Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study

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  • 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 .Accessed: 18/07/2014 00:21

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  • 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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  • 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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  • 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 (Ta...

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