Impact of exotic macroalga on shorebirds varies with foraging specialization and spatial scale

Abstract

Exotic species may increase or decrease native biodiversity. However, effects of exotic species are often mixed; and indirect pathways and compensatory changes can mask effects. Context-specific assessments of the indirect impacts of exotic species are also needed across multiple spatial scales. Agarophyton vermiculophyllum (previously Gracilaria vermiculophylla), an exotic, invasive macroalga, has established throughout the western hemisphere with reported positive or neutral impacts on biodiversity. Shorebirds are an important group for conservation in areas invaded by A. vermiculophyllum. We assess the impacts of this invader on shorebirds by measuring behavior and habitat selection at spatial scales ranging from algal patches to the entire study region. Birds were considered either flexibleforagers that used diverse foraging techniques, or specialized-foragers that employed fewer, more specialized foraging techniques. Responses were scale dependent, with patterns varying between spatial scales, and between behavior and habitat selection. However, a general pattern of habitat selection emerged wherein flexible-foraging shorebirds preferred A. vermiculophyllum habitat, and for specialized-foragers, habitat selection of A. vermiculophyllum was mixed. Meanwhile, flexible-foraging birds tended to neutrally use or avoid uninvaded habitat, and specialized-foraging birds mostly preferred uninvaded habitat. Shorebird behavioral response was less clear; with flexible-foragers spending less time on bare sediment than expected, the only significant response. Shorebird response to A. vermiculophyllum differed by foraging mode; likely because flexible, opportunistic species more readily use invaded habitat. Increases in A. vermiculophyllum could result in functional homogenization if the bare habitat preferred by specialized-foragers is reduced too greatly. We hypothesize the effect of scale is driven by differences among tidal flats. Thus, tidal flat properties such as sediment grain size and microtopography would determine whether foraging from A. vermiculophyllum was optimal for a shorebird. Specialization and spatial scale are important when assessing the biodiversity conservation impacts of invasive A. vermiculophyllum.

Mudflat geomorphology determines invasive macroalgal effect on invertebrate prey and shorebird predators

“…The positive effects of Agarophyton on shorebird foraging occur because some species preferentially forage for invertebrate prey directly within Agarophyton mats (Haram et al 2018, Besterman et al 2020). However, shorebird foraging traits mediate this effect, with birds using primarily visual cues and responding more poorly to macroalgal mats compared with those using tactile cues (Green et al 2015, Haram et al 2018, Besterman et al 2020.…”

“…The positive effects of Agarophyton on shorebird foraging occur because some species preferentially forage for invertebrate prey directly within Agarophyton mats (Haram et al 2018, Besterman et al 2020). However, shorebird foraging traits mediate this effect, with birds using primarily visual cues and responding more poorly to macroalgal mats compared with those using tactile cues (Green et al 2015, Haram et al 2018, Besterman et al 2020. Even for tactile-foraging shorebirds the preference strength for algal mats varies by mudflat and on a spatial scale (Haram et al 2018, Besterman et al 2020, with Agarophyton abundance only weakly related to total shorebird numbers (Besterman et al 2020).…”

Leveraging the Interdependencies Between Barrier Islands and Backbarrier Saltmarshes to Enhance Resilience to Sea-Level Rise

“…These sand bodies are depauperate and predominantly occupied by Agarophyton vermiculophyllum (previously Gracilaria vermiculophylla)-Diopatra cuprea polychaete associations (Thomsen et al, 2009). Besterman et al (2020) have studied the value of this association in terms of biological resources for shorebirds, a major conservation concern for the region. The association is common in the Virginia coastal lagoons and provides prey resources to generalist shorebirds, but lower value resource quality for specialist foragers (Besterman et al, 2020).…”

“…Besterman et al (2020) have studied the value of this association in terms of biological resources for shorebirds, a major conservation concern for the region. The association is common in the Virginia coastal lagoons and provides prey resources to generalist shorebirds, but lower value resource quality for specialist foragers (Besterman et al, 2020). These data and analyses allowed us to identify three flood-tidal delta bodies located nearby to our project site that may be used as potential borrow sites (at locations of cores CEDV-29, −31, and −32; Figure 10).…”

Intraspecific diversity and genetic structure in the widespread macroalga Agarophyton vermiculophyllum

“…2018, Besterman et al. 2020). The success of this species likely depended on adaptive shifts in tolerance to temperature and salinity (Hammann et al.…”