Tropical Ecosystems

Alfred Russell Wallace

• Early Indo-West Pacific explorer
• Father of biogeography
• Independently theorized evolution by natural selection

Charles Darwin

Alexander Von Humboldt

• Nearer approach the tropics - greater increase in variety of structure, grace or form,

Biodiversity highest in tropics - many hypotheses as to why

Early ecological hypotheses - related to area effects (larger and more connected), species coexistence (nich partitioning) and temperature effects

Later evolutionary (geographic differences between speciation and extinction rates) and historical hypothesis (tropics older than temperature latitudes)

More likely, latitudinal gradient best described as intergration of ecological, evolutionary and historical factors

HIGHEST SPECIES RICHNESS AT THE EQUATOR

James Brown - hypothesised marine species diversity gradients explained by combining metabolic theory with biogeography

Tropical areas warmer than temperate areas = increased evolutionary speed and:

• Increased metabolic rate
• Reduced life span
• More/shorter generation times = higher DNA mutation rates
• Speeds up growth rates
• Shorter larval development = shorter dispersal distances

Larval dispersal + high temperature = difficulty for marine species to disperse between island in tropics 

= Islands metaphorically 'father' away (larval development faster and dispersal lower)

Explains Indo-West Pacific marine biodiversity hotspot

Over large timescales - big efffect on evolution of new species = allopatric speciation

In order to discuss factors driving diversity gradients - conceptual models of geographic scenarios of bio-diversification. 5 scenarios:

• Cradles - extinction constant, speciation highest in tropics
• Museums - extinction low in tropics, speciation constant
• Out of the tropics - high speciation, low extinction, dispersal out of tropics
• Casino - high speciation, high extinction, dispersal in tropics
• Intro the tropical museum - low extinction, speciation constant, dispersal into tropics

In general, regional or global patterns of biodiversity are function of geographical differences between speciation, extinction and dispersal rates over space and time.

Data needs to be tested - which geographical scenario applicable to study system

Use fossils or moden records

Eastern Pacific rocky intertidal gastropod latitudinal diversity gradient and worldwide diversity of marin bivalves examples - high diversities, well known taxonomy/ecology and excellent fossil record

Latitudinal diversity gradient = into the tropical museum hypothesis

Low extinction in tropics, constant speciation rates and dispersal into the tropics

Study restricted to gastropods from one habitat type - rocky shores - very few of these habitats in tropics = difficulty to immigrate

Species originating in rocky shore 'islands' in temperate regions:

• Short distances between habitats
• Longer dispersal distances
• Evolved in highly seasonal environments
• Capable of dispersal into tropics over time

Species originating in tropics:

• Long distances between habitats
• Shorter dispersal times
• Evolved in warm environments
• Less capable of dispersing out of tropics

Bivalve species pooled across depths

= Out of tropics scenario = high speciation rates, low extinction rates and net dispersal of larvae out of tropics

Study included many different habitat types 

Pooling across habitats erases distance between islands - affect results

Controlling for habitat is key

Hermatypic corals = reef building corals

Phylum Cnidaria, class Anthozoa, order Scleractina - hard, stony corals

Contain zooxanthellae - limited to photic zone

Ahermatypic corals - non-reef building with no zooxanthellae

Environmental limits - shallow water tropic regions;

• Temperature - 18-36C, optimum 26-28C
• Light - <10m but can be <30m - light for zooxanthellae photosynthesis
• Salinity - high salinity 33-35
• Wave action - high wave energy but constant risk of storms/hurricanes = growth/erosion cycle
• Sediment load - no sediments, causes smothering, shading and abrasion (no estuaries)

Earliest fossils - Triassic Period (~240mya), after late Permian mass extinction

Scleractina not first reef-building animals - rugrose corals, stromatolites and bivalves can also build reefs

Hermatypic corals have symbiotic dinoflagellates - only in photic zone

Transfered via reproduction or direct from water

Benefits for both organisms - mutualism

Zooxanthellae

• Provide growth energy in form of carbohydrates, lipids, oxygen during photosynethsis
• Rapid uptake of CO2 - enhanced CaCO3 deposition at coral base = reef building
• Remove organic waste

Algae obtain nutrients from host and direct seawater upyake

Coral hosts:

• Give access to sunlight for photosynthesis
• Give stable,protected environment
• Provide coral metabolic waste

Bleaching - generalised response to stress = zooxanthellae ejected from polyp

= decreased growth and reproduction = death

If temperature of water too high, bleaching occurs - during El Nino and climate change

Warm water temperatures higher than average summer for 4 weeks = bleaching

Excessive sunlight, calm seas and low tides also = bleaching

Now in 3rd coral bleaching event (lasted since 2014) - longest bleaching event ever recorded

Fringing reef - close to shore

Barrier reef - often surrounds tropical islands and seperated from land by 1-10km wide lagoon

Atoll reef - form around volcanic islands - start off as volcanic island with fringing reef as island subsides/sea level rises, coral grow rapidly on outer edge = barrier reef. Eventually island is lost but outer ring-shaped atoll remains

Patch reefs (bommies) - smalls areas of reef in shallow lagoon area

Divisions of a reef:

• Shore - sandy beach
• Back reef - shallow lagoon areas - 10sm to 1000m (small patch reefs and seagrass beds)
• Reef flat - strong wave action w/ smooth coral free-area of encrusting coarline algae
• Fore reef - building most rapid (ideal conditions), dominated by elkhorn coral or staghorn if calm
• Drop off - plate corals catch more light due to less wave action and more sediment

Coral reefs - extremely diverse systems - large variety of primary producers with high biomass

Species richness not uniform - divided into 3 major global regions

• Indo-west pacific - biggest area/most diverse
• Eastern Pacific - smallest area
• Western Atlantic - impoverished fauna

Most nutrients cycle internally - close coupling of plants and animals (inc. symbiosis) = low f-ratio (little regenerated nutrients) and low net production (respiration)

Space limiting - competition for space and light

• Expoitative competition = corals shading out others
• Interference competition = coral extrude gastric filaments and sweeper tentacles kill of polyps of neighbour (branching corals)
• Massive corals counter by being shade tolerant = competitive hierarchies with depth

Predatory pressure constant 

Corallivorous, herbivorous and omnivorous fish influence community structure