Wild rice. Oryza meridionalis Ng. semipalmata) and the dusky plains rat (Rattus colletti). It may be. Tracheobionta (Vascular plants)

Transcription

1 Wild rice (Oryza meridionalis) FS12020 Kingdom: Subkingdom: Super-Division: Phylum/Division: Class: Order: Family: Genus & species: Common name: Plantae (Plants) Tracheobionta (Vascular plants) Spermatophyta (Seed plants) Anthophyta/Magnoliophyta (Flowering plants) Liliopsida (Monocotyledons) Cyperales (Grasses and sedges) Poaceae (Grasses) Oryza meridionalis Ng Wild rice Wild rice (Oryza meridionalis) is an annual, emergent grass, which grows on the floodplains during the annual wet season, and persists only in the soil seed-bank during the dry season. Fig. 1. Inflorescences of wild rice O. meridionalis is both widespread, and abundant on Top End floodplains during the wet season. It is an important food resource for vertebrate fauna, especially the magpie goose (Anseranas semipalmata) and the dusky plains rat (Rattus colletti). It may be considered the fuel underpinning the floodplain vertebrate food chain Penny Wurm

2 Phenology The growing season for O. meridionalis starts in the early wet season. Seedlings emerge at the soil surface after approximately 150mm of rain has fallen. Seedlings then have to keep pace with rising water levels, as the floodplain soils become saturated and then inundated by standing water. Young seedlings can survive short period of complete inundation, but this may kill a proportion of them Surviving plants continue to grow as the water levels rise. Their emergent leaves make sure that photosynthesis and growth are maintained, in preparation for flowering. Inflorescences start emerging in February, and seed shed starts in April. However, as long as the soil is flooded or even wet, plants will continue to produce inflorescences. Large numbers of seeds are shed into standing water or onto wet soil. In nursery studies, seed production of 2,300 seeds per plant was observed. So as you can see, like most grasses, individual rice plants are very fecund. Each plant has a tremendous capacity to replace itself, or make up for losses of other individuals in the population, in a particular year. 2

3 Field measurements of seed production taken a t two sites on the South Alligator River floodplain, between range from 26 kg ha -1 to 263 kg ha -1. (Table 1). Table 1: Calculations of gross seed production (kg ha -2 ) of O. meridionalis at Water Recorder Point and Boggy Plain in 1992, 1993 and WRP BP This seed is a precious resource for floodplain vertebrates and much of it is eaten by rats and magpie geese. Magpie geese browse on emergent inflorescences, and strip off any mature seed. This often flattens (onto the water) areas of otherwise erect plants. Although magpie geese are an important rice seed predator, their impact is local, and plant can recover after being grazed. Rats eat a vast number of the seed produced forage on the soil surface for seeds but can even swim and access some of the seed produced in flooded areas. In a study in which seed caches were placed on the ground in the field and checked each morning, it was found that almost 75% of the seed was consumed. Small wire exclosures placed around a subset of the seed caches indicated that rats were the culprits! 3

4 However much seed escapes predation. Stiff, hook-shaped hairs covering the husk of each seed, ensure that any escaping predation works its way into the soil. Even though the soil is wet or flooded at the time of seed shed, seed does not germinate in response to the moisture. Thus, seed that makes its way into the soil seed-bank remains dormant until the following wet season. Field studies have shown that during the dry season, seed dormancy is broken in some 50% of Oryza seeds, which are then available to germinate in the first wet season after shed. A further 40% germinate at the beginning of the second wet season after entering the seed-bank, and 10% die in the soil during these 2 dry seasons. References Wurm P.A.S. (1998). A surplus of seeds: high rates of post-dispersal seed predation in flooded grassland in monsoonal Australia. Australian Journal of Ecology 23: Wurm P.A.S. (1998). The population ecology of Oryza meridionalis Ng on the South Alligator River floodplain, Kakadu National Park, monsoonal Australia. PhD thesis, Northern Territory University, Darwin. 4

5 Fig. 3. Conceptual model of the life-cycle of Oryza meridionalis. Life-cycle stages are represented in squares, regulating effects in shaded ovals and external variables in unshaded ovals. Transitions between life-cycle stages are represented by block arrows, impacts of external variables on transition probabilities are indicated by line arrows, and influences of population regulating factors are indicated by broken lined arrows. The returned arrow at the seed-bank stage of the life-cycle represents those seeds which persist in the soil for more than one dry season. 5