selection of diet by metamorphic and juvenile western toads (bufo boreas… · 2013-03-28 ·...

Herpetological Conservation and Biology 4(1):85-95.Submitted: 10 March 2008; Accepted: 30 January 2009

SELECTION OF DIET BY METAMORPHIC AND JUVENILEWESTERN TOADS (BUFO BOREAS) IN NORTHEASTERN OREGON

EVELYN L. BULL AND JANEL. HAYES

Pacific Northwest Research Station, 1401 Gekeler Lane, La Grande, Oregon 97850, USA, e-mail: ebu11(iv(s.(edlls.

Abstract.-The decline of Bufo boreas, the Western Toad, in portions of its range has accentuated the need for morecomplete information on all life stages of this species. Our objectives were to describe the diet of recentlymetamorphosed and juvenile (one-year old) B. boreas and then compare it to the available arthropods. Metamorphs (n= 200) and juveniles (n = 176) preyed on small arthropods (< 5 mm) from among > 20 families within 10 orders ofinsects, two orders of arachnids, and a few gastropods. The majority of the numbers of prey taken by metamorphsconsisted of adult Hemiptera (24%), adult Hymenoptera (19%), and Arachnida (11%). Juveniles primarily fed onCollembola (48%), Hymenoptera (20%) and Coleoptera (11%). We found that the biomass of the diet sample wascorrelated with the mass of the toad. Metamorphs and juveniles selected prey in a nonrandom manner. Metamorphsfed on a higher proportion of Hemiptera (specifically Aphididae), Hymenoptera (specifically Formlcldae), Lepidoptera,Coleoptera, and Arachnida and on a lower proportion of Diptera compared with available arthropods. Juveniles fed ona higher proportion of Hymenoptera (specifically Formicidae), Coleoptera, Diptera, and Arachnida, but on a lowerproportion of Collembola compared to available arthropods.

Key Words=Bufo boreas; diet; juvenile; metamorph; Western Toad

INTRODUCTION

Populations of Bufo boreas (Western Toads) haveexperienced severe population declines and evenextirpations in the mountains of Colorado (Carey 1993;Livo and Yeakley 1997; Muths et al. 2003), GreatCentral Valley of California (Fisher and Shaffer 1996),northern Utah (Corn et al, 1997; Thompson et a1. 2003),and the northern Great Basin (Wente et aI. 2005). InColorado, declines are closely linked to outbreaks ofchytridiomycosis (Carey et a1. 1999; Muths et al. 2003;Scherer et al. 2005), a skin disease caused by the fungalpathogen Batrachochytrium dendrobatidis.

These declines suggest that recruitment into thebreeding populations may also be problematic, yet weknow little about the survival and habitat needs ofmetamorphic and juvenile B. boreas (Muths 2005). Inthe central Oregon Cascade Range, Olson (1992)observed masses of dead metamorphic toads at breedingsites, with "boom" recruitment years occurring rarely.Few studies target young toads because of the difficultyof monitoring the movements of small size-classes oftoads once they emerge from breeding pools. Ourobjectives were to identify the diet of metamorph andjuvenile B. boreas and compare the diet with availablearthropods.

MATERIALS AND METHODS

Study areas.- We described the diets of metamorphsfrom three reservoirs and the surrounding areas innortheastern Oregon in 2006 and 2007: Balm

Copyright © 2009. Evelyn L. Bull. All Rights Reserved.

(44°58'55" N, 117°29'73" W; elevation 1368 m; Fig. 1),Pine (44°49'30" N, 118°4'58" W; elevation 1966 m),and Fish (45°2'92" N, 117°5'41" W; elevation 1992 m).We compared metamorph diet with available arthropodsat Balm only. Dietary data for juveniles (one-year oldtoads) came from Balm and Pine because we found onlyone juvenile at Fish. We selected these three reservoirsas study areas because they had a high density ofbreeding toads (Bull and Marx 2002; Bull 2006). Themajority of the water in these reservoirs comes fromsnow-melt run-off and seasonal rains, although at leastone perennial stream flows into each reservoir. All threereservoirs serve for cropland irrigation downstream inthe summer. Consequently, irrigation needs drain thereservoirs in some years. These reservoirs are 12 to 36ha in size at full capacity and are located in BakerCounty, Oregon, USA.

All study areas are in mountainous, forested terrainwith undulating uplands and moderately or steeplywalled drainages. Wallowa-Whitman National Forestmanages the surrounding forests, which consist primarilyof Lodgepole Pine (Pinus contorta) and Subalpine Fir(Abies lasiocarpa) except at Balm, where PonderosaPine (Pinus ponderosa), Douglas-fir (Pseudotsugamenziesii), Western Larch (Larix occidentalis) andGrand Fir (A. grandis) are present. Non-arborealvegetation along drainages consists primarily of grassesand sedges with some shrubs. Density of vegetation andproximity to forest stands differ among locations.Livestock grazing occurs around Balm and Fish.Drainages in all three areas contain substrate dominatedby gravel, cobble, and some boulders.

85

esipp

Text Box

This file was created by scanning the printed publication. Text errors identified by the software have been corrected: however some errors may remain.

Bull and Hayes.-Diets of Metamorphic and Juvenile Western Toads

FIGURE 1. Primary breeding site of the Western Toad (Bufo boreas) at Balm Reservoir in northeastern Oregon, USA. Metamorphosed toadsused the small drainage in the foreground for dispersal. (Photographed by Evelyn L. Bull)

Diet Samples.-We collected diet samples frommetamorphs at each of the three study areas in 2006 andat Balm in 2007. In August and September 2006, wecollected 88 samples from metamorphs at the breedingsites or along streams within 1 km of the breeding sites.In July 2007, we collected 112 diet samples frommetamorphs at Balm along two streams used fordispersal within 200 m of the breeding site. Wecompared these 2007 samples with available arthropodsto determine selection of prey.

We collected diet samples from juvenile toads at Pinein August and September 2006 and from juveniles atBalm in late April and May 2007 . We distinguishedjuveniles from metamorphs based on date and size.After a metamorph had overwintered, we considered it ajuvenile the next spring. At the time larvae transformed,juveniles (toads about one year old) were 25-50 mmsnout-vent length (SVL) and at least 1.1 g (Fig. 2).Juveniles occurred near the breeding sites, along streamsflowing into the reservoirs, and in wet meadows up to 1km from the reservoirs.

We collected diet samples by stomach flushing with a

plastic flexible catheter (1.5 mm outside diameter)inserted through the mouth and esophagus of themetamorph/juvenile with the opposite end attached to a35 cc syringe filled with water (Legler and Sullivan1979; Bull 2006). After flushing the stomach contents,we palpated the stomach to confirm that it was empty.We recorded SVL (in mm) and mass (g) for eachmetamorph/juvenile sampled (Table 1). We preservedfood samples in 70% ethanol, and identified prey itemsat least to order using a dissecting scope. We sortedprey items into three size classes (1-4.5, 5-9.5, and > 10mm) after we measured them with an ocular micrometer.Prey size refers to the actual length of the prey item (i.e.,whole insect, mite, or spider); partial prey items were notmeasured. We quantified the numbers of arthropods bycounting head capsules or forewings if they werepresent. We counted other body parts if the former weremissing. For example, six ant legs constituted one ant ifthere were other ant body parts present; identificationwas not based on legs alone. Each diet sample wasoven-dried for 24 h at 40 C. We determined biomassusing a Mettler HP35 scale (Mettler Instrument

86

Herpetological Conservation and Biology

FIGURE 2. We distinguished juvenile (larger toad) Western Toads (Bufo boreas) from metamorphs on the basis of size. (Photographed by EvelynL. Bull).

Corporation, Hightstown, New Jersey, USA) with anaccuracy of 0.1 mg. We estimated the mass of dietsamples at 0.0001 mg if it was smaller than the detectionlimit of the electronic balance. Diet composition wasdefined as the percentage of items of a particular preytype out of the total number of dietary items.

Available Artltropods.- To determine the terrestrialinsects and other arthropods available to metamorphsand juveniles at Balm, we positioned sticky traps on theground where juveniles and metamorphs were active inMay and July 2007, respectively. Traps consisted ofwhite poster board (20.5 cm x 30.5 cm) with a stickylayer on one side (Intercept Varroa Mite TrapsTM,IPMTechnologies Inc, Portland, Oregon). We positionedtraps on the ground where young toads forage;metamorphs could extricate themselves from the traps(pers. obs.)

We sampled arthropods available to juveniles in Mayusing sticky traps at three plots at l00-m intervals alongeach drainage where there were more than 200 juveniles.

At each plot, we positioned three sticky traps inlocations having juveniles. These were typically in thesun and within 0.5-5 m of water. We placed anotherthree sticky traps about 10m from the water, but in areastypically lacking juveniles, (e.g., in the shade and ondrier ground). We left sticky traps out for seven daysand collected them on 4 and 11 May 2007.

We sampled arthropods available to metamorphs inJuly at three plots along streams used for dispersal (twoplots 70 m apart on one stream and one plot on a secondstream) within 200 m of the breeding site. We placedtraps in the same arrangement as described above andcollected them after 7 days on 7 Jul 2007.

We returned traps to the lab and refrigerated them (T ~6°C) until processed for arthropod identification.Arthropods were measured and classified as previouslydescribed for diet samples above. Numbers ofindividuals were directly counted except for Collembola,which occasionally occurred in extremely large numberscovering the entire trap. On Collembola-saturated traps,we counted all of the individuals found in four random

87


0.25 cm2 squares and took an average of the foursquares, and then extrapolated this to the total area toestimate number of Collembola in the sample. Weexpressed the abundance of each prey category in termsof relative frequency.

Altalysis.- Our data analysis was generated usingSAS/STAT software, Version 9.1 (SAS Institute Inc.,Cary, North Carolina, USA). Statistical significance wasestablished at a = 0.05. We tested data for normalityusing the Shapiro-Wilk statistic calculated within PROCUNIVARIATE of SAS. For comparisons where datawere structured as one-way analysis of variance,heterogeneity of variance was determined using Levine'stest, which is insensitive to non-normality. This wasexecuted within PROC GLM of SAS, which was usedonly to generate this test statistic. Deviation fromnormality was the rule within all data, and varianceswere heterogeneous among treatment factors in mostcomparisons. Thus, non-parametric analysis procedureswere used.

We conducted three statistical tests to evaluaterelationships among treatment factors. In experimentsfitting into the framework of one-way analysis ofvariance with more than two treatments, we conductedKruskall-Wallis tests. We used Mann-Whitney U testsin experiments of this sort with only two treatments. Weassessed strength of relationships between twoquantitative response variables by calculatingSpearman's correlation coefficients.

We compared the number and size of prey items, preybiomass, and diet composition of metamorphs andjuveniles among study areas in 2006 using Kruskal-Wallis tests. We compared the number and biomass ofprey between metamorphs and juveniles using Mann-Whitney U tests. Spearman's correlation coefficientswere calculated to assess relationships between: (1) preysample biomass and number of prey items; (2) toad SVLand mass with number of prey items; and (3) toad SVLand mass with biomass of prey. We compared the SVLof metamorphs and juveniles among study areas usingKruskall-Wallis tests and between years at Balm usingMann- Whitney U tests.

We compared composition of diet: (1) amongmetamorphs in three study areas (2006) using Kruskal-Wallis tests; (2) between metamorphs at Balm in 2006and 2007; (3) between metamorphs and juveniles at Pine(2006) and at Balm (2007); and (4) between juveniles atPine (2006) and Balm (2007) using Mann-Whitney Utests (Zar 1999). We compared the size of prey taken:(1) among study areas and between years formetamorphs using Kruskal-Wallis tests; (2) betweenjuveniles at Pine (2006) and Balm (2007); and (3)between juveniles and metamorphs at Pine (2006) and atBalm (2007) using Mann-Whitney U tests. Wecompared the number and size of available arthropodscaptured on traps at Balm in 2007 between weeks inMay and between traps in close proximity tometamorphs and juveniles versus those traps at somedistance using Mann-Whitney U tests. We used a< 0.05to establish significance. Standard error (SE)accompanies all mean values.

RESULTS

Larvae metamorphosed into terrestrial metamorphs atapproximately the same size at each reservoir (Table 1).Most metamorphs left the breeding sites within severaldays of metamorphosis and started traveling in massesalong intermittent or permanent streams flowing into orout of the reservoir. Juvenile toads were difficult tolocate, so we were only able to collect diet samples atPine in 2006 and at Balm in 2007.

Prey Items and Biomass.-More than 90% of themetamorphs we sampled contained at least one fooditem. Across all metamorphs, 200 diet samplescontained an average of 15.2 ± 1.06 prey items (range =0-79) with a mean sample biomass of 4.67 ± 0.65 mg(Table 1). In general, we could identify approximately90% of the contents of individual diet samples; -55%was partially digested but recognizable parts, 35% wasnearly whole arthropods, and 10% was mostly digestedunidentifiable material.

For all juveniles captured at Pine in 2006 and Balm in2007, 176 diet samples contained an average of 23.4 ±

88


2.69 prey items (range= 0-313) with a mean biomass of 6.90, P == 0.05, respectively); at Balm in 2007, only the12.2 ± 3.83 mg. In a comparison between metamorphs number of prey items differed between metamorphs andand juveniles found within two study areas in the same juveniles (Z == 8.29, P < 0.01). The biomass of dietyear, number of prey items and biomass of diet samples samples correlated with toad SVL (r == 0.41, P < 0.01)differed significantly at Pine (Z= -6.91, P< 0.01; Z= and mass (r = 0.38,P< 0.01).


Diet Composition.-Metamorphs fed on arthropods inat least 20 families within 10 orders of insects, inaddition to two orders of arachnids and a fewgastropods. For all study areas combined, the orderswith more than 5% of the prey items of metamorphsincluded Hemiptera (23.5%), Hymenoptera (19.1%),Arachnida (11.4%), Collembola (8.8%), Coleoptera(7.6%), Diptera (7.0%), Lepidoptera (6.8%), andThysanoptera (5.6%; Table 2; Fig. 3). The families thatwere in the highest number of prey items wereAphididae (11.9%; order Hemiptera) and Formicidae(8.6%; order Hymenoptera). Diet composition of alljuveniles included Collembola (47.9%), Hymenoptera(20.4%), Coleoptera (11.1%), and Arachnidae (5.4%)each making up more than 5% of the diet (Table 2, Fig.3). Formicidae (order Hymenoptera) was the familywith the most prey items consumed by juveniles.

Diet composition of metamorphs differed amongstudy areas and between years at Balm. In 2006, dietdiffered significantly among study areas in number ofprey items made up of the orders Hymenoptera,Coleoptera, Psocoptera, Collembola, Thysanoptera, and

Tricoptera (Tables 2 and 3). Metamorph diet at Balm in2006 and 2007 differed significantly with highernumbers of the order Hymenoptera and familyCicadellidae in 2006 and higher numbers of the ordersHemiptera and Lepidotera in 2007. The order with thegreatest representation in each area and year wasHymenoptera (52%) at Balm in 2006, Hemiptera (36%)at Balm in 2007, Thysanoptera (33%) at Pine, andCollembola (30%) at Fish (Table 2). Composition ofprey also differed between metamorphs and juveniles inthe two study areas where both were sampled in thesame year (Tables 2 and 3).

Size of Prey.-Size of prey of metamorphs in 2006differed significantly among study areas and betweenyears in Balm for items 1-4.9mm (X2 = 24.43, P < 0.01;Z = -3.87, P < 0.01, respectively) and for items 5-9.9mm (X2 = 18.26, P < 0.01; Z = -4.77, P < 0.01,respectively; Table 4). Similarly, metamorph SVLdiffered significantly among study areas in 2006 (X2 =57.54, df= 2, P < 0.01) and between years at Balm (Z =5.70, P < 0.01). The mean SVL ofmetamorphs ranged

90


from 16.2 at Fish to 24.5 at Balm in 2006 (Table 1). In2006, a higher percentage of prey from metamorphs atBalm was in the 5-9.9 mm size class when compared tometamorphs at Pine and Fish (Table 4). At Balm,metamorphs consumed larger prey in 2006 compared to2007, although metamorphs sampled in 2006 were anaverage of 6 mm larger than those in 2007 (Table 4).Toad SVL was significantly correlated with thepercentage of prey 1-4.9 mm in 2007 (r = 0.19, P = <0.01) and with prey 1-4.9 mm and 5-9.9 mm in 2006 (r= 0.35, P < 0.01; r = 0.56, P < 0.01, respectively) in allthree study areas.

Size of prey taken by juveniles at Pine in 2006 and atBalm in 2007 differed significantly (Z = 9.39, P < 0.01)with larger prey being taken at Pine where juvenilesaveraged 34 mm SVL compared to juveniles thataveraged 22 mm SVL at Balm (Table 4). Juvenile SVLdiffered significantly between Pine and Balm (Z = 9.39,

P < 0.01). The percentage of prey 1-4.9 mm wassignificantly correlated (r = 0.33, P = 0.04) with juvenileSVL at Pine.

Compared to juveniles in the same area, metamorphswere significantly smaller at both Pine in 2006 (Z = -6.91, P < 0.01) and Balm in 2007 (Z = -11.44, P < 0.01)and fed on significantly smaller prey at Pine (Z = -6.91,P< 0.01) and at Balm (Z= 11.44, P< 0.01; Table 4).

Available Arthropods.-We collected a total of94,409 arthropods in the Insecta and Arachnida on stickytraps at Balm belonging to at least 10 families in 12orders (Table 5). Of the families and orders representedon the sticky traps, metamorphs andlor juveniles preyedon all except Orthoptera. The order with the greatestrepresentation on sticky traps was Collembola with anestimated 89,023 captured in May (120 on 4 May and88,903 on 11 May), although only 26 were trapped inJuly. Excluding Collembola, adult Diptera andHemiptera, specifically Cicadellidae, made up themajority of arthropods in both time periods. Owing tothe few differences (described below) in the followingstatistical comparisons, we pooled the data forsubsequent analyses on arthropods collected: (1) onsticky traps during the two weeks in May; (2) fromsticky traps near juveniles and those away fromjuveniles in May; and (3) from sticky traps nearmetamorphs and those away from metamorphs in July.In 16 comparisons by order or family of availablearthropods collected on sticky traps the week of 4 Mayand 11 May, only Hymenoptera (Z = 2.11, P = 0.03),Collembola (Z = -4.86, P < 0.01), and adult Diptera (Z =-3.29, P < 0.01) differed significantly. Hymenopteraand Collembola were more abundant the second week,and adult Diptera was more abundant the first week.These differences are likely to reflect the differentemergence times of some of the arthropods.

In 14 comparisons by orders and families ofarthropods collected on sticky traps placed nearjuveniles and away from juveniles in May, a significantdifference occurred only in the abundance ofCicadellidae (Z = -2.03, P = 0.04). This family wasmore than twice as abundant on traps near juvenileswhen compared to traps away from juveniles. Similarly,in 15 comparisons by order and families of arthropodscollected on sticky traps placed near metamorphs andaway from metamorphs in July, a significant differenceoccurred in the abundance of Hymenoptera (Z = -2.26, P= 0.02) and adult Diptera (Z = 2.65, P < 0.01) with moreHymenoptera on traps away from metamorphs and moreDiptera on traps near metamorphs. These differences arelikely to reflect microhabitat differences of thesearthropods.

In a comparison of sizes of available arthropods, therewere significantly more arthropods 1-4.9 mm the weekof May 11 (Z = 3.93, P < 0.01) and more arthropods 5-

91


9.9 mm the week of May 4 (Z = -3.93, P < 0.01).However, arthropods 5-9.5 mm in length made up 12%of the arthropods on sticky traps near metamorphs versusonly 4% on traps away from metamorphs in July (Z =-2.40, P = 0.02).

Metamorphs at Balm used a higher proportion ofHemiptera (specifically Aphididae), Hymenoptera(specifically Formicidae), Lepidoptera, Coleoptera, andArachnida and a lower proportion of Diptera thanoccurred on sticky traps (Fig. 3); the available Hemipteraconsisted primarily (80%) of the family Cicadellidae(Table 5). Juveniles at Balm preyed on a higherproportion of Hymenoptera (specifically Formicidae),Coleoptera, Diptera, and Arachnida than occurred onsticky traps (Fig. 3). Only Collembola was used in alower proportion than occurred on sticky traps. Inaddition, we did not collect representatives of four orderson traps although they occurred in diet samples of preytaken by metamorphs or juveniles. Typically, the ordersnot collected on traps made up a small proportion of thediet with less than 10 prey items taken of Ichneumonidaeand Symphyta in the order Hymenoptera; Anobiidae andScarabidae in the order Coleoptera; Miridae, Saldidae,Pentatomidae, Nabidae, Tingidae, Thyreocoridae andBerytidae in the order Hemiptera; and the orderNeuroptera. Exceptions included two families of beetles(Curculionidae and Chyrsomelidae) where 35 and 36

items were taken as prey, but none were found on thetraps. Lepidoptera and Diptera adults, but no larvae,were captured on traps, yet metamorphs and juvenilespreyed on 193 and 95 larvae of these orders,respectively.

Size of prey taken by metamorphs and juveniles inBalm was approximately the same proportion thatoccurred on sticky traps (Table 4). However,metamorphs preyed on slightly more prey < 5 mm inlength and juveniles preyed on slightly more prey > 5mm compared to arthropods on the traps (Table 4).

DISCUSSION

No previous studies exist on the diet composition ofB. boreas metamorphs or juveniles, although diets ofother species of Bufo juveniles (family Bufonidae) havebeen reported. Stomachs of seven newlymetamorphosed B. woodhousei fowleri in Connecticutcontained 66% Collembola, 20% Aphidae, 10% Diptera(adults and larvae), 2% Acarina, and 1% Formicidae(Clarke 1974). In South Dakota, 50 metamorphs of B.woodhousii and B. cognatus preyed primarily onCollembola and Coleoptera (Flowers and Graves 1995).Metamorphs of 17 B. cognatus in Texas preyed on morescarab beetles and formicid ants, while 13 Speamultiplicata (family Pelobatidae) preyed more onchysomelid and elaterid beetles (Smith et al. 2004).Newman (1999) found that 26 Scaphiopus couchiimetamorphs in Texas fed on collembolans andArachnida (primarily mites) while the largest items instomachs were generally soft-bodied caterpillars orbeetle larvae. Our study differed from others bysampling diet in a large number of live toads (200metamorphs and 176 juveniles), which likely contributedto a greater diversity of prey being reported.

In our study, Hemiptera (particularly Aphididae),Hymenoptera (particularly Formicidae) and Arachnidamade up over 50% of the prey taken by metamorphs.However, metamorphs of B. boreas in our studyconsumed a large variety of prey items within a smallsize range; over 80% of prey items were smaller than 5mm. These findings contradict Newman's (1999)suggestion that there may be little scope for dietvariation because of the small range of size variationamong metamorphs in a population. Differences in preyconsumed among our three study areas suggest thatmetamorphs were versatile in foraging on different preypresent at different locations. Although somedifferences were found between available arthropods anddiet samples, it seems that the diversity of metamorphand juvenile diets reflect the arthropod diversity in theappropriate size classes in our study area at the time.

Despite the breadth of prey taken, metamorphs andjuveniles at Balm appeared to select prey non-randomly(Fig. 3), assuming that our sticky traps adequately

92


sampled available arthropods. Other explanations wouldinclude the patchy distribution of arthropods and/oryoung toads. An example of patchiness in time andspace is the extremely high number of Collembolacaptured in May where 98% were captured on four of 36traps. Of the Collembola captured, approximately 0.1%were captured during the first week compared to thesecond. Other factors include the relative size andmobility of potential prey. Although two prey itemsmay be equally numerous, young toads may consumemore of one than another if it is easier to capture andconsume because it is comparatively smaller and/or lessmobile. Our results differed from those of Newman(1999) who reported that the diet of Scaphiopus couchiimetamorphs was in almost complete accordance in theidentity of prey items with arthropods captured on stickytraps. Other factors that may contribute to differencesbetween diet and available arthropod samples, is thecondition of the diet sample and digestability ofarthropods. Some soft-bodied arthropods may digestmore rapidly than others may. Although the majority ofthe obvious stomach contents were identifiable, it ispossible that easily digested and/or absorbed prey wereoverlooked.

Foraging strategies may change as metamorphs maturein terms of movement, time of foraging, prey/predatorsize relationship, and energetic needs (Christian 1982).Toft (1981) found foraging mode differed with age ofthe toad, with metamorphs actively searching for preyduring the day while adult toads, which were primarilynocturnal, sat in one place and consumed passing prey.The diurnal activity of these metamorphs may have beena function of the combination of warmer temperatures inour study areas during the day and because small soft-bodied insects are active in the day increasingmetamorph probability of encountering smallerarthropods. Metamorphs in our study did not selectlarger prey, most likely, because of their inability tocapture and consume it, but also perhaps because theydid not encounter it during the day.

Size of metamorph is influenced by the size atmetamorphosis and time since metamorphosis andaffects the size of prey taken because of a size-relatedforaging ability. Both size of prey taken by metamorphsand size of metamorphs differed among study areas andwere correlated. Studies on both Scaphiopus couchii(Newman 1999) and B. woodhousii (Flowers and Graves1995) found that prey size differed among metamorphsizes. However, while neither study found a correlationbetween metamorph size and number of prey items, themass of prey in stomachs did increase with metamorphsize with Scaphiopus couchii.

Not surprisingly, the diet ofmetamorphs and juvenilesdiffered markedly from that of adults in our study areaswhere adults foraged primarily on larger ants and groundbeetles in northeastern Oregon (Bull 2006), which are

also common items in the diet of adult Bufo spp. inOklahoma (Smith and Bragg 1949). In Colorado, B.boreas adults ate mostly ants, beetles, and spiders(Campbell 1970) and moths, grasshoppers, ants, deerflies, mosquitoes, and beetles (Burger and Bragg 1947).In Montana, seven stomachs of B. boreas contained 75%Hymenoptera, 23% Coleoptera, 3% Arachnida, and lessthan 1% of four other insect orders (Miller 1978). Basedon analysis of 47 scats of B. boreas found in the CascadeMountains of Oregon, diet at one area consisted almostexclusively of carpenter ants (Camponotus spp.; Hayesand Hayes 2004).

Overall, prey availability would not appear to be alimiting factor for B. boreas metamorphs and juvenilesin these study sites during the two time periodsconsidered. Our studies were conducted near breedingsites, and it is unknown how prey availability may varyas young toads disperse away from these sites. Knownsources of mortality of metamorphs and juveniles in theimmediate vicinity of the breeding sites includepredation, trampling (by cattle and vehicles), anddesiccation. The presence of chytridiomycosis in adulttoads at these sites has been confirmed (Bull 2006), aswell as in metamorphs and juveniles (Bull, unpubl. data).Dispersal and survivorship of toads prior to recruitmentinto the breeding populations warrants furtherinvestigations in light of declining numbers in some sitesin northeastern Oregon and elsewhere.

Acknowledgments.-We thank Christopher Pearl, JayShepherd, and Lia Spiegel for reviewing earlier drafts ofthis manuscript. Patricia Johnson identified ingested andavailable arthropods. Jay Shepherd and Steven Smithconducted statistical analyses. Funding was provided bythe Pacific Northwest Research Station and u.s. Fishand Wildlife Service. Toads were handled under OregonPermit Numbers 1-06 and 84-07.

LITERATURE CITED

Bull, E.L. 2006. Sexual differences in the ecology andhabitat selection of Western Toads (Bufo boreas) innortheastern Oregon. Herpetological Conservation andBiology 1:27-38.

Bull, E.L., and D.B. Marx. 2002. Influence of fish andhabitat on amphibian communities in high elevationlakes in northeastern Oregon. Northwest Science76:240-248.

Burger, W.L., Jr., and A.N. Bragg. 1947. Notes on Bufoboreas (B. and G.) from the gothic region ofColorado. Proceedings of the Oklahoma Academy ofScience 27:61-65.

Campbell, J.B. 1970. Food habits of the Boreal Toad,Bufo boreas boreas, in the Colorado Front Range.Journal of Herpetology 4:83-85.

Carey, C. 1993. Hypothesis concerning the causes of the

93


disappearance of Boreal Toads from the mountains ofColorado. Conservation Biology 7:355-362.

Carey, C., N. Cohen, and L. Rollins-Smith. 1999.Amphibian declines: an immunological perspective.Developmental and Comparative Immunology23:459-72.

Christian, K.A. 1982. Changes in the food niche duringpostmetamorphic ontogeny of the frog Pseudacristriseriata. Copeia 1982:73-80.

Clarke, R.D. 1974. Food habits of toads, Genus Bufo(Amphibia: Bufonidae). American Midland Naturalist9:140--147.

Com, P.S., M.L. Jennings, and E. Muths. 1997. Surveyand assessment of amphibian populations in RockyMountain National Park. Northwestern Naturalist78:34-55.

Fisher, R,N., and H.B. Shaffer. 1996. The decline ofamphibians in California's Great Central Valley.Conservation Biology 10:1387-1397.

Flowers, M.A., and B.M. Graves. 1995. Prey selectivityand size-specific diet changes in Bufo cognatus and B.woodhousii during early postmetamorphic ontogeny.Journal of Herpetology 29:608-612.

Hayes, M,P., and C.B. Hayes. 2004. Bufo boreas boreas(Boreal Toad). Scats and behavior. HerpetologicalReview 35:369-370.

Legler, J.M., and L.J. Sullivan. 1979. The application ofstomach-flushing to lizards and anurans.Herpetologica 35:107-1 10.

Livo, L.J., and D. Yeakley. 1997. Comparison of currentwith historical elevational range in the Boreal Toad.Herpetological Review 28:143-144.

Miller, J.D. 1978. Observations on the diets of Ranapretiosa, Rana pipiens, and Bufo boreas from westernMontana. Northwest Science 52:243-249.

Muths, E. 2005. Bufo boreas Baird and Girard, 1852 (b)Western Toad. Pg. 392-396 In Amphibian Declines:The Conservation Status of United States Species.Lannoo, M. (Ed.). University of California Press,

Berkeley, California, USA.Muths, E., P.S. Corn, A.P. Pessier, and D.E. Green.

2003. Evidence for disease related amphibian declinein Colorado. Biological Conservation 110:357-365,

Newman, R,A. 1999, Body size and diet of recentlymetamorphosed Spadefoot Toads (Scaphiopuscouchii). Herpetologica 55:507-515.

Olson, D.H. 1992. Ecological susceptibility ofamphibians to population declines. Pp. 55-62 InSymposium on Biodiversity of NorthwesternCalifornia. Harris, R.R., and D.C. Erman, (Tech.Coord.), Report 29. Wildland Resources Center,University of California, Berkeley, California, USA.

Scherer, R.D., E. Muths, B.R. Noon, and P.S. Com.2005. An evaluation of weather and disease as causesof decline in two populations of Boreal Toads.Ecological Applications 15:2150--2160.

Smith, C.C., and A. Bragg. 1949. Observations on theecology and natural history of Anura, VII. Food andfeeding habits of the common species of toads inOklahoma. Ecology 30:333-349.

Smith, L.M., M.l Gray, and A. Quarles. 2004. Diets ofnewly metamorphosed amphibians in west Texasplayas. Southwestern Naturalist 49:257-263.

Thompson, P., B, Hadolski, and P. Chase. 2003. BorealToad (Bufo boreas boreas) and Spotted Frog (Ranaluteiventris) distributional surveys and monitoring innorthern Utah, 2002. Utah Division of WildlifeResources, Salt Lake City, Utah, USA. PublicationNumber 03-02.

Toft, C.A. 1981. Feeding ecology of Panamanian litteranurans: patterns in diet and foraging mode, Journal ofHerpetology 15:139-144.

Wente, W,H., M.J. Adams, and C.A. Pearl. 2005.Evidence of decline for Bufo boreas and Ranaluteiventris in and around the northern Great Basin,western USA. Alytes 22:95-108.

Zar, IH. 1999. Biostatistical Analysis. Prentice Hall,Upper Saddle River, New Jersey, USA.

94


EVELYN L. BULL is a Research Wildlife Biologist with thePacific Northwest Research Station, U.S.D.A. Forest Service.She received her B.S. in Zoology at University of Wisconsin atMadison, her M.S. in Fisheries and Wildlife at Oregon StateUniversity at Corvallis, and her Ph.D. in Wildlife Ecology atUniversity of Idaho at Moscow. Her research focuses on theeffects of natural and human disturbances on Western Toads andColumbia Spotted Frogs, and on old-growth dependent speciesincluding Pileated Woodpeckers and other cavity nesting birds;Great Gray Owls, Vaux's Swifts, and American Martens.(Photographed by Dave Wyland).

JANE L. HAYES is a Research Biological Scientist withthe Pacific Northwest Research Station, U.S.D.A. ForestService. She received her B.S. from Biological Sciencesat Stanford University and her Ph.D. in Entomology fromthe University of Kansas. Her research examinesarthropod biological and ecological interactions at thestand-level and across forest landscapes. Her studiesfocus on the role of disturbances, like insect outbreaks, inecosystems, and how their desirable and undesirableeffects might best be managed. (Photographed byanonymous).

95

selection of diet by metamorphic and juvenile western toads (bufo boreas… · 2013-03-28 ·...

Documents