Bottom-up and top-down interactions in coastal interface systems

BAKKER, J.P.; NIELSEN, K.J.; ALBERTI, J.; CHAN, F.; HACKER, S.D.; IRIBARNE, O.O.; KUIJPER, D.P.J.; MENGE, B.A.; SCHRAMA, M.; SILLIMAN, B.R.

Trophic Ecology

Cambridge University Press

Año: 2015; p. 157 - 200

The two coastal interface ecosystems we have focused on in this chapter share many characteristics. First and foremost, both experience the rhythm of the tides, which sets the scene for most of the abiotic conditions and biotic factors that influence these ecosystems. In these systems, community structure and the relative importance of top-down and bottom-up factors result from differences in oceanographic and climatological conditions, geographical location, substrate type, and local and regional species´ pools.     A striking difference between rocky shore and salt marsh ecosystems is the opposed orientation of their major vertical stress gradients: abiotic stress on rocky shores is strongly linked to the duration and conditions of atmospheric exposure during low tides, whereas stressful conditions on the salt marsh arise from the duration of inundation by seawater (Fig. 7.1). Rocky shores generally become less physiologically stressful for macrophytes and sessile invertebrates lower on the shore, where periods of inundation are longer, whereas abiotic stress for salt marsh plants generally decreases with increasing salt marsh elevation. For rocky shore macrophytes, winds and insolation result in desiccation, and thermal and light stresses are greater higher on the shore, where nutrients are also less readily available. In contrast, the soils of salt marshes are highly heterogeneous with respect to salinity, nutrients, and redox potential, and these factors, absent from rocky shores, are very important determinants for plant community composition in salt marshes. However, mobile marine consumers in both ecosystems are negatively influenced by atmospheric conditions at higher elevations, and both rocky intertidal and salt marsh communities are vulnerable to terrestrial consumers. On rocky shores, these are primarily birds, whereas mammals and birds are common on salt marshes.    Further, one of the most fundamental differences between salt marsh and rocky shore ecosystems is the difference in substrates, noted above. Detritus does not accumulate in rocky intertidal ecosystems and contribute to the formation of soil; instead, it is largely exported to adjacent beaches (Fig. 7.2) and other benthic marine ecosystems. As a result, there is limited opportunity for consumers to influence nutrient recycling or ecosystem nutrient supply rates. Rather, exchange or inputs of nutrients and energy with adjacent ecosystems are both common and more influential. In contrast, salt marsh plants and animals can influence nutrient dynamics and sediment properties through uptake and sequestration of nutrients in plant biomass, bioturbation and excreta (e.g., pseudofeces of mussels at the edge of the marsh), and the brown food web. Top-down and bottom-up processes differ markedly between these systems. Trophic interactions on rocky shores can be strong, but also vary as a function of environmental stress and oceanographically mediated subsidies of energy, materials, and propagules from adjacent ecosystems. Some of these processes mediate bottom-up effects (e.g., oceanographic processes that affect nutrient supply or light availability), but many are spatially structured subsidies transported among ecosystems by ocean currents. Adjacent pelagic ecosystems serve as "nurseries" that also provide avenues of dispersal for larvae. Additionally, the marine macro- and microalgae that are the dominant primary producers on rocky shores have lower C:N ratios and thus higher forage quality than the vascular plants that dominate salt marshes. Although this makes rocky shore autotrophs more palatable and nutritious, these macrophytes often go uneaten, and in some places, especially where nutrients and light are plentiful, they can dominate the shore. Mobile rocky shore herbivores seem to have the greatest impact early in succession before macrophytes grow large, escaping vulnerability. Suspension-feeding invertebrates do not make much of a dent in nearshore phytoplankton concentrations, yet phytoplankton abundance fuels the growth and fecundity of suspension feeders, and of their invertebrate predators. Thus, energy from cross-ecosystem subsidies plays a major role in supporting rocky shore consumer populations.    In comparison, the greatest impacts of herbivores in salt marshes occur at intermediate stages of succession in European salt marshes (Kuijper and Bakker, 2005), and during early and intermediate stages of succession in South American salt marshes (Alberti et al., 2008, 2010a; Daleo et al., in press). Top-down forces in salt marshes often play a relatively small role in structuring communities. However, this strongly depends on the geographical range. On the vast low-lying western Atlantic marshes, herbivore top-down control can play a major role; consumer assemblages in these areas are often dominated by marine grazers (for example, snails and crabs), or a mix of terrestrial and aquatic grazers (such as aphids, stem-borer moths, nutrias, wild guinea pigs, and other rodents). In the extensive lower marshes, plants can be completely devoured by consumer fronts of snails, which overwhelm the vascular plants. Low-lying salt marshes in South America are also top-down controlled by invertebrate grazers (e.g., crabs). In contrast, in northeastern Atlantic marshes (Europe), which are dominated by high marsh, grazers are mostly terrestrial vertebrates, such as hares and geese. Grazing by these vertebrates, especially in early successional stages, can have a profound effect on vegetation development. In later stages of salt marsh development, only grazing by large vertebrate herbivores (livestock) can have a similar impact. In general, top-down control by carnivores is rare in salt marshes.     Ecosystems of the land?sea margin are characterized by their strong connectivity with adjacent pelagic, estuarine, shoreline, and terrestrial ecosystems (Menge et al., 1997, submitted; Polis et al., 1997 (and references therein); Menge, 2000; Valiela et al., 2002; Dugan et al., 2003; Silliman and Bertness, 2004; Schrama et al., 2012, 2013a; Menge and Menge, 2013). Empirical studies in rocky shore and salt marsh (and other) land-sea margin ecosystems have made important contributions to our increasing appreciation of the interplay between bottom-up and top-down processes, and how they structure communities and influence ecosystem functioning. In addition, these systems highlight the importance of spatial and temporal scales (see Chapter 11) in determining the interaction of bottom-up and top-down processes at the land-sea boundary.