Parasites primarily affect food web structure through changes to diversity and complexity. However, compared to free-living species, their life-history traits lead to more complex feeding niches and altered motifs.

There is mounting evidence that parasites can influence the composition and structure of natural animal communities. In spite of this, it is difficult to assess just how important parasitism is for community structure because very few studies have been designed specifically to address the role of parasites at the community level, no doubt because it is difficult to manipulate the abundance of parasites in field experiments. Here, we bring together a large amount of published information on parasitism in intertidal communities to highlight the potential influence of parasites on the structure and biodiversity of these communities. We first review the impact of metazoan parasites on the survival, reproduction, growth and behaviour of intertidal invertebrates, from both rocky shores and soft-sediment flats. Published evidence suggests that the impact of parasites on individuals is often severe, though their effects at the population level are dependent on prevalence and intensity of infection. We then put this information together in a discussion of the impact of parasitism at the community level. We emphasize two ways in which parasites can modify the structure of intertidal communities. First, the direct impact of parasites on the abundance of key host species can decrease the importance of these hosts in competition or predator-prey interactions with other species. Second, the indirect effects of parasites on the behaviour of their hosts, e.g. burrowing ability or spatial distribution within the intertidal zone, can cause changes to various features of the habitat for other intertidal species, leading to their greater settlement success or to their local disappearance. Our synthesis allows specific predictions to be made regarding the potential impact of parasites in certain intertidal systems, and suggests that parasites must be included in future community studies and food web models of intertidal ecosystems.Key words : Altered behaviour, castration, food webs, host survival, rocky shores, soft-sediment flats, trematodes. Parasitism is now widely recognized as a factor that can influence the composition and structure of natural animal communities (Minchella & Scott, 1991 ; Combes, 1996 ; Hudson & Greenman, 1998 ; Poulin, 1999). The presence of certain species, or their abundance relative to that of other species in a community, may be entirely dependent on the action of parasites. In addition to the numerous studies that have shown that parasites can decrease the survival or reproductive output of their host, there is evidence that the outcome of interspecific competition between hosts (Hudson & Greenman, 1998) and the interaction between predators and prey (Lafferty, 1999) can be modified by parasites. The latter effect often involves parasite-mediated changes in host behaviour that increase their probability of being captured by a predator ; these changes, however, can also impact on communities in other ways (Poulin, 1999). Despite this accumulating evidence, it is still difficult to ...

Summary 1.Despite their documented effects on trophic interactions and community structure, parasites are rarely included in food web analyses. The transmission routes of most parasitic helminths follow closely the trophic relationships among their successive hosts and are thus embedded in food webs, in a way that may influence energy flow and the structure of the web. 2. We investigated the impact of parasitism on the food web structure of a New Zealand intertidal mudflat community. Different versions of the food web were analysed, one with no parasites, one with all parasite species and several other versions, each including a single parasite species. We measured key food web metrics such as food chain length, linkage density and proportions of top, intermediate and basal species. 3. The inclusion of all parasite species in the food web resulted in greatly increased mean and maximum food chain length, but had little impact on linkage density and realized connectance. The main change caused by introduction of parasites was the relegation of a number of species from top predators to intermediate status, although the addition of parasites as top predators left the actual ratio of predators to prey relatively unchanged. 4. When individual parasites were added to the food web, their effect on food web properties was generally minimal. However, one trematode species that affected several host species, because of its complex life cycle and low host specificity, produced food web properties similar to those in the web version including all parasite species. 5. The respective effect of individual parasite species was roughly proportional to the number of host species they affected, and thus the life cycle characteristics of parasites determine to a large extent their impact on food web structure. The next step would be to quantify how they affect energy flow through the web.

R. 2005. Parasites boosts biodiversity and changes animal community structure by trait-mediated indirect effects. Á/ Oikos 108: 344 Á/350.Parasitism has long been emphasised as an important process structuring animal communities. However, empirical evidence documenting the impact of parasites in other than simple laboratory settings is lacking. Here we examine the trait-mediated indirect effects of echinostome trematodes on a New Zealand soft bottom intertidal community of macroinvertebrates. Curtuteria australis and a second related but undescribed trematode both utilise the cockle Austrovenus stutchburyi as second intermediate host in which the parasites infect the foot tissue. Heavily infected cockles are therefore more sessile than lightly infected individuals, and, unable to bury, often rest on the sediment surface. We utilised these behavioural changes in two long term field experiments, respectively manipulating the parasite load of buried cockle (i.e. bioturbation), and the density of surfaced cockles (i.e. surface structures and seabed hydrodynamics). Both high parasite loads in buried cockles and the presence of surfaced cockles increased species richness and generally also the density of certain species and of major systematic and functional groups of benthic macroinvertebrates. Species diversity (alpha) peaked under intermediate densities of surfaced cockles. Our results demonstrate that parasites, solely through their impact on the behaviour of a single community member, can be significant determinants of animal community structure and function.

The population status of harbor porpoises has been of concern for several years, and the establishment of Marine Protected Areas (MPAs) has been suggested as a method to protect the harbor porpoise (Phocoena phocoena, Linneaus 1758) and other small cetaceans. In order to designate MPAs, high-density areas for the species must be identified. Spatial distribution of small cetaceans is usually assessed from ship or aerial surveys. As a potentially more accurate alternative, this study examined the movements and area preferences of 64 harbor porpoises, satellite tagged between 1997 and 2007, in order to determine the distribution in the North Sea, the western Baltic, and the waters in between. Results show that harbor porpoises are not evenly distributed, but congregate in nine high-density areas within the study area. Several of these areas are subject to significant seasonal variation. The study found no differences in the home range size of males and females, but immature harbor porpoises have larger home ranges than mature porpoises. The use of satellite telemetry for 230 SVEEGAARD ET AL.: SATELLITE TRACKING HARBOR PORPOISES 231 identifying areas of high harbor porpoise density can be of key importance when designating MPAs.

The effect of larval trematodes on growth, fecundity, egestion and locomotor activity in naturally infected Hydrobia ulvae (Pennant) was studied in the laboratory. Infected snails showed increased growth rates (shell height, body wet weight) compared with uninfected controls. C/N analysis of the snails suggested that the additional growth of infected specimens included shell material as wet1 as dry soft tissue. infection caused a significant reduction in penis size and an almost complete cessation of oviposition. As judged by their egestion rates, food consumption rates of infected and uninfected snails were roughly similar, but infection caused a significant reduction in locomotor activity. It is suggested that the energetic basis for parasite metabolism and excess host growth is in part reallocated reproductive energy following castration, and in part energy saved by reduced locomotor activity. Some previously reported hypotheses explaining the equivocal evidence on p~asite-induced growth enhancement are discussed. It is argued that the phenomenon is not necessarily speciesspecific, but should also be addressed at the level of subpopulations both in and outside the snails' reproductive season.

Circumstantial evidence has suggested that marine mlcrophallid ti-ematodes using gastropods Hydrobia spp. as flrst lnterlnedlate hosts, amphipods Corophlum spp. as second intermediate hosts, and various waterblrds as final hosts, may cause temperature-dependent epizootics and eventually local extinction of ~ntel-mediate host populations. Therefore, we examined experirnentally the impact of the microphallid trematode Mar~trenia subdolum on Corophluni volutator wlth special relerence to the influence of temperature, during the parasltes' transmission from s n a~l to amphipod. Trematode infected snails and amphipods were estabhshed togtlther in experimental aquaria at temperatures of 15, 2 2 and 24°C for 9 d. Amphipod surface activ~ty, survlvdl rate, parasite prevalence, lnfectioil rate, and paraslte dlstnbution both within and between amphlpod ~nti~vlduals were recorded d u n n g , or at the end of, the experiment The snalls' shedding rate of larval ti-clnatodes as a function of temperature was also e x a m~n e d . Increasing temperature resulted 111 higher Infection levels and parasite-induced mortality in the amphipods, most Ilkely governed by a similar temperature-dependent emergence of larval trc'matodes from the snails. No transmission occurred at Ij'C, whereas at 24°C the paraslte prevalence approached 100",1 and the parasite-induced mortality exceeded 50%) in relation to controlsAs assessed by the transparency of the amphipods' gills, infestation inflicted anaemia was the llkely mechanism behind the Increased surface activity observed among infected specimens. This paraslteinduced behavioural change niay facilitate transmission of infective stages to shorebird hosts feeding on C. volutator The results demonstrate that microphalhd trematodes are able to induce a significant additive mortality in C. volutator populatlons. The temperature mediated mortality emphasizes the potential significance of a density-~ndependent process in controlling the impact of parasltes on host organisms and thereby host population dynamics.

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