Biology Reference
In-Depth Information
gut features may not occur at all in some species and can be compensated for by
strong expression of the other characteristics. For example, high enzymatic activity
compensates for a short, stomachless, alkaline digestive tract in the silverside
Atherinops affinis
(Horn et al.
2006
). Much less is known about the physiological
processes associated with digestion and assimilation in invertebrate herbivores
feeding on seaweeds (Targett and Arnold
2001
). Many invertebrate guts contain
an array of proteinase, amylase, and lipase enzymes. Similar to herbivorous fishes,
microflora is supposed to assist with carbohydrate polymer breakdown, while algal
lysis with gut acids may be a less likely mechanism in invertebrate herbivores. The
field of seaweed digestion by invertebrate herbivores still requires much research so
that ecological and evolutionary frameworks of grazer-macroalgal interactions can
be more fully developed.
8.3 Grazer Impacts on Seaweeds and Seaweed Communities
Macrograzer feeding can remove large amounts of seaweed biomass, with strong
ecosystem-wide effects such as trophic cascades best known from temperate kelp
forests and tropical coral reefs (Pinnegar et al.
2000
). Sea urchin grazing on kelps
can be one cause for phase shifts from kelp beds with an abundance of kelps, other
fleshy algae, and associated invertebrates and fishes to an urchin barren state, where
few seaweeds except for encrusting corallines prevail (Steneck et al.
2002
). For
example, in the Alaskan Aleutian Islands, these phase shifts are closely linked to the
presence/absence of sea otters, the keystone species that preys on urchin grazers
(Estes and Duggins
1995
). Under reduced predation pressure in the absence of sea
otters, urchins grow unchecked and overgraze the kelp, causing urchin barrens. The
barren phase is stable over long time periods, until a major disturbance, e.g.,
reintroduction of a predator or disease, reduces sea urchin numbers and kelps can
grow back into stable kelp beds. Besides the indirect benefits to kelp forests of sea
otter predation on urchins, it also provides positive feedbacks for the otters, which
will find more nutritious prey in the complex kelp forests. Trophic cascades
including grazing by urchins and/or herbivorous fishes also are well documented
in tropical coral reef community shifts between coral-dominated and seaweed-
dominated phases (Mumby et al.
2006
). Grazing removes seaweeds from coral
reefs, thus facilitating space for coral growth and recruitment. When grazing is
reduced, such as after the mass die-off of the urchin
Diadema antillarum
in the
Caribbean in 1983 (Carpenter
1990
) or by overfishing, competitively dominant
seaweeds smother corals and dominate the reef community (see Chap.
16
by Mejia
et al.). A greater diversity of reef grazers is typically more effective in removing
algal biomass, thus increasing overall coral reef resilience (e.g., Duffy et al.
2003
).
The feeding activity of small mesograzers is typically nonlethal to large
seaweeds, but their grazing activity can still affect host fitness (e.g., reduce growth
and reproductive output), and influence overall community structure (Arrontes
1999
). The overall community effects of algal biomass removal by mesograzers
