Ecological Archives E095-261-A2

Fiia Haavisto, Veijo Jormalainen. 2014. Seasonality elicits herbivores' escape from trophic control and favors induced resistance in a temperate macroalga. Ecology 95:3035–3045. http://dx.doi.org/10.1890/13-2387.1

Appendix B. Description of the method used to collect herbivores, data for herbivore species and their densities in the study site, and results on herbivore density variation.

To estimate herbivore densities, we collected herbivores at the end of the experiment. We placed the cages inside a mesh-net bag (mesh diameter 1 mm) underwater, thereby preventing herbivore escape. All herbivores were identified to the level of species or genera and their numbers were counted (Table B1).

Among-seasons variation was found in the densities of Gammarus spp. and Idotea chelipes (Fig. B1, Table B2). Fish predation reduced gastropod Hydrobia spp. in spring, with the decrease possibly caused by abundant three-spined stickleback (Gasterosteus aculeatus) aggregating into shallow areas to spawn and shown to control small gastropods feeding on filamentous algae or periphyton (Eriksson et al. 2009). The gastropod Theodoxus fluviatilis density was reduced by fish predation in the autumn (Fig B1, Table B2).

Table B1. Herbivore species/ genera collected at the end of the manipulation period from the experimental cages in the autumn (18th September 2008) and in summer (16th June 2009). Numbers were first counted for the base area of the cage (40 × 60 cm) and then generalized for the area of m2. Here are represented the mean densities/m² ± SE in Open and Fish Exclosure cages for the two seasons.

 

Autumn 2008

Spring 2009

 

Open, n = 6

Fish Exclosure, n = 6

Open, n = 6

Fish Exclosure, n = 6

Gammarus spp.

937.5 ± 95.3

754.2 ± 78.8

168.8 ± 34.5

137.5 ± 30.3

Hydrobia spp.

2151.4 ± 368.4

2888.9 ± 439.5

708.3 ± 165.4

2957.7 ± 631.1

Jaera spp.

30.6 ± 8.7

15.9 ± 4.9

42.4 ± 11.5

24.3 ± 6.7

Idotea balthica Pallas

508 ± 79

338 ± 45

13 ± 4

38 ± 11

Idotea chelipes Pallas

12.5 ± 4.6

0

60.4 ± 10.0

51.4 ± 12.8

Idotea granulosa Rathke

2.2 ± 1.5

0

0

0

Praunus spp.

3.5 ± 1.7

6.9 ± 4.5

0

1.4 ± 1.4

Radix

0.5 ± 0.5

2

0

0

Theodoxus fluviatilis L.

1232.6 ± 162

2115.3 ± 232.4

638.9 ± 74.1

745.8 ± 44.5

 

Table B2. The effects of Season (Autumn vs. Spring), Treatment, Trm (Open vs. Fish Exclosure) and their interaction on the densities of the most abundant herbivores in our experimental cages in the general linear models. Cage(Trm) was included into the model when it improved the model fit (smallest AIC) to account for possible among-cages variation.

 

Gammarus spp.

Hydrobia spp.

Jaera spp.

I.chelipes

T.fluviatilis

Fixed factors

F

P

F

P

F

P

F

P

F

P

Season

186

<0.0001111

4.48

0.06

1.62

0.23

34.58

<0.0001111

43.94

0.0001

Treatment, Trm

1.88

0.20

8.33

0.016

3.49

0.09

1.62

0.22

11.16

0.003

Season × Trm

2.24

0.17

5.43

0.042

0.05

0.83

0.04

0.84

6.86

0.016

Random factors

 

 

 

 

 

 

 

 

 

 

Cage (Trm)

1.81

0.089

2.1

0.073

0.1

0.38

 

 

 

 

 

FigB1

Fig B1. Densities of the four common herbivores in the experimental cages, showing statistically significant seasonal or among-treatment density variation. The box represents 25% and 75% quartiles with the middle line showing the median density and the whiskers minimum and maximum densities.


 

Literature cited

Eriksson, B. K., L. Ljunggren, A. Sandstro, G. Johansson, J. Mattila, A. Rubach, S. Råberg, and M. Snickars. 2009. Declines in predatory fish promote bloom-forming macroalgae. Ecological Applications 19:1975–1988.


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