Ecology, 92(2), 2011, pp. 422–431 Ó 2011 by the Ecological Society of America

The ecological impact of invasive cane toads on tropical snakes: Field data do not support laboratory-based predictions

1 GREGORY P. BROWN,BENJAMIN L. PHILLIPS, AND RICHARD SHINE School of Biological Sciences A08, University of Sydney, NSW 2006, Australia

Abstract. Predicting which species will be affected by an invasive taxon is critical to developing conservation priorities, but this is a difficult task. A previous study on the impact of invasive cane toads (Bufo marinus) on Australian snakes attempted to predict vulnerability a priori based on the assumptions that any snake species that eats frogs, and is vulnerable to toad toxins, may be at risk from the toad invasion. We used time-series analyses to evaluate the accuracy of that prediction, based on .3600 standardized nocturnal surveys over a 138- month period on 12 species of snakes and lizards on a floodplain in the Australian wet–dry tropics, bracketing the arrival of cane toads at this site. Contrary to prediction, encounter rates with most species were unaffected by toad arrival, and some taxa predicted to be vulnerable to toads increased rather than declined (e.g., death adder Acanthophis praelongus; Children’s python Antaresia childreni). Indirect positive effects of toad invasion (perhaps mediated by toad-induced mortality of predatory varanid lizards) and stochastic weather events outweighed effects of toad invasion for most snake species. Our study casts doubt on the ability of a priori desktop studies, or short-term field surveys, to predict or document the ecological impact of invasive species. Key words: alien species; Australia; biological invasion; Bufo marinus; bufotoxins; cane toad; predator–prey; Rhinella marina; time-series analysis; tropical snakes.

INTRODUCTION extrapolated to predict invader effects on native species There is broad scientific consensus that invasive and areas not yet invaded (Kolar and Lodge 2001, species threaten biodiversity (Mack et al. 2000, Pimentel 2002). In some cases, a prior invasion event has provided et al. 2000). However, invader impacts are highly empirical data on changes in abundance of native heterogeneous, with some native taxa benefited rather species coincident with (and putatively, due to) invader than imperiled by the arrival of an exotic species (e.g., arrival (Ricciardi 2003, Kraus 2007, 2009). Unfortu- Wonham et al. 2005, King et al. 2006). For management nately, that empirical evidence often is fragmentary or purposes, we need to determine which native species are anecdotal, and at best usually involves short-term threatened by an invader and which are not, so that surveys that may be insufficient to detect clear trends. limited budgets for conservation can be allocated In some situations, as with terrestrial plants or sessile toward the former group. Unfortunately, the details of marine invertebrates outcompeting native taxa for invader impact remain unclear for most cases, compli- space, the effects of the invader on native species may cating attempts to either predict or document invader be straightforward to quantify. If the putative impact impact. Typically, management agencies are forced to falls on mobile native fauna, however, documenting rely on desktop evaluations (based on published data impact by demonstrating a shift in abundance is and anecdotal reports) to predict species vulnerability; logistically challenging, especially if the taxon believed or to base those predictions on laboratory-based to be at risk is rare, difficult to census, and/or exhibits measures thought to affect vulnerability; and to rely highly variable population sizes through time and space on short-term field surveys to actually test the robust- even in the absence of the invader (Caughley 1977, ness of those predictions. Such approaches may be Watson and Woinarski 2003). The resulting high subject to considerable error. variances around count data make it difficult to detect Predicting species vulnerability to an invasive organ- (let alone assign causation to) any fluctuations in ism usually is based on hypotheses about the causal abundance. mechanism of impact (e.g., dietary habits of a feral The invasion of cane toads (Bufo marinus; see Pramuk predator, habitat requirements of an introduced weed), 2006 for alternative nomenclatural allocation to Rhinella marina) across tropical Australia (Urban et al. 2007, 2008) exemplifies these logistical difficulties in predicting Manuscript received 14 March 2010; revised 15 July 2010; and detecting impact. Invading cane toads could directly accepted 26 July 2010. Corresponding Editor: D. A. Spiller. 1 Corresponding author. impact populations of native vertebrates in several ways. E-mail: [email protected] For example, cane toads sometimes ingest small 422 February 2011 ECOLOGICAL IMPACT OF INVASIVE TOADS 423 vertebrates, including lizards and snakes (Lever 2001). most frequently encountered taxa in our nocturnal More importantly, many native predators are fatally surveys and cover a considerable range in mean adult poisoned when they attempt to eat the highly toxic body sizes, vulnerability to bufotoxins, and dietary toads, and population densities of some species have habits (Table 1; see Cogger 2000 for distributions and plummeted as a result (Lever 2001, Oakwood 2004, descriptions of these species). Based on experimental Griffiths and McKay 2007, Letnic et al. 2008, Doody et studies, two colubrids (Stegonotus cucullatus and Tropi- al. 2009, Ujvari and Madsen 2010). The most robust donophis mairii) are very tolerant of bufotoxins whereas evidence of such declines involves large-bodied preda- a third colubrid (Boiga irregularis) is not; elapid and tors that are relatively easy to survey (crocodiles, pythonid snakes have low resistance to bufotoxins varanid lizards) or trap (quolls), providing reliable (Phillips et al. 2003). Dissections of preserved specimens estimates of population densities (see Shine 2010 for a and palpation of freshly caught animals show that the review). But what of animals that are more difficult to proportion of the diet composed of anurans ranges from survey in a quantitative fashion? For example, Phillips et .95% in the keelback Tropidonophis mairii to 0% in the al. (2003) attempted to predict the impact of cane toads mammal-specialist Liasis fuscus (Shine 1991, Madsen et on Australian snakes by collating data on geographic al. 2006; see Table 1). Based on toxin resistance and distributions, feeding habits and resistance to bufotoxins anurophagy, we predict that four species should be of Australian snake species. They (Phillips et al. highly vulnerable to toad invasion (frequently eat frogs, 2003:1745) concluded that ‘‘the invasion of cane toads low toxin resistance: Acanthophis; see Plate 1), Pseude- has caused, and will continue to cause, massive mortality chis, Boiga, Antaresia), three species should be at lower among snakes in Australia.’’ risk (rarely eat frogs, low toxin resistance: Pseudonaja, It is clearly a major conservation concern to have so Cryptophis, Morelia), and three species should be at no many Australian snake species identified as at potential risk (toxin-tolerant: Stegonotus, Tropidonophis) (never risk from cane toad invasion. However, few data are eat anurans: Liasis). Small head sizes of the remaining available to test that prediction. Snakes typically are taxa (Furina, Lialis) should reduce their vulnerability to cryptic, often are inactive for long periods, and many toad invasion, by restricting maximal ingestible toad size species are rare (Seigel 1993). Those problems are (Phillips et al. 2003), but the small body sizes of these exacerbated in the Australian wet–dry tropics, where animals may render them more vulnerable to ingestion spatial and temporal variation in monsoonal rainfall by toads (specimens of Lialis burtonis have been found patterns generates immense variation in the food supply, in the stomachs of field-collected toads; T. Madsen, growth rates, survival rates, and population densities of personal communication). snakes (Madsen and Shine 2000, Brown et al. 2002, Madsen et al. 2006, Shine and Brown 2008). How can Rates of encounter in the field we detect the impact of an invasive species on snakes Nightly surveys for snakes and cane toads were that are rare and difficult to survey, and whose conducted during fieldwork on the Adelaide River population densities fluctuate massively from year-to- floodplain of the Northern Territory (128310 S, 1318200 year and place-to-place? In particular, can desktop E) 70 km southeast of the city of Darwin, between May studies (such as those of Phillips et al. 2003) accurately 1998 and October 2009 (total number of nightly counts predict which species will be affected by toad invasion; ¼ 3619; range ¼ 3–31 nights per month, average 26.2 and can short-term survey data reliably document such nights per month for all 138 months over this period, declines if they occur? 87% of all nights over the 4178 day total duration). In the present paper, we explore the impact of cane From August 2002 onward, we also counted the toads on an assemblage of 12 reptile species in tropical pygopodid lizard Lialis burtonis, a species that is Australia. Based on published literature on their morphologically and ecologically convergent with am- physiological resistance to bufotoxins, and the impor- bush-foraging snakes (Wall and Shine 2007). Counts tance of anuran prey in the diet (i.e., the criteria used by were made along 1.5 km of the wall of Fogg Dam, a low Phillips et al. 2003), these species vary widely in their earthen wall that separates the dam from the floodplain likely vulnerability to lethal toxic ingestion following below, and along the 1.3-km section of roadway through toad invasion. We examine a long-term data set on rates mixed woodland that leads to the Fogg Dam wall. of encounter with these snake species at a site that was Surveys were conducted between 19:00 and 22:00 hours invaded by cane toads partway through the monitoring from a slow-moving vehicle or on foot. Both adult cane period. Our data enable us to evaluate the accuracy of toads and these squamate species are primarily noctur- our a priori predictions. nal in our study area (Freeland and Kerin 1998, Brown

METHODS et al. 2002). Only live animals were included in survey counts (i.e., roadkills were excluded). Study species Encounter rates are affected not only by abundance, We focus on five species of elapid snakes, three but also by animal behavior (and hence, observability). colubrids, three pythons, and a snake-like lizard from For two snake species for which we have robust mark– the Australian wet–dry tropics. These species were the recapture data over a long period, we can test the 424 GREGORY P. BROWN ET AL. Ecology, Vol. 92, No. 2

TABLE 1. Body sizes, predicted vulnerability to cane toad (Bufo marinus) invasion (based on bufotoxin tolerances and dietary habits), and survey counts of snake species studied on the Adelaide River floodplain, Northern Territory, Australia.

Lethal dose Proportion Predicted Mean adult of toad of anurans impact Total Species Common name SVL (cm) toxin (lL/g) in diet of toads no. seen Elapids Acanthophis praelongus death adder 50 0.77 0.37 decline 90 Pseudechis australis king brown 170 0.69 0.20 decline 82 Pseudonaja nuchalis western brown 86 low? 0.06 decline 12 Cryptophis pallidiceps small–eyed snake 41 low? 0.06 decline? 68 Furina ornata moon snake 33 low? 0 none 15 Colubrids Boiga irregularis brown tree snake 100 0.70 0.35 decline 40 Stegonotus cucullatus slatey-grey snake 90 15.02 0.25 none 2263 Tropidonophis mairii keelback 54 37.99 0.97 none 6131 Pythons Antaresia childreni Children’s python 71 0.82 0.33 decline 43 Liasis fuscus water python 138 low? 0 none 15294 Morelia spilota carpet python 141 low? 0.01 decline 18 Pygopodid lizard Lialis burtonis Burton’s legless lizard 19 unknown 0 none? 216 Notes: Listed are mean adult snout–vent length, SVL (from Shine [1991] and included references), physiological tolerance to cane toad toxins (estimated LD50 dose from Phillips et al. [2003]; inferred from a congeneric species’ tolerance in the case of Pseudechis australis; inferred to be low [,1.0] for other python and elapid species, based on confamilial taxa that have been tested), the proportion of the diet of free-ranging individuals composed of anurans (from Shine [1991] and included references, plus R. Shine, unpublished data), and the mean (followed by mininum and maximum) frequency of encounter (mean number of nights between successive sightings of a species during field surveys) before and after toad invasion in 2005. assumption that encounter rates reflect abundance. In ecology (Rasmussen et al. 2001, Druckenbrod 2005). both cases, estimated population sizes for each year were The approach requires a large amount of pre-interven- positively correlated with mean encounter rates in that tion data (usually N . 50 observations) to properly year (slatey-grey snakes, N ¼ 10 years, P ¼ 0.035; model baseline patterns of variation in the dependent keelbacks, N ¼ 11 years, P , 0.0001). variable. Specifically, we used a segmented regression For each survey night that a species was encountered, approach to compare the slopes and elevations of the we also calculated the time (number of surveys) since relationships between encounter rates and time, during that species was last seen. two periods, prior to and subsequent to the arrival of toads in our study area. Our segmented regression Statistical analyses model was as follows: Encounter rates from our surveys showed strong seasonal effects (Appendix A) and high serial correlation Y ¼ B0 þ B1ðtimeÞþB2ðinterventionÞ among observations and among residuals (based on þ B3ðtime after interventionÞþerror visual analysis of scatterplots, confirmed by Durbin- Watson tests for autocorrelation). Such autocorrelation where time is a continuous variable denoting the number among residuals violates the assumption of independence of months since the beginning of the study. Intervention of errors implicit in ordinary least squares analyses. is a dummy variable distinguishing the pre-toad part of Therefore we used time-series analyses, which are the study from the post-toad study period (month designed to incorporate complex patterns of autocorre- numbers 1–91 were coded ‘‘0’’ and month numbers 92– lated errors into regression models. Specifically, we used 138codedas‘‘1’’).Timeafterinterventionisa time-series intervention (or interrupted time-series) anal- continuous variable, composed of 0’s until month ysis using an autoregressive integrated moving average number 93 and then increasing to denote number of (ARIMA) model. Time-series intervention analyses use a months since toad arrival. The parameter B0 represents segmented regression approach to identify changes in the the pre-intervention y-intercept, B1 represents the pre- level or slope of a dependent variable (encounter rates in intervention slope, B2 represents the post-intervention our case) over time before and after a perturbation or level change, and B3 represent the post-intervention intervention introduced at a known time (in our case, the slope change. In Appendix B we present a simple arrival of cane toads in our study area). numerical example to illustrate the calculation and Time-series intervention analyses are commonly used interpretation of these four parameters. in econometrics, social sciences (Huitema and McKean The error terms in the time-series intervention models 2000), and clinical studies (Wagner et al. 2002, Matowe were assigned specific ARIMA structure such that the et al. 2003, Shardell et al. 2007) but less frequently in model residuals exhibited ‘‘white noise’’ or ‘‘random February 2011 ECOLOGICAL IMPACT OF INVASIVE TOADS 425

TABLE 1. Extended. obscuring any general trends in the time series. Encounter rates tended to be higher in the wet season than the dry season for some snake species (e.g., death Mean no. Mean no. surveys between surveys between adders), whereas the reverse was true for other taxa (e.g., encounters pre-toad encounters post-toad water pythons) (Appendix A). The autocorrelated errors in our encounter rate data 57.0 (1–620) 26.7 (1–210) were described by relatively simple ARIMA models. In 33.6 (1–272) 155.3 (43 to .533) the case of two species (Furina ornata and Antaresia 429 (76–1000) 162.5 (10–650) childreni), errors were not autocorrelated, and in all 45.6 (1–210) 76.2 (1–416) 171.8 (1–448) 372.2 (149–1159) other cases the error structure was best incorporated in ARIMA models that contained only autocorrelation 160.5 (11–463) 45.4 (1–233) components (i.e., p and/or P). 2.5 (1–31) 2.3 (1–13) Average weather conditions (rainfall and tempera- 1.6 (1–36) 2.6 (1–21) ture) did not shift significantly at the time that toads first arrived (Table 2), simplifying interpretation of our 101.9 (1–376) 58.8 (1 to .266) survey data. Intervention analyses of the encounter-rate 1.2 (1–8) 1.2 (1–7) time-series data revealed statistically significant changes 317.1 (47–844) 132.5 (3–556) in rates of encounter following toad invasion for six of the 12 reptile species surveyed. Death adder (Acantho- 13.1 (1–117) 10.7 (1–117) phis praelongus) encounter rates had been increasing prior to toad arrival, and continued to do so subsequent to toad arrival, but displayed a steeper slope (Fig. 1b, process’’ characteristics. We assessed randomness of Table 2). Encounter rates with small-eyed snakes error terms by inspecting normality plots as well as (Cryptophis pallidiceps) were stable prior to toad autocorrelation functions (ACFs) and partial autocor- invasion but began to increase after toads arrived (Fig. relation functions (PACFs). Overall randomness of the 1c, Table 2). When toads arrived, a decreasing trend in ACF was determined using Ljung-Box tests. Seasonal encounter rates with keelbacks (Tropidonophis mairii) ARIMA models contain two sets of three parameters: stopped abruptly and began to increase (Fig. 2c, Table one set of parameters concerns the nonseasonal compo- 2). Encounter rates with Children’s pythons (Antaresia nent of the model and the other set relates to the childreni) exhibited a significant increase in level seasonal component. The three parameters describe (intercept) concurrent with the arrival of toads (Fig. orders of autoregression, differencing, and moving 3a, Table 2). In contrast, encounter rates with water averaging, denoted by p, d, and q, respectively, for the pythons (Liasis fuscus), which had been increasing over nonseasonal component and P, D, and Q for the time, began to decrease after the arrival of toads (Fig. seasonal component. Standard notation for ARIMA 3b, Table 2). The rate at which we encountered carpet models is of the form ( p,d,q)(P,D,Q)s, where s indicates pythons (Morelia spilota) showed a sharp increase in the length of the seasonal cycle (e.g., s ¼ 12 in the case of level soon after toad arrival, followed by a decline over monthly sampling). For all of our models, only time thereafter (Fig. 3c, Table 2). For the other six autoregressive components were required to reduce species that we surveyed, there were no statistically residuals to white noise. Thus, for example, an ARIMA significant time trends or shifts in encounter rates before vs. after toad arrival (Table 2). model denoted as (1,0,0)(2,0,0)12 would indicate a nonseasonal autoregressive component of p ¼ 1 and a How closely did shifts in encounter seasonal autoregressive component of P ¼ 2, with no rate correlate with toad arrival? differencing or moving average components. This model would yield a time series wherein the mean encounter If toad impact was the reason for snake population rate for a species in a given month is partly determined densities to change subsequent to December 2005 (the date of toad arrival), we would expect to see a shift in level by the mean encounter rate in the previous month (e.g., or slope of our encounter-rate data at that time or shortly p ¼ 1), as well as the mean encounter rate 12 and 24 thereafter. Of the 12 species surveyed, six showed a change months previously (e.g., P ¼ 2). in level or slope of encounter rates concurrent with or RESULTS subsequent to the arrival of toads. Three of these species (Acanthophis, Cryptophis, Tropidonophis) all showed Overall shifts in rates of encounter significant increases in encounter rates after arrival of before vs. after toad invasion toads (Table 2). Another species (Antaresia) showed a Cane toads were first recorded in our surveys in significant increase in level corresponding to the timing of December 2005, and sightings increased rapidly there- toad arrival. The only cases of negative impacts (decreases after (Fig. 1a). Seasonal and year-to-year variation in in slope or level) occurred in the pythons Liasis and encounter rates was substantial for most reptile taxa, Morelia, both of which exhibited significantly decreasing 426 GREGORY P. BROWN ET AL. Ecology, Vol. 92, No. 2

FIG. 1. Encounter rates with cane toads (Bufo marinus) and three species of elapid snakes as counted on regular nocturnal surveys on a floodplain in tropical Australia, over a 12-year period. Lines represent trends fitted from the time-series intervention analysis. The vertical arrow for 2005 shows the year when cane toads first arrived at this study site. Values are back-calculated to the original scale from ln-transformed values used in analyses.

TABLE 2. Results of statistical analysis of ln-transformed encounter rates and weather variables using time-series intervention analysis.

Pre-toad Pre-toad Post-toad Post-toad intercept slope intercept change slope change Species and Total no. ARIMA model weather variables encountered Estimate P Estimate P Estimate P Estimate P ( p,d,q)(P,D,Q) Acanthophis praelongus 90 0.7022 0.0009 0.002 0.0441 0.059 0.0023 0.004 (3,0,0)(1,0,0) Pseudechis australis 82 0.6158 0.0007 0.158 0.0030 0.934 0.0004 0.727 (2,0,0)(1,0,0) Pseudonaja nuchalis 12 0.6922 0.0001 0.604 0.0082 0.462 0.0002 0.535 (1,0,0)(0,0,0) Cryptophis pallidiceps 68 0.6572 0.0001 0.422 0.0145 0.358 0.0017 0.001 (2,0,0)(0,0,0) Furina ornata 15 0.6929 0.0000 0.142 0.0003 0.149 0.0000 0.960 none Boiga irregularis 40 0.6882 0.0001 0.609 0.0319 0.060 0.0005 0.331 (2,0,0)(0,0,0) Stegonotus cucullatus 2263 0.1859 0.059 0.0026 0.146 0.1298 0.351 0.0044 0.365 (1,0,0)(1,0,0) Tropidonophis mairii 6131 0.8065 0.002 0.0063 0.105 0.3244 0.124 0.0239 0.007 (1,0,0)(2,0,0) Antaresia childreni 43 0.6704 0.0001 0.524 0.0556 0.003 0.0010 0.086 none Liasis fuscus 15 294 1.010 0.0078 0.043 0.1608 0.443 0.0221 0.011 (1,0,0)(2,0,0) Morelia spilota 18 0.6905 0.0001 0.688 0.0392 0.0005 0.0010 0.004 (0,0,0)(1,0,0) Lialis burtonis 216 0.8357 0.002 0.0044 0.237 0.0638 0.545 0.0012 0.813 (1,0,0)(1,0,0) Rainfall 184.49 0.6300 0.469 20.24 0.647 0.1661 0.930 (1,0,0)(2,0,0) Mean air temperature 8.776 0.0058 0.513 0.1388 0.743 0.0014 0.942 (1,0,0)(2,0,0) Notes: Four parameters were estimated from a model containing variables for pre-toad intercept and slope (encounter rates vs. time), a level change response coinciding with the arrival of toads, and a post-toad slope change variable (see Appendix B for details of the four parameters). Each cell for pre-toad slope and the change (pre-toad vs. post-toad) in slope and intercept shows the estimate for that variable, and (on the line below) the statistical significance of that estimate’s departure from a null value of zero. Significant results (P , 0.05) are shown by boldface font. The ARIMA (autoregressive integrated moving average) model indicates the structure of the model used to reduce the error structure of the time series to ‘‘white noise.’’ The three parameters describe orders of autoregression, differencing, and moving averaging, denoted by p, d, and q, respectively, for the nonseasonal component and P, D, and Q for the seasonal component. ‘‘None’’ listed under ARIMA model indicates species for which the error structure of the data was not autocorrelated and thus did not require ARIMA modeling. The time-series data for 11 snake species contained survey data for 138 consecutive months from May 1998 to October 2009. The time-series data for the lizard Lialis burtonis consisted of survey data from 87 consecutive months from August 2002 to October 2009. Weather data are mean monthly values between May 1998 and October 2009. February 2011 ECOLOGICAL IMPACT OF INVASIVE TOADS 427

FIG. 2. Encounter rates with three species of colubrid FIG. 3. Encounter rates with three species of pythons as snakes as counted on regular nocturnal surveys on a floodplain counted on regular nocturnal surveys on a floodplain in tropical in tropical Australia, over a 12-year period. Lines represent Australia, over a 12-year period. Lines represent trends fitted trends fitted from the time-series intervention analysis. The from the time-series intervention analysis. The vertical arrow vertical arrow for 2005 shows the year when cane toads (Bufo for 2005 shows the year when cane toads (Bufo marinus) first marinus) first arrived at this study site. Values are back- arrived at this study site. Values are back-calculated to the calculated to the original scale from ln-transformed values used original scale from ln-transformed values used in analyses. in analyses. the Adelaide River floodplain would be imperiled by encounter rates after the arrival of toads. In the case of cane toad invasion. In contrast, our survey data show Morelia, however, the pattern was complex: a significant that those putatively threatened taxa generally have increase in level concurrent with toad arrival, followed by increased rather than decreased in abundance (or at a decline from this elevated level. least, are encountered more frequently) since toad arrival. Only two species exhibited declines in the level Comparison between field data and predictions or slope of encounter rates after the arrival of toads, and Based on toxin tolerance and feeding habits, we one of these (Morelia) also exhibited an immediate predicted that six of the nine common snake species on positive effect prior to the later decline. The only species 428 GREGORY P. BROWN ET AL. Ecology, Vol. 92, No. 2

PLATE 1. The floodplain death adder (Acanthophis praelongus) is a dangerously venomous elapid snake species that was predicted to decline precipitously when highly toxic cane toads (Bufo marinus) arrived at our study site (based on laboratory data on feeding responses and toxin tolerances, and by radio-tracking work at a nearby site). Contrary to prediction, encounter rates with death adders have increased not decreased since toad arrival. Photo credit: G. P. Brown. for which our data show the kind of decline predicted by would be available for most other study systems. toad impact is Liasis fuscus, the water python. Encoun- Previous analyses of reduction in predator abundance ter rate with this species was increasing significantly over following the invasion of cane toads have relied on data time prior to the arrival of toads, but upon the arrival of gathered over briefer periods (Doody et al. 2006, 2009, toads this trend reversed and encounter rates began to Letnic et al. 2008, Phillips et al. 2010, Ujvari and decrease significantly. However, a direct negative impact Madsen 2010). Two main conclusions emerge from our of toad invasion on water pythons is inconsistent not data: (1) fewer snake species declined in abundance after only with a priori predictions, but also with other toad invasion than we predicted a priori based on toxin evidence (see Discussion: Failure of prediction). For one- tolerances and feeding habits; and (2) many of the shifts half of the species we surveyed (six of 12 species), there in snake abundance that occurred at about the time of were no statistically significant changes in encounter toad invasion likely were not causally driven by toad rates before vs. after toad arrival. None of the species impact. that were predicted to suffer a negative impact of cane toad invasion actually exhibited a significant decline in Failure of prediction encounter rates (Table 1). Our a priori predictions were dramatically in error. DISCUSSION Three putatively ‘‘highly vulnerable’’ snake species either Our results paint a stark cautionary tale about the increased in numbers after toad invasion (Acanthophis, logistical obstacles to inferring which native taxa are Antaresia) or tended to increase (Boiga; P ¼ 0.06). Two impacted by an invasive species, and which are not. Our of the ‘‘minor risk’’ species also showed an increase in data set is unusually large (standardized surveys of encounter rate following toad arrival (Morelia, Crypto- faunal encounter rates on 87% of nights over 12 years phis). The only species for which encounter-rate data bracketing the arrival of invasive cane toads), providing suggested a strong negative impact was one (Liasis a more robust opportunity to detect invader impact than fuscus) that had been predicted to be at no risk from February 2011 ECOLOGICAL IMPACT OF INVASIVE TOADS 429 toad invasion. Without exception, then, our a priori adaptive shifts in the morphology, physiology, and predictions about toad impact were wrong. behavior of blacksnakes from toad-infested areas, in Why did this happen? The extensive background ways that reduce the snake’s vulnerability to lethal toxic information on snake ecology at this study site (see ingestion of toads (Phillips and Shine 2006b), support Shine and Brown 2008 for a review) clarifies the relative the idea that toads pose a major threat to these large roles of toad invasion vs. other factors. For example, elapids. Anecdotal evidence of mortality and population declines in encounter rates with water pythons (Liasis decline is available for the king brown also (Scott-Virtue fuscus) were the result of climatic events, notably a and Boulter 2007; P. Fisher, personal communication), major flood in March 2006 that drowned the native rats but our intervention analysis revealed no significant (Rattus colletti) that sustain python populations (G. P. impact of toad arrival on encounter rates with Pseu- Brown, unpublished data; T. Madsen, personal commu- dechis. Exploratory analysis indicated that a significant nication). A direct causal influence of cane toad invasion step decrease (P ¼ 0.0028) in encounter rates with this also appears unlikely to explain fluctuations in survey species occurred in January 2003. Thus, the number of counts for most other snake species that we surveyed. king browns may have been unusually low when toads The apparent increase in numbers of five species, arrived 35 months later, resulting in encounter rates so coincident with toad arrival, may reflect indirect rather low that any additional impact imposed by toads did not than direct effects of the toads’ invasion. Within a year register. We have not encountered any king browns at or two of their arrival, cane toads fatally poisoned most all during the last 18 months of our surveys. These data of the large varanid lizards (Varanus panoptes) in our neither document an impact of cane toads on king study area (Ujvari and Madsen 2010), and these large browns, nor provide convincing evidence of any lack of lizards are generalist predators that often consume impact. snakes (Shine 1986, Phillips et al. 2010). Plausibly, Although our failure to predict which snake species reduced varanid predation enhanced snake survival rates would decline following toad invasion is explicable in and thus, abundances (as occurred for turtle egg survival light of other information, our case study is discourag- rates when varanids were killed by cane toad invasion at ing for attempts to predict which species will be another site; Doody et al. 2006). In other cases, the imperiled based on laboratory studies on toxin resis- vulnerability of a species to toads may be reduced by tance combined with field data on dietary composition. morphological or behavioral traits. For example, some Phillips et al. (2003) predicted that ;30% of Australia’s taxa are too small to ingest toads of most body sizes snake species may be at major risk from cane toad (e.g., Cryptophis pallidiceps) or rarely attempt to ingest invasion. Our data paint a more encouraging picture, in toads (Boiga irregularis; B. Phillips, unpublished data). In suggesting no major decline in abundance in an array of the case of Boiga irregularis, trends in encounter rates putatively vulnerable species. are inconsistent with our a priori prediction because of an error in an assumption underlying that prediction Documenting toad impact with field survey data (i.e., we assumed that a species that consumes native Our long-term data on encounter rates with reptiles anurans also will consume cane toads). on the Adelaide River floodplain also enable us to assess One puzzling case of a species increasing in numbers the adequacy of shorter-term data sets (the only type despite being expected to suffer a severe negative impact that are usually available) to reveal the impacts of from toad invasion involves the floodplain death adder invasive species. As for the desktop analyses, our results (Acanthophis praelongus; Webb et al. 2005). This species are discouraging in showing how strongly the duration readily attacks toads, has low tolerance to the toxins of data acquisition can affect conclusions. For example, (Table 1), and 22% of radio-tracked death adders were encounter rates with carpet pythons (Morelia) declined killed by ingesting cane toads at a site only 10 km away over time after toads arrived, suggesting a negative (Phillips et al. 2010). Counts along a highway near that impact. However, this decrease was preceded by a step site showed a massive (89%) decline in adder numbers increase in encounters at the time of toad arrival. Thus, (Phillips et al. 2010), in striking contrast to the increased a short-term study carried out from 2005 to 2007 would adder numbers recorded since toad arrival at Fogg Dam suggest a strong positive impact of cane toad invasion (Fig. 1b). These data suggest small-scale spatial vari- on carpet pythons, whereas a study that began a year ability in invader impact, as shown previously at larger later might reach the opposite conclusion. Hence, spatial scales for other predator taxa affected by cane inferences about the effect of cane toads on encounter toads (Letnic et al. 2008). rates with carpet pythons could be reversed depending Another puzzling case involves the king brown snake, on the duration and frequency of data-gathering prior to Pseudechis australis. A congeneric species in eastern toad invasion. Australia (the blacksnake, Pseudechis porphyriacus) with In addition to the logistical problems associated with similarly general dietary habits is killed by ingesting detecting a shift in abundance at about the time of toads (Fearn 2003) and has been widely (but anecdot- invader arrival, a reliance on survey data introduces ally) reported to decline in abundance when cane toads ambiguity about causation. The rates at which we invade an area (Phillips and Shine 2006a). Rapid encountered snakes fluctuated markedly through time 430 GREGORY P. BROWN ET AL. Ecology, Vol. 92, No. 2 even at the same study site, with the same methodology, Brown, G. P., R. Shine, and T. Madsen. 2002. Responses of and without any influence of the invasive toads (Figs. 1– three sympatric snake species to tropical seasonality in 3). Long-term studies on water pythons, keelbacks, and northern Australia. Journal of Tropical Ecology 18:549–568. Caughley, G. 1977. Analysis of vertebrate populations. Wiley, filesnakes (Acrochordus arafurae) at our study site show New York, New York, USA. that annual variations in rainfall patterns drive demo- Cogger, H. G. 2000. Reptiles and amphibians of Australia. graphic variation in snakes, via changes to feeding Sixth edition. Reed Books, Sydney, Australia. opportunities for these predators (Shine and Madsen Doody, J. S., B. Green, D. Rhind, C. M. Castellano, R. Sims, and T. Robinson. 2009. Population-level declines in Austra- 1997, Brown and Shine 2007, Shine and Brown 2008, lian predators caused by an invasive species. Animal Ujvari et al. 2010). Hence, stochastic abiotic variation Conservation 12:46–53. provides a plausible alternative explanation for any Doody, J. S., B. Green, R. Sims, D. Rhind, P. West, and D. year-to-year shifts in snake abundance, including Steer. 2006. Indirect impacts of invasive cane toads (Bufo changes that occur at about the same time that cane marinus) on nest predation in pignosed turtles (Carettochelys insculpta). Wildlife Research 33:49–54. toads invade an area. Druckenbrod, D. L. 2005. Dendroecological reconstructions of forest disturbance history using time-series analysis with Conclusion intervention detection. Canadian Journal of Forest Research In summary, the invasion of cane toads at our study 35:868–876. Fearn, S. 2003. 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In complex and highly variable systems such as tropical Letnic, M., J. K. Webb, and R. Shine. 2008. Invasive cane toads floodplains, desktop studies and short-term surveys are (Bufo marinus) cause mass mortality of freshwater crocodiles unlikely to provide robust predictions or conclusions (Crocodylus johnstoni) in tropical Australia. Biological Conservation 141:1773–1782. about invader impact. Lever, C. 2001. The cane toad: the history and ecology of a successful colonist. Westbury Academic and Scientific, Otley, ACKNOWLEDGMENTS West Yorkshire, UK. We thank the members of Team Bufo—especially J. K. Mack, R. N., D. Simberloff, W. M. Lonsdale, H. Evans, M. Webb, D. J. Pearson, and M. Greenlees—for discussions, and Clout, and F. Bazzaz. 2000. Biotic invasions: causes, the Northern Territory Lands Corporation for logistical epidemiology, global consequences and control. Issues in support. The work was funded by the Australian Research Ecology 5:1–20. Council. Madsen, T., and R. 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APPENDIX A Seasonal differences in encounter rates of snake and lizard species in the wet–dry tropics (Ecological Archives E092-037-A1).

APPENDIX B Interpretation of time-series intervention analysis parameters (Ecological Archives E092-037-A2).