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Migratory Pollinators Program


The Benefit of Saguaros to White-winged Doves
Stable Isotopes: The use of different food types by animals is usually estimated from pollen loads, fecal samples, analysis of crop or stomach contents, or foraging observations (Collins et al. 1990).  However, increasingly, the incorporation of the natural isotopic signatures of resources into a consumer's tissues is used to track use (Gannes et al. 1998).  Stable isotopes are powerful tools for dietary analysis when applied to producer-consumer systems only if isotopically distinct food sources are available to consumers (Gannes et al. 1997, Tieszen and Boutton 1989).  Saguaros exhibit such isotopically distinct signatures.  The nutrients contained in saguaros have a distinct carbon signature and the water in nectar and fruit of saguaros has a unique hydrogen signature.

The isotopic signature of saguaro is the result of its crassulacean acid metabolism (CAM) photosynthetic pathway.  Stem-succulent cacti, such as the saguaro, tend to exhibit obligate-CAM photosynthesis and thus have isotopic signatures similar to those of C4 species that are relatively enriched in 13C  ( Bender 1971, Ehleringer 1989).  The alternative food sources of saguaro contain a different isotopic signature. An extensive survey of white-winged dove crop contents revealed that the seeds from eight plant species other than saguaro appeared frequently in the dove's diet (Fig. 2, see also Haughey 1986).  The saguaro and these C3 plant species accounted for more than 90% of the white-winged dove's crop contents.  These eight other plant species all have a C3 photosynthetic pathway, which has a carbon isotopic composition distinguishable from the saguaro's CAM carbon isotopic composition. 

Nutrient Source: Saguaro is not only the most frequent item in the dove's diet (Fig. 2), it is also the primary source of incorporated carbon for a large fraction of the breeding season (Fig. 3).  In July, during the peak of saguaro fruit consumption, the isotopic composition of the doves' tissues was almost indistinguishable from that of saguaro.   In other words, during this time of year, doves are deriving nearly all of their nutrients from saguaro fruit.  In fact, the distribution of breeding western white-winged doves broadly coincides with the distribution of saguaros in Arizona and Sonora (Cottam and Trefethen 1968, Turner et al. 1995).

Water Source: Two isotopically distinct sources of water are available for desert-dwelling white-winged doves: the nectar and fruit of saguaro, and surface water in man-made tanks and in tinajas (natural water catchments).  We have found that the water from the nectar and fruit of the saguaro is greatly enriched in deuterium, a heavy isotope of hydrogen, relative to surface water (Table 9).  The body water of white-winged doves becomes deuterium-enriched with increased incorporation of saguaro carbon (Fig. 4).  Thus, white-winged doves derive water as well as nutrients from saguaro.

The Benefit of White-winged Doves to Saguaros
Pollination: Western white-winged doves visit both the flowers and fruit of saguaros, and they are both pollinators and seed dispersers.  When saguaros are in bloom, white-winged doves visit their flowers frequently and appear to carry large pollen loads (Fig. 5).  Only saguaro pollen was found on white-winged doves, and these doves are the most frequent avian visitors to the saguaro's flowers and fruit (Haughey 1986).  They have also been observed to visit saguaro flowers an order of magnitude more frequently than nectar bats (Fleming et al.1996).  The only species found visiting saguaro flowers more frequently on some sites is the European (domestic) honey bee  (Apis mellifera L.) (Schmidt and Buchmann 1986, Fleming et al. 1996).  White-winged doves are clearly important pollinators of saguaros. 

Dispersal?: These doves also consume saguaro fruit, which is filled with saguaro seeds.  They regurgitate saguaro fruit pulp to their squabs (Neff 1940, Wolf and Martínez del Rio unpublished data) and, in the messy process, some seeds fall intact beneath their nests (Olin et al. 1989).  Desert-dwelling white-winged doves nest in large trees such as mesquites (Prosopis sp.), palo verdes (Cercidium sp.), and ironwoods (Olneya tesota), which serve as nurse plants for saguaro seedlings (McAuliffe 1984).  It has therefore been hypothesized that doves may play an important saguaro seed dispersal role (Olin et al. 1989, Sosa 1997).  However, direct corroboration of this hypothesis has not yet been achieved.

Seed Predation: The white-winged dove has a powerful gizzard (Goodwin 1983) and they destroy the vast majority of the saguaro seeds that they ingest (reviewed by Sosa 1997).  Also, relative to the number of seeds destroyed by doves after ingestion, the number of seeds dropped under nests is minute.  Olin et al. (1989) reported that the number of seeds spilled under nests ranged from 57 to 675 per season.  A single dove ingests approximately 3,400 g of saguaro fruit pulp in a season, containing about 280,000 seeds (Wolf and Martínez del Rio 2000).  Thus, even though white-winged doves are the most important consumers of saguaro fruit pulp, they are better characterized as seed predators than as seed dispersers.  The bulk of effective saguaro seed dispersal is probably performed by birds with gentler guts, such as Gila woodpeckers (Melanerpes uropygialis), verdins (Auriparus flaviceps), ash-throated flycatchers (Myiarchus cinerascens), curve-billed thrashers (Toxostoma curvirostre), and cactus wrens (Campylorhynchus brunneicapillus; Sosa 1997) and bats.  Therefore, saguaros and white-winged doves maintain a lopsided interaction.


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