Experimental test of predator and herbivore food preference (Tables S2–S5)
Feeding Assays Methods: To estimate the feeding preferences of consumers, we
conducted a series of feeding assays at the University of North Carolina at Chapel Hill’s
Institute of Marine Science (IMS) in Morehead City, NC, October through December of
2004. Each of the five predator species and several herbivore species used in the
diversity experiment were provided with potential food sources in controlled,
independent trials. Predator food sources were four macroinvertebrate herbivore
species and seven species of macroalgae. Herbivores were assayed with all seven
species of macroalgae (Table S2). We used two additional species of macroalgae
(Ulva lactuca and Gracilaria verrucosa) in the feeding trials not included in the diversity
experiment. All species used in these assays are common in local hard substrate
benthic marine habitats. We included the herbivore Ampithoe longimana in these trials
even though it was not abundant in the diversity experiment because the results of other
studies in this system indicate that it is an important herbivore and a strong interactor
(Duffy & Hay 2000). Several species that were abundant in the field at the time of the
mesocosm experiment (e.g., Hyale spp. and Corophium spp.) were not abundant
enough two weeks later for use in the feeding trials. All of these herbivores are
macroscopic crustaceans known to consume macroalgae, microalgae or epiphytic
algae, although many of the details of their feeding ecology are not well understood
(Duffy & Hay 2000).
We manipulated consumer and prey identity in a crossed factorial design (n = 5
for each consumer/prey combination). Containers were 2 L in volume and were
maintained in indoor water tables with aeration and fresh, cycling seawater supplied
from the adjacent Bogue Sound. All consumers were acclimated to the container
environment for 24 hours without food prior to the experiment. We maintained the
predator treatments for 24 hours and the macroinvertebrate consumer treatments for 72
hours. Time, water temperature, and predator weights and lengths were recorded for all
315 trials. At the end of each trial, we counted remaining animal prey and measured
remaining macroalgae biomass. Algal wet mass in the predator assays was measured
after removing excess water using a salad spinner (60 revolutions). To measure
consumption by macroinvertebrate herbivores, we estimated the change in algal surface
area by calculating initial and final area using digital image analysis software (ImageJ
version 1.28v by NIH Image). We analyzed data with a two-way ANOVA (both factors
fixed: consumer and prey identity) and LSM planned contrasts using SAS JMP 5.0.1a.
The response variable was the amount of prey consumed and results are reported as
the mean percent change in alga surface area over the course of the experiment.
Feeding Assays Results: There is evidence for selective predation of
macroinvertebrate prey species by the five predator species (Table S3). Overall,
predators consumed more amphipods than isopods (LSM planned contrast: P < 0.001).
The number of macroinvertebrate prey consumed varied among predator species, and
L. rhomboides consumed more prey than other predator species (P < 0.001). There
were no significant interaction effects between the predator and prey treatments (Table
S3).
Most predators did not consume algae (Table S4). There was a significant
interaction between predator species and algae species. L. rhomboides preferentially
consumed the green alga Enteromorpha linza and was the only predator species that
consumed more than 30% of any algae (Table S3). E. linza was the most consumed
algae across all predator treatments (P < 0.0001).
Feeding preferences differed significantly among herbivore species (Table S3
and S5). Paracerceis spp. and A. longimana consumed brown and green algae and less
red algae, while E. levi consumed red and green algae (Table S5). A. marcuzii was the
most selective, only consuming the brown alga S. filipendula. Codium fragile was the
least likely to be consumed by any macroinvertebrate herbivore (P < 0.0001). Across all
species tested, macroinvertebrate herbivores as a group do not appear to prefer a
single species or functional group of macroalgae (Table S5).
Table S2. Species used in feeding experiment.
Trophic level
Species
Common name
Predator
Portunus spinimanus
Swimming crab
Penaeus aztecus,
Predator
P. setiferus
Shrimp
Predator
Lagodon rhomboides
Pinfish
Predator
Fundulus heteroclitus
Killifish
Hypleurochilus geminatus,
Hypsoblennius hentzi,
Predator
Starksia ocellata
Blennies
Herbivore
Ampithoe longimana
Amphipod
Herbivore
Elasmopus levi
Amphipod
Herbivore
Ampithoe marcuzii
Amphipod
Herbivore
Paracerceis spp.
Isopod
Primary producer
Codium fragile
Green algae
Primary producer
Dictyota menstrualis
Brown algae
Primary producer
Hypnea musciformis
Red algae
Primary producer
Sargassum filipendula
Brown algae
Primary producer
Enteromorpha linza
Green algae
Primary producer
Gracilaria verrucosa
Red algae
Primary producer
Ulva lactuca
Green algae
Table S3. Two-way ANOVA results for single species feeding trials.
Predator – macroinvertebrate prey
SS
df
F
P
Consumer effect
186.34
4
6.74
0.0001
Prey effect
112.36
3
5.42
0.0019
Interaction effect
44.32
12
0.53
0.8860
Consumer effect
0.0131
4
4.58
0.0017
Algae effect
0.0654
6
15.25
< 0.0001
Interaction effect
0.0916
24
5.34
< 0.0001
Consumer effect
0.1052
3
19.17
< 0.0001
Algae effect
0.0803
6
7.31
< 0.0001
Interaction effect
0.2843
18
8.63
< 0.0001
Predator – algae
Macroinvertebrates – algae
Table S4. Percent of herbivores and macroalgae consumed after 24 hours. Shaded
boxes indicate treatments in which at least 3 of the 5 predators consumed 30% or more
of the prey.
Herbivores
Consumer:
Shrimp
Crabs
Killifish
Blennies
Pinfish
Paracerceis spp.
40
40
50
70
70
Elasmopus levi
50
50
80
100
100
Ampithoe longimana
80
60
80
90
100
Ampithoe marcuzii
70
70
80
70
100
Sargassum filipendula
Dictyota menstrualis
Macroalgae
Ulva lactuca
Enteromorpha linza
Codium fragile
Hypnea musciformis
Gracilaria verrucosa
70
Table S5. Percent of algae consumed after 72 hours. Shaded boxes indicated
treatments in which at least 3 of the 5 herbivores consumed 30% or more of the prey.
Consumer:
E. levi
Paracerceis
A. marcuzii
A. longimana
Sargassum filipendula
85
100
100
Dictyota menstrualis
81
76
Ulva lactuca
53
99
64
Enteromorpha linza
92
100
80
Codium fragile
73
Hypnea musciformis
96
Gracilaria verrucosa
100
41
76
Literature Cited
Duffy J.E. & Hay M.E. (2000) Strong impacts of grazing amphipods on the organization
of a benthic community. Ecological Monographs, 70, 237-263