External Pressures and Unnatural Presences
A natural preserve surrounded by advancing urban and suburban development would inevitably face threats to its integrity. On the whole, Rock Creek Park was ably defended by its military custodians from adverse external pressures and encroachments.
The first major threat of encroachment was an 1897 proposal by the District of Columbia Water Department to construct a reservoir in the park. Finding the proposal objectionable, the Board of Control referred it to Attorney General Joseph McKenna for an opinion that it hoped would buttress its position. McKenna did not disappoint, replying that under the Rock Creek Park legislation the board was precluded from permitting any such development foreign to the stated park purposes. 
The reservoir proponents thereupon drafted new legislation to authorize their objective. They contended that the reservoir would be an attractive addition to the park. Faced with likely enactment of the authorization, the Board of Control negotiated a happy compromise. The park boundary in the vicinity of the desired reservoir site, north of Blagden Avenue and west of 16th Street, was uneven. If the Water Department would purchase certain tracts, the board would exchange an equal or lesser amount of parkland for them so as to leave the department with an adequate reservoir site and the park with a straightened boundary.
Authorization for this bargain was incorporated in an act or Congress approved June 6, 1900, which resulted in construction of the Brightwood Reservoir and net enlargement of Rock Creek Park by seven square feet.  (The reservoir became obsolete in the 1930s, and its site is now occupied by tennis courts and ball fields.)
In January 1898 Rep. Alfred C. Harmer of Pennsylvania and Sen. Francis M. Cockrell of Missouri introduced bills that would authorize each state to erect in the park an exhibition building for "any and all articles or things connected with its natural or industrial resources or evidencing its social, scientific, or artistic progress and development." The states would be given from one to six acres apiece for their buildings.  The office of the District commissioners recommended against passage, stating that the development would conflict with the intended park purposes. The scheme did not threaten further.
In 1911 the United States Forest Service obtained permission to plant trees for experimental purposes north, south, and east of Camp Good Will. The Board of Control asked that the trees be set in irregular patterns to avoid the appearance of artificial cultivation. Several species of willow and a few poplars were installed the next spring. By 1920 the Forest Service had planted about 2000 trees comprising 170 species and planned to continue plantings from all parts of the world. 
The Forest Service and other parties supporting this venture hoped to expand and formalize it, with congressional approval, as the National Arboretum and Botanic Garden. The Fine Arts Commission thought otherwise. A 1917 report prepared by its landscape architect member, Frederick Law Olmsted Jr., declared the project incompatible with the natural qualities for which Congress had established Rock Creek Park. "It does not now, and it never will, look like a part of the natural scenery," the report said of the existing arboretum. "It is distinctly out of harmony with it." Olmsted repeated his stand in his report prepared for the park's managers a year later. As a result, the Office of Public Buildings and Grounds in 1920 disapproved further extension of the arboretum. 
The office continued its own planting of certain exotic vegetation including Japanese honeysuckle to stabilize embankments from erosion. In May 1920 Charles Moore of the Fine Arts Commission wrote Colonel Ridley to warn of the spreading, destructive nature of the plant: "It will kill anything but the largest trees, and unless pains are taken to keep it down, for it cannot be exterminated, it will ruin Rock Creek Park." At the same time Moore called Ridley's attention to the problem of people carrying dogwood and other flowering plants from the park. 
The Board of Control was also willing to allow introduction of non-native birds to the park. According to its 1912 report:
Through the subscription of private individuals and at the suggestion of Dr. Cecil French, D.V.S, some wild ducks, wild geese, and black, white, and gray swan, were presented last fall to the park and are generally prospering. A few have disappeared and there does not seem to be much mating. Some English and golden pheasants were also presented, but were almost all killed by some unknown animal. 
The board spent $164.51 during fiscal years 1907-1909 for feed for wildfowl. It was less hospitable to certain other exotic animals, however. In 1911 it reprimanded the Chevy Chase Club, an exclusive country club nearby in Maryland, for fox-hunting with a pack of hounds through the park.