Tropical forest conservation and estuarine ecology

Tropical forest conservation and estuarine ecology

Biological Conservation23 (1982)247-259 TROPICAL FOREST CONSERVATION AND ESTUARINE ECOLOGY ARCHIE CARR III New York Zoological Society, Bronx, New Yo...

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Biological Conservation23 (1982)247-259

TROPICAL FOREST CONSERVATION AND ESTUARINE ECOLOGY ARCHIE CARR III New York Zoological Society, Bronx, New York 10460, USA

ABSTRACT

Basic ecological characteristics of a tropical estuary (Tortuguero, Costa Rica) are reviewed with special emphasis on the seasonal (wet season-dry season) variability of abiotic and related biotic parameters. The relationship of estuarine dynamics to the general character of the watershed are discussed, and a proposal to protect the remaining coastaljbrest cover of the Tortuguero watershed is offered. It is argued that the well-being of tropical estuaries is determined by the conservation of tropical .[brests.

INTRODUCTION

Deforestation of tropical lands is a subject receiving increasingly anxious attention from the world community. Tropical forests harbour a disproportionate share of the earth's genetic material and living biomass, and, like the oceans, their conservation is a matter of global ecological security. The current emphasis within the debate on tropical forest conservation rests largely upon the loss of in situ values associated with the forestper se: the vegetation, the animal life, the soils. This paper strives to show that ecosystems that are distinct from, but contiguous to, tropical forests, such as rivers and estuaries, are ecologically dependent upon the forests and, therefore, upon their management. It is an application of conventional principles of watershed management to a tropical setting.

THE SITE

Field studies were conducted in 1974 75 on the seasonal dynamics of parts of the Tortuguero Estuarine System located on the Caribbean coast of Costa Rica,Central America ( C a r l 1979; Fig. 1). Two linked basins (Figs. 1, 2) comprise the aquatic environment periodically 247 Biol. Conserv. 0006-3207/82/0023-0247/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

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exhibiting estuarine properties (i.e., brackish water) in Tortuguero, forming the Tortuguero Estuarine System. The northern basin, the Tortuguero Estuary, is a 6;Skin elongate channel with a notably uniform depth of approximately 7m (Nordlie & Kelso, 1975), and a width of 300-400 m. The northern extreme of this basin communicates with the Caribbean Sea via a shallow pass through which fresh and salt water are exchanged. At its southern end, the estuary is joined with the Tortuguero Lagoon at the confluence of two rivers, the Tortuguero and the Lagunas Penitencia. This junction is locally known as Quatro Esquinas, and hydrologically separates the Tortuguero Estuary from the Lagoon. The Tortuguero Lagoon exhibits estuarine attributes throughout its 16km length. Like the estuary, it is an elongate, narrow (300 m) basin with an average depth of 4m.

ECOLOGY OF THE ESTUAR1NE SYSTEM

The ecology of the estuarine system at Tortuguero is heavily influenced by the intensity and seasonality of precipitation (Carr, 1979). Annual rainfall ranges between 5000-6000ram (Hirth, 1963; Portig, 1965; Cart, 1979), with a strongly bimodal distribution (Fig. 3). The rainfall produces surface runoff, bearing dissolved and organic and inorganic I00(3

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particulate matter from the land, through the rivers to the estuarine system. The seasonal delivery of flesh water provides for two distinct aquatic environments in the single basin complex in the wet and dry seasons respectively, with extreme variations in physical, chemical and biological properties, as summarized in Table 1. In an earlier work elsewhere in Central America (Lake Izabal, Guatemala), Brinson (1973) demonstrated that algal productivity (primary production) in a tropical estuarine system is also strongly influenced by the seasonality of precipitation. In tropical estuarine systems where air and water temperatures and solar periodicity are relatively constant, aquatic ecology is disproportionately influenced by seasonal rainfall. This influence is all the more acute in the case of Tortuguero, where the total rainfall is enormous. Deforestation of the Tortuguero catchment basin would be ruinous, and one is led to the conclusion that management of the Tortuguero Estuary must begin inland with management of the watershed. THE PRINCIPLE OF WATERSHED MANAGEMENT

The purpose of watershed management is to control the gravitational movement of water, chemicals, and particulate matter from the terrestrial ecosystem, downslope toward the estuary (Likens & Borman, 1974). Alterations in the terrestrial segment of a land-water system by, for example, deforestation, agricultural expansion, or urbanization, generally result in increased flows of water, nutrients, and particulates to the aquatic segment. These increases alter the aquatic system both physically and chemically. The relationships have been generally recognized by aquatic ecologists for many years (Cronin, 1967), and have been expressed in concise and management-related terms by Likens & Borman (1974). THE TORTUGUERO WATERSHED

There are seven major watersheds along the northeastern slope of Costa Rica. These have been delineated by Nuhn & Perez (1967) in a study of the general geography of the North Atlantic Zone of Costa Rica. The Tortuguero basin is the second largest of these, encompassing 2061 km 2, and its drainage area is entirely within the coastal plain, below the foothills of the interior mountains (Fig. 4). It is a wedge-shaped area with the apex inland and the broad base along the coast; flat to gently rolling, and reaching an altitude in excess of 200 m only near the apex surrounding the town of Guapiles. The mid-section ranges in altitude from 100-200 m, and the coastal areas are extremely low, ranging from 0 20m, with the exception of isolated hills or small hill ranges.

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Fig. 4. Drainage basins of northeastern Costa Rica illustrating the existing Tortuguero National Park and the proposed forest reserve (slanted lines outlined with dotted border). Stippled areas indicate palm swamps.

Rainfall is greater toward the coast (5000-6000 mm year- x) than near the apex of the wedge (3500 mm year-1). Approaching the coast along a transect through the basin, there is an increase in the number of streams, a reflection of the low topography and local rainfall distribution. This coastal half of the Tortuguero basin is the critical watershed. The streams ultimately empty into the lagoons of Penitencia and Tortuguero, and their combined waters disgorge at the Tortuguero inlet. The natural vegetation of the lower Tortuguero basin is of two basic types. Yolillo palms Raphia taedigera blanket broad pans of regularly flooded gley soils and are

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perhaps the most striking feature of the lower coastal plain. On better drained alluvial soils in the lower basin, a mixed hardwood forest arises which continues inland to the Guapiles region with phenological changes associated with the increasingly well-drained soils (Nuhn & Perez, 1967). The river system arises within this originally heavily forested terrain, not in the distant mountains. In the coastal half of the basin the erosive potential of the heavy rainfall is absorbed by the forest canopy, the dense root mats of the rainforest, and the shallow pans of palm swamps. The restrained flood waters run off gradually, continuing into periods of reduced rainfall. The quieted waters permit mineral sediments to settle and organic compounds from forest litter to dissolve into the surface flow. LAND USE IN THE WATERSHED

The watershed of the Tortuguero basin is no longer entirely in its natural state. In the lower basin there are extensive tracts of intact, if not virgin forest, but extensive development has occurred in the upper basin, and along the coastal fringe. Present land use falls into five major categories: 1. 2. 3. 4. 5.

Extensive agriculture Shifting agriculture Urban development Selective logging National park reserves.

Extensive agriculture takes three forms. Along the coast, adjacent to the beach, are found traditional coconut plantations owned and managed either by coastal villagers or by absentee landowners. These plantations are stable and require little maintenance apart from undergrowth clearing. The sale of coconuts and copra has been a principal source of cash for coastal families for several generations (Frost, 1974). A second traditional major agricultural practice is the cultivation of bananas on plantations more or less in the upper third of the drainage basin, and continuing beyond the boundaries of the Tortuguero basin. The banana monocultures totally exclude native flora over hundreds of square kilometres; the soils of the farms are kept bare of even small shrubs and grasses. Trees not only occupy desirable agricultural space, but pose a hazard to fumigation aircraft. Thus, even trees along rivers banks and in marginal areas of the plantation are usually removed. To shed the excessive rainy season water from these cultivated expanses, networks of ditches and canals are dug to nearby rivers. Quantities of pesticides and other chemicals are applied to the plantations, many of which inevitably yield residues to runoff. In addition, the runoff carries substantial volumes of top soil, adding to the sediment load of streams and rivers.

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The threat of banana plantations to the Tortuguero estuarine system is alleviated by two related circumstances. One of these is that the present plantations are located inland of the zone of heaviest rainfall. Thus, the potential for erosion and stream alteration is less than it might be were the plantations further east. Secondly, according to Standard Fruit Company and other banana company executives (pets. comm.), the eastward expansion of the plantations has already reached its limits, the more coastal land being currently unsuitable for banana cultivation due to inferior soils, excessive rain and poor drainage. Thus, the ecological hazard of banana agriculture appears to be contained, and recommendations to reduce its current impact on aquatic ecosystems will be presented below. However, the third form of extensive agriculture, cattle ranching, is just beginning to spread. Rangelandfor cattle, to provide beef for export, has expanded eastward during recent years, beyond the area of banana plantations. According to a distinguished cattle man of the Tortuguero basin, Teodoro Quiros (pers. comm.), the lush pastures of the Atlantic slope support four times as many cattle per hectare as the traditional, arid, Pacific coast rangeland. Mr Quiros, previously Minister of Agriculture, is optimistic about the future and has encouraged road construction and other developmental projects to support the increasing Atlantic beef industry. Resident terrestrial ecologists do not share his optimism. According to Dr Leslie Holdridge and Dr Joseph Tosi of the Tropical Science Center in San Jose (pers. comm.), soils of the Atlantic slope will not survive cattle ranching due to erosion following removal of the forest canopy and to soil compaction by cattle hooves. Notwithstanding uncertainty on the sustainability of cattle ranching, it is proceeding, and the long-term impact on soil and water have yet to be precisely identified. The problem is that the 'experimental plots' are measured in thousands of hectares, their expansion is uncontrolled, and they require the irretrievable removal of forest. As best as can be determined, cattle ranching is projected to extend to a northeast-southwest line 20-25 km inland from the coast. Assuming full development, this would commit approximately 50 ~,,, of the Tortuguero watershed to extensive agriculture, either to bananas or to cattle ranching. Under present Costa Rican land-use custom, all of the forest over this area can be assumed to be earmarked for clearing. Labour forces for banana growing and packaging, cattle ranching, and for assorted support enterprises, have led to a gradual growth of residential and urban sites. Most of this development has occurred around Guapiles and Roxana located at the heads of major rail spurs connecting with the railroad line to the Caribbean port of Limon. New and improved roads have facilitated automobile and bus traffic into this isolated region in recent years. Residential expansion in the Tortuguero watershed follows agricultural development and spreads over land already deforested by farming. Accordingly, during early stages of urbanization, the estuarine manager is concerned less with municipal

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alterations to terrestrial ecology than with the wastes released from the cities to the aquatic system. Given what must be assumed to be a great dilution factor in the Tortuguero river system, municipal effluents have not yet become a serious factor, but as these effluents increase and are combined with the nutrient loads emanating from the banana and cattle farms, enrichment and eutrophication in the estuary may become serious, dilution rates notwithstanding. At present, there are no obvious signs of eutrophication in the Tortuguero estuary. It is very possible that the swamp and aquatic macrophytic assemblages between the developed area and the estuary can absorb substantial concentrations of nutrient enrichment without significant ecological change. If so, it is an unplanned, fortuitous phenomenon worthy of protection and further study. Within the forested fraction of the Tortuguero watershed, one encounters traditional, shifting agriculture. Single families follow the river courses, find suitable sites, clear the area and plant corn and a few other vegetables. These farms are small, and in the past, were abandoned when productivity dwindled. More recently, a practice has arisen whereby two or more squatters develop adjoining plots, farm them for a few years, and then sell the combined land to individuals who speculate on the cattle ranching potential. This frontierism is popular in political circles, but it has the random aspect of unplanned growth, and results in the destruction of forest communities and the loss of forest products before there is any demonstrable social need for the growth or the losses. Timber harvesting in the lower Tortuguero watershed is an important enterprise. The growth of interior cities and the export demand for plywood and other wood products make the timber market virtually insatiable, and there is a steady flow of log booms down the canals and rivers to the rail and truck terminals at Moin near Limon. Timber has been cut by Tortuguero residents for over 50 years. During this period a series of small sawmills have operated near the village. When these are not functioning, saw logs are shipped to Moin. The traditional timber enterprise has two important characteristics: (1) it is labour intensive, providing substantial employment to the small coastal communities; and (2) it focuses upon only a few tree species. Consequently, after the lumberers leave an area, the forest remains despite the loss of individual trees. Even the selected timber species are not annihilated: both law and the market place relegate against the cutting of young trees. Thus, both ecological qualities important to the estuarine manager, and future economic resources of the forest, are preserved. Less balanced forestry proposals have been advocated for the lower Tortuguero basin. American, European, and Japanese interests have, from time to time, attempted to obtain tracts of land for plantation forestry (Guillermo Cruz, pers. comm.), wherein the existing forest is levelled and replanted with even-age stands of exotic trees. Large developments of this type are presently underway in the Amazon basin (Goodland & Irwin, 1975) and elsewhere in the tropics.

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The risk inherent in these proposals is that in the heavy rainfall zone of the lower Tortuguero watershed, plantation forestry may contribute as much to erosion, siltation and hydrological instability to the estuarine system as banana growing. Another entrepreneurial recommendation is the exploitation of the palm swamps, either for paper or for stakes to support banana trees, or for cattle or water buffalo grazing after water-tolerant grasses are planted in place of the palms (Guillermo Cruz, pers. comm.). Fortunately, none of these proposals for the lower watershed has yet materialized for various political or economic reasons. A final major commitment of land in the Tortuguero watershed is the 19,000 ha Tortuguero National Park (Fig. 4), bounded to the east by the Caribbean Sea, and including the entire Tortuguero Lagoon--although the 6.5 km lower estuary is not within the park boundaries--extensive palm swamps and substantial areas of mixed rainforest. Apart from its value as a refuge for marine turtles and rare terrestrial and aquatic mammals (Ugalde, 1975), the park is a great encouragement to the prospects for a healthy estuarine system. LAND USE AND MANAGEMENT RECOMMENDATIONS

Land management proposals all too frequently emerge after irreversible commitments of land are made to both food and bad courses of development. The present proposal is no exception with respect to the upper watershed of the Tortuguero river system. In the lower basin, application of ecological and economic research might yet offer guidelines for the most judicious utilization of the land. The lack of any detailed plans by the government for land use in the lower basin may work to the benefit of the estuarine manager if he is able to explain the significance of the forests and palm swamps of this region to the continued productivity of the estuary. Concern for the rational management of the Tortuguero area has been expressed by the Ministry of the Presidency of Costa Rica in the terms of agreement of a 1980 research contract with the Tropical Science Center, San Jose, Costa Rica, entitled 'Potential areas for national parks and equivalent reserves in Costa Rica' (Joseph Tosi, pets. comm.). Broad land-use recommendations can be made for each of the three major terrestrial components of the Tortuguero watershed: (1) In the agriculturally developed upper basin, emphasis must be placed upon containment of sewage (point source pollution) and agricultural runoff (non-point source pollution), including the erosion of topsoil. Vigorous implementation of existing statutes would be helpful. (2) The coastal strand is lightly populated, and development there must be carefully integrated with other land-use concepts, such as the formation of a forest reserve for the lower Tortuguero basin. (3) Appropriate protection for the forests and palm swamps of the lower Tortuguero basin could be achieved by designating the entire area outside the

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National Park as a national forest or equivalent reserve. The forest reserve would extend from a line on or near the coast, inland approximately 25 kin, and stretch the width of the Tortuguero drainage basin (Fig. 4). Under this type of administrative classification two general goals could be achieved: (a)

(b)

Yolillo palm swamps could be protected from any form of commercial exploitation or disturbance in the interest of soil and water conservation, flood control, and fish and wildlife production. The ecological integrity of the rainforest could be maintained, while permitting regulated selective logging and harvests of other forest products.

Plantation agriculture and plantation forestry would be specifically excluded from the forest reserve. Managed in this way, the lower basin would provide an economic yield, one that is sure to grow in value with the passage of time; and the ecological relationship of the forest to the estuarine system would be preserved. In a study sponsored by the conservation-minded Sierra Club, Hamilton (1976) recommended a similar, selective harvest approach as the most rational manner in which to conserve the complex resources of the excessive rainforests of Venezuela. It might be proposed that the lower basin be added to the Tortuguero National Park and managed under far more restrictive policies. From the standpoint of pure conservation, this might indeed be a desirable goal. However, if the swamps are protected and clearcutting of the forest is prohibited, the major ecological considerations of concern to estuarine management would be provided for by creation of a forest reserve. Furthermore, the protected forest would benefit the National Park, serving as an extensive buffer zone and added terrain for numerous species of characteristically wide-ranging birds and mammals. The forest would also provide opportunities for high quality recreation, chiefly in the form of fishing and nature viewing along the numerous forested streams. These are activities which are increasing in popularity in Tortuguero, irrespective of forest management policy. Water-borne outdoor recreation within the forest would provide an additional asset to the profits of timber and other forest products. Considering the Tortuguero estuarine system in toto, an area generally encompassing both the near-shore ocean, the estuary and lagoon, and the rivers and streams leading to them, a lucrative outdoor recreation package recommends itself. West (1976) arrived at this conclusion independently, and called for national and international planning for recreational enterprises along the Atlantic coast of Central America, while minimizing other forms of high-impact development. Realization of these values would be assured by the proposed forest reserve. It would hold soils in place, ameliorate water runoff, maintain the present salinity regime, provide a supply of organic matter to the estuary, and buffer the effects of intensive land development inland of the forest. Were it not for the amount of rain (5-6 m) in Tortuguero, commitment of such

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large tracts of land to forest reserves would not be as critical to the m a i n t e n a n c e of the aquatic ecosystem. However, there is n o t h i n g that can be d o n e a b o u t the weather. It is therefore i n c u m b e n t u p o n m a n to adjust land use appropriately.

ACKNOWLEDGEMENTS Research leading to this report was supported by grants from the N a t i o n a l Science F o u n d a t i o n (P4A032700 a n d DES7306453), an award from the Society of Sigma Xi, a n d a fellowship from the O r g a n i z a t i o n of A m e r i c a n States. I a m also i n d e b t e d to the O r g a n i z a t i o n for Tropical Studies, the C a r i b b e a n C o n s e r v a t i o n C o r p o r a t i o n , the N a t i o n a l Park Service of Costa Rica, a n d particularly the m a n a g e r s of the T o r t u g u e r o N a t i o n a l Park for vital assistance in the field; a n d to the New York Zoological Society for travel support.

REFERENCES

CARR, A. F., III (1979). The ecology of the prawn, Macrobrachium acanthurus (Weigmann) and its implications for tropical estuarine management. PhD dissertation, University of Michigan, Ann Arbor, Michigan. BRINSON, M. M. (1973). The organic matter budget and energy flow of a tropical lowland aquatic ecosystem. PhD dissertation, University of Florida, Gainesville, Florida. CRONIN, L. E. (1967). The role of man in estuarine processes. Pubis AmAss. Advmt Sci., 83, 667-89. FROST,M. D. (1974). Man-wildliferelationships in a tropical lagoon area: The case of Tortuguero (Costa Rica). Revista Geografica, Mexico, $1, 155-64. GOODLAND,R. J. A. & IRWIN,H. S. (1975). Amazon jungle: Green hell to red desert? In Developments in landscape management and urban planning, 1. Amsterdam, Elsevier Scientific Publishing Co. HAMILTON,L. S. (1976). Tropical rainforest use and preservation: A study of problems and practices in Venezuela. Sierra Club Special Publication International Series, No. 4. HIRTH, H. F. (1963). The ecology of two lizards on a tropical beach. Ecol. Monogr., 33, 83-112. LIKENS,G. E. & BORMAN,F. H. (1974). Linkages betweenterrestrial and aquatic ecosystems. BioScience, 2, 4747-56. NORDLIE,F. G. & KELSO,D. P. (1975). Trophic relationships in a tropical estuary. Revta Biol. trop., 23, 77-99. NUHN,H. & PEREZ,R. S. (1967). Estudio Geografico Regional: Zonal A tlantico Norte de Costa Rica. San Jose, Costa Rica, Instituto de Tierras y Colonizacion. PORTIG,W. H. (1965). Central American rainfall. Geogr. Ret,., 55, 68-90. UGALDE,A. F. (1975). Fund application to 1UCN for the protection of Tortuguero National Park. San Jose, Costa Rica, Fundacion de Parques Nacionales (Unpublished). WEST,R. C. (1976). Conservation of coastal marine environments. Revta Biol. trop., Suppl. 1,187-209.