1.3.1 PLANTS  

The Galapagos vegetation can be divided into four main zones depending on the altitudinal humidity gradient. The narrow littoral zone is followed by an arid zone which extends roughly to a height of 100 m. The following transition zone gradually merges at about 200 m into a humid zone higher up the mountainside. On the highest islands a secondary dry zone follows above about 500 m, which is mainly covered with ferns and sedges (Wiggins and Porter, 1971). Many of the smaller islands are so low that their vegetation belongs entirely to the arid zone. 

   The flora of the Galapagos islands consists of about 500 vascular plants, roughly 30–40% of which are endemic. The majority of these plants (about 60%) have supposedly been transported to the islands on birds or in their guts, and only about 8% might have arrived by oceanic drift (Porter, 1983). The torrential rains caused by El Niño on the mainland of South America must have flushed a lot of seeds and whole plants from rivers such as the Guayas out to sea, which then drifted along with the currents in the general direction of Galapagos. Drift plants (such as mangroves, Scaevola plumieri, Batis maritima, Salicornia fruticosa, and Ipomoea pes-caprae, for example) adapted to life in the mangrove, and littoral vegetation may thus arrive in increasing numbers at the islands during El Niño. Speciation and the number of endemics in the littoral vegetation is, however, quite limited, suggesting that arrival of dissiminules from the mainland is quite frequent and sufficient to prevent genetic differentiation. Therefore, additional arrivals of seeds or other propagules of plants of the littoral zone during El Niño events may not be very important to the ecosystem. 

  Arriving on an island is, however, only half or perhaps less of the battle to get established on it. Most of the seeds that get washed ashore by oceanic currents, or are carried to the islands by long-distance migrant birds, arrive in a very inhospitable, arid climate. The migratory birds which carry seeds are mostly coastal birds; 20 of 22 species listed as regular migrants to the Galapagos by Porter (1983, after Levèque et al., 1966) are living in the littoral zone. There is a definite advantage, then, if the arriving propagule has the ability to remain dormant until conditions become better. McMullen (1986) showed that 62.5% of the indigenous plants had pronounced seed dormancy (27 species out of 18 families were tested). Thus it seems that many of the plants that became established on the Galapagos were able to wait for quite a while after seed arrival until conditions became suitable for germination. El Niño may help the initial establishment of newcomer plants by providing a mesic habitat in the normally arid zone of the Galapagos. The nine months rain which poured down during the 1982-83 El Niño most likely could break the dormancy of seeds which had arrived earlier, and thus lead to initial establishment of a plant species. Hamann (1985) found that many unusual species of herbs, grasses, sedges, and vines were growing during El Niño in a number of regularly surveyed quadrats (see below), again indicating that a large seed reserve was waiting in the arid zone until moist conditions arrived. About 30% of these plant species had never been recorded in regularly checked quadrats in the previous 17 years (Hamann, 1985). Since most of these plants were able to fruit during El Niño a new seed reserve was deposited. 

   Since the plants of more mesic habitats cannot survive through dry periods in the arrival site, they either have to produce seeds and return into dormancy in situ or else disperse through birds or other animal agents up into the more mesic highlands of the Galapagos. One way in which seeds could be transported into the highlands can be deduced from Cayot's (1985) report on the behavior of giant tortoises (Geochelone elephantopus) during the El Niño rains. She observed tortoises on Santa Cruz descending from their normal mesic habitat half-way up the mountain to coastal sites where they fed on the abundant weedy vegetation and fruits until the rains ceased. As the coastal zone dried up the giant tortoises returned to their normal habitat. In this kind of migration they could easily carry seeds or fruits internally or externally up into their mesic habitat. Through this complex interplay of El Niño-related changes, 'stranded' seeds could end up in mesic habitats where a permanent population might become established. Hamann (1984) mentions a few species of drift plants that show very patchy distribution in the Galapagos islands which may have colonized in this way (Table 1.1). For example, Hippomane mancinella is a typical oceanic drift plant which has colonized both coastal and inland habitats in the Galapagos. Hippomane fruits may easily have been transported inland by giant tortoises. Tortoises were observed to eat Hippomane fruits, which then, often after several days, get passed almost unchanged in appearance (Hamann, 1984). 

   A second category of plants with drifting propagules occurs only inland. This category includes such species as Mucuna rostrata, Dioclea refiexa, Sapindus saponaria, Stictocardia tiliifolia, and Ipomoea alba. In the Galapagos these plants have restricted and spot-like distributions. This distribution pattern is difficult to explain in relation to the plants' effective dispersal mechanism. This may reflect that although they are well adapted for long- distance dispersal through oceanic drift, they could be less well adapted to establish in an arid coastal region such as that on Galapagos, because they require mesic habitat. Thus, once arrived on the beach, they would have to get a rare opportunity to cross the barrier of dry land to become established. This could limit their success as colonizers. 

   Hamann(1984) concludes: 'The present spot-like distribution patterns of some notorious drift seed plants in the Galapagos suggest that successful colonization by these species is a rare phenomenon. However, extreme environmental conditions such as those in a Niño year could be a decisive factor.'