Does Size Really Matter?

How many different species do you think are within 1 square foot of you right now?  How about 10 square feet? Or a square mile?

Now how do you think that would change if you were standing in the middle of nowhere with nothing close by?

The answers to these questions have often been explained by 2 well known theories in biology:

  1. The species-area curve, where the number of species increases with area size
  1. The insular biogeography theory (aka the island biogeography theory), where the idea is that less species arrive at more isolated areas

 

Screen Shot 2017-04-10 at 10.24.30 PM

Basic idea behind the island biogeography theory (less arrows, or ‘species’, reach the farther island).  Picture from https://en.wikipedia.org/wiki/Insular_biogeography

Sometimes these theories aren’t completely true in all ecosystems though so I was interested to see if they were for Acadia Beach, Vancouver.

Since beaches are constantly being disturbed by changing tides and large weather events, the animals that live there have to be able to return to rocks quickly.  Does the size and remoteness of a rock change the number and speed of animals returning to rocks after a disturbance?

This led to 2 questions I decided to investigate:

  • Will there be consistently more invertebrate species on bigger rocks?
  • Will rocks that are farther away from other rocks (more isolated) have less invertebrates return to them after being cleared?

To answer these questions, I counted the species richness (# of species) on the bottoms of rocks of varying sizes (measured by surface area).  I then cleared the rocks (scraped off all the animals on the bottom) and moved 20 to an empty, “isolated” zone on the beach, while leaving 20 rocks in the same spot I found them.

Screen Shot 2017-04-10 at 2.03.26 PM

Rocks before and after being cleared

Screen Shot 2017-04-10 at 2.28.28 PM

“isolated zone” circled in red

1 day later, and again 1 week later, I came back and counted species richness on the moved and unmoved rocks.

The most common things I came across were barnacles and mussels (on the first day only, since they take a while to come back), crabs, isopods and amphipods.

Here are the take-aways I got after analyzing my data

  • The size of a rock (ie the surface area) did affect the species richness. More species were found on larger rocks (about 1 extra species for every 250cm2 of rock).
  • A week after rocks were cleared, species richness increased on non-moved rocks but returned to the same value on moved/isolated rocks… Maybe the isolation made it so there were less species that went back to the moved rocks than the non-moved rocks, but I can’t be sure from these results.

 

Overall I found that size does matter (sorry guys), but the effects of isolation weren’t as clear.  These are just the results for 1 specific ecosystem, how do you think the effects of area size and isolation would change in an ecosystem near your home?  If you’re curious, try doing a little science experiment like mine and discover something new about the place you live!

 

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