Herbivory on macro-algae affects colonization of beach-cast algal wrack by detritivores but not its decomposition doi:10.5697/oc.55-2.339 OCEANOLOGIA, 55 (2), 2013. pp. 339 – 358. C Copyright by Polish Academy of Sciences, Institute of Oceanology, 2013. KEYWORDS Induced anti-herbivore defence Macro-algal wrack Marine-terrestrial ecotone Spatial subsidy Trophic link Philip Eereveld1 Lena Hübner1 Gesa Schaefer1 Martin Zimmer1,2,⋆ 1 Institute of Zoology, Christian-Albrechts University, Am Botanischen Garten 9, 24118 Kiel, Germany 2 Paris-Lodron University, (FB Organismische Biologie), Hellbrunnerstr. 34, 5020 Salzburg, Austria; e-mail: martin.zimmer@sbg.ac.at ⋆ corresponding author Received 4 October 2012, revised 20 February 2013, accepted 26 February 2013. Abstract Spatial subsidies have increasingly been considered significant sources of matter and energy to unproductive ecosystems. However, subsidy quality may both differ between subsidizing sources and vary over time. In our studies, sub-littoral herbivory by snails or isopods on red or brown macro-algae induced changes in algal tissues that affected colonization of beach-cast algal wrack by supra-littoral detritivores (amphipods). However, microbial decay and decomposition through the joint action of detritivores and microbes of algal wrack in the supra-littoral remained unaffected by whether or not red or brown algae had been fed upon by snails or isopods. Thus, herbivory on marine macro-algae affects the cross-system The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ 340 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer connection of sub-littoral and supra-littoral food webs transiently, but these effects diminish upon ageing of macro-algal wrack in the supra-littoral zone. 1. Introduction Spatial subsidies (sensu Polis et al. 1997) cross boundaries between seemingly distinct ecosystems and provide nutrients and energy to the recipient system. In this respect, cross-system subsidies have strong implications for species interactions and food web dynamics. Subsidies of materials and organisms can affect all trophic levels of food webs either directly or indirectly. Many food webs rely on cross-system subsidies of detritus for sources of energy and nutrients (Huxel & McCann 1998). Recently, the marine-terrestrial ecotone around the intertidal area of marine coasts has received increasing attention in this respect (e.g. Polis & Hurd 1995, 1996, Rose & Polis 1998, Kawaguchi & Nakano 2001, Fariña et al. 2003, Roth 2003), having been spawned in large part by the now well-documented salmon-bear-forest interaction (Ben-David et al. 1998, Willson et al. 1998, Cederholm et al. 1999, Naiman et al. 2002). The major contributor to marine subsidies into the supralittoral terrestrial zone is beach-cast macrophyte wrack (Orr et al. 2005, and references therein), which provides food (e.g. Mews et al. 2006) and shelter (e.g. Lewis et al. 2007) to invertebrate inhabitants of sand and cobble beaches. Considering the input of senescent or dead biomass of primary producers from herbivory-influenced standing stocks into detritus pathways, it is not surprising that herbivory-induced changes in plant tissue chemistry also affect detritivores and detritivore-mediated decomposition processes (e.g. Bardgett et al. 1998, Chapman et al. 2003), since similar plant traits affect herbivores that feed on living plant tissue and detritivores that consume detrital plant material (Cornelissen et al. 1999, Wardle et al. 2002, Cortez et al. 2007). In terrestrial systems, there is increasing evidence for the slowed decomposition of leaf litter derived from herbivore-affected trees through herbivore-induced chemical defences of plants (Schweitzer et al. 2005, Fonte & Schowalter 2005), and for herbivore-specific herbivory effects on decomposition (Kay et al. 2008). Examples of herbivore-induced defence have also been reported in marine algae (Van Alstyne 1988, Yates & Peckol 1993, Cronin & Hay 1996, Pavia & Toth 2000, Rohde et al. 2004, Rothäusler et al. 2005). Algal responses to herbivory depend on both the herbivore species and the algal species. Both direct feeding by the isopod Idotea balthica and feeding on neighbouring plants induced chemical defence in the bladder wrack Fucus vesiculosus, whereas the snail Littorina littorea only induced defence by direct grazing (Rohde et al. 2004). This species has also Herbivory on macro-algae affects colonization of beach-cast . . . 341 been proven to down-regulate defence in the absence of herbivores (Rohde & Wahl 2008). The brown alga Lessonia nigrescens responded to amphipods but not to sea urchins, and another brown alga, Glossophora kunthii, showed inducible defence against one species of amphipods (Parhyalella ruffoi). The red alga Grateloupia doryphora did not respond to herbivory by any of the tested grazers, whereas another red alga, Chondracanthus chamissoi, responded to one species of amphipods (Hyale hirtipalma) and an isopod (Isocladus bahamondei) (Rothäusler et al. 2005). Ceh et al. (2005) observed reduced consumption of Hypnea pannosa (red alga), Sargassum asperifolium (brown alga) and Cystoseira myrica (brown alga) by the amphipod Cymadusa filosa following previous herbivory by that same mesograzer. Furthermore, the effects of induced defences on potential herbivores may vary among herbivores (Pavia & Toth 2000, Amsler 2001). The sea urchin Tetrapygus niger was not affected by induced defences in L. nigrescens, but H. hirtipalma was (Rothäusler et al. 2005). However, the debate surrounding the structural basis of chemical defence in marine macrophytes is controversial (for discussion, Jormalainen et al. 2003, Kubanek et al. 2004, Macaya et al. 2005), and essentially nothing is known about how herbivore-induced changes in macrophyte tissue chemistry translate into decomposition processes. Marine detritus frequently subsidizes the terrestrial fringe of coastal ecotones, as much of the annually produced sub-littoral macro-algal biomass is exported into adjacent littoral and supra-littoral habitats. The deposition of macrophyte wrack, detached thalli and blades of macro-algae and seagrasses, is of major significance as food and/or habitat for invertebrates of the relatively unproductive supra-littoral zone (e.g. Inglis 1989, Colombini et al. 2000, Orr et al. 2005, Mews et al. 2006, Lewis et al. 2007, Rodil et al. 2008). Species-specific decomposition rates of beach-cast wrack through microbial and detritivore action (Mews et al. 2006) may in part be explained by interspecific differences of algal tissue in defence against herbivory. To this end, detritivores prefer aged wrack to fresh wrack (Pennings et al. 2000), possibly because of the decreasing levels of defensive compounds upon ageing (Cronin & Hay 1996, Pennings et al. 2000, Norderhaug et al. 2003). However, the effects of herbivory on the value of macrophyte wrack as a spatial subsidy have, to the best of our knowledge, not been examined experimentally to date. Differences among macrophyte wrack with respect to habitat quality (cf. Bonte et al. 2003) or nutritive value at different stages of ageing and decomposition (cf. Pennings et al. 2000, Olabarria et al. 2007) affect the population dynamics and activity of invertebrates in stressful environments (cf. Ford et al. 1999, Rossi & Underwood 2002). In particular, wrack 342 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer patches of native vs. invasive algae have recently been described as being colonized, and probably utilized, differently by beach invertebrates (Rodil et al. 2008). Hence, we used common herbivores [the isopod Idotea balthica (Pallas) (Isopoda: Valvifera) and the snail Littorina littorea (Linnaeus) (Gastropoda: Neotaenioglossa)] that feed upon the common brown alga [Fucus vesiculosus L. (Phaeophyta)] and a currently establishing non-native red alga [Gracilaria vermiculophylla Papenfuss (Rhodophyta)]. Induced defences have been demonstrated for both Fucus (see above; Rohde et al. 2004, Rohde & Wahl 2008) and Gracilaria (Nylund et al. 2011, Rempt et al. 2012), being active against, e.g. isopod and snail herbivores. Such generalist herbivores (e.g. I. balthica and L. littorea) prefer F. vesiculosus over G. vermiculophylla, but readily feed upon the latter when no alternative food source is available (Weinberger et al. 2008, Nejrup et al. 2012), albeit with low growth rates (Nejrup et al. 2012). However, daily biomass uptake by grazers hardly exceeds 10% of the average daily net growth of G. vermiculophylla in natural stands (Weinberger et al. 2008), resulting in lower herbivore pressure than on native macroalgae (Nejrup et al. 2012). Hence, G. vermiculophylla accumulates in shallow and sheltered waters from where it can be washed ashore during storm events. The invasive G. vermiculophylla differs remarkably from the native F. vesiculosus in both chemistry and morphology. Both the chemistry and the morphology of macrophytes have been coined major determinants of colonization and utilization by invertebrates as food and habitat (cf. Rodil et al. 2008). We hypothesize that changes in algal tissue induced by herbivory in the sub-littoral persist beyond detachment, deposition ashore and ageing of wrack, and affect colonization and decomposition of beach-cast algal tissue in the supra-littoral. We test this hypothesis in laboratory (colonization by detritivorous amphipods) and field (decomposition through microbial and macro-detritivore activity) studies. 2. Material and methods With a combination of field and laboratory studies, we aimed to disentangle the short- (lab) and long-term (field) effects of sub-littoral herbivory on colonization and decomposition, respectively, of beach-cast macroalgal wrack. 2.1. Algae Algae were collected in spring in coastal waters along the Kiel Fjord (Fucus: 54◦ 27′ 17′′ N, 10◦ 11′ 53′′ E; Gracilaria: 54◦ 21′ 08′′ N, 10◦ 08′ 32′′ E). Prior to the experiment, they were kept in a climate chamber (11◦ C; Herbivory on macro-algae affects colonization of beach-cast . . . 343 simulated natural long-day light conditions) in separate systems with a continuous flow-through of filtered seawater from the Kiel Fjord for four weeks. Algae of each species were randomly designated separately to three distinct groups in separate flow-through systems: algae without herbivores to prevent or reduce any induced defence against herbivory (cf. Rohde & Wahl 2008); algae with snails (Littorina littorea (L.), Gastropoda) to induce defence against snail herbivory; algae with isopods (Idotea balthica (Pallas), Isopoda) to induce defence against isopod herbivory. 2.2. Animals For studying the colonization of freshly deposited wrack by detritivores, we chose beach fleas (Amphipoda: Talitridae: Orchestia gammarellus (Pallas)), frequently considered primary macrofaunal colonizers of fresh beach-cast algal wrack on sand and cobble beaches (Griffiths & StentonDozey 1981, Inglis 1989, Colombini et al. 2000, Pelletier et al. 2011). Beach fleas play a major role as consumers of Fucus wrack (Adin & Riera 2003). Both detritivores (O. gammarellus) and herbivores (L. littorea, I. balthica) were collected in spring in the supra-littoral and sub-littoral, respectively, of a sandy beach of the Kiel Fjord (54◦ 27′ 17′′ N, 10◦ 11′ 53′′ E). Herbivores were immediately added to flow-through systems that contained the algae to be manipulated (see above). Detritivores were kept in a climate chamber (15◦ C, low-light long-day conditions) for up to two weeks with their natural food source, mixed algal wrack from the site they had been collected, ad libitum, before they were used for colonization preference tests. 2.3. Experimental design 2.3.1. Colonization Plastic boxes (20 × 12 × 13 cm3 ; N = 10 for each assay) were filled with 2 cm wetted sand that had previously been washed thoroughly and ovendried (60◦ C, 48 h). On one (randomly chosen) side of the sand area, we placed a piece of algal tissue (either Fucus or Gracilaria) that had been in contact with either isopods or snails; on the other side, we placed a similarsized piece of algal tissue (of the same species) that had not been in contact with herbivores for four weeks. Control boxes, free of algae, enabled any intrinsic spatial preference of beach fleas to be excluded (no preference for any side of the box: p > 0.6). In an approach to test the effects of wrack ageing, which is accompanied by leaching of water-soluble compounds (experimentally simulated by 344 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer drying and rewetting), we air-dried half of the algal tissue obtained from the pre-experimental treatment with or without herbivores (see above) to constant weight and re-wetted it in seawater prior to placing it in the preference boxes. To each box (N = 10 for each treatment level) we added 20 amphipods, mimicking the natural densities of ca 800 m−2 in natural drift lines in the vicinity of Kiel (authors’ observation). The amphipods remained inside the box for 24 h (15◦ C) and were kept in the dark to prevent any visual orientation or disturbance. After 24 h, a shield was quickly placed along the longitudinal centre of each box to separate the areas of herbivory-treated and control algae and to keep the amphipods in place. Subsequently, the amphipods in each side of the box were counted. Beach fleas find algal wrack through olfactory cues and colonize freshly deposited patches within < 1 h (Pelletier et al. 2011). They use wrack patches both as habitat and feeding ground and remain underneath or in the close vicinity of these patches, where they are most frequently found in the field (cf. Lewis et al. 2007, Pelletier et al. 2011). Adult beach fleas can consume up to 60% of their dry body mass per day (Griffiths et al. 1983), and the composition of beach-cast wrack affects their small-scale distribution on sand and gravel beaches (Crawley & Hyndes 2007, Olabarria et al. 2007). Taking all this together, we hold that wrack patches, once colonized by beach fleas, remain densely inhabited (at least in the upper intertidal where patches are rarely submerged: Pelletier et al. 2011). Minor fluxes of immigration and emigration will result in a steady state of colonization until the ageing and decomposition of a particular wrack patch render it less attractive as a habitat and feeding ground. To evaluate colonization preferences, we calculated the percentage of amphipods on each side of the box and chose the value that exceeded 50% (i.e. equal distribution in both sides) as the estimator of the preference for one algal source or the other. For testing the statistical significance of preferences, we used absolute amphipod counts. To avoid fallacies in the statistical analyses of the preference tests (cf. Roa 1992, Manly 1993), colonization preferences were analysed through resampling statistics (poptools: http://www.poptools.org) according to Storry et al. (2006). In brief, resampling within data pairs (single preference boxes) was performed as ‘Shuffling’ (i.e. without replacement). Subsequently, a Monte Carlo Analysis with 9,999 iterations was performed on the numerical difference between the numbers of amphipods on both sides of the box. As evaluation criterion we chose ‘resampled values’ > ‘experimental values’. The number of resampled cases that met this criterion divided by the number of iterations provided the p value. Herbivory on macro-algae affects colonization of beach-cast . . . 345 2.4. Decomposition The decomposition of wrack ashore requires the joint action of microbial decomposers and detritivores that support each other (cf. Mews et al. 2006). Jędrzejczak (2002) and Feike (2004) did not find any significant contributions of macrofaunal decomposers to the breakdown of beach-cast seagrass on sandy beaches in Poland or Germany respectively. In contrast, Griffiths et al. (1983) found that on South African sandy beaches half the kelp wrack input was consumed by talitrid amphipods. On western Canadian shores, the contribution of the macrofauna to macrophyte wrack decomposition depended on the wrack composition (Mews et al. 2006). These authors observed an up to fourfold acceleration of decomposition by detritivorous invertebrates as compared to microbial decay in animal-free mesh bags. Feeding rates by intertidal macrofauna change significantly once beach-cast wrack starts to age (Pennings et al. 2000). Hence we studied the medium- (1 week) and long-term (3 weeks) decomposition of beach-cast wrack in situ within the supra-littoral of the same beach where amphipods had been captured for the preference tests (54◦ 27′ 17′′ N, 10◦ 11′ 53′′ E). Samples (N = 10 for each treatment) of freshly detached wrack of Fucus or Gracilaria (either fed upon by snails or isopods or without contact with herbivores for four weeks; see above) were air-dried at room temperature to constant weight (6 days) – to simulate natural ageing within the drift line – and weighed. Dry algal tissue was placed in a mesh bag (10 × 10 cm2 ). In order to separately quantify the effect of microbes and macro-detritivores (cf. Mews et al. 2006), we used two mesh sizes, 1 × 1 mm2 and 4 × 4 mm2 . Rather than using larger meshes (e.g. Mews et al. 2006), we chose this mesh size to minimize loss of algal fragments but to allow for the immigration of amphipods and other macrodetritivores. Bags, filled with wrack of either herbivore-treated or untreated red or brown algae, were deposited within the existing drift line of mainly Fucus wrack that was home to high densities (ca 800 m−2 ) of amphipods. After 1 week (medium-term decomposition) and 3 weeks (long-term decomposition), we collected the mesh bags. In the laboratory, algal remains were dried to constant weight and weighed for calculating the difference from the initial dry weight of the same sample. Owing to severe storms during the field incubation, some samples were lost so that the initial replicate number of 10 was reduced to 6–9; in the case of snail-treated Fucus none of the samples could be recovered in sufficient quantity after three weeks. Mass loss differences (separately for 1- and 3-week treatments) were analysed by two-way ANOVA (separately for Fucus and Gracilaria) to separate the effects of induced anti-herbivore defence and presence (large 346 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer mesh size) vs. absence (small mesh size) of macro-detritivores on decomposition rates. For the sake of simplicity, mass loss data are depicted as the percentage of the mass loss we obtained from control (herbivore-free) algal tissue as a result of microbial decay without macro-detritivores (small mesh size). 3. Results 3.1. Colonization Pre-experimental feeding by isopods on macro-algal tissue did not affect the attractiveness of freshly detached and deposited Fucus wrack as a habitat and/or food source for amphipods (Figure 1a), but amphipods significantly more densely (62%) colonized the wrack of Fucus that had not been in contact with herbivores than that of snail-treated Fucus (Figure 1b; negative effect of snail herbivory: p = 0.04). In contrast, the freshly deposited wrack of snail-treated Gracilaria was preferentially colonized (64%) by amphipods (Figure 1d; positive effect of snail herbivory: a b isopods on Fucus snails on Fucus fresh fresh 30 20 10 10 20 30 30 treatment 20 10 20 30 control preference [%] aged aged c d isopods on Gracilaria snails on Gracilaria fresh fresh 30 20 10 10 20 30 treatment 30 control 20 10 10 20 30 treatment preference [%] aged 10 treatment control preference [%] control preference [%] aged Figure 1. Colonization preference (% of total number) of amphipods (Orchestia gammarellus) for wrack of algae that had not been in contact with herbivores for 4 weeks (‘control’) versus a) wrack of Fucus vesiculosus after feeding by isopods (Idotea balthica), b) wrack of Fucus after feeding by snails (Littorina littorea), c) wrack of Gracilaria vermiculophylla after feeding by isopods, d) wrack of Gracilaria after feeding by snails; grey bars indicate a significant (α = 0.05) preference for wrack of herbivore-treated (bar to the left: ‘treatment’) or untreated (bar to the right: ‘control’) algae; bars represent mean + SD of 10 replicates Herbivory on macro-algae affects colonization of beach-cast . . . a 140 1 week * 120 mass loss (% of control) 347 * 100 80 p = 0.04 60 40 20 0 FC- FC+ FI- FI+ FL- FL+ b mass loss (% of control) 400 GC- GC+ GI- GI+ GL- GL+ 3 weeks 300 * 200 p = 0.03 100 0 FC- FC+ FI- FI+ FL- FL+ GC- GC+ GI- GI+ GL- GL+ Figure 2. Mass loss of wrack of algae (F – Fucus vesiculosus: left, G – Gracilaria vermiculophylla: right) that had been in contact with snails (L – Littorina littorea) or isopods (I – Idotea balthica) or without contact with herbivores (C – control) for four weeks prior to the experiment in in situdecomposition studies for 1 week (a) and 3 weeks (b). Wrack was either prone to microbial decay (–) or was accessible to macro-detritivores (+); the dotted line (100%) indicates microbial decay rates of wrack of herbivore-free control algae; box plots show minimum and maximum values, and first, second (median) and third quartiles; owing to heavy storms during the incubation, none of the snail-treated Fucus samples (FL−/FL+) could be recovered in sufficient quantity from the field. Asterisks indicate statistical differences between treatments (α = 0.05); grey boxes designate treatments with mass loss that differs significantly from that of the respective control; p-values indicate significance levels of the herbivory effect detected through within-algal species two-way ANOVA p = 0.02). On the other hand, Gracilaria wrack was preferentially colonized by amphipods (60%) when algal tissue had been fed upon by isopods only 348 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer Table 1. Effects of herbivory and the presence of detritivores on mass loss (twoway ANOVA) of Fucus wrack (a) after 1 week and (b) after 3 weeks in situ (a) df model detritivores herbivores detritivores × herbivores error total 4 1 1 1 32 36 F 435.912 0.861 0.499 0.017 p < 0.001 0.359 0.484 0.896 (b) model detritivores herbivores detritivores × herbivores error total df F p 4 1 1 1 20 24 4.967 0.599 0.001 0.195 0.006 0.448 0.998 0.663 after the wrack had been dried and rewetted to simulate ageing and leaching of soluble compounds (Figure 1c; positive effect of isopod herbivory after wrack ageing: p = 0.02). 3.2. Decomposition During three weeks in situ, Fucus wrack lost up to 30% of its initial mass, whereas Gracilaria wrack was reduced by up to 50%. Compared to the herbivory-free controls, pre-experimental feeding by isopods tended to reduce decomposition rates (both with and without detritivores) during the first week, but insignificantly so (Figure 2a, left), and pre-experimental feeding by snails slightly increased microbial decay rates (small mesh: without detritivores) of Gracilaria wrack (Figure 2a, right; effect of snail herbivory on microbial decay: p = 0.09). After three weeks, detritivores had increased decomposition rates of control Fucus two-fold but insignificantly, and pre-experimental herbivory by isopods decreased decomposition of Fucus wrack by detritivores (Figure 2b, left; negative effect of isopod herbivory on detritivores: p = 0.06). Pre-experimental herbivory reduced decomposition rates of Gracilaria wrack over three weeks, but significantly so (negative effect of snail herbivory on microbial decay: p = 0.02) only in the case of snail-treated algae when macro-detritivores did not have access to algal wrack (Figure 2b, right). 349 Herbivory on macro-algae affects colonization of beach-cast . . . Table 2. Effects of herbivory and the presence of detritivores on mass loss (twoway ANOVA) of Gracilaria wrack (a) after 1 week and (b) after 3 weeks in situ (a) df model detritivores herbivores detritivores × herbivores error total 4 1 1 1 38 42 F 353.803 5.141 0.090 0.575 p < 0.001 0.029 0.766 0.453 (b) model detritivores herbivores detritivores × herbivores error total df F 4 1 1 1 32 36 8.654 1.027 1.718 0.167 p < 0.001 0.318 0.199 0.686 Overall, however, there was no consistent pattern of effects of the herbivory- or mesh bag-treatment on the decomposition of Fucus (Table 1) or Gracilaria (Table 2) wrack over 1 or 3 weeks. Thus, induced anti-herbivore defence in macroalgae did not affect the decomposition of beach-cast wrack in situ, irrespective of which herbivore had fed on the algal tissue (not shown). It was only Gracilaria wrack decomposition after 1 week that was affected by the presence of macro-detritivores (Table 2a), with on average slightly reduced decomposition when macro-detritivores were present (Figure 2a). 4. Discussion As early as 1996, Grime et al. demonstrated a causal connection between anti-herbivore defence in terrestrial vascular plants and the decomposition rate of the leaf litter of various plant species, and this pattern was confirmed across bio-geographical and climatic zones by Cornelissen et al. (1999). In this respect, herbivorous and detritivorous consumers of terrestrial vegetation respond to the same plant traits in terms of preference and consumption (e.g. Cornelissen et al. 1999, Wardle et al. 2002, Cortez et al. 2007), and chemical changes in vegetal composition in response to herbivory will also affect detritivores and decomposition rates (e.g. Schweitzer et al. 2005, Fonte & Schowalter 2005). 350 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer Our present results suggest that this pattern only partially holds true for the decomposition of marine macro-algal wrack in the terrestrial realm. Whereas initial colonization of freshly deposited wrack patches by beach fleas is affected by herbivore-induced changes in algal tissue, differences between anti-herbivore defended and undefended algal tissue seem to diminish over time and to not affect the decomposition of algal wrack. 4.1. Persistence of herbivore-induced changes in algal tissue Obviously, herbivore-induced changes in algal chemistry persisted following deposition of algal tissue ashore (since colonization of algal wrack by detritivores was affected by herbivory). However, in contrast to the above findings for terrestrial vegetation, the effects of algal anti-herbivore defence (1) depended on both the algal and herbivore species as far as colonization patterns are concerned, and (2) changed with ageing of algal wrack ashore. In view of the controversial debate regarding which algal compounds are involved in chemical anti-herbivore defence (Pavia et al. 1997, Deal et al. 2003, Jormalainen et al. 2003, Kubanek et al. 2004, Macaya et al. 2005), as well as the lack of chemical analysis within the present study, we can but speculate about the nature of such herbivoreinduced changes. High concentrations of phenolic compounds, for instance, correlate with low densities of surface-associated microbes (Van Alstyne et al. 1999), and phenolic compounds are known to impair decomposition processes (Palm & Sanchez 1991, Mafongoya et al. 1998, Northrup et al. 1998). 4.2. Colonization of beach-cast wrack by detritivores While herbivory by snails reduced the colonization of freshly beachcast Fucus wrack by amphipods, the colonization of fresh Gracilaria wrack was promoted following herbivory by snails. Ageing diminished any effect of herbivory by snails, but highlighted the effect of isopod herbivory on Gracilaria, with increased attractiveness of isopod-grazed algal tissue to detritivorous amphipods. Hence, isopods and snails induce different chemical defences in Fucus, as has already been shown by Rohde et al. (2004). Further, snail-induced chemical defences differ between Fucus and Gracilaria, as they hampered colonization of Fucus wrack but promoted colonization of Gracilaria wrack. Both effects vanished with wrack-ageing, suggesting the involvement of chemical compounds that are prone to microbial or physico-chemical degradation. Herbivory on macro-algae affects colonization of beach-cast . . . 351 Similarly, isopod-induced chemical compounds in Gracilaria appear to degrade with wrack-ageing, but their degradation products increase the attractiveness of the wrack to beach fleas. Pennings et al. (2000) found an increased preference of two amphipods and an isopod from the supra-littoral zone for aged wrack of seven different macro-algae. They hypothesized increasing organic and mineral contents with ageing to mediate these preferences. Alternatively, or additionally, reduced levels of defensive compounds may have made aged wrack more palatable than fresh wrack (for brown algae: Cronin & Hay 1996, Pennings et al. 2000, Norderhaug et al. 2003). Given sufficient time, there are successional changes in the wrack fauna (Griffiths & Stenton-Dozey 1981, Inglis 1989, Colombini et al. 2000, Olabarria et al. 2007) that might be explained by changes in wrack chemistry. Although transient, the effects of sub-littoral herbivory on supra-littoral colonization of macro-algal wrack may affect the marine-terrestrial ecotone. Amphipods in the wrack line provide food to various predators, both marine and terrestrial (Lewis et al. 2007). Differences among wrack patches in terms of attractiveness to supra-littoral detritivores may therefore translate into differences in cross-system nutrient fluxes, when herbivore pressure becomes regionally massive. 4.3. Decomposition of beach-cast wrack Decomposition rates clearly differed among algal species, but herbivory did not affect decomposition. In contrast to previous findings of the significant acceleration of decomposition by detritivores (e.g. Griffiths et al. 1983, Williams 1984, Chown 1996, Mews et al. 2006), detritivores either had no effect on or reduced the mass loss of algal wrack (Gracilaria during the first week of decomposition). Similarly, Jędrzejczak (2002) and Feike (2004) found no significant contributions of macrofaunal decomposers to the breakdown of beach-cast seagrass. Nevertheless, in pair-wise comparison, detritivores promoted mass loss of Fucus wrack after herbivory by isopods in comparison to the herbivorefree control during three weeks, somewhat paralleling the increased attractiveness of isopod-treated Gracilaria wrack upon ageing. Microbial decay of Gracilaria wrack after herbivory by the isopod was faster than after herbivory by snails during the first week. We hold that this was the result of enhanced microbial activity, possibly due to the snail mucus left on algal surfaces (cf. Zimmer et al. 2004; Mews et al. 2006, Ewers et al. 2012). The same effect would explain the significant preference of amphipods for freshly snail-treated Gracilaria wrack. However, detritivores significantly hampered mass loss of Gracilaria wrack after herbivory by snails. These 352 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer findings appear to stand in contrast with the enhanced colonization of snailgrazed Gracilaria wrack, but overall they show that any chemical changes in the algal tissue due to herbivory attenuate rapidly during decomposition ashore (see above; cf. Cronin & Hay 1996, Pennings et al. 2000, Norderhaug et al. 2003). Both Fucus (Mews et al. 2006; this study) and Gracilaria (Hanisak 1993; this study) decompose slowly enough to persist in the wrack line for long enough to lose any herbivore-induced chemical compound. Deposits of other species of algal wrack, however, decompose completely within just a few days (Mews et al. 2006). In these fast-decomposing species, any herbivore-specific effect on the fate of algal wrack (see above) that had diminished in Fucus and Gracilaria after one week ashore may affect short-term decomposition processes. Further studies with additional algae and additional herbivores may provide valuable insights into this particular aspect of the marine-terrestrial ecotone. 5. Conclusions We provide evidence for herbivory resulting in herbivore- and algaspecific changes in algal chemistry that persist until algal tissue is deposited ashore as wrack. Thus, herbivory on macro-algae in the sub-littoral transiently affects the fate of freshly beach-cast algal wrack by detritivorous invertebrates in the supra-littoral. However, provided that macro-algal wrack remains ashore for long enough, any effect of herbivore-induced change in algal chemistry vanishes early during decomposition. Acknowledgements We are grateful to Florian Weinberger (GEOMAR, Kiel) for providing space and equipment in a climate chamber with flow-through technique and for valuable comments on our experimental design and data interpretation. We thank Chris Swan (University of Maryland, Baltimore) for his helpful comments on a previous draft of the manuscript. Lena Gieschen, Steffi Hatzki and Verena Tams (Universität zu Kiel) helped a great deal during parts of the field study and the trial-and-error improvement of the experimental set-up for preference tests. References Amsler C. D., 2001, Induced defenses in macroalgae: the herbivore makes a difference, J. Phycol., 37 (3), 353–356, http://dx.doi.org/10.1046/j. 1529-8817.2001.037003353.x. Adin R., Riera P., 2003, Preferential food source utilization among stranded macroalgae by Talitrus saltator (Amphipod, Talitridae): stable isotopes study Herbivory on macro-algae affects colonization of beach-cast . . . 353 in the northern coast of Brittany (France), Estuar. Coast. Shelf Sci., 56 (1), 91–98, http://dx.doi.org/10.1016/S0272-7714(02)00124-5. Bardgett R. D., Wardle D. A., Yeates G. W., 1998, Linking above-ground and below-ground interactions: how plant responses to foliar herbivory influence soil organisms, Soil Biol. Biochem., 30 (14), 1867–1878, http://dx.doi.org/10. 1016/S0038-0717(98)00069-8. Ben-David M., Hanley T. A., Schell D. M., 1998, Fertilization of terrestrial vegetation by spawning Pacific salmon: the role of flooding and predator activity, Oikos, 83 (1), 47–55, http://dx.doi.org/10.2307/3546545. Bonte D., Lens L., Maelfait J. P., 2003, Sand dynamics in coastal dune landscapes constrain diversity and life-history characteristics of spiders, J. Appl. Ecol., 43 (4), 735–747, http://dx.doi.org/10.1111/j.1365-2664.2006.01175.x. Cederholm C. J., Kunze M. D., Murota T., Sibatani A., 1999, Pacific salmon carcasses: essential contributions of nutrients and energy for aquatic and terrestrial ecosystems, Fisheries, 24 (10), 6–15, http://dx.doi.org/10.1577/ 1548-8446(1999)024<0006:PSC>2.0.CO;2. Ceh J., Molis M., Dzeha T. M., Wahl M., 2005, Induction and reduction of anti-herbivore defenses in brown and red macroalgae off the Kenyan coast, J. Phycol., 41 (4), 726–731, http://dx.doi.org/10.1111/j.1529-8817.2005. 00093.x. Chapman S. K., Hart S. C., Cobb N. S, Whitham T. G., Koch G. W., 2003, Insect herbivory increases litter quality and decomposition: an extension of the acceleration hypothesis, Ecology, 84 (11), 2867–2876, http://dx.doi.org/10. 1890/02-0046. Chown S. L., 1996, Kelp degradation by Paractora trichosterna (Thomson) (Diptera: Helcomyzidae) at sub-Antarctic South Georgia, Polar Biol., 16 (3), 171–178, http://dx.doi.org/10.1007/s003000050042. Colombini I., Aloia A., Fallaci M., Pezzoli G., Chelazzi L., 2000, Temporal and spatial use of stranded wrack by the macrofauna of a tropical sandy beach, Mar. Biol., 136 (3), 531–541, http://dx.doi.org/10.1007/s002270050713. Cornelissen J. H. C., Pérez-Harguindeguy N., Dı́az S., Grime J. P., Marzano B., Cabido M., Vendramini F., Cerabolini B., 1999, Leaf structure and defence control litter decomposition rate across species and life forms in regional floras of two continents, New Phytol., 143 (1), 191–200, http://dx.doi.org/10.1046/ j.1469-8137.1999.00430.x. Cortez J., Garnier E., Pérez-Harguindeguy N., Debussche M., Gillon D., 2007, Plant traits, litter quality and decomposition in a Mediterranean oldfield succession, Plant Soil, 296 (1–2), 19–34, http://dx.doi.org/10.1007/ s11104-007-9285-6. Crawley K. R., Hyndes G. A., 2007, The role of different types of detached macrophytes in the food and habitat choice of a surfzone inhabiting amphipod, Mar. Biol., 151 (4), 1433–1443, http://dx.doi.org/10.1007/s00227-006-0581-0. Cronin G., Hay M. E., 1996, Induction of seaweed chemical defenses by amphipod grazing, Ecology, 77 (8), 2287–2301, http://dx.doi.org/10.2307/2265731. 354 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer Deal M., Hay M. E., Wilson D., Fenical W., 2003, Galactolipids rather than phlorotannins as herbivore deterrents in the brown seaweed Fucus vesiculosus, Oecologia, 136 (1), 107–114, http://dx.doi.org/10.1007/s00442-003-1242-3. Ewers C., Beiersdorf A., Wieski K., Pennings S. C., Zimmer M., 2012, Predator/prey-interactions promote decomposition of low-quality detritus, Wetlands, 32 (5), 931–938, http://dx.doi.org/10.1007/s13157-012-0326-4. Fariña J. M., Salazar S., Wallem K. P., Witmnan J. D., Ellis J. C., 2003, Nutrient exchanges between marine and terrestrial ecosystems: the case of the Galapagos sea lion Zalophus wollebaecki, J. Animal Ecol., 72 (5), 873–887, http://dx.doi.org/10.1046/j.1365-2656.2003.00760.x. Feike M., 2004, Die Bedeutung des Strandanwurfes für das Ökosystem Sandstrand, Ph. D. thesis, Univ. Rostock. Fonte S. J., Schowalter T. D., 2005, The influence of a neotropical herbivore (Lamponius portoricensis) on nutrient cycling and soil processes, Oecologia, 146 (3), 423–431, http://dx.doi.org/10.1007/s00442-005-0203-4. Ford R. B., Thrush S. F., Probert P. K., 1999, Macrobenthic colonisation of disturbances on an intertidal sandflat: the influence of season and buried algae, Mar. Ecol.-Prog. Ser., 191, 163–174, http://dx.doi.org/10.3354/meps191163. Griffiths C. L., Stenton-Dozey J. M., 1981, The fauna and rate of degradation of stranded kelp, Estuar. Coast. Shelf Sci., 12 (6), 645–653, http://dx.doi.org/10. 1016/S0302-3524(81)80062-X. Griffiths C. L., Stenton-Dozey J. M. E., Koop K., 1983, Kelp wrack and the flow of energy through a sandy beach ecosystem, [in:] Sandy beaches as ecosystems, A. McLachlan & T. Erasmus (eds.), Junk Publ., The Hague, 547–556. Grime J. P., Cornelissen J. H. C., Thompson K., Hodgson J. G., 1996, Evidence of a causal connection between anti-herbivore defence and the decomposition rate of leaves, Oikos, 77 (3), 489–494, http://dx.doi.org/10.2307/3545938. Hanisak M. D., 1993, Nitrogen release from decomposing seaweeds: species and temperature effects, J. Appl. Phycol., 5 (2), 175–181, http://dx.doi.org/10. 1007/BF00004014. Huxel G. R., McCann K. S., 1998, Food web stability: the influence of trophic flows across habitats, Am. Nat., 152 (3), 460–469, http://dx.doi.org/10.1086/ 286182. Inglis G., 1989, The colonisation and degradation of stranded Macrocystis pyrifera (L.) C. Ag. by the macrofauna of a New Zealand sandy beach, J. Exp. Mar. Biol. Ecol., 125 (3), 203–217, http://dx.doi.org/10.1016/0022-0981(89) 90097-X. Jędrzejczak M. F., 2002, Stranded Zostera marina L. vs wrack fauna community interactions on a Baltic sandy beach (Hel, Poland): a short-term pilot study. Part I. Driftline effects of fragmented detritivory, leaching and decay rates, Oceanologia, 44 (2), 273–286. Jormalainen V., Honkanen T., Koivikko R., Eränen J., 2003, Induction of phlorotannin production in a brown alga: defense or resource dynamics?, Oikos, 103 (3), 640–650, http://dx.doi.org/10.1034/j.1600-0706.2003.12635.x. Herbivory on macro-algae affects colonization of beach-cast . . . 355 Kawaguchi Y., Nakano S., 2001, Contribution of terrestrial invertebrates to the annual resource budget for salmonids in forest and grassland reaches of a headwater stream, Freshwater Biol., 46 (3), 303–316, http://dx.doi.org/10. 1046/j.1365-2427.2001.00667.x. Kay A. D., Mankowski J., Hobbie S. E., 2008, Long-term burning interacts with herbivory to slow decomposition, Ecology, 89 (5), 1188–94, http://dx.doi.org/ 10.1890/07-1622.1. Kubanek J., Lester S. E., Fenical W., Hay M. E., 2004, Ambiguous role of phlorotannins as chemical defenses in the brown alga Fucus vesiculosus, Mar. Ecol.-Prog. Ser., 277, 79–93, http://dx.doi.org/10.3354/meps277079. Lewis T. L., Mews M., Jelinski D. E., Zimmer M., 2007, Detrital subsidy to the supratidal zone provides feeding habitat for intertidal crabs, Estuar. Coast., 30 (3), 451–458. Macaya E., Rothäusler E., Thiel M., Molis M., Wahl M., 2005, Induction of defenses and within-alga variation of palatability in two brown algae from the northern,central coast of Chile: Effects of mesograzers and UV radiation, J. Exp. Mar. Biol. Ecol., 325 (2), 214–227, http://dx.doi.org/10.1016/j.jembe. 2005.05.004. Mafongoya P. L., Nair P. K. R., Dzowela B. H., 1998, Mineralization of nitrogen from decomposing leaves of multipurpose trees as affected by their chemical composition, Biol. Fert. Soils, 27 (2), 143–148, http://dx.doi.org/10.1007/ s003740050412. Manly B., 1993, Comments on design and analysis of multiple-choice feedingpreference experiments, Oecologia, 93, 149–152. Mews M., Zimmer M., Jelinski D. E., 2006, Species-specific decomposition rates of beach-cast wrack in Barkley Sound, British Columbia, Canada, Mar. Ecol.Prog. Ser., 328, 155–160, http://dx.doi.org/10.3354/meps328155. Naiman R. J., Bilby R. E., Schindler D. E., Helfield J. M., 2002, Pacific salmon, nutrients, and the dynamics of freshwater and riparian ecosystems, Ecosystems, 5 (4), 399–417, http://dx.doi.org/10.1007/s10021-001-0083-3. Nejrup L. B., Pedersen M. F., Vinzent J., 2012, Grazer avoidance may explain the invasiveness of the red alga Gracilaria vermiculophylla in Scandinavian waters, Mar. Biol., 159 (8), 1703–1712, http://dx.doi.org/10.1007/s00227-012-1959-9. Norderhaug K. M., Fredriksen S., Nygaard K., 2003, Trophic importance of Laminaria hyperborea to kelp forest consumers and the importance of bacterial degradation to food quality, Mar. Ecol.-Prog. Ser., 255, 135–144, http://dx. doi.org/10.3354/meps255135. Northrup R., Dahlgren R. A., McColl J. G., 1998, Polyphenols as regulators of plant-litter-soil interactions in northern California’s pygmy forest: a positive feedback?, Biogeochemistry, 42 (1–2), 189–220, http://dx.doi.org/10.1023/A: 1005991908504. Nylund G. M., Weinberger F., Rempt M., Pohnert G., 2011, Metabolomic assessment of induced and activated chemical defence in the invasive red alga 356 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer Gracilaria vermiculophylla, PloS One, 6, e29359, http://dx.doi.org/10.1371/ journal.pone.0029359. Olabarria C., Lastra M., Garrido J., 2007, Succession of macrofauna on macroalgal wrack of an exposed sandy beach: effects of patch size and site, Mar. Environ. Res., 63 (1), 19–40, http://dx.doi.org/10.1016/j.marenvres.2006.06.001. Orr M., Zimmer M., Jelinski D. E., Mews M., 2005, Wrack deposition on different beach types: spatial and temporal variation in the pattern of subsidy, Ecology, 86 (6), 1496–1507, http://dx.doi.org/10.1890/04-1486. Palm C., Sanchez P., 1991, Nitrogen release from the leaves of some tropical legumes as affected by their lignin and polyphenolic content, Soil Biol. Bioch., 23 (1), 83–88, http://dx.doi.org/10.1016/0038-0717(91)90166-H. Pavia H., Cervin G., Lindgren A., Åberg P., 1997, Effects of UV-B radiation and simulated herbivory on phlorotannins in the brown alga Ascophyllum nodosum, Mar. Ecol.-Prog. Ser., 157, 139–146, http://dx.doi.org/10.3354/meps157139. Pavia H., Toth G., 2000, Inducible chemical resistance to herbivory in the brown seaweed Ascophyllum nodosum, Ecology, 81 (11), 3212–3225, http://dx.doi. org/10.1890/0012-9658(2000)081[3212:ICRTHI]2.0.CO;2. Pelletier A., Jelinski D. E., Treplin M., Zimmer M., 2011, Colonisation of BeachCast macrophyte wrack patches by Talitrid Amphipods: a primer, Estuar. Coast., 34 (4), 863–871. Pennings S. C., Carefoot T. H., Zimmer M., Danko J. P., Ziegler A., 2000, Feeding preferences of supralittoral isopods and amphipods, Can. J. Zool., 78 (11), 1918– 1929, http://dx.doi.org/10.1139/z00-143. Polis G. A., Anderson W. B., Holt R. D., 1997, Toward an integration of landscape and food web ecology: the dynamics of spatially subsidized food webs, Ann. Rev. Ecol. Syst., 28, 289–316 , http://dx.doi.org/10.1146/annurev.ecolsys.28.1.289. Polis G. A., Hurd S. D., 1995, Extraordinarily high spider densities on islands: flow of energy from marine to terrestrial food webs and the absence of predation, Proc. Nat. Acad. Sci. (USA), 92 (10), 4382–4386, http://dx.doi.org/10.1073/ pnas.92.10.4382. Polis G. A., Hurd S. D., 1996, Linking marine and terrestrial food webs: allochthonous input from the ocean supports high secondary productivity on small islands and coastal land communities, Am. Nat., 147 (3), 396–423, http://dx.doi.org/10.1086/285858. Rempt M., Weinberger F., Grosser K., Pohnert G., 2012, Conserved and species-specific oxylipin pathways in the wound-activated chemical defense of the noninvasive red alga Gracilaria chilensis and the invasive Gracilaria vermiculophylla, Beilstein J. Org. Chem., 8, 283–289, http://dx.doi.org/10. 3762/bjoc.8.30. Roa R., 1992, Design and analysis of multiple-choice feeding-preference experiments, Oecologia, 89, 509–515. Rodil I. F., Olabarria C., Lastra M., López J., 2008, Differential effects of native and invasive algal wrack on macrofaunal assemblages inhabiting exposed sandy Herbivory on macro-algae affects colonization of beach-cast . . . 357 beaches, J. Exp. Mar. Biol. Ecol., 358 (1), 1–13, http://dx.doi.org/10.1016/j. jembe.2007.12.030. Rohde S., Molis M., Wahl M., 2004, Regulation of anti-herbivore defence by Fucus vesiculosus in response to various cues, J. Ecol., 92 (2), 1011–1018, http://dx.doi.org/10.1111/j.0022-0477.2004.00936.x. Rohde S., Wahl M., 2008, Temporal dynamics of induced resistance in a marine macroalga: time lag of induction and reduction in Fucus vesiculosus, J. Exp. Mar. Biol. Ecol., 367 (2), 227–229, http://dx.doi.org/10.1016/j.jembe.2008.10. 003. Rose M. D., Polis G. A., 1998, The distribution and abundance of coyotes: the effects of allochthonous food subsidies from the sea, Ecology, 79 (3), 998–1007, http://dx.doi.org/10.1890/0012-9658(1998)079[0998:TDAAOC]2.0.CO;2. Rossi F., Underwood A. J., 2002, Small-scale disturbance and increased nutrients as influences on intertidal macrobenthic assemblages: experimental burial of wrack in different intertidal environments, Mar. Ecol.-Prog. Ser., 241, 29–39, http://dx.doi.org/10.3354/meps241029. Roth J. D., 2003, Variability in marine resources affects arctic fox population dynamics, J. Animal Ecol., 72 (4), 668–676, http://dx.doi.org/10.1046/j. 1365-2656.2003.00739.x. Rothäusler E., Macaya E., Molis M., Wahl M., Thiel M., 2005, Laboratory experiments examining inducible defense show variable responses of temperate brown and red macroalgae, Rev. Chil. Hist. Nat., 78 (4), 603–614, http: //dx.doi.org/10.4067/S0716-078X2005000400001. Schweitzer J. A., Bailey J. K., Hart S. C., Whitham T. G., 2005, Nonadditive effects of mixing cottonwood genotypes on litter decomposition and nutrient dynamics, Ecology, 86 (10), 2834–2840, http://dx.doi.org/10.1890/04-1955. Storry K. A., Weldrick C. K., Mews M., Zimmer M., Jelinski D. E., 2006, Intertidal coarse woody debris: a spatial subsidy as shelter or feeding habitat for gastropods?, Estuar. Coast. Shelf Sci., 66 (1–2), 197–203, http://dx.doi.org/ 10.1016/j.ecss.2005.08.005. Van Alstyne K. L., 1988, Herbivore grazing increases polyphenolic defenses in the brown alga Fucus distichus, Ecology, 69 (3), 655–663, http://dx.doi.org/10. 2307/1941014. Van Alstyne K. L., McCarthy J. J., Hustead C. L., Duggins D. O., 1999, Geographic variation in polyphenolic levels of Northeastern Pacific kelps and rockweeds, Mar. Biol., 133 (2), 371–379, http://dx.doi.org/10.1007/s002270050476. Wardle D. A., Bonner K. I., Barker G. M., 2002, Linkages between plant litter decomposition, litter quality, and vegetation responses to herbivory, Funct. Ecol., 16 (5), 585–595, http://dx.doi.org/10.1046/j.1365-2435.2002.00659.x. Weinberger F., Buchholz B., Karez R., Wahl M., 2008, The invasive red alga Gracilaria vermiculophylla in the Baltic Sea: adaptation to brackish water may compensate for light limitation, Aquat. Bot., 3 (3), 251–264, http://dx. doi.org/10.3354/ab00083. 358 P. Eereveld, L. Hübner, G. Schaefer, M. Zimmer Williams S. L., 1984, Decomposition of the tropical macroalga Caulerpa cupressoides (West) C. Agardh: field and laboratory studies, J. Exp. Mar. Biol. Ecol., 80 (2), 109–124, http://dx.doi.org/10.1016/0022-0981(84)90007-8. Willson M. F., Gende S. M., Marston B. H., 1998, Fishes and the forest, BioScience, 48 (6), 455–462, http://dx.doi.org/10.2307/1313243. Yates J. L., Peckol P., 1993, Effects of nutrient availability and herbivory on polyphenolics in the seaweed Fucus vesiculosus, Ecology, 74 (6), 1757–1766, http://dx.doi.org/10.2307/1939934. Zimmer M., Pennings S. C., Buck T. L., Carefoot T. H., 2004, Salt marsh litter and detritivores: a closer look at redundancy, Estuaries, 27 (5), 753–769, http://dx.doi.org/10.1007/BF02912038. View publication stats