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Population dynamics with life history modelling of rare and common Grevillea species in Western Australia

Armstrong, Paul Gerrard (2002) Population dynamics with life history modelling of rare and common Grevillea species in Western Australia. PhD thesis, Murdoch University.

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The management of rare plant species often involves the maintenance of small populations. This is a difficult task as management prescriptions are limited due to an inability to trial different options on the species where few individuals are involved. ln small populations there is no resilience to failure if an unsuccessful management decision is made. This thesis reports on an investigation of selected factors that may have or have had an impact on the ecology and biology of four Grevillea species from the northern sandplains kwongan of southwestern Western Australia over a nine year period.

The aims of the study were twofold. Firstly, to compare several rare species with a common species to highlight and investigate general principles that might be involved in rarity and to examine how different features of the ecological and reproductive biology of a species relate to rarity. Secondly, from information obtained, to develop and test a model that could be used to examine the effects of different management scenarios on the plant populations without impacting on existing populations.

The relatively common and wide spread Grevillea armigera is an obligate seeder species which has a range of 125 km by 110 km. It flowers pro I ifically (> I 00,000 flowers per plant) mostly in spring but some flowers can be found all year, resulting in up to 300 seeds produced per plant per year. Many seedlings are produced following a fire but they are also frequent at other times. With a short juvenile phase and a high survival rate many plants flower and produce seed at two years of age.

The rare species encompass both seeders and resprouters. Grevillea calliantha is a critically endangered facultative resprouter species with a very restricted range ( < 10 km diameter) and is found mostly as small populations. It produces large numbers of flowers (>70,000) in spring to summer but seed production appears to be limited, typically averaging 120 seeds per plant per year. Seed-set was very variable between populations and individuals, with the less disturbed sites producing more seeds per plant than populations on degraded roadside sites. Germinations occurred predominantly in the post-fire period, from smoke inoculation or from other disturbances. Of the moderate numbers of seedlings produced, few survived the five years needed to reach maturity. Following a fire all adults regenerated from lignotubers and were flowering and setting seed three years later. In addition, what seemed to be root-suckers of the mature plants were recorded post-fire.

Grevillea christineae is an endangered obligate seeder with six populations from three disjunct areas (separated by 140 km and 250 km) mostly with moderately small numbers. It produces abundant flowers (>25,000) in winter and spring but fertilisation and seed-set appeared to be limited, typically producing 70 seeds per plant per year. No fires were recorded from any of the populations. However some seedlings were recorded, with no apparent soil disturbance. Of these, few survived the three years to maturity.

Grevil/ea saccata is a rare obligate seeder species with moderate abundance and a restricted range (140 km N-S and 20 km E-W). It produced many flowers (>500) from winter to spring, but fertilisation and seed-set were limited, typically producing 64 seeds per plant per year. No fires were recorded at any of the population sites. However, only a few seedlings were recorded and these were all the result of soil disturbance. Of these, few survived the three years to maturity.

Based on field observations over a five to eight year period, a generalised individual based. stochastic, density dependant, and non-linear simulation model was produced, called Individual Plant-Population Simulator Model (IPPSM). IPPSM used a reductionist approach, with as many factors and processes treated as practical considerations allowed. These factors and processes include the plant’s growth and survival rates, seed-set, seed survival and germination rates. In addition to these factors, environmental processes such as rainfall, frequency and season of fires, responses to smoke inoculation, inbreeding depression and catastrophes, were modelled.

IPPSM was used to simulate a two hundred year period for a population, the results of which included the mean number of adults, juveniles, seedlings, germinations and seedbank size for each year of the simulation. The cumulative probabilities of the population becoming extinct by that year and the population falling below a given size (quasiextinction) could also be considered. In addition the parent plant replacement potential was determined from the number of germinations required to produce a plant to that age, and the number of seeds estimated in the seedbank. Approximately three hundred scenarios on each species were investigated to examine the following: standard conditions equivalent to field conditions with an assumed normal fire regime of 15 year fire cycle; sensitivity analysis; environmental changes where processes of the environment were altered (for example fire frequency) and active management intervention.

Using the standard conditions, IPPSM produced what could be considered realistic results based on the limited time of field observations. The rare species declined in population over time. resulting in a moderate extinction probability, while the common species had a stable population and a low extinction probability. Results from the sensitivity testing indicated that habitat factors and adult survival were the most critical factors for the survival of a population.

Of the environmental changes studied the manipulation of fire frequency could greatly degrade or enhance the survival of a species. For example fire frequencies between seven to twelve years produces a stable population with low extinction probabilities for G. calliantha. Yet the same regime would result in rapidly declining numbers and a high extinction probability for G. saccata. Conversely, for long fire-free periods G. saccata produces an almost stable population with moderate extinction while G. calliantha rapidly becomes extinct without fires.

Active management tested the use of smoke inoculation to boost declining populations under reduced rainfall and under fire exclusion conditions. Appropriate uses of these were found to be beneficial in reducing the population decline and extinction probabilities. In addition, the use of supplemental plantings was found to produce stable populations with low extinctions for the normally declining species.

When comparing the common species to the rare species the most obvious differences were that the common species produced much more seed that germinated more readily. Once germinated the seedlings had a far greater survival rate and reached maturity sooner. This contrasted to the rare species producing less seed; germinations were fewer post-fire and rarely occurred at other times. Those seedlings that were produced had a lower survival rate and took longer to reach maturity than the common species. The differences related to the time required to reach maturity were likely to be associated with the differences in the basic biology of each species.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Hobbs, Richard and Ladd, Phil
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