All City wildlands are managed towards healthier ecosystems that perform specific functions, be it enhancement of recharge to the Barton Springs segment of the Edwards Aquifer or habitat preservation for endangered species that live in our area. Native wild animals are a natural part of those ecosystems but domestic animals and pets are not. These non-native animals are of particular concern on the Balcones Canyonlands Preserve where they can prey on and disrupt native wildlife. Water quality in conserved wildland can also be degraded from feces left by domestic animals and pets or dumped over boundary fences.

Many domestic cats (and possibly some dogs) will hunt whether or not they are well fed. While dogs can usually be contained by a homeowner's property fence, cats defy containment. Their nature is to roam and to hunt and they can easily cross into protected preserve land to do so. Thus, allowing a cat outside subjects birds, small mammals, and lizards to injury and death from these efficient little predators. Further, their predation puts them in competition with native animals and can disrupt the natural food chain. A national campaign called "Cats Indoors" is conducted by the American Bird Conservancy and is focused on the benefit to native birds. However, it also advocates on behalf of healthy cats and educates pet owners about the hazards facing cats outdoors. Cars, poisoning, prey for wildlife, human abuse, traps, greater risk for contracting diseases such as rabies, feline leukemia, and spreading of diseases that affect humans are all possible dangers for cats allowed to roam outside.

The problems associated with dogs on preserve land tend to come from trail users in dual management lands that disobey leash rules and allow their dogs to roam off trail. Dogs leave scent trails similar to predators that can disturb the behavior of wildlife and cause confusion about territories. Dogs are also capable predators that can scare an animal away from its burrow or foraging area and have been observed chasing and attacking native wildlife.

Urban cats and dogs released to "roam free" are also harmful to native wildlife. In Preserves bordered by residential development, they can exist in high numbers. Feral cats (wild cats surviving without owners) are particularly hard on small vertebrates such as lizards, field mice and birds and become even more efficient predators since it is vital to their survival. Feral dogs form packs that can be dangerous not only to wildlife but to staff. When they venture into neighborhoods, they can be dangerous to residents and may see small cats and dogs as prey.