Changes of the Main Vernal Pool on the Mesa de Colorado Vs. Time

This page summarizes how the depth of the Main Pool changes with time and how the number of various Pool inhabitants change with time. See Introduction to the Main Vernal Pool for an overview of the pond inhabitants discussed here.

In all the plots below, individual years are color-coded. Individual observations are not connected with lines, since the data between observations may not be well fit by a straight line.

The data are parameterized by the number of days from the existence of the Pool. Thus day 1 is the first full day that the Pool exists.

The densities of the pond inhabitants are given in number in the water column per square foot of surface area. That is, take a square one foot on each side on the water surface, and count the number of inhabitants within those boundaries from the surface to the bottom of the pool. In general, the numbers given below are the maximum densities observed along the boardwalk.

Depth of the Main Pool
Fairy Shrimp
Gray Shrimp
Red Shrimp

Depth of the Main Pool Vs. Time

The depth of the Main Pool depends strongly on the amount of rainfall vs. time. Forming the pool in the first place requires a certain amount of rain in the last one and two months. If no rain fell after the formation of the Pool, the Pool would evaporate at the rate of ~5-6"/month. This rate is influenced by both transpiration from plants growing in the pool as well as evaporation.

plot of pool depth vs. time since 1996

The shortest lifetime of ~90 days was in 1999-2000, when it essentially rained only in February. The longest lifetime of ~180 days was in 1997-1998, a year of heavy prolonged rain. This extreme variation of a factor of two in pond lifetime allows us to see how pond inhabitants respond to the lifetime of the Pool.

Number of Fairy Shrimp Vs. Time

The number of fairy shrimp has varied significantly with year and time:

plot of the number of fairy shrimp vs. time since 1996

See also plot with expanded scale to see the points representing the lowest non-zero observed densities.

The largest number of fairy shrimp were observed in 1999-2000, after the absence of a Pool in 1998-1999, and they were also observed over the longest period of ~40 days. The first fairy shrimp was observed on day 10 of that year and the last one at around day 50. In contrast, the numbers were significantly lower in the other two years, with their densities fluctuating significantly.

In 2000-2001, the jump in fairy shrimp density at day 42 was after the die-off of the largest shrimp, and the existence of one spot along the boardwalk where the smaller shrimp began to congregate to mate. It probably is not associated with any real increase in the density of the smaller shrimp.

The shrimp must reach ~1/20" in length to first become visible on around day 5. At that point, they look just like little dots moving in the water, and can't be identified from body features or their habit of swimming upside-down. When their length reaches ~1/10" one can see their appendages beating at high speed to propel them upside-down.

The maximum observed length of the fairy shrimp increases approximately linear with time, until the largest species dies out:

plot of the maximum length of fairy shrimp vs. time since 1996

The minimum observed length of the fairy shrimp was always about half the maximum observed length, until the maximum observed length reached 1.0". After that time, the minimum observed length was always 0.5" when the shrimp were present. This is due to the presence of two species of fairy shrimp, one of maximum length 1.0" which completely dies out before the other species, which has a maximum length of 0.5".

When the fairy shrimp become sexually mature, one can see the egg sac for the females and the gonads for the male. This plot shows the percentage of the population of the largest shrimp which are mature:

plot of the % of fairy shrimp that are sexually mature vs. time

At most, about 10% of the entire fairy shrimp population are observed to have eggs in an egg case at any given time. Presumably this means that about 20% of the female fairy shrimp population has eggs at any given time.

Number of Gray Shrimp Vs. Time

The number of gray shrimp has also varied significantly with year and time:

plot of the number of gray shrimp vs. time since 1996

Again, the largest number were observed in 1999-2000. The distribution shows sharp peaks in the number anywhere from days 30-50, with a constant low number observed over a period of about two months.

Number of Red Shrimp Vs. Time

plot of the number of red shrimp vs. time since 1996

Again, the largest number were observed in 1999-2000. Red shrimp are usually the most numerous of the post inhabitants, with a total of eight observations having densities over 25 per square foot compared to four each for fairy and gray shrimp. They can generally be observed until the Pool is nearly dried up.

Number of Springtails Vs. Time

Springtails are the first of the pond's inhabitants to appear, and have the largest variation with time:

plot of the number of springtails vs. time since 1996

It is curious that there seem to be two peaks in the population. At the time of the second peak in 2000-2001, it was clearly due to another generation, since their size was very small at the beginning of that peak.

Also see plot with an expanded scale.

Number and Length of Tadpoles Vs. Time

I haven't made as many observations of tadpoles as for the other inhabitants prior to 1999, so it is more difficult to draw conclusions about their behavior with time. They were observed over a period of about a month in 1999-2000, and extensively from 2000-2001 onward.

Maximum number of tadpoles per square foot:

plot of the number of tadpoles vs. time since 1996

The number of the tadpoles fluctuates heavily, due to the balance between predation by the two-striped garter snakes and continued hatching of eggs. The predation effects can be seen clearly on the next plot.

Length of the main body of the largest tadpoles (excludes the tail):

plot of the maximum length of tadpoles vs. time since 1996

The larger tadpoles are easier to find and be eaten, and it looks like the entire first generation of eggs got eaten in 2000-2001, causing the observed decrease in maximum size. Note that this measurement is highly uncertain after the tadpoles reach 0.5" in length, since only the most skittish survive to be that long, which makes the length measurement quite difficult in situ.

The tadpoles began to develop rear legs at about Day 72 in 2000-2001.

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Copyright © 2001 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 29 April 2001.