A central theme in all environments (terrestrial, as well as marine): what determines spatial and temporal patterns of diversity?

Thus far, we have focused on the physical environment.... now consider how biological interactions may influence patterns of marine diversity.

Inter-specific competition and predation/herbivory/parasitism coupled with environmental factors often play important roles in structuring marine communities.

Competition and predation

The competitive exclusion principle suggests that there should be little diversity: how it it maintained?

Remember, food and space can both be important limiting resources for marine organisms.

Niche partitioning

Character displacement

Predation/herbivory/parasitism

Intense patterns of exploitation can reduce the abundance of certain species: sponges,

  Keystone predation

Classic example: Pisaster seastar in intertidal habitats prey preferentially on mussels, allowing others

Another "local" example: Sea otters, urchins, and kelp. 

Disturbance

As in terrestrial habitats, community succession following disturbance is commonly observed.

Intertidal habitats offer opportunities to examine physical and biological interactions

Intertidal habitats are characterized by periodic (predictable) exposure to air during low tide.

Marine animals within these habitats must cope with desiccation, heat stress, and exposure to terrestrial predators

As mentioned in past lectures, strategies include: "Clamming up", evaporative cooling (increases the risk of desiccation) and moving to cracks or tide pools

Strategies for coping with wave stress/shock (abrasion, pressure changes, drag) include flexibility, low profiles, hard shells, and strong muscles or hold fasts for attachment

Because of the constant and predictable fluctuation in exposure to water along a vertical gradient, zonation is common in all tideland habitats

Zonation is often organized by:

Physical limitations/tolerances at upper ranges of distribution

Larval preferences (cracks and crustose coraline red algae often induce settlement)

Competition for space: overgrowing and undercutting

Predation: Immersion times often limit the effectiveness of predators

 

Several tideland habitats are of interest to marine biologists, including: the rocky intertidal, estuaries, spartina salt marshes, and mangrove habitats

Rocky intertidal regions are perhaps the best studied

Some important lessons from intertidal studies:

Keystone predation

Complex food webs are well studied in intertidal regions

Indirect interactions within intertidal food webs can influence distribution and abundance

Cthamalus have an inducible morphological defense against predation by Acanthina

 

Estuaries and tide flats

These large expanses of relatively loose sediment are characterized by extensive flats and a network of drainage channels.

Because of their flat topography, estuaries are relatively ephemeral habitats (geologically), strongly influenced by relatively small changes in sea level.

In the short-term, they represent extremely rich, productive habitats: combining high levels of nutrient input with shallow, relatively protected marine habitats.

A gradient of salinity occurs in most estuaries: this gradient shifts with the tides.

Some species (e.g. crustaceans) have adapted to low salinities (10-15 o/oo) while others (e.g., many echinoderms) have not.

A critical salinity range of 3-8 o/oo limits exchange between fresh and marine spp. Salty enough to inhibit purely freshwater spp, but too dilute for marine adapted spp.

With time to adjust (acclimate), some spp (e.g. salmon) can make the transition from salt to fresh or vice versa.

Lots of work on physiology to understand how these tolerances change.

Large estuaries can be important nursery area, retaining larvae while they develop. Some species show larval adaptations to remain within estuaries (e.g. staying close to the bottom during outgoing tides)

Many estuary residents are adapted to life on soft bottoms rather than hard substrate

Estuary habitats are extremely important coastal wet lands

They are threatened by development, pollution, and agricultural practices that divert water flow.

Spartina/salt marshes

Spartina grasses are salt tolerant species that bind sediment with their rhizomes (like roots).

Rhizomes allow uptake of nutrients and vegetative reproduction.

The presence of grasses builds substrate... and where they have been introduced, their spread threatens native spp.

Spartina salt marshes show distinct patterns of zonation that reflect levels of competition and physiological tolerances

Mangrove habitats (mangels)

Mangrove habitats are intertidal regions in tropical and subtropical latitudes

Mangrove trees extend into areas of moderate tide flux by means of prop roots.

These roots reduce water flow and promote sedimentation increasing shoreline building processes.

Because of their quiet, rich, spatially complex waters, mangroves are extremely important "nursery" habitats for many juvenile fishes.

They also support unique assemblages of filter feeding sponges and bivalves