ABSTRACT

The endangered herb Rutidosis leptorrhynchoides from south-eastern Australia has had its population numbers and sizes severely reduced by habitat loss and fragmentation. Assessment of allozyme variation in 18 diploid and five tetraploid remnant populations shows a strong positive relationship between the logarithm of population size and allelic richness for diploid populations, but not for tetraploids, which generally maintain higher diversity. Diploid populations smaller than 200 plants also show reduced S allele richness at the self-incompatibility locus, which constrains mate availability. Reduced mate numbers are associated with increased variance in male fitness in small isolated populations as reflected by high correlated paternity and increased deviation in allele frequencies between adult and pollen gene pools. Such changes reduce effective population sizes in small populations below their census size, further exposing them to effects of genetic drift.

Stochastic matrix projection models based on two years of demographic data from five populations show that population persistence is related in a general way with both higher genetic diversity and lower correlated paternity. Variation in population performance is primarily due to differences in the magnitude of the variance in transition probabilities rather than their mean values, which are generally not significantly different.

Measurement of genetic variation and fixation indices suggests that small re-established populations suffer the same constraints as small remnant populations. Conservation strategies for R. leptorrhynchoides should focus on maintaining population sizes above 200 plants to avoid demographic effects of S allele erosion and future re-establishment efforts should source seed broadly for the same reasons.

ABSTRACT

In the past, single-population approaches have dominated ecology and evolutionary biology. However, populations are not isolated either in time or space, but are connected by among-population processes such as migration and gene flow. While this concept is not new, until recently, there have been relatively few studies that have explicitly investigated the effects of spatial structure on demographic and genetic processes in the context of conservation. The metapopulation framework explicitly recognises and provides a conceptual tool for dealing with the interactions of within- (e.g. birth, death, competition) and among-population processes (e.g. dispersal, gene flow, colonisation and extinction). The ever-growing diversity of empirical and theoretical studies that demonstrate the importance of spatial structure in determining ecological and evolutionary trajectories also indicates that long-term conservation programmes need to focus on regional rather than local within-population persistence. In this regard, it is important to realise that ultimately all populations are ephemeral, and therefore colonisation processes must also be preserved. Clearly, not all species whose populations have undergone fragmentation fit the definition of a metapopulation. Nevertheless, a metapopulation approach to conservation biology is likely to provide a useful tool for developing management strategies as it addresses genetic, species and community effects of fragmentation in a single framework, thereby making explicit questions regarding extinction, population connectedness, species behavioural patterns and the survival of coevolved systems. In essence, a metapopulation perspective ensures a processoriented, scale-appropriate approach to conservation that focuses attention on among-population processes critical for persistence of many natural systems.

THEMETAPOPULATION PARADIGM: AN INTRODUCTION

Traditionally, ecological studies of natural systems have been dominated by a focus on single populations, or several populations sampled at a single point in time.