NSF - LUQUILLO LTER IV Project:
Understanding Change in Ecosystems of Northeast Puerto Rico
Long-term Ecological Research (LTER)
Location/Duration: Caribbean National
Forest, Puerto Rico (2006-2011)
Current Graduate Students: Katherine Smith (PhD),
Pedro Torres (PhD)
Previous Graduate Students: James March (PhD),
Jonathon Benstead (PhD), Effie Greathouse (PhD), Kelly Crook (MS)
Summary: The Luquillo Long-Term Research Program
(LUQ) focuses on understanding factors driving long-term change in
tropical forest ecosystems in the Luquillo Mountains of Puerto Rico.
Building from an earlier emphasis on natural disturbance (hurricanes,
landslides, droughts, floods) and ecosystem response to disturbance,
LUQ is continuing studies of ecosystem structure and processes in
mid-elevation tabonuco forest, extending research into other forest
types along an elevation gradient, and beginning investigations of
regional-scale processes affecting the Luquillo Mountains. For
approaches are being used: long-term measurements and experiments,
comparative analyses among different forest communities, gradient
analysis from forests to urban ecosystems and synthesis using
conceptual and simulation models.
Mounting
evidence suggests that increasing hurricane intensity, declining
rainfall in the mountains and rising temperature in urbanized areas in
the nearby lowlands can have significant effects on the ecosystems of
the Luquillo Mountains. In this context we are asking: How do changes in disturbance regime
and climate alter biogeochemical cycles, biotic structure, and
ecosystem services in the Luquillo Mountians and northeastern Puerto
Rico? This
overarching question leads to three specific questions that address key
elements of our long-term conceptual framework: (1) What controls
variation in C and nutrient fluxes and how are these variations
modified by disturbance?; (2) Are changes in temperature, rainfall,
light and wind (climate) along the Luquillo elevation gradient
sufficient to explain variation in biogeochemical processes and biotic
structure?; and (3) How important are changes in land-use in
determining long-term ecosystem biogeochemistry, biotic structure, and
services?
Research
will provide an improved scientific framework for the management of
tropical ecosystems and ecosystem services. It will do so both through
conceptual advances and documentation of human disturbance and
ecosystem response. The project will continue to produce a cadre of
young and minority scientists who are versed in linking population and
ecosystem approaches to evaluating environmental change, and will
provide them with skills that can be applied to tropical regions or
elsewhere, LUQ has developed a comprehensive education program
involving teachers at a network of six high schools and with a
web-based middle school curriculum for teaching ecology. Additional
outreach activities are directed at improving thegeneral public's
appreciation of the water resources provided to surrounding towns by
steams draining the Luquillo Mountains.
Selected
Publications:
Pringle, C. M., G. A. Blake, A. P. Covich, K. M. Buzby and A.
Finley. 1993. Effects of omnivorous shrimp in a montane tropical stream: Sediment
removal, disturbance of sessile invertebrates and enhancement of understory algal biomass. Oecologia
93: 1-11.
Pringle, C. M., and G. A. Blake 1994. Quantitative effects
of atyid shrimp (Decapoda: Atyidae) on the depositional environment in a tropical
stream: Use of electricity for experimental exclusion. Canadian Journal of Fisheries and Aquatic
Sciences 51: 1443-1450.
Pringle, C. M. 1996. Atyid shrimps (Decapoda: Atyidae)
influence the spatial heterogeneity of algal communities over different scales in tropical
montane streams, Puerto Rico. Freshwater Biology 35: 125-140.
Pringle, C. M. 1997. Exploring how disturbance is
transmitted upstream: going against the flow. Journal of the North American Benthological
Society 16:425-438.
Pringle, C. M., N. H. Hemphill, W. McDowell, A. Bednarek, and J.
March. 1999. Linking species and ecosystems: Different biotic assemblages cause
interstream differences in organic matter. Ecology 80: 1860-1872.
Benstead, J. P., J. G. March, C. M. Pringle, and F. N. Scatena.
1999. Effects of a low-head dam and water abstraction on migratory tropical
stream biota. Ecological Applications 9: 656-668.
March J. P., C. M. Pringle, M. J. Townsend, and A. I. Wilson.
2002. Effects of freshwater shrimp assemblages on benthic communities along
an altitude gradient of a tropical island stream. Freshwater Biology 47: 1-14.
March J. P., C. M. Pringle, M. J. Townsend, and A. I.
Wilson. 2002. Effects of freshwater shrimp assemblages
on benthic communities along an altitude gradient of a tropical island stream. Freshwater
Biology 47: 1-14.
Greathouse, E. , C. M. Pringle, and W. McDowell.
2006. Do small-scale exclosure/enclosure experiments predict effects of
large-scale extirpation of freshwater migratory fauna? Oecologia. 149: 709-717.
Greathouse, E. A., C. M. Pringle, W. H. McDowell, and J. G. Holmquist.
2006. Indirect upstream effects of dams: consequences of migratory
consumer extirpation in Puerto Rico. Ecological Applications 16:
339-352.
Greathouse, E. A., and C. M. Pringle.
2006. Does the River Continuum Concept apply on a tropical island? Longitudinal variation in a Puerto
Rican stream. Canadian Journal of Fisheries and Aquatic Sciences.
63: 134-152.
Smith, K. L., I. Corujo-Flores, and C. M. Pringle. In press.
: A comparison of the current and historical fish assemblages in a Caribbean
island estuary: Conservation value of historical data. Aquatic Conservation x:xx-xx..
Crook, K., F. Scatena, and C. M. Pringle. 2007. Water withdrawn from the Luquillo Experimental Forest.(2004). Technical report IITF-GTR-34, 25 p.
(Newsletter article written by Pringle for the LTER Network Newsletter, Fall 2000 issue: "Science Management connections help protect stream ecosystems at the Luquillo LTER Site, Puerto Rico")
There
is
increasing pressure on the Luquillo Experimental Forest (northeastern
Puerto Rico) to provide drinking water for human populations in the
lowlands: at least 600,000 people are dependent on water withdrawn from
rivers draining the forest. It was not until 1994 that a water-use
budget was constructed for Luquillo. Creation of this water budget was
facilitated by longterm discharge data available from 12 established
stream gages located within or closely adjacent to the forest. The
water budget indicated that on an average day, more than 50% of
riverine water is diverted into municipal water supplies before it
reaches the ocean. A total of 21 water intakes are operational within
forest boundaries, and 9 large intakes are located in lower stream
reaches outside the forest. Many streams have no water below their
intakes for much of the year and it is increasingly common for saline
waters from the ocean to intrude 2-3 km upstream in the absence of
riverine inputs of freshwater (Benstead et al. 1999). Conditions are
particularly severe during drought periods and severe water rationing
to local communities is common. Stream dewatering also results in less
water to dilute wastewater discharged by sewage treatment plants [into
lower river reaches outside of the national forest and is an
increasingly important constraint to tourism development in the coastal
lowlands (Pringle and Scatena 1999).
The
sheer magnitude of hydrologic changes being proposed for streams
draining the forest has kept resource managers busy evaluating and
providing scientific information relevant to poorly-planned proposals
(F. Scatena, USDA Forest Service, International Institute of Tropical
Forestry, San Juan, Puerto Rico, personal communication). Conflicts for
water usage between local communities and tourist complexes geared to
off-island residents are aggravated by the fact that the dry season
coincides with the tourist season. These conflicts can only be expected
to increase as tourism expands. The island has recently experienced
severe water shortages, despite its historical reputation as "the land
of many rivers" (Pringle and Scatena 1999). In 1994, the drought became
so severe that Puerto Rico was declared an agricultural disaster area
by the federal government and water rationing was imposed in the major
metropolitan area of the island.
Managers
of the Caribbean National Forest and scientists from both the Forest
Service and academia are concerned about the aforementioned water
resource issues - particularly how effects of human disturbance are
transmitted upstream and how they affect the biotic integrity of the
national forest itself (e.g., Pringle 1997). While the forest has some
of the last undeveloped water supplies on the island, water withdrawals
conflict with other functions that the forest fulfills (e.g.,
recreation, research, and the maintenance of the original biodiversity
of the island). For example, since all of the fish and shrimp species
that inhabit the streams of the Caribbean National Forest are migratory
(i.e., traveling from stream headwaters to estuaries or the ocean and
back at some point in their life cycle), dams, water abstraction, and
pollution along stream continua can affect their populations (e.g.,
Holmquist et al. 1998). This can, in turn, have potentially important
effects on ecosystem dynamics in headwater streams since research shows
a strong linkage between species assemblages and ecosystem properties
(Pringle 1996, Pringle et al. 1999, March 2000, Crowl et al. 2001).
Scientists
and managers have brought water resource issues facing the Luquillo
Experimental Forest to both local and national attention. Projects
funded by NSF and/or the USDA Forest at this LTER site include: (1)
longterm population monitoring of shrimp populations (e.g., Covich et
al. 1996); (2) evaluation of instream flow and habitat requirements of
shrimps (Scatena and Johnson in press) and fishes (N. Hemphill and E.
Garcia, Caribbean National Forest, unpublished data); (3) the timing of
larval shrimp migration to estuaries (March et al. 1999); (4) estuarine
shrimp larval development and upstream post-larval migration (Benstead
et al. 2000); (5) effects of dams and water withdrawals on shrimp and
fish mortality (Benstead et al. 1999); (6) genetics of shrimp
populations between rivers (T. Crowl et al., Utah State University,
unpublished data); (7) ongoing studies on effects of different types of
water intakes on shrimp and fish mortality (J. March et al., University
of Georgia, unpublished data); and (8) ongoing studies on the impacts
of water extraction and sewage releases on water quality in the Mameyes
estuary (e.g., Scatena in press, and unpublished data).
Results
of this research are being directly linked to management of hydrologic
connections across the boundaries of the Caribbean National Forest. For
example, Benstead et al. (1999) showed that water extraction over a two
month period, from a major water intake located outside of the forest
on the lower Espiritu Santo River, resulted in high mortality (42%) of
drifting first-stage shrimp larvae by entrainment during downstream
migration. One hundred percent of drifting larvae were entrained by the
intake during low flows when no water was discharged over the dam
(Benstead et al. 1999). Field measurements of larval shrimp mortality,
combined with a 30-yr discharge record, were used to model the
long-term impacts of different intake management strategies on shrimp
mortality at the water intake. Results indicated that long-term mean
daily entrainment mortality of shrimps ranged from 34 to 62%, depending
on estimates of the water amount extracted from the river. A companion
study, on temporal patterns of shrimp migration (March et al. 1998),
showed that larval shrimps drift during the night with a nocturnal peak
occurring a few hours after dusk. This combined information was used to
make recommendations for mitigation of negative environmental effects
caused by water abstraction. Recommendations include: (1) 3-5 hour
stoppages in water abstraction during peak nocturnal (i.e. post-dusk)
larval drift ; (2) up-keep of functional fish ladders; (3) maintenance
of minimum flows over dams; and (4) evaluation of different types of
water withdrawal systems (March et al. 1998; Benstead et al. 1999).
As a
result of these findings, the designs of two new water withdrawal
systems have been altered by the Puerto Rican Aqueduct and Sewage
Authority to minimize mortality of migrating aquatic biota. Intakes
also operate when stream flows are high so that base flows are
maintained. Equally encouraging is that water withdrawal from another
intake on the Culebrinas River has been prohibited from 7 to 11 PM and
a fish ladder has been required. These small successes aside, what are
the cumulative impacts of water withdrawals on the biotic integrity of
streams and rivers draining the Caribbean National Forest ? How will
droughts and island-wide water shortages exacerbated by burgeoning
human populations affect water withdrawals and the biological integrity
of the forest in the longterm? These are some of the questions that
longterm research at the Luquillo LTER site will help address.
In
summary, the Luquillo Experimental Forest (a.k.a. the Caribbean
National Forest) is like many public lands throughout the U.S. in that
it is increasingly threatened by human alteration of hydrologic
connections outside of its boundaries. Expanding human populations
require more water and they often look to public lands to meet their
demands. On a global scale, humans have already appropriated half of
the accessible freshwater runoff, and conservative estimates indicate
that this appropriation could climb to 70% by the year 2025.
Correspondingly, there is less fresh water available for the
environmental needs and integrity of ecosystems that are not dominated
by humans. Of great concern are situations where hydrologic alterations
outside of the boundaries of managed areas end up controlling the
hydrology and/or biology of those areas.
This
is the topic of a recent paper which just came out in a special issue
of Ecological Applications devoted to the land-water interface and
science for a sustainable biosphere: "Threats to U.S. public lands from
cumulative hydrologic alterations outside of their boundaries" (Pringle
2000). In this paper, the Luquillo Experimental Forest LTER site is
provided as a case study which highlights the critical need to address
cumulative longterm effects of hydrologic alterations on public lands
and to illustrate the synergism that can occur between field managers
and scientists in implementing localized solutions.
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River, Puerto Rico |
Xiphocaris elongata, a freshwater shrimp found in Puerto Rican streams |
tributary (Bisley), Puerto Rico