Wax Myrtle and Myrtle Warblers: Reciprocal Specialization and its Consequences in a Temperate Fruit-Frugivore Interaction

Abstract

Interactions between fruiting plant and frugivorous species are considered to be highly generalized, resulting in diffuse mutualisms. Specialization has frequently been found to be either asymmetrical or the result of restricted options for frugivores. This dissertation documents a highly unusual case of reciprocal specialization between the myrtle group of yellow-rumped warblers (Dendroica coronata coronata) and wax myrtle (Myrica cerifera). Far from being one of a group of ecologically redundant dispersers, these warblers are the most valuable quantitative and qualitative contributors to this plant's recruitment on Assateague Island National Seashore, U.S.A.

Fecal samples collected over four years from migrant and wintering passerines and feeding trials in two years demonstrated that wax myrtle fruit was both the most preferred fruit and a consistently major food item for myrtle warblers throughout a 7-month residence. Abundance of wax myrtle fruit significantly affected this warbler's abundance in all years. Similarly, compared to all other frugivorous species, myrtle warbler were the most frequent and consistent consumer of wax myrtle fruit in all years. Wax myrtle seed dispersal was significantly affected by yellow-rumped warbler abundance.

In order to evaluate disperser contributions to wax myrtle, I examined factors influencing seedling recruitment. Data derived from three sources 1) seed trap data from three replicated habitats, 2) experimental evaluation of the effect of time and place of seed deposition, and 3) seedling surveys confirmed that deposition was wide-spread, lasting until late April. Recruitment was greatest in both scrub and meadow habitats, but seeds deposited in meadows, especially in spring, established at higher rates than in scrub because of post-dispersal predation rates associated with microhabitat seed deposition patterns of predation among habitats.

Fecal samples and observations indicated that wax myrtle has three primary dispersers: myrtle warblers, gray catbirds and tree swallows. Myrtle warblers, the only documented disperser after December, provided the greatest quantitative dispersal services. Although the germination rate and time of ingested seeds were unaffected by species identity of dispersers, post-foraging observations demonstrated that myrtle warblers were most likely to be the agent of wax myrtle seed emigration from established thickets to sites suitable for colonization.