The smallest variability values of CR and species composition occurred at the first two sampling intervals (Fig 2.4, 2.5) Largest variability occurred during the main recruitment period (April-June).
Summary
No clear differentiation in resource utilisation among functional groups suggests the lack of resources partitioning mechanisms. Similarly, no differences in temporal variability among diversity treatments were found.
CR of mussels seems to decrease when is present in polyspecific assemblages, hinting to effects of species density on community functioning.
Effects of functional group diversity on size &
stability of hard-bottom community functioning
Nelson Valdivia
1, Markus Molis
1, Stuart Jenkins
2, and Kate de la Haye
3Introduction
Species diversity is suspected to beget stability in community functioning. Niche complementary (e.g. resources partitioning) and sampling effects are two processes by which diversity may operate to cause positive effects on functional community responses.
Hereby, we ask whether size and stability (= temporal variance) of community function (= clearance rate) is higher in poly- (3 species) than in the respective monospecific assemblages and whether such patterns could be explained by niche complementary or sampling effects.
Material and methods
In field experiments clearance rates (CR) of mono- and polyspecific communities were measured. The experiment was replicated at 2 locations: Helgoland island, Germany and Plymouth, UK.
Artificial substrata were made up of 2.5 × 2.5 cm PVC-settlement panels, which were exposed at a range of depths and locations with each site to natural colonisation for 6 months. Colonisers were categorised into 3 functional groups, covering panels monospecifically. Panels were assembled into experimental units containing 9 panels to create assemblages of defined functional diversity. These consisted of the 3 monocultures and one polyculture including all groups (Fig 1). Assemblages were kept in the sea at 1 m depth for 5 months after
assemblage, allowing for natural settlement.
Clearance rates and species composition of developing assemblages were 5 and 7 times measured during the assay period, respectively.
Temporal variability was expressed as SD Log(y) for CR and the Bray-Curtis dissimilarity index for species composition, which were calculated between 2 consecutive sampling events. Variability measures were summed across all sampling intervals to produce gross variability of CR and species composition during the experiment.
Results
Mussels preformed largest clearance rates (CR), followed by polyculture, colonial ascidians, and barnacles (Fig 2.1, ANOVA p = 0.014). The smallest differences were observed between mussels and polycultures (Fig 2.1).
There was a non-significant trend for barnacles preferring larger- and mussels preferring smaller-sized prey (Fig 2.1).
Neither gross variability of CR nor of community composition significantly depended upon functional group diversity (Fig 2.2, 2.3, ANOVA, p > 0.19).
However, variability of monocultures of colonial ascidians tended to be higher than in all remaining treatments (Fig 2.2, 2.3). Temporal variability of CR and species composition significantly changed over the experimental period (Fig 2.4, 2.5,ANOVA, p < 0.008).
Fig 2. (1)Clearance rates (CR) of functional groups and polyculture on three microalga species; letters denote sig.
difference between functional groups.
Gross temporal variability (across all sampling events) of (2) clearance rates and (3)species composition. (4, 5) Changes in temporal variability over time.
NOTE: values are given as MEAN±SEM, n = 3
Alfred Wegener Institute for polar- and marine research.
Helmholtz Association www.awi.de
Biologische Anstalt Helgoland Kurpromenade 201 27498 Helgoland nelson.valdivia@awi.de Fig 1 Polyculture including 3 functional
groups, namely barnacles, colonial ascidians, and mussels.
2.5 cm
Resources (microalga spp.) Isochrysis galvana Cryptomonas rostrella Prorocentrum micans +Tetraselmis suecica
Clearance rate (CR) [ml g-1h-1]
0 10 20 30 40 50 60 70 80 90
Barnacles Colonial ascidians
Mussels Polyculture
1 Key for groups
Barnacles Colonial ascidians Mussels Polyculture
Main recruitment period
0.0 0.2 0.4 0.6 0.8 1.0 1.2
time [days]
CR variability [SD Log(CR)] 4
0 13 27 125 146 20-Feb 19-Mar 4-Jun 16-Jul
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Gross CR variability [∑SD Log(CR)]
2
0.8 1.2 1.6 2.0 2.4 2.8 3.2
Gross community variability [∑Bray-Curtis]
functional groups 3
Max length [µm]
0 10 20 30 40
1Biologische Anstalt Helgoland, Alfred Wegener for Polar and Marine Research, Seaweed Biology Section, Marine Station, Kurpromenade 201, 27498 Helgoland, 2School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5AB, United Kingdom. 3Marine Biological Association, The Laboratory, Citadel Hill, Plymouth, PL1 2PB, United Kingdom
a b
c d
Community variability [Bray-Curtis]
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
time [days]
0 13 27 72 104 125 146 20-Feb 19-Mar 4-Jun 16-Jul
5
0 0 0
