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Articles An Ecological Perspective on the Biodiversity of Tropical Island Streams GORDON C. SMITH, ALAN P. COVICH, AND ANNE M. D. BRASHER Ecological processes of insular tropical stream drainages are not well understood. Furthermore, it is not yet clear how the concepts and paradigms derived from studies of continental stream ecosystems in the temperate zone can be effectively applied to understanding and managing tropical island watersheds. The articles in this special section describe important aspects of how tropical stream ecosystems are structured and how these systems function and change over time in response to natural processes and human activities. Keywords: tropical island, stream ecology, island biogeography, watershed, aquatic resources I mages of exotic tropical streams and waterfalls are commonly used to denote the beautiful and unique features associated with island landscapes. Yet the continued existence of such scenic habitats is being threatened in a variety of ways. The ecology and the potential for sustainability of insular tropical stream and river drainages are not well understood. Furthermore, it is not yet clear how the concepts derived from studies of continental stream ecosystems in the temperate zone can be effectively applied to understand and manage streams and rivers on islands in tropical areas. With ever-increasing development pressure in these areas, there is a growing need to establish more unified themes for research, management, and conservation of tropical stream ecosystems. This special section of BioScience presents recent research on different types of tropical streams and emerging themes in this area. The articles describe important aspects of how tropical stream ecosystems are structured and how these ecosystems function and change over time in response to natural processes and human activities. Several articles emphasize the magnitude of impacts resulting from human alteration of the physical and biological characteristics of these ecosystems. There is growing recognition, from studies of both temperate continental and tropical insular ecosystems, of the importance of conserving the biological resources of watersheds as well as the other “ecosystem services” they provide, such as a clean water supply, fishery resources, and recreation. In recent years, integration of watershed-scale 1048 BioScience November 2003 / Vol. 53 No. 11 considerations into management and conservation planning has gained momentum. As these articles illustrate, tropical stream ecologists are developing new approaches to understand, manage, and conserve these environments. These articles also clearly document that the main differences in biodiversity among tropical island streams reflect regional variations in geology, terrain, climate, and land use. However, across a wide range of latitudes, most tropical streams differ significantly from those found on continents by having relatively short, straight, and steep channels in comparatively small, narrow watersheds. Low-latitude island watersheds are often influenced by major fluctuations in precipitation and generally show the effects of exposure to tropical storms, hurricanes, and occasional major droughts (Covich et al. 2003, Oki and Brasher 2003). In contrast to the more seasonally driven temperate stream systems, tropical stream networks tend to be more event driven, and these events may be relatively unpredictable. Because tropical Gordon C. Smith (e-mail: [email protected]) is a fish and wildlife biologist at the US Fish and Wildlife Service’s Pacific Islands Fish and Wildlife Office, 300 Ala Moana Boulevard, Honolulu, HI 96850. Alan P. Covich is a professor and director of the Institute of Ecology, University of Georgia, Athens, GA 30602. Anne M. D. Brasher is an aquatic ecologist at the Water Resources Division of the US Geological Survey, 2329 West Orton Circle, Salt Lake City, UT 84119. © 2003 American Institute of Biological Sciences. Articles insular streams and rivers differ from their mainland counterparts in a wide range of geologic, climatic, biogeographic, and anthropocentric features, there is great need for comparative studies. Geologic and biological processes Tropical island stream ecosystems offer insights into processes that occur on geologic and evolutionary time scales. The geological sequence of the origins and development of tropical oceanic islands was first postulated by Darwin. This sequence begins with the eruption and formation of a high and often steep-sided island. The initial phase of formation is followed by eventual erosion, subsidence, and concurrent development of fringing coral reefs on the island’s perimeter. Although some islands in tropical and subtropical regions are formed by uplifted sedimentary material (Bass 2003) or were once connected to continental landmasses (Benstead et al. 2003), most oceanic islands are volcanic in their origin and closely follow this physical progression of creation and decline (Craig 2003). For example, the evolutionary adaptation of a group of aquatic flies to the specialized stream microhabitats that they inhabit in the Society Islands of French Polynesia is an example of how species respond to the development and eventual demise of the streams in which they are present (Craig 2003). The evolutionary histories of the organisms found in these streams depend on the geologic history of the islands. This evolutionary pattern is distinctly different from the patterns of speciation and evolution of the aquatic organisms that inhabit the geologically more permanent streams and rivers located on continental landmasses. Although insular tropical streams and rivers generally lack the exceptionally high diversity of fishes observed in the largest continental tropical rivers, such as the Amazon and Mekong, their biotic communities reflect complex geological and biological processes. Biologists need to understand these long-term processes better to enhance their understanding of freshwater biodiversity and island biogeography. The species–area relations that are derived from these studies are at the core of several broad principles of biogeographic theory. For example, the associations between island area and species richness have been well studied for many terrestrial species, yet researchers know little about these species–area relations among fishes and aquatic invertebrates in insular lotic habitats (Haynes 1987, 1990, Donaldson 2002, Donaldson and Myers 2002, Bass 2003, Craig 2003). The number of species that are observed to inhabit islands in relation to their landmass has been a topic of in-depth study for several decades. The islands of the Caribbean region reflect a mosaic of different geologic histories and exhibit a considerable variety of different landforms. Consequently, they provide locations for comparative studies to examine such associations. Bass (2003) discusses the biodiversity of aquatic invertebrates in this region. Species richness was generally related to area and island altitude, consistent with biogeographic theory. However, the ability or inability of certain groups of aquatic organisms to disperse widely at some stage in their life history is a strong determinant of biogeographic patterns of distribution. Mechanisms of dispersal strongly shape the life histories of parasitic organisms (Font 2003). Given the geographical isolation and geologically young age of the island archipelago of Hawaii, the successful colonization of parasites on these islands would presumably have been an extremely unlikely event before the arrival of humans. However, a number of helminth parasites are present in organisms occurring in Hawaiian stream systems. In fact, because native fish fauna diversity is low and aquatic community structure is relatively simple, the likely mechanisms by which native and introduced helminths colonized the islands are inferred more readily than in more complex continental systems. The Hawaiian Islands serve as a model for examining the global spread of parasites in aquatic systems, both through natural processes and through human introductions. Conservation biologists recognize tropical streams and rivers as critical, complex habitats for diverse species that have developed in many different ways (Bass 2003, Benstead et al. 2003). Some islands, such as Borneo, Madagascar, Trinidad, and Tobago, were once connected to continental landmasses; streams on these continental islands are similar to species-rich continental freshwater streams. Islands that are continental in their geologic origin, or that were formed by processes of uplift and emergence, have drainage patterns shaped by unique geologic characteristics and local hydrology. These islands provide useful biogeographic contrasts between true oceanic islands and those with more complex histories of continental connections. True oceanic tropical island stream networks often occur on high islands of volcanic origin (Craig 2003). They tend to have short, steep drainages with frequent torrential flows compared with stream networks in continental settings. Most true oceanic islands are relatively recent in their geological histories and are relatively small in area compared to continental islands. These islands have well-recognized biotic differences based on their physical isolation, with the Hawaiian archipelago being the most isolated in the world. Freshwater species that colonize these distant tropical islands rely on a variety of dispersal mechanisms to exploit their remote aquatic habitats. Several groups of aquatic insects, such as those in the orders Odonata (dragonflies and damselflies) and Diptera (black flies and midges), have sufficient capacity for sustained flight to cross the considerable distances between islands or island groups, leading to colonization. In contrast, native species of true “primary” freshwater fishes (those taxa not derived from marine or diadromous ancestors), and many groups of aquatic insects, often have low diversity on oceanic islands, because most continental species have limited dispersal abilities. For example, in the Hawaiian Islands these freshwater colonizers include numerous dipteran species such as the midge Telmatogeton torrenticola (Benbow et al. 1997). Many freshwater island species are phylogenetically derived from marine ancestors. Neritid snails, atyid and November 2003 / Vol. 53 No. 11 BioScience 1049 Articles palaemonid shrimp, and some fishes, including gobies, sleepers, and eels, are widespread because their planktonic larvae drift long distances among islands in oceanic currents. As juveniles or postlarvae, they migrate upstream considerable distances over steep terrain and climb up high waterfalls. These taxa are especially well adapted for completing major periods of their life histories in tropical stream drainages, where they play essential roles in aquatic food webs (Covich and McDowell 1996, Crowl et al. 2001, March et al. 2003a, Williams 2002). These life-history characteristics have resulted in a significant number of shared common genera of fishes, decapods, and gastropods in many streams throughout the tropical Pacific and Caribbean (Fievet et al. 2001, Leberer and Nelson 2001, Debrot 2003, March et al. 2003b). Despite their widespread, even pantropical, distribution, researchers know little about how these groups’ complex life cycles or their ecological roles can be disrupted by events such as natural droughts, global warming, or management decisions regarding water diversions and waste disposal. Human influences Streams on tropical islands are particularly vulnerable to changes in ecosystem properties because of the spatially concentrated human populations that characterize many of these rapidly changing inhabited landscapes. There is an urgent need for studies on tropical island stream drainages, because the freshwater resources they contain are in great demand. As a result of their physical and chemical properties as well as their biotic communities, these ecosystems may be transformed and irreversibly changed by fast-growing human populations (Brasher 2003, March et al. 2003b). Because larger temperate and tropical continental stream and river ecosystems are also under increasing stress from dense coastal populations, these smaller island drainages provide an excellent source of comparative studies for better understanding these critical ecosystems. Protection of “reference watersheds” is essential before these ecosystems are so severely altered that sufficiently diverse comparative studies are not possible. Human alteration of land and hydrologic systems on many tropical islands occurs primarily along the coastal perimeter. However, profound changes to island stream habitats resulting from domestic and agricultural diversion of stream water occur throughout the islands, even in remote areas with relatively pristine forests and other terrestrial habitats (Brasher 2003, March et al. 2003b). Although extensive research has documented the impacts of hydrologic alterations on biotic communities in temperate systems, far less is known about biotic responses to hydrologic modifications in tropical systems (Pringle et al. 2000). The amphidromous life histories of many island stream macrofaunal species require passage both downstream and upstream (Fitzsimons et al. 2002). However, passage of aquatic organisms through physically altered streams in both directions is significantly affected by habitat modification (McDowall 1995). Brasher (2003) presents an overview of land-use and water-diversion patterns occurring in Hawaii that have led to alterations in biotic 1050 BioScience November 2003 / Vol. 53 No. 11 communities. The numerous introduced aquatic organisms, well adapted to the conditions in these modified habitats, cause additional detrimental effects through complex interspecific interactions, such as predation and competition for space and other resources (Larned et al. 2003). Human demand for water resources continues to grow on most tropical islands. In the past, water diversion from stream channels was often implemented without regard for conservation of the aquatic communities that were affected either by the loss of water in the channel or by the disruption of migratory pathways resulting from channel blockage or entrainment into offstream ditches and impoundments (Concepçion and Nelson 1999, McIntosh et al. 2002). Engineered fish-passage structures and watershed-wide management strategies that lessen the negative effects of water diversion to stream ecosystems need to be developed if tropical stream environments and the organisms that inhabit them are to be conserved. March and colleagues (2003b) describe the effects of dam configuration and operation on the amphidromous fish and shrimp fauna of the streams and rivers of Puerto Rico. During the last hundred years, a considerable amount of fish-passage design and implementation has been incorporated into the operation of dams in continental settings, particularly for salmonids. However, the features of fish-passage design that are required for tropical island systems, such as the effective fish-passage structures and data on minimum flow regimes needed to allow upstream passage of migrating postlarval fish and invertebrates (which climb rather than jump), have yet to be developed. Ultimately, an understanding of these diversion bypass systems and their associated ecology is necessary. Implementation of water-conservation strategies—and a strong commitment to sustainable use of water resources—will be needed to conserve and restore tropical island stream networks that are degraded by water withdrawal. In terms of its size and geologic origin, Madagascar, the world’s fourth largest island, lies at the conceptual opposite extreme from the small, mostly volcanic, oceanic islands that are the subject of the articles discussed above. Benstead and colleagues (2003) discuss newly described patterns of biodiversity of the aquatic macrofauna of Madagascar. These patterns stem from Madagascar’s continental origins combined with the geologically long isolation of the island from its parent landmass. The remarkable evolutionary history of various groups of aquatic organisms that inhabit stream systems in Madagascar is demonstrated by the presence of many phylogenetically basal species (i.e., progenitors that radiated into new species). The frequent microendemism and the array of adaptive radiation among certain groups are likewise notable. Throughout the island of Madagascar, overfishing, sedimentation (a result of land clearing), forest loss and fragmentation, and introduction of alien species continue to threaten flowing waters. Conservation priorities include strengthening and directing resources to local institutions that undertake resource conservation; examining patterns of biodiversity to effectively set priorities and conservation goals; and incorporating watershed-wide planning into Articles identification of conservation actions for natural area reserves and other managed landscapes. Research on the combined responses of organisms and biological communities to the effects of habitat isolation and resource limitations is providing a strong basis for effective stewardship of tropical island streams. The isolated assemblages of species in these habitats provide many opportunities for regional and comparative studies of general ecological and evolutionary biology. However, time is limited before invasive species and other disturbances alter these communities in ways that will be difficult or impossible to reverse. Acknowledgm ents We wish to thank all participants in the symposium titled “Conservation Research in Tropical Island Stream Systems: Importance of a Watershed Perspective,” convened at the Society for Conservation Biology’s 2001 annual meeting, and particularly those who presented papers: David Bass, Jonathan Benstead, Douglas Craig, William Font, Michael Kido, Robert Kinzie, Daniel Lindstrom, James March, Robert Nishimoto, Dan Polhemus, and Marcus Wishart. 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