Patterns of Diversity Flashcards
What four conceptual concepts are included in a measure of diversity?
- Number of entities or compositional diversity – species richness
- Categorising how many different things there are.
- Distribution of abundances or structural diversity – evenness
- Degree to which entities differ - divergence (molecular)/disparity (morphological)
- how they are built / what they look like
- Functional role entities play in ecosystems – trophic, metabolic, habitat-forming
- If you lose a species, and there are lots of other things forming the same role, does it matter for the functioning of the ecosystem?
In principle, all four components can be quantified. Functional role - database of organisms traits - fecundity / how often do they intriduce
The richness of any region is a consequence of two factors;
- The richness of the smaller areas that compose it
- a rocky shore (richness of gullies, under seaweed, on seaweed, sheer rock ect)
- Turnover of species composition among them
- How much does the species change from one area to another? Two areas may have the same richness yet totally different fauna.
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- How much does the species change from one area to another? Two areas may have the same richness yet totally different fauna.
Dependent on their spatial scale a variety of terminology has been used to describe richness but it is not consistent.
Diversity can be characterised at various focal scales including;
- Point diversity – richness of a subset of a community
- α diversity – richness within the full extent of a single community
- we are most familiar with
- γ diversity – richness of a landscape comprising >1 community
- ε diversity – richness of a broad geographic area, >1 landscape
Remember though these are only arbitrary distinctions along a continuum of possible focal scales
’ I want you to understand that alpha diversity is basically our species richness’
3 additional measures of diversity relate to turnover among focal units
Local β diversity – change in species composition among subsets of a community
β diversity – change in species composition among different communities in a landscape or along a gradient
δ diversity – change in species composition along a climate gradient (big) or among geographic areas
- Latitude and depth are the two largest environmental gradients on the planet that encompass multiple communities and reflect climatic and environmental characteristics
- But again as these terms are somewhat arbitrary in a biogeographic context you will see β diversity adopted as a popular term for turnover among focal units within the communities
Latitudinal diversity patterns have been examined across taxa but with a disproportionate analysis on aquatic invertebrates, birds and mammals
What is the general agreement?
The majority of published analyses (>70%) corroborate a pattern whereby the latitudinal gradient is one where species richness increases towards the tropics
This association has been described using a range of diversity metrics from point to gamma
And also for a range of organismal groups including coral (Harriot & Banks 2002, Coral Reefs 21: 83-94), molluscs (Rex et al. 1993, Nature 365: 636-639), decapods (Steele, 1998, Int. Rev. Hydrobiol. 73: 235-46) and fishes (Macpherson, 2002, Proc. R. Soc. B 269: 1715-20)
key papers ‘if you want to have a look’
Latitudinal trends – the classical pattern for marine
- Fish species - not restricted to one particular area
- In fish species richness has been shown to increase towards the equator in the Atlantic (Angel, 1993, Conserv. Biol. 7: 760-772), Pacific (Stevens, 1996, J. Biogeogr. 23: 149-154) and the Sea of Japan (Kafanov, 2000, J. Biogeogr. 27: 915-933)
- Significant latitudinal gradients for taxonomic subsets of fish fauna i.e. teleosts and elasmobranchs (Macpherson & Duarte, 1994, Ecography, 17: 242-248)
- Well established for molluscs (Crame, 2002, Paleobiology, 28: 184-207) - account for significant variation in number of species, genera and families as well as number of species within specific functional groups i.e.
- Benthic marine gastropods and bivalves form the eastern Pacific (Roy et al., 1996, Philos. Trans. R. Soc. B, 35: 1605-1613; Roy et al., 2000, Proc. R. Soc. B, 267: 293-299), predatory gastropods on the eastern Atlantic shelf (Taylor & Taylor, J. Biogeogr. 4: 73-81), prosobranch gastropods from western Atlantic and eastern Pacific (Roy et al., 1998, PNAS, 95: 3699-3702)
Why do you see slightly higher diversity (on the humpback diversity distribution against latitude) in the antarctic than the arctic
Age of the habitats
Antarctica has been isolated as a continent for longer periods of geological time
Arctic is younger
Out of the 30 hypotheses for the cause of latitudinal trends in diversity, which four does ben want us to focus on?
These hypotheses have the most support and potential and are the least easy to refute based on current data
- Geographic area hypothesis – originated by Terborgh (1973), developed an amplified by Rosenzweig (1995)
- Productivity hypothesis – idea dating back to 1959. Wright (1983) advanced the species-energy hypothesis as a general extension of the species-area theory of MacArthur & Wilson (1963; 1967)
- Rapoport-Rescue hypothesis – based on an extension of Rapoport’s Rule where size of distributional ranges is inversely related to latitude
- Evolutionary speed hypothesis – proposed by Rhode (1992)
Geographic area
- The tropics support more species than other regions as they comprise more area – coupled with greater levels of productivity in tropical regions and zonal ‘bleeding’ in the tropics - the idea of getting overspill from areas just outside the tropics.
- Spherical planet – hemispherical separation of zones
- Species richness increases with the area – as area increases so do number of habitats, biomes and biogeographic provinces within it
- Decreased likelihood of extinctions – more individuals and populations - because of the regions above
- Increased likelihood of barriers to gene flow – enhance speciation rates. Thermal barriers, lots of different habitats separating, fringing reefs - possibly more barriers at lower latitudes
- More diverse habitats – development of specialisation, adaptation etc
- Examples against – Eurasian freshwater fishes (not more diverse in bigger lakes) and deep oceans (big, not so diverse
Productivity Hypothesis
- Annual input of solar radiation determines energy availability, productivity, and biomass - inverse relationship to latitude
- Not been widely accepted as an important cause of latitudinal patterns
- It fails to elucidate on mechanism as to why species richness would increase to a maximum set by energy availability as opposed to population densities simply increasing in magnitude
- Highly productive environments often exhibit great species richness but can also exhibit low richness in some situations
- There is very little generality about the form of the relationship between species richness and productivity (positive, negative, modal)
- Only agreement is that it is very scale dependant
Latitudinal trends – Rapoport-Rescue hypothesis
- Several taxa exhibit the pattern associated with Rapoport’s rule, including fish, amphipods and molluscs
- You have more seasonal variation at higher latitudes, more variability in your environment.
- This creates organisms with broad temperature tolerances.
- A broad tolerance results in large ranges.
- Animals with larger ranges are more likely to have competitive interactions, spread, outcompete for resources
- Narrower tolerances of tropical organisms result in the environment being more heterogeneous (from their perspective) and they disperse or spillover into unfavourable areas
- essentially saying that the high species richness across the tropics is augmented in unfavourable areas by the addition of ‘rescued’ ‘accidentals’, with spillover from their desired range into slightly unfavourable
- Less substantiated in tropics than temperate zones (Gaston et al., 1998, Trends Ecol. Evol. 13: 70-74) and more recent empirical and modelling evidence suggests underlying logic is flawed or only applicable under restrictive circumstances
- the first part of the argument holds really true for temperate ranges
- not for topics as much
- being suggested that it fits more for some taxa
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Latitudinal trends – evolutionary speed hypothesis
Species richness increases toward the tropics because of temperature-induced increases in rates of speciation. Animals are doing things faster, have a greater metabolic rate when temperatures are warmer.
- Suggests proliferation of animals with shorter generation times, higher mutation rates and accelerated selection in the tropics this enhances species richness through speciation
- Number of generations is negatively related to latitude in some taxa (arctic animals have very long generation times – but short generation times ≠ faster evolutionary rates
- Little data evidence to support or refute this hypothesis
- Suggested that the tropics have been a source of evolutionary novelty throughout geologic time – higher origination rates of post-Paleozoic marine orders in tropics compared to temperate zones
Latitudinal patterns in the deep-sea - what did they study
Case study – Rex et al. (1993). Global-scale latitudinal patterns of species diversity in the deep-sea benthos. Nature, 365: 636-639
- ‘in somewhere as isolated and homogeneous as the deep ocean would we still expect to see the same trends in diversity and latitude’
- This paper examined 97 epibenthic sled samples collected between 37oS and 77oN
- Latitudinal gradients unexpected
- Deep-sea bivalves, gastropods and isopods show clear latitudinal diversity gradients in the North Atlantic and interregional variation in South Atlantic
- Bivalves are often used, as you can get to species level just from the shell, however, the shell may not have originated from that space. (but give a good example of a large dataset for these trends.
- Many, often incompatible, mechanisms have been proposed to explain deep-sea diversity
- Regular global patterns imply that these mechanisms must operate at different spatial scales
- 97 epibenthic sled samples collected between 37oS and 77oN
Latitudinal patterns in the deep-sea - what did they find?
Case study – Rex et al. (1993). Global-scale latitudinal patterns of species diversity in the deep-sea benthos. Nature, 365: 636-639
- Much of the variation reflects species turnover and changes in diversity in the bathyal zone
- Attributing changes in diversity to changes in species turnover
- Controlled for variation in-depth – relationships between diversity and latitude still highly significant (can be confident it is a real result)
- Patterns of diversity inferred from individual samples of deep-sea communities normalised to correct for variation in sample size
- Normalised expected number of species E(Sn) is extensively used
- Affected by species number and relative abundance but sensitive to rare species. Diversity is low but evenness is very skewed.
Zonation and depth related patterns
What are some of the reasons for zonation?
- Physical factors - pressure or temperature-related thing.
- Local habitat factors, sediment condition, changes in the tropic strategy. You tend to see deposit feeders dominating when you get deeper.
- Changes in trophic strategy along a gradient of food energy
- Biological interactions – enhanced competition for resources
- Also got to think about biological interactions - where you get competition for resources. Which can act to boost species richness by animals becoming so well adapted to make use of different components of a resource so they are not in competition with each other? This is one of the reasons that you get elevated diversity of sea cucumbers in the very deep ocean.
All of these things are going to operate a different evolutionary and ecological timescales.
Zonation and depth-related patterns
What is a good example of zonation causes in the deep sea?
A big dataset by carrier Howell, in the North Atlantic, collected in 2002.
Extensive dataset study to look at changes in beta diversity.
Looking at variability in ranges of individual species. Some species have very large ranges, whereas others have very narrow restrictive ranges.
Very few species are truly deep sea - so you see big compositional changes in their community.
interestingly there is a similar humpback distribution with depth as for latitude, with a peak in diversity for the mid bathyal slope (1700m)
The anomaly of HIGH species richness
When was this high species richness shown?
- Hessler & Sanders (1967); Sanders & Hessler, (1969) showed that previous trends of decreasing diversity with depth were a result of sampling bias and low densities of organisms
- If you just sample the first bit of the curve it will only show part of the humpback distribution.
- They then produced these rarefaction curves (Jumars & Hessler, 1976)
- These are a method of estimating the number of species based on the number of individuals you collect, and how that varies between habitats.
- They have a range of data from ocean basins and have a relationship between the number of individuals and habitat area.
- Found little similarity in species richness between ocean basins
- Wide differences even between geographically close areas
- Reduced diversity at Santa Catalina basin – low O2 conditions (oxygen minimum zone).
- have depressed diversity even with high individuals., and that it is close to the San Diego trough
- Fewer species in trenches – catastrophic effects of slumps and slides, food availability, geographically isolation
