Florida Sand Scrub Habitat Hosts Pliocene-Age Relicts

About 2 million years ago, an exceptionally dry climate phase occurred over most of North America.  Grasslands and scrub habitat stretched in a continuous belt from southern California to Florida.  Most of the large vertebrates that thrived in such an environment such as llamas, camels, flat-headed peccaries, pronghorns, horses, and donkeys have been rendered extinct or extirpated from the southeast, but the sandhill habitats of north and central Florida still host scores of relict invertebrates in addition to the Florida scrub jay.  Specimens found at the Inglis fossil site in Citrus County, Florida provide a glimpse of the fauna formerly inhabiting the once extensive arid grassland and scrub habitat that existed across the southeast during the late Pliocene.  In addition to the above mentioned species, the antelope jack rabbit (Lepus alleni), now confined to the American southwest, was a common component of the ecosystem when the climate was drier.

Antelope jack rabbits and an extinct species of jack rabbit lived in southeastern North America during the late Pliocene when the climate was much drier than it is today.

Pleistocene glacial cycles also fostered drier climates and an increase in scrubland and grassland habitat, but these environments never again formed an unbroken corridor from west Texas to Florida.  I hypothesize that pine and oak forest species evolved a greater drought tolerance and were able to grow in some areas with favorable conditions, thus forming interdicting fingers of habitat that prevented some scrubland species, such as jack rabbits, from recolonizing the southeast.

Location of Florida Scrub (peninsular)

The shaded black areas indicate sand scrub habitat–an environment that once stretched from southern California to Florida.  A continous sand scrub belt hasn’t existed for at least 1 million years.  Many small species have become isolated in these relict habitats.

Most of Florida is so low lying in elevation that numerous high sea stands have inundated much of the state.  High sea stands have occurred on many occasions dating as far back as the Miocene and as recently as the Sangamonian Interglacial of the late Pleistocene.  High hilltops, however, remained above sea level as islands surrounded by sandy beaches.  The sand scrub areas of today are simply remnants of these sandy beaches.

The Florida scrub jay (Aphelocoma coerulescens) used to be considered the same species as the Western scrub jay (A. californicus).  About 20 years ago scientists declared they were a separate species because they have a shorter broader bill and are less able to disperse following an acorn crop failure.  Fossil evidence shows that Florida scrub jays were a distinct species as long as 2 million years ago.

The Florida scrub jay is an example of a scrubland species now isolated from populations of its ancestral species–the Western scrub jay.  The fossil evidence from Inglis shows that by 2 million years ago, the Florida scrub jay was already a distinct species from the Western scrub jay.  Florida scrub jays are habitat specialists that will not even travel through unsuitable habitat.  They are endangered today because much of their habitat has been transformed into subdivisions and citrus orchards.  Families of scrub jays living on 1 patch of remnant scrubland will not fly through an orange orchard to reach another patch of scrubland.  Studies show that for the bird to survive, they will need corridors of protected scrubland habitat to prevent extinction through inbreeding.

Scrub jays are tame birds, known for taking food from people’s hands.  Offspring help care for young, a habit that makes them semi-communal birds.  Scrubland habitat in Florida provides a reliable crop of acorns which along with seeds, insects, and small lizards makes up the bulk of their diet.  Despite growing stunted, sand live oaks (Quercus germinata). myrtle oaks (Q. myrtlefolia), and scrub oaks (Q. iopina) provide plenty of mast, unlike Rocky Mountain oaks which may fail to produce acorns in the harsher climate there.  This accounts for the behavioral difference between Florida scrub jays and Western scrub jays. The former never evolved the habit of dispersing when the acorn crop fails because in Florida’s climate that seldom happens, but the latter did of necessity and is therefore more widespread and not endangered.

Over 70 species of invertebrates are also unique to the Florida scrublands.  Because it’s such a harsh environment, most plants growing there are high in toxins and have evolved thorny structures to discourage herbivores.  But these defenses don’t deter many of the insects that have co-evolved with them.  Scrub rosemary is a toxic plant that unwillingly hosts species of a grasshopper, moths, and beetles. 

Florida sand scrub wolf spider killing an insect.  Note the spider is the same color as the sand.

Florida sand hair ant (Componotus floridanus).  The hair enables them to travel through sand without sinking.

Other unusual invertebrates are specially adapted to living in sand.  They have waxy armor that protects them from being shredded by sharp grains of sand, and they have stiff hairs that help them locomote through sand without slipping backward.  One species of harvester ant has hair under its mandible in the shape of a basket to carry sand when they excavate their 3 foot deep nests.  Hunting wasps are common, and they actively defend their paralyzed prey because in the thinly vegetated habitat, it’s more likely to be discovered by other carnivores such as tiny yellow predatory ants, wireworms, and robber fly larvae which abound under the sand. 

 Each isolated sand scrub community has its own species of short-horned scrub grasshopper…an ideal case study for biologists interested in evolution.  The wealth of unique arthropod species found in the Florida scrub attracts entomologists and evolutionary biologists who consder the scrublands a mecca of potential new discoveries.  And just think–these invertebrate species once shared a wider range with now extinct megafauna.

Reference:

Deyrup, Mark and Thomas Eisner

“Last Stand in the Sand”

Natural History Magazine (102) 12 December 1993

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2 Responses to “Florida Sand Scrub Habitat Hosts Pliocene-Age Relicts”

  1. jrobertsmith Says:

    I’ve seen the scrub jay in some state parks in Florida. The park I visited that seemed to have the highest number of them was Blue Springs State Park north of Orlando. We had a troublesome raccoon at our site so the park ranger set a live trap to capture her (and move her elsewhere). The trap was baited with hot dog pieces, but a scrub jay flew in and and stole the bait (after a possum had already done the same, but getting caught)–the jay was able to take the bait without tripping the spring.

    It’s a strikingly pretty bird–the blue is quite vibrant. Every scrub jay that I’ve seen has a tag.

    (You can see a photo of a scrub jay at our campsite if you do a search on my blog for The Little Asshole–referring to the troublesome raccoon.)

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