Ecosystems, paleoecology and human disturbance in subtropical and tropical america

Quaternary Science Reviews, Vol. 6, pp. 115-128, 1987. 0277-3791/87 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1987 Pergamon Journals Ltd.

ECOSYSTEMS, PALEOECOLOGY AND HUMAN DISTURBANCE IN SUBTROPICAL AND TROPICAL AMERICA

Michael W. Binford Graduate School of Design, Harvard University, 48 Quincy Street, Cambridge, MA 02138, U.S.A.

Mark Brenner, Thomas J. Whitmore, Antonia Higuera-Gundy, E.S. Deevey Florida State Museum, University of Florida, Gainesville, FL 32611, U.S.A.

and

Barbara Leyden Department of Geology, University of South Florida, Tampa, FL 33620, U.S.A.

Human disturbances of ecosystems last a long time and have quantifiable influences on the structure and function of the systems. If long records (e.g. paleoecoiogical) of both disturbances and the responses are available, the array of disturbances provides quasi-experimental treatments useful for the study of factors which govern ecosystems. This paper examines the paleoecology of a series of lake-drainage basin ecosystems that have been subject to disturbances which vary through time and space. In all cases studied, it has been demonstrated that human activities have increased the movement of materials from the catchment to the lake. Examples in Guatemala, Haiti and Florida demonstrate that the flow of macronutrients (nitrogen and phosphorus) is proportional to human population sizes, and that the flow of inorganic particulates is related to the nature of both the disturbances and the catchment. Lake eutrophication is driven by growing human populations, but the rate of increase can be slowed by activities such as urbanization, which increases siltation. Several tropical ecosystems have recovered from severe disturbances, but the rate of recovery was related to the severity and temporal extent of the disturbances.

INTRODUCTION

Ecology is a science of observation and experimen- tation on extant ecosystems. It is also a Quaternary science because evolutionary histories of ecosystems take place in geological time. The structure and function of ecosystems are molded by processes that occur at different rates (Delcourt et al., 1983), and understanding of the mechanisms requires study at appropriate spatial and temporal scales (O'Neill et al., 1986). Studies of some long-term or very slow processes are most effectively accomplished by paleoecological techniques (Binford et al., 1983).

Tropical ecosystems in particular are under increasing stresses brought about by a combination of social, political, economic and ecological factors. It is essential that ecologists understand, and be able to predict, the effects of these stresses in order to play a role in the planning and management of development in the tropics. Development will occur; human population growth in lesser developed countries is simply too rapid to think otherwise.

The objectives of this paper are to review the theoretical bases for paleoecological studies of ecosystems, and to review several studies in tropical and subtropical America as examples. In doing so, we intend to clarify the role of paleolimnologically based ecosystem studies in the tropics.

We are concerned only with the period of the Quaternary during which natural systems have been stressed significantly by Homo sapiens, which is roughly the past 10 ka. Specifically including humans is important for two reasons. First, agriculture, urbaniz-

ation, waste disposal and other cultural activities are as much a part of ecosystem processes as decomposition or nitrification by microbial communities. Every ecosystem on earth, except perhaps hydrothermal vents on the ocean floor, is influenced by humans in some way. Second, different kinds and magnitudes of human disturbances provide 'treatments' for quasi-experimental studies suitable for hypothesis testing (e.g. Deevey, 1969).

ECOSYSTEMS AND SCALE

Large ecosystems are usually long-lived, and changes of the entire system usually occur slowly. The relationship between the size of a system and the temporal scale of disturbances and biological responses has been described by Delcourt et al. (1983), and incorporated into the principles of landscape ecology by Forman and Godron (1986). These two treatments are concerned mostly with vegetation, but the relationship between spatial and temporal scales is applicable to functional aspects of ecosystems. 'Vegetation' is one of many ecosystem components that are influenced by, and in turn influence, system-scale processes.

Ecologists have only recently begun to study the long-term factors which govern ecosystems, and while they are found to be important, in most cases the historical information required for rigorous definition of past environmental conditions is lacking or inadequate (Likens, 1983). A few exceptions to this statement are the classical studies of chronosequences for demonstration of succession (Cowles, 1899), examination of the development of biological production and

115

116 M.W, Binford et al.

nutrient cycling during succession (Gorham et al. , 1979; Gholz and Fisher, 1982), and use of early surveys for studying pre-disturbance forests (Whitney, 1986; Del- court and Harris, 1980). Yet even these studies are concerned with contemporaneous systems of different ages, and assume that all factors besides the one being studied are identical.

THEORETICAL FOUNDATIONS

The unit of study for paleolimnologicai work is a lake and its associated drainage basin. Drainage-basin ecosystems have proven useful in physical geography (Oldfield, 1978), geomorphology (Chorley, 1969), water resources studies (Black, 1970; Gower, 1980), and ecosystem studies (Likens, 1984; Lotspeich, 1980). Lake-drainage ecosystems are definable in both space (on the landscape) and in time. If chosen properly, they have discrete, mappable divides that separate them, at least on the land's surface, from other drainage systems. Even if the subsurface drainage does not correspond with surface topography, the hydrological divide is, in principle, mappable.

Once the boundaries of the system are defined, we can measure the flux of materials and energy as well as the patterns of biological diversity, land uses and other ecosystem properties. For example, linkages between terrestrial and aquatic systems are factors that control ecological patterns and processes in lakes (Deevey, 1984; Likens, 1985). Material (e.g. nutrients) fluxes are often controlled by flowing water, whether surface or subsurface. Thus, measurement of hydrological conditions within the drainage basin is part of whole- system studies.

A unique characteristic of lake-drainage ecosystems is that they contain an historical record within the sediments of the lake (Frey, 1969; Pennington, 1981). This encoded record substitutes for, and in many cases is better than, historical documents. When appropriate

historical data do exist, care must be taken to ensure the comparability of modern and obsolete methods. Tech- niques developed in the past few years were not available for earlier studies and many scientific questions being asked today were not even considered until a few years ago.

While other kinds of records exist, lake sediments meet the criteria of continuousness, stability and interpretability necessary for studies of long-term processes in individual ecosystems. Physical, chemical and biological characteristics of sediments are determined by environmental conditions within both the lake and its drainage basin. The sediments, which accumulate in an ordered manner through time, contain a continuous record of processes and events of the past. Modern analogs of the sedimentary characteristics are found in extant lakes and are useful for both quantitative and qualitative interpretations.

Following Likens and Bormann (1974), we take ~ process-oriented view of lake-drainage basin ecosystems (Fig. 1). Materials come into a lake by atmospheric fallout, geological weathering, and biological transfer. They are lost from the lake water by volatilization, outflows, emerging organisms, or sequestering in the sediments (from which material can cycle back into the water column). Within the lake there are many transformations that affect the distribution and abundance of elements and organisms. These latter transformations are the primary subject of limnology.

Of the inputs, transformations and outputs, paleo- limnologists are most interested in the transfer of material to the sediments. We assume that relationships exist between fluxes of materials from land to lake water and from lake water to the sediments. Once the relationships are defined quantitatively, they can be worked backwards from measurements of sedimentary properties to estimates of limnological or drainage- basin conditions (Fig. 2).

WEATHERING

PREC~ITATION

EVAPORATION

DRY FALLOUT

SEEPAGE SEDIMENTATION

OUTFLOW

REGENERATION

SEEPAGE

FIG. 1. Diagrammatic representation of a lake and its drainage, showing the major pathways by which water and other substances enter and leave the lake. Investigation of historical conditions in the lake and watershed is accomplished by stratigraphic analysis of accruing lake sediments.

Ecosys t ems , Pa l eoeco logy and H u m a n Di s tu rbance in A m e r i c a 117

PROCESSES

Fm

2" rT / / / C ~ / / / / /

M O D E L S

Gin= f ( F m )

F~n = f (C m)

G~= f (F~n)

.'. C~n = f (F m )

c~c F~,

I F m

e,~ - m 1

3" f ~ ~ ~ C~cC ~ ~ ~ ~ ~m-~-°d" / / / / / / / /

F = f l u x C = c o n c e n t r a l i o n m = any m a t e r i a l

FIG. 2. Thcorctical expressions of quantitative material flux between terrestrial and aquatic systems and from water column to sediments. The sedimentary variables can bc mcasurcd palcolimno]ogically. ] f the functions arc known, they can be worked backward to estimate past fluxes from land to water or past water concentrations. Model I illustrates the relationship between flux of a material (Fro), e.g. phosphorus, from the land, and lakcwater concentration (Cm). The function C = f(F) can take any form. One of many useful functions is that developed by Dillon and Rigler (1974), which predicts phosphorus concentration as a function of annual loading rate, water renewal time, and loss to sediments. Model 2 expresses the relationship between the flux of a material (F~) from the water column to the sediment, and the concentration (C~) of the material in the sediment, assuming no mixing of the upper layer and constant sedimentation rate (r). Again, the function may take any form, and in this case is based on material flux and sedimentation rate only. Model 3 assumes a constant depth of mixing (d), which is a modification to the

second model (designated by double prime).

The basic questions of long-term ecosystem dynamics are concerned with the magnitude and rate of change of each of the processes and patterns. According to Deevey (1984), 'The objective of general ecosystem theory is explanatory description and prediction of a system's trajectory in time'. The specific trajectory in which we are interested is that of ecosystem-scale material flux. Nutrient flux to the sediments is examined as a function of flow to the lake, and thus a measurement of output from the drainage basin.

Our working hypothesis is that changes of material flux to sediments are consequences of changes of the flux to and through a lake (Fig. 2). Any disturbance in a drainage basin changes fluxes, usually increasing them. Naturally occurring flux from terrestrial systems in various stages of succession or development are measured at times before cultural disturbance occurred. Changes in material flow caused by cultural disturbances are studied by knowing the type and magnitude of disturbance and measuring the timing, rate of change, and magnitude of change of accumulation rates of several components of the sediment (see Fig. 3 in Binford et al., 1983).

Our approach to these questions has been summar- ized in two papers (Deevey, 1984; Binford et al., 1983), and will not be recounted here. Instead, we will present three cases that illustrate effects of human activities on material-flux aspects of long-term ecosystem dynamics. The study areas are all located around the rim of the Gulf of Mexico and the Caribbean Sea (Fig. 3). The studies address the effects of human activities on

different temporal scales, but all stay in the spatial context of lakes and drainage basins.

PALEOECOLOGICAL STUDIES OF MAYAN GUATEMALA

The Central Peten Historical Ecology Project (CPHEP) is an assessment of the impact of long-term human settlement on a tropical lowland, Karst environment. Maya civilization arose, persisted for several millennia, and then collapsed in this ecological context. The project studied several basins in the Peten region of northern Guatemala by paleolimnological and archaeological techniques.

The study sites lie at 17°N latitude, and range along a line about 40 km either side of 89°30'W longitude (Fig. 3). The hilly terrain varies from 100-300 m above m.s.l., and is underlain by Cretaceous and Tertiary limestones. At some sites the rock contains high amounts of gypsum or dolomite (Vinson, 1962; West, 1964). The basins of the major lake chain are located in a graben. Mean annual temperature in the region exceeds 25°C (Vivo Escoto, 1964). Annual precipi- tation varies from 900-2500 mm, with a regional mean of about 1600 mm/year (Deevey, 1978).

Forest soils, mostly black calcareous lithosols or calcimorphic rendzinas, cover 89.9% of the Peten region. Savanna and low-lying 'bajo' soils, usually hydromorphic clays, occupy 9.8% of the region. The remaining 0.4% of the area is represented by lake surface (Simmons et al., 1959; Stevens, 1964).

The Peten is located in a tropical lowland dry forest life zone (Holdridge, 1947). The forest vegetation is diverse, varies locally, and is typically evergreen, but the amount and distribution of rainfall influences deciduousness. Lundell (1937) referred to the vegetation as 'quasi-rain forest'. Much of the forest belongs to the zapotal association, which is dominated by Manilkara (chico zapote) (Wagner, 1964). Other common high-forest taxa include Brosimum, Swietenia, Calophyllum, Pouteria, Cecropia, Bursera, Spondias, Crysophila, Ficus, and Piper (Lundell, 1937; Leyden, 1984).

Maya Prehistory The record of earliest Maya settlement in the region

is at Cuello, in northern Belize, where occupation was established by the 3rd millennium B.C. (Hammond et al., 1976). The oldest known Maya settlements, at sites such as Altar de Sacrificios and Seibal (Adams and Culbert, 1977) in the Peten date to Middle Preclassic times (1000-250 B.C.). Middle Preclassic house mounds have also been found in all lake-drainage basins examined by the CPHEP (Rice, 1976, 1986; Rice and Rice, 1980). Following initial settlement in the Middle Preclassic, populations grew and expanded into new areas. At many sites, populations grew steadily, while at others they declined during the Terminal Preclassic (100 B.C. to 250 A.D.) and Early Classic (250-550 A.D.). Many of the abandoned sites were

118 M.W. Binford et al.

• , , . . . . . ,

.2oo. G. , ,o , . . . ,oo \ o ,oo ,ooo

I . . . . . . . . . . . . . . . . . . . ~ . . . . -¢romC o,

\ . ,x ,co \ } ¢, o . - .

P.©I ' I c Oo.,In I ~ " f ( / " ~ ' " " ~ SOUTH AMERICA

, o o : . . o : . , ,

FIG; 3. Regional map of the Caribbean Basin showing the locations of paleoecological projects discussed in the text. Within the tropic, efforts have been focused on the Peten lake district in the Maya lowlands of northern Guatemala, and at Lake Miragoane in the southern peninsula of Haiti.

Outside the tropic, Florida's numerous subtropical basins have been examined.

reoccupied in the Late Classic (550-850 A.D.). The entire region was densely occupied near the end of the Late Classic (Rice and Rice, 1980; Rice and Puleston, 1981). Regional estimates of Late Classic populations in the Maya lowlands vary from 3 × 106 persons (Thompson, 1966) to 14 × 106 (Adams et al., 1981).

Classic Maya civilization ended abruptly in the 9th century A.D., and the population declined significantly (Willey, 1956). Several hypotheses, including warfare, disease, earthquakes, and soil degradation have been proposed to account for the decline (Adams, 1973). Postclassic settlements persisted at some sites (Rice and Rice, 1984; Rice, 1986), but populations never recovered to levels attained in the Late Classic. The campaign of Hernan Cortes in 1525 A.D. marked European entry into Peten, and colonial domination was secured in 1697 A.D. with the defeat of the Itza Indians. Population densities have remained relatively low during the last three centuries, although recent immigration of Guatemalan highlanders accounts for substantial population growth over the last few decades. The modern population has increased from 25,910 people in 1964 to over 200,000 in 1980 (Schwartz, 1977; Castellanos Lopez, 1980).

Archaeological Methods Maya population densities were estimated by a

program of archeological survey and pit-sampling of house mounds. Calculations assume that 84% of the mapped mounds were residential structures and that 5.6 people lived in each house (Haviland, 1%9). Reported population densities are those reached at the close of an archaeological period. Figure 4 shows demographic histories of five Peten drainage basins.

Field work over a number of years raised several sediment cores from six basins (Lakes Yaxha, Sacnab, Quexil, Macanche and Salpeten; earlier studies examined cores from Lake Petenxil: Cowgill et al.,

? E

O U ~

0 )

n

200 SACNAB

20!t 10 QUEXlL

i i

l oo 1oo r . . . .

0 , , , ,

200300 1100 SALPETEN /

o , r ~ - ~ - ~ 1 1000 0 1000 2000

B.C.~,--- (Date) - - - - ~ A.D.

Age

FIG. 4. Crude historical Maya population dcnsitics for five Peten watersheds that have been examined archaeologically and paleolimnologically. Densities are numbers achieved at the termination of an archaeological period. Values omit nucleated Postclassic settlement confined to islands or peninsulas. Population reconstructions are based on archaeological data provided by D.S.

and P.M. Ricc. (Figure modified from Brenner, 1983a).

1966). Most of the cores are either short, sediment- water interface samples that represent less than 1000 years of sedimentation or longer Holocene sections. Two long cores taken from Lake Quexil and Lake Salpeten in 1980 included sediments from the late Pleistocene (Deevey eta/., 1983; Leyden, 1984).

Maya Impact On Riparian Ecosystems Pollen analysis of deposits from both Quexil and

Salpeten shows that late Glacial vegetation consisted of marsh, savanna and juniper scrub (Leyden, 1984).

Ecosystems, Paleoecology and Human Disturbance in America 119

Immediately after an apparent climatic change about 10,750 radiocarbon years BP, the surrounding vegetation resembled temperate forest, which was soon replaced by mesic tropical vegetation that persisted until felled by the earliest Maya inhabitants about 3000 BP. During Maya times (3000-400 BP), the high forest taxa (Brosimum, Chlorophora, Ficus, Manilkara, Bombax, Thouinia and Sapium) declined relative to non-arboreal (Gramineae, Ambrosiae) and grassland- arboreal ( Byrsonima, Quercus, Melastomataceae) types (Deevey et al., 1979; Leyden, 1984; Vaughan et al., 1985). The high forest was reestablished throughout the Peten during the past four centuries, and modern vegetation disturbance is localized near present population centers.

Water in all the Peten lakes contains a considerable amount of carbon derived from the dissolution of limestone, which contains no 14C. Organic matter that incorporates the 'dead' carbon, settles to the bottom of the lake, and is later used for radiocarbon dating, may appear to be older than its true age. This 'hard-water lake error' (Deevey and Stuiver, 1964) has confounded dating Peten sediments since the inception of the project.

Alternative methods, anchored On a couple of reliable dates from fragments of terrestrial wood, were developed. Horizons in cores from Lakes Yaxha, Sacnab and Quexil were assigned ages by correlation of pollen stratigraphy and archaeological studies (Deevey et al., 1979; Vaughan et al., 1985). Changing pollen percentages between 3000 and 400 BP reflect human disturbance of vegetation, and the degree of deforestation is assumed to be associated with shifting population density and cultural development. Dates assigned to pollen zones are therefore based on the cultural chronology for the region.

Forest removal was associated with a change of the lithological composition of the lake sediments. De- forestation for agriculture and construction caused increased erosion of upland topsoils, which was trans- lated to increased net accumulation rates of phosphorus and inorganic matter in the sediments (Table 1). When organic surface soils were depleted, sediments became

increasingly inorganic and dominated by siliceous silts and clays, as well as carbonates (Brenner, 1978; Deevey et al., 1979; Brenner, 1983a), and the m e a n

particle size of eroded material deposited on the bottom decreased by a factor of four (Binford, 1983b). When the riparian land was abandoned about 400 BP, organic deposits once again accumulated on the lake bottoms.

Maya populations around Lakes Yaxha and Sacnab grew steadily from Middle Preclassic through Late Classic times, and phosphorus accumulation rates tracked the riparian populations (Brenner, 1978; Deevey et al., 1979). Phosphorus input to sediments appeared to proceed at a rate of about 0.5 kg P per person per year, which is roughly equal the annual physiological output rate for humans (Vollenweider, 1968). Although the rate was consistent with physiological output, most of the phosphorus reaching the lake edge probably arrived as colluvium or alluvium. Analyses of soil profiles from Petcn watersheds demonstrate that phosphorus is much more concentrated in the easily eroded surface soils than in lower horizons (Deevey et al., 1979; Brenner, 1983b).

Accumulation rates of inorganic materials (mostly carbonates and silicates), also increased with growing population numbers. Inorganic exports reflect down- wasting of the disturbed regolith as well as the number of people in the basin and the nature of their activities (e.g. swidden agriculture vs. causeway and temple construction: Deevey and Rice, 1980; Rice et al., 1985).

Depopulation of the drainages associated with Euro- pean contact permitted regrowth of the vegetation. Soil erosion stabilized, and the rate of nutrient and inorganic export slowed. Accumulation rates of phosphorus and inorganic materials at Lakes Yaxha and Sacnab were higher than during pre-Maya times, indicating either a lag between the cessation of disturbance and recovery of the ecosystem or continuation of sufficient disturbance of the riparian landscape for sustained high delivery of the materials to the lake- shore. Another possibility is that the pollen zonation may be in error, and revegetation may have begun much earlier (Wiseman, 1985).

TABLE 1. Rates of phosphorus loading and siltation for three Peten lakes during pre-Maya and Maya times. For computation, core zones were delimited based on changes in organic matter content (LOI 5500C) or organic carbon

content

Core Zone

Inorganic Phosphorus Years Depth in Accumulation Accumulation

in Core Rate Rate Zone (cm) (mg cm -2 year -t) (~g cm -2 year -t)

Quexil Maya 2600 230-80 8.9 2.9 Shallow-Water Pre-Maya 7000 642-230 2.4* 1.6*

Quexil Maya 2600 590-210 38.7 12.0 Deep-Water Pre-Maya 7000 920-590 4.1" 3.2*

Salpeten 80-1 Maya 2600 845-160 138.1" 39.4* Pre-Maya 7000 1025-845 5.7 4.1

Macanche 80-1 Maya 2600 585-85 67.0 24.1 Pre-Maya 7000 1070-585 29.7* 20.7*

* Incompleteness of section causes slight underestimate of computed zonal accumulation rates.


Figure 5 (a and b) summarizes the effects of Maya activities on several ecosystems. Contrary to our original expectations, as phosphorus loading increased, aquatic productivity as measured by accumulation rate and concentration of aquatic microfossils decreased. Brenner (1983a) speculates that higher silt load in the water column could have decreased primary production by shading and could also have diminished secondary production by clogging the feeding apparatus of aquatic herbivores.

PALEOECOLOGY OF LAKE MIRAGOANE, HAITI: SHORT CORE

The Lake Miragoane basin (Fig. 3) lies in an area that is of interest for several reasons in addition to the ecosystem questions. First, paleoclimatic studies in mainland Central and South America demonstrate an arid climate in the late Pleistocene, during which much of the tropical forest may not have existed (Binford, 1983a; Bradbury et al., 1981; Leyden, 1984, 1985). If the climate of the Caribbean islands was as arid, then many of the tests of theories of island biogeography are invalid because changing environmental conditions would not allow the biota to reach an assumed equilibrium. Also, with lowered sea level, many of the Caribbean islands may have been interconnected and thus not strictly isolated from each other - - immigration and exchange of species would have been much easier. The only paleoecological information available in the Caribbean has been from deep-sea cores, in which the temporal resolution over the last 18 ka is relatively coarse. A more detailed picture is desirable for high-resolution description of changing climates.

Second, biogeographical studies by Woods (in press) and Woods and Ottenwaider (1983) suggested that Lake Miragoane lies between two regions that may have been refugia for forest mammals and birds during Pleistocene glaciations. One of the tests of refugium theory is the stratigraphic demonstration that regions between postulated refugia were not good habitat for forest biota (Livingstone, 1981), and Miragoane is ideally sited for such a test.

Third, much of the work in Maya Guatemala has proceeded without modern analogs. The human population density around Lake Miragoane and the general geology of the drainage basin might represent conditions similar to Maya times in Central America.

Fourth, the history of human habitation in the Antilles is not well understood (Meggers and Evans, 1983). Evidence of agriculture or other human activities in a stratigraphic context would be a significant contribution to the understanding of human migration in the Americas over the past 10 ka.

Geology, Vegetation and Human Occupation Lake Miragoane lies on limestone terrain in a narrow

(20-40 km) peninsula of southwest Haiti (Fig. 3). The lake surface is 20 m above sea level within a lowland,

dry tropical forest zone with mesic forest and pine communities growing at higher elevations.

The history of human activity in the Caribbean may have begun about 4-5 ka BP with the immigration of early paleoindians (Meggers and Evans, 1983). Ceramics of different styles have been excavated from three locations near the lake. The oldest pottery is dated 600 A.D. Numerous archaeological sites contain- ing more recent pottery (900-1500 A.D.) have been unearthed along the margins of the peninsula, including a site near the lake (Rouse and Moore, 1984). This archaeological information does not indicate whether human occupation of the Miragoane area has been continuous since 600 A.D. People of the ceramic age, the Arawak, practiced fishing and cultivated cassava (Manihot), as the staple crop, and maize (Rouse, 1948).

The aboriginal population of Hispaniola at the time of Columbus's arrival was estimated to be 4 million people by de Las Casas, but it could have been as high as 8 million people (Cook and Borah, 1971). Based on the latter figure, the average population density of the island was 105 people per km 2. Contact with European activities and diseases devastated the natives, and they were almost extinct by 1535.

Modern population density of Haiti is about twice the average estimated for the whole island upon European occupation. Densities of up to 800 people per km 2 of agricultural land have resulted in severe deforestation and soil erosion in most of the nation.

Paleoecological studies of sediments from Lake Miragoane provide evidence that the present environmental deterioration of the watershed began some 500 years ago. All the work to date is based on a single, short core from the approximate center of the lake (Higuera-Gundy et al., 1986). The 72-cm core from the sediment-water interface records the past 1000 years of the ecological history of the lake and its surrounding vegetation (Fig. 6). Chronology of the core was established by extrapolating 2~°Pb dates.

The periods of human activities are clearly divided into pre-Columbian and European. The pre-Colum- bian environment (1000(?)-1500 A.D.) is recorded in the bottom 30 cm of the core (Fig. 6). Sediments are rather organic and low in carbonate content, with no major lithologic changes. Very high concentrations of pollen prevail at all levels, which suggests the persist- ence of a widespread vegetation cover and slow lacustrine sedimentation,

Palynological evidence indicates presence of open forests in the watershed of Lake Miragoane. Upland mesic forests included Cecropia and several genera of Moraceae. Dry forest communities included trees such as Alchornea, Phyllostylon, Bursera and Caesalpinia. The sparsity of pollen of these taxa, along with presence of shrubs (mostly Melastomataceae) and abundant grasses suggests open vegetation. Trema, a successional tree, and Celtis were common.

Although archaeological data indicate settlements near the lake between 600-1500 A.D. (Rouse and

High 'll MAYA POPULATION

DENSITY 1

Low ..............

i

Forest ~'1 I

VEGETATION

Savanna

/- l l

High

T SOIL EROSION

l Low Iiiiiiiiiiiiiii .................................................... ,

10000 4000 3o'oo 2ooo ooo o

YEARS B.P.

FIG. 5(a). Summary diagram showing the impact of long-term Maya settlement on the terrestrial environment of the Peten lowlands. Generalized trend for Maya population increase begins in Peten about 3 ka BP and continues until the Late Classic collapse, about 850 A.D. Dotted lines indicate local population declines detected in several watersheds (see Fig. 4). Tropical dry forest, stable since the early Holocene, was progressively removed by expanding populations of Maya agriculturalists. Forests recovered following European contact and virtual depopulation of the region. Soil erosion was enhanced as Maya populations expanded and agro-cngincering efforts intensified. Population declines and forest

rcgrowth lcd to stabilization of local topsoils.

High

SEDIMENTATION RATE

Low .................................... , Organic

'lt SEDIMENT CHEMISTRY

Inorganic Z~Z~ .................................

High

PHOSPHORUS LOADING

Low

High 'll LACUSTRINE PRODUCTIVITY

Low 10000 4000

i

i t ' I t

I I

3000 2000 1000

/-

YEARS B.P.

FIG. 5(b). Summary diagram showing the impact of long-term Maya settlement on the Peten lakes. Bulk sedimentation rates and phosphorus loading accelerated as Maya population densities increased. Sediments deposited during the first 7000 years of the Holocene contain higher organic content than silt- and clay-rich muds of the Maya period. High export rates of inorganics (siltation) during the Maya episode not only changed proximate composition of the sediment, but appear to have suppressed lacustrine productivity. The lakes are currently mesotrophic and are

accruing organic deposits.

121

122 M . W . B in fo rd et al.

LAKE MIRAGOANE, HAITI

MUD/WATER INTERFACE CORE

Age

(years) 0

100

500

1000

20

c 40-

g a

60 o i

Total Pollen Concentration (grains g-1 dry sediment)

2__

(x 10 5)

Trees

---7

'/

Pollen (%)

Herbs

/

;-"-/U~-iU ' -10

Organic Matter (% L.O.I. 550"C)

0 20 40

Carbonates

(% L.O.I. 600-990"C)

20 40

FIG. 6. Data from the Lake Miragoanc, Haiti, mud-wa te r interface corc summarizing pollen and lithologic changcs during the last millennium. Total pollen concentration includes all taxa. The plot of arboreal taxa (trees) includes Moraceac, Cehis, Trema, and Bursera, but excludes Pinua and Cecropia, a common successional tree. Weeds include grasses, composites, and pollen of the cheno-am group. Deforestation is reflected in the declining representation of tree taxa in the upper half of the core and is associated with a shift from organic to more carbonate-rich deposils.

Moore, 1984), there is no evidence of human-induced disturbance of the local environment during the period. The settlements may have been too small, sparse, brief, or too far from the basin to leave a detectable evidence in the paleolimnological record. Alternatively, because the human populations at the time of European contact were large, the section we consider to be pre-disturbance may represent significantly deforested landscapes. We are examining a 7.4-m core with a basal date greater than 10,000 radiocarbon years BP to answer this question.

The sediments of the shorter core are rich in Cecropia pollen and carbonized plant fragments. Both parameters are indicators of swidden agriculture (Vaughan, 1979; Higuera-Diaz, 1983). In Lake Mira- goane's pre-Columbian deposits, however, these two fossils seem to reflect natural conditions: Cecropia-rich mesic forests, and fires in dry plant communities.

Pollen and chemical data from the top half of the core, which represents the post-Columbian environment (1500 A.D. to present), record a long episode of local deforestation and soil erosion in the Lake Mira- goane basin (Fig. 6). The beginning of the present environmental deterioration is contemporaneous with early European occupation of the watershed. Forest clearance has prevailed for the past 500 years and continues at present.

The transition zone (at 30 cm) into the episode of deforestation is marked by a decline in total pollen concentration, from 6.2 to 0.13 x 105 grains/g dry sediment. Low pollen content and carbonate enrich- ment characterize the sediments in the interval from 30 to 10 cm. This may be a result of both lower pollen production because of vegetation removal and higher sediment accumulation rate which is a result of increased soil erosion. The time period corresponds to Spanish (1500-1700 A.D.) and French (1700-1800 A.D.) occupation of Haiti and, apparently, the Lake Miragoane basin. Expansion of forest clearance and more intensive use of open land are evident from a

gradual decline of arboreal pollen and a four-fold increase in weeds in the zone.

Near the top of the core (6-8 cm), pollen of terrestrial plants attains concentrations similar to those recorded prior to apparent forest disturbance (below 30 cm). This increase in pollen suggests a brief period of forest recovery. Return of the forest may be related to changes in the use of agricultural land during the 19th century. After independence (1804 A.D.), Haitians divided the large plantations and returned to subsistence agriculture in small farms (Holdridge, 1945). Relaxation of land use and perhaps human emigration from the basin favored the partial recovery of the forest. Sediments of this interval show an increase in organic matter and decline in carbonates. The lithologic change reflects reduced erosion of the reforested soils.

Surface sediments contain only small amounts of plant pollen, including weeds. Under the pressure of a high human population, the watershed is now com- pletely deforested, with severely eroded soils and exposed bedrock. These conditions prevail in all the lowlands and most of the highlands of Haiti. The most vegetated area near the lake is the fringing marsh.

EFFECTS OF RIPARIAN DISTURBANCE ON THE TROPHIC STATE OF LAKES IN THE FLORIDA

PENINSULA

The lakes district of Florida (U.S.A.) stretches from about 30 ° to 27°N latitude, and from 81 ° to 86°W longitude (Fig. 4). Surface topography of the peninsula is formed by Karsted limestone buried under recent deposits. Although limestone bedrock underlies much of the state, many lakes do not contain hard water (Huber et al., 1982; Brenner et al., in press). Surficial material in which the lakes lie is mostly Pleistocene and Hoiocene sands with very low cation exchange capacity, although phosphate-rich marine deposits


occasionally outcrop. The lakes are hydrologically separated from the limestone by aquicludes of broad extent, which are breached irregularly.

Lakes of the district form one of the most limno- logically diverse groups in the world (Linthurst et al., 1986; Brenner et al., in press). Within a short drive one can find lakes of varied origin, morphometry, water chemistry, and biological productivity. The latter two characteristics are controlled largely by local geology (Canfield, 1981), and human activities (e.g. construction, phosphate mining, sewage disposal).

Modern human activities around the lakes provide a system for the development and testing of quantitative, paleolimnological measurements of ecosystem change as a function of disturbance. In terms of human population, Florida is the fastest-growing state in the U.S. Between 1970 and 1980, 7000 people moved permanently to the state each week (Morris, 1983; Brenner et al., in press). Although many people may settle in coastal communities, subdividing lakeside areas is a major activity of real-estate developers. The limnological effects of changing land use are concerns of limnologists, riparian landowners, government agencies and planners.

Extensive limnological studies in Florida lakes did not begin until the late 1960s and early 1970s. Many of the lakes may have undergone severe eutrophication by that time due to cultural impact, but the alterations were not documented as the changes took place. Consequently, studies of the effects of human activities must use a paleolimnological approach.

Social and economic conditions surrounding the development of basins provide an excellent set of 'experimental conditions'. Major disturbances began less than 100 to 150 years ago and are at a peak today. Census records, aerial photographs, phosphate mining atlases, tax records, government land-use records, agricultural surveys, and insurance records can be used to document land uses since Florida became a state in 1831. Pre-disturbance environmental conditions were qualitatively well-described (Bartram, 1791) and not as difficult to determine paleoecologically as conditions of much earlier times. Chronological resolution on a 21°pb time scale is relatively good, and pollen horizons corresponding to known introductions of exotic plants have been described (Binford and Snodgrass, 1983). Lakes without recent basin disturbances exist in the district, and many of them are protected.

Modern limnological research provides an extensive series of measurements on variables directly related to the ecosystem-level characteristics that are of the most interest, e.g. trophic state. Huber et al. (1982) compiled limnological measurements reported before 1981 on a sample of nearly 600 lakes. This collection contains measurements of water chemistry, biologically important nutrients, algal biomass, and other aspects of the biology of a wide spectrum of lake types. Preliminary studies suggested that the C:N ratio of sediments was negatively correlated with trophic state (Flannery et al., 1982). Brenner et al. (in press) and Williams et al.

(1985) summarize the state of limnological understanding.

We approach the questions of ecosystem responses to human disturbances by a two-phase program. First, the relationships between limnological and sedimentary characteristics in a wide range of lakes are defined. This procedure is a synoptic, or cross-sectional (sensu Reckhow and Chapra, 1983) study, and provides quantitative analogs of paleolimnological measurements which are represented by the function described in Fig. 2 as C' = f ( F ) . These quantitative relationships are called transfer functions (Sachs et al., 1977).

The transfer functions are applied to measurements made in 2t°pb-dated core sediments to reconstruct quantitatively the state and rate of change of limnological variables in lakes that have been differently disturbed (the 'experimental treatments'). This procedure is the diachronic study, which tests the modern analogs when predicted land-use changes are compared with observed patterns.

Trophic state, although a nebulous concept, is a quantifiable and well known description of limnological conditions. We chose Carlson's (1977) trophic state indices (TSI) for nutrients and chlorophyll a as a quantitative measure of the ecosystem property. TSI is an index derived by log (base 2) transformation and scaling of Secchi disk transparency and the concentrations of nitrogen (Kratzer and Brezonik, 1981), phosphorus, and chlorophyll a. Higher TSIs indicate more eutrophic conditions.

Cross-sectional studies examined the relationships between water chemistry and lake morphometry on the one hand, and chemical and biological fossils in surface sediments on the other. All the work is based on grab samples of surficial sediments in 97 lakes, short cores of the sediment-water interface in 15 other lakes, and lake-specific medians of water-chemistry variables in the data set compiled by Huber et al. (1982).

Concentration-based studies define the function C' = f (C) in Fig. 2, which assumes that sediment concentration of a material is a function of its concentration in the water column. However, correlations between water and sediment concentrations of C, N, P, S, photosynthetic pigments, and major metallic cations are either not statistically significant or have very little explanatory power (Table 2). Sedimentary concentrations of many of the elements, e.g., C and N, S, Ca and Mg are correlated with the percent organic fraction of the sediments (Brenner and Binford, 1986), and some of the elements may be associated primarily with organic matter (Brenner and Binford, in prep.) . How- ever, the proportion of organic matter varies between lakes from 0.8% to 82.3% and is not predictable by any other trophic-state variable.

The inorganic fraction of these sediments consists primarily of siliceous sands, which resemble the soils of the drainage basins (except in a single lake, Pana- soffkee, in Sumter county, which has a 66% carbonate fraction in the sediments). Sand accumulation dilutes other sedimentary constituents both volumetrically and


TABLE 2. Selected transfer functions expressed as simple or multiple regression equations for sedimentary variables in Florida lakes. Independent variables in each equation except the diatoms are expressed as accumulation rates. FFSIAVG, FI'SICHL, MEDPH, LNFL-TSI, SRPLANK, and LGFL-MPH are all various combination variables used with diatom remains, and are defined in the text and in Whitmore

(1985)

Dependent Variable Regression Equation

TSI-chlorophyU a

Chlorophyll a (mg m -3-)

Weed Area (ha)

VFSIAVG (see text)

FTSICHL (see text)

MEDPH (see text)

49.9 + 336.9 × Organic Matter (g cm -2 a -~) R 2 = 0.26 (Deevey et al., 1986)

50.94 + 28.82 x S (mg cm -2 a --~) R 2 = 0.44 (Brenner and Binford, 1986)

45.43 + 10.59 x N (mg cm -2 a-l) + 25.20 x P (mg cm --2 a- ~) -2.92 x C (mg cm-2a ~) + 11.06 × SCDU (cm--2a - ~) R 2 = 0.69 (Binford and Brenner, 1986)

56.38 - 0.27 x Alona verrucosa + 0.003 × Eubosmina spp. (# cm 2 a l) R 2 = 0.69 (Binford, 1986)

7.07 - 0.71 x Alona karua + 0.91 ×Alona quadrangularis - 0.08 x Aloha rustica - 0.20 × Alona verrucosa + 0.22 x Ephemeroporus hybridus + 0.004 × Eubosmina spp. (# cm -2 a- l)

R ~ = 0.95 (Binford, 1986)

86.6 + 85.0 x Alona karua - 7.6 x Ephemeroporus hybridus + 0.86 × Leydigia leydigi (# c m 2 a ~) R 2 = 0.99 (Binford, 1986)

39.52 + 20.65 x LNFL-TSI R 2 = 0.78 (Whitmore, 1985)

29.22 + 0.009 x SRPLANK R 2 = 0.78 (Whitmore, 1985)

6.77 - 0.68 x LGFL-MPH R 2 = 0.75 (Whitmore, 1985)

gravimetrically. Consequent ly , concentra t ions of materials in lake water are poor ly related to concentrations of the same materials in the sediments.

Correla t ions be tween limnologicai variables and concentra t ions of sedimentary const i tuents are not s trong enough for paleoecological inference in these subtropical lakes. The situation may be confined solely to the Florida lakes district because of the nature of the drainage basin soils and hydrologies. These results illustrate two fundamenta l problems of using concentration variables to study rate processes. First, concentrations are static measurements and cannot express fluxes or rates of chang& Second, concentra t ions are componen t s of a closed universe. If the concent ra t ion of one c o m p o n e n t increases, the concentra t ion of every other must decrease. This p roblem is analogous to that encounte red with interpreta t ion of pol len-percentage diagrams. In terpre ta t ion must take into considerat ion the in te rdependence of all the variables.

Al though theoretical unders tanding of the relationship between water concent ra t ion of an e lement , its loading rate into the lake, and its rate of loss to sediments is still unclear and controversial (Edmond- son and Lehman , 1981; Lehman and E d m o n d s o n , 1983; Chapra and Reckhow, 1983), accumulat ion rates of each e lement are much bet ter predictors of water- column concentra t ions (Binford and Brenner , 1986).

Because the actual form of the function F ' = f (C) (Fig. 2) is unknown, results of the cross-sectional study derive empirical relationships be tween water concentrations and sedimentary accumulat ion rates of dif-

ferent materials by regression analysis. Simple linear regression equat ions of sedimentary C, N and P explain 34%, 73% and 49%, respectively, of the variance of TSI (chlorophyll a, ni trogen and phosphorus) in 34 lakes studied (Binford and Brenner , 1986). However , a multiple regression equat ion that uses accumulat ion rates of total ni trogen and phosphorus in sediments as independent variables explains 69% of the variance of TSI-chlorophytl a (Table 2). This strongly predictive equat ion comprises the transfer funct ion that we use for reconstruct ing past TSIs.

Deevey et al. (1986) a t tempted quanti tat ive recon- struction of TSI-chlorophyl l a in 14 lakes with a prel iminary transfer function based only on the accumulat ion rate of organic matter . The organic mat ter -TSI regression is one of the weaker , but still statistically significant relationships (R 2 = 0.26) described by Binford and Brenner (1986). It is most useful if the accumulat ion rate of organic mat ter is a direct function of system-wide biological product ion. The model predicts accurately the mode rn TSIs for mesotrophic and eutrophic lakes, but overest imates modern TSIs in ol igotrophic lakes. Of the five lakes with human activity in their drainage basins, only two showed a recent increase in t rophic state. Of the o ther three, at least one (Lake Waube rg in Alachua County) is known to have been eutrophic in the early 1930s (Carr , 1934), and the lakewater passes over phosphate- rich bedrock before seeping into the lake (Opper , 1982).

Sediment chemistry is subject to alteration by many


processes which are not measurable by paleolimnological methods (Berner, 1980; Engstrom and Wright, 1984). Inferences based on chemical accumulation rates are hypotheses, and require testing by other, independent methods. Examination of different kinds of biological remains in sediment cores are independent lines of evidence against which inferences from chemical measurements can be tested (Pennington, 1981).

Several studies have examined Cladocera and diatom remains in surficial and core sediments from Florida lakes. Earlier studies proposed subtle relationships between relative abundances of animal microfossils and several limnoiogical variables, including lake water alkalinity, conductivity, and phosphorus concentration. The relationships were not clear and required sophisti- cated statistical methods to describe (Crisman, 1980). Multivariate statistics that Crisman (1980) used have many assumptions that are not valid when used with relative-abundance data, and the number of lakes examined was inadequate for the number of variables included in the statistical models.

Analysis of accumulation rates of Cladocera remains is not subject to the assumptions of multivariate statistics nor the interdependency of relative abundances; thus, accumulation-rate data provide a more accurate view of the relationships between living biota and sedimentary assemblages (Binford, 1986). Accumulation rates of two genera of Bosminidae are closely correlated with chlorophyll a concentration in the water column, whether expressed as TSror mg/m 3. Other water chemistry was not predicted well by accumulation rates of chydorids or bosminids, but dependent variables such as the area of weedbed, and even the size of the lake, were. Multiple regression equations which use individual taxa as independent variables were successful for predicting trophic state (Table 2). For example, the accumulation rates of two taxa, A l o n a verrucosa and E u b o s m i n a spp. explained more than 2/3 of the variation of TSI-chlorophyll a.

Whitmore (1985) examined the empirical relationships between recent diatom thanatocoenoses and

modern TSIs in 30 of the Florida lakes by linear regression analysis (Table 2). One regression model used a diatom index (called FL-TSI) based on auteco- logical nutrient preferences of taxa in the assemblages. The index resembles pH indices that are used widely (Nygaard, 1956; Renberg and Hellberg, 1982). The most effective model of this sort used the log-transformed index (LNFL-TSI) as the independent variable, and an averaged TSI as the dependent variable (FTSIAVG is the average of TSIs for nitrogen or phosphorus, chlorophyll a, and secchi disk transparency) .Whether TSI-N or TSI-P is used depends on the N:P ratio (Huber et al., 1982). The best model accounted for 78% of the variance of trophic state (Whitmore, 1985). Trophic state and lakewater pH tend to covary in Florida lakes (Canfield, 1981; Huber et al., 1982), but partial correlation analysis showed that the relationship between the diatom index and pH was not significant when trophic state was held constant (Whitmore, 1985).

The second transfer function relates diatom accumulation rates to TSIs. Diatom accumulation rates were determined by the 2]°Pb-dilution method (Binford and Brenner, 1986). The best predictive model resulted from the regression of the square-root transformation of accumulation rates of planktonic diatoms (SRPLANK) with FTSICHL, a t roph ic state index based on water concentration of chlorophyll a (Huber et al., 1982). Predicted values of this transfer function were not correlated with lakewater pH.

Paleolimnological inferences for Lake Parker obtained with the FL-TSI index (Fig. 7a) were consistent with accumulation rates of phosphorus and organic matter. About one-third of Lake Parker's drainage basin is strip-mined for phosphate rocks. Mining activity began in the 1950s, and increased through the 1960s to a peak in the late 1970s. Increases in TSI and the accumulation rates of P and organic matter are coincident with mining activity.

Whitmore (1985) also developed a transfer function for inferring pH values from fossil diatom assemblages. The diatom index used in this function (FL-MPH) was

20 o

~4o

60

Lake Parker a. / 0

20

72 .o

Lake Parker b. / - ~ '

/

60

4"o 5"o e'o i o 8o 9o 6.5 7".o ",(s 8.o 8~6 FTSIAVG MEDPH

FIG. 7(a). Historical TSI inferences for Lake Parker, Polk County, Florida, based on down-core application of the FL-TSI diatom index. Based on the 21°Pb chronology, sediments at 60 cm depth were deposited in calendar year 1875.

FIG. 7(b). Historical pH inferences for Lake Parker, Polk County, Florida, based on down-core application of the FL-MPH diatom index.


based on Nygaard's (1956) alpha-index, although certain periphytic taxa were subtracted to reduce the variance, and circumneutral taxa were added to the numerator and denominator to stabilize the index (Renberg, 1976). The transfer function was then constructed by linear regression of the log-transformed index values (LGFL-MPH) with median pH values for 30 Florida lakes. The resulting predictive model accounts for 75% of the variability of pH, in a group of lakes with a pH range of 4.7 to 9.0. Partial correlation coefficients showed that the relationship between the diatom pH index and trophic state was not significant when pH was held constant.

The diatom pH transfer function was also applied to the sediment core from Lake Parker. Historical pH inferences (Fig. 7B) show that lake pH increased over the past 30 years, a likely consequence of carbonate- rich runoff from phosphate strip mines in the drainage basin.

DISCUSSION

Each of these research programs has demonstrated that human activities increase the flux of material, both organic and inorganic, from terrestrial to aquatic systems. The response of the aquatic systems differs depending on the nature of both the activity and the drainage basins. The Maya were builders of cities, and their activities resulted in tremendous silt loads in adjacent lakes. Consequent soil erosion and colluvi- ation removed a large fraction of the nutrients available for agricultural plants on land, but the nutrients were delivered to the lake in a form unavailable to aquatic plants. Instead, the material was buried in deep sediments from which it is not recycled.

Human activities in Haiti probably did not begin to disturb the terrestrial systems until discovery by Europeans. The rate of land clearance then may have been similar to that of early Maya times, but Haitians have not built cities on the banks of Lake Miragoane. Instead, open forests have given way to wide-spread subsistence agriculture. Massive siltation, such as demonstrated in Guatemala, has not occurred in the Lake Miragoane basin, except perhaps in the very early days of European activities. Lake Miragoane is not eutrophic today, despite the dense human population around the lake.

Work in Florida demonstrates that human activities can drive an increase in trophic state. Two lakes with human-induced changes of trophic state are both in regions with phosphorus-rich bedrock and subsoils. Disturbance of the soil and rock matrix, by whatever mechanism, increased the flux of available nutrients to the lakes, and the lacustrine biota responded with increased productivity. Several lakes that were expected to show increases of trophic state did not, at least one of these is naturally eutrophic, and probably always has been. These observations point out the dependency of lacustrine systems on their geological setting.

ACKNOWLEDGEMENTS

We thank Kathleen Dorsey for help with the final preparation of the manuscript. Work in Guatemala was supported by several grants to E.S. Deevey from the National Science Foundation: BMS 72- 01859, DEB 774)6629, EAR 79-26330, EAR 82-14308, and EAR 84- 19506. Field work in Guatemala was supported in 1974 by a grant from the American Philosophical Society. Florida research was funded by N.S.F. grants DAR 79-24812 and DEB 82-11380, and continues with support from the Whitehall Foundation. Work in Haiti was supported by NSF grant BSR 85-00548 to M.W. Binford and E.S. Deevey.

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Ecosystems, paleoecology and human disturbance in subtropical and tropical america

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