Patch Formation Among Israeli Crucifers: How do They Get Away with it?

Abstract

Many crucifers in the Mediterranean and desert floras of Israel frequently form large monospecific patches but do not suffer severe herbivore damage. This situation is unexpected from some recent theory of insect-plant interactions which predicts that such resource concentration should attract high herbivore loads, and that herbaceous plants should rely on unpredictability in time and space as a defense against herbivores. In Israel, Mediterranean crucifers flower earlier than members of other species-rich families with similar growth forms (Lamiaceae, Asteraceae, Liliaceae, Solanaceae, Poaceae, Papilionaceae, Apiaceae). This is not true for desert crucifers; their phenologies are unpredictable from year to year. We hypothesize that their displaced phenologies and possession of potent allelochemicals allow Mediterranean patch-forming species to produce monocultures while not sustaining high levels of herbivory. In contrast, desert patch-forming crucifers rely on unpredictable phenologies as well as allelochemicals as defenses against herbivores. Patch formation may be reinforced by plant-pollinator interactions. Among Mediterranean crucifers, patch formers are taller and have larger petals than do non-patch formers. In addition, patch formation is correlated with high levels of floral ultraviolet reflectance and patterning. These differences may result from reduced interspecific but enhanced intraspecific competition for pollinators among patch formers. These patterns are not found among desert species, although desert patch formers do initiate flowering before other crucifers. The family Brassicaceae (Cruciferae) has figured prominently in the development of theory concerning insect-plant interactions. Responses of phytophagous insects to experimental manipulations of one or a few crucifer species (e.g., Tahvanainen & Root, 1972; Root, 1973; Slansky & Feeny, 1977), as well as considerations of family-wide attributes (Feeny, 1976, 1977), comprise much of the empirical and theoretical support for concepts such as associational resistance (Tahvanainen & Root, 1972) and plant apparency (Feeny, 1976). Feeny (1977) also cited family-wide attributes of crucifers to support his contention that escape in time and space and allelochemic diversity protect herbaceous plants from potential herbivores. Several characteristics of crucifers make them particularly amenable for studies of herbivore-plant interactions. They are well represented in floras of many regions of the world (Hedge, 1976), all species thus far examined contain glucosinolates (Kjaer, 1976), and many species are important cultivated crops. The family contains approximately 400 genera and 3,000 species, most of which are annual herbs (Vaughan et al., 1976). The Irano-Turanian region is home to about 900 crucifer species and was probably the center of origin for at least the Old World taxa (Hedge, 1976). Brassicaceae is a dominant and conspicuous family in terms of species diversity and abundance in both the Mediterranean and desert regions of Israel, particularly in late winter and early spring (Shmida & Auerbach, 1983). Most of the 111 or more species native to Israel are Irano-Turanian or are descendants of Irano-Turanian stock (Zohary,