Reptiles: Ecology

The reptiles of the islands of the Sea of Cortés have provided many opportunities to test ecological and biogeographical hypotheses because they support a diverse fauna with much insular endemism; are numerous and of varying ages and degrees of isolation; are relatively undisturbed by human activity and introduced species; and have a relatively well-understood geological history (see chap. 2). In particular, contrasts of mainland and island reptile populations in the region have resulted in significant progress in testing theories of island biogeography, principles of ecological character displacement, ecological release, density compensation, and vicariance biogeography (see chap. 8). The reptiles, being conspicuous in these arid habitats, have attracted relatively more research attention than other vertebrates, and today we have a reasonably complete picture of at least which species are on which islands. Since the first edition of this book, nearly 20 years ago, there have been only 15 new records for the major islands, of which all but one are of snakes. In this chapter I review the basic elements of reptilian island biogeography in the Sea of Cortés with an emphasis on ecological factors shaping distributions and evolutionary trajectories. I first examine the patterns of species diversity and association across islands. I then take a closer look at some particular island forms, reviewing features of their life history that seem divergent from mainland relatives. In this regard I present some new data from a long-term study of two insular species of chuckwallas. Finally, I review patterns of population density across islands and their possible determinants. A recurrent debate in island biogeography centers on the relative importance of contemporary and ongoing ecological factors relative to historical circumstances in accounting for the number and the identities of species on islands. Historical biogeographers typically view the number of species on an island as being determined by the availability of appropriate habitats. They see changes in species composition chiefly as a consequence of alteration of the mix of habitats due to climatic change (e.g., Pregill and Olson 1980; Olson and Hilgartner 1982); extinctions are posited to occur in waves, as old habitats disappear and new ones become available.

Rocky-Shore Fishes

Marine systems have provided little empirical or theoretical support for the equilibrium theory of island biogeography introduced by MacArthur and Wilson (1967; hereafter referred to as MacArthur-Wilson equilibria). In particular, although marine islands represent isolated habitats for shoreline-restricted marine organisms, it is clear that they do not have impoverished biotas relative to adjacent mainland shores as do their terrestrial counterparts. Additionally, it is not clear that colonization rates based on distance from propagule sources, and extinction rates based on island size, play a substantial role in determining the number and kind of species that may exist here. In this chapter we ask whether the gulf islands are biogeographic islands to rockyshore fishes as they are to terrestrial plants and animals. Although the adults and juveniles of most marine shore fishes cannot readily cross the deep waters separating landmasses, most marine fishes have pelagic eggs and larvae which are often found great distances from shore (Leis and Miller 1976; Leis 1991). Certain families of teleostean fishes (e.g., the blennioids and gobioids) have demersal eggs that are attached to a substrate, and only the larvae are dispersed by ocean currents. Some of these fishes have short-lived larvae that are normally found only close to shore (Brogan 1994). Considering such different types of dispersal mechanisms, one must conclude that distance over open water must be as formidable a barrier to dispersal in some fishes as it is to terrestrial organisms. In line with this conclusion, shore-fish faunas of oceanic islands show high degrees of endemism—for example, 23% in Galapagos shore fishes (Walker 1966), 23.1% and 22.2% in Hawaiian and Easter Island fishes, respectively (Randall 1998). It is well known that the marine insular environment differs considerably from the mainland or continental environment (Robins 1971). Essentially, the former is characterized by a more stable, predictable physical regime with moderate fluctuations in physical factors such as sea temperature, salinity, and turbidity, whereas the latter usually has wider and more unpredictable fluctuations in physical parameters. Robins (1971) compared the difference in species richness between insular and continental fish faunas of the tropical western Atlantic to that between a tropical and a temperate forest, respectively.

Plants

This chapter deals with the general features of plant diversity and distribution on the Sea of Cortés islands and to a lesser extent with adaptive features of plants such as morphology and phenology. This review is based mainly on the plant lists in appendixes 4.1-4.5 describing the island floras, endemics, and relicts. In our interpretations of these lists we draw from various floristic, systematic, and distributional works that pertain to the Gulf of California region and beyond. After an introductory section, we work from broader biogeographical questions to matters that have more local, specific, or taxonomically restricted perspectives. There must be few experiences in the biologist’s world to compare with approaching a “new” island by boat. The ingredients are adventure and suspense, mystery and perhaps even a little danger. There are feelings of discoveries to be made, knowledge to be extended, curiosity to be both piqued and satisfied. Such feelings are shared not only among natural historians but by any adventurous and curious traveler; the more difficult the island is to reach, the keener the excitement of the visit. The islands in the Sea of Cortés would seem ideally qualified to generate this sort of bioadventurism. They are mostly uninhabited and have been little explored biologically, and the mounting of small-scale expeditions to successfully reach (and leave) the islands is not always a trivial matter. First appearing as blurry, near-colorless breaks between the unrelenting blue of gulf and sky, the islands leave early impressions of abrupt topography and a seeming lack of vegetation. The dull-green smudge of plant life on foothill outwash fans and in the arroyos eventually becomes apparent, but almost up to the point of a landing the islands preserve the impression of rock masses broken only occasionally by bajadas on which a few cardons (Pachycereus pringlei) are conspicuous. Islands larger than a few square kilometers have well-developed drainage courses that reach the coasts as dry arroyos and provide breaks in the generally steep coastal cliffs. The beaches where the larger arroyos reach the coast are logical landing points, and here the visitor gets the first close look at the vegetation.

Geology and Ages of the Islands

Before middle Miocene times, Baja California was attached to the rest of the North American continent. Consequently, most of the terrestrial fauna and flora of the peninsula had its origins in mainland Mexico. However, the separation of the peninsula and its northwestward displacement resulted in a variety of distribution patterns, isolations, extinctions, origins and ultimate evolution of fauna and flora in several ways. The islands in the Gulf of California have been colonized by species from Baja California and mainland Mexico. Some workers (Soulé and Sloan 1966; Wilcox 1978) consider that many of these islands originated as landbridges. Geographically, most of the islands are closer to the peninsula than to the mainland. Therefore, it has been assumed that the Baja California Peninsula was the origin of most of the organisms inhabiting them (Murphy 1983). Islands separated by depths of 110 m or less from the peninsula or mainland Mexico apparently owe their current insular existence to a rise in sea level during the current interglacial period (Soulé and Sloan 1966). In contrast, little information exists for deep-water islands. Any complete analysis of the distribution and origin of several organic groups inhabiting the Gulf of California islands should involve the consideration of several contrasting models arguing in favor of or against the equilibrium theory (MacArthur and Wilson 1967). In any model, one of the most important features to consider is the relationship between the species inhabiting the gulf islands and the physical and geological processes of formation of the islands, as well as their age, size, and distance from either the peninsula or the mainland. Understanding colonization, migration, and distribution, particularly in some groups, requires information on whether a particular island was ever connected to a continental source. For example, to explain some characteristics of the populations of any island, which presumably had a recent (<10,000-15,000 years) connection to a continental source, it is necessary to evaluate the coastal erosion or the relative rise in the sea level. These factors might contribute to effectively isolating an insular habit or to forming landbridges.

Human Impact in the Midriff Islands

Although many students of island biogeography consider the midriff islands one of the world’s last major refuges of pristine desert-island biota, humans have been a part of that ecosystem for possibly 10,000 years or more. Humans have long affected the terrestrial and inshore marine biota, but the most serious injuries they have caused apparently began in the mid- to late nineteenth century with the start of guano mining on Patos, Rasa, and San Pedro Mártir islands. Since then, most of the major human impacts affecting the Midriff are related to rapid population growth in northwestern Mexico and increasing demands for the Midriff’s fishery and tourist resources. This chapter offers both historical and ecological perspectives on the human occupancy of the Midriff, so that a cultural-historical foundation will be available for ecological studies in the region as well as for land-use planning and conservation. The Midriff, located between 28° and 29°45´N and 112° and 114°W, includes the coasts of Lower California and Sonora and 39 islands and islets. Tiburόn, with an area of 1223.53 km2 and a maximum elevation of 1219 m, and Ángel de la Guarda, with an area of 936.04 km2 and a maximum elevation of 1315 m, are among the largest and most mountainous islands of Mexico. The other major islands of the Midriff, in order of decreasing size, are San Esteban (40.72 km2), San Lorenzo (33.03 km2), Smith (Coronado) (9.13 km2), San Lorenzo Norte (Las Ánimas) (4.26 km2), San Pedro Mártir (2.9 km2), Mejía (2.26 km2), Partida Norte (1.36 km2), Dátil (Turner) (1.25 km2), Alcatraz (Tassne or Pelícano) (1.2 km2), Salsipuedes (1.16 km2), Estanque (Pond) (1.03 km2), Rasa (0.68 km2), and Patos (0.45 km2) (Murphy, unpublished data). The entire region is extremely arid, and Tiburón is the only island that has permanent potable water, found in a few springs or in tinajas, although several tinajas on Ángel de la Guarda may contain water for long periods. The only island permanently inhabited since initial European contact is Tiburón, the historic stronghold of the Seri Indians or Comcáac, once a seminomadic, nonagricultural, seafaring, hunting, fishing, and gathering people.

Land Birds

Very few of the early scientific explorers in the Gulf of California had much to say about the land birds. There might be two reasons for this: first, the land birds in arid, desert regions are sparse and in general unbecoming, and second, the species encountered are by and large those seen in the much more accessible regions of southwestern North America. Chapter 1 introduced János Xántus, who is recognized as the pioneer ornithologist (or at least bird collector) in the cape area of Lower California, whose contributions (e.g., 1859, in which the first description of the Gray Thrasher, Toxostoma cinereum, was published) are appropriately commemorated in the Xantus Hummingbird, the most spectacular endemic on the peninsula. Lawrence (1860) first described the species as Amazilia xantusi (thence Hylocharis xantusii, and now Basilinna xantusii), and P. L. Sclater announced the discovery to Ibis readers in the same year. By the end of the nineteenth century, several ornithologists had collected in the southern peninsula and reported their findings (e.g., Baird 1870; Belding 1883; Bryant 1889; Ridgway 1896), but very little of this work referred to the islands in the gulf. Brewster’s (1902) report on the cape region avifauna was the most comprehensive of the earlier studies. Serious attention was first paid to the gulf island birds by Maillard (1923) and Townsend (1923), and the latter’s 1911 island-hopping trip in the Albatross served as a model for many similar expeditions later. The first distributional synthesis of their work, and especially that of Nelson (1921), Lamb (e.g., 1924), and Thayer (e.g., 1907), was published by Joseph Grinnell in 1928 in a monograph that is still the standard reference for the peninsula and gulf area. The last 50 years have seen little progress beyond the accumulation of further distributional records and the description of new subspecies (e.g., van Rossem 1929, 1932; Banks 1963a,b,c, 1964, 1969). The island birds remain rather poorly known; even species lists are likely to be incomplete, and ecological studies of the island populations have scarcely begun. In this chapter we report on the results largely of our own field work.

Ants

The distribution and abundance of ants on islands has attracted considerable attention from ecologists and biogeographers, especially since the classic studies by Wilson on the ants of Melanesia and the Pacific islands (Wilson 1961; Wilson and Taylor 1967a,b; see also updates by Morrison 1996, 1997). The species-area curve for Polynesian ants was an important contribution in the development of island biogeography theory (MacArthur and Wilson 1967). Subsequent studies of other island ant faunas, such as those of the Caribbean (Levins et al. 1973; Wilson 1988; Morrison 1998a,b), Japan (Terayama 1982a,b, 1983, 1992), Korea (Choi and Bang 1993; Choi et al. 1993), and island archipelagos in Europe (Baroni Urbani 1971, 1978; Pisarski et al. 1982; Vepsäläínen and Pisarski 1982; Ranta et al. 1983; Boomsma et al. 1987) and North America (Goldstein 1976; Cole 1983a,b), have confirmed the general features of this relationship, although the underlying causative agents and the relative contribution of stochastic and deterministic processes to ant community composition remain points of controversy. The islands in the Sea of Cortés are particularly interesting from a biogeographic standpoint because they vary considerably in size, topography, and isolation. In addition, both oceanic and landbridge islands occur in the gulf, allowing comparisons between faunas that resulted from colonization (assembly) versus relaxation. Nevertheless, the ant assemblages of the gulf islands have received little study. There are a few scattered island records in taxonomic and faunistic papers (Smith 1943; Cole 1968; MacKay et al. 1985). Bernstein (1979) listed 16 ant species from a total of nine Gulf of California islands, but a number of evident misidentifications occur in her list. To the best of our knowledge, no other publications have appeared on the ant communities of these islands. In this chapter, we document the ant species known from islands in the Sea of Cortés and analyze species composition in a selected subset of the better sampled islands. Most of the data come from recent collections made within the last two decades.

History of Scientific Exploration in the Sea of Cortés

The Sea of Cortés (el Mar de Cortés), also known as the Gulf of California, is the body of water that separates the California peninsula from the mainland of Mexico. It extends in a northwest-southeast axis for 1070 km, varying in width from 100 to 200 km. The gulf was formerly much longer, but sediments carried by the Colorado River created a delta and dammed off its upper end, forming what is now the Imperial Valley. The western side of the gulf is dotted with islands, the longest of which is Ángel de la Guarda, measuring 67 km long, up to 16 km wide, and 1315 m high (see app. 1.1 for a list of island names and measurements). Most of the islands are geological remnants of the peninsula's separation from the mainland, a continuing process that started 4 or more million years ago. One central gulf island, Tortuga, is an emerged volcano, whereas San Marcos Island to its west is largely gypsum, possibly precipitated from an ancient lake. The largest island in the gulf is Tiburón, with an area of approximately 1000 km2. It is barely separated from the mainland to the east and has a curiously mixed biota of peninsular and mainland species. One tiny island, San Pedro Nolasco, is only 13 km off shore in San Pedro Bay, Sonora, but has an unusual flora that includes a high percentage of endemics. The isolation of organisms that colonized or were established previously on the Sea of Cortés islands provided an opportunity for genetic and ecological change. In one plastic and rapidly evolving plant family, the Cactaceae, about one-half of the 120 species found on the islands are endemic. Similarly, populations isolated by climate on peninsular mountains are well differentiated. Because of the topographical diversity of the area and its effect on the disruption and integration of populations, the Sea of Cortes and its islands have been called a natural laboratory for the investigation of speciation.

Cultural Dispersal of Plants and Reptiles

The equilibrium theory of island biogeography (MacArthur and Wilson 1967) gives little attention to the human forces that have contributed to shape the biota of archipelagos. Most of the studies that have been done to test the theory, however, concentrated mostly on natural forces and less on the ancient influences of sea-faring cultures on island biodiversity. Although many biologists have followed MacArthur and Wilson’s lead by charting the natural processes shaping the island biogeography of the midriff islands in the Sea of Cortés (Soulé and Sloan 1966; Case and Cody 1983), the cultural dispersal of native plants and animals across the gulf has hardly been taken into account in these pattern analyses of the region’s biota. Nevertheless, new opportunities have emerged. Analyses made possible by novel genetic tools can now be combined with recent revelations of oral history from Seri Indian seafarers who have frequented the midriff islands and who know of their ancestors’ activities on the islands. Archaeologists have found indigenous remains on San Esteban, Ángel de la Guarda, San Lorenzo Norte and Sur, and Tiburón, with dateable occupation sequences on San Esteban for a minimum of 350 years (Bowen 2000). We can now begin to reconcile data from cultural geography, genetics, and biogeography to track cultural dispersal with new precision. A cohesive but curious story has begun to emerge from this unlikely partnership of genetic analyses performed in laboratories and oral history documentation in the field: historic seafarers of this arid region have carried with them flora and fauna that became established on islands other than those accessible by natural routes of dispersal (Grismer 1994; Petren and Case 1996, 1997; Nabhan in press). This should come as no surprise to scientists who read beyond their own area of interest: similar cultural dispersal dynamics have been documented in Polynesia and Melanesia (McKeown 1978; Fisher 1997; Austin 1999) and in Central America and the Caribbean (Bennett 1992; Case 1996). Factoring indigenous cultural dispersal into island biogeography has led to very different views of biotic origins and migrations than those offered by a purely biological perspective.

Island Food Webs

Most of this book focuses on the biogeography and ecology of plants and various animal taxa on islands in the Sea of Cortés. These chapters highlight the historical and biogeographical factors that contributed to the patterns of species distribution and co-occurrence among islands. However, these patterns also reflect the action of ecological processes because the species present interact, directly or indirectly, within the food web that occurs on any given island. Island food webs may also be unique from other communities in the degree to which their structure and dynamics are also strongly influenced by the surrounding ocean. We believe that a deeper appreciation of the trophic connections between the sea and the land, and the resulting effects on the structure and dynamics of island food webs, is key to understanding the biogeography of species on islands. Many factors that operate through the food web can enhance or depress populations in a way that affects their local distribution and persistence, and, as a consequence, affects patterns of diversity on a biogeographical scale. Of these, we recognize three as being particularly important: the availability and quality of resources, competition, and consumption (i.e., by herbivores, predators, parasites). Bottom-up factors (nutrients, primary productivity, and food availability to consumers) set limits on island productivity and hence on the potential abundance of a particular group. Within a given community, secondary productivity and population density are subsequently constrained by top-down (i.e., consumption) and competitive effects. One of our goals in this chapter is to show how processes that influence productivity of gulf islands determine patterns of abundance of organisms on islands and affect interactions among species and trophic levels in these systems. Our second goal is to demonstrate the importance of spatial and temporal variability in productivity in determining the structure and dynamics of island food webs. Using our long-term studies of plants and consumers on islands in the northern gulf, we show that productivity varies greatly, both among years and islands, as a result of both local conditions and global climatic factors. Such variable productivity markedly affects food web dynamics and ultimately the abundance of species on the islands in the Sea of Cortés.