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Sublethal effects of a rapidly spreading native alga on a key herbivore.
Bradley DJ
,
Boada J
,
Gladstone W
,
Glasby TM
,
Gribben PE
.
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Multiple anthropogenic stressors are causing a global decline in foundation species, including macrophytes, often resulting in the expansion of functionally different, more stressor-tolerant macrophytes. Previously subdominant species may experience further positive demographic feedback if they are exposed to weaker plant-herbivore interactions, possibly via decreased palatability or being structurally different from the species they are replacing. However, the consequences of the spread of opportunistic macrophytes for the local distribution and life history of herbivores are unknown.The green alga, Caulerpa filiformis, previously a subdominant macrophyte on low intertidal-shallow subtidal rock shores, is becoming locally more abundant and has spread into warmer waters across the coast of New South Wales, Australia.In this study, we measured (a) the distribution and abundance of a key consumer, the sea urchin Heliocidaris erythrogramma, across a seascape at sites where C. filiformis has become dominant, (b) performed behavioral field experiments to test the role of habitat selection in determining the local distribution of H. erythrogramma, and (c) consumer experiments to test differential palatability between previously dominant higher quality species like Ecklonia radiata and Sargassum sp. and C. filiformis and the physiological consequences of consuming it.At all sites, urchin densities were positively correlated with distance away from C. filiformis beds, and they actively moved away from beds. Feeding experiments showed that, while urchins consumed C. filiformis, sometimes in equal amounts to higher quality algae, there were strong sublethal consequences associated with C. filiformis consumption, mainly on reproductive potential (gonad size). Specifically, the gonad size of urchins that fed on C. filiformis was equivalent to that in starved urchins. There was also a tendency for urchin mortality to be greater when fed C. filiformis.Overall, strong negative effects on herbivore life-history traits and potentially their survivorship may establish further positive feedback on C. filiformis abundance that contributes to its spread and may mediate shifts from top-down to bottom-up control at locations where C. filiformis has become dominant.
FIGURE 1. Map of study locations (i.e., Mona Vale, Bulli, and Wollongong) on the east coast of Australia (a), New South Wales. The top right photograph shows a shallow rocky landscape dominated by turfing algae with the presence of Sargassum spp and Eklonia radiata (b) while the bottom right photograph (c) shows Mona Vale, dominated by C. filiformis
FIGURE 2. (a) Density of urchin H. erythrogramma in the studied region inside, at the edge and outside C. filiformis patches. (b) Density of homing scars inside, at the edge and outside C. filiformis patches. (c) Percentage of homing scars occupied by an individual of H. erythrogramma. Data in plots represent the values for all 3 locations
FIGURE 3. Number of urchins found at each position at the end of experiment of those place (a) inside C. filiformis, (b) at the edge between the two habitats, and (c) outside C. filiformis patches at the beginning of the experiment
FIGURE 4. Urchin consumption (grams consumed) of each of C. filiformis, E. radiata and Sargassum spp in no‐choice food assays. Dark lines in boxplot represent the median values found per treatment
FIGURE 5. Two‐choice experiment results when urchins were offered (a) E. radiata & Sargassum spp, (b) C. filiformis & Sargassum spp, and (c) C. filiformis & E. radiata in total grams consumed. The black line in boxplots represents the median values. The top x‐axis represents the Wilcoxon test results on the preferences between the two‐choice items
FIGURE 6. Lethal and nonlethal effects of feeding on C. filiformis compared with E. radiata or starved. (a) Sea urchin mortality at the end of the experiment, (b) calcareous body proportion weight in grams, (c) gonad weight in grams, and (d) the ratio of gonad to calcareous body weight
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