COLORADO WESTERN TIER WILDFLOWERS

 Jim Fuchs

Plant Spreadsheet

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Sclerocactus (Cactaceae)

There are two species of Sclerocactus that are commonly encountered on the western slope of Colorado: S. glaucus and S. parviflorus. They look very much alike and are best distinguished by DNA analysis. Supposedly S. glaucus has a straight central spine while S. parviflorus has a hooked central spine. I commonly find varying degrees of central spine curvature on both species. Supposedly there are 1-3 central spines on S. glaucus and 4-6 on S. parviflorus. I am not sure how consistant the number of spines are. So, with that being said, if you don't agree with my photos I encourage you to get some DNA studies to clarify things!

Sclerocactus glaucus

Sclerocactus

Sclerocactus glaucus

Sclerocactus flower

Sclerocactus glaucus

Sclerocactus flower

Sclerocactus glaucus

Sclerocactus

Sclerocactus glaucus

Sclerocactus

Sclerocactus glaucus

Sclerocactus

They can hide quite well

Haltictidae pollinator

Family Haltictidae pollinator

Sclerocactus parviflorus

Sclerocactus

Sclerocactus parviflorus

Sclerocactus

Sclerocactus parviflorus

Sclerocactus

Sclerocactus parviflorus

Sclerocactus beetle kill

This cactus was killed by a beetle that ate the inside

Sclerocactus

Cactus was beetle killed and hollowed out. Grass was growing from inside the shell.

Sclerocactus vs.Small Mammals

I first saw Cactus 601 in 2009. It was getting along nicely until 2015 when mice and rabbits decided to eat it. In this instance the cactus was girdled, but I have seen other cacti that have been eaten from the top downwards to the ground. Lots of cacti have been eaten this autumn. It is often said that rabbits eat cacti for moisture but I am sure that they also eat cacti for nutrition because this autumn has been wet. After Cactus 601 had been eaten I noticed that nearby Cactus 447 was being eaten. Once eaten to the ground the cactus is dead. Videos were captured with a trail cam on loan from wildlife biologist Van Graham.

The map image below is of a hill about one kilometer from the hill where the videos were filmed. It shows the change in cacti status from March to September 2016. Rabbits eat Sclerocactus in the spring and summer too!

rabbit herbivory on Sclerocactus

Click to see what a rabbit did to Cactus 601 (left of the dog)

rabbit herbivory on Sclerocactus

Click to see what a rabbit did to Cactus 447

video of mouse herbivory on Sclerocactus

A few frame time lapse (with days reversed) of a mouse and Cactus 447. Mouse enters from lower R.

time lapse video of rabbit herbivory on Sclerocactus

Time-lapse video of rabbit eating at the base of Cactus 447

video of rabbit herbivory on Sclerocactus

Video of rabbit eating Cactus 447 which is now almost fallen over

map of Sclerocactus decline

Top - March 2016. Bottom - September 2016.

Left: green = alive; red circle = dead from beetles or unknown; red FSM = dead from rabbit herbivory; green FSM = eaten by rabbits but not yet dead. Read more below.

The area of the cactus map above is a small portion of a larger study area that contains 130 ha. The large study area had 143 known live cactus in 2012. In 2016 only 30% of those cacti were still alive. 26% had been eaten by rabbits and 44% had been killed by beetles or other agents. There are some cacti in the area that were not found in 2012 because they were either well hidden or were very small at that time. They are not included in the 143 live count. In 2016 successful reproduction was noted as evidenced by tiny plants that have germinated in the past few years. There are no plans at this time to measure reproductive success.

From my observations Sclerocactus, if present, are the preferred cacti for rabbit herbivory. They will eat Sclerocactus to the ground before doing any severe damage to Opuntia. Rabbits are also fond of Escobaria missouriensis cacti and will even eat the underground portion. I have not seen any herbivory on Echinocereus.