How is Tropical Diversity Maintained?

Question 1

What is the competition hypothesis?

Answer 1

Competition hypothesis • Each different species has a different ecology or niche • Occupy the same space by using it differently • Populations in equilibrium with resources at carrying capacity, only when resources are changed would the species competition change. • A section of habitat will act as a template and a set community structure would inhabitant a set habitat. If a species was removed, the same species would recolonise and the habitat would not be open to all.

Question 2

Competition hypothesis: weaknesses?

Answer 2

• All species are slightly different • Measuring amount of food available is very difficult What about larval stages? Series of experiments demonstrated how random larval production is - lottery hypothesis

Question 3

Lottery hypothesis: characteristics

Answer 3

• Species may be ecologically similar to each other • Randomness of larvae - community structure depends on who arrives at the scene first, described as chaotic • Whilst individual populations would not be in equilibrium with resources, the guilds of species would. A particular patch of habitat could only fit in so many individuals of competing species, and that total size of guild would be in equilibrium with resources Species composition changes stochastically (chaotic)

Question 4

Lottery hypothesis: weaknesses

Answer 4

• No easy way at looking at whole population scale, data from small scale studies. Small areas will have greater variability. • Strong associations occur between particular species, when studies have taken place on a larger scale, suggesting something more orderly is going on.

Question 5

Is there any evidence for closed recruitment?

Answer 5

Classic study - Jones GP et al. 1999 Nature 402: 802-804 on lizard island on the northern GBR. • The females lay eggs on clean surfaces, so eggs were collected on PVC tiles • Tiles with eggs put into plastic bags with tetracycline which is fluorescent and taken up by otoliths. ○ Around 10 million embryos marked in field ○ No evidence for effect on larval survivorship • Collect recruits from light traps, subsample 5000 • Presence of fluorescent band near otolith centre mean fish was marked on home reef - 15 (Larval life • Reef fish larvae mean swimming speed 13.7 body lengths/sec (olympic swimmers ~1 bl/s!) maximum 65 cm/s (currents 10-30 cm/s) • Late stage reef fish larvae: mean swimming distance without food or rest 40km (max 140km). Mean exhaustion time 83 h (max 288 h))

Question 6

If the equilibrium between population and resources (space, food, habitat)is not reached then the population could be kept below the carrying capacity by,

Answer 6

1. Predation (predation hypothesis)

• Population kept below carrying capacity by a high level of predation ○ Competitive exclusion doesn’t occur - the idea is the species don’t reach the population sizes in which they compete
• Population size fluctuates driven by changes in predator numbers/effectiveness ○ Community structure driven partly by the selectivity of predators dependant on how generalist the predators are.

2. Larval recruitment (Recruitment Limitation Hypothesis)
   * Population kept below carrying capacity with a low level of recruitment ○ Competitive exclusion doesn’t occur - the idea is the species don’t reach the population sizes in which they compete Population, community structure and overall size vary ‘stochastically’ (chaotically) The hypothesis could be reconciled in terms of competition (present or past) and the level of post settlement process going on. Shows we are not looking at a single model, but instead a mix of model.

Question 7

Summary

Answer 7

A number of hypothesis that explain how species manage to coexist in the environment. Across all the situations and species involved and mix of hypothesis will help explain the high level of coexistence, particularly in these very diverse settings in the tropics. The idea of closed and open systems is an important principle, relating to a number of things, such as species outcompeting each other.

Question 8

WIDER READING: Evidence of self-recruitment

Answer 8

Evidence of self-recruitment in demersal marine populations, S. Swearer et al, 2002.

Most demersal species have a two-phase life cycle: a pelagic larval stage and a benthic, sedentary adult stage. Due to the fluid medium in which the larvae exist in, it was widely thought that local marine populations were open, and recruitment could occur from a multitude of different sources. However, its now accepted that larvae are able to recruit to their natal source population, or can self-recruit.

The evidence:

• Endemic species:
  
  • A species is only found in one specific area, eg. Seamounts, therefore recruitment must be due to local reproduction.
  • Levels on endemism on seamounts can be between 29-34%, showing little overlap with nearby seamounts (less than 1000km).
• Introduced species:
  
  • Establishment of a non-native taxa can only take place If the introduced species can self-recruit.
  • Since the 1900s, 33 species of salt tolerant fish have been introduced to the Hawaiian Islands, with at least 48% of these species being self-recruiters, and therefore having the capability to establish a larger population.
  • Intertidal mussel species Mytilus galloprovincialis quickly spread in south Africa. It was estimated that the larvae were capable of dispersing to distances of up to 100km, however after 4 years of the mussel being introduced, 90% of the population was found within 5km of the original parent population. This is highly indicative of self-recruitment. (McQuaid and Phillips 2000)
• Environmental and chemical markers:
  
  • Jones et al (1999), marked millions of damselfish embryos which were spawned on plastic tiles with tetracycline which appeared in their otoliths. They then used light traps to capture the late stage larvae just before settlement and looked if the marker was present or not. From 5000 fish, 15 were marked, and using these figures along with how many fish they originally marked, were able to conclude that 16-60% of the fish were self-recruiters.

Question 9

WIDER READING: sensory mechanisms of a clownfish

Answer 9

Coral reef fish smell leaves to find island homes, D. Dixon et al, 2008.

The study:

• Looks at the sensory mechanisms used by clownfish species Amphiprion percula to help them find settlement sites, which are either natal reefs or distant reefs.
• Carried out experiments using paired-choice flumes to help investigate the importance of olfactory cues in finding islands.
• Explores the facts that there is a high level of self-recruitment in many tropical reefs.

The results:

• Recently settled clownfish showed a preference for water from reefs with islands, water collected near islands and water treated with anemones or rainforest vegetation.
• Lab reared juveniles of the same species also exhibited these preferences, especially to anemones- inferring that using water-born olfactory cues is innate and not a learned behaviour. This suggests that they use the olfactory cues to find vegetates island which explains the high level of self-recruitment on island reefs, as apposed to reefs with no islands.
• A. percula exhibits an innate olfactory attraction to rainforest vegetation and hence is attracted to reefs which are associated with an island.

This study is supported by Almany et al 2007- At Kimbe island in Papua New Guinea, around 60% of the juvenile clownfish that settled at the island were offspring of already resident adults. This highlights self-recruitment in this area.

Question 10

WIDER READING: Recruitment, loss and coexistence

Answer 10

Recruitment, loss and coexistence in a guild of territorial coral reef fishes. P. Sale, 1979.

Guild = a group of species that have similar requirements and play a similar role within a community.

The study:

• Looks at three species of territorial pomacentrid fishes which all occupy adjacent territories on a rubble patch on a shallow reef slope.
• The three species are: Eupomacentrus apicalis, Plectroglyphidodon lacrymatus, and Pomacentrus wardi
• They study explores recruitment of the three species onto bare patches.

Question 11

WIDER READING: Competitive coexistence of coral-dwelling fishes:

Answer 11

Competitive coexistence of coral-dwelling fishes: the lottery hypothesis revisited. P. Munday, 2004.

The study:

• Lab and field experiments were used to test the mechanisms of co-existence between two coral-dwelling goby species: Gobiodon histrio and G. erythrospilus. The two species are found at Lizard island on the Great Barrier Reef and have almost identical behaviours regarding habitat use.
• They can coexist in this area due to a competitive lottery.
• The lottery hypothesis states that species with similar niches can coexist through chance recolonization of vacant space, if there are temporal or spatial fluctuations in abundance of recruits available to occupy vacant habitats (Chesson and Warner 1981)
• The results:
• If one species is removed, the other species increases in abundance as they are competing for vacant space.
• Coral colonies limit the colonisation of both goby species and therefore they exist due to the competitive lottery in which vacant habitat is colonised by the recruit which is first available in that area.
• Due to their high reliance on coral, and the impending increasing in SST which is also resulting in mass coral bleaching, the 2 goby species are very susceptible to population decline if there is not enough coral for both species to survive.
• The probability of extinction for these species will probably be a lottery too, as it will depend who is the superior competitor at the specific time.