Journal of Arid Environments (2003) 53: 115–124 doi:10.1006/jare.2002.1029, available online at http://www.idealibrary.com on
Seed removal by birds, rodents and ants in the Austral portion of the Monte Desert, Argentina Sergio Leonardo Saba%w* & Alicia Toyosw %
Facultad de Ciencias Naturales, Universidad Nacional de la Patagonia San Juan Bosco, Sede Puerto Madryn, Boulevard Almirante Brown 3700 9120, Puerto Madryn, Chubut, Argentina wCentro Nacional Patago´nico, Boulevard Almirante Brown s/n. 9120, Puerto Madryn Chubut, Argentina (Received 6 June 2001, accepted 2 April 2002) The relative importance of granivory by birds, rodents and ants in the southern Monte Desert (Argentina) was examined using an experimental seed–dish technique. Monthly seed-removal rates were recorded during two consecutive years, from September’ 1995 to August’ 1997. Birds were the main and most constant seed eaters during the study period. There was no seed removal by ants in September’ 95, April, June, and July’ 96, and May, June and August’ 97. There was no consumption by rodents from March to May’ 96. Rodents consumed significantly more seeds than did ants during both winters, and spring’ 96. Conversely, seed removal by ants was significantly higher than that of rodents in summer 96. Overall removal rates were significantly different between both annual cycles studied. The highest seed-removal rate was recorded in March 1997. We compare our results with those obtained by several authors at the northern and central Monte Desert, as well as other arid and semi-arid regions of the World, reevaluating conclusion about granivory in South American arid zones. # 2002 Elsevier Science Ltd.
Keywords: granivory; desert ecology; Monte desert; seed removal; Patagonia; ants; birds; rodents
Introduction Comparisons with other ecosystems are essential to identify which functional units have truly general attributes (Morton & Davidson, 1988). Orians & Solbrig (1977) proposed that the structure and functioning of desert biotic communities are convergent. Many authors conclude that although ecological–evolutionary convergence between communities can occur, it is difficult to demonstrate conclusively (Cody, 1975; Karr & James, 1975; Mooney, 1977; Orians & Solbrig, 1977, Mares *Corresponding author. E-mail: [email protected]
0140-1963/03/010115 + 10 $35.00/0
# 2002 Elsevier Science Ltd.
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& Rosenzweig, 1978; Cody & Mooney, 1978; Brown et al., 1979; Mares, 1980; Orians & Paine, 1983; Morton, 1985; Kerley & Whitford, 1994; Kelt et al., 1996). Other authors argue against the validity of ecological–evolutionary convergence on the grounds that unique histories or subtle environmental differences would have produced many of the observable differences in current community organization (Pianka, 1973; 1975; Schall & Pianka, 1978). Recent research in several arid regions of the world challenges the convergence paradigm (Morton & Davidson, 1988; Kerley, 1992; Mares, 1993; Morton, 1993; Kerley & Whitford, 1994; Medel, 1995; Lo´pez de Casenave et al., 1998). Post-dispersal seed predators are prominent components of desert ecosystem throughout the world (Brown et al., 1979). Seed predation has long been recognized as one of the key biotic factors influencing seed survival and subsequent seedling establishment (Janzen, 1971) with implications for entire biotic communities (Brown et al., 1986). Granivory by rodents, birds and ants has been shown to be one of the major avenues of seed loss in desert systems. Since the seminal experimental study of Brown et al. (1975) in North America using seed-dish techniques, differences in the levels of granivory between different desert systems have been usually measured in terms of seed-harvesting rates (Mares & Rosenzweig, 1978; Abramsky, 1983; Morton, 1985; Kerley, 1991; 1992; Lo´pez de Casenave et al., 1998). The seeds of plants constitute a relatively dependable resource for a diverse array of ants, rodents, and birds, which show marked differences in richness and abundance over deserts on different continents. For example, whereas ants and rodents appear to be the most important granivores in North American and Israeli deserts (Mares and Rosenzweig, 1978; Brown et al., 1979; Abramsky, 1983), mammalian seed eaters are insignificant in the Australian arid zone where ant and avian granivores predominate (Morton, 1979; 1985). Mares & Rosenzweig (1978) reported a set of results on the rates of seed removal by granivorous animals experimentally assessed in a locality of the northern Monte Desert of Argentina, and they compared those results with similar data from the Sonoran desert of North America. They concluded that granivory is ‘much depressed’ in South America. Almost all the studies on granivory rates in warm deserts published in the last 20 years have taken this statement as given. Seed-removal rates, however, should be examined for consistency within large geographic areas, because granivorous assemblages may exhibit important spatial variations. Also, seed-dish data should be interpreted with caution, due to differential responses by rodents, birds and ants (Parmenter et al., 1984). Based on Mares and Rosenzweig’s data, it is frequently stated in the specialized literature that Argentina’s Monte Desert has few granivorous ants or rodents (Abramsky, 1983; Morton, 1985; Brown & Ojeda, 1987; Morton & Davidson, 1988; Kerley, 1991; Kerley & Whitford, 1994). After Mares and Rosenzweig’s initial experimental study, Lo´pez de Casenave et al., (1998), examined seed-removal rates by granivores in other locations of Monte Desert. They found that in the central Monte ˜ acun˜a´n, 341 020 S, 671 580 W), the rates of seed removal by granivores varied Desert (N with season. Ants were the most important granivores in spring and summer, but removed few seeds during autumn and winter. Conversely, birds, followed by mammals, were the most important consumers in autumn and winter, but removed fewer seeds in spring and summer. The objectives of this study were: (i) evaluate seed removal rates by the granivorous guild (ants, birds and mammals) at the southern part of the Monte Desert; (ii) compare our results with those obtained at northern and central locations of the Monte Desert, analysing for consistency within a large geographic area; (iii) reexamine the prior Mares and Rosenzweig’s conclusion about granivory in South American arid zones. Also, we compare our data with those coming from other deserts of the World, in order to check the Orians and Solbrig convergence hypothesis.
GRANIVORY AT THE MONTE DESERT, ARGENTINA
Methods Study area The experiments were carried out in a ranch located 40 km west of Puerto Madryn, Chubut Province, Argentina (421 390 S, 651 23´W; 115 m. a.s.l). Mean annual precipitation is 188 mm, with high interannual variation. The mean annual temperature is 13?71C (15 year series, Barros & Rivero, 1982). Soils are a complex of Typic Palaeorthid-Typic Caliorthid (Soil Survey Staff, 1992). In the upper soil layer, clay content ranges from 4% to 6% beneath vegetation patches, and 7–8% in exposed areas (Bertiller, 1998). This area is included in the southern portion of the Monte phytogeographic province, which occupies the northern Patagonian territory. The dominant vegetation is the steppe of Larrea divaricata and Stipa spp. and displays a patchy structure of long-lived perennial shrubs, forbs and annuals (Ares et al., 1990; Bertiller et al., 1991). Sheep have grazed these grasslands extensively since the beginning of 20th Century. Grazing has reduced the cover of perennial grasses and of some shrubs, leading to changes in the floristic structure and spatial organization of vegetation, such as fragmentation of large patches, extinction of grass patches and formation of new incipient vegetation patches (Bisigato & Bertiller, 1997).
Sampling design Seeds were provided to granivores at three plots. Because we could not afirm that in the study area rodents have only nocturnal habits, and ants diurnal ones, we applied the following design: on each plot, 15 feeding trays were activated, five accessible only to ants, five only to birds and five only to rodents. We placed five control trays (all seed consumers excluded) at each plot to determine possible effects of loss on seed mass due to wind. The feeding trays were plastic Petri dishes (9 cm diameter). Ant trays were covered with 6 mm mesh hardware cloth to exclude vertebrate consumption, and they were set at ground level with small metallic thread’s rim to facilitate access by ants. Rodent trays were set 2–3 cm off the ground surface and without the metallic thread’s rim to exclude consumption by ants, and were covered by a square stainlesssteel roof (25 25 cm) arranged 5 cm above each dish to exclude bird consumption. Bird’s trays were set 25 cm above the ground to prevent ant and rodent access. Control trays were set 25 cm above the ground, and covered with 6 mm mesh hardware cloth. All the trays were filled up with 10 g of commercial seed (Phalaris canariensis), and only checked 1 week later. In spite of this sampling design limitation in our study (seed extraction was not recorded daily) there was not taxa that took so many seeds. The rate of seed removal by tray was determined by weighing the rest of the seeds and subtracting this value from the initial weight. Although winds are usually very strong at these latitudesFfrequently reaching more than 30 km/h1Fseed loss calculated on control trays was negligible. Therefore, we did not subtract any value from the dishes offered to the granivores. Fifteen dishes were available to each taxon for 7 days, exposed every month during two annual cycles, from spring of 1995 to winter of 1997, except January 1997. When seasonal analysis was performed, we considered that data of December–January– February corresponded to summer, of March-April–May to fall, of June-July-August to winter, and that of September–October–November corresponded to spring. Inter-taxa and inter-season differences in mean seed removal rates, and differences in mean absolute seed removal per annual cycle were evaluated with one-way ANOVA (Sokal & Rohlf, 1981). We used Tukey’s studentized range test (HSD) to determine
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maximally nonsignificant subset (p 0?05). When descriptive statistics were needed, we computed these ( x, S.D, 95% confidence limits of means). Rodent species present at the study site were studied over a 10 10 traps, separated by 10 m each one, using Sherman and Blair live capture traps baited with generalized bait. The grid was activated during each sampling period. Notes were kept on birds and ants that were observed removing seed from dishes.
Results Two potential granivorous mammal species (Muridae: Sigmodontinae) were recorded at the study area: Eligmodontia typus, Calomys cf. musculinus. Another sigmodontinae, Graomys griseoflavus, also was captured. The only species of ant observed carrying seeds was Acromyrmex lobicornis. Three bird species of the family Emberizidae, considered to be among the most important seed-eater organisms at the Central Monte Desert (Marone et al., 1998), were observed at the study site: Zonotrichia capensis, Diuca diuca, Phrygilus carbonarius. The Patagonian Mackingbird Mimus patagonicus was also frequently observed removing seeds from dishes. Figure 1 shows monthly seed removal rates recorded for birds, rodents, and ants. Granivory was a continuous activity in the study area. The highest seed removal rate was March 1997. Birds were the main and most constant seed eaters during the study period. There was no seed removal by ants in September’95, April, June, and July’96, and May, June and August’97. Finally, there was no consumption by rodents from March to May’96. Table 1 shows mean seasonal seed removal by each granivorous taxon considered. We obtained mean values from the 3-month records of each dish, except summer’97 (without January data). Thus, we have n = 15/taxon data set for each season. Bird seed removal rates were significantly greater than that of rodent and ants all times. Rodents consumed significantly more seeds than did ants during both winters, and spring’96. Conversely, seed removal by ants was significantly higher than that of rodents in summer’ 96 (Table 1). The overall seed consumption between the two annual cycles studied was compared. We consider the former cycle from spring’95 to winter’96, and the second from spring’ 96 to winter’ 97. Seed removal was significantly different, F (1,22) = 7?70, po0?011, between both annual cycles, being higher in the last one. There were
Figure 1. Monthly seed removal rates (g week1) by birds, rodents and ants in the southern Monte Desert.
GRANIVORY AT THE MONTE DESERT, ARGENTINA
Table 1. Seasonal seed removal rates (g dish week1) by birds (B), rodents (R), and ants (A) in the southern Monte Desert.
Conf. lim. (95%)
Spring’95 n = 15, F (2, 42 ) = 42?42* B R A
4?888 a 0?469 b 1?395 b
1?793 0?458 1?529
3?895–5?881 0?216–0?723 0?548–2?241
Summer’96 n = 15, F (2, 42) = 20?82* B R A
2?493 a 0?278 b 1?240 c
1?468 0?190 0?690
1?680–3?306 0?173–0?383 0?857–1?621
Fall’96 n = 15, F (2, 42) = 80?47* B R A
2?715 a 0?097 b 0?335 b
1?037 0?085 0?299
2?141–3?290 0?050–0?144 0?170–0?501
Winter’96 n = 15, F (2, 42) = 93?88* B R A
9?200 a 4?417 b 0?442 c
0?495 2?974 0?358
8?926–9?474 2?770–6?064 0?244–0?640
Spring’96 n = 15, F (2, 42) = 98?09* B R A
8?573 a 3?458 b 1?334 c
0?768 2?279 0?755
8?148–8?999 2?196–4?720 0?916–1?752
Summer’97 n = 15, F (2, 42) = 24?89* B R A
6?217 a 1?513 b 2?296 b
2?220 1?208 2?258
4?988–7?447 0?844–2?182 1?046–3?546
Fall’97 n = 15, F (2, 42) = 59?11* B R A
5?993 a 1?164 b 2?237 b
1?000 1?257 1?521
5?439–6?546 0?468–1?860 1?395–3?080
Winter’97 n = 15, F (2, 42) = 2514?73* B R A
9?339 a 0?524 b 0?053 c
0?520 0?467 0?050
9?052–9?627 0?266–0?782 0?026–0?081
Within a season, means with the same letter do not differ significantly (Tukey’s studentized range test, p40?05).
significant differences among seasons in the former cycle, but not in the second one (Table 2). In cycle 1, the highest consumption occurred in the winter, followed by the previous spring. Seed removal in summer and fall were not significantly different (p>0?05). In Table 3, each taxon’s seed removal rate is desegregated, and then is compared between equivalent seasons of the two cycles. Birds extracted significantly more seeds during spring, summer, and fall of the second cycle, but there were no significant
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Table 2. Comparison of the absolute seed removal (birds+rodents+ants) between seasons of the same annual cycle
Cycle 1;F (3,8)=86?29 po0?001 x S.D.
33?03 a 5?60
49?32 a 5?86
49?61 a 1?26
Cycle 2; F ( 3,8 ) = 2.92 p =0?10 x S.D.
Values are the means (expressed in grams) obtained from the three plots (5 dishes by plot 3 month in each season, except summer 97 with 2 month recorded), so n = 3. Within each cycle, means with the same letter have no significant differences (Tukey’s studentized range test, p40?05).
Table 3. Comparison between seasonal seed-removal rates, discriminated by taxon, paired between equal seasons of the two annual cycles analysed in this work
x Spring 1995
53?56 24?80 0?019
o 0?001 o 0?001 0?8913
29?36 15?30 3?01
o 0?001 o0?001 0?094
5?993 1?164 2?237
77?61 10?76 22?59
o0?001 o0?001 o0?001
9?339 0?524 0?053
0?57 25?09 17?36
0?458 o0?001 o0?001
Birds Rodents Ants
4?888 0?469 1?395
8?573 3?458 1?334
Birds Rodents Ants
Summer 1996 2?493 0?278 1?240
Summer 1997 6?217 1?513 2?296
Birds Rodents Ants
2?715 0?097 0?335 Winter 1996
Birds Rodents Ants
Winter 1997 9?200 4?417 0?442
Means are expressed in g week1 periods. One-way ANOVA analysis.
differences between the two winters. Rodents also consumed significantly more seeds during spring, summer, and fall of the second cycle, but they extracted significantly more seeds in the former winter than the latter one. Seed consumption by ants was more erratic. While it was significantly higher in the second fall than the former, the opposite was done in winters. There were not any significant differences between springs and summers.
GRANIVORY AT THE MONTE DESERT, ARGENTINA
Discussion The results of this study show some clear differences with those previously reported for the Monte Desert in particular, and other deserts in general. Overall removal rates were higher than those reported by Mares & Rosenzweig (1978) at the northern part of Monte Desert, especially for birds and mammals. Our results contrast with Mares and Rosenzweig’s claim that these taxa have a small impact in South America, an idea that was thereafter restated by several authors (Abramsky, 1983, Morton, 1985; Brown & Ojeda, 1987; Kerley, 1991; Kerley & Whitford, 1994, Whitford, 1999). Bird seed consumption during the summer was higher than that reported by Lo´pez de Casenave et al. (1998) at the central part of Monte Desert for the same season. Also contrasting with their findings, we did not find in our study a higher consumption by ants than by birds in spring–summer. In relation to other deserts of the World, our results show differences too. Ants and rodents appear to be the most important granivores in the North American and Israeli deserts (Mares & Rosenzweig, 1978; Brown et al., 1979, Abramsky, 1983), but mammalian seed eaters would be less important consumers in other arid zones of the world. Thus, in Australian deserts, as soon as in the karoo, ant granivores are dominants (Morton, 1979, 1985; Morton, & Davidson, 1988; Kerley, 1991), while our current findings and that of Lo´pez de Casenave et al., (1998) for the Monte Desert show that birds are the most important consumers all the year around, and in part of the year, respectively. Both historical and ecological factors have been proposed to explain these disparities in the proportional representation of the three granivorous classes between Australia and North America (Morton, 1979), or Australia, North America and South America (Medel, 1995). For example, the most prominent family of the North American seed-eating rodents (Heteromyidae) originated in the Oligocene, when the desert flora was still developing (Axelrod, 1950), but rodents were probably absent from Australia until the Pliocene (Lee et al., 1981). Hence, the development of vegetation during the Oligocene or Miocene (Truswell & Harris, 1982) would have long preceded the occupation of Australian deserts by rodents, and food niches for granivores might have been preempted by ant and birds. Greater variability in precipitation and primary productivity in Australian deserts (Leeper, 1970; Gentilli, 1971) may have magnified the advantages of ants and birds over granivorous rodents. Bipedal rodents, morphologically convergent with the North American heteromyids, exist in arid Australia (Watts & Aslin, 1981) but they are not significant seed removers (Morton, 1985; Morton Davidson, 1988). To account for the differences in granivorous assemblages between North and South America’s arid zones, Mares & Rosenzweig (1978) proposed that the extinction of the marsupial family Argyrolagidae in South America had brought about an abrupt decline of seeds that, in turn, led to impoverished assemblages of granivorous ants. The general value of the data that led to this conclusion should be reexamined. In particular, seed-removal rates should be examined for consistency within continents, because granivorous assemblages may exhibit large geographic variation (e.g., Medel & Va´squez, 1994; Va´squez et al., 1995; Kelt et al., 1996; Lo´pez de Casenave et al., 1998). The Monte Desert exhibits great latitudinal variations. In the northern part the only significant granivores are ants, and seed consumption is depressed (Mares & Rosenzwieg, 1978). Consumption rates are higher in the central Monte, where there is a seasonal alternation of taxa taking the main role as granivores (Lo´pez de Casenave et al., 1998). Finally, this study shows that birds are the main consumers all year-round at the southern part of the Monte Desert. The most up-to-date literature review about rates of seed removal by granivores in arid zones is that of Lo´pez de Casenave et al., (1998). They included data from the
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Sonora Desert, North America (Mares & Rosenzweig, 1978), semi-arid Karoo, South Africa (Kerley, 1991), Australian Desert (Morton, 1995), Chilean Matorral, Chile (Va´squez et al., 1995), northern Monte Desert (Mares & Rosenzweig, 1978) and central Monte Desert (Lo´pez de Casenave et al., 1998). Our study is the first that shows birds as the main seed consumers all year-round. This observation is consistent with the diversity of the different granivorous taxa. There is only one species of ant (Acromyrmex lobicornis, Formicidae, Attini) that remove seeds. This ant, which has the broadest distribution in Argentina, has a broad trophic spectrum and is the only leaf cutting ant in the world found so far south (Farji-Brener & Ruggiero, 1994). The rodents Calomys cf. musculinus and Eligmodontia typus (small nocturnal seedeating mouses of open habitats, Pearson (1995), and authors cited therein), and Graomys griseoflavus (a large nocturnal desert mouse that forages on green vegetation) were captured in the study area. Their densities here are usually very low. Saba et al., (1995) reported that on an effort of 8281 trap-nights, only 54 specimens were captured. Pardin˜as et al., (2000) analysed owl pellets of Tyto alba from Late Holocene (1830 7 70 yr B.P.), and compared them with modern samples from different Patagonian phytogeographic districts. They claim that human activities like overgrazing by sheep and expansion of agriculture are probably the most important agent of the modification of the environment, with a concomitant decrease of the rodent diversity. In contrast to North American deserts, where most seed-eating birds are seasonal migrants that breed outside deserts and feed on insects while breeding, the southern Monte Desert contains avian granivores that are year-long residents, as the Emberizidae Zonotrichia capensis, Diuca diuca, Phrygilus carbonarius and Saltatricula multicolor. These species were observed all along the year at the study area, but it is possible that its densities show fluctuations by migrations. This study confirms that levels of seed consumption by small mammals do differ between different deserts. These divergences in what is generally regarded as a fundamental process in desert systems are particularly surprising in view of the apparent similarities of the physical environment of these deserts. As was pointed out by many authors previously, some aspects of Orians & Solbrig’s (1977) hypothesis about the structure and function of desert biotic communities are convergent, are refuted by the present study. Thus, the overall seed consumption, and the composition of the granivorous guild are very changing, both through a same desert, as well as between different deserts of the World. In the composition of granivorous guilds, both historical factor and human activities can play a greater roll than abiotic factors do. We thank V. Quiroga, and A. Torres for helping us during field work. F. Sarasa for authorizing the development of this research project at his ranch. We greatly appreciate Dr L. Marone, and M. Pascual by the earlier lecture, discussion and review of the manuscript, and two anonymous reviewers. This research project was supported by the Universidad Nacional de la Patagonia San Juan Bosco. We thank Centro Nacional Patago´nico (CENPAT-CONICET) for facilitating the installations of laboratory works.
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