43 of 59, 72.9%) than dams (n = 41 of 80, small body size of <2 cm) were only encountered 51.3%; v2 = 11.9, df = 1, P < 0.001) had ingested at dams. dung beetles. Cane toads preyed on all of the four Dung beetle densities per liter of dung were 9.6 dung beetle species found in the area (O. gazella, E. times lower at dams (22.9 ± 3.3) than tanks intermedius, O. alexis, O. viridulus); the ratio of dung (220.8 ± 24.9; GLMM, F1,108 = 22.36, P < 0.001; beetle species ingested by toads did not differ be- Figure 2B). All beetles belonged to one of four tween toads from dams and tanks (v2 = 1.9, df = 3, species, with O. gazella being the most abundant P = 0.6) and did not differ from the ratio of dung (3109 individuals, 93.2%); E. intermedius (130, beetle species found in dung samples (v2 = 4.24, 4.2%), O. alexis (41, 1.3%), and O. viridulus (40, df = 3, P = 0.24; Table 2). However, E. intermedius 1.3%) occurred at low densities (Table 2). Species was poorly represented in toad stomachs (4.2% of composition did not differ between dams and tanks dung beetles in dung pats; 0.2% of dung beetles (v2 = 1.52, df = 3, P = 0.7). The four species belong ingested by toads). The low occurrence of E. inter- to the functional group of nocturnal tunnelers, medius could be the result of its smaller body

Figure 2. A Number of cane toads Rhinella marina per 100 m2 at the two different reservoir types: tank (n = 8) and dam (n = 5). B Number of dung beetles per liter of fresh dung at the two different reservoir types: tank (n = 64 dung pats) and dam (n = 46 dung pats). Error bars indicate ±1SE. B. Feit and others

Table 2. Total Number and Ratio of Dung Beetle Species Found in Dung Pats and Ingested by Cane Toads Rhinella marina

Onthophagus gazella Euoniticellus intermedius Onitis alexis Onitis viridulus

In dung pats (n = 110) 3109 130 41 40 93.20% 4.20% 1.30% 1.30% Ingested (n = 138) 1719 3 30 18 97.10% 0.20% 1.70% 1.00% size compared to the other dung beetle species differ between dams (3.5 ± 0.5) and tanks (4.4 ± (E. intermedius: 7–9 mm, O. gazella: 10–13 mm, O. 1.0; GLMM, F1,131 = 0.02, P = 0.9; Figure 3A). alexis: 13–20 mm, O. viridulus: 18–23 mm) that Dung beetles were a more substantial component of made it less likely to be detected by cane toads while the diet of toads at tanks (70.5% of the total number moving in or on dung pats but also be attributable to of prey items) than at dams (36.3%; v2 =21.8, their diurnal flight pattern and thus lower overlap df = 1, P < 0.001). with the activity period of cane toads. Overall, individual cane toads ingested 6.6 times Predation Pressure on Dung Beetles more dung beetles at tanks (25.8 ± 4.9) than at dams (3.9 ± 0.6; GLMM, F1,131 = 31.02, P < 0.001; Estimates of the predatory impact of cane toads on Figure 3A). The highest number of dung beetles dung beetles differed significantly between dams ingested by a single toad was 160, found in the and tanks with 6.6 times stronger predation pres- stomach of a female cane toad (snout-vent length sure at dams (-0.0059 ± 0.0015) than tanks 118 mm) at a tank. Average numbers of other prey (-0.0009 ± 0.0003; LMM, F = 20.23, df = 1, items (mostly other beetle species and ants) did not P < 0.001, Figure 3B).

Figure 3. A Number of dung beetles and other prey items consumed by cane toads Rhinella marina at the two different reservoir types: tank (n = 58 toads) and dam (n = 80 toads). Error bars indicate ±1SE. B Estimated cane toad impact, calculated as the predatory impact of cane toads Rhinella marina on dung beetles at the two different reservoir types: tank and dam. Error bars indicate ±1SD. Introduced Predator Disrupts Dung Decomposition

Effects of Predictor Variables on correlation among the Pearson residuals generated Population Densities of Dung Beetles by the best model (Mantel test, r = 0.10, P = 0.9). Differences in the population densities of dung beetles between the 13 AWP were best explained DISCUSSION by the model including only the interaction be- Our results showed that population densities of tween the predictors reservoir type and population toads were lower at tanks than dams, that density of cane toads (Table 3). Model coefficients population densities of dung beetles were higher at showed that the best predictor for the population tanks than dams and that the predatory impact of density of dung beetles was reservoir type with toads on dung beetles was greater at dams than higher densities at tanks (b = 2.76 ± 0.72), tanks. In turn, the rate of cattle dung removal was whereas population densities of cane toads had a greater at AWP fitted with tanks than dams. Taken negative influence on the population density of together, these findings support the hypothesis that dung beetles at tanks (b = -6.06 ± 2.58; Figure 4). dams, by providing toads with unrestricted access We found no spatial autocorrelation among the to water for rehydration, facilitate higher popula- Pearson residuals generated by the best model tions of toads and hence rates of toad predation on (Mantel test, r = 0.04, P = 0.9). dung beetles which results in lower population densities of dung beetles and lower rates of dung Dung Decomposition Rates decomposition. Our findings have implications for water management within the invasive region for Dung beetle densities were relatively low at all sites cane toads, suggesting that using tanks as reservoirs due to the experiment being conducted during the at AWP as an alternative to dams may not only dry season when dung beetle activity is quite low. limit population densities of cane toads (Letnic and However, the number of dung beetles found per others 2014, 2015), but also reduce the negative dung pat at the end of the experiment was higher impacts that cane toads have on dung beetles and at tanks (3.6 ± 0.5) than dams (0.1 ± 0.1; GLMM, the ecosystem services they provide. F1,57 = 5.56, P < 0.05, Figure 5A). After 48 h, Earthen dams provide cane toads with virtually 13.4% more mass was lost in dung pats in the vi- unlimited access to standing water for rehydration cinity of tanks (51.7 ± 6.1%) than dams and hence can support large populations of toads (38.2 ± 4.9%; LMM, F = 30.25, df = 1, P < 0.01, (Florance and others 2011; Letnic and others Figure 5B). Inspection of model coefficients from 2014). Closed tanks, on the other hand, provide model averaging showed that decomposition rates toads with little and in some cases no water to re- were best explained by the predictors reservoir type hydrate because the only water usually available is (AICw = 1.00) and dung beetle density (AICw = that emanating from small leaks in pipeline joints, 0.87, Table 4). Ambient temperature and dung the seams of tanks, and at bore heads (Letnic and availability were weak predictors; relative humidity others 2014). However, because these pools of was not a predictor in any of the models in the best water are short lived (that is, a few days), cane model subset (Table 4). We found no spatial auto- toads are unable to reproduce and increase

Table 3. Model Selection Results for a Candidate Set of Mixed Effects Generalized Linear Effects Models with a Poisson Distribution and Log-link Function of the Population Density of Dung Beetles at Artificial Water Points

Model AICc DAICc AICw

Type of reservoir * Cane toad density 1424.4 0.0 0.55 Type of reservoir + Cane toad density 1426.9 2.5 0.16 Type of reservoir 1426.9 2.6 0.15 Type of reservoir + Type of soil 1428.4 4.1 0.07 Global model 1429.8 5.4 0.04 Cane toad density 1430.1 5.7 0.03 Null model 1439.7 15.4 0

Selected models comprise a total relative likelihood (AICw) of 0.95. Type of reservoir includes dams and tanks, type of soil includes sand and clay. The global model consisted of a combination of all predictors used in the model subsets. AICc is the Akaike Information Criterion corrected for small sample sizes. DAICc is the difference in AICc in relation to the best model. All models include the random factors: AWP identity and year of survey. B. Feit and others

Figure 4. Number of dung beetles in relation to cane toad Rhinella marina abundance at the two different reservoir types: tank and dam. Points indicate the number of dung beetles per liter of fresh cattle dung. The shaded region indicates the 95% confidence region of the generalized linear mixed model.

Figure 5. A Number of dung beetles found in artificial dung pats at the two different reservoir types tank (n = 30 dung pats) and dam (n =30 dung pats). B Percentage of mass loss of artificial dung pats after 2 days of exposure at the two different reservoir types tank (n = 30 dung pats) and dam (n = 30 dung pats). Error bars indicate ±1SE.

population densities at water tanks as the devel- Hence, we contend that toad populations at tanks opment from eggs to metamorph cane toads takes a were much lower than at earthen dams because minimum of about 1 month (Zug and Zug 1979). there was less water available for rehydration and Introduced Predator Disrupts Dung Decomposition

Table 4. Model-averaged Parameter Estimates and Relative Predictor Likelihood (AICw) from the Final Subset of Linear Mixed Effects Models Explaining Dung Removal Rates

Parameter Mean SD AICw

Intercept 37.76 12.07 Type of reservoir (Tank) 11.40 3.26 1.00 Dung beetle density 1.05 0.42 0.87 Dung availability -1.20 1.24 0.61 Ambient temperature 0.60 0.70 0.36

Type of reservoir includes dams and tanks; temperature equals ambient temperature at time of survey, type of soil comprises sand and clay. AICc is the Akaike Information Criterion corrected for small sample sizes. DAICc is the difference in AICc in relation to the best model and AICw the relative likelihood of the model. All models include the random factors AWP identity and year of survey.

no water available for reproduction at tanks. This tion, and ambient temperature were weak predic- explanation is supported by experimental studies in tors of dung beetle density. All sites included in the similarly arid regions showing that during the an- analysis were located in an area of less than nual dry season, toads seek water in dams on an 3000 km2 with little variation climatic factors such almost daily basis to rehydrate and cannot survive as rainfall or relative humidity. Due to heavy more than 3 days without access to water (Florance grazing impact, there was no grass or shrub layer in and others 2011; Webb and others 2014) and by the immediate surroundings of the AWP, but even the absence of metamorph and juvenile cane toads if vegetation cover had differed between AWP, during our surveys at tanks. dung beetle species in the functional group of Previous studies have shown that cane toads can tunnelers were unlikely to be affected (Koike and suppress the abundances of their invertebrate prey others 2014). Thus, it is likely that the major dif- (Greenlees and others 2006) and that toads can ference between the two reservoir types influenc- exploit high prey densities by ingesting large ing the population density of dung beetles was the numbers of prey items at a time. For example, Zug population density of cane toads. Furthermore, the and Zug (1979) reported an average of 43.2 in- population density of beetles at tanks was corre- vertebrates ingested per toad per night in their lated negatively with the density of cane toads. natural habitat in Panama. We found 160 dung Although our results provide support for the hy- beetles in the stomach of one toad (and an average pothesis that predation by cane toads suppresses of 13.1 dung beetles per toad), presumably ingested dung beetle abundance, we caution that without in a single night as all beetles were still intact and having conducted an experimental manipulation of thus digestion not far advanced. Our inspections of cane toad abundance, causation remains difficult to stomach contents confirmed the findings of previ- attribute because it remains possible that con- ous studies showing that toads readily prey upon founding factors could have influenced our results. dung beetles (Figure 5; Gonza´lez-Bernal and others Experimental studies that remove or exclude cane 2012; Gonza´lez-Bernal and Greenlees 2013). toads are required to confirm that cane toads’ Our results provide support for the hypothesis suppress dung beetle abundance. that the low population densities of dung beetles A plausible scenario explaining the importance observed at dams were due to high predation of the predictor of reservoir type on population pressure by toads. Our estimates of cane toad pre- densities of dung beetles is that, after the initial dation pressure on dung beetles suggest that cane colonization and subsequent increases in toad toads had a greater predatory impact on dung population density, the high predatory impact by beetles at dams than tanks. Even though individual toads suppresses dung beetle populations at dams. toads at dams consumed more dung beetles than In the vicinity of tanks, however, population den- toads at tanks (Figure 3A), their total impact on sities of cane toads remain low after colonization, dung beetles was much greater because of the resulting in a low overall impact on dung beetle higher cane toad population densities at dams populations. (Figure 3B). Reservoir type and its interaction with The results of our dung removal experiment population densities of cane toads were the best suggest that cane toad predation on dung beetles predictor of dung beetle population density. The can translate to reduced rates of cattle dung de- type of soil, the availability of dung for coloniza- composition. This interpretation is supported by B. Feit and others our findings that the best predictors for mass loss of nutrient deposition (Croll and others 2005). Our dung pats in our dung removal experiment were study furthers understanding of predators’ effects the population density of dung beetles and reser- on ecosystem processes because it provides evi- voir type with the rate of dung removal being dence that consumption of detritivores by an in- greatest at tanks where toad populations were low. troduced predator can retard the rate of herbivore This interpretation is also supported by the negative dung decomposition and potentially hinder the correlation between population densities of dung functioning of rangeland ecosystems. beetles and cane toads, and the fact that toads’ predatory impact on dung beetles was greater at ACKNOWLEDGMENTS dams than tanks. The weak explanatory power that Funding was provided by the Hermon Slade temperature and humidity had in our linear mixed Foundation. We thank Graeme Sawyer (Frogwatch effects models suggest that the rate of dehydration NT), Tom Nichols (Parks & Wildlife NT), Georgia, of dung pats was similar between AWP and thus and Mick Underwood of Riveren Station, and the did not influence our findings. managers of Dungowan and Camfield stations for High levels of cane toad predation on dung their support. Christopher Turbill and Anna Feit beetles are likely to diminish the ecosystem services provided helpful comments on earlier versions of that dung beetles provide for livestock producers. the manuscript. Because cattle avoid feeding in areas surrounding dung, a reduction of dung decomposition rates can translate into reduced pasture utilization (Dohi and others 1991). Furthermore, dung beetle activity has REFERENCES been linked to reduced recruitment of cattle para- Anderson P, Hoffman M. 2007. The impacts of sustained heavy sites that require dung to complete their life cycles, grazing on plant diversity and composition in lowland and such as buffalo flies (Fay and Doube 1983; Fay and upland habitats across the Kamiesberg mountain range in the Macqueen 1990) and nematodes (Bryan 1976). Succulent Karoo, South Africa. J Arid Environ 70:686–700. Thus, higher availability of habitat for dung- Asner G, Elmore A, Olander L, Martin R, Harris A. 2004. Grazing systems, ecosystem responses, and global change. Annu Rev breeding parasites could have adverse effects on Environ Resour 29:261–99. cattle production if it results in higher parasite Baillie C. 2008. Assessment of evaporation losses and evapora- loads, reduced live weight and diminished repro- tion mitigation technologies for on farm water storages across ductive success (Byford and others 1992). In addi- Australia. Cooperative Research Centre for Irrigation Futures, tion, by distributing nutrient-rich organic matter Irrigation Matters Series 05/08. through upper soil layers, dung beetle activity Bornemissza G. 1960. Could dung eating insects improve our benefits plant productivity by promoting nutrient pastures? J Aust Inst Agric Sci 26:54–6. cycling (Nichols and others 2008). Depletion of Bryan R. 1976. The effect of the dung beetle, Onthophagus gazella, on the ecology of the infective larvae of gastrointestinal ne- dung beetle populations by prolific cane toad matodes of cattle. Aust J Agric Res 27:567–74. populations at earthen dams could thus create a Burnham K, Anderson D. 2002. Model selection and multi- situation similar to that which existed prior to the model inference: a practical information-theoretic approach. introduction of dung beetles, when large areas of New York: Springer. pasture were unavailable to livestock, cattle para- Byers J. 2002. Impact of non-indigenous species on natives en- sites had uncontrolled access to readily available hanced by anthropogenic alteration of selection regimes. Oi- dung to complete their life cycles and the release of kos 97:449–58. nutrients from dung was slow (Bornemissza 1960). Byford R, Craig M, Crosby B. 1992. A review of ectoparasites and their effect on cattle production. J Anim Sci 70:597–602. There is a growing body of evidence showing Christianson D, Creel S. 2010. A nutritionally mediated risk ef- how predators can influence the flow of energy and fect of wolves on elk. Ecology 91:1184–91. matter through ecosystems and that removal of Creel S, Winnie J Jr, Maxwell B, Hamlin K, Cree M. 2005. Elk native predators or introduction of novel predators alter habitat selection as an antipredator response to wolves. can disrupt ecosystem dynamics (Creel and others Ecology 86:3387–97. 2005; Schmitz and others 2010). Predators can af- Croll DA, Maron JL, Estes JA, Danner EM, Byrd GV. 2005. In- fect ecosystem dynamics because their consump- troduced predators transform subarctic islands from grassland tive and non-consumptive effects on prey species to tundra. Science 307:1959–61. can, for example, control the intensity of con- Crooks J. 2002. Characterizing ecosystem-level consequences of biological invasions: the role of ecosystem engineers. Oikos sumption by herbivores (Strickland and others 97:153–66. 2013), the type of forage consumed (Christianson Davis A. 1996. Seasonal dung beetle activity and dung dispersal and Creel 2010), the assimilation of food (Hawlena in selected South African habitats: implications for pasture and Schmitz 2010), and spatial distribution of improvement in Australia. Agric Ecosyst Environ 58:157–69. Introduced Predator Disrupts Dung Decomposition

Didham RK, Tylianakis JM, Gemmell NJ, Rand TA, Ewers RM. Letnic M, Koch F, Gordon C, Crowther MS, Dickman CR. 2009. 2007. Interactive effects of habitat modification and species Keystone effects of an alien top-predator stem extinctions of invasion on native species decline. Trends Ecol Evol 22:489– native mammals. Proc R Soc B 276:3249–56. 96. Letnic M, Webb JK, Jessop TS, Florance D, Dempster T. 2014. Dohi H, Yamada A, Entsu S. 1991. Cattle feeding deterrents Artificial water points facilitate the spread of an invasive emitted from cattle feces. J Chem Ecol 17:1197–203. vertebrate in arid Australia. J Appl Ecol 51:795–803. doi:10. Doube B. 1991. Dung beetles of southern Africa. In: Hanski I, 1111/1365-2664.12232. Cambefort Y, Eds. Dung beetle ecology. Princeton: Princeton Letnic M, Webb JK, Jessop TS, Dempster T. 2015. Restricting University Press. p 135. access to invasion hubs enables sustained control of an inva- Duncan RP, Cassey P, Blackburn TM. 2009. Do climate envelope sive vertebrate. J Appl Ecol . doi:10.1111/1365-2664.12390. models transfer? A manipulative test using dung beetle in- Losey J, Vaughan M. 2006. The economic value of ecological troductions. Proc R Soc B 267:1449–57. doi:10.1098/rspb. services provided by insects. Bioscience 56:311–23. 2008.1801. Mack M, D’Antonio C. 1998. Impacts of biological invasions on Edwards P. 2009. Introduced dung beetles in australia 1967– disturbance regimes. Trends Ecol Evol 13:195–8. 2007: current status and future directions. Melbourne: The Mellanby K. 1939. Low temperature and insect activity. Proc R Orica Community Foundation. Soc B 127:473–87. Fay H, Doube B. 1983. The effect of some coprophagous and Nakagawa S, Schielzeth H. 2013. A general and simple method predatory beetles on the survival of immature stages of the for obtaining R2 from generalized linear mixed-effects models. African buffalo fly, Haematobia thirouxi potans, in bovine dung. Methods Ecol Evol 4:133–42. Z Angew Entomol 95:460–6. Nichols E, Spector S, Louzada J, Larsen T, Amezquita S, Favila M. Fay H, Macqueen A. 1990. Impact of fauna on mortality and size 2008. Ecological functions and ecosystem services provided by of Haematobia spp. (Diptera: Muscidae) in natural dung pads in Scarabaeinae dung beetles. Biol Conserv 141:1461–74. Australia and South Africa. Bull Entomol Res 80:385–92. Pimentel D, Zuniga R, Morrison D. 2005. Update on the envi- Florance D, Webb JK, Dempster T, Kearney MR, Worthing A, ronmental and economic costs associated with alien-invasive Letnic M. 2011. Excluding access to invasion hubs can contain species in the United States. Ecol Econ 52:273–88. the spread of an invasive vertebrate. Proc R Soc B 278:2900–8. Russell B, Letnic M, Fleming P. 2011. Managing feral goat im- Freeland W. 1986. Populations of cane toad, Bufo marinus,in pacts by manipulating their access to water in the rangelands. relation to time since colonization. Wildl Res 13:321–9. Rangel J 33:143–52. Gonza´lez-Bernal E, Greenlees MJ. 2013. Interacting biocontrol Schmitz OJ, Hawlena D, Trussell GC. 2010. Predator control of programmes: invasive cane toads reduce rates of breakdown ecosystem nutrient dynamics. Ecol Lett 13:1199–209. of cowpats by dung beetles. Austral Ecol 38:891–5. Strickland MS, Hawlena D, Reese A, Bradford M, Schmitz OJ. Gonza´lez-Bernal E, Greenlees MJ, Brown GP, Shine R. 2012. 2013. Trophic cascade alters ecosystem carbon exchange. Proc Cane toads on cowpats: commercial livestock production fa- Natl Acad Sci 110:11035–8. cilitates toad invasion in tropical Australia. PloS one 7:e49351. Tyndale-Biscoe M. 1990. Common dung beetles in pastures of Greenlees MJ, Brown GP, Webb JK, Phillips BL, Shine R. 2006. south-eastern Australia. Melbourne: CSIRO Publishing. Effects of an invasive anuran [the cane toad (Bufo marinus)] Vitousek PM. 1997. Human domination of earth’s ecosystems. on the invertebrate fauna of a tropical Australian floodplain. Science 277:494–9. Anim Conservat 9:431–8. Webb JK, Letnic M, Jessop TS, Dempster T. 2014. Behavioural Hawlena D, Schmitz OJ. 2010. Herbivore physiological response flexibility allows an invasive vertebrate to survive in a semi- to predation risk and implications for ecosystem nutrient dy- arid environment. Biol Lett. doi:10.1098/rsbl.2013.1014. namics. Proc Natl Acad Sci 107:15503–7. Wootton J. 1997. Estimates and tests of per capita interaction James CD, Landsberg J, Morton SR. 1999. Provision of watering strength: diet, abundance, and impact of intertidally foraging points in the Australian arid zone: a review of effects on biota. birds. Ecol Monogr 67:45–64. J Arid Environ 41:87–121. Zug G, Zug P. 1979. The marine toad, Bufo marinus: a natural Koike S, Soga M, Nemoto Y, Kozakai C. 2014. How are dung history resume of native populations. Washington: Smithso- beetle species affected by deer population increases in a cool nian Institution Press. temperate forest ecosystem? J Zool . doi:10.1111/jzo.12138.