Plant biodiversity associated with representative corn cropping systems in the Philippines: modern corn genebanks or dispersal barriers and death traps

A comprehensive inventory and assessment of plant diversity associated with 15 selected representative corn cropping systems in Isabela Province (Luzon, Philippines) showed that a relatively high total plant diversity level exists within the province, with H' value of 4.15 and a species richness of 268. On a per locality/study site basis, the species diversity index of study plots ranged from H'= 1.91 to 3.70 and species richness ranging from S =16 to 60. Comparison of these values with those obtained from the border areas of study plots showed that the latter had a much higher species diversity index, ranging from H'=1.11 to 7.20; the highest value being nearly twice the highest value recorded from the study plots. The border areas had species richness ranging from S = 19 to 96 which is also much higher than the highest value obtained from the study plots. Using Sorensen's Similarity Index (S.1) formula, the similarity in species composition per study plot and their border area basis ranged from 6.5 to 100%. On the other hand, a similarity index of 71% was obtained for the entire Isabela Province where 147 species were found common to the study plots and their border areas out of a total of 268 species recorded from the 15 study localities. In terms of taxon composition, the 151 species recorded for the 15 corn cropping systems in Isabela Province belong to 123 genera and 40 families. Of the 40 families, the top 5 families in terms of number of species are Papilionaceae (29 spp). Graminae (25 spp.), Euphorbiaceae (16 spp.), Compositae (12 spp.), and Moraceae (11 spp.). The rest of the 35 families had each a species ranging from 1 to 9. In terms of habit composition, the 151 species are composed of 25 grasses, 80 herbs, 2 lianas or woody vines, 3 large trees, 6 medium-sized trees, 12 shrubs,10 small trees, and 13 vines. Despite having the second highest number of species present in the 15 corn cropping systems in Isabela Province, 24 of 25 species of Family Gramineae do not pose as potential recipients of B.t. corn pollen in the event that corn plants of this type will be cultivated on a large scale in Isabela Province. Even C. lacrymajobi L. (Job's tears or Tigbi), although diploid and outcrosser, will not hybridize with modern corn due to the fact that these two grass species are spatially, ecologically, temporally as well as cytologically isolated from each other. Plants of Job's tears are usually found along watercourses such as banks of irrigation canals and frequently, in association with rice cropping systems instead of corn ecosystems. By virtue of being inherently phylogenetically unrelated, the other 39 families of flowering plants will not serve as potential recipients nor temporary storage units of B.t. corn pollen grains. By and large, species belonging to these 39 families, particularly those shrubs and tall trees growing on border areas of corn fields, will serve as physical barrier to dispersal of B.t. corn pollen grains and eventually, deathtraps for these. Conversely, the 80 herbaceous species present in the 15 corn cropping systems, mostly widespread annual weeds, would not be affected by the B.t corn pollen as these are normally removed by the farmers on a regular basis long before the flowering stage of the corn plants. As far as conservation of biodiversity is concerned, the 15 corn cropping systems had a remarkably low level of endemism, approximately 3%. In fact, many of the study sites had no endemic species. For those study sites with some endemic species, only a few of these are considered to be endangered