Tropical Diversity

Question 1

What are the different hypotheses for latitudinal patterns?

Answer 1

• Temperature hypothesis: temperature increases metabolism which promotes speciation or relaxed temperature constraints in warm waters, especially ectotherms.
  
  • Seals, for example, do not show this pattern as they are endotherms.
  • Some groups of species such as fishes and corals are more abundant in the tropics
• Productivity-richness hypothesis: primary productivity facilitates population size, averts extinction and/or supports niche specialisation
  
  • In a productive system it will be more likely that you will have big populations present. In big populations it will be less likely populations will go extinct.
• Correlations also with coastline length, evidence that habitat area is important also.
• These are just correlations are don’t address the actual causation. Works for some groups, and not others.

Question 2

Tony paper of distrbution of species diversity latidtudionally

Answer 2

Chaudhary C et al. 2016 Trends in Ecology & Evolution 31: 670-676

• Look at a number of published studies and look to see whether, across all the groups of marine organisms what the overall pattern is.
• Of 18 global marine species studies, 9 show one or more taxa with unimodal (single peak around equator) distributions, 9 not unimodal, 2 bimodal (Ocean Biogeographic Information system, OBIS)
• Fewer species around the equator (grey boxes), more at higher latitudes (black boxes)
• Most species evolved through adaptation to temperature at the edges of the tropics?

Question 3

What is the reason for this large number of species within the Indo Pacific region? They come up with 4 reasons - give 2

Tony paper

Answer 3

1. Centre of Origin Hypothesis
2. Centre of Overlap Hypothesis

Mora C et al. 2003. Nature 421: 933-936

Question 4

Centre of Origin Hypothesis : IPR a major centre of speciation from which species disperse outwards

Answer 4

Mora C et al. 2003. Nature 421: 933-936

• Centre of Origin Hypothesis : IPR a major centre of speciation from which species disperse outwards
• Within that region something species happens which means that it becomes the centre of origin of species. Therefore, we should find species that have originated within that region and dispersed east and west.
• If you look at varying sites with different numbers of species across the Indo Pacific as a whole, a very high proportion of those species can be attributed to those Indo Philippine species, the idea of a centre of origin and species spreading out seems to work.
• The number of species goes down, the further away you travel from the IPR. Mean pelagic larval duration also varies spatially, further away from the equator has a longer larval duration. You can conclude a strong negative relationship between the mean pelagic larval duration and the log of the number of species by combining these two sets of data.
• Highest endemism in the IPR: supports Centre of Origin Hypothesis, but significant endemism hotspots also outside the IPR: not just one Centre of Origin?

Question 5

Centre of Overlap Hypothesis: IPR high diversity lies in overlap of distributions from various biogeographic provinces where speciation occurs, dispersal in all directions

Answer 5

Mora C et al. 2003. Nature 421: 933-936

• Not the region itself which is acting a source for new species, but instead the region itself is doing something that means species arise on either side of that region, and then subsequently mixing back into that region.
• Examples of different reef based species with strikingly similar colouration patterns. (butterfly fish, rabbit fish, eels, parrot fish, wrasse)
• One sister species occurs on the pacific side and one sister species occurs on the Indian ocean side, suggests there is something going on aside from the IPR being the centre of species as a whole. The IPR may both act as a physical barrier and a source of species. Periods would have occurred when due to changing sea levels or tectonic movement the IPR would have been more of a barrier separating the two ocean basins. Dark grey areas represent overlap of the two species.
• The east Indian ocean also has areas of upwelling and not much reef, so would not provide the right habitat for larvae moving through - adding to the barrier.

Question 6

Describe Parapatric speciation

Answer 6

In contrast to these ideas, the notion that things might have to be geographically separated is not a universal process. This idea is parapatric speciation.

• Parapatric ecological speciation where natural selection in alternative environments within locations overrides gene flow
• Possibility increased by
• populations being more self-recruiting than expected
• active habitat choice by larvae (salmon go back to rivers in which they were born)
• evidence of reduced gene flow over short distances (for whatever reason)
• With parapatry
• evolutionary differences should align with habitat distributions
• contact zones between different groups should coincide with mixed environmental conditions

Question 7

What Tony paper describes parapatric speciation?

Answer 7

Rocha LA et al. 2004 Proceedings of the Royal Society B 272: 573-579

• Molecular study of 5 wrasse species (Halichoeres spp.) in W Atlantic with similar biogeographies, currents, PLDs
• Four factors routinely used to explain genetic structure in marine species:
  
  • Biogeographic barriers
  • Ocean currents
  • Ocean distance
  • Pelagic larval duration (PLD)
• Only one of the 5 species showed genetic differences varying with geographical distance
• Genetic variation explained in part by habitat specialisation – some generalists, others reef specialists
• Strongest genetic differences within biogeographic provinces not between (e.g. oceanic and Caribbean sites separated by 2000-4000 km similar in two species, but differences between sites separated by only 300 km)
• Thus distance does not consistently explain genetic variation among populations

Question 8

WIDER READING: Global patterns and predictors of marine biodiversity

Answer 8

Global patterns and predictors of marine biodiversity across taxa, D. Tittensor et al, 2010.

The study:

To examine global patterns and predictors of species richness across 13 main species groups.

• Coastal and oceanic species were compared.
• The results:
• Coastal species showed maximum diversity in the Western Pacific. Pinnipeds were an exception here as they are only found at higher latitudes, eg in the Arctic, therefore have low tropical diversity.
• Oceanic species consistently had high diversity across mid-latitudinal bands in all oceans and showed mostly pantropical distribution.
• Sea surface temperature was the only environmental factor related to diversity. This is because higher metabolic rates promote diversity.
• Species richness was driven by fishes.
• Hotspots were determined to be coastal areas around Japan, China, India, South Africa, and the Caribbean amongst other places, which coastal fish species being found in especially high densities in Southeast Asia.
• Diversity also increased with coastline length, which indicates that habitat complexity and habitat area increases diversity, due to a higher number of niches.
• Concludes by saying that stopping anthropogenic activity and trying to reduce rising sea surface temperatures are vital in maintaining a high level of biodiversity in the future.

Question 9

WIDER READING: Repeat patterns and processes in reef fish diversity

Answer 9

Patterns and processes in reef fish diversity, C. Mora et al, 2003.

The study:

• Explores ideas behind speciation and dispersal of reef fish in the Indo-Pacific.
• Looks at how a centre of origin can cause large scale gradients in species richness and how dispersal from these origins can affect structure of communities.
• The results:
• The Indonesian and Philippine region is the centre of high diversity, this is referred to as the IPR.
• Past this point, species richness decreases along both latitude and longitude.
• There are 3 hypotheses to explain the patterns of species richness past the IPR:
  
  • Centre-of-origin hypotheses –
    
    • the most supported hypothesis.
    • Species disperse from the IPR.
    • The IPR is home to many endemic species.
    • The IPR is a highly connected area so dispersal of larvae to surrounding areas is facilitated.
    • Allopatric speciation
  • Centre-of-overlap hypotheses
    
    • IPR is very diverse due to many different areas overlapping. These species disperse from all directions
  • Centre-of-accumulation hypotheses
    
    • Speciation occurs to areas surrounding the IPR and these species disperse in the currents to the IPR. These species disperse from one direction towards the IPR.
• As you move away from the IPR the species richness declines (for fish families Labridae and Pomacentridae) which suggests there are limitations in the dispersal mechanisms of the fish.
• PLD determines how far from the IPR larvae can disperse which in turn determines the overall composition of local communities outside of the IPR.
• 86% of fish species which characterised reefs in the Indian and the Pacific Ocean were also found in the IPR, which supports the centre-of-origin hypothesis.

Question 10

WIDER READING: High connectivity among habitats

Answer 10

High connectivity among habitats precludes the relationship between dispersal and range size in tropical reef fishes, C. Mora et al, 2011.

The study:

• Collected data on PLD, evolutionary ages and range sizes of seven tropical fish families.
• Explores the effects that PLD and evolutionary age have on range size.
• Explored why PLD and dispersal has little effect on range size by quantifying connectivity between several tropical reefs by using the global ocean circulation model.
• Looked at seven families of tropical fish (Pomacentridae, Labridae, Pomacanthidae, Chaetodontidae, Anguilidae, Serranidae and Sparidae)
• The results:
• When evolutionary age was controlled, PLD had very little effect on range size.
• If PLD is higher, then the species can disperse over a larger distance and therefore increase its range size. (this is correct is the larvae is not affected by other abiotic factors such as sharp changes in temperatures or currents)
• older species ought to have had more time to disperse and if dispersal influences range size then older species would be expected to have larger ranges than younger species
• Only Pomacentridae showed a weak significant relationship between range size and PLD but in the other 6 families the relationship was statistically non-significant.
• Travel times between reefs in tropical oceans are from 30-100d and the average PLD of tropical fish species is 42.6d.
• Shallow reef habitats are variable in isolation but tend to be within reach of most tropical fish species PLD’s.
• The quick colonisation of lion fish species Pterois voltians (which has a PLD of between 20-40d) throughout the coasts of Southeast America, Bahamas and other Caribbean islands focuses on the fact that isolation of a habitat is not important in a quick colonisation of a new species.
• Habitat isolation plays a minor role in reducing the geographical expansion of reef fishes through dispersal.