Insect activity patterns of are often affected by the environmental temperature.  For example low environmental temperatures in montane or arctic habitats may restrict the period of insect flight to the warmest seasons and times of the day.  Thermal limitation of insects may generate top-down control on pollination systems and exert convergent selection on several floral traits including mating system, flower size, and time of floral receptivity.  In contrast, reproductive interference (e.g. competition for pollination) between similar plant species may generate bottom-up influences and divergent selection on flowers size or shape, and when flowers open and pollinators can visit.  The balance of these two types of forces may have generated clines in floral traits as well as patterns in local and regional diversity and community structure.