Iridaceae - Ecology

Ecology

Members of the Iridaceae occur in a great variety of habitats. About the only place they do not grow is in the sea itself, although Gladiolus gueinzii occurs on the seashore just above the high tide mark within reach of the spray. Most species are adapted to seasonal climates that have a pronounced dry or cold period unfavorable for plant growth and during which the plants remain dormant. As a result most species are deciduous. Evergreen species are restricted to subtropical forests or savannah, temperate grasslands and perennially moist fynbos. A few species grow in marshes or along streams and some even grow only in the spray of seasonal waterfalls.

The above-ground parts (leaves and stems) of deciduous species die down when the bulb or corm enters dormancy. The plants thus survive periods that are unfavorable for growth by retreating underground. This is particularly useful adaptation for growth in areas like grasslands and fynbos which regularly have fires in the dry seasons— the plants are dormant and their bulbs or corms are able to survive underground. Veld fires clear the soil surface of competing vegetation and fertilize it with ash. With the arrival of the first rains, the dormant corms are ready to burst into growth, sending up flowers and stems before they can be shaded out by other vegetation. Many Iridaceae species which grow in grassland and fynbos flower best after fires and some fynbos species will only flower in the season after a fire.

The family has a very diverse pollination ecology. Although the majority of species of African Iridaceae are pollinated by Hymenoptera (mostly bees), the remaining species are pollinated mainly, or solely, by insects in the orders Coleoptera (beetles), Diptera (short- and long-proboscid flies) and Lepidoptera (butterflies and moths), or by passerine birds (Nectarinidae). It is now known that pollination systems are predominantly specialized: plants rely on a single species or a few ecologically analogous species for pollination. By contrast, generalist species, which are pollinated by a range of pollinators from at least three pollinator groups, are rare among southern African Iridaceae. In consequence, almost all genera of any size exhibit a range of pollination syndromes, with similar patterns of floral variation having developed repeatedly within different genera.

Most significantly, the diversity of pollination systems increases primarily with floral complexity and secondarily with genus size. Thus, Aristea (approximately 56 species), which has radially symmetric, mostly blue flowers, has three different pollination systems, whereas Sparaxis (15 species), with both zygomorphic and secondarily radially symmetric flowers, in a variety of colors, exhibits five different pollination systems, and Gladiolus, with a similar array of floral types but over two hundred species, exploits seven different pollination systems, some of which have evolved multiple times. As is usual in predominantly specialist pollination systems, floral attractants and rewards correlate closely with pollinator profile, resulting in the development of distinct floral syndromes. Attractants are primarily perianth pigmentation, complemented by a range or floral odors in many species, but flower shape and tepal orientation, in particular functional floral symmetry, may be equally important for some pollinators. The reward to visitors in the majority of species is nectar, but in others it is pollen, and one species offers non-volatile oil. In the case of hopliine beetles (Scarabaeidae, Hopliinae), flowers provide a stable platform on which to congregate, and the value of pollen, which beetles sometimes consume, as a reward is uncertain.