Processes that affect ecological community measurements, such as abundance, species richness or species diversity, form the framework of ecology. For years these processes were investigated by examining local factors (explanatory variables) that may affect the relationship between individuals or species on a local scale. Over the last two decades, due to the rise of advanced scientific theory and practice, new computational and statistical approaches have been developed, allowing ecologists to examine community structure and measurements using several scale-dependent variables and processes. Despite much research conducted in this field, reptiles as a biological group have been somewhat neglected compared to other groups (e.g. birds, insects, rockpools).
Over the years, various studies have been conducted on the structure of ecological communities within agricultural systems, using diverse approaches, including advanced methods of spatial ecology. However, contiguous farmland is rarely spread over a sharp climatic gradient, so the effect of such a gradient on community structure within an agricultural system is currently understudied. In addition, in cases where a climatic gradient exists, it is usually accompanied by changes in elevation, ground composition, and geology, and, as expected, in the agricultural activity itself.
The study area for this thesis, the Southern Judea Lowlands, is characterized by a very sharp climatic gradient over a short distance, with no significant change in elevation, soil, geology, human history or agricultural activities. This PhD thesis examines scale-dependent variables that affect the reptile community within natural patches in the fragmented agricultural system of the Southern Judea Lowlands.
The first chapter addresses the effect of different spatial scale variables on the reptile community located in natural patches within an agricultural matrix. Three 3.2×4 km land units were chosen, located from north to south – Galon, Lachish and Dvir. These land-units reflect the north-south climatic gradient that exists at the landscape scale. Patches of varying size, shape and spatial configuration were identified within the land-units. Within these patches, I marked 100×50 m equal-sized plots which were used for sampling reptiles. This sampling method allowed me to examine how a series of variables (e.g. plot heterogeneity, patch size and spatial configuration), which operate at different spatial scales and may be related to different ecological processes, affect the reptile community. By using an AICc-based (Akaike Information Criterion with correction for finite sample sizes) model selection approach, I examined which variables most affect community structure. The models that offered the best explanation for the three community measurements – abundance, species richness and species diversity – were all multiple scale models. However, all three community measures were strongly affected by local scale variables which suggested a strong influence of local hetrogeneity on reptile communities. Moreover, for all three community measures, both grazing and climatic gradients together were found to be an important variable, which indicates the significance of this combined environmental phenomenon.
The second chapter deals with one of the most common agricultural activities within my research area -- seasonal grazing of sheep (Ovis aries) and cattle (Bos primigenius). Grazing takes place mainly in the stubble after harvest, but herds also find natural patches located within the agricultural matrix. Previous studies have examined the effect of domestic animal grazing on reptile communities, but few of these have examined the integrated effects of grazing and climate, especially within a similar area. In Chapter 2, I examine the combined effect of grazing and climatic gradient on the reptile community of the Southern Judea Lowlands with an additional site to the south – Rahat. I list the proportion of species according to their biogeographical origin. The results indicate a decrease in the percentage and relative abundance of Mediterranean species and an increase in the percentage and abundance of desert species with decreasing precipitation. The effect of grazing itself also changed according to the location of the plot along the climatic gradient. Within the Galon land-unit, which belongs to the Mediterranean climate, grazing was found to increase plot heterogeneity and species richness. In contrast, in the southern area, near Dvir, a negative effect of grazing on plot heterogeneity and species richness was found. At Dvir, grazing was also found to affect community composition. Mediterranean-oriented species richness was negatively affected by grazing intensity. In contrast, arid-oriented species richness was positively related to grazing intensity at Dvir.
Apart from grazing, the study area is also characterized by the presence of vast wheat fields. In the third chapter of the work I address the effect of wheat fields on the reptile communities. I chose to focus on plots located in the center of wheat fields which were all concentrated in the northern land-unit Galon, in order to avoid in this analysis the impact of the climatic gradient. In addition, due to the paucity of observations of other species, this part of the study focused on the lizard Bridled Mabuya (Trachylepis vittata) only. Arthropod abundance found in the early spring within the wheat fields was significantly higher than in natural patches. In addition, I found a significant movement of reptiles from natural patches to the wheat fields, but very little in the opposite direction. The physical condition of the individuals who left the natural patches for the fields was significantly better than that of the individuals who remained in the natural patches. Finally, no individuals were found within the wheat fields after the harvest in contrast to the natural patches. These results indicate that wheat fields act as an ecological trap for T. vittata. This third chapter has been accepted for publication in the journal Biological Conservation.
In conclusion, the results of this work suggest that the reptile community within a fragmented agro-ecosystem is affected by many scale-dependent ecological processes. The results also suggest that the presence of a reptile community within an agricultural area is affected by the type of agricultural crop and other agricultural practices. My research highlights the need to consider various scale-dependent ecological variables, as well as the type of agricultural activity, when investigating ecological communities within an agricultural area.