DOI: 10.5061/dryad.pnvx0k6nc
title: Barriers to restoration: Pollution alters nurse effects for an ecosystem engineer

description: Nurse plants modify the environment to facilitate the recruitment of
 propagules, and are potentially valuable tools for ecological restoration.
 Yet empirical tests, particularly in polluted environments, remain rare.
 The few studies that have examined nurse-effects in polluted environments
 report exclusively positive effects, but most tests have focused on
 pollution-tolerant species in metal contaminated environments. Biotic
 interactions are highly context-dependent, however, such that
 extrapolations to other suites of species and pollutant types appear
 premature. We examined changes in intraspecific nurse-effects across
 pollution regimes for a pollution-sensitive, macroalgal ecosystem engineer
 that is a target for intertidal restoration. In a manipulative field
 experiment, we out-planted propagules in the presence and absence of
 conspecific canopies at (i) unpolluted control shores and (ii) shores that
 received a partially-remediated (low toxicity) sewage effluent, and
 monitored the performance (survival and growth) of these propagules
 over-time. Algal canopies facilitated the survival and early growth of
 propagules at control sites but not at partially-remediated sites, where
 propagules performed poorly irrespective of canopy
 presence. Synthesis and applications:  Using a
 pollution-sensitive ecosystem engineer, we show for the first time that
 nurse-plant effects can be contingent on pollution regime, and that
 exposure to pollution (even at low levels) can erode nurse effects. We
 caution that nurse-plants are unlikely to be a universal tool for
 ecological restoration, even in systems where facilitative nurse-effects
 naturally occur (in our case, algal canopies on unpolluted rocky reefs).
 Management practitioners should carefully consider the pollution tolerance
 of target species, and test whether nurse-plant effects are maintained
 under current pollution regimes, before large-scale translocations are
 attempted.
description: Data from our field experiment that investigated changes in
 intraspecific nurse-plant effects across pollution regimes for the
 intertidal macroalgal ecosystem engineer, <em>Hormosira
 banksii</em>. We outplanted <em>Hormosira</em>
 propagules on experimental plates (5 x 5 cm) that were positioned beaneath
 a conspecific canopy (Canopy treatment) or without a canopy (Exposed
 treatment) at three sites adjacent to a partially-remediated sewage
 outfall (Partially-remediated sites; SH = Shelley Beach; O = Outfall; OE =
 Outfall East) and three unpolluted (Control) sites that did not recieve
 any sewage effluent (BY = Backyards; BT = Booties; C = The Craggs). We
 measured two performance metrics (survival and growth) of these outplanted
 propagules across two post-settlement times (4 and 12 days after
 outplanting). Survival was measured as the density of propagules remaining
 on the plates at each post-settlement time, which we quantified as the
 summed density of propagules within 25 haphazardly selected squares of a
 25-cm<sup>2</sup> grid (i.e. values represent the number of
 propagules per 6.25 cm<sup>2</sup>). Propagule density was log
 (x +1) transformed for analysis. Growth was measured as the average size
 of propagules on the plates at each post-settlement time, which we
 estimated by measuring the widest diameter (in µm) of ~10 haphazardly
 selected propagules per plate where possible, or less than 10 propagules
 for plates where fewer propagules were available for measurement (due to
 high mortality). Missing values for propagule size in the dataset
 indicates that no propaugles were available for measurement from those
 plates. Note that each site initially recieved 10 plates per canopy
 treatment and post-settlement time combination (10 replicates x 2 canopy
 treatments x 2 post-settlement times x 6 sites = 240 plates total), but
 several plates were lost during the experiment, resulting in slight
 inbalances in replicates across treatment combinations and
 sites.