Seascapes and foraging success: Movement and resource discovery by a benthic marine herbivore

Abstract

Spatially concentrated resources result in patch-based foraging, wherein the detection and choice of patches as well as the process of locating and exploiting resource patches involve moving through an explicit landscape composed of both resources and barriers to movement. An understanding of behavioral responses to resources and barriers is key to interpreting observed ecological patterns. We examined the process of resource discovery in the context of a heterogeneous seascape using sea urchins and drift kelp in urchin barrens as a model system. Under field conditions, we manipulated both the presence of a highly valuable resource (drift kelp) and a barrier to movement (sandy substratum) to test the interacting influence of these two factors on the process of resource discovery in barren grounds by urchins. We removed all foraging urchins (Strongylocentrotus droebachiensis) from replicate areas and monitored urchin recolonization and kelp consumption. We tested two hypotheses: (1) unstable substratum is a barrier to urchin movement and (2) the movement behavior of sea urchins is modified by the presence of drift kelp. Very few urchins were found on sand, sand was a permeable barrier to urchin movement, and the permeability of this barrier varied between sites. In general, partial recolonization occurred strikingly rapidly, but sand slowed the consumption of drift kelp by limiting the number of urchins. Differences in the permeability of sand barriers between sites could be driven by differences in the size structure of urchin populations, indicating size-specific environmental effects on foraging behavior. We demonstrate the influence of patchy seascapes in modulating grazing intensity in barren grounds through modifications of foraging behavior. Behavioral processes modified by environmental barriers play an important role in determining grazing pressure, the existence of refuges for new algal recruits, and ultimately the dynamics of urchin-algal interactions in barren grounds.

Keywords: Gulf of Saint Lawrence; Strongylocentrotus droebachiensis; drift kelp; foraging behavior; northwest Atlantic; rocky subtidal; sand substratum; urchin barren.

FIGURE 1

Study sites in the Gulf of Saint Lawrence (Quebec, Canada). Two sites are in the Mingan Archipelago on the north shore (Petite Île au Marteau [PIM] and Île aux Goélands [IG]) and one site on the south shore of the maritime estuary (Baie de Pointe‐Mitis [BPM]).

FIGURE 2

Experimental setup for in situ subtidal manipulations. (a) Schematic representation of the manipulated areas showing both the inside and outside zones; (b) Sand‐substratum treatment with central marker and subsurface float; (c) Control‐substratum treatment with kelp; (d) Rebar procedural‐control treatment with kelp.

FIGURE 3

Barriers to movement: The number of large urchins in both the inside and outside zones after 48 h. Large points are means ± 1 standard error, and individual data points are shown as smaller and paler points (n = 3 per treatment at each of the two sites which are indicated by different shapes). The dashed red line in each panel indicates the overall mean pre‐manipulation large urchin density (see Table 4 for more detail).

FIGURE 4

Temporal and spatial patterns of urchin aggregation on kelp. Small points are data from individual experimental units. Lines are splines of large urchin densities through time ± 1 standard error. The dashed line in each panel indicates the overall mean density of large urchins across all sites.

FIGURE 5

Proportion of kelp consumed per 24 h in all manipulations (barriers experiment and attraction/retention experiment combined). (a) Boxplots showing the distribution of values per substratum treatment; box widths are proportional to the square roots of the number of replicates (Sand n = 12, Procedural Control n = 12 and Control n = 34). (b) Proportion of kelp consumed as a function of the number of urchins in contact with the kelp. Points are colored by substratum treatment, and shapes indicate site‐experiment blocks. Unfilled points with values >0.8 were not included in the model, and lines show predicted values.

FIGURE 6

No evidence for a relationship between relative water movement (dissolution rates of clod cards) and consumption of kelp during the study period.

FIGURE 7

Size and biomass distribution for the three experimental sites. (a) Size distributions are shown as urchins.m⁻² within 5‐mm bins of test. (b) Biomass distribution within the same 5‐mm bins of test diameter. The red dashed line indicates the limit defined as large actively foraging urchins in the present study (test diameter > 20 mm).