Anthropogenic causes of high mortality and low recruitment in three Acacia tree taxa in the Negev desert, Israel

Ward, David; Rohner, Christoph

doi:10.1023/b:bioc.0000010408.90955.48

Cited by 85 publications

(86 citation statements)

References 27 publications

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“…The increased Acacia/Faidherbia tree density in the vicinity of the river may be due to rare mass recruitment of these trees after the large floods in the period between the photographs (esp. in 1934) and probably has little to do with human activities (Ward & Rohner, 1997). It is clear from the photographs that the piosphere (sacrifice zone) around the watering points near the town have enlarged over this century (to about 0)5 km radius) and that many trees have been chopped down in the area north of the town.…”

Section: Discussionmentioning

confidence: 96%

Perceptions and realities of land degradation in arid Otjimbingwe, Namibia

Ward

Ngairorue

Apollus

et al. 2000

Journal of Arid Environments

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Section: Discussionmentioning

confidence: 96%

Perceptions and realities of land degradation in arid Otjimbingwe, Namibia

Ward

Ngairorue

Apollus

et al. 2000

Journal of Arid Environments

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“…Wiegand et al 2000b), in particular because of the wide spacing among trees, which prevents suppression of small individuals by larger individuals as found in forests. Annual survival of older trees has been estimated as 98% (Ward and Rohner 1997). The assumption of age-independent survival of older A. raddiana has been shown to be reasonable by Wiegand et al (2000b) based on an analysis of the size distribution of dead trees in the field and based on simulation experiments.…”

Section: Model Systemmentioning

confidence: 99%

“…The starting point of our study concerns the Acacia trees in the Negev desert of Israel. A lack of recruitment leads to concern regarding the long-term survival of these trees in the Negev (Ashkenazi 1995;Ward and Rohner 1997;Rohner and Ward 1999). Wiegand et al (1999Wiegand et al ( , 2000aWiegand et al ( , 2000b have developed a spatially explicit, individual-based simulation model, SAM, to better understand the population dynamics of the Acacia trees.…”

Section: Introductionmentioning

confidence: 99%

Minimum recruitment frequency in plants with episodic recruitment

Wiegand¹,

Jeltsch

Ward

2003

Oecologia

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There is concern about the lack of recruitment of Acacia trees in the Negev desert of Israel. We have developed three models to estimate the frequency of recruitment necessary for long-term population survival (i.e. positive average population growth for 1,000 years and < 10% probability of extinction). Two models assume purely episodic recruitment based on the general notion that recruitment in arid environments is highly episodic. They differ in that the deterministic model investigates average dynamics while the stochastic model does not. Studies indicating that recruitment episodes in arid environments have been overemphasized motivated the development of the third model. This semi-stochastic model simulates a mixture of continuous and episodic recruitment. Model analysis was done analytically for the deterministic model and via running model simulations for the stochastic and semi-stochastic models. The deterministic and stochastic models predict that, on average, 2.2 and 3.7 recruitment events per century, respectively, are necessary to sustain the population. According to the semi-stochastic model, 1.6 large recruitment events per century and an annual probability of 50% that a small recruitment event occurs are needed. A consequence of purely episodic recruitment is that all recruitment episodes produce extremely large numbers of recruits (i.e. at odds with field observations), an evaluation that holds even when considering that rare events must be large. Thus, the semi-stochastic model appears to be the most realistic model. Comparing the prediction of the semi-stochastic model to field observations in the Negev desert shows that the absence of observations of extremely large recruitment events is no reason for concern. However, the almost complete absence of small recruitment events is a serious reason for concern. The lack of recruitment may be due to decreased densities of large mammalian herbivores and might be further exacerbated by possible changes in climate, both in terms of average precipitation and the temporal distribution of rain.

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“…For further details on the life history of A. raddiana in the Negev, see Ward and Rohner (1997), Rohner and Ward (1999), and Wiegand et al (1999Wiegand et al ( , 2000a. For further information about model construction, see Electronic supplementary material (ESM) 1.…”

Section: The Modelsmentioning

confidence: 99%

Interaction between ungulates and bruchid beetles and its effect on Acacia trees: modeling the costs and benefits of seed dispersal to plant demography

2011

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Integrative studies of plant-animal interactions that incorporate the multiple effects of interactions are important for discerning the importance of each factor within the population dynamics of a plant species. The low regeneration capacity of many Acacia species in arid savannas is a consequence of a combination of reduction in seed dispersal and high seed predation. Here we studied how ungulates (acting as both seed dispersers and herbivores) and bruchid beetles (post-dispersal seed predators) modulate the population dynamics of A. raddiana, a keystone species in the Middle East. We developed two simulation models of plant demography: the first included seed ingestion by ungulates and seed predation by bruchids, whereas the second model additionally incorporated herbivory by ungulates. We also included the interacting effects of seed removal and body mass, because larger ungulates destroy proportionally fewer seeds and enhance seed germination. Simulations showed that the negative effect of seed predation on acacia population size was compensated for by the positive effect of seed ingestion at 50 and 30% seed removal under scenarios with and without herbivory, respectively. Smaller ungulates (e.g., <35 kg) must necessarily remove tenfold more seeds than larger ungulates (e.g., >250 kg) to compensate for the negative effect of seed predation. Seedling proportion increased with seed removal in the model with herbivory. Managing and restoring acacia seed dispersers is key to conserving acacia populations, because low-to-medium seed removal could quickly restore their regeneration capacity.

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Anthropogenic causes of high mortality and low recruitment in three Acacia tree taxa in the Negev desert, Israel

Cited by 85 publications

References 27 publications

Perceptions and realities of land degradation in arid Otjimbingwe, Namibia

Perceptions and realities of land degradation in arid Otjimbingwe, Namibia

Minimum recruitment frequency in plants with episodic recruitment

Interaction between ungulates and bruchid beetles and its effect on Acacia trees: modeling the costs and benefits of seed dispersal to plant demography

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