Transcript
Page 1: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

Population Dynamic Consequences of Habitat Heterogeneity: An Experimental StudyAuthor(s): Ronen KadmonSource: Ecology, Vol. 74, No. 3 (Apr., 1993), pp. 816-825Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1940808 .

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Page 2: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

Ecology, 74(3), 1993, pp. 816-825 ? 1993 by the Ecological Society of America

POPULATION DYNAMIC CONSEQUENCES OF HABITAT HETEROGENEITY: AN EXPERIMENTAL STUDY'

RONEN KADMON Department of Evolution, Systematics and Ecology, Institute of Life-Sciences,

The Hebrew University, Jerusalem, 91904 Israel

Abstract. Population dynamic consequences of habitat heterogeneity were investigated in a population of the desert annual Stipa capensis by measuring demographic responses of subpopulations inhabiting different habitats (slopes, depressions, and wadis) to natural and experimental changes in the amount of yearly rainfall.

The results indicate that rainfall fluctuations affect the dynamics of the studied popu- lation by influencing both the percentage of germination and the number of seeds produced per germinated plant. However, the effect of changes in rainfall on both demographic parameters depends on habitat conditions, with slope subpopulations exhibiting the largest, and wadi subpopulations the smallest, effects. The fact that demographic responses to rainfall fluctuations are habitat dependent has two major implications. First, subpopula- tions inhabiting different habitats show considerable differences in their year-to-year fluc- tuations in density. Secondly, since seed production per seedling is habitat dependent, the distribution of the seedling population among the various habitats is a major determinant of the total number of seeds produced by the population in a given year. The results further indicate that most of the seeds (75-99.9%, depending on rainfall conditions) are produced in the depressions and the wadis, which taken together account for only 10% of the total area. This finding indicates that the ecological conditions in these spatially restricted hab- itats are critical for the dynamics of the whole population. The overall results suggest that taking into account factors such as the number and types of habitats available, the relative area occupied by each habitat and the distribution of the individuals among the available habitats may be important in explaining observed patterns of population dynamics.

Key words: demography; desert annuals; habitat heterogeneity; Jordan Rift Valley; population dynamics; rainfall fluctuations.

INTRODUCTION

Most plant and animal species may be found in a variety of habitat types, even within relatively small geographic regions. As a result, individuals in different local subpopulations of the same species may experi- ence different probabilities of survival and reproduc- tion, depending on which habitat they occupy (Mack and Pyke 1983, Fowler 1984, Silvertown and Wilkin 1983, Ungar 1987, Weiss et al. 1988). If individuals of the same population inhabit different habitats and experience habitat-specific demographic rates, then the relative area of each habitat, as well as the distribution of the individuals among the various habitats, become major determinants of the overall population dynam- ics (Pulliam 1988). Yet, although habitat heterogeneity in natural landscapes has often been emphasized (Wiens 1976, Turner 1989, Kotliar and Wiens 1990), very few studies have been designed to test how habitat-specific demography interacts with landscape structure and composition (sensu Turner 1989) to affect the dynam- ics of natural populations (Fahrig and Paloheimo 1988, Rykiel et al. 1988, Weiss et al. 1988).

I Manuscript received 26 August 1991; revised and ac- cepted 7 July 1992.

This study was designed to experimentally test how habitat heterogeneity interacts with rainfall fluctua- tions to affect the dynamics of the desert annual Stipa capensis. I demonstrate that (1) habitat conditions affect the demographic responses of individuals to vari- ation in rainfall, (2) variation in rainfall affects the distribution of the individuals among the various hab- itats, and (3) the distribution of the individuals among the various habitats is important in determining the dynamics of the overall population.

METHODS

Study species

Stipa capensis is a tufted annual grass, very common in desert and semidesert regions of the Middle East and North Africa. It occurs in areas receiving annual precipitation of 20-700 mm, but it is most abundant and often forms extensive monospecific stands in areas receiving an average of 100-200 mm annual rainfall. Germination usually occurs after the first rains in Oc- tober-November, and flowering takes place in March- April. Inflorescences are 3-10 cm contracted, narrow panicles. Spikelets are one flowered. The spear-like grain is armed with a very sharp callus and a long slender awn, which twists or untwists as the air humidity var-

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Page 3: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

April 1993 HABITAT-DEPENDENT POPULATION DYNAMICS 817

ies. When these awns are caught up in the vegetation, the twisting action helps drive the grass seed into the soil (Feinbrun-Dothan 1986).

Study area

The study was conducted at the Jericho research site in the Jordan Rift Valley, 7 km south of Jericho (Fig. 1). The region is -270 m below sea level. The area has an extremely dry Mediterranean climate, with an av- erage annual rainfall of 100 mm that varies greatly among years (cv = 110%). Mean maximum daily tem- perature is 14'C in January and 350C in August (Ro- senan 1970).

The bedrock is the Lisan formation (Neev and Em- ery 1967). In the study area this formation is composed mainly of fine sediments that produce a landscape of very gentle slopes dissected by small wadis (drainage channels that are usually dry). Height differences be- tween the wadi beds and the surrounding slopes are on the order of 1-2 m. Shallow depressions, commonly 1-10 cm deep and 2-4 m wide, are scattered separately over the slope areas.

The fine sediments rapidly form a crust after wetting by rain. This feature leads to a considerable redistri- bution of rainfall water and creates local run-off/run- on gradients, which are correlated with the microto- pography of the landscape. Slopes represent the driest habitat conditions. They receive only direct rainfall and contribute some of that water to the depressions and wadis. The wadis may receive run-off water from relatively large areas in addition to direct rainfall. This results in much more favorable soil water conditions (Kadmon 1989, Kadmon and Shmida 1990a). The depressions represent intermediate soil water condi- tions, as they receive run-off water but from relatively restricted drainage areas.

Experimental design

The study was based on a system of rainfall manip- ulation experiments that were conducted during two successive years, 1985/1986 and 1986/1987. An area of 100 x 100 m typical of the study site was selected for the experiments. This area was made of very gentle slopes in which shallow depressions and small wadis occurred as distinct units. The relative area of each habitat within the selected site was sampled using a line-transect method. A total of 20 transects running from north to south at intervals of - 5 m were sampled. A calculation of the cumulative length of each habitat along these transects indicated that the fraction of area occupied by slopes, depressions, and wadis within the study site was 90, 8, and 2%, respectively.

The general structure of the experimental design is described in Fig. 2. Before the study began, 18 plots were selected in each habitat type and were marked for subsequent measurements and experiments. Plots were round and their diameter varied between 2 and 3 m, depending on habitat structure and expected pop-

320

JERICHO

JERUSALEM

study site

LU

00 LO~ ro EIN GEDID a

10km

310

FIG. 1. Location map.

ulation density. On the slopes, where densities were lowest, plot diameter was 3 m in all cases. In the de- pressions, plot diameter varied between 2 and 3 m, depending on the size and shape of the particular de- pression. Wadi channels were usually narrow (up to 3 m in width) and in order to reduce edge effects their plots were 2 m in diameter. An attempt was made to spread the plots over the entire area. The distance be- tween the borders of neighboring plots was -3 m. The 18 plots of each habitat (54 in all) were then divided randomly into three rainfall manipulation treatments: control (natural rainfall), a supplementation of 30 mm water, and a supplementation of 80 mm water. Given the amount of natural rainfall in 1985/1986 (60 mm), this resulted in yearly amounts of 60, 90, and 140% of the annual average, respectively. Such deviations from the annual mean are common in the studied area (Ro- senan 1970).

After the 1st year of the experiments, the control plots and the 80 mm plots of each habitat were divided again into two treatment groups (Fig. 2): three plots from each treatment were designated to receive only natural rainfall (as with the previous season) and the remaining three plots to receive a supplementation of 30 mm water. Natural rainfall in 1986/1987 was 85 mm and by supplementing 30 mm of water we created rainfall regimes that were 15% below (the control plots) and 15% above (the experimental plots) the annual mean.

In total, 9 different combinations of habitat and rain- fall regime, each represented by 6 plots, were produced in 1985/1986, and 12 habitat/rainfall combinations, each represented by three plots, were produced in 1986/ 1987.

Water was supplemented using a system of overhead sprinklers established in the study site. In order to reduce evaporation, the simulated rainstorms were cre- ated on relatively cloudy days. Sprinkling intensities

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Page 4: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

818 RONEN KADMON Ecology, Vol. 74, No. 3

All plots

Habitat Slope Depressions Wadi Type (18) (18) (18)

Rainfall Manipulation In 1985-1986 Cont. +30mm +80mm Cont. +30mm +80mm Cont. +30mm +80mm

(6) (6) (6) (6) (6) (6) (6) (6) (6)

Rainfall Manipulation In 1986-1987

Cont. +30 mm Cont. +30 mm Cont. +30 mm Cont. +30 mm Cont. +30 mm Cont. +30 mm (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3)

FIG. 2. Design of the rainfall manipulation experiments. Values in parentheses denote number of plots in a given habitat/ rainfall combination.

were -4 mm/h, a rate that is common in natural rain- storms occurring in the study site (Kutiel 1978). The duration of simulated rainstorms (3-5 h) was also ad- justed to mimic the conditions of natural rainstorms. Figs. 3 and 6 represent the dynamics of the natural and simulated rainstorms during 1985/1986 and 1986/ 1987, respectively.

Demographic measurements

Two demographic variables were measured in each plot: seedling density at germination and per-capita seed production. Densities ofgerminating seedlings were measured in five or more 10 x 10 cm quadrats that were placed randomly in each plot after each germi- nation event (Figs. 3 and 6). In general, relatively large plots and plots in which germination was particularly patchy were sampled more intensively. In order to de- termine per-capita seed production, five or more ran- domly chosen seedlings were marked in each plot im- mediately after germination by placing a ring of very thin iron wire around their bases. The ring was tied to a nail that was inserted into the ground 10 cm from the marked seedling. All the marked plants were cut during the period of seed set and were taken to the laboratory for seed production measurements. Per- capita seed production was defined as the number of seeds produced by a germinated seedling. Thus, seed- lings that had died before producing any seed were considered to have zero per-capita seed production and were included in the calculation of plant means.

Statistical analysis

The density and seed production data were log(x + 1)-transformed since both variables exhibited positive correlations between means and variances and includ- ed zero values (Sokal and Rohlf 1981). The variances of the transformed variables did not show any pattern in relation to the means. Differences among habitats

and treatments in seedling density were analyzed using ANOVA models with mean density per plot as the dependent variable, habitat type and rainfall manip- ulation as fixed effects, and among-plot variation as the error term. Since the design was not completely factorial, two separate models were used for the anal- ysis. The first model was constructed to test the com- bined effects of habitat type and rainfall manipulation on seedling density in 1985/1986 (N= 54). The second model was constructed to test the combined effects of habitat type, rainfall manipulation in 1985/1986, and rainfall manipulation in 1986/1987 on seedling density in 1986/1987 (N = 36). Sums of squares were decom- posed with each effect being adjusted to all other ef- fects. A third model was constructed to test for differ- ences in seedling density between years. This aspect of the variation was tested using repeated-measures anal- ysis with habitat type as a between-subject factor and year as a within-subject factor. Only data from the nine plots that were left as a control in both years were used in this analysis.

The seed production data were analyzed using sim- ilar approaches, but with mean per-capita seed pro- duction per plot as the dependent variable. Variances of the linear models were tested for homogeneity using Cochran's C statistic in the case of the ANOVA mod- els, and Box's M statistic in the case of the repeated- measures analysis (SPSSX 1986). None of these sta- tistics were ever significant (Table 1).

RESULTS

The 1985/1986 season

The first rainstorm of the 1985/1986 season occurred on 17 November 1985 (Fig. 3a). This rainstorm caused germination only in the wadi habitat. Subpopulations of the remaining two habitats on the control plots ger- minated separately, later in the season: the depression

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April 1993 HABITAT-DEPENDENT POPULATION DYNAMICS 819

TABLE 1. Results of tests for variance homogeneity in the six linear models.* The test statistic is Cochran's C in ANOVA (single-year) models and Box's M in repeated-measures models (comparisons between the two years).

Seedling density Per-capita seed production

Statistic df P Statistic df P

1985/1986 models C= 0.28 5,9 .168 C= 0.20 5,9 .935 1986/1987 models C= 0.32 2,12 .161 C= 0.32 2,12 .173 1985/1986-1986/1987 models F = 0.92 6,897 .482 F = 0.46 6,897 .834

* The 1985/1986 models test the effects of habitat type and rainfall manipulation in 1985/1986 on seedling density and per-capita seed production in 1985/1986. The 1986/1987 models test the effects of habitat type, rainfall manipulation in 1985/1986 and rainfall manipulation in 1986/1987 on seedling density and per-capita seed production in 1986/1987. The 1985/1986-1986/1987 models test the effects of habitat type and year on seedling density and per-capita seed production in plots that received natural rainfall in both years.

subpopulations after the rainstorm of mid-December and the slope subpopulations after the heavy rain- storms of February (Fig. 3a). There was no germination of S. capensis before, or after, the main germination event in any of the control plots. The last rainstorm of 1985/1986 (13 mm on 1 May 1986) occurred after all the annual vegetation had completely withered and was therefore unimportant for the dynamics of the studied population.

A different pattern of germination was detected in the watered plots (Fig. 3b,c). The simulated rainstorms of 1 December 1987 and 3 December 1987 caused germination of S. capensis in the slope and the de- pression habitats, but not in the wadis (where S. ca-

pensis had already germinated). No germination was observed following later rainstorms, natural or simu- lated, in any of the experimental plots (Fig. 3b,c).

Density of the germinating seedlings was lowest in the slope habitat, moderate in the depressions, and highest in the wadi habitat under all rainfall manipu- lation treatments (Fig. 4). Water supplementation had a positive effect on seedling density, but the intensity of this effect was habitat dependent. It was most pro- nounced in the slope habitat, less pronounced in the depressions, and negligible in the wadis. As a result of the habitat-dependent germination responses, the mag- nitude of among-habitat differences in density was highest under natural rainfall conditions and lowest in

I Germination event in a given habitat

0 Natural rainstorm

3 Simulated rainstorm

(Wadi) (Depressions) (Slope) a. 20- * i V

10 i n n 11 S n n 1Hn |

(Depressions) E (Wadi) (Slope) b 0

-', 20i E

Ef 1? 1

o -"Am ,D li

.co i:

(Depressions) (Wadi) (Slope)

20 - * C.

10 I I 1 I 1 7 I 1 n nnl | | n ln

Nov Dec Jan Feb Mar Apr May 1985 1986

FIG. 3. Dynamics of rainstorms and germination events in the 1985/1986 season. (a) Plots received only natural rainfall. (b) Plots received a total water supplementation of 30 mm. (c) Plots received a total water supplementation of 80 mm.

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Page 6: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

820 RONEN KADMON Ecology, Vol. 74, No. 3

TABLE 2. Analysis of variance for the effects of rainfall manipulation and habitat type on seedling density and per-capita seed production in 1985/1986.

Seedling density Seed production

Effect ss df F ss df F

Rain 0.3 2 851.0*** 3.8 2 24.5*** Habitat 29.6 2 1684.1*** <0.1 2 0.1 Rain x habitat 0.2 4 4.9** 2.4 4 14.9*** Error 0.4 45 1.8 45 Total 30.5 53 8.0 53

** P < .0 1; *** P < .00 1.

plots that received a supplementation of 80 mm water (Fig. 4). The interaction between the effects of habitat type and rainfall manipulation was highly significant (Table 2).

Per-capita seed production varied among habitats, as well as among rainfall manipulation treatments (Fig. 5). Water supplementation increased per-capita seed production in all habitat types, but as with the ger- mination response, the effect of water supplementation was highest in the slope habitat, moderate in the de- pressions, and negligible in the wadis. As a conse- quence of this habitat-dependent effect, the qualitative pattern of among-habitat differences in per-capita seed production varied among rainfall manipulation treat- ments. Under natural rainfall conditions, per-capita seed production was lowest on the slopes and highest in the wadis, while under the two experimental rainfall treatments it was lowest in the wadis and highest on the slopes. The interaction between the effects of hab- itat type and rainfall manipulation treatment was high- ly significant (Table 2). It is also worth noting that both

W Slope : Depressions Wadi

100.00 _

50.00

0

250 10000 ;

5.00+ 0

5.2.50 ::

1.00 *

0.50-

0.25-

O 0.10 0

0.05

0

0.01 Control +30 mm +80 mm

Rainfall manipulation FIG. 4. Seedling density in the 1985/1986 season as a

function of habitat type and rainfall manipulation treatment (mean and 1 SE). Note the logarithmic scale.

the lowest and the highest values of per-capita seed production were found in the slope habitat.

The 1986/1987 season

The first rainstorm of the 1986/1987 season occurred on 1 October 1987 and did not cause germination in any of the habitats (Fig. 6a). Subsequent rainstorms appeared in the beginning of November. These heavy rainstorms caused massive germination of S. capensis in all habitat types. Later rainstorms did not lead to any germination of S. capensis. A few individuals ger- minated in the slope habitat following the simulated rainstorms performed in the beginning of January, but as a result of their rarity (7 seedlings in all plots), their effect on density and seed production was negligible.

In 1986/1987, seedling density and per-capita seed production were measured in plots representing four

W Slope ElDepressions Wadi

-5-

UI)

Co 25 - + (I)

C;

0

0

5 -

ci)

Control +30 mm +80 mm

Rainfal I manipulation FIG. 5. Per-capita seed production in the 1985/1986 sea-

son as a function of habitat type and rainfall manipulation treatment (mean and 1 SE). Note the logarithmic scale.

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Page 7: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

April 1993 HABITAT-DEPENDENT POPULATION DYNAMICS 821

Germination event in a given habitat I Natural rainstorm | Simulated rainstorm

(Slope)

(Depressions)

(Wadi) a.

30

20-

10 n E

? n n n n MI 07 I in

E (Slope) E (Depressions)

(Wadi) (Slope)

CI 30 4D+b 20

10 1i 0 I

Oct Nov Dec Jan Feb Mar Apr 1986 1987

FIG. 6. Dynamics of rainstorms and germination events in the 1986/1987 season. (a) Plots received only natural rainfall in 1986/1987. (b) Plots received a total water supplementation of 30 mm in 1986/1987.

combinations of rainfall treatments (Figs. 7 and 8): 1) control in 1985/1986; control in 1986/1987; 2) a sup- plementation of 80 mm in 1985/1986; control in 1986/ 1987; 3) control in 1985/1986; a supplementation of 30 mm in 1986/1987; 4) a supplementation of 80 mm in 1985/1986; a supplementation of 30 mm in 1986/ 1987. As in the previous year, seedling densities were always lowest on the slopes and highest in the wadis (Fig. 7). The effect of 30 mm water supplementation in 1986/1987 on seedling density was statistically in- significant. A supplementation of 80 mm in 1985/1986 had a positive effect on seedling density in 1986/1987, but the intensity of this effect varied between habitats: it was highest in the slopes, intermediate in the de- pressions, and negligible in the wadis. The interaction between the effects of habitat type and rainfall manip- ulation in 1985/1986 was highly significant (Table 3).

The effect of 30 mm water supplementation on per- capita seed production was not significant. A supple- mentation of 80 mm water during the previous, 1985/ 1986 year had a significant negative effect on per-capita seed production, but the interaction of this effect with habitat type was also statistically significant (Table 3). This negative effect was greatest on the slopes and least in the wadis. Differences among habitats were statis-

tically significant, but the pattern of between-habitat variation was not uniform and varied considerably be- tween the various treatments (Fig. 8).

Variation between years

In the control plots in all habitat types, natural den- sities in 1986/1987 were higher than those of 1985/ 1986. The increase in density was > 10-fold on the slope habitat, almost 4-fold in the depressions, and

2-fold in the wadi habitat. A repeated-measures anal- ysis performed on the data from the nine plots that received only natural rainfall in both years indicated that the interaction between the effects of year and habitat type was highly significant (F2 = 85.8, P < .0001).

Per-capita seed production also varied between years in a habitat-dependent manner: variation was highest on the slopes, moderate in the depressions, and neg- ligible in the wadis. As with density data, the inter- action between the effects of year and habitat type was highly significant (F2 = 1633, P < .0001).

Among-habitat distribution of the population

Based on the information we had on the relative abundance of the various habitats in the study area

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822 RONEN KADMON Ecology, Vol. 74, No. 3

TABLE 3. Analysis of variance for the effects of rainfall manipulation in 1985/1986, rainfall manipulation in 1986/1987, and habitat type on seedling density and per-capita seed production in 1986/1987.

Seedling density Seed production

Effect ss df F ss df F

Rain (1985/1986) 0.68 1 51.5*** 0.23 1 11.8** Rain (1986/1987) 0.01 1 0.5 0.04 1 2.3 Habitat 21.50 2 804.5*** 0.40 2 10.1*** Rain (1985/1986) x habitat 0.26 2 9.7*** 0.32 2 8.1** Rain (1986/1987) x habitat 0.02 2 0.9 0.05 2 1.3 Rain (1985/1986) x rain (1986/1987) 0.01 1 0.5 0.04 1 2.1 Rain (1985/1986) x rain (1986/1987) x habitat 0.02 2 0.7 0.02 2 0.6 Error 0.32 24 0.48 24

Total 22.81 35 1.60 35 ** P < .01; *** P < .001.

and the demographic data obtained from the rainfall manipulation experiments, we calculated the effect of rainfall conditions on the distribution of (1) the ger- minating seedlings and (2) the newly produced seeds among the various habitats. The results (Fig. 9) indi- cated that the contribution of each habitat to the overall abundance of the population (expressed either as ger- minated seedlings or newly produced seeds) was highly dependent on rainfall. The general effect of increasing the amount of the yearly rainfall was to increase the relative contribution of the slope habitat and to de- crease the relative contribution of the wadi habitat. Such an effect was documented for both natural (be- tween-year) and experimental changes in the amount of the yearly rainfall (Fig. 9). It can also be seen that the slopes, in spite of occupying 90% of the area, main- tained only a small portion of the overall population

W Slope I Depressions * Wadi

250.00

100.00_

-~50.00-

E25.00-

095186oo +8 m Coto.+0m

o 10.00-

Ct 5.00 m

o2.50-

1.00- 0.50-

0.25

o0 0.10

0.0

0.0I 1985 1986 -Control +80 mm Control +80 mm 1986 1987 --Control Control +30 mm +30 mm

Rainfall manipulation

FIG. 7. Seedling density in the 1986/1987 season as a function of habitat type and rainfall manipulation treatment (mean and 1 SE). Note the logarithmic scale.

of S. capensis in the study site. In contrast, wadis, which made up only 2% of the total area, were of considerable importance in contributing to the total abundance of the population.

The distribution of the newly produced seeds among the various habitats was more sensitive to natural or experimental changes in rainfall than the correspond- ing distribution of the germinated seedlings. For ex- ample, a supplementation of 30 mm water in 1985/ 1986 increased the relative contribution of the slopes by less than one order of magnitude if the abundance of the germinated seedlings is being considered, but by more than two orders of magnitude if the abundance of the newly produced seeds is being considered (Fig. 9). This difference reflects the habitat-dependent seed production responses of the germinated seedlings.

Finally, the results indicate that water supplemen-

W Slope Depressions MWadi

C.

25 -

C5

01 i '5

CL

C-,

1985-1986 - Control +80 mm Control +80 mm

1986-1987 -Control Control +30 mm +30 mm

RainfallI manipulation

FIG. 8. Per-capita seed production in the 19-86/1987 sea- son as a function of habitat type and rainfall manipulation treatment (mean and I SE). Note the logarithmic scale.

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Page 9: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

April 1993 HABITAT-DEPENDENT POPULATION DYNAMICS 823

Area 81% -2%/

1985-1986 Season 1986-1987 Season

Rainfall Demographic parameter Rainfall Demographic parameter manipulation manipulation

Germination Seed Germination Seed .Rroduction ,pro Uction

3.9% - J4.8% 0.9% 0.1%

1985-1986 Control

1986-1987 ContrDI i .

8.1% 14.1% 9C

1985-1986 +80 mm 2%

+30mm 524% %1986.1987 00Jn11V1 36......:

+30~~~~~37

3~~~~~~~~~~~~~~~~~~31 1985-1986 C3mm 199% 201

1986-1987 3mm 5-5

1986::19 t+30 mm SLope ,,2 Depesso r Wad

FIG. 9. Effect of rainfall manipulation on the distribution of germinating seedlings and newly produced seeds among the various habitats.

tation in 1985/1986 had a considerable effect on the distribution of the seedling population among the three habitats in 1986/1987. This result indicates that the among-habitat distribution of the studied population was influenced not only by rainfall conditions at the same year, but also by rainfall conditions at the pre- vious year.

DISCUSSION

Using rainfall manipulation experiments we were able to demonstrate that rainfall fluctuations affect the dynamics of S. capensis in a habitat-dependent man- ner. These experimental results suggest that local spa- tial heterogeneity in habitat conditions is a crucial fac- tor in "translating" fluctuations in rainfall into fluctuations in population density. Moreover, because germination responses to rainfall are habitat depen- dent, changes in rainfall lead to related changes in the distribution of the seedling population among the var- ious habitats. Such rainfall-induced changes in the spa-

tial structure of the population influence the total num- ber of seeds produced by the population since seed production per seedling is also habitat dependent. Thus, rainfall affects the dynamics of the studied population not only directly (by determining the values of de- mographic parameters), but also indirectly, by influ- encing the distribution of the seedling population among the various habitats.

Habitat-specific demography

Previous studies on the demography of desert an- nuals have emphasized the role of rainfall, but ignored the potential implications of spatial redistribution of the rainfall water (Went 1948, 1955, Tevis 1958, Beatly 1967, 1969, 1974, Friedman and Orshan 1974, Inouye et al. 1980, Evenari et al. 1982, Gutterman 1983, Gu- tierez and Whitford 1987, but see Loria and Noy-Meir 1979/1980). In deserts, small differences in microto- pography may lead to considerable redistribution of the rainfall water (Ayyad and Ammar 1973, 1974, Yair

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Page 10: Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

824 RONEN KADMON Ecology, Vol. 74, No. 3

and Danin 1980, Schlesinger and Jones 1984, Kadmon et al. 1989) and consequently, spatial heterogeneity in microtopography is expected to be important in "trans- lating" fluctuations in rainfall into fluctuations in population density (Shmida et al. 1986). The results obtained in this study demonstrate that the effect of rainfall on both germination density and per-capita seed production of S. capensis depends on the position of the particular habitat along the run-off/run-on gra- dient. For example, a supplementation of 80 mm in 1985/1986 resulted in a 971-fold increase in seed pro- duction on the slopes, a 6.9-fold increase in the de- pressions, and a 1.4-fold increase in the wadis (Fig. 4). The results of the statistical analysis indicate that whenever the effect of rainfall manipulation was sta- tistically significant, its interaction with habitat con- ditions was also significant (Tables 2 and 3).

It should be emphasized that while differences in seedling density among treatments within the same habitat can only be related to variation in germination percentage (as determined by soil-water conditions), differences among habitats may also reflect underlying variations in seed density. The results of the present study do not allow us to separate these effects, but they suggest that at least some of the observed, among- habitat variation in seedling density was caused by habitat-dependent germination percentage.

To demonstrate that demographic responses to rain- fall are important for population dynamics, one has to show that manipulating the amount of water available to the individuals of one generation affects the de- mography of the next generation. Two types of such indirect effects can be expected. First, an increase in seed production owing to water supplementation in one year can lead to higher seedling densities in the subsequent year. Secondly, if seed production is density dependent, the increase in seedling density can cause a decrease in per-capita seed production. The experi- mental results support both predictions. A supplemen- tation of 80 mm water in 1985/1986 had a positive effect on seedling density in 1986/1987, and as can be expected, this effect was strongest on the slopes, mod- erate in the depressions, and negligible in the wadis (Fig. 7). On the slopes, where this effect was most pro- nounced, the experimental increase in population den- sity was followed by a 66% reduction in per-capita seed production (relative to the control plots). This finding suggests that density-dependent effects on per-capita seed production may be important in stabilizing den- sity fluctuations caused by year-to-year variations in rainfall.

The experimental results suggest that subpopula- tions inhabiting the various habitats may differ con- siderably from each other in the amplitude of their year-to-year fluctuations in density, with slope sub- populations exhibiting the largest, and wadi subpop- ulations the smallest, fluctuations. The study reported here covered only 2 yr, but the natural changes in den-

sity that were detected between the 2 yr, as well as the highly significant interaction obtained between the ef- fects of year and habitat type support this prediction.

Among-habitat distribution of the population

In the landscape we studied, most of the area is dominated by slopes and only a small portion of the total area is occupied by depressions and wadis. Yet, owing to higher densities there, depressions and wadis maintain a relatively large and disproportionate frac- tion of the S. capensis population. Moreover, it has previously been found (Kadmon and Shmida 1990b) that input of seeds produced in the wadi and depres- sions habitats may account for >90% of the abundance of S. capensis on the slope habitat. These findings in- dicate that demographic processes operating in the wa- dis and the depressions may be critical in determining patterns of population dynamics observed on the sur- rounding slope areas.

Previous studies on the demography of desert an- nuals have usually focused on populations inhabiting slopes or other dry habitats (Went 1948, Tevis 1958, Beatly 1967, Friedman and Orshan 1974, Inouye et al. 1980, Gutterman 1983, Guttierez and Whitford 1987). The reason for this is that such habitats dominate the landscape and are assumed to represent the "typical" conditions available for desert plants. The results de- tected for S. capensis demonstrate that relatively fa- vorable habitats that occupy a very small portion of the total area may maintain a large and disproportion- ate portion of the whole population. This finding points to the importance of identifying the types of habitats available, their relative abundances, and the between- habitat distribution of the individuals when studying the dynamics of natural populations. Moreover, since fluctuations in the environment may lead to related fluctuations in the spatial structure of the population, this population characteristic should be regarded as a dynamic, rather than a fixed property of the system.

AcKNOWILEDGMENTS

I thank P. Bierzychudek and two anonymous reviewers for providing valuable comments on a previous version of this manuscript. The study was conducted as a part of a Ph.D. research under the supervision of A. Shmida, and was sup- ported by grant number DISUM 00012 GR 00621 from the National Council for Research and Development, Israel, and the G.S.F., MUnchen, Germany.

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