Ecosystem Services

6.3.3 Biological control
Shallow, wave sheltered bays and inlets of the northern Baltic Sea serve as essential
habitat for several species of coastal fish, including large predatory fish, i.e. piscivorous,
fish-eating, species. The ecological function of fish production can be considered
among the most important services provided by these ecosystems, since the predatory
fish in turn provide a large number of ecosystem services that either directly or indirectly
benefit human well-being and life style (Holmlund and Hammer 1999).
Perhaps one of the most important indirect services of large predatory fish lies in the
regulatory service of top down control, whereby decline of predatory fish promotes the
production of nuisance algae by decreasing invertebrate grazer control, leading to worsened
eutrophication symptoms (Eriksson et al. 2009, 2011). A recent meta-study assessed
the magnitude of bottom-up processes relative to the loss of top-down control and
showed that top-down effects are on average on par with fertilization effects on ephemeral
algae (Östman et al. 2016), and that the loss of large predatory fish can yield similar
eutrophication symptoms as nutrient enrichment. Increased overgrowth of ephemeral,
filamentous algae on larger, more structurally complex vegetation, reduces the quality of
the habitat for fish production (Snickars et al. 2010), and filamentous algae are generally
perceived as negative for the recreational value of the water (Söderqvist et al. 2005). The
supporting service of biological (top-down) control provided by predatory fish thus impacts
the food web and, hence, influences several other services.
The system is, however, more complicated than that. Biological control exerted by
predatory fish on mesopredators, such as three-spined sticklebacks (Gasterosteus aculeatus),
is multidirectional but size dependent. Sticklebacks also reproduce in shallow
coastal habitats and the timing in spring, when they arrive from the open sea, can have
severe consequences for the recruitment of perch, since sticklebacks can feed on larvae
and the juvenile stages of perch (Bergström et al. 2015, Byström et al. 2015). However,
as perch and other predatory fish grow in size, they instead feed on sticklebacks. The
preservation of bays and inlets, and their associated ecosystem components, is therefore
vital for the long-term provision of several ecosystem services, including both supporting
services.
86 Ecosystem Services