The basis of our comparative analysis of Islamic and Christian societies in the Iberian peninsula of the high middle ages rests on the perception that the two societies during the period when the frontier between them was stabilized (until approximately the mid-eleventh century) were ecologically differentiated. Thus our understanding of the agricultural economies and patterns of resource utilization pursued in each sector follows from a macroscopic view of human ecological adjustments, viewed in terms of regional geographic variation. This approach, which stresses cultural factors preeminently, seems more valid than any simplistic division of the peninsula into physically defined zones, the most usual being "wet" and "dry" Spain. For, whatever variable one might choose to define an ecosystemic frontier (vegetation cover, temperature, or precipitation), the frontier will differ in each case. It is, rather, the culture itself, through its patterns of settlement, that defines what frontier or which ecological limits are meaningful.
This is not to ignore significant climatological distinctions. It is well to bear in mind, for example, that the number of rain days per year typically diminishes as one moves from north to south along the Mediterranean coast. The pattern in the central and western regions is different: as much rain falls in the campiña of Andalusia as falls in Galicia, both well in excess of the figure for the Castilian meseta. The mean temperature generally rises from north to south. But when the two indices are conjoined in an attempt to delimit pluvious and arid zones, each attempt has produced different cartographic patterns.(1)
Our understanding of the traditional agrarian economics presently to be described is based upon three interrelated assumptions:
(1) The basic unit of analysis is the agroecosystem. All processes comprising an agricultural regime (whether agronomic, hydrologic, technological, or economic) constitute an interdependent system, each element of which is linked systemically to all the others. 
(2) Following Peter H. Freeman, we assume that "From an ecological perspective, traditional farming techniques are almost without exception well suited to environmental potentials. They could be viewed as ecologically 'harmonious' with the important proviso that human and domestic animal populations have not exceeded the carrying capacity (under traditional practices) of the environment."(2) Such a formulation provides a standard against which agricultural change can be analyzed and assessed.
(3) When movements of culturally distinctive populations are involved, as they were throughout the course of the high middle ages, we assume that such migratory groups will tend to fill ecological niches which are most nearly in accord with their cultural traditions. We define "niche" broadly and with a cultural connotation: it is a zone with discrete ecological characteristics which a cultural group recognizes as familiar. In this view, when a human group fills an ecological niche an element of choice is implied. Anthropologists have recognized that migrating groups tend to move into areas "where their particular ecology is feasible."(3)
Further, this choice is conditioned by traditional perceptions of the environment, and by institutions and technologies developed for the pursuit of specific styles of agriculture.
When a group attempts to expand its old niche (for example, by the introduction of a new technique or crop) or when it, through conquest or migration, changes niches, then its success in the new situation will be a function of its capacity to adapt. A variety of cultural strategies are available: learning new techniques; attempting to continue an old style in an ecologically inappropriate situation (if a crop is valued highly enough, a low yield may be preferable to abandoning the crop completely); or selecting only those elements of a traditional system which will survive in new conditions.
In early medieval Iberia there were a number of significant ecological frontiers, separating zones in which the components of agroecosystems (dry farming, irrigation, herding, arboriculture) either differed or were conjoined in a different balance.
One frontier was delimited by the southern slopes of the Cantabrian-Pyrenean mountain system, separating a mountain herding and forest ecology from a sector characterized by cereal dry-farming, vineyards, supplementary irrigation, and local herding (e.g., the Duero Valley, the Plain of Vich). Another is that described by the northern limits of olive  cultivation, which corresponded more or less to the stabilized political frontier between Islamic and Christian zones to A.D. 1000. After the opening of the Tajo Valley to Christian settlement in the wake of the conquest of Toledo (1085), the Tajo constituted a boundary between an agrarian system in which cereal crops were dominant, lying to the north, and a more typical Mediterranean landscape -- grapevines, olive trees, irrigated "oases" -- lying to the south, where cereal grains played a relatively minor role.
In the following sections, the dynamics of agricultural growth and change in Islamic and Christian Spain will be analyzed, as will the transitions necessitated by the breaching of ecological frontiers.
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