This article is an extract from a paper Population dynamics, distribution and health assessment of Macrozamia fraseri (Cycadales) in Kensington Bushland Reserve, Perth, Western Australia by Curtin University students Jeyda Aktepe, Jake Batt, Jessica Bayley, Neasa Flynn, Catelin Roberts and Zoe Rose.
Context
Macrozamia fraseri, a cycad native to southwestern Australia, is at risk due to the urbanisation of Perth. Clustered distribution, high seedling mortality and a lack of seed dispersal vectors, compounded by environmental changes, may threaten its long-term survival and recruitment.
Data for this study were collected in the Kensington Bushland Reserve (Bush Forever site 048) in September 2024. The density of M. fraseri at the study site was 12.7 individuals/ha, which was lower than expected for the species. The population had a median and mode health rating of four, however, there was a high mortality rate of 15.97%. The population had the highest proportion of seedlings (56.52%) compared to other Perth region study sites. Among adult plants, 61.36% were of identifiable sex by the presence of recent or remnant cones. Of the 11 females, 54.54% had identifiable seedling or juvenile offspring.
The study concluded that clustered distribution, high seedling mortality and a lack of seed dispersal vectors, compounded by environmental changes, may threaten the population’s long-term survival and recruitment.
Introduction
Macrozamia fraseri, a member of the Zamiaceae family, is a dioecious cycad native to southwestern Australia (Western Australian Herbarium 1998). M. fraseri is a long-lived, large-seeded species common in the Perth metropolitan region and is found predominantly in sandy soils of the Swan Coastal Plain and Geraldton Sandplains (Atlas of Living Australia 2024). Although its conservation status is currently not listed as threatened, increasing urban sprawl and habitat disturbance in the region pose potential risks to its long-term survival (Nathan and Muller-Landau 2000; Carlo and Morales 2008). The Kensington Bushland Reserve provides an important refuge for M. fraseri, as well as other endemic species. M. fraseri rely on large frugivores such as mammals and birds to disperse its fruit long distances, though dispersal vectors are limited in this reserve (Snow and Walter 2007).
M. fraseri is a slow-growing, scatter-dispersing species (Burbidge 1987; Howe 1989). These factors, combined with the dioecious nature of cycads—where male and female reproductive structures are found on separate plants—can lead to imbalanced sex ratios, which may further limit reproductive success (Gerlach 2012). In fragmented habitats like the Kensington Bushland Reserve, genetic bottlenecks, inbreeding and reduced gene flow may also contribute to decreased population viability.
Materials and methods
- Due to the small scale of the study site, a comprehensive census survey was conducted across the entire reserve. Each plant was marked with a unique number using flagging tape, and GPS coordinates and measurements (see data collection) were recorded.
- Data collection took place on the 2nd and 3rd of September, 2024. For each plant, measurements included the plant diameter and height using an 8 m tape measure. Plant height was measured from the ground to the tip of the tallest leaf, while plant diameter was measured, from leaf tip to leaf tip, across the widest part of the plant. Plant height aided in determining an estimated age class (age in this study refers to plant developmental maturity): seedling, juvenile and adult. Where possible, the sex of individuals was recorded. The adult plants were determined as male or female based on the presence of cones or remnants of cones within the leaf crown and around the base of the plant. Male cones have more numerous thinner, elongated scales, and female cones have fewer larger, broader scales. Sex was classed as indeterminate for all seedlings and all juveniles, as well as some adults that had an absence of cones.
Male and female Macrozamia fraseri cones. (a) Male cones have more numerous thinner, elongated scales compared to female cones; and (b) Female cones have fewer larger, broader scales. Images taken 03/09/2024.
- Plant health was assessed by observation, using a rating scale from 1 (dead) to 5 (perfect condition). In addition, the number of dead and alive leaves, number of other M. fraseri individuals within a 5 m radius, vegetation density and soil texture were recorded. For seedlings and juveniles, the distance from the likely parent (female) plant was recorded with an 8 m tape measure to quantify dispersal distance.
Results
- There was a total of 138 individuals in the study site, at a density of 12.7 individuals/ha. Of this, 78 were seedlings (0–40 cm tall), 16 were juveniles (41–80 cm tall) and 44 were adults (>80 cm tall). Plant height ranged from 10 cm to 326 cm and the median value was 34 cm. Plant diameter ranged from 2 cm to 580 cm and the median value was 15 cm. The total number of leaves, which is the sum of the dead and alive leaves, ranged from 1 to 186 leaves, the mean was 22.97 leaves, the mode was one leaf and the median was three leaves.
- All individuals that were seedlings and juveniles were of indeterminate sex. Of the individuals that were adults, 16 were male, 11 were female and 17 were of indeterminate sex. The sex ratio for adults of identifiable sex was approximately 1.45 males per female.
- The population had a mean health rating of 3.28 and a median and mode health rating of 4. Adults had significantly better health than seedlings. There were 19 individuals with a health score of 1, meaning 15.97% of the population were dead.
- It appears that there are two distinct corridors where individuals most commonly occur. These corridors run parallel to each other, along the north-east edge and south-west edge of the study area.
- The distribution of ages of individuals across the site appears to be random. The observed mean distance between seedlings and juveniles was approximately 3.56 m. The expected mean distance for a random distribution is approximately 20.04 m. Seedlings and juveniles are generally closer to other juveniles and adult females than to adult males. However, they are still most clustered around their nearest seedling or juvenile neighbours.
Discussion
At a density of 12.7 individuals/ha, the site has a vastly lower population density than observed by Gerlach (2012) at Kings Park, with 341.9 individuals/ha (424 individuals over 1.25 ha) (Fig. 2). Initial observation of the comparatively low population at Kensington Bushland Reserve may indicate a declined population from higher population densities such as those seen at Kings Park. This proves the hypothesis that the species is low in numbers.
In some flora species, low population densities have been found to be beneficial for survival due to higher pollinator success and lower resource competition, however self-incompatible species, such as M. fraseri, may be more vulnerable at low density (Mustajärvi et al. 2001).
The M. fraseri population at the study site is highly clustered. Corridors of individuals are observed along the south-west and north-east edges of the site. This type of clustering may be due to environmental factors or a lack of dispersal vectors.
The emu (Dromaius novaehollandiae) has been identified as the major dispersal vector for M. fraseri, with the common brushtail possum (Trichosurus vulpecula) and the Australian raven (Corvis coronoides) being key dispersal vectors where the emu is absent (Burbidge and Whelan 2006; Gerlach 2012). The study site has no presence of emus, so possums and ravens are expected to be the key dispersers of seeds. Gerlach (2012) observed ravens dispersing seeds up to 390 m. Observations from Kensington Bushland Reserve found that the furthest seedling from any adult female was only 8.5 m, with the vast majority of seedlings being less than 3 m from their adult female parent plant.
The proportions of each age class in the population are 56.52%, 11.59% and 31.88% for seedlings, juveniles and adults, respectively. Avon Valley, Sawyers Valley and Kings Park had 12% to 22% seedling populations (Gerlach 2012). The higher seedling proportion at the study site (56.52%), indicates there is greater recruitment. The three sites surveyed by Gerlach (2012) identified approximately 57% to 70% juveniles, compared to 11.59% at Kensington Bushland Reserve. The lower proportion of the population that are juveniles, compared to other sites, may highlight an issue in the survival of seedlings at the study site (Gerlach 2012). This age structure identifies issues in seedling mortality, which further justifies the need for intervention for the species survival at the study site.
Population demographics of Macrozamia fraseri (seedlings, juveniles, indeterminate adults, male adults and female adults) at different study sites across Perth. Note: all seedlings and juveniles are of indeterminate sex. Adapted from Gerlach (2012).
Although the median and mode health score was four (Fig. 4; Fig. 8), the proportion of individuals with a health score of one (dead) was higher for the study site (15.97%) compared to other sites in the Perth region (Table 1; Gerlach 2012). Sawyers Valley’s higher mortality level (4.02%) was attributed to introduced Phytophthora dieback in the area, but Kensington Bushland Reserve is not affected by Phytophthora dieback (Gerlach 2012; Eco Logical Australia 2018).
Table 1. Level of mortality of Macrozamia fraseri the study site in comparison to other sites in Perth (Gerlach 2012).
| Study location | Number of dead individuals | Number of alive individuals | Number of dead individuals as a proportion of population (%) |
| Kensington Bushland | 19 | 119 | 15.97 |
| Kings Park | 0 | 424 | 0.00 |
| Avon Valley | 2 | 918 | 0.22 |
| Sawyers Valley | 73 | 1743 | 4.02 |
Intervention
The high seedling mortality could be explained by intraspecies competition, with seedlings not dispersing from the base of the maternal plant (Burbidge 1987; Howe 1989).
It was determined that no intervention in the near future will be needed. However, if future population studies of M. fraseri at Kensington Bushland Reserve determine that the populations is unhealthy, relevant intervention is required. One intervention strategy could be anthropogenic seed dispersal, to address seedling clustering and intraspecies competition.
Cycads are considered to be globally declining due to climate change and their inability to outcompete angiosperms (Laidlaw and Forster 2012). In the south-west of the state, between 1910 and 2023, the average temperature increased by 1.1°C, rainfall has declined by 20% and fire risk has increased (Department of Primary Industries and Regional Development 2023). The increase in warmer, drier conditions may lead to the decline or even extinction of Macrozamia species, as many cycads are considered to be in an evolutionary dead-end awaiting extinction (Laidlaw and Forster 2012). However, Macrozamia species are long-lived and a longer time period is likely needed to observe major impacts to population structure (Gerlach 2012).
Conclusion
This study aimed to assess the population dynamics, distribution and health of the M. fraseri population in Kensington Bushland Reserve, examining whether environmental factors or plant characteristics influence these parameters, and to determine if intervention is needed to ensure the species’ long-term viability. The density of M. fraseri was lower than expected for the species. This proves the hypothesis that the species is low in numbers. The health of the species was high, which disproves the hypothesis that the species has low health, however there was also a high mortality rate. The population had the highest proportion of seedlings compared to other sites in the Perth region. Of the adult plants, the majority were of identifiable sex due to the presence of recent or remnant cones. Additionally, the majority of females had identifiable seedling or juvenile offspring, meaning the hypothesis that there are limited or no signs of recruitment is disproved.
As population dynamics are studied on a temporal scale, using a single time point for determining population structure is a limitation of this study. Follow up research is required to measure changes to the population distribution and demographics over time, especially since M. fraseri does not have a well-researched definition of species maturity. This study is the first to investigate M. fraseri at Kensington Bushland Reserve, providing valuable insight into the population dynamics, distribution, health, sex and dispersal patterns of the species. This can assist with the creation of well-informed management plans to ensure the species’ long-term viability.
Acknowledgments
Experiment design and conduction assisted and supervised by Shane Turner, School of Molecular and Life Sciences, Curtin University, Kent Street, Bentley, 6102, Western Australia, Australia. Data analysis assisted by Michael Just, School of Molecular and Life Sciences, Curtin University, Kent Street, Bentley, 6102, Western Australia, Australia.
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