Principles of Ecology

Week 11: Biodiversity patterns

Latitudinal diversity gradient

• Motivated a huge body of work in ecology and evolution
• Lots of hypotheses to explain this pattern
  • Climate
  • Age of landmasses
  • Amount of area conducive to biological activity
  • Habitat heterogeneity
  • …

As we often do, let’s simplify the complexity

• What about biodiversity patterns at smaller spatial scales?

• Start at the simplest level: an isolated area

  • i.e. an island that recently formed
  • what determines how many species live on it in the long term?

Processes that govern species richness:

• \(\uparrow\) by immigration of new species

• \(\downarrow\) by local extinction (‘extirpation’) of existing species

• \(\uparrow\) by local speciation

• Let’s assume speciation is very slow and not relevant to our dynamics

Processes that govern species richness:

• \(\uparrow\) by immigration of new species
• \(\downarrow\) by local extinction (‘extirpation’) of existing species

• What determines rate of immigration of new species and local extinction?

Rates of immigration

What determines rate of immigration?

• Proximity to source (“mainland”)
  • When an island is closer to the source, more new species can end up there
• Number of species already on the island
  • If all the species from the mainland are already present on the island, then nothing new can immigrate in

Rates of local extinction

What determines rate of local extinction?

• Size of island
  • On smaller islands, species are more likely to go extinct just by randomness (“stochastic” extinction of smaller populations)
• Number of species already on the island
  • If there are lots of species on the island, that means the number of possible extinctions is higher

Putting the two together

Actual islands are not the only “islands” out there

This week’s activity

Using island biogeography to interpret patterns of soil bacterial and fungal biodiversity on islands

Island Biogeography, Day 2

Logistics

Next week:

• Formal proposal for Semester Project due on Sunday Nov 12th
• No Weekly Activity next week so that you can focus on the proposal
  • Following week’s activity will be worth 10 points
  • Review of the ideas we covered over the past ~5 weeks (competition, consumer-resource dynamics, diversity measures, island biogeography etc.)

Next Monday

Dept. of Biological Sciences seminar speaker, Dr. Orou Gaoue

1-2pm in LSA 101

“Dr. Gaoue’s lab uses mathematical models, field observations, experiments and ethnobotanical methods to study the drivers and conservation implications of plant-human interactions in a changing world”

Which island should have more species richness at equilibrium?

Which island should have more species richness at equilibrium?

Which island should have more species richness at equilibrium?

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

On the following graph, draw 2 lines: one for a big island, one for a small island (assuming both islands are equally far away from the mainland)

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

On the following graph, draw 2 lines: one for a near island, one for a far island (assuming both islands are of the same size)

Putting the two together

Extending island bioegeoraphy to conservation

The island biogeogaphy concept might well have languished in the cabinet of academic curiosities had not conservation biologists realized a decade or so ago that human activities, by fragmenting natural habitats, were creating island from previously continuous populations.

Some insights extend relatively cleanly

• Bigger ‘islands’ (like big national parks) are more likely to support a diversity of organisms than smaller ones

• Conservation areas that are near large “sources” are likely to be more diverse than isolated conservation areas

But of course, complications arise when we want to generate insights for conservation

• If you know that you can set aside 500 square km for conservation, should you…
  • Have one big conservation area (a big ‘island’)?
  • Or, have 10 smaller conservation areas of 50 square km each?
  • Or, have 100 tiny conservation areas?
• “SLOSS” (Single Large Or Several Small) debate - all the rage in the 1970s-80s in conservation

Team Single Large

• Bigger conservation areas ensure lower extinction rates, because each species can grow to larger population sizes (rather than a bunch of small populations, each of which might blip out)

Team Several Small

• One big conservation area might miss some habitat heterogeneity, and only select for a small group of species
• Even if a species goes locally extinct (‘extirpated’) from one site, it can immigrate in from another area

Team Single Large

• Some species, especially large mammals and many trees need large areas to survive – e.g. for a wolf pack to have enough food, there needs to be enough prey - so a small patch might be useless

and so on, and so on…

Eventually, conservation has become a very ‘local’ problem:

• What is possible to conserve in any one locality?

• What are the key species, and what is their population distribution?

• How can we maximize total amount of land and hetereogenity of habitat?

• What does a “large” conservation area even mean? How do roads change the landscape?

• Almost as important as total area, is the connectivity between islands.