Spatial and Temporal Segregation of Juvenile and Mature Garlic Mustard Plants (Alliaria Petiolata) in a Central Pennsylvania Woodland

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ABSTRACT.-

Garlic mustard (Alliaria petiolata) is an invasive, spring-germinating, obligately biennial plant. In a central Pennsylvania forest, two distinctive plant patch types exist simultaneously: patches with mixed juveniles and adults and patches that are exclusively juvenile. We experimentally tested two hypotheses to explain this peculiar distribution. The habitat limitation hypothesis proposes that garlic mustard, like other biennials, is limited to rarely available habitats and can only reach maturity in a subset of the patches where its seeds germinate. The intraspecific competition hypothesis proposes that juveniles only survive to maturity in patches lacking adults. We mapped the natural distribution of ten juvenile and ten mature patches and used transects to assess juvenile and adult density in 2 y. We found that patches cycled annually from juvenile to mixed, supporting the intraspecific competition hypothesis. We looked for biotic (interspecific competition) and abiotic (shading) factors that might contribute to habitat limitation, but none affected juvenile performance. We also looked for evidence of intraspecific competition by monitoring juvenile growth in juvenile, mixed and mixed with adults-removed patches. We found that juveniles from juvenile patches have a higher probability of survival than juveniles from mixed patches and adult removal improved juvenile survival in mixed patches. We conclude that the spatial and temporal segregation of juvenile and adult age classes is maintained by intraspecific competition.

INTRODUCTION

Biennials often have unstable population dynamics and two general classes of hypotheses have been advanced to explain this, one involving extrinsic factors and the other involving intrinsic factors. Meta-analyses of plant distributions have suggested that biennials exploit intermittently disturbed habitats (Harper, 1977), and that these habitats are transient, limiting biennial distribution (Hart, 1977; Viswanathan and Aarssen, 2000). Furthermore, within these habitats, biennial populations commonly go locally extinct, persisting only at the metapopulation level (see Van der Meijden et al., 1992 for review). However, in field studies, habitat limitation successfully explains local extinctions sometimes (Gross, 1980), but not always (Van der Meijden, 1989; Van der Meijden et al., 1992). In fact, other metaanalyses have downplayed the importance of habitat limitation (Silvertown, 1984) by showing that some plant families have many biennials. Thus, the extrinsic hypothesis of habitat limitation has mixed empirical support.

More recently, a number of theoretical explorations have suggested that biennial life histories can be intrinsically unstable due to intraspecific competition between juvenile and adult cohorts, and that this instability can lead to periodic or chaotic behavior (Bulmer, 1977; Ebenman, 1987, 1988; Cushing and Jia, 1992; Kostova et al., 1999; Davydova et al., 2003). For example, Cushing and Jia (1992) analyzed a population with two life history stages, juvenile and adult. They demonstrated that such populations, when relatively stable in size, can either have synchronously living juveniles and adults or alternately living juveniles and adults. They showed that if there is competition between age classes, the synchronous state is unstable, but the alternating (periodic) state is stable. These results lead to the prediction that biennials that are regulated by density-dependent processes and that experience intraspecific competition between juveniles and adults should have temporally segregated life history stages. In fact, temporal segregation of life history stages due to intraspecific competition between them has been described as a special example of the competitive exclusion principle (Nisbet and Onyiah, 1994). Thus, biennial populations may be unstable due to the effects of intraspecific competition between juveniles and adults.

There are few known examples of organisms with temporally segregated life history stages, despite theoretical predictions. …