Grazers on Benthic Seaweeds - Seaweed Biology - page 161

the increase in nutrients from herbivore excretions may have positive effects on
seaweed productivity (Carpenter 1986 ). Lastly, the preferential feeding of
amphipods on reproductive algal tissue can lead to an increased release and
recruitment of spores (Buschmann and Vergara 1993 ).
8.4 Seaweed Defenses Against Grazing
Algal defenses against herbivores present the other side of the coin in
grazer-seaweed interactions, where defenses presumably have developed in
response to selective pressures of grazers. The large array of algal defense
mechanisms ranges from tolerance and fast replacement of lost tissue, to temporal
and spatial escapes, to structural and chemical defenses. Several different defense
strategies may be employed simultaneously in some seaweeds, and many of these
defenses may also have functions other than herbivore deterrence (e.g., antifouling,
antimicrobial defense, allelopathic functions). These defenses are commonly dis-
tinguished as non-coexistence and coexistence strategies, with the latter
encompassing structural and chemical defenses (e.g., Lubchenco and Gaines
1981 ; Duffy and Hay 1990 ; Table 8.3 ).
In non-coexistence strategies the encounter of seaweeds with herbivores is
minimized. These “strategies” do not imply a specific, directed behavioral response
in seaweeds but likely are a result of chance or are evolutionary developments from
situations where spatial and temporal occurrence of seaweeds coincided with
minimal herbivore activity. Overall, grazing pressure is variable over time and
space, and seaweeds can escape grazing by occurring in places, or by completing
important life history phases such as initial growth or reproduction during times,
where and when herbivores are not present or active. These strategies are most
effective at driving co-evolution in systems where temporal and spatial distribution
of herbivores is predictable. Escape in size occurs when seaweeds outgrow the
sporeling stage as a bottleneck of high grazer susceptibility and reach a size where
they are unaffected by the specific feeding mode of the abundant herbivore. Lastly,
macroalgae also can escape herbivores by association with other organisms that
provide them chemical or structural protection from grazers (for examples of the
above see Table 8.3 ).
Coexistence strategies lower the herbivore's attraction to a seaweed because of
specific structural or chemical traits of the alga. The most common example of
structural defenses, also referred to as mechanical or morphological defenses,
involves the inclusion of calcium carbonate into algal cells (Paul and Hay 1986 ).
In their functional-form model, Steneck and Watling ( 1982 ) suggest that calcareous
seaweeds, specifically crustose coralline forms, have the highest grazer resistance
of all algal forms, and grazing on calcareous forms requires specialized mouthparts
in the herbivore. The main herbivores able to graze on coralline algae are chitons,
limpets, sea urchins, and parrot fishes. Coralline algae often are abundant despite
high herbivore densities, demonstrating that herbivore damage is minimal.