Rev. BioL Trop.,41 (1): 131-141,1993

Trophic relationships among four cricetid in rice fields

Angela M.G. Martino' and Marisol Aguilera Departamentode Estudios Ambientales,Un iversidad Sim6n Bolívar,Apartado 89.000, Caracas IOSI-A-Venezuela. , Present Address: Centrode Investigacionesen Ecología y ZonasAridas, Universidad Nacional Experimental Francisco de Miranda,,Apartado 7506,Coro, Edo. FaIcát, Venezuela.

(Rec. 23-1X-1991. Acep. 25-vm-I992)

Abstraet: The food habits of lIenezuelae. delicalus, Sigmodon alslani and mi· crolÍllus were studied by storoaeh cootents anaIysis in aged Venezuelan rice-fie1ds. H. lIenezuelae is mainiy herbivo­ rous,feeding primarly on rice. The importance of other food items depends on their availability. S. alslani, is omnivo­ rous,its diet consisting oírice seeds (40%) and invenebrates (27%). O. delicalus is granivorous,and Z. microlÍllus is chiefly insecnvorous (70.6%). Diet diversity (Shannon-Weaver index) is low in H. lIenezuelae. O. delicatus and Z. microlÍllus while S. alslani has high values. All speciespresented medium to low niche width vaIues (petraitis Index) in all crupstages. The.vaIuesfor Hom's index ofniche overlapbetween species becamegreater with food avai­ labi.lity. Only S. alslaniand Z. microlÍllus seemed to interactsi gnificantly when food availability was low. Diets of the four species were related to cropdeve10pmental stage,vegetation composition,and characteristics of the habitat su­ rroundingthe crop.

Key words: rodent,rice-fie1d, Holochilus. Sigmodon, Oryzomys.Zygodontomys. trophic relationships.

Over the last decade. researeh on eommuni­ dealing with multiple species trophic relations­ ties of small terrestrial has ooen in­ hips have been limited because of diffieulties creasingly focussed on spatial patterns (Dueser in: a) identifying slOmaeh eontents (mainly in and Shugart 1979. M'Closkey 1980, Meserve herbivores and frugivores) and, b) obtaining 1981a, Ostfeld el al. 1985, Seagle 1985) and adequatesample sizes (Meserve 1981b). food resouree utilization (Hubert and Adams Current researeh in feeding eeolagy tries lO 1981, Kinkaid and Cameron 1982a. Butet answer mainly three questions: a) how related 1986). species in the same habitat eould share food re­ In turo, trophie studies have been eentered sourees? (Reiehman 1977, Flake 1973, (Flake 1973, Kinkaid and Cameron 1982a,b, Houteooper 1978), b) what are the trophie rela­ Meserve 1981a, Rodgers and Lewis 1986)not tionships between species? (Meserve 1981b, onIy in descriptions of ingested food, but also Meserve 1988), e) is eompetition for food re­ on ealorle and energetie requirements (Chew sources one of the interactions that affect eom­ andChew 1970, Soholt 1973). Optimization fe­ munity assemblages? (Smartt 1978, Brown and eding models have been propossed (Reiehman Liebermann 1973, Trombuleek 1985). Most of 1977, Rau et al. 1981). Nevertheless, studies these researeh has been done in natural habitats, -

132 REVISTA DE BIOLOGIA TROPICAL

but it will be interesting to know !he pattem of fo­ ges of rice crops and 3) to establish trophic re­ od resource utilization in less complex ecosys­ lationships ,between the rodent species found in tems, such asagrosystems. Al!hought agrosystems the rice-field. seem evidentIy uniform and stable, they suffer pe­ riodical changes caused by crop cycles. such chan­ MATERIAL AND METHODS ges would be reflected on dispersion patterns of !he small fauna living !here, as well as in Study area: The study was conducted in their habitat and/or food resource utilization two rice plantations near Acarigua city, in (Barrett el al . 1990). Portuguesa State (Venezuela): Los Mamones In Venezuela, one of !he most common neo­ (9023'3"N, 6902'10"W), and El Porvenir tropical crops is rice (Oryza saliva). A cricetid (9023'37"N, 68053'16"W). Annual temperature rodent community (H% chilus venezue/ae. in the region was 22-29° C, and mean annual Sigmodon alstoni and Oryzomys delicatus) was precipitation was 1400 mm with a marked dry found in association with rice-fields in the season from December to April. rice in diffe­ country (Cartaya and Aguilera 1985), but infor­ rent stages of development was the dominant mation on population biology and food habits vegetation in both sites. On, river and stream for the in volved species is scarce. Some of banks. a minimal amount oC natural vegetation them are considered pests in many South was maintained. American regions (Hershkovitz 1955, Massoia Methods: Sampling took place between 1974, Twigg 1965). In Venezuela, H. venezue­ July 1983 and April 1984, with one sampling lae. S. a/stoni and Zygodontomys microtinus period per month. Two sampling periods (a and are considered rice-field pests (Aguilera 1985, b) on each crop stage of development were ma­ Cartaya and Aguilera 1985). de. Phase 1: between the end of !he active vege­ Under this scenario, the objectíves of the tative phase and the beginning of the, slow ve­ present study were: 1) to obtain food habit getative phase (crop age: 1 month); Phase II: informatíon on H. venezuelae. S. alstoni and Slow vegetative phase (crop age: 2 months); O. delicatus, 2) to relate diet variations with Phase JI: Reproductive· phase and ripening of potential foodresources in different growth sta- rice grain (crop age: 3 months) (Fig.' 1).

' P H A S E 1 PHASE II P H A S E III

! GALLERY lit S U GA R CANE [gJ SLOW o ACTIVE o MATURATION FOREST FIELO VEGETATIVE VEGETATIVE PHASE FIELO PHASE FIELO PHASE FIELO

Fig. 1. Grid location in lhe study area. Letters A and Bpoint out lhe state of development of adjacent rice-fields al a given sampling periodo Ref: Reaped field; Raf: Raked field; G: grid; S: Sown ground. MARTINO, & AGUILER.A: Trophicrelationships of rodents 133

One trapping grid with 256 stations (15 m of the rice field was slighOy different: into the spacing) with two snap traps (17.3 x 8.5 cm crop area 20 random plots of 1 m2 were chosen, and 9.0 x 4.6 cm) per station was established in while in the dams, 0.25 m2 plots, 10 m apart, each sampling period of rice fieId. Traps were were examined along the dam border. In both baited with a mixture of oatrneal and sardine cases, recording of plant coverage data ceased and checked twice nighOy for five nights each when the species-area curve stabilized horizon­ month .. Standard autopsy data (weight, sex and taUy. reproductive condition) were recorded; whole Diet diversity was estimated with the stomachs were removed and fixed in 5% for­ Shannon-Weaver diversity index (H') both malino for planfparts CH'p) and species (H's) inges­ Analysis of stomach contents followed stan­ ted. Plant resoruse niche width was estimated dard procedures (Dusi 1949). Subsamplesfrom with the Petraitis index (Pi) (Petraitis 1979), each stomach were processed and five sUdes which shQWS very small bias when few re­ were prepared per subsample; 20 randomly 10- sources are abundant, as in our case. cated fields per slide were examined at lOOx Invertebrates were not included in the niche using a microscope. Frequency and volume width calculations because their availability percentage (mean percent area occupied per was not quantified. fragments in the mkroscopic fieId) were deter­ In order to establish trophic relationships mined. Idc.mtification was made with a photo­ between species, niche overlap was ca1cula- . micrograph key prepared from reference slides ted using a modification of Morisitas index of the plants and invertebrates found in the (CA) (Hom 1966), calculated for plant part study area (see also Martino and Aguilera ingested (CAp) and for the whole plant inges­ 1989). ted (CAs). Vegetation cover was analyzed by the point Parametric and non parametric ANOVAS ínterception method (Mueller-Dumbois and were used to test differences on diet between Ellemberg 1974). Thesurvey made in each area species and/or samplings periods.

TABLE 1

Speciescompdsitionand number 01 animalscaptured in the several samplingphases at a rice-plata/ion. . Roman numbers indica te crop phases, letters denote samplingperiods

Species 1 TI m Total

a b a b a b N %

H: venezuelae 40 111 15 19 101 103 389 82.2

Z. microtinus O 31 O O O O 31 6.6

S,alsloiti 2 11 O O 7 11 31 6.6

O. delicatus 2 8 O 10 22 4.6

483 100.0 134 REVISTA DE BIOLOGIATROPICAL

100 A

80 ... 411 > 60 o u 40 [ill] C)'peroc.oe

.... D Dlcot)"edoft• c: o 11 Other monocofyledoM eL

15'0•••• : .... o B B C.,odloto

411 lllI D. lIo,l%ontoll. 0\ � E.colonum o .... Ilill� c: 411 U O !!:.!.!!!!! ... D Otll., 411 a..

a b a b a b

Pho s e 1 Phose II Pho se III

Pig. 2. Percentage of plant coverage of lile different species and/orplant groups found in Ihesludy area. A: Cropor cultivated area. B: Dam. Sampling periods are denoted by letters a and b.

RESULTS and Leptochloa scabra (Graminae) covered 21 % each, and C. iria a lower percentage and plant species composition: (16%). H.venezuelae was the most common species in In Phase IlI, weed coverage increased, aH crop phases; the others reached low percen­ Fimbrislilis miliacea (Cyperaceae) reached tages and were not caught in a11 crop phases 62% in period IIIb, while rice coverage was (Table 1). S. alstoni. O. delicalus and Z. mi­ maintained or tended to decline in comparison crotinus were almost exclusively captured on wiLh Phase n. In all phases dicotyledons were dams and/or dry areas of the crop when floo­ scarce and most spedes found were smaH se­ ding was minimal, while H. venezuelae was miaquatic plants. trapped in all the grid area. The plant species On dams, species abundance and composi­ composition in crop area was different from Lion were greater and more variable (Fig. 2B). that found in dams (p < 0.05)(Fig.2). A total of The usually predominant group was dicotyle­ 47 plant species was recorded. In Phase 1, rice dons, wiLh exception in sampling periods la, lb covered, into the crop area, about 50%. Water, and Ha. Coverage of Cyperaceae was generally bare ground, and weeds comprised the remai­ low, excluding Phase 1, when it was almost ning (Fig. 2A). among weeds, Cyperus iria 90% (Fig. 2B). (Cyperaceae) reached the highest coverage va­ The abundance and importance of grass spe­ lue (8%). des were also variable: in Phase 1 Chloris ra­ During Phase II rice was approaching its diata coverage was high (sampling period la), maximum vegetative growth and coverage in­ in Phase II Digitaria horizontalis was the do­ creased (Fig. 2A). weed coverage also Íncrea­ minan grass, and in Phase III there was no do­ sed: Saggitaria planitiana (Monocolyledoneae) minance. MARTINO.&; AGun..ER A: TrophicJelaIianJhipl d rodenu 135

TABLB 2

D;',comporition of 10111'speciUol cricetitl rotklltS capl lll'lId¡,. ,ice Mida in Venez_la. Ruul,an upnued ¡,.pel'Cenl4,é witla vol. 6ItIttdard. dniation ¡,.panntltuil (,= tracu; < 0.1,*,) lteml H.Yflnez__ O.delical/18 S.aIst(Jflj ZJflÜ:rocUr.118

N=202 N=13 N=24 N=17

Gruses 84.3(1.4 ) 76.3(1.7) 57.8 (S.8) 24.3 (1.0)

O. satiYtl 66.5 (U) 0.8(0.4) 1.1 (9.4) 1.0(0.8) Stems 28.6(1.7) 5.5(4.2) 9.2 (3.0)' 4.8(4.7) Seeds 24.9 (U) 63.9(1 0.0) 3S.8(�I) 15.8 (5.2)

L.scabra 5.4(0.9) 3.0(2.1) 1.0 (0.5) Stems 2.0(0.3) 0.6(0.4) Brácts 2.6(0.8) 1.0(LO) 0.4(0.2)

E.colo_ 1.1 (0.4) 3.3 (2,6) Stems 0.1 (0.04)

C.,adüJta 1.1(0.4) 0.4(0.2) Sientl 0.4(0.1) 0.1 (0,1)

S.lIa lepense 0.8(0.4) 0.4(0.4) 2.1 (U) Leaves 0.4(0.2) t Seedl 0.2(0.2) 0.4(0.4) 2. 1 (1.1)

D. lIDrizonl4lil 0.3(0.1) 1.4(1.3)

1.",,- 0.2(0.04) 0.1(0. 1) 1.4 (1.1)

C. dactyl(Jfl (stems) 0.1 (0.1) 1

E.plllll:ltIltI 0.1(0.03) 0.1 (0.1)

UDidenr.ified 8.9(0.6) 1.6(0.9) 2.7(1.2) 0.4(0.2) c,peraceae 6.0(1.0) 1.4(l.0) 6.0(1.1) 0.1(0.1)

CYP'TII8sp. 1.9(O.S) 0.5(0.5)

F. milÜII:M (seeds) 0.1 (0.03) 0.1(0.1) 0.5(0.5)

', ' , UDidenr.ified 4.0(0.8) 1.3(0.9) 4.9 (1.4) 0.1 (0.1) l>icotyledans 1.7(0.4). 12.6(6.6) 9.4(3.3) 4.1 (2.0)

E.alba 0.3(0.1) 2.3 (1.3)

BupbOlbiac:eae t 5.4(4.8) 1.1 (0.9) .1.9 (1,2)

UDidenr.ified 1.4 (0,4) 7.2(4.2) 4.2 (1.6) 2.2(1.6)

Invertebratel S.8(0.8) 6.8(2.4) 25.9(63) 70,6(1.2) "

Unidenlified 1.9(O.S) 2.9(1.6) I.S'(1.1)' 1.1 (0.6) 136 REVISTA DE BIOLOGIA TROPICAL

Rodent diet: H. venezuelae eonsumed diet). stems and seeds of other grasses, mainly plant material"particularly grass, foUo­ Cyperaeeae and dk:otyledons, mainly wed by Cyperaeeae.,3Ild dicotyledons; ihverté� Euí>horbiaceae. con1prlsed the remaining items. brates were only 5.8% of its diet (Tabie 2). Theproportions of rice inseeds itsdiet did notdif­ Rice was the most commonly eaten grass, fo� feebetween sampling periods (p > 0.1) (pig. 3D). llowed by L. scabra. E. c;olonum andC. radia­ Z. microtinus was mainly insectivorous, its ta (Table2). O. saliva stems, followed by seeds diet eonsisting of invertebrates, followed by were the most consumedparts, and eonsidering grasses, dieotyledons and Cyperaceae. rice.se­ all grasses, the stems were the item most eaten eds were the main plant material (72% over to­ (p< 0.0001). Cyperaeeae and dieotyledons we­ tal rice ingested) followed by Sorghum hale­ re eaten in lower quantities in spite of their pense and Euphor1.>iaeeae seeds (TabIe2) . . abundancein someeropS "(Table 2 and Fig. 2). Grasses were found in 94% of the stomachs S. aistoni was an omruvorous species. gras­ analized, rice seeds in 88% of the individúals. ses were an important fO()d item, followed by all indívidualseonsumed invertebrates. A com­ invertebrates, dieotyledons and Cyperaeeae parisonof dietsof the rodent species studied is (Table 2). Frequeney and abundanee in which shown in Fig. 4; differencesbetween their diets these items were ea ten varied signifieantly were significative(p < 0.05). (p<0.05)from one eropto another(pig. 3A-C). as in the dietof H. venezuelae, secd eonsump­ % " % tion inereaseswith eropage.

% A. 100 so 'l��il'l���� "I¡III 60 H o s Z Ii o S Z H o S z

Fig. 4. Percentage Ófvolume of main items ingestedby ro­ dent species in the study area. A: pertaxa. B: pergrass species. C: perpartsóf O. sativa. H: H. venezuelae; o: o . eEDlcoty,.dons fIID InVfJrt.b,af.. • � o O. sativo delicatus; s: S. alstoni;Z: Z. microtinus.

11Unid flltltt_d [[ll] E cotonum D Othou Diet diversity and niche width:. The diet % % e lOO o lOO diversity of H. venezuelae at H'p level waS in­ termediate (Table 3) and more or less eonstant so 00 throughout riceerop phases. Values for H's we­ 60 60 re slightly lower. H'p values forH. venezuelae were slightly higher than for S. alstoni H'p va­ "" 40 lues, but for H's valúes the situationwas inver-· ro 20 se. For O. delicatus. estimates of both H'were highly variable, ránging from low to interme­ A B A B 1 m diate values.

• aree'. rnn\.. .ove:! f?JJSceda O SIom. []UntunHfled Niche width lvalues (pi, Table 3) for H. ve­ nezuelae were intermediate with few varia­ Fig.3. Percentage ofvolume oC main items ingested by: A: tions, low in sampling period la and high in S. alsloni per taxa. B: S. alslom pergrass species. C: S. sampling period S. aIstoN also shows in-. afstoni perparts of O. saliva. D: O. delicatus. perpalti óf Illb. O. saliva. termediate Pi values, meaning that their niche width not ehange throughout the crop phases, O. delicatus was mainly granivorous, its diet in spite of resouree variability. O. delicatus included grasses, followed by dicotyledons; showed intermediate niehe width and Z:'ini­ Cyperaceae,and invertebrates(Table 2). This spe­ crotinus showed the highest Pi values, but one cies fed primarly on rice seeds (63.9% of total of thelowest diet diversities. The reason o( this MARTINO,& AGUILERA:Trophic relationships ofrodents 137

TABLE3

Measuresof diversityand niche widJhfor roqenJsin several samplingperiods. H'p: Shannon- Weaver diversity,index per partofplant (p)andper species ofplanJ (s) inges/ed. Pi: Petraitisniche breadJhindexo N: Samplesize. Remannumbersde- no/e cropphases, lellers sampling period

Phases N H'p H's Pi

H. venezuelae

a 18 1.00 0.72 0.33 1 b 46 0.70 0.41 0.75

a 12 0.73 0.60 0.77 Il b 15 0.90 0.67 0.53 a 53 0.76 0.39 0.57 ID b 58 0.91 0.55 1.10

O. de/iea/u.<;

a 2 0.21 0.20 0.59 1 b 8 0.90 0.75 0.59 a 1 0.03 0.03 0.89 ID b 10 0.70 0.19 0.90

S. als/oni a 2 0.96 0.93 0.28 1 b 11 0.49 0.40 1.24 a 7 0.82 0.57 0.89 ID b 11 0.78 0.66 0.76

Z. mierotinus 1 b 17' 0.46 0.39 1.24

result is that Pi measures, niche width in rela­ lae and S. alstoni a1ways shared high values. tion lO availability of plant resources. In this During IIIb. overlap values were even higher way, rodent species that eat few invertebrates than in the previouscrop stage, and similar willshow subestimate Pi values andvice versa. among rodentspecies. Niche overlap: In áll stages the CAp index was lower thanthe CM index (Table 4). Values DISCUSSION of niche overlap estimated for O. delícatus ver­ sus the other rooent species were high in sam­ A summary of trophic telationships descri­ S. alstoni bed for this rodentcommunity is presented in pling period la for CAp, while and . Z. microtinus shared the highest values in pe­ Fig.5. riod lb. InllIa niche values were higher than in The dominant species wasH. venezuelae, an period 1 for all species involved. The lowest herbivorous rodent which can ingest important overlap vahies were presented by O. delicatus amounts oí animal matter. The importanceof regarding the other species and considering plant parts and species consumed varied with only the plant parts eaten (C A p). H. venezue- crop stage (Martino and Aguilera 1989). Other 138 REVISTADE BIOLOGIAmOPICAL

TABLE4

Horn.'s (1966)n¡che erlapi.ndex (CY } jor lheJOIlT speciesoj rodenlSsludied in differelll crop pllases.Cyp: nkheoverlap . � . Indaper parlofplanl mgested. CYs: mehe overlap indexper speciesplalll of ingested.Roman numbers dellOtecrop pllases, letlers samplingperiods

O. de/katllS S. alslon¡ Z. microt¡nus

Species Phases CAS CAp CAS CAS CAS CAS

a 0.73 0.31 0.75 0.70

b 0.68 0.06 0.27 0.09 0.31 0.08 H.venezuelae a 0.97 0.66 0.94 0.91 m b 0.95 0.84 0.97 0.97

a 0.44 0.21 l. b 0.29 0.24 0.33 0.28 O. delicatus a 0.87 0.64 m b 0.91 0.91

S. alstoni 1 b 0.99 1.00

INVERTEBRATES O. delicatus was found to be mostly grani­ vorous; values of diet diversityand niche width suggest specialization in the useof this vegeta­ ble resource. Other species, like O. longicau­ datus in Chile, eat primarily seeds during the dry season, and pollen and flowers during the wet season (Me serve 1981a,b, Murua and

Slgmod o n Gonzalez 1981). O. palustris texensis eats se­ !.!.!!.2.!!1. * eds, green vegetation (Svihla 1931) and large

Oryzomy& quantities of insects and crustaceans (Sharp venezuela, � 1967, Kinkaid and Cameron 1982b), both spe­ GREEN * --*- PARTS '--______� seEOS des also showedsmall niche widths. Z. microtinus was a sporadic species in this Fig. 5. Trophic realtionships oí four species of rodents in rice-fields of Venezuela determined by stomach content study, since it only occurred in one sampling analyses. Proximity of points 10 apexes oítriangle denotes period (Table 1). It appeared lO be strongly in­ relative importanceof threemajor food categories. sectivorous, but was a generalist in the use of plant resources (see H and Pi values, Table 3). There is Httle information about this rodent, Holochilus species were also found lo be mainly herbivorous (Hershkovitz 1955 andit ís not clear if the diet observed is a result Twigg 1965). Second in importance is S � of the particular conditions in which it was captured (see Results and Fig. 1). In Guarico alstoni. which is omnivorous. Related spe­ state, Vivas (1984) found that microtinus cies, like S. hispidus, feed primarly on gras­ Z. ses, leaves and seeds during the summer, eats plant material, but the proportion of this ítem in the diet is unknown. when invertebrates are less important (Kinkaid and Cameron 1982b, Vivas and Diet variations found during the study could Calero 1985, 1988). be explained by three faclOrs: MARTINO,& AGUILERA: TropbicreIationsbips ofrodents 139

l. Crop developmental stage: This explains overlap values. Tbese change with environmen­ differences in item ingestion between the spe­ tal conditions, indicating the opportunistic be­ cies and séemingly affects mainly H. venezue­ haviour of these radents in relation to resource loe and S. alstoni. since they eat large amounts availability. In similar studies in other �sys­ of O. sativa seeds doring advanced crop phases tems it hasalso been observed thatrodents tend (Phase I1I, Fig. 3C) wh�re seeds are more avai­ to profit from using resources oP!J(>rtunistically lable and stems become less palatable (Martino (Houtcooper 1978, Truszkowski 1982). and Aguilera 1989). Tbe stage of development Finally. this stu(ly points out that H. venezue­ in the rice fields also explains differentiaI ro­ loe feeds mainly on stems, followed by seeds of dent catches, as they relate to variations of wa­ O. sativa. Since it is the most abwldant rodent terlevel. Such variations á might openpotential species in the study area, its impact on rice field habitats that could be used temporally by O . damage could be considerable (Cartaya and delicatus. S. alstoni and Z. microtinus. which Aguilera 1985, Candellet 1984). Damage invol­ are not adapted to semiaquatic habitats. On the ves all crop phases, whether the wholeplant is other hand, H. venezuelae is able to use this en­ destroyed in the early phases by eating stems, or vironment quite successfully (Hershkovitz by eating seeds and stems in later phases, which 1955). Niche overlap was small in early crop in any case decreases croPproduction. The other phases, increasing with the stage of the crop, rodeot species are less abundant, but they could probably because the increase of foad resources become pests. especially in phases where seeds leadthe to use them more extensively. are present (phaseIII). 2. Composition of plant species; This factor seems to affect S. alstoni more than other spe­ cies. In crops where dams had abundant vegeta­ ACKWOWLEDGMENTS tion, more dicotyledons were consumed by S . alstoni. which had a wide niche; this pattem This research was partly supported by a was not found in crops where vegetation was grant from Consejo Nacional de scarce or nil, (pig. 1 and Fig. 3A). For H. ve­ Investigaciones Científicas y Tecnológicas nezueloe. diet composition was mainly affected (CONICIT-Venezuela). We thank Antonio Da by variation of percentage of weeds in the crop Grossa and Jose Leopardi who lent facilities area (Martino and Aguilera 1989), and the re­ during the study. P. Jackson and M. Zelwer for sulting niche width values were high. O. deli­ the translation assistance, J. Ojasti for revie­ catus consumed more seeds of Euphorbiaceae wing the earliest draft, and the anonymous re­ when the availability of rice seeds was low viewers who led to substantial improvements in (phase 1). Tbese results may be explained by the manuscripts. the way these species exploit the horizontal space: S. alstoni and O. delicatus dwell in dry weedy areas while H. venezuelae occupy RESUMEN mainly wet cultivated areas. 3. Surrounding crop area at sampling time: Mediante análisis de contenidos estomaca­ Changes in the vegetation of the surrounding les, se estudiaron los hábitos alimentarios de crop area also influence the diet of radents. The Holochilus venezuelae. Oryzomys delicatus. ingestion of seeds, when free-seed crops were Sigmodon alstoni y Zygodontomys microtinus. sampled. indicate that the animals were feeding roedores presentes en diferentes edades de cul­ in places different from their capture sites. This tivos de arroz en Venezuela. H. venezuelae fue was also evidenced by the occurrence of plants principalmente herbívoro, siendo el arroz el in stomach contenI noI found in the grid vege­ mayor renglón consumido. la importancia de tation survey. otros renglones ingeridos dependió de su dispo­ Agrosystems might be used as models of va­ nibilidad en el área de estudio. S. alstoni pre­ riation in rodent food habits. because they sentó una dieta principalmente omnívora que allow a rapid species-response to change inside consistió básicamente en semillas de arroz the ecosystem. Changes could be observed not (40%) e invertebrados (27%). O. delicatus fue only in foad ingested. but also in habitats sha­ granívoro y Z. microtinus consumió principal­ red at a given moment and in niche width and mente insectos (70.6%). H. venezuelae. O. de- 140 REVISTA DESIOLOGIA TROPIC AL " licatus y Z. microtinus mostraron baja diversi­ Flake,L.D.1973. Food habits oí four species of rodents on dad dé dieta (IndiceShannon-Weaver) mientras a short-grass prairie in Colorado. J.Mamm. 54: 636- 647. que S. alstom presentó valores inás aItos. Para todas las esPecies y'en todas las edades del cul­ Hershkovitz,P. 1955. Soudl American marsh rats,genus tivo. se encontraron ainplitudes de nicho con Holochilus widl a summary of rodenis. vaÍÓl'es inemos a bajos (lndice' de Petraitis) y Field.Zoo1.37 : 639-673. valores de Sólapamientó de nicho proporcíona­ Hom, H.S.1966. Measu rement of"over1ap" in comparalive les ala disponibilidad' del alimento. S610 S . ecoll:lgica1 studies. Amer.NaL, 100:419-424: cilstoni y t. micrótinus tendieron a mostrar altas interacciones cuando la disWnibilidad del ali­ Hootcooper,W;C. 1978.Food habits of rodents in a cultiva­ mento file baja. Las dietaS de todas las especies ted ecosystem. J: Mamm:59: 427-430. se relacionan con el estado de' desarrollo del JIubert,G. & F. Adams.1981. Cycle annuel du regime ali­ cUltivo, lácomposici6n de la vegetación y las meqtalre des trois principales espéces de rongeurs (Rodentia; Gerbillidae et Muridae) de Bonde,Senegal. características del área que rodea al cultivo en .. él momento del muestreo. Mammalia 45: 1�20.

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