Abstract
In Europe and North America waterfowl are major dispersers of aquatic and terrestrial plants, but in New Zealand their role has yet to be investigated. Mallards were introduced to New Zealand in the late 1800s, and today they are the most abundant and widespread waterfowl in the country. To assess seed dispersal, we radiomarked 284 female mallards from two study sites during the pre-breeding (June–August) and breeding (August–December) periods in 2014–2015, and examined movements that occurred within 24, 48 or 72 h when seed dispersal by endozoochory is considered likely. During June and July 2015, we collected 29 faecal samples from individual female mallards during radiomarking and 24 samples from mallard flocks. We recovered 69 intact seeds from the faecal samples and identified 12 plant taxa. Of the plant seeds identified and dispersed by mallards in this study, 40% were members of the Asteraceae family, nine plant species were alien to New Zealand, and the indigenous-status of three unidentified taxa could not be determined. Two taxa (and 9% of seeds) were germinated following gut passage: an unidentified Asteraceae and Solanum nigrum. During the pre-breeding and breeding periods, movement of females within 24 h averaged 394 m (SD = 706 m) and 222 m (SD = 605 m) respectively, with maximum distances of 3,970 m and 8,028 m. Maxima extended to 19,230 m within 48 h. Most plant species recorded are generally assumed to be self-dispersed or dispersed by water; mechanisms that provide a much lower maximum dispersal distance than mallards. The ability of mallards to disperse viable seeds up to 19 km within 48 h suggests they have an important and previously overlooked role as vectors for a variety of wetland or grassland plant species in New Zealand.
Highlights
Endozoochory, the dispersal of propagules through the gut by an animal, is a common dispersal strategy of many plant species (Reynolds & Cumming, 2016; Janzen, 1984; Wenny et al, 2016)
We aimed to (i) identify plant species dispersed by mallards, (ii) determine the indigenous status and germinability of seeds egested by mallards, (iii) assess whether these plant species are recognized as having the capacity to disperse via birds, (iv) evaluate the likely dispersal distances for seeds by endozoochory, as determined by radio-tracking of mallards
Many of the alien species we identified have been reported throughout New Zealand, but there have been few records of Agrostis stolonifera, Polygonum aviculare, and Persicaria maculosa occurring in the Southland region, which suggests that these species have recently extended their range southwards (AVH, 2017)
Summary
Endozoochory, the dispersal of propagules (e.g., seeds) through the gut by an animal, is a common dispersal strategy of many plant species (Reynolds & Cumming, 2016; Janzen, 1984; Wenny et al, 2016). Only 12% of native plant species produce fleshy fruits, and endozoochory of other plants depends mainly on granivory (Thorsen, Dickinson & Seddon, 2009). Granivory is often an over-looked pathway of seed dispersal (Costa et al, 2014; Green et al, 2016; Farmer et al, 2017), and its significance has been recognized in New Zealand, no previous studies have documented seed dispersal by granivorous waterfowl there (Thorsen, Dickinson & Seddon, 2009). Alien plants can disperse by endozoochory and this could have negative consequences for native biodiversity and ecosystem functions (Reynolds, Miranda & Cumming, 2015; Green, 2016; Van Leeuwen, 2018)
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