Distribution of Atta (Hymenoptera - Formicidae) in "terra-firme" rain forest of Central Amazonia: density, species composition and preliminary results on effects of forest fragmentation

DISTRIBUTION OF A t t a (HYMENOPTERA - F O R M I C I D A E ) IN "TERRA-FIRME" BAIN FOREST OF CENTRAL AMAZONIA: D E N S I T Y , SPECIES COMPOSITION AND PRELIMINARY RESULTS ON EFFECTS OF FOREST F R A G M E N T A T I O N . (*)

H e r a l d o Luís de Vasconcelos (**)

SUMMARY

Otic

hundfiad and ^ounteen hectaAeA o{, a "teAAa-^iAti\z" Ααλη

j o t M t 70 fem

noAth o&

Ma-'ίαιν·},

A m a z o i a i ,

Bnaztl, mne sunoeijed ion lea^-cutting ant colonies

(Atta

spp) . One. hal^

this <viQ_a teas ai íòola-ted ^oneit fragmento [suAAoundid by poAtunes on second gnouth]

.two sizes:

J

and 10 ha.

Títe

otheA halfi

a6

i

non-isolated fragments I connected to a

iange poach frneAt] the same sizti . Only turn species occuned in this fio test: Ktta

iexdem Aexdoi* L. and

A. ciphaJlsiteJs L. The, fi-Oiit was the most abundant species with a

mean density ο&

0.35

colonies peA ha. The mean density o$

A. c.ephaJÍote.A colonieò was 0.03

pen., ha.

The density o£ colonies wai net significantly difâcient between the isolated

fragment* and the continuous fonest. FnAXhesunotie, the species composition did not change

with isolation. Howe.veA, pAe-isolation data and long tenm motuto/Uug cfte. neceAiaJiy to

conclude that the isolation 0$ a potest fragment has no effect upon Atta colonies.

The non-uni^oAm ipatiai dl.stnAbuti.on o& Ktta colonies within the "-Wuia-&.inme."

fonest trust be taken into account when selecting conieAvation

ateai

in the

Amazon,

in

ondeA to pnesenve t'nis important qnoup o$

anXi

togetheA with theÁA native habitat.

INTRODUCTION

The leaf-cutting ants of the genus A t t a are the most evolved group of the Neotropical tribe Attini (Weber, I 9 6 6 ) , a unique tribe among ants that cultivates fungi as a food source. The A t t a species construct large and conspicuous subterranean n e s t s , which can support a population of a few million ants (Weber, 1 9 8 2 ) ,

Little is known about the distribution of the A t t a s p e c i e s w i th i η the Amazon region and the few w o r k s about this subject are most concerned w i t h agro-forestry systems (Gon c a l v e s , 1 9 5 7 ; R i b e i r o & W o e s n e r r , 1 9 7 9 ) , since some species are considered major pests

(*) Part of M. Sc. Thesis presented to the Graduate School, of Instituto Nacional de Pes-quisas da A m a z ô n i a and F u n d a ç ã o U n i v e r s i d a d e do A m a z o n a s ,

(**) Instituto Nacional de Pesquisas da Amazônia - Ι Ν Ρ Λ , Manaus - AM.

of crops and p l a n t a t i o n s . Information on undisturbed areas is laching. Moreover, in the last two d e c a d e s , the Amazon forest has been rapi dly cl ea red for ag r i cu i ture and ranching (Fearnside, 1 9 8 2 ) , causing the fragmentation of a once c o n t i n u o u s h a b i t a t . The c o n s e ­ quences of the habitat destruction itself and that of the subsequent isolation of the forest remnants for the original A t t a populations is still largely u n k n o w .

This paper deals with the distribution of A t t a in the dominant forest habitat of the Amazon region, the " t e r r a - f i r m e " rain f o r e s t . Some preliminary data on e f f e c t s of iso­

lation of forest fragments upon the distribution of A t t a are a l s o presented.

METHODS

This study w a s conducted at the Fazendas D i m o n a , Esteio and Porto A l e g r e (Fig. ]) located 70 km north of M a n a u s , A m a z o n a s , Brazil ( 202 5 ' S , 5 9° 5 0' W ) as part of the M i n i m u m Critical Size of Ecosystem Project (MCSEP) (Lovejoy, I 9 8 O ) . The c l i m a t e is tropical and the annual precipitation is approximately 2 1 0 0 mm (Ribeiro, ϊ97β) . T h e vegetation,a "ter ra-f?rme" (never-flooded) rain f o r e s t , is primary and e v e r g r e e n . In these ranches(fazen d a s ) the primary forest in being cleared to plant p a s t u r e s . As Brazilian law requires that one haif of the area o f the ranches m u s t be preserved as f o r e s t , th i s p e r m i t t e d the MCSEP to create forest fragments of various s i z e s . O r i g i n a l l y , the f r a g m e n t s of smaller sizes w e r e named as 1 and 10 ha f r a g m e n t s , although they are somewhat larger: 1.7 and 13 ha respectively, if w e a l s o include a 5 m w i d e corridor (c1 eared of vegetation in the isolated fragments) which delimits the f r a g m e n t . In this paper the original nomenclature will be fol1 owed .

Five isolated (fragments number 110*», 1 1 1 2 , 2 1 0 7 , 2 1 0 8 , 311*0 and 5 non-isolated (fragments n? 1101, 1 1 0 2 , 1 1 0 5 , 1109, 1 1 1 3 ) 1 ha fragments (Fig. 1) and k isolated (n9 1202, 1 2 0 7 , 2 2 0 6 , 3209) and k non-isolated (n? 1 2 0 1 , 1 2 0 4 , 1 2 0 8 , 1210) 10 ha fragments were surveyed for A t t a colonies between O c t o b e r 1985 and January 1 9 8 6 . A non-isolated fragment is one connected to a large patch of forest and an isolated fragment i s o n e sur rounded by pastures o r second g r o w t h . The 10 ha fragments w e r e surveyed by w a l k i n g tran s e c t s , parallel to each other and 20 to 35 m a p a r t , pausing every 15 t o 20m to carefully look for nests or foraging t r a i l s , which w e r e used as an indicator of nest p r o x i m i t y . In the 1 ha f r a g m e n t s , following the same p r o c e s s , the most external 25m of the fragments w a s Surveyed (i.e. an area of nearly 1 h a ) . Thi s m e t h o d 1imited the survey to m i d - a g e and mature n e s t s , estimated a s having a surface area of up to 0.5 square m e t e r s .

F i g . 1. L o c a l i z a t i o n of the study a r e a . Each forest fragment is identified by a n u m b e r with f o u r d i g i t s . T h e first digit indicates the F a z e n d a (ranch)(1 = E s t e i o ; 2 = D i m o n a ; 3 = P o r t o A l e g r e ) , the second indicates the size of the fragment (1 = 1 h a ; 2 = 10 h ) and the two last d i g i t s are u n i q u e n u m b e r s for the fragments given in order of d e l i m i t a t i o n .

R E S U L T S

Density and s p e c i e s c o m p o s i t i o n

T h e mean density of A t t a in " t e r r a - f i r m e " forest w a s 0.38 c o l o n i e s per h e c t a r e . T h i s e s t i m a t e is result of a survey of 114 ha w h i c h included b o t h : isolated and n o n ­

isolated f r a g m e n t s . Only two species occured in this forest: A t t a s e x d e n s sexdens L. and A . c e p h a l o t e s L. The first one w a s the m o s t abundant species and had a m e a n density of 0.35 colonies per ha. The m a x i m u m density of A . s . s e x d e n s w a s 2 c o l o n i e s per h a . For A . c e p h a l o t e s the m e a n density w a s 0.03 c o l o n i e s per ha and the m a x i m u m density was I colony per h a .

E f f e c t s o f habitat isolation

T h e r e was no di fference i η the dens i ty of A t t a colonies betvjeen the 1 ha isolated and non-isolated f r a g m e n t s . Within the 5 isolated fragments there w e r e 3 colonies with a maximum of 1 colony per h a . The same occured w i th the 5 non-isolated fragments (Table 1 ) . The species composition did not c h a n g e wi t h isolation. All the colonies of the 1 ha isolated fragments w e r e of A . s. s e x d e n s , the same species found in the non-isolated o n e s .

The 10 ha isolated fragments showed a density of A t t a colonies 2.4 times greater than the non-isolated fragments of the same size (Table 2 ) . H o w e v e r , th i s d i f f e r e n c e w a s not signiticant (Kolmogorov-Smirnov T e s t , Kd = 3> ρ > 0.05; S i e g e l , 1 9 5 6 ) , because the

number of colonies w a s variable between the isolated fragments {range: 2 to 1 3 ) . A t t a c c p h a l o t e s only occured in t h e isolated f r a g m e n t s , but all the 3 colonies w e r e i π the same fragment (n<? 3209) •

T a b l e ) . Number of A t t a col on ies in i s o l a t e d a n d non-isolated forest fragments of 1 h a . All colonies a r e of A , s . s e x d e n s . I s o l a t e d N o n - i s o l a t e d F r a g m e n t C o l o n i e s Fragman t C o l o n i es

ι ιοί)

η

Π

T a b l e 2. Ν umber of A t t a col on ies in i s o l a t e d a n d non-isolated fores t f ragmen ts of 10 ha. Except for 3 colonies of A . c e p h a l o t e s in the fragment number 3 2 0 9 , al 1 the co onies are of A . s . s e x d e n s

1 s o l a t e d N o n - i s o l a t e d F r a g m e n t C o l o n i e s F r a g m e n t C o l on i e s 1 202 5 1201

k

13

(•'•) Kol rragorov-Smi rnov T w o Samp I e Tes t , Kd -- 3 , N. S.

DISCDSSION

The m e a n density of A t t a colonies in Central Amazonia ( 0 , 3 8 per ha) is similar to that found by Cherrett (1968) in an evergreen forest of Trinidad ( 0 . 6 colonies per h a ) , but smaller than that found by Rockwood ( 1 9 7 3 and included references) in evergreen forests of Costa Rica ( 1 . 0 to 2 . 5 colonies per ha) and by Leston ( 1 9 7 8 ) in B a h i a , Brazil (3 colonies per h a ) . Although not providing density e s t i m a t e s , Cherrett ( 1 9 8 l ) found equal numbers of A . c e p h a l o t e s and A , s . s e x d e n s colonies in a newly felled "terra-fir m e " forest in the Rio Jari forestry project (Northern A m a z o n i a ) , It would be interesting

T h e o c c u r e n c e of A . c e p h a l o t e s in only o n e (n? 3209) of the four 1 0 ha isolated fragnenls p r e v e n t s g e n e r a l i z a t i o n about the effects of isolation upon the species c o m p o s i t i o n of the fragments o f this s i z e . M o r e o v e r , this species a l s o inhabits the c o n t i n u o u s forest ( V a s c o n c e l o s , 1 9 8 7 ) , but probably w a s not found in the non-isolated fragments due to its low density ( 0 . 0 3 c o l o n i e s per ha) and clumped d i s t r i b u t i o n ( V a s c o n c e l o s , 1 9 8 7 ) . T h e latter may a l s o be responsible for the presence of 3 colonies i η the fragment number 3 2 0 9 .

A l t h o u g h no significant d i f f e r e n c e w a s found between the isolated fragments and the c o n t i n u o u s forest, w e cannot conclude that the isolation of a forest fragment has no effect upon A t t a . Even w i t h i n the same size category there is a lack of uniformity in the c o n d i t i o n s o f the isolation of the f r a g m e n t s . They differ in time since isolation (l, 3 or 5 y e a r s ) , surrounding vegetation (pasture or second g r o w t h ) and iη the distance to the c o n t i n u o u s f o r e s t . In this way w e c a n n o t c o n s i d e r n_ i sol ated fragments as n_ repl_i_ cates and so w e must to h a v e pre-isol at ion data to confidently test this h y p o t h e s i s .

O n e possible response of A t t a to isolation w o u l d be the immigration of c o l o n i e s into the f r a g m e n t s . H o w e v e r , m i g r a t i o n of A t t a c o l o n i e s is an uncommon phenomenon and c a n lead to the death of the migrant colony (Rockwood, 1 9 7 3 ) . A n o t h e r e f f e c t of forest isolation on A t t a colonies could occur at the border of the f r a g m e n t s , w h e r e an increased tree m o r t a l i t y rate and subsequent invasion of secondary plant species is observed (Lo-vejoy e t a l . , 1 9 8 4 ) . Vegetation changes normally affect the density and/or species c o m p o sition of leaf-cutting ants ( C h e r r e t t , 1 9 8 1 ; F o w l e r , 1 9 8 3 ) .

This w o r k , although not conclusive on the effects of habitat isolation upon A t t a , provides us w it h some information that m a y be used for the c o n s e r v a t i o n of leaf-cutting a n t s in the Amazon f o r e s t . One point that should be considered in the s e l e c t i o n of c o n s e r v a t i o n areas is that A t t a is not distributed uniformily w i t h i n the " t e r r a - f i rme''

f o r e s t . Even areas as large as 1 3 ha may be free of A t t a species (ex: fragment n? 1 2 0 8 ) . For A . c e p h a l o t e s this is a factor that needs special a t t e n t i o n not only b e c a u s e it is the

rarest s p e c i e s , but a l s o because it is typical only of rain forests ( C h e r r e t t , 1 9 6 8 ; R o c k w o o d , 1973)· A l s o , as Fowler e t a l . (in p r e s s ) have pointed o u t , not all s p e c i e s of leaf-cutting a n t s a r e agricultural p e s t s . Indeed most of them are endemic taxa w h i c h must be preserved together w it h their native h a b i t a t s .

A C K N O W L E D G M E N T S

I am grateful to D r . I. Walker by the s u p e r v i s i o n . D r s . L. C. F o r t i . H . G . Fowler and R. W . Hutchings m a d e valuable c o m m e n t s on the m a n u s c r i p t . T h i s w o r k w a s supported b^ the M i n i m u m Critical Size of Ecosystem Project ( I N P A / W W F - U S ) , T i n k e r Foundation and by the C o n s e l h o Nacional d e P e s q u i s a s . This paper represents publication number 56 in the MCSE Project Technical S e r i e s .

RESUMO

Um levantamento de colônias de Saãva (Atta spp) {oi fretto

em Π

4 hectares

c/e

uma

falo resta chuvosa de terra-{irme situada a 70 km ao noite,

cie

Manaus, Amazonas, Brat>il

. Me

tade desta área era formada por {ragmentos {loreAtais bolados (limitados por pastagem

ou capoeira) de dois tamanhos: 1 £ 10 ha.

A

outra metade consistia

em

{ragmentos não

isolados {conectados a uma grande porção

de

{loresta intacta) . Somente. 2 ei.peti.ei, de

sauva ocorreram nesta {loresta: Atta sexdens sexdens l.

e

A . cephalotes L.

A

primeira

{oi mato abundante,

e

teve uma densidade media de 0,35 colônias por ha.

A

densidade

me-dia, de colônias de A. cephalotes {oi de 0,03 por ha.

A

denòidade de colÔni.as não {oi significativamente diferente entre. oi> {ragmeyitos

isolados e a {loresta continua. Também, a composição de espécies não mudou em função do

isolamento. Entretanto, dados históricos [prévios ao isolament.o) e. um monitorameiito ã

longo prazo são necessários para concluir que o isolamento de. um fragmento {lores tal não

tem efeito sobre as colônias de sauva.

A

distKÁhuição espacial não uniforme das colônias

cie

Atta dentro da {loresta de

terra-{irme deve seA levada em consideração ao selecionar áreas de conservação na

Amazô

nia, a {im de preservar este importante grupo de formigas juntamente com seu habitat na

tivo.

R e f e r e n c e s

C h e r r e t t , J. M . - 1 9 6 8 . Some aspects of the d i s t r i b u t i o n of pest species of l e a f - c u t t i n g ants in the C a r r i b e a n . P r o c . A m . S o c . H o r t . S c i . T r o p . R e g i o n , 1 2 : 295 - 3 1 0 .

- 1 9 8 1 . The interaction of wild v e g e t a t i o n and crops in l e a f - c u t t i n g ant a t t a c k . I n : P e s t s , pathogens and v e g e t a t i o n . T h r e s h , J. M. ( e d . ) . P i t m a n Advanced Publishing. Program, B o s t o n , M a s s a c h u s s e t t s , U S A : 3 1 5 - 3 2 5 .

F e a r n s i d e , P. M. - 1 9 8 2 . D e s m a t a m e n t o na A m a z o n i a b r a s i l e i r a : com q u e i n t e n s i d a d e vem ocorrendo? A c t a A m a z ô n i c a , 1 2 : 579 - 5 9 0 .

F o w l e r , Η. G. - 1 9 8 3 . D i s t r i b u t i o n p a t t e r n s of Paraguayan leaf-cutting a n t s (Atta and Acronryrmex) (Formicidae - A t t i n i ) . Stud. N e o t r o p . Fauna and E n v i r o n . , 1 8 : 1 2 1 - 1 3 8 .

F o w l e r , H. G.; P a g a n i , Μ. 1.; S i l v a , 0 . A . d a ; F o r t i , L. C . ; Pereira da S i l v a , V.; V a s ­ c o n c e l o s , Η. L. d e - (in p r e s s ) . A pest is a pest is a pest? T h e d i l e m a of Neotropi cal leaf-cutting a n t s : key-stone taxa of natural e c o s y s t e m s . E n v i r o n . M a n a g .

G o n ç a l v e s , G. R. - 1 9 5 7 . O b s e r v a ç õ e s sobre as sauvas da A m a z o n i a . R e v t a . S o c . B r a s . A g r o n . , 1 2 : 4 3 - 5 2 .

L e s t o n , D. - 1 9 7 8 . A Neotropical ant m o s a i c . A n n . E n t o m o l . S o c . Am., 7 1 : 649 - 6 5 3 .

L o v e j o y , Τ . E . - 1 9 8 0 . D i s c o n t i n u o u s w i l d e r n e s s : m i n i m u m areas for c o n s e r v a t i o n . P a r k s , 5: 13 - 1 5 .

L o v e j o y , Τ . E . ; R a n k i n , J. M.; B i e r r e g a a r d , R. 0 . J r . ; B r o w n , K . S . J r . ; E m m o n s , L . H . ; Van der M o o t , Μ. E . - 1 9 8 4 . Ecosystem decay of A m a z o n forest r e m n a n t s . I n : E x ­ tinctions. N i t e c k y , Μ. H . ( e d . ) . U n i v e r s i t y of C h i c a g o P r e s s , Chicago,London:295-325.

R i b e i r o , G. Τ . & W o e s s n e r , R. Α . - 1 9 7 9 . Teste de e f i c i ê n c i a de seis Sauvicidas n o c o n trole das saüvas (Atta spp) na J a r i , P a r á , B r a s i l . A n . S o e . E n t o m o l . B r a s . , 8 : 77-84.

R i b e i r o , M , d e N . G. - 1 9 7 6 . A s p e c t o s c l i m a t o l ó g i c o s d e M a n a u s . A c t a Amazônica, 6: 2 2 9 -2 3 3 .

R o c k w o o d , L . L . - 1 9 7 3 . D i s t r i b u t i o n , density and d i s p e r s i o n of two species of A t t a (Ilymenoptera - F o r m i c i d a e ) in G u a c a n a s t e P r o v i n c e , Costa R i c a . J. A n i m . E c o l . , 4 2 : 803 - 8 1 7 .

S i e g e l , S. - 1 9 5 6 . N o n p a r a m e t r i c statistics for the behavioral Bciencea. M c G r a w - H i l l Book C o m p a n y , Ν . Y. 312 p. V a s c o n c e l o s , Η . L. de - 1 9 8 7 . A t i v i d a d e f o r r a g e i r a , d i s t r i b u i ç ã o e f u n d a ç ã o d e c o l ô n i a s de s a ú v a (Atta spp) em f l o r e s t a d a A m a z ô n i a C e n t r a l . Unpublished M . S . T h e s i s , F u n d a -ção U n i v e r s i d a d e do A m a z o n a s e Instituto N a c i o n a l de P e s q u i s a s da A m a z ô n i a , M a n a u s . 62 p . W e b e r , N . A . - 1 9 6 6 . P u n g u s - g r o w i n g a n t s . S c i e n c e , 1 5 3 : 587 - 6 0 4 . - 1 9 8 2 . F u n g u s a n t s . I n : Η . R. H e r m a n n ( e d . ) . S o c i a l I n s e c t s , v o l . 4 . A c a d e m i c P r e s s , L o n d o n and Ν . Y . 255 - 363 . (Aceito para p u b l i c a ç ã o em 05/7/88)