Perturbation pathways affecting the avian lotic predator, blue duck, following a volcanic eruption

Abstract

1. Perturbation pathways affecting interactions between feeding habitat, food supplies and diet of a lotic avian predator, blue duck (Hymenolaimus malacorhynchos Gemlin), were tracked in a New Zealand river following substantial inputs of sediment from a volcanic eruption. Sediment impacts were separated temporally into two distinct phases: (i) deposition of fine ash from volcanic fall-out, and (ii) pulsed releases of volcanic sand and gravel retained by an upstream dam. 2. Levels of interstitial suspendable inorganic sediments increased by several orders of magnitude following ash inputs to the river, but returned to low levels within 3 months. Flushing of volcanic sand and gravel retained by the dam resulted in sediment deposition upstream of a large island where coarse material lodged firmly between larger benthic substratum elements in blue duck feeding habitat. 3. Changes in algal biomass appeared to largely reflect seasonal and hydrological influences, but the percentage inorganic content of periphyton increased significantly during both posteruption phases. 4. Diversity and abundance of blue duck food supplies on boulders and on the riverbed declined following the eruption, whereas significant impacts on biomass were only detected in benthic habitats which were more susceptible to sediment deposition. However, percent composition of the benthic and boulder invertebrate food resources appeared largely unaffected by the eruption. 5. There was no apparent association between the eruption and the composition or electivity (E*) of blue duck diet, but there was a significant increase in the percent inorganic content of faeces, suggesting a reduction in the quality of food rations. 6. Changes in food quantity and quality associated with deposition of volcanic sediments were linked to an observed decline in blue duck population density and fecundity following the eruption. An interacting continuum of adaptive responses to perturbation intensity and impact duration is proposed for blue duck, and potentially other long-lived, riverine birds, that enable them to cope with disturbances in highly variable and heterogenous lotic environments.

Visual fields in Blue Ducks Hymenolaimus malacorhynchos and Pink‐eared Ducks Malacorhynchus membranaceus: visual and tactile foraging

“…Studies of Blue Duck foraging behaviour and diet demonstrate that foraging occurs within specific microhabitats and that invertebrate prey have heterogeneous spatial and temporal distributions within the foraging areas (Veltman & Williams 1990, Collier 1991,Veltman et al . 1995, Collier 2004. The pronounced temporal variations in the compositions of benthic invertebrate communities (which are mostly a consequence of flood events and subsequent patterns of invertebrate recolonization) are reflected in changes in relative species composition in the diet.…”

“…Are these food items detected by tactile cues when 'blind searching' among rocks and stones of the riverbed, or are Blue Ducks detecting individual items by sight either on the substrates or in the water column? This would introduce a previously unconsidered factor into the selection of sites for population enhancement or re-introductions, a current conservation focus (Collier 2004). They are classified as an IUCN Red Listed Endangered species with an estimated world population of 2440 individuals (BirdLife International 2006).…”

“…If Blue Ducks are primarily visual feeders their foraging may be significantly disrupted by changes to water clarity. This would introduce a previously unconsidered factor into the selection of sites for population enhancement or re-introductions, a current conservation focus (Collier 2004).…”

Combining surveys of river habitats and river birds to appraise riverine hydromorphology

“…Many bird species utilize river channels and the adjacent habitats to complete part, or all, of their life‐cycles. Links with river production, quality and hydromorphology have been increasingly realized not least by freshwater biologists (Ormerod & Tyler, 1991; Lock & Naiman, 1998; Iwata, Nakano & Murakami, 2003; Collier, 2004; Feck & Hall, 2004; Baxter, Fausch & Saunders, 2005; Mattsson & Cooper, 2006). Globally, around 60 specialist river species and up to 23% of all bird species use freshwaters, including rivers (Ormerod & Tyler, 1993; Buckton & Ormerod, 2002).…”

Evaluating the Framework of a New River Bird Survey Method

“…This suite of indicators is sensitive to changes in flow regime, water level fluctuation and the presence of dams that alter these systems (Books, 1985;Stevens et al, 1996;Royan et al, 2013). Thus, riverine birds and trends in metrics such as species richness may be proxies for key ecosystem features such as productivity, hydrogeomorphology or water quality (Ormerod and Tyler, 1993;Iwata, 2003;Collier, 2004;Feck and Hall, 2004;Mattson and Cooper, 2006), and because birds function at larger spatial and temporal scales than many other taxa (e.g. plants, insects, fish), they are highly relevant to understanding the linkages between river, riparia and watershed (Robinson et al, 2002).…”