What is macroecology?
Field of ecology that deals with the study of relationships between organisms and their environment at large spatial scales to characterise and explain statistical patterns of abundance, distribution and diversity. Species area curves and latitudinal gradietns
What is a species area curve?
As the area studied increases, the number of species recorded increases. The increase isn't linear. Follows the relationship:
S = cAZ -> Log S = Log C + z Log A.
Z is the slope and Log C is the intercept of the straight line.
"one of ecology's few genuine laws"
What are the explanations for the species area curve?
- Habitat diversity hypothesis
- Passive sampling hypothesis
- Equilibrium model of island biogeography
What is the habitat diversity hypothesis?
If there is a wider range of habitat types, a wider range of species can live in that area. Predicts that larger areas will have more habitats and that species diversity will increase with habitat diversity.
Support for this hypothesis where area and habitat diversity can be separated.
Often, but not always explains the pattern.
What is the passive sampling hypothesis?
Species have a random probability of colonising an area, and as area increases, the probability of more species increases.
Fails to explain why small islands have few species, as the richness should grow over time.
What is the equilibrium model of island biogeography?
The number of species on an island could be predicted as the rate of colonisation and extinction of the island.
Ignore rates of speciation. The equilibrium number of species depends on area and isolation.
Immigration rates: when there are more species on the island, the chance of a species colonising is lower (competition) and it might already be there.
Extinction rates: when there are more species, competition leads to increased extinction due to finite resources.
Smaller islands are more prone to extinction. Islands closer to mainland will have higher immigration rates.
More realistic models have curved immigration and extinction functions.
Predictions of the equilibrium model of island biogeography?
- There should be substantial turnover in species composition, but species number should be relatively constant.
- All else being equal, the number of species present on an island should decrease with increasing isolation.
How do mainlands differ to islands?
They are self contained regions where species originate entirely by speciation within the region. This is in contrast to islands where immigration is very important.
The slope of the species area curve (z) is lower as on the mainland you can have species present as "sink populations". On an island, these wouldn't exist as if the island isn't a suitable habitat, they would go extinct. This means that new species are encountered at a lower rate on the mainland as you include their home land (already been sampled so doesn't increase the richness of the area).
How can species area curve be used for conservation?
Predict how many extinctions will occur following area reduction and suggest the best ways of slowing the loss.
Assumes that species richness is the primary object of conservation.
Facts about tropical deforestation?
Rainforest loses 1.8% a year, 50% will be gone by 2022.
10-22% of all rainforest species are predicted to go extinct within this population.
Is it better to have a single large area for species or several smaller areas?
Depends on the degree to which the species overlap between areas. Sometimes, many small islands can collectively have more species than an equivalent large area.
Where are the greatest threats to biodiversity?
Areas of high diversity, in tropical habitats
Biodiversity in the tropics?
Rainforests - cover 7% of the Earth's surface and account for over half of the Earth's biodiversity.
Coral reefs - cover 1% of the Earth's surface and account for 25% of all marine fish species.
Why do you need to adjust for area when sampling latitudinally?
The number of species recorded depends on the geographical area surveyed. Sampling areas based on lines of latitude and longitude decrease towards the poles.
What biotic explanations exist for the longitudinal gradient in species richness?
- Productivity/Species Energy Hypothesis
- Competition Hypothesis
What abiotic explanations exist for the longitudinal gradient in species richness?
- Time and Stability
- Effective evolutionary time
What null models exist for the longitudinal gradient in species richness?
What is the productivity energy hypothesis?
The hottest, most humid places have the greatest NPP. This therefore can support more species.
However, this doesn't necessarily lead to high biomass or abundance.
Tropical seas have a low NPP but high richness.
Eutrophic lakes have a high NPP but low richness.
What is the competition hypothesis?
There is a constant climate in the tropics, meaning that most species grow towards their carrying capacity, so there is intense competition.
Can lead to narrowing of niche breath, and closer species packing so that more species can coexist.
Difficult to test.
What is the Connell/Janzen hypothesis?
Predator mediated coexistence. More intense mortality from natural enemies in the tropics due to stable climate.
Population sizes of their prey are kept low, reducing competition. Allows overlap in resource use and prevents any one species from becoming locally dominant.
Seedling mortality is strongly density dependent within species due to pathogens or herbivores, enhancing diversity.
What is the time and stability hypothesis?
Communities diversify with time and so temperate regions have younger communities due to glaciation. Whereas the tropics have remained relatively unchanged for at least 150 x106 years. Also stable in the sense of low seasonality.
Outside of the tropics there are empty niches, and no time for species to reinvade (ecological time) or evolve to fill them (evolutionary time).
Many exceptions where stable habitats have low diversity (tropical mountain tops) and many tropical environments have seasonally fluctuating environments.
What is the effective evolutionary time?
Faster speciation in the tropics AND a longer time for evolution to occur.
Higher rates of mutation, shorter generation times and more intense selection. Little direct evidence on the speed of evolution.
Problems with latitudinal gradient hypotheses?
Make qualitative not quantitative predictions.
Sample size is small (only 1!).
What is the area null model for latitudinal gradient hypothesis?
Tropics cover a greater area than any other major ecological zone. The longitude peaks at the equator.
If the tropics have higher speciation rates and lower extinction rates due to their large area, they will have a high diversity.
If diversity were equal in tropics and subtropics, the average tropical species would have a larger geographical area. Lower extinction rates due to larger population and refuges from environmental change.
Geographical barriers in tropics allow allopatric speciation.
More chance of a rare mutation leading to more rapid evolution.
Area alone cannot explain all aspects of the gradient.
What is the mid domain model for latitudinal gradient hypothesis?
Boundedness of the Earth results in a latitudinal gradient. There are Northern and Southern boundaries to where species can exist.
If you place species ranges at random on the Earth within these boundaries, you inevitably get more species towards the tropics.
Different assumptions don’t get rid of pattern, just affect quantitative results. Makes quantitative results which are easier to test.
Fits data well but not perfectly so not the sole explanation.