Ecological and biogeographical correlates of rarity in two narrow endemics in Tasmania : Spyridium microphyllum (F. Muell. ex Reisseck) Druce and Spyridium obcordatum (Hook. f.) W.M. Curtis
Creators: name: Coates, F
Publisher: name: University Of Tasmania
Year: 2023

description: Spyridium microphyllum (F. Muell. ex Reissek) Druce and Spyridium obcordatum (Hook. f) W. M. Curtis are narrow endemic shrubs which are confined to Tasmania and are rare within their present ranges. Spyridium microphyllum is an obligate seed regenerator, whilst S. obcordatum is apparently able to regenerate both by seed and by resprouting. Ecological and biogeographical explanations for rarity in the two species were investigated by analysing their biological, bioclimatic, phytosociological and phytochorological correlates. Spyridium microphyllum was not significantly affected by grazing and is unlikely to be rare because of predation. Spyridium obcordatum is moderately to heavily grazed in summer but is able to recover biomass the following spring. Seedlings are likely to be particularly vulnerable. Seed production was not significantly affected at one site where the grazing trial was conducted, where rabbits were the main predator and the ground layer consisted of a high proportion of herbs and grasses. However, concentrations of native animals may significantly affect reproductive output where herbaceous food sources are scarce. Consequently, predation may contribute to rarity under these circumstances. Phenological observations revealed markedly different patterns relating to flowering, fruit maturation and time to reproductive maturity, suggesting that the two species may have different evolutionary histories. Spyridium microphyllum flowers in mid to late summer and seed is released one year later. This may enable plants to avoid competition for pollinators in spring, as well as maximise seed storage in the soil when ant activity is highest. Although seeds are exposed to threats for most of the year after fertilisation, the same pattern is known from widespread Spyridium taxa and does not explain rarity in S. microphyllum. However, plants may be disadvantaged by this regime where there is a reduced seed bank. Spyridium microphyllum does not set seed for at least four years after germination and may be eliminated by a series of successive fires. Spyridium obcordatum flowers in early spring and produces seed within two months. In this respect, it is similar to a number of widespread taxa within the Rhanmaceae and other families. Plants become reproductively active within two years. Thus, phenological patterns do not explain rarity in this species. Seed dormancy results from physical characteristics of the seed coat. Seed viability is high, and seed of both taxa germinate after scarification of the testa, or after stimulus by heat within similar ranges required to break dormancy in widespread congeners. In some instances, the two rare species responded to a wider range of heat stimuli than some widespread species. However, there were genecological differences in the germination behaviour of populations from different sites. In spite of these, rarity could not be explained by poor seed viability or lack of stimulus for germination. The loss of soil stored seed may partially contribute to rarity in the two species. Seed burial trials revealed S. microphyllum seed is only partially retained in the soil. Spyridium obcordatum was less vulnerable to seed bank depletion. However, losses are within the ranges recorded for other taxa and are replaced annually. Seed burial did not increase germinability in either of the two species. Consequently, loss of soil stored seed may be significant only if seed banks are not replenished annually because of repeated disturbance by fire or excessive predation. An analysis of the geographic patterns of distributions of Spyridium taxa determined that centres of endemism and species richness within the genus are in the South West Province of Western Australia and in southern South Australia. Eastern Tasmanian Spyridium species comprise an outlying concentration of endemics. There are no endemic centres in eastern Australia. The majority of taxa had no topographic or edaphic preference but were mainly associated with heathland or shrubland. A comparison of the geographic and climatic distributions of Spyridium taxa enabled hypotheses to be constructed which might explain rarity in the genus. Spyridium microphyllum and S. obcordatum were geographically and climatically narrowly distributed. In respect of the latter, they were similar to some geographically widespread Spyridium taxa and it was concluded that the two species are unlikely to be entirely restricted by climate. Recent divergence, inadequate time for dispersal, drastically reduced ranges or poor competitive ability were more likely explanations. Analysis of quadrat data using multivariate statistical methods showed that the two rare taxa are restricted by environmental conditions. Sites supporting the same species differ floristically and environmentally, and perturbations resulting from fire, soil disturbance or periodic flooding are influential events. Edaphic controls maintain environments where more competitive vegetation, especially trees and tall shrubs, are excluded. In combination, these characteristics ensure the perpetuation of shrub dominated vegetation supporting S. microphyllum or S. obcordatum. The frequency of such habitats in Tasmania and the range of habitats supporting populations, suggests that rarity in S. microphyllum and S. obcordatum cannot be attributed to confinement to a rare habitat. Thus, combinations of locally specific environmental conditions and interspecific competition, rather than a unique habitat, appears to be linked to rarity in both taxa. A phytochorological analysis determined that the two Spyridium species are associated with aggregates of taxa with restricted ranges. Spyridium microphyllum was primarily associated with other narrow endemics or with rare non - endemics concentrated in eastern Australia. Spyridium obcordatum had a stronger relationship with taxa concentrated in southern and southwestern Australia, some of which are rare in Tasmania. Where soil disturbance was substantial at one site, also thought to have been a refugium for forests during the late Pleistocene, S. obcordatum was associated with taxa widespread in southeastern and southern Australia. The results suggested that sources of interspecific competition have a basis in the disturbance and biogeographical histories of sites. Ultimate causes of rarity in S. microphyllum and S. obcordatum are best considered from a biogeographical viewpoint. Spyridium microphyllum is likely to be relictual under the present climate, having been a member of shrub dominated vegetation formerly more widespread in Tasmania under more arid conditions than exist at present. Spyridium obcordatum populations are likely to have been more or less continuous as part of a Bassian flora which is now extinct, the relicts of which have been incorporated into coastal heathland or open rocky environments. Thus, the most parsimonious explanation for rarity in both species is temporal remoteness from optimal conditions. Stochastic events relating to local extinction, establishment and maintenance of populations have influenced their survival. Ecological explanations for rarity in the two species are most likely to be a consequence of their present relictual status. These relate to poor competitive ability under conditions of improved soil moisture, or in closed or undisturbed vegetation, and a lack of safe sites for regeneration. Land use practices since European occupation of Tasmania further endanger populations at present.