The Lomas Formations of Coastal Peru: Composition and Biogeographic History

Summary

For nearly 3500 km along the western coast of South America [5°-30°S lat.], the Peruvian and Atacama deserts form a continuous, hyper-arid belt, broken only by occasional rivers valleys from the Andean Cordillera. Native vegetation of the deserts consists of over 1000 species, many highly endemic and largely restricted to the fog-zone or lomas formations. To provide a backdrop for discussions of human occupation in coastal Peru and interpreting changes over the last 10,000 years, a synthesis of the present-day coastal environments and summary of paleoclimatic data are used to reconstruct the conditions likely to have persisted when the earliest humans arrived. Our attempt to reconstruct past climates and vegetational patterns draws upon both current composition and proxy data from a variety of sources. Floristic diversity and levels of endemism found within the coastal deserts suggest that arid environments have existed along the Peruvian coast for over 100,000 years with some areas influenced by pluvial cycles. Relationships within the flowering plant genus Nolana (Solanaceae), a group of over 80 species distributed predominately in the lomas formations of Peru and Chile, have been examined in detail. The putative phylogeny of Nolana provides a framework for examining the coastal environments and processes important in their evolution, including effects of glacial cycles, sea level changes, and the historical development of the ENSO ("El Niño") weather pattern.

Introduction

The greater part of the western coast of South America [5°- 30°S latitude] is occupied by the Peruvian and Atacama Deserts which form a continuous belt for more than 3500 km along the western escarpment of the Andes, from northern Peru to northernmost Chile. The Peruvian desert is a narrow band 1250 miles (ca. 2000 kms) long [5°-18° S latitude] and 30-60 miles (50-100 km) wide. This arid expanse is broken by occasional rivers valleys from the Andean Cordillera that reach the coast with little water and support riparian vegetation common to the inland river valleys.

A combination of factors is responsible for the development of the hyper-arid conditions along the coast. The coast is isolated from weather patterns to the east by the Andean Cordillera which reaches an average height of over 3000 meters. There is remarkable temperature homogeneity along the entire latitudinal extent of the desert which results from the influence of cool, sea-surface temperatures associated with the south to north flow of the Humboldt (Peruvian) Current, and a positionally stable subtropical anticyclone, resulting in a mild, uniform coastal climate with the regular formation of thick cloud banks below 1000 m during winter months.

Where the coastal topography is low and flat, this stratus layer dissipates inward with little biological impact, but where isolated mountains or steep coastal slopes intercept the clouds, a fog-zone develops with a stratus layer concentrated against the hillsides (Fig. 1). These fogs termed "garúa" in Peru and "camanchaca" in Chile, are the key to the floristic diversity of the western coast. In Peru alone, there exist over 40 such discrete localities (Fig. 2) supporting vegetation, including several offshore islands (e.g., Las Viejas, San Gallan, and San Lorenzo). The actual area covered by vegetation, even during periods of maximum development, is probably less than 16,000 acres (ca. 8000 hr). These communities are called "lomas formations" and consist of desert vegetation, that is highly endemic and largely restricted to the fog-zones (Dillon 1997, Ferreyra 1953, Rundel et al. 1991).

Lomas Vegetation

Lomas communities occur as islands of vegetation and are separated by varying distances of hyperarid habitat devoid of plant life. Since plant growth is dependent upon available moisture and drought tolerance of individual species, a combination of climate, physical topology, and ecophysiology of each species of plant ultimately determines community composition. These communities are highly variable and consist of mixtures of annual, short-lived perennial, and woody scrub vegetation. Current estimates for the total flora represented within the Peruvian lomas formations are ca. 600 species of flowering plants, gymnosperms and ferns. These species can be roughly grouped into broad categories: 1) wide-ranging pan-tropical or weedy species, many introduced during historical times (including shoreline species); 2) amphi-tropic, long-distance disjunctions from North American deserts; 3) Andean disjunctions from the nearby cordillera, and 4) lomas endemics, found only in coastal deserts, sometimes restricted to only a few localities. Endemism at the level of species often exceeds 40% in individual communities. The greatest number of endemic genera are found in southern Peru between 15°-18°S latitude. These include the Peruvian genera Islaya (Cactaceae), Weberbaueriella (Fabaceae), Mathewsia (Brassicaceae), and Dictyophragmus (Brassicaceae), and endemic species in a wide range of genera including, Ambrosia (Asteraceae), Argylia (Bignoniaceae), Astragalus (Fabaceae), Nolana (Solanaceae), Palaua (Malvaceae), Calceolaria (Scrophulariaceae), Cristaria (Malvaceae), Tiquilia (Boraginaceae), Leptoglossis (Solanaceae) and Eremocharis (Apiaceae).

The influence of man on the lomas formations, especially over the last 1500 years, can not be underestimated. Many native woody species have been severely depleted for firewood and construction. It may be assumed that native tree species, such as Caesalpinia spinosa ("tara"), Carica candicans ("mito"), or Myrcianthes ferreyrae have had wider distributions prior to the arrival of man. Movement of livestock between the interior and coast has led to the introduction of many Andean weeds (Sagástegui & Leiva 1993). Historical introduction of alien or exotic species, such as Australian trees (Eucalyptus and Casuarina), has changed the character of the landscape. Perhaps the worst plague that man as set upon the lomas formations since the arrival of Europeans, are herbivores such as goats, which indiscriminately decimate native communities.

We have examined patterns of similarity within the overall flora of the lomas formations and have shown that the coastal deserts of western South America are not uniform (Duncan & Dillon 1991; Rundel et al. 1991). Our analysis supports three primary floristic segments which appear to have independent histories: (1) a northern Peruvian unit from 7°55'S to 12°S latitude, (2) a southern Peru unit from 12°S to 18°S latitude, and (3) a northern Chilean unit from 20°S to 28°S. The area between 18°S and 20°S is nearly devoid of vegetation (Rundel et al. 1991) and is suggested to have been a barrier to coastal dispersal for an extended period (Alpers & Brimhall 1988).

In our search for the forces that act upon the coastal regions, we have examined short-term climatic fluctuations, such as El Niño events (5-50 year cycles), and longer-term climatic change associated with glacial cycles (13,000-200,000 year cycles). These phenomena are potentially important factors in shaping the flora we see today (Dillon 1989).

El Niños and Lomas Formations

In addition to the seasonal fogs, the coastal region is also influenced by periodic and recurrent El Niño events. The physics behind the El Niño Southern Oscillation (ENSO) phenomenon is complex and represents a world-wide weather perturbation. El Niño conditions prevail when the normally cold waters of the coast of western South America are displaced by a warmer, western Pacific surface and subsurface body of water that stimulates brief periods of heavy rainfall and relatively high temperatures. This influx of available moisture has profound effects within the lomas formations and has undoubtedly helped shape their composition and structure. Primarily, this moisture stimulates massive germination of seeds leading to large blooming events that replenish seed banks for annual and perennial plants. These events also provide opportunities for seed dispersal and establishment, which would expand distributions under favorable conditions. The impact of El Niños on these communities is obvious (Dillon & Rundel 1990), and one can only wonder what the coastal vegetation would resemble in the absence of these conditions. Potentially, levels of floristic diversity would be much lower and migration and establishment more difficult. In the western Pacific, the reverse effects of recurrent droughts and rainfall variability have been implicated in the evolution of vegetation patterns in Australia (Nicholls 1991).

El Niño events have been recorded in both historical (Quinn & Neal 1987) and Holocene periods (Fontugne et al. 1999; Rodbell et al. 1999; Sandweiss et al. 1996, 1999). Longer-term records of El Niño events are more difficult to establish but results from fossil coral suggest that El Niño-like conditions may have existed for 124,000 years (Hughen et al. 1999). Our studies of modern vegetation does not allow for estimations of the onset of El Niño conditions, but regardless of their age, they have undoubtedly played an important role in shaping the present coastal communities.

See Northcoast Peru Storms

Glacial Cycles & Sea-level Changes

Another phenomenon, which predate the arrival of man and that of El Niño, was glacial cycles which had global effects throughout the Pleistocene (+/- 1.8 million years ago). It is estimated that there have been at least 20 glacial events during the Pleistocene, each with cycles of approximately 200,000 years. The formation of glaciers on mountain tops has caused sea levels to fluctuate dramatically (Matthews 1990). Estimates of sea-level fluctuation range from between 400-750 ft (120-230 m) and this lowering would have significantly changed the position of the seashore in relation to that today. This drop would have exposed a considerable area of the continental shelf and displaced lomas plant communities, especially from 5° to 15° S latitude (Fig. 3). This would have resulted in species shifting their ranges in relation to the near-ocean environments, adapting to changing conditions in situ, or undergoing range reductions and extinction. Glacial cycles would also have had a profound influence on the evolution of the flora and fauna of the coastal deserts by providing geographic isolation and opportunities for merging species thereby allowing for gene exchanges.

Nolana Studies

Within the lomas formations, the genus Nolana (Solanaceae-Nolaneae) stands out as one of the most wide-ranging and conspicuous elements of the flora. Nolana is largely confined to coastal Andean South America from central Chile to northern Peru and one distinctive species occurs in the Galápagos Islands (Fig. 4). It is the only genus to be encountered in nearly all lomas formations within the coastal deserts. Nolana species are often important members of their respective communities and dominate in the numbers of individuals present. Their showy flowers are beautiful and species display various types of habits, e.g., annuals, perennials, or shrubs, and variable corollas sizes and shapes (Fig. 5).

Ecologically, Nolana species prefer arid and semi-arid habitats with their greatest concentration in near-ocean habitats within a few kilometers of the shoreline, often directly on the beach (e.g., Nolana galapagensis). The establishment of a putative phylogeny in Nolana provides a framework for examining hypotheses of isolation events in desert communities. This pattern is reflected in the Nolana clade, where the distribution of species by latitude reflects the pattern in the overall flora, with three distinctive units (northern Peru, southern Peru, and northern Chile). Only three species have distributions that span the 18° S latitude segment. One, Nolana adansonii, most likely originated in southern Peru and dispersed to Chile, while the other two, N. jaffuelii and N. lycioides, appear to be essentially Chilean with small range extensions into southern Peru. The presence of two major groups (clades) in the genus Nolana, one Peruvian and the other Chilean, points to long-term isolation of the genus above and below 18° South latitude.

Reliable data on speciation rates for desert plants is largely lacking, however, the development of endemic genera and species, and the morphological and physiological adaptations they manifest, suggest long-term aridity along the coast of Peru, at least from 12°-28° S latitude (Rundel & Dillon 1998). The timing of vicariant events (separation) can be estimated with molecular divergence data to establish a molecular clock (Tago, 1999). For the genes investigated, all estimates for the first appearance of Nolana is lower Tertiary (Miocene, 10.6-11.6 mya). These data also suggest that N. galapagensis potentially reached the Galapagos Islands sometime between 4-8 mya (late Miocene to early Pliocene). Further, due to morphological character distribution in the mainland members of Nolana, it appears that N. galapagensis was pre-adapted to arid habitats prior to its dispersal to the island chain (Dillon & Tago 2000). The geographic origin of this remote island endemic remains a mystery, but comparative morphology points to Chilean ancestors.

Recent archeological findings from the northern Atacama desert have recorded Nolana mericarps (seeds) in rodent middens dating to 35,000 years before present (Claudio Latorre, pers. com.). These mericarps are comparable to those we find in this desert locality today. Therefore, the divergence data from molecular studies and the presence of Nolana in desert habitats for no less than 35,000 years, suggest that 10,000 years ago, the overall character of the coastal flora was similar to that of found today. The frequency of strong El Niños and demonstrated sea-level changes suggest that these phenomena have played a role in stimulating evolution in the plants of the lomas formations.

Conclusions

The vegetation of coastal Peru is diverse, highly endemic, and has multiple origins. Given available paleoclimatic data and divergence times suggested by gene sequences, Nolana occupied coastal desert environments prior to the Pleistocene glacial events. Further investigations in additional taxonomic groups will be necessary to test hypothesis of the age for the desert, but our preliminary studies point to western South America as an arid region with great antiquity, well over 35,000 years. The flora of the lomas formations has been shaped by the effects of man and short- and long-term climatic changes. Most data suggest more or less stabilized aridity for coastal Peru since before the arrival of its first inhabitants, but with dynamic periods with much greater available moisture.

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