Journal of Experimental Marine Biology and Ecology
Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community
Introduction
Understanding how nutrients and grazing influence the structure of ecological systems is both practically and conceptually important. The widespread use of nitrogenous fertilizers has significantly increased the amount of available nitrogen in global systems, with important implications for the structure and functioning of a wide array of ecosystems (Vitousek et al., 1997, Suding et al., 2005). Similarly, the reduction in abundance or complete removal of species that play key roles in systems can have dramatic effects on ecosystems (Paine, 1966, Power et al., 1996, Jackson et al., 2001, Myers and Worm, 2003). The effects of both top-down effects of consumers and bottom-up effects of nutrients have elicited a good deal of ecological inquiry, but an understanding of the conditions under which we expect each to reign remains elusive (McQueen et al., 1989, Hillebrand, 2002, Gruner, 2004, Schiel, 2004, Vinueza et al., 2006).
In marine systems, evidence that differences in productivity can lead to differences in the structure and dynamics of rocky intertidal communities comes primarily from examinations of the effects of large-scale oceanographic processes (Bosman et al., 1987, Bustamante et al., 1995, Menge et al., 1997, Menge et al., 2003, Nielsen and Navarrete, 2004, Vinueza et al., 2006). Experimental manipulations of nutrient availability in rocky intertidal systems are less prevalent—some have documented important effects (Bosman et al., 1986, Worm and Sommer, 2000, Nielsen, 2001, Worm et al., 2002, Kraufvelin et al., 2006), while others have seen little to no effect of enrichment (Wootton et al., 1996, Bokn et al., 2003, Pfister and Van Alstyne, 2003).
Grazing is a key determinant of algal community structure on rocky shores (reviewed by Lubchenco and Gaines, 1981, Hawkins and Hartnoll, 1983). Molluscan grazers, in particular, play key roles in determining algal distribution and abundance on intertidal rocky shores (Underwood, 1980, Lubchenco, 1983, Cubit, 1984, Underwood and Jernakoff, 1984, Freidenburg et al., 2007). Given the strength of top-down effects in these communities, an important question is: are there conditions under which bottom-up effects temper or alter the defining effects of grazers?
The effects of bottom-up and top-down control on primary producers are often co-dependent (Menge et al., 1997, Proulx and Mazumder, 1998, Menge, 2000, Lotze et al., 2001). Three meta-analyses of aquatic systems have examined how productivity affects the interactions of top-down and bottom-up forces (Hillebrand, 2002, Worm et al., 2002, Burkepile and Hay, 2006). One emerging synthetic result from these studies is that in nutrient-poor environments consumers appear to exert extremely strong control on primary producers but have less control in nutrient-rich environments. A second emerging result from these meta-analyses is that in nutrient-rich environments, enrichment tends to affect both abundance and diversity of primary producers, while in nutrient-poor environments enrichment increases diversity but not abundance.
Here, we examine the effects of experimentally manipulating both nutrient enrichment and grazing on the abundance and diversity of algal communities in a nutrient-poor system on the South Island of New Zealand. We tested two hypotheses: H1 at low (ambient) nutrient levels, consumers will depress both algal abundance and diversity H2 with enrichment, top-down control will weaken and algal diversity will be independent of, or enhanced by, consumers.
Section snippets
Study site
We conducted the experiment in the mid- to high-intertidal zone at Blue Duck, a rocky reef north of Kaikoura (42°25′S, 173°42′E), on the northeast coast of the South Island of New Zealand. The reef is composed of greywacke bedrock and some large, immobile boulders. The rock is extremely hard and relatively smooth. The site is moderately wave exposed.
At Blue Duck, the low-zone is dominated by the large fucoids Durvillaea antarctica and D. willana, and encrusting coralline algae. The mid- to
Efficacy of treatments
The manipulation of limpet densities was successful (Table 2a). High grazing plots averaged (mean ± SE; snail averages reported are after the snail removals began) 40.3 ± 1.2 limpets and 42.5 ± 6.5 snails, intermediate grazing plots averaged 26 ± 0.9 limpets and 29.6 ± 3.6 snails, and low grazing plots averaged 5.6 ± 0.6 limpets and 15.1 ± 1.5 snails. Throughout the experiment, there were more limpets in the high grazing treatment than there were in the intermediate grazing treatment. Although a few limpets
Discussion
This experiment had two major, somewhat contrasting results. First, at the level of the overall community, herbivory, but not nutrient enrichment, had consistent and strong effects on the abundance and diversity of algae in this system. Second, at the level of foliose algae, a major but not the dominant component of the community, grazing and nutrients interactively influenced algal abundance and biomass.
The dominant structuring force of herbivory has been well documented in many temperate reef
Acknowledgements
This work would not have been possible without able assistance in the field and lab from numerous individuals. In particular, we would like to thank R. Russell, and S. Lilley. We thank A. Milligan for running the C:N samples. D. Schiel, and J. Van Berkel provided logistical assistance. J. Lubchenco, J. Lawler, R. Russell, and F. Chan were all essential intellectual sounding-boards. For financial support, we thank the U.S. National Science Foundation Graduate Research Fellowship program (to
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