1. Multiple anthropogenic stressors are causing a global decline of foundation species including macrophytes often resulting in the expansion of functionally different more stressor-tolerant macrophytes. Previously subdominant species may experience further positive demographic feedbacks 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 is unknown.

2. The green alga, Caulerpa filiformis, previously a subdominant macrophyte on low tidal-shallow subtidal rock shores is becoming locally more abundant and has spread into warmer waters across the coast of New South Wales, Australia. 

3. In this study, we measured 1) the distribution and abundance of a key consumer, the sea urchin Heliocidaris erythrogramma, across a seascape at sites where C. filiformis has become dominant, 2) performed behavioral field experiments to test the role of habitat selection in determining the local distribution of H. erythrogramma and 3) 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. 

4. 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, whilst 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.

5. Overall, strong negative effects on herbivore life-history traits and potentially their survivorship may establish further positive-feedbacks on C. filiformis abundance that contribute to its spread and may mediate shifts from top-down to bottom-up control at locations where C. filiformis has become dominant.

The aim of the study was to explore the effects of a native spreading alga (Caulerpa filiformis) along the coast of New South Walse, Australia on populations of a key herbivore, the sea urchin Heliocidaris erythrogramma. 

Surveys: This sheet contains data related to patterns of urchin abundance inside and outside C. fliformis. At each of the three locations (column 1), we compared the abundance of H. erythrogramma and the number of homing scars on rocky platforms within patches of C. filiformis to those in surrounding algal habitats (“outside”). These two variables were also measured at the edge of the two habitats. Abundances of H. erythrogramma (column 4) and homing scars (column 5) were counted in n = 5 replicate quadrats (N=1 per patch; 50 x 50 cm, column 3) in each position respect to patches (inside, edge and outside; column 2) at each location. The percentage of occupancy of homing scars was also accounted (column 6).

Movement: This sheet contains the results of a behavioural experiment testing sea urchin habitats preference. To determine how C. filiformis influenced urchin movement, we tagged 60 H. erythrogramma of ~5 cm test diameter (TD, without spines). We haphazardly placed 20 urchin individuals in the middle of large a C. filiformis patch, 20 on the edge and 20 outside (columns 2, 3 and 4). All urchins were randomly allocated to positions within the patch at sufficient distance to maintain independence among individuals. One hour after placing urchins, we recorded the position of all tagged urchins as either middle, outside or on the edge of a C. filiformis patch (Column 1, rows).

No-choice experiment: This sheet contains information regarding the amount of algae consumed (column 3, consumption) in experiments in which only one food item was offered to sea urchins. We placed one urchin (~5 cm TD, column 2) together with a ~5 g piece of either C. filiformis, E. radiata or Sargassum vestitum in each container (n=20, column 1, algae).

Preferences: this sheet contains information from 3 preference experiments in which two food items were offered at the same time to sea urchins. When presented with two species H. erythrogramma were placed in the centre of the container with algal species at either end. Water flow in the containers was directed as such to cause minimal interference to algae upon initiation of the experiments. After 48 h, any remaining algae were collected and reweighed as above. The amount of algae consumed for each food item offered (columns ‘Algae’) is presented in columns ‘Consumption’ in grams. Negative values represent net increases in wet weight. 

Sea urchin mortality: to test the effects of C. filiformis consumption on urchins, we placed 30 individuals of H. erythrogramma ~5 cm test diameter in each of nine 54 L tanks with free-flowing filtered 300 µm seawater. We randomly assigned treatments to each of the tanks in which urchins were fed with either nominally low-quality food (C. filiformis),high quality food (E. radiata), or nothing (starvation; n = 3 tanks per treatment). Algae were supplied to tanks ad libitum over the course of the experiment and replaced/replenished seaweed, and removed older uneaten algae when necessary for a total of 18 weeks. This sheet contains the status of individual urchins (column 3) placed in tubs (column 2) and feed with E. radiatta, C. cilindracea or starved (column 1, Food Item). The status at the end of the experiment is presented as a binomial variable (0 being dead urchins and 1 being urchins alive). 

Sublethal effects: At the conclusion of the experiment on the effects of C. filiformis consumption (see above) total weight, gonad weight and test weight was measured in a portion of surviving urchins. This sheet contains this information for 25 urchins fed with E. radiata, 35 fed with C. filiformis and 7 starved. Gonads and calcareous material were placed in pre-weighed weigh trays and dried at 60°C for 48 hours before being re-weighed. The food item offered to urchins is stated in column 1, while the dry weight in grams of the tests and gonads are presented in columns 2 and 3 respectively. Column 4 indicates the tub in which urchins were placed at the beginning of the experiment. Column 5 indicates the exact size of sea urchin (TD without spines).