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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
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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
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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
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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
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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. Development
of this special section was supported in part by the US Fish
and Wildlife Service, Pacific Islands Fish and Wildlife
Office.
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