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


    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.


    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.


    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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    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-




    study site


    00 LO~ ro EIN GEDID a



    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