NSW Department of Environment and Conservation, 2005
ISBN: 1 7412 2143 9
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
Z. granulata is a tall bushy shrub or small tree that can grow to 6 metres in ideal conditions (M. Robinson, pers. comm.). The longevity of the species is not known although, given the large girth of the trunks that have been observed on some mature specimens, it is suspected of being in excess of 50 years (M. Bremner, DEC, pers. comm.).
6.2.1 Vegetative Reproduction
Vegetative reproduction from subsurface epicormic buds (suckering) has not been reported for Z. granulata (Armstrong 2002). The species has been observed to reshoot from the base of mature stems (coppice) in response to mechanical disturbance (M. Bremner, DEC, pers. comm.). The age at which the species is capable of coppicing is not known.
6.2.2 Breeding System
Zieria flowers are morphologically hermaphrodite, having both male and female organs present in each flower. The condition of dichogamy (ie the maturation of the stigma and anthers at different times) is prevalent in the genus and is frequently coupled with herkogomy (ie the spatial separation of anthers and stigma) (Armstrong 2002). These conditions act to reduce the chances of self-fertilisation and enhance cross-pollination. However, it is not known whether these conditions operate in Z. granulata.
A genetic study of Z. granulata plants at Tabbagong Forest and Bombo found that a low level of genetic diversity was present, indicating that significant cross-pollination was likely to be occurring within that study area (Sharma & Young 2000).
Armstrong (1990) states that the flowering period of
Z. granulata is from late spring to summer although Robinson (1999) has observed the species flowering in early spring. It is not known whether individuals flower regularly or sporadically. The species failed to flower at many sites during the drought of 2002/2003 (M. Bremner, DEC, pers. comm.).
Z. granulata is known to possess functional pollen although deformed pollen has been recorded for the species (Armstrong 2002). The species open floral structure indicates that pollination by a generalist pollinator is likely, although it is possible that the pollinator is a specialist (Robinson 1999).
Nectar seeking flies (viz. Biblio imitator, Biblionidae; Callophora hilli, Calliphoridae) have been observed on Z. granulata flowers and appear to be effective pollen vectors (Armstrong 2002). These flies are considered to be generalist pollinators and strong fliers that are capable of traversing open spaces (Robinson 1999).
Robinson (1999) has also observed an Eastern Spinebill (Acanthorhynchus tenuirostris) probing Z. granulata flowers.
6.2.5 Seed ecology
Seed dispersal in Zieria spp. is by forcible ejection from the mature coccus (fruit) (Armstrong 2002). The presence of an elaiosome (an ant-attracting appendage to the seed) indicates that secondary dispersal by ants (termed myrmecochory) may also occur (Robinson 1999).
It appears likely that some form of dormancy mechanism exists for Z. granulata seed and that a disturbance related trigger is required to break this dormancy (see Section 6.4). However, low levels of seedling recruitment have been observed in the inter-disturbance period (Robinson 1999; A. Bofeldt, Wollongong Botanic Gardens, pers. comm.).
There is no published information about the longevity and viability of Z. granulata seed and these are considered to be critical aspects of the species biology requiring investigation.
6.2.6 Reproductive maturity
The age at which Z. granulata is capable of flowering and producing seed is not known. This is a critical aspect of the species biology that requires investigation, as it will assist in determining disturbance intervals that the species may be capable of surviving.
Robinson (1999) makes the following unquantified observations of flowering Z. granulata plants at Tabbagong Forest:
- Flowering was observed in individuals less than 1 m tall (although these individuals may have had their height reduced by grazing);
- Flowering was most common for individuals between 1 and 2 m tall; and
- Flowering was typically more prolific for individuals greater than 2 m tall.
6.3.1 Total population size
At least 11,739 Z. granulata plants (including both mature and immature individuals) have been observed at 77 of the 97 recorded Z. granulata sites (see Appendix 2). It should be noted that in calculating this figure; the lowest stated value has been used for sites where a size range has been recorded, and a value of 2000 plants has been attributed to each of the two sites that have been described as containing thousands of plants.
It should also be noted that many sites have not been surveyed for over ten years and some of these sites may have been lost, or experienced significant changes in plant numbers, during this period.
Following the IUCN Red List Criteria (1994), the total population of a species is considered to be the number of mature individuals only. Using height class data collected at 14 Z. granulata sites (Robinson 1999; Kevin Mills & Associates 2000b;
M. Bremner, DEC, pers. comm.), it is estimated that 68% of Z. granulata plants are mature individuals (based on the assumption that all plants under 1 m in height are immature). Consequently, the total population size of Z. granulata is estimated to be approximately 8000 mature individuals.
6.3.2 Population structure
The number of Z. granulata plants that have been recorded at individual sites varies from a single plant to thousands of plants. As illustrated in Table 10 below, the majority of sites contain a very low number of plants.
|Number of individuals||Number of sites|
|1 to 10||23 (30%)|
|11 to 100||32 (42%)|
|101 to 300||12 (16%)|
*using lowest estimate of total plant numbers at 77 of the 97 Z. granulata sites
A total of 26 Z. granulata populations (as defined in Section 5.1) can be delineated from existing records. The number and size of the sites that form these populations is detailed in Table 11.
|Population||Sites in population||No. of Sites in population||No. of sites with plant counts||Total plants recorded in population*|
|Albion Park||Zg2 to Zg5||4||3||615|
|Mount Terry||Zg7, Zg8, Zg96||3||3||43|
|Dunmore Hills||Zg9 to Zg21, Zg37, Zg93, Zg97||16||9||4213|
|North Killalea||Zg22 to Zg25||4||4||424|
|Bass Point||Zg27, Zg28||2||2||5|
|Minnamurra Rainforest||Zg29, Zg30||2||1||10|
|Jamberoo Road||Zg31, Zg32||2||2||17|
|Terragong||Zg33 to Zg36, Zg38 to Zg43, Zg91, Zg92||12||8||441|
|Wallaby Hill Road||Zg45 to Zg47||3||3||450|
|Minnamurra Bends||Zg49, Zg50||2||2||4|
|North Kiama Tabbagong||Zg51 to Zg57, Zg94||8||8||597|
|West Kiama||Zg58 to Zg67, Zg95||11||8||1496|
|East Kiama||Zg68, Zg69||2||1||27|
|Mount Brandon Jerrara Dam||Zg71 to Zg76, Zg90||7||7||602|
|Saddleback Mountain||Zg77 to Zg79||3||3||14|
|East Saddleback||Zg80 to Zg82||3||2||69|
|Upper Foxground||Zg84, Zg85||2||2||12|
|Toolijooa||Zg87 to Zg89||3||3||2355|
*using the lowest estimate of total plant numbers at the 77 sites for which plant counts exist. A value of 2000 plants has been attributed to each of the two sites that have been described as containing thousands of plants.
Zieria granulata has been observed to respond to physical disturbance (including grazing and slashing) by coppicing from the base of damaged stems (M. Bremner, DEC, pers. comm.). However, the age at which plants are capable of coppicing in response to disturbance in not known.
It can be reasonably assumed that frequent disturbances (as a consequence of grazing, slashing, fire or other activities that destroy the above ground parts of the plant and prevent seed production) may lead to local extinctions of the species in the long term. This will occur if the disturbances are of a frequency that prevent the plants that germinate in response to an initial disturbance from developing to a stage where they are capable of producing seed and/or coppicing in response to subsequent disturbances.
Some form of occasional disturbance may be required to break seed dormancy mechanisms and encourage seedling recruitment of the species. Circumstantial evidence for this is the high levels of seedling recruitment observed at some sites following mechanical disturbance (Robinson 1999; Mills & Jakeman 1993; QEM 1994; B. LaFoe, RTA, pers. comm.). It is likely that natural disturbances
resulting from storm damage, animal scratching and possibly fire, would provide such recruitment opportunities for the species in a healthy ecosystem.
Research into how seedling recruitment is influenced by the frequency and timing of disturbance events is required to inform site management decisions.
There have been no recorded observations of the effects of fire frequency, intensity or seasonality on
Z. granulata or its soil seed bank. As Z. granulata appears to be incapable of suckering, it is unlikely that the species would survive a fire that was of great enough intensity to destroy the above ground parts of the plant. The intensity of fire (if any) that the Z. granulata soil seed bank is capable of surviving is not known.
Research into the fire response of the species (including its soil seed bank) is considered to be a critical aspect of the species biology requiring investigation.