patterns of species and community distributions related to environmental gradients in an arid...

Vegetatio 117: 69-79, 1995. 69 (~) 1995 Kluwer Academic Publishers. Printed in Belgium.

Patterns of species and community distributions related to environmental gradients in an arid tropical ecosystem

Robin S. Patten* & James E. Ellis Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80521, USA *Present address: Biology Department, Montana State University Bozeman, NIT 59717, USA

Accepted: 7 December 1994

Key words: Acacia spp., Community identification, Gradient analysis, Kenya, Landscape heterogeneity

Abstract

The heterogeneous vegetation mosaic of the South Turkana region of north Kenya is associated with diversity in the region's physical environment. The abundance and distribution of the dominant species are related to gradients in those abiotic factors that influence water availability, including precipitation, soil texture, and topographic relief. Research focused on three Acacia species that are a major component of the Turkana vegetation; A. tortilis, A. senegal, and A. reficiens. These species each exhibit a different response to variations in abiotic factors. Consequently, species abundance varies independently across the landscape, creating a continuum of intergrading populations. Community types can be identified within the mosaic of intergrading populations. Although community borders are not discrete due to continual change in species abundance, types are identifiable and are repeated in areas with similar environmental conditions. The landscape patterns are representative of Whittaker's (1953) climax- as-pattern, with communities created by individual patterns of populations responding to environmental gradients, creating a continuum of community change across the landscape.

Introduction

The vegetation patterns of arid regions are often patchy, with variation in the abundance and distribution of both species and community types across the landscape (Walker & Noy-Meir 1982; Wiens 1985; Belsky 1989). Because arid zones are characterized by minimal precipitation and frequent droughts (Mabbutt 1977; Cole 1986), availability of water may be one of the primary factors controlling the distribution of species (Noy- Meir 1973; Yair & Danin 1980; Bornkamm & Kehl 1989). Consequently, variations in the physical factors that influence water availability may drive heterogeneity in the vegetation mosaic.

Sections of northern Kenya exhibit patchy vegetation patterns that are believed to be created by gradients in physical factors (Ellis & Dick 1985). Research was completed in this area to examine the relationship between heterogeneity of the abiotic environment and the distribution of both individual species and community types. This study also examined the nature of

the vegetation mosaic to determine if it is a continuum of intergrading communities created by overlap in individual species distributions.

Gradients in abiotic factors can be a primary control over plant distributions (Brown 1988). In arid regions, the most critical gradients may be those related to water availability (Noy-Meir 1973; Bornkamm & Kehl 1989), including mean annual precipitation, soil characteristics, and topography. Precipitation is a major component of the overall input of water into the system, creating sources of surface water and also recharging ground water aquifers. Once water is put into a system, its availability to plants is influenced by both soils and topographic relief (Noy-Meir 1973; Westoby 1979; Yair & Danin 1980; Noy-Meir 1981; Coppinger 1987; Ellis & Dick 1985). Soil characteristics such as clay content will influence infiltration rates and the ability of plants to take up water. Topo- graphic variation may redistribute water in such a way that runon zones will receive a greater amount of input than would be available from direct precipitation alone.

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Precipitation, soil texture, and topographic relief are therefore key factors that may affect overall vegetation patterns in an arid region and are used in this study to examine the relationship between the abiotic environment and the vegetation mosaic.

Because of the significance of precipitation, soils, and relief to plant growth in arid regions, species abundance is expected to vary across gradients in these factors. Yet, gradients in plant abundance associated with physical gradients may be different for each species, creating a vegetation mosaic of populations intergrading across the landscape (Noy-Meir 1973; Westoby 1979; Brown 1988; Cody 1989). Although population distributions may overlap in a pattern of continual change, it is still possible to define community types within the continuum (Daubenmire 1966; Austin 1980; Grieg-Smith 1983; Noy-Meir & Van der Maarel 1987; Austin & Smith 1989). Species with similar environmental tolerances may cluster together in like environments. This combination of species may be repeated in areas of similar environmental conditions and can therefore be classified as a type. The resulting landscape pattern is a mix of definable communities that blend without discrete boundaries. How distinct the boundaries of the intergrading communities are is a function of the rate of change in physical gradients. Community boundaries may be more defined in areas where physical gradients change rapidly, as species abundance will also vary more abruptly at these points (Austin 1980; Austin & Smith 1989).

Whittaker (1953) developed the 'climax as pattern' concept to describe vegetation patterns that result from individual species response to physical factors. This hypothesis states that communities are a result of individual patterns of populations responding to environmental gradients, creating a continuous gradation of climax patterns across the landscape. Because physical gradients related to water availability may be a primary control over species distributions in arid regions (Noy-Meir 1973; Cole 1986; Bornkamn & Keh11989), the idea of 'climax as pattern' may be an appropriate description of the vegetation mosaic of such areas.

Objectives. The vegetation mosaic of the South Turkana region in north Kenya exhibits the heterogeneous vegetation patterns observed in other arid regions. It is believed that the patchiness of the mosaic is a result of variability in the area's physical environment. The goal of this study was to analyze the relationship between gradients in abiotic factors and both species and community distributions and subse-

quently determine the nature of the resulting mosaic. Specific objectives were to:

A. Analyze species distributions: - Identify what abiotic factors influence species distributions and determine whether species respond individually to variations in these factors. - E x p l a i n species distributions across the landscape based on species response to gradients in significant abiotic factors.

B. Analyze community composition and distribution: - Use multivariate analyses to determine how factors that influence individual species distributions influence community composition. - Explain distribution of communities across the landscape based on observed relationships between community composition and abiotic factors.

C. Describe the mosaic structure: - Integrate information on species, communities, and abiotic gradients to determine if the Turkana landscape can be described by the climax-as- pattern concept, i.e. a continuum of intergrading communities created by overlap in individual species distributions.

This research focused on the relationship between three common species and three physical factors believed to be related to water availability. The vegetation of this region is dominated by Acacia tortilis, Acacia reficiens, and Acacia senegal, each of which appears to be related in different ways to physical gradients across the landscape. Analysis of the response of these species to variations in mean annual precipitation, soil texture, and topographic relief was used to determine how heterogeneity in the physical environment can drive diversity in vegetation patterns. This information was then combined with analysis of community composition and distribution to examine the nature of the Turkana vegetation mosaic.

S i t e d e s c r i p t i o n

Research was conducted in the Ngisonyoka section of the South Turkana District of North Kenya, an approximately 9000 km 2 area within the East African Rift Valley, 70 km southwest of Lake Turkana (Fig. 1). The climate is semi-arid to arid with spatial variation in mean annual precipitation ranging from 200 to 600 mndyear. Precipitation varies along a regional gradient from the wetter southwest to the dry northeast due to the presence nearby of the high altitude rift in

71

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KITALE

~____TURKANA /

/ DISTRICT /

NGISONYOKA TURKANA

KM 1 I I 0 100 200

@

NAIROBI

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Fig. 1.

/ INDIAN OCEAN

The study area, located in the Ngisonyoka Turkana region of north Kenya.

the south. There is a seasonal rainfall pattern, with a long rainy season from March to May and short rains around November, although drought periods of one year or more are not uncommon. Mean dally temper- atures are around 30 °C, with a mean maximum near 35 °C and little seasonal variation.

The Ngisonyoka Turkana landscape is physical- ly complex due to its diverse climate and geology. Three geological systems occur within the region (Jou- bert 1966); a basement complex of metamorphic rock (often covered with a mantle or thin veneer of alluvium), more recent alluvial deposits, and Miocene basalt flows. Elevations range from 370 m on the plains to over 2100 m in the mountains. Soils are generally poor- ly developed and range from sandy to sandy clay loams. Stream channels dissect the region, but streams are ephemeral, running at most once or twice per year.

A majority of the Ngisonyoka Turkana study area consists of either the mantle or alluvial substrates. Mantles are erosional or transportational area with thin, gravelly soils while the alluvium consists of depositional areas with sandy soils.

Both the mantle and the alluvial substrates contain several landforms with unique smaller scale properties related to topographic features, soils, and drainage patterns (Fig. 2). Two major landforms are found within

Fig. 2. A cross section of the eastern slopes of the Ngisonyoka Turkana region, showing the three substrates and the principal landforms.

the mantle, the mountains and pediment. The mountains are high elevation areas, generally steep with high relief and more precipitation than other landforms. The pediments are more gently sloping bedrock plains with a thin veneer of skeletal soils. The alluvium includes three major landforms, the upper bajada, lower bajada, and alluvial plain, distinguishable by soils and topography.

Many of the Ngisonyoka Turkana landforms occur in sequence on the eastern slopes of the study region (Fig. 2), creating a catena typical of arid regions

72

(Cooke & Warren 1973; Chorley et al. 1984). At the top of the eastern catena are landforms derived from a mantle substrate, the mountains and pediment. Below these landforms are broad plains of alluvial deposition, with the sandy bajada gradually leveling out to form the alluvial plain. Several abiotic gradients are associated with this catena (Patten 1991). Precipitation is greater at higher elevations due to orographic effects and soil texture is progressively finer from the upper bajada to the alluvial plains. Topographic relief also declines downslope, with the least relief found on the alluvial plains (Patten 1991).

Methods

Sampling was done at 103 sites spread throughout the region. Sites were randomly selected to sample all landforms, gathering data on both vegetation and physical attributes.

Woody canopy cover was measured using a Bit- terlich stick (Cooper 1957) which measures woody cover of plants six inches to 30 feet in crown diameter. A reading was taken at six points spaced 100 meters apart, with the points divided into two lines of three points each. These points surrounded the soil auger and/or pit locations (described later). Species and total woody canopy cover were recorded at each point.

Community types were identified using both ordi- nation and cluster analyses of the vegetation data. Detrended Correspondence Analysis (DCA) was used to determine four axes scores based on woody species cover at each site (Hill 1979). Community types were identified using cluster analysis of the four DCA scores. Community types were thus objectively defined using multivariate analysis, and were subsequently plotted against the DCA axes to illustrate relationships between community composition and abiotic gradients. A separate discriminant analysis was completed using the defined groups characterized by both DCA axes scores and percent cover of six common species to confirm that these community types are unique and significantly different from each other.

Precipitation values were derived from a model using elevation and regional location to predict mean annual precipitation (Coughenour et al. 1990). Topo- graphic maps provided elevation (m) for the model.

Topographic relief was measured at each site to quantitatively record how rough the land surface is. To measure relief, eight 25 meter strings were laid along two 100 m transects oriented along contour lines. The

end of each string was held one meter above the ground, creating a straight line above the uneven ground. The distance from the ground to the string was measured at five meter intervals for 25 meters to determine the mean difference between successive measurements. Areas with a high relief show a greater mean difference between measurements than areas of low relief, pro- viding a measure of small scale topographic variation at each site.

At each site three to five one meter soil holes were augered at fifty meter intervals, with a soil pit dug at several sites to more accurately describe the soil profile. Horizon depth, texture, color, CaCO3 presence, and structure were recorded for each horizon in the profiles. Field estimates of texture class were deter- mined for all horizons at all sites, with clay and sand percentages derived from a soil texture triangle. The percent clay reported here is an average of the clay percent throughout the one meter profile based on field estimates and should be considered a relative com- parison of clay content rather than an absolute clay percentage. Laboratory texture analysis was done for the B horizon at 55 of the 103 sites to verify the overall accuracy of the field texture classifications.

Mean values of the measured abiotic variables (precipitation, relief, and percent clay) were calculat- ed within each landform to characterize their general physical environment (Table 1).

Correlation analysis was used to analyze the relationship between species abundance and abiotic factors. The results of these analyses were used to explain the variation of species abundance down the eastern slope of the study area. The distribution of communities down the eastern slope was analyzed using the percent of sites of each community type for all sites sampled on each landform.

Results

Species

Species distributions in the Turkana study region were found to be related to gradients in precipitation, soil texture, and topography (Table 2), yet each of the three dominant Acacia species exhibits different relationships to these factors. Although the average percent clay of the soil is the factor most closely correlated to A. tortilis and A. reficiens cover, there is a negative correlation between A. tortilis and clay and a positive correlation between A. reficiens and clay. A. senegal

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Table 1. Mean and standard deviation (sd) of abiotie factors, woody canopy cover, and species cover.

Ppt. Clay Relief A. tor. A. ref A. sen.

ALL SITES X 380 13.4 7.1 4.2 7.7 3.3

sd 142 9.1 6.2 4.8 8.5 6.8

LANDFORMS

Mountain X 594 19.2 17.4 1.8 6.4 14.4

sd 13 7.1 13.1 1.3 6.2 8.1

Pediment X 383 9.7 12.4 4.0 7.1 6.3

sd 145 4.0 7.0 3.9 4.5 4.6

Upper X 269 7.3 5.4 4.0 5.0 0.1

Bajada sd 80 7.5 2.5 4.1 5.9 0.2

Lower X 354 10.4 4.8 3.4 6.5 0.2

Bajada sd 144 7.2 2.7 4.0 7.4 0.6

Alluvial X 231 18.6 3.1 1.4 15.4 0.0

Plain sd 67 11.1 0.6 2.5 12.2 0.0

Plat = precipitation in mm, Clay = average percent clay of soil; Relief = variation in relief at one meter; A. tor.= percent cover ofA. tortilis; A. ref = percent cover ofA. reficiens; A. sen. = percent cover of A. senegal.

Table 2. Correlation coefficients between the percent cover of the three dominant Acacia species, DCA axes scores, and abiotic factors.

Ppt. Clay Relief

A. tortilis 0.01 --0.33** -0 .21

A. reficiens - 0 . 0 2 0.35** -0 .09

A. senegal 0.40** 0.04 0.48**

Axis 1 0.28* 0.27* 0.55**

Axis 2 0.27* --0.39** 0.06

* p<0.01; ** p<0.001.

cover is significantly related to both relief and precipitation, yet shows no relationship to percent clay. These results demonstrate the unique relationship between each species' distribution and physical features of the landscape.

Gradients in the abundance of the three dominant Acacia species are associated with the physical gradients of the landforms of eastern Turkana (Table 1, Fig. 3). The landforms lie in a sequence from the mountains through the pediment and bajada to the low-lying alluvial plains (Fig. 2). Continuous changes occur in precipitation levels, soil clay content, and relief across these landforms (Fig. 3A). Clay content is relatively high in the shallow rocky mountain soils due to in situ development. Pediment soils are less clayey than

the adjacent mountains. The upper and lower bajada are sandy alluvial landforms with low clay content. Below the bajada lies the alluvial plain where deposition of fine materials increases clay content of the soils. Relief steadily declines downslope and is highest in the mountains and lowest on the plains.

Species' distributions across the landforms are a function of their relationship to gradients in precipitation, soils, and relief (Table 2, Fig. 3B). A. tortilis is negatively correlated with clay and is found on sandy soils where clay percentages are not much higher than 10%, primarily the pediment and bajada. A. reficiens is positively correlated with clay and has peak abundance in the clayey alluvial plains. A senegal is positively correlated with both relief and elevation and is primarily a mountain and pediment species, with little cover on the bajada or alluvial plains.

These results show relatively low levels of A. reficiens cover in the mountains despite high soil clay content. To explain this pattern, a separate correlation analysis was done using mantle sites. This analysis showed that A. reficiens has a negative correlation to relief on the mantle substrate (r=-0.46,p<0.01). Cov- er of A. reficiens may not rise substantially with the higher clay levels in the mountains as it does not favor these areas of high relief.

74

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BAJADA BAJADA PLA I N Fig. 3. Gradients in abiotic factors and species cover across the landtorms ot me eastern catena. (A) i'llyslcal gradients showing high clay content in the mountains and alluvial plains, and decreasing precipitation and relief down the slope. (B) Species distributions downslope, demonstrating that each species varies independently across the physical gradients.

Communities

The cluster analysis of DCA scores identified eleven vegetation types within Turkana (Fig. 4). The com-

munity types can be distinguished by the relative dominance of five species: A. tortilis, A. reficiens, A. senegal, Balanites orbicularis, and Acacia nubica. A majority of sites have communities dominated

by one of the three primary Acacia species: A. tortilis, A. reficiens, or A. senegal. Communities in Ngisonyoka Turkana are therefore a function of the occurrence of these three Acacia species. Communities represented in the study area are:

1. A. tortilis: A. tortilis is the dominant species, with cover of 28% to 92% of the total cover by all species.

2.A. tortilis-Commiphora trothae: A. tortilis is the dominant species, with cover of 32%-33% of the total. Commiphora trothae is present, with 10% to 20% of the total cover.

3. A. tortilis-A, reficiens: A. tortilis and A. reficiens occur in approximately equal amounts, ranging from around 10% to 40% of the total cover. A. nubica is often present.

4. A. reficiens: A. reficiens is dominant, ranging from 40% to 92% of the total cover.

5. A. reficiens-Acacia mellifera: A. reficiens is dominant, with A. mellifera ranging from 15% to 40% of total cover. C. africana is often present.

6. A. reficiens-A, tortilis-A, senegal: A. reficiens is generally dominant, yet A. tortilis and A. senegal are present in significant amounts. Together, A. reficiens, A. tortilis, and A. senegal make up approximately 60% of the total cover.

7. A. senegal: A. senegal is dominant, ranging from 22% to 87% of the total cover.

8.13. orbicularis-A, reficiens: B. orbicularis is dominant with A. reficiens present at approximately 15% of the total cover.

9.13. orbicularis: B. orbicularis is dominant, making up over 50% of total cover.

10. A. nubica-A, reficiens: A. nubica dominant, ranging from 33 to 78% of the total, with A. reficiens present in amounts greater than 10%.

11. A. nubica: A. nubica dominant.

DCA axes scores for each site represent the mix of species occurring at that site, thus the scores were used to determine relationships between community composition and precipitation, clay, and relief. Axis 1 scores are significantly correlated with all of these factors (Table 2), most strongly with relief. Axis 2 scores are only significantly correlated to clay percent. Community composition represented by the DCA axes scores thus appears to be related to gradients in all of these abiotic factors. Communities are aligned along the environmental gradients associated with the DCA axes scores, showing how composition is influenced by relief, clay, and precipitation (Fig. 4).

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The distribution of communities across the eastern slope of the study region (Fig. 5) demonstrates the relationship between communities and environmental gradients implied in the DCA plot. In the mountains where elevation and relief are high, A. senegal communities are dominant. The mixed A. tortilis-A, reficiens- A. senegal community occurs on 50% of the pediment sites. This landform is an environment transitional between the mountains and bajada and has both shallow erosional soils and deeper depositional soils in close proximity. Pediments therefore have substrate characteristics similar to both the mountains and the bajada and can support species found on both of these other landforms. Consequently a large portion of the pediment area is covered by the mixed or transitional A. tortilis-A, reficiens-A, senegal community.

The bajada has primarily A. tortilis and A. reficiens communities, depending on the soil texture. Soil types are relatively homogenous over large areas on the bajada with disjointed occurrence of sandy and clayey soils. Because of the separation of soil properties, communities tend to be dominated by either A. tortilis or A. reficiens, reducing the incidence of the mixed A. tortilis-A, reficiens community. A. tortilis communities dominate sandy areas while A. reficiens communities occur in more clayey zones. The A. reficiens community dominates the alluvial plain where relatively clayey soils are more common than sandy soils.

Discussion

Species

Gradients in precipitation, soil, and topographic relief appear to be primary factors driving the distribution of the three dominant Acacia species of the Turkana area. The abundance of these species is significantly related to these factors, with each species varying independently across gradients in the physical environment.

The different response of each Acacia species to variation in precipitation, soils, and relief may be due to differences in the species morphology or life history. These differences allow species to coexist as they can exploit different sources of water (e.g. deep vs. shallow water) or tolerate different environmental conditions (e.g. sandy vs clayey soils) (Noy-Meir 1973; Westoby 1979; Brown 1988; Cody 1989). For example, A. tortilis is a deep rooted species that utilizes ground water

76

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1 1 / 6 6 " W _ I ' 1 . ~

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0 50 i00 150 200 250 300 350

RELIEF AND PRECIPITATION m,,--

Fig. 4. The DCA plot showing the relationship between community composition and precipitation, relief, and clay. Axis 1 scores are primarily related to relief and precipitation along the x-axis. Axis 2 scores are related to decreasing clay content on the y-axis. Community types are: (1) A. tortilis; (2) A. tortilis- Commiphora trothae; (3) A. tortilis-A, reficiens; (4) A. reficiens; (5) A. reficiens-Acacia mellifera; (6) A. reficiens-A, tortilis-A, senegal; (7) A. senegal; (8) B. orbicularis-A, reficiens; (9) B. orbicularis; (10) A. nubica-A, reficiens, and (11 ) A. nubica.

sources (Lind & Morrison 1974), thus this species occurs in sandy environments where water rapidly infiltrates. In contrast, the shallow rooted A. reficiens uses surface water provided by incident precipitation or sheetflow (Lind & Morrison 1974; Coughenour et al. 1990), accounting for its presence in clayey environments where water infiltration is reduced. The third dominant species in the area, A. senegal, is best adapt- ed to coarse rocky soils of mountains, possibly due to its greater water demands. In the mountain environment, there is not only higher levels of precipitation, but also greater relief that serves to concentrate water in runon zones.

Although each of the three dominantAcacia species has a unique response to the environmental gradients of Ngisonyoka Turkana (Fig. 3), their distributions overlap across the landscape. A. senegal dominates the mountain areas, yet all three major Acacia species are found on the adjacent pediment. On the more sandy, low relief bajada, A. reficiens and A. tortilis are both

relatively abundant yet A. senegal is absent. A. tortilis gradually declines on the alluvial plain as clay levels increase and A. reficiens becomes dominant. The extensive abiotic gradients of Turkana thus produce environments which both favor a single species, such as the mountains and alluvial plains, as well as environments where species coexist in more equal abundance, such as the pediment and lower bajada.

Community composition and distribution

The communities of Ngisonyoka Turkana are similar to those found in other parts of Kenya (Barkham & Rainy 1976), with the three major community types dominated by one of three widespread Acacia species A. tortilis, A. reficiens, and A. senegal. Other 'inter- mediate' communities also exist, those with a mixed assemblage of species without a single dominant.

Community composition is significantly related to variations in precipitation, clay percent, and relief.

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80

70

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COMMUNITY TYPE

lWl OTHER

A. senegal

Mixed Acacia

A. reficiens

l i l l ' l ! l

A. tort-A, ref

A. tortilis

MT. PEDIMENT UPPER LOWER ALLUVIAL

BAJADA BAJADA PLAIN The percent of sites of each community type found within the landforms of the eastern slope of Ngisonyoka Turkana.

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Because the species that compose these communities are independently associated with these physical factors, changes in community composition are related to complex gradients, involving more than one factor. This relationship is illustrated in the DCA plot (Fig. 4) which shows a change in community composition associated with all three factors across the first axis and with two factors on the second axis (Table 2).

The DCA plot demonstrates that communities in this area intergrade and are not highly discrete due to the individualistic nature of species response to environmental gradients. There is a continuum in community types across the DCA axes, from communities dominated by A. tortilis to A. reficiens to A. senegal across the X-axis and from A. nubica and A. reficiens to A. tortilis and B. orbicularis along the Y-axis. The A. tortilis communities lie in areas of the plot which represent sandy, fiat environments. A. reficiens communities are in high clay and low to moderate relief areas of the plot, and A. senegal communities are primarily in high relief areas. Mixed communities without a single dominant (e.g.A. tortilis-A, reficiens-A, senegal communities) lie between the single-dominant communities on the DCA plot, indicating they are tran-

sitional, occurring in environments where there is an overlap in the distribution of the species that form the single-dominant communities.

The pattern of intergrading communities implied by the DCA plot are demonstrated in the landscape pattern of the Turkana region (Fig. 5). Communities composed primarily of single species are most abundant in those environments which favor that particular species. For example, A. senegal community types are most common in the mountains, and A. reficiens communities dominate the alluvial plains. In contrast, mixed communities are found in environments where species distributions overlap, such as the lower bajada and pediment.

The vegetation mosaic

These results demonstrate that the 'climax pattern hypothesis' developed by Whittaker (1953) is appli- cable to the Turkana landscape. This hypothesis con- siders vegetation patterns to be a continuous gradation of climax patterns, with communities a result of the 'pattern of populations, variously related to one anoth- er, corresponding to the pattern of environmental gra-

78

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MOUNTAIN PEDIMENT UPPER

BAJADA

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LOWER ALLUVIAL

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Fig. 6. A model for vegetation change across the landforms of eastern Ngisonyoka Turkana. Community types are created by patterns of intergrading populations. Communities with mixed dominants occur where distributions overlap.

dients'. The Turkana vegetation mosaic is composed of a continuum of communities that are created by overlapping species distributions related to broad environmental gradients. These communities intergrade to produce vegetation patterns that conform to Whittak- er's hypothesis.

A conceptual model of Turkana's physical gradients, species abundance, and community gradients, summarizes the continuity of patterns across the landscape that fit the idea of climax-as-pattern (Fig. 6). The abundance of the three dominant species varies con- tinuously in response to the changing environment. More abrupt changes occur in species cover at landform boundaries where physical factors change more rapidly. This results in a mosaic of identifiable patches, some of which are dominated by a single species, while others have a mixture of species due to the over-

lap in distributions. These patches have fairly indis- tinct boundaries, yet can be identified as community types as they have a characteristic composition and are repeated throughout the study area in areas of similar environmental conditions (Daubenmire 1966; Grieg- Smith 1983). The continuum of change is driven by abiotic factors and is appropriately described by Whit- taker's concept.

Acknowledgments

This research was partially funded by a National Sci- ence Foundation graduate fellowship. Additional fund- ing was provided by a National Science Foundation research grant. My thanks to the people of Ngisonyoka Turkana for welcoming us to their country and to

Mohammed Bashir, Achuka, Lopayone, and Eliud Lowoto for their assistance in field work. Special thanks to Richard Lamprey, Joao Queiroz, and Robin Reid for their help and support.

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patterns of species and community distributions related to environmental gradients in an arid...

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