Ecological Observations on the Woodrat, Neotoma floridana Part 1

Ecological Observations on the Woodrat, Neotoma floridana.

by Henry S. Fitch and Dennis G. Rainey.

Introduction

The eastern woodrat exerts important effects on its community a.s.sociates by its use of the vegetation for food, by providing shelter in its stick houses for many other small animals, and by providing a food supply for certain flesh-eaters. In the course of our observations on this rodent on the University of Kansas Natural History Reservation, extending over an eight-year period, from February, 1948, to February, 1956, these effects have changed greatly as the population of woodrats has constantly changed in density, and in extent of the area occupied.

This report is concerned with the population of woodrats on the Reservation, the changes that the species has undergone, and the factors that have affected it. Our two sets of field data, used as a basis for this report, supplement each other and overlap little, either in time or s.p.a.ce. Fitch's field work which covered approximately the western half of the Reservation, was begun in September, 1948, and was pursued most intensively in the autumn of 1948 and in 1949, with relatively small amounts of data obtained in 1950 and 1951 because of the great reduction in numbers of rats. Rainey's field work began in the spring of 1951 and was continued through 1954, concentrating on a colony in the extreme northwestern corner of the Reservation and on adjacent privately owned land. In actual numbers of rats live-trapped and for total number of records the two sets of data are comparable. Fitch's field work consisted chiefly of live-trapping while Rainey's relied also upon various other approaches to the woodrat's ecology. Rainey's findings were incorporated originally in a more comprehensive report (1956), from which short pa.s.sages have been extracted that are most pertinent to the present discussion. Our combined data represent 258 woodrats (153 Fitch's and 105 Rainey's) caught a total of 1110 times (660 Rainey's and 450 Fitch's). Rainey's records pertain, in part, to woodrats outside the Reservation but within a few hundred yards, at most, of its boundaries.

_Habitat_

In the autumn of 1948 the population of woodrats was far below the level it had attained in 1947 or earlier, but the rats were still abundant and distributed throughout a variety of habitats. Almost every part of the woodland was occupied by at least a spa.r.s.e population. Also, many rats lived beyond the limits of the woodland proper, in such places as deserted buildings, thickets, roadside hedges, and tangles of exposed tree roots along cut banks of gullies. All these situations are characterized by providing abundant cover, a limiting factor for this woodrat.

In 1947, when the population of woodrats was especially high, plant succession on the wooded parts of the Reservation may have been near the optimum stage for the rats. For some 80 years, since the time the land was first settled and prairie fires were brought under control, woody vegetation has been encroaching into areas that were formerly gra.s.sland.

About 1934 the University changed its policy with regard to treatment of the tract that was later made the Reservation. Up to that time, most of the area had been used as pasture and subjected to heavy grazing, but several fields had been fenced and cultivated. Under the new policy the hillsides and hilltop edges with open stands of various deciduous trees were enclosed with stock fences and protected from grazing. Successional trends were greatly altered. Woody vegetation, already favored by protection from the prairie fires originally important in the ecology of this region underwent further development as a result of protection from browsing. Thickets of shrubs and saplings sprang up throughout the woodland, forming a dense understory layer beneath the discontinuous canopy of the relatively scattered mature trees. The composition and density of the undergrowth varied markedly in different parts of the woodland. The parts that were formerly most open acquired the most dense understory. Blackberry, honey locust, osage orange, and p.r.i.c.kly ash formed in places th.o.r.n.y tangles almost impenetrable to humans. This thicket stage reached its peak in density in the middle to late forties coinciding approximately with the time of maximum abundance of the rats.

In the past eight years, under continued protection from burning, cutting and browsing, the forest has developed further; sizable trees 20 feet or more high and up to eight inches in trunk diameter have grown from seedlings during the period of protection. An almost continuous canopy of foliage has developed, shading the understory and thinning it by killing shrubs and saplings. In those situations where the canopy is most dense, as on north slopes having stands of young hickory averaging twenty feet high, the understory is now largely lacking, but in other situations, particularly on south slopes, the understory thickets are still dense. On the whole, however, habitat conditions have become less favorable for the woodrat.

Within the woodland the population of woodrats was not evenly distributed even at its maximum density; only those situations that provided sufficient overhead shelter were occupied by woodrats. The hilltop limestone outcrop, which was the refugium of the survivors when the population was at low ebb, also supported the greatest concentration when the population was high. The number of individuals living along any particular stretch of ledge could be determined only by intensive live-trapping, whereas residences of individuals could be more readily identified in most other situations away from the ledge. Stick houses of woodrats are, characteristically, large and dome-shaped in woodland, but along the ledges they usually lacked this typical form and consisted of a much smaller acc.u.mulation of sticks, often merely filling a small crevice. Sticks carried into such places where they were partly or wholly protected from moisture and suns.h.i.+ne were much less subject to decay than those in more open situations, and remained long after the rats themselves were gone. Acc.u.mulations of droppings in depressions in rock surfaces beneath overhanging ledges likewise have lasted for many years. The rock outcrop provided a continuous travelway along the hilltops, and even parts that were not permanently occupied usually had some sign. The following types of situations were found to be especially favorable for occupancy: deep crevices beneath overhanging projections of the ledge; large flat boulders broken away from the main ledge; thick clumps of brush (usually fragrant sumac, _Rhus trilobata_) providing shelter and support for the house; logs fallen across the ledge providing support and protection for the house structure.

A second outcropping limestone stratum approximately 20 feet below the level of the hilltop was just as extensive as the upper outcrop, but it was little used by the rats because the exposed rock surface was more regular, lacking the jagged cracks and deep fissures of the hilltop outcrop; and it lacked the overhanging projections which provided overhead shelter for the rats along the upper outcrop. More than ninety per cent of the rats that were recorded as a.s.sociated with the outcrops were at the hilltop stratum.

Second in preference to the hilltop outcrop as a house site was the base of an osage orange tree in thick woods. This tree occurs throughout the woodland of the Reservation, having become established when the leaf canopy was more open, and the whole area was subject to grazing, with less development of the understory vegetation in the woodland. Houses were most often situated in those osage orange trees that had been cut one or more times, and had regenerated with spreading growth form, the multiple branching stems offering substantial support. Occasionally houses were built in crotches from two to six feet above ground.

Blackberry thickets also are favorable locations for houses. These thickets grew up mostly in fenced areas from which livestock were excluded, but where there was not dense shade--hilltop edges and level or gently sloping ground adjacent to creek banks. The houses were usually in densest parts of the thickets where they were almost inaccessible. Mats of dead canes more or less horizontal, with the live canes growing up through them, provided effective overhead protection, while the ground beneath was relatively open. Houses built in the thickets were so well concealed that they were usually not detected until after leaves were shed in autumn. In most cases the blackberry thickets were small and well isolated. Houses of the rats were sometimes unusually near together suggesting that these thickets provided especially favorable habitat conditions.

Hollow trees are often utilized, the acc.u.mulation of sticks for the house being largely inside the cavity. To be suitable for a house site, the snag must have an opening near ground level, and another higher on the trunk, providing emergency outlets in two directions. Most of the hollow trees utilized were black oaks (_Quercus velutina_).

In 1948 there were many houses in cut tops of trees left from small scale lumbering operations a few years earlier. The densely branched tops of elms, oaks and hickories had satisfied the requirement for support of the house and nearby shelter. The houses built in them were in open woodland well separated from otherwise favorable situations. By 1948 the tops were disintegrating and no longer provided effective shelter. The houses built in them were falling into disrepair and were not permanently inhabited but were often used temporarily by wandering individuals.

Along cut banks of gullies where trees were partly undermined by erosion, the exposed, tangled root systems provided sites for occupancy.

In these situations the acc.u.mulations of sticks were small and lacked the typical domed shape, consisting essentially of a lining to the cavity beneath the roots.

Two small buildings at the Reservation headquarters were accessible to woodrats and were utilized off and on throughout much of the period of this study, despite the fact that most other sites of occupation away from the hilltop outcrops were deserted in the same period. One small building used as a laboratory had an enclosed wooden box five feet square housing an electric water pump. The interior of this box was accessible to the rats from beneath the floor. Litter of sticks and stems and various food materials were carried in by the rats. The nest thus protected and enclosed was not surrounded by the usual acc.u.mulation of sticks. An old garage 30 feet from the laboratory building was also occupied, sometimes by a different individual. The nest and food stores were behind boards propped against the wall.

In October, 1948, live-trapping was begun on a heavily wooded slope facing northwest, and a ten-acre area was trapped rather thoroughly in the succeeding weeks. Because few traps were then available, this was the only area that was well sampled in 1948, although diffuse trapping was carried on over some 200 acres. On the ten-acre tract a total of 17 adult and subadult woodrats were caught, four along the hilltop rock outcrop, six along the gully at the bottom of the slope, and seven at intermediate levels on the slope. Judging from the many unoccupied houses, the population on this tract had been much higher before the study was begun. On the basis of this sample it seems that in 1947 a population of several hundred woodrats lived on the wooded parts of the square mile where the Reservation is located.

_Reduction of Population_

The abrupt reduction in the population of woodrats on the Reservation cannot be explained conclusively with available data. Probably weather played a major part, but other unknown factors must have been important also. It is certain that the population of woodrats was high, if not at an all-time peak, in 1947. In late February, 1948, when one of us (Fitch) first visited the area on a preliminary inspection trip (not concerned primarily with woodrats), houses of these rats were found to be unusually numerous and those seen seemed to be occupied and well repaired. Possibly the population was drastically reduced within the next few weeks, as unseasonably cold and stormy weather occurred in early March. For the first 12 days of March, 1948, temperature averaged 20 below that of average March weather, and even colder than the average for January or February. A reading of -5F. on March 11 set a new low locally for the month since records were begun in 1869. The record low temperatures were accompanied by 12.8 inches of snow. This spell of unusually severe weather in early March coincided with the period in which first litters of young usually are born, as most females breed in early February and the gestation period is in the neighborhood of five weeks. That most of these first-litter young may have been eliminated by the unfavorable extreme of weather at the most critical stage in the life cycle may be readily imagined although definite proof is lacking. However, the mortality must have extended beyond newborn young. Loss of first litters ordinarily would be compensated for by the end of the season, since a female usually breeds more than once in the course of a season. In any case, by autumn, when the actual field study of woodrats was initiated, many houses were already deserted and in disrepair. Although the rats were still moderately abundant, they were, seemingly, much below the population peak of the preceding year.

Further drastic reduction of adults and subadults took place in the winter of 1948-49. In the course of live-trapping operations from mid-October into early December, 51 individuals were caught and marked.

Chiefly because of unfavorable weather conditions, field work was discontinued in mid-December, and live-trapping was not resumed until early March. Subsequently, only 12 of the woodrats previously marked could be recaptured, and the population had become noticeably spa.r.s.e.

Seemingly, more than three-fourths of the population present in late autumn had been eliminated in the interval. In January, weather was exceptionally severe; on the ninth and tenth the worst sleet storm in twelve years occurred. Sleet fell in small granules, while the temperature remained several degrees below freezing. Partial thawing on January 12, 13 and 14 was followed by a steady drizzling rain on the fifteenth. On the following day the temperature dropped to -7F. Ice still remained from the sleet storm, and the slush again froze. On the night of January 18, there was one of the worst snow storms on record and temperature reached a low of 2F. Exceptionally low temperatures persisted through January 24, with more sleet on January 25. Ice from the earlier storm still remained. On January 30, the temperature dropped to -7 and a three-inch cover of snow still remained over the coat of ice. The month of January, 1949, had the heaviest precipitation in 81 years (5.09 inches) and a cover of ice remained for at least 21 days.

There were other sleet storms of lesser proportions on February 2 and again on February 21.

Ordinarily sleet would not seriously damage woodrats living in houses in woodland habitats and less suitable hedge rows because it usually freezes as it falls and coats only the surface of the house. Gradual thawing would allow normal runoff without much penetration. Because the sleet during the storm described above did not form a glaze as it fell, the ice particles penetrated many houses. It has been observed many times that captive woodrats refused food that was frozen or were unable to eat it. Woodrats in live-traps in winter rapidly weaken unless a large supply of food is available. If food supplies became sealed over by ice, woodrats would have died by starvation or by falling an easy prey to predators. The rats were more accessible to several predators than were smaller mammals such as meadow voles which were difficult to obtain because of the coating of ice over the fields.

The decimated population surviving into the breeding season of 1949 failed to make substantial gains. In fact, during the following four-year period the general trend of the population over the Reservation as a whole seemed to be one of gradual further decline.

In November, 1949, the rats were almost gone from the area of north slope and hilltop in oak-hickory-elm woodland where the most intensive live-trapping and other field work had been done the previous year. The following descriptions of houses remaining on the area at that time give some idea of the habitat, and of the course of events correlated with the fluctuations in numbers of woodrats.

No. 1. At the hilltop outcrop, partly on a substrate of limestone boulders, built around an elm of two-foot DBH, which lent support to one side. A hackberry sapling one inch in stem diameter grew through the middle of the house, providing further support. The house was two feet high and six feet in diameter, and was in obvious disrepair, with a hole several inches in diameter in its top. It had been occupied in the autumn of 1948. It was constructed mainly of sticks, ranging in diameter from approximately one inch to straw size. Many of the sticks, from .4 to .5 inches in diameter and one to two feet long, seemingly would have been heavy burdens for a rat, although they were of light-weight wood, sumac and elm. Mixed with the sticks were quant.i.ties of dry leaves, bark, and chips of wood, all material appearing old and weathered. This house was in elm-oak-hickory woods 50 feet from a cultivated field on the hilltop to the east and south. To the north and west the escarpment sloped away abruptly. There was a coralberry thicket beneath the trees on the adjacent hilltop.

[Ill.u.s.tration]

FIGURE 1

(A) Map of part of University of Kansas Natural History Reservation, showing first-capture sites for all woodrats live-trapped in the autumn of 1948. Because of the short time involved and the few traps available, much of the area shown was not thoroughly trapped. Woodrats were abundant, though much less so than in 1947, as shown by the large number of deserted houses.

(B) Map of woodrat study area, same as shown in (A), showing first-capture sites for all woodrats live-trapped in 1949. Woodrats were still moderately abundant, but much below the level of the previous year. Triangles indicate those capture sites not sampled in 1948.

(C) Map of woodrat study area, same as shown in (A), showing first-capture sites for all woodrats live-trapped in 1950. Numbers were medium-low, having undergone drastic reduction from the peak level.

Triangles indicate those capture sites where trapping was not done in earlier years.

(D) Map of woodrat study area, same as shown in (A), showing first-capture sites for all woodrats live-trapped in 1951. The population was low, but had not yet reached its lowest ebb.

(E) Map of woodrat study area, same as shown in (A), showing first-capture sites for all woodrats live-trapped in 1952, when the population had declined to relatively low numbers and disappeared from much of its former habitat.

(F) Map of the 590-acre Natural History Reservation, showing the area where woodrats were studied.

No. 2. On gently sloping hilltop edge 15 feet from the outcrop and escarpment, built around a forked walnut sapling having both trunks approximately five inches in diameter.

The sapling, coming up through the center of the house at a 45 angle, evidently had been bent by the acc.u.mulated weight of the debris at an early stage of its growth, many years before. Trees were small in this part of the woods, with a well developed understory thicket of coralberry and sumac.

This house approximately one foot high and six feet wide, was constructed mainly of sticks and was similar in composition to No. 1, but appeared considerably older with all the sticks blackened and rotten. In the autumn of 1948 this house was used by woodrats, but probably only as a temporary stopping place, because it was already in disrepair then.

No. 3. At edge of escarpment, 25 feet from No. 2, on a flat boulder approximately six feet long, three feet wide and one foot thick. The decaying and much flattened ma.s.s of sticks was mainly on top of the boulder, but also spilled over its edges. Fresh sign was noted at this house in the autumn of 1948, but the house was already in disrepair then, and seemingly it was used only as a stopping place.

No. 4. At the hilltop outcrop where an elm had fallen across it. The decaying log remaining was approximately 12 feet long and 15 inches thick. This log pa.s.sed diagonally through the house, providing its main support. The house was approximately 39 inches high, its summit extending a little above the level of the top of the outcrop. The house was approximately seven feet wide along the outcrop. This house was somewhat intermediate between the typical dome-shaped stick piles that the rat builds in open situations and the formless acc.u.mulations of sticks with which some rats living in deep rock crevices line the entrances. Part of the acc.u.mulation was beneath the limestone boulders and outcropping slabs. Approximately half of the material used in the house consisted of sticks and the remainder of pieces of bark and chips of wood, mostly gathered from the fallen elm. This house had shrunken noticeably from decay and settling in the months since it was occupied, in the autumn of 1948. The house was surrounded by a thicket of fragrant sumac, dogwood, and hackberry saplings.

No. 5. At edge of a protruding boulder one foot thick at the hilltop outcrop of the west facing escarpment, and 100 feet back in the woods from the edge of a corn field, in undergrowth of dogwood, wild currant, and coralberry. The house consisted of a pile of rotten twigs, 3 inches deep and 30 inches wide on the upper side of the boulder, and a lining of similar material at the lower edge of the boulder, partly blocking the crevice beneath it. The twigs composing the house were old and rotten. However, a few dry but still green hackberry leaves were stored in the crevice beneath the boulder. In a bare s.p.a.ce atop the boulder were several recent woodrat droppings, small and obviously produced by an immature individual, which, perhaps, had recently settled at this old house site.

No. 6. In hilltop woods, 30 feet from a corner adjoining a pasture and a corn field, at the base of an osage orange tree of one foot DBH, and also over a hollow cottonwood log one foot in diameter and three feet from the osage orange tree. Suspended mats of grape and smilax vines, and the th.o.r.n.y, dead, lower branches of the tree provided additional shelter. The house was composed of sticks and twigs, mostly of osage orange, with spines still present; slabs of bark, wood chips, and dry leaves also made up part of it.

Materials on the exterior of the house appeared old and weathered, but the house was conical and solid. Seven fresh corn cobs were on the house or near its base, suggesting that corn from the nearby field had figured importantly in the diet of the occupant. A well beaten path led from the base of the house alongside the log, to a large cottonwood tree 15 feet from the house. This evidence that the house was occupied was verified by live-trapping the occupant.

Late in 1948, also, the house was occupied by another individual, but seemingly was deserted for a period of months thereafter.

No. 7. On upper part of north slope where a hickory seven inches in diameter had fallen across an old sunken log approximately one foot in diameter. The house, composed mainly of hickory twigs 1/4-inch to 1/2-inch in diameter, mixed with bark, wood chips, and leaves, was partly decayed, with no fresh sign and was in a thicket of greenbrier, saplings of hickory and hackberry, and cut tops of hickories. The top was flattened to less than four inches above the level of the supporting hickory log. There were large cavities in the side of the house. When first discovered in the autumn of 1948, this house was occupied by a subadult female rat, but she moved away permanently, and the house had been deserted for approximately a year when these observations were recorded.

No. 8. In middle of northwest slope, in thick branches of broken top of a black oak. This house had become flattened by decay and settling to form a mound approximately one foot high and five feet in diameter. Only the top protruded through the carpet of dry leaves. Once well protected and partly concealed by the branches and twigs of the oak top, this house was now fully exposed by the disintegration of the top. The house consisted chiefly of oak twigs. In October, 1948, a woodrat was live-trapped at this house, but probably it was a wanderer. The house had then already undergone much deterioration.

_Natural Enemies_

Some 56 species of animals that regularly prey on small vertebrates live on the Reservation. Many of the larger kinds may take woodrats occasionally. Because of size, habitat preferences and the time and manner of hunting, five species stand out as the more formidable enemies--the horned owl (_Bubo virginia.n.u.s_), prairie spotted skunk (_Spilogale putorius_), long-tailed weasel (_Mustela frenata_), pilot black snake (_Elaphe obsoleta_) and timber rattlesnake (_Crotalus horridus_).

Throughout the study horned owls were common on the area, but their numbers were highest in 1948. Samples of pellet collections have shown that the cottontail is the staple food, being represented in almost every pellet. Various rodents also are important in the diet, the cotton rat, prairie vole, or white-footed mouse being most prominent according to the time and place of collection. The woodrat is approximately optimum size for prey, and it const.i.tutes one of the most preferred food sources. Remains of only two woodrats were found in the pellets examined, but at times when the pellets were collected woodrats were so scarce that they const.i.tuted only an insignificant percentage of the bioma.s.s of potential prey. On several occasions woodrats in live-traps were attacked by horned owls, as shown by the overturned and displaced trap and quant.i.ties of fine down adhering to them and to nearby objects.

The horned owl lives in the same habitat as does the woodrat. In other regions woodrats are known to figure prominently in the diet of the horned owl. At the San Joaquin Experimental Range in California, for instance, _N. fuscipes_ was found 240 times, more frequently than any other kind of prey, in 654 pellets of the horned owl, and this owl was shown to be the one most important natural enemy of the rat, although many kinds of carnivores, raptors and snakes also took toll from its populations. On the Reservation the population of horned owls has been fairly stable from year to year, with roughly one pair to 100 acres of woodland. Some territories have been maintained continuously throughout the eight-year period of observation, though changing to some extent in size, shape and area included. In 1948, when livestock grazed on the area, and the ground cover of herbaceous vegetation was relatively spa.r.s.e, cottontails were much less abundant than they were later when the vegetation was protected. Small rodents including voles, cottonrats, and deer mice, were also less abundant then, and the numerous horned owls may have been supported in part by the high population of woodrats.