NSW Department of Environment and Conservation, 2005
ISBN: 1 7412 2145 5
6 Biology and Ecology
- 6.1 Habit and longevity
- 6.2 Reproductive biology
- 6.3 Population size and structure
- 6.4 Response to disturbance
- 6.5 Fire ecology
6.1 Habit and longevity
I. trypherus is an annual or short-lived perennial herb with an erect to spreading, often multi- stemmed, habit. In ideal conditions, individual plants can survive for up to three years with stems reaching 250 cm in length (A. Bofeldt, Wollongong Botanic Gardens, pers. comm; M. Bremner, DEC, pers. comm.).
6.2.1 Vegetative reproduction
I. trypherus has been observed to reshoot from damaged stems following light grazing, trampling and extended dry periods (M. Bremner, DEC, pers. comm.). The species possesses a long taproot (to 30 cm long) and does not reproduce vegetatively from stolons (M. Bremner, DEC, pers. comm.). It is therefore expected that I. trypherus is wholly reliant upon seed production for its dispersal and recruitment.
6.2.2 Breeding system
The flowers of I. trypherus are morphologically hermaphrodite (ie they contain both male and female reproductive parts) although it is unknown whether the species is capable of self-pollination. Genetic self-incompatibility is the major out-crossing mechanism in the Brassicaceae family and has been reported to operate in the Sisymbrieae tribe to which I. trypherus belongs (Kusaba et al 2001).
The pollination mechanisms of the species are unknown although wind and insect pollination have been reported for other Brassicaceae species (Treu & Emberlin 2000). Brassicaceae flowers are considered to be attractive to insect pollinators because they possess nectaries and are abundant on individual plants.
Two types of flies have been observed visiting I. trypherus flowers and are considered to be potential pollinators. One species is a type of hoverfly (family Syrphidae, subfamily Syrphinae) and is almost certainly a generalist pollen feeder. The other species could be identified only as a type of beefly (family Bombyliidae) (D. McAlpine, Australian Museum, pers. comm.).
6.2.3 Phenology and fecundity
The flowering period of I. trypherus extends from December to June with fruit maturing from February (M. Bremner, DEC, pers. comm.; R. Johnstone, DEC, pers. comm.). The species flowers annually and appears to produce prolific quantities of seed.
Observations indicate that the failure of I. trypherus flowers to develop into fruit is more likely to be the result of damage to the flower than a failure of its pollination mechanisms (M. Bremner, DEC, pers. comm.).
6.2.4 Seed dispersal
The seeds of I. trypherus are numerous, oblong- ovoid in shape and 0.7 to 1mm long (Hewson, 1982). Seed is not stored on the plant but is released upon maturity from the dry dehiscent fruit. The fruits mature and release seed gradually along the inflorescence, ensuring that seed is dispersed over an extended period (R. Johnstone, DEC, pers. comm.).
Seed dispersal mechanisms for the species are not known although, given the small size of the seeds, gravity and water are potential dispersal mechanisms. I. trypherus seed becomes sticky when wet and may be dispersed by sticking to the fur or feathers of fauna (A. Bofeldt, Wollongong Botanic Gardens, pers. comm.).
6.2.5 Seed viability and longevity
No studies to investigate seed longevity or viability in I. trypherus have been undertaken. Three year old I. trypherus seed was successfully germinated at the Wollongong Botanic Gardens although germination rates were low (A. Bofeldt, Wollongong Botanic Gardens, pers. comm.).
It can be reasonably assumed that, as a short-lived obligate seeding species that does not store seed on the plant, I. trypherus is likely to form a soil seed- bank. The rediscovery of I. trypherus plants at Macquarie Pass (Ir2) in 2004 in a location where no plants had been observed during targeted surveys in 1998, 2001 and 2002 supports this assumption. The length of time that the I. trypherus seedbank can persist is unknown. Given the potential management implications of this, it is considered to be a critical aspect of the species ecology requiring investigation.
6.2.6 Seed dormancy and germination cues
The seed dormancy mechanisms and germination cues for I. trypherus are unknown. Germination of I. trypherus seed at the Wollongong Botanic Gardens was observed approximately one month after the seed was sown. It has been speculated that the chemical compound that covers each seed (and which becomes sticky when moistened) acts to inhibit germination, and leaching with water will break this dormancy (A. Bofeldt, Wollongong Botanic Gardens, pers. comm.). Daylight length may also play a role in triggering germination of the species (A. Bofeldt, Wollongong Botanic Gardens, pers. comm.).
Disturbance (discussed in Section 6.4 below) and light levels are expected to influence recruitment of the species. The density of I. trypherus plants tends to decrease as canopy cover increases at a site (M. Bremner, DEC, pers. comm.). It is likely that sufficient light levels for recruitment are maintained at least partially by the topography of many sites (ie cliff lines and steep rocky slopes).
6.3.1 Total population size
The total population size of I. trypherus (based on the most recent observation available for each site) is estimated to be 1120 plants, comprising at least 267 mature individuals. The short-lived nature of I. trypherus renders it susceptible to considerable fluctuations in population size over time. Such fluctuations are considered to be a natural response to changing environmental conditions, including weather (and particularly rainfall) in the short term and changes in the structure and floristics of associated vegetation over longer periods.
The fluctuations in plant numbers that have been recorded at I. trypherus sites are illustrated in Table 5. The extent to which natural fluctuations in population size are being exacerbated by the effects of threatening processes remains unknown and this is a critical aspect of the species ecology requiring investigation.
6.3.2 Population structure
The following populations (as defined in Section 5.1) can be delineated from I. trypherus records:
- North Marshall Mount (IR1);
- Macquarie Pass (IR2);
- Johnstons Ridge (IR3);
- Stockyard Mountain (IR4 to IR9);
- Minnamurra Rainforest (IR10, IR11 and IR14);
- Kangaroo Valley (IR12);
- Ettrema Gorge (IR13)
- Upper Kangaroo River (IR15);
- Lake Yarrunga (IR16); and
- Barrengarry (IR17).
No extant I. trypherus plants were located during recent surveys of the sites that constitute three of these populations (ie North Marshall Mount, Johnstons Ridge, and Ettrema Gorge).
I. trypherus has been observed to reshoot from damaged stems in response to physical disturbance including trampling and light grazing. The species has also been observed to reshoot from the base of desiccated stems following extended dry periods (M. Bremner, DEC, pers. comm.).
Small natural disturbances occur regularly on the steep, rocky slopes and cliff lines that are occupied by I. trypherus. Such disturbances are created by rock fall and the movement of water, animals and humans across unstable slopes.
It is possible that these disturbances create the regular recruitment opportunities that this short-lived species requires if it is to persist at a site in the long-term.
Disturbances at I. trypherus sites that are considered to be undesirable include those that result in the following:
- The removal or death of I. trypherus plants;
- The removal of any part of the soil seed bank;
- A reduction in the fecundity of I. trypherus plants;
- An increase in the projected canopy cover at a site or in proximate habitat; and
- The clearing of proximate habitat that will limit opportunities for the migration of the species.
I. trypherus is likely to be killed by fire and is considered to be an obligate seed regenerator. Post- fire recruitment of the species will therefore rely on regeneration from the in-situ soil seedbank and/or migration from nearby unburnt areas. A critical aspect of the species ecology requiring investigation is what intensity of fire (if any) the soil seedbank is capable of surviving.
The habitat of I. trypherus is not considered to be fire-prone although occasional burning of its sclerophyll forest habitat may assist in maintaining the structure and floristics of that habitat. This in turn may assist in maintaining light levels that are suitable for the recruitment and growth of the species. It is possible however, that light levels at I. trypherus sites are maintained by the topography of the sites rather than by fire.
Recruitment of I. trypherus occurs readily in the inter-fire period (M. Bremner, DEC, pers. comm.).