“If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed ten thousand years ago. If insects were to vanish, the environment would collapse into chaos.” – Edward O. Wilson
Invertebrates are animals without a backbone, ranging anywhere from sea urchins to snails to scorpions. Numerous types of invertebrates can be seen on the Bull Creek Nature Preserve, including insects, spiders, and worms. Although invertebrates often do not receive as much favorable attention as larger vertebrate species, they are incredibly diverse and interesting creatures, making up 95 percent of the species on earth and six of the eight endangered species protected by the Balcones Canyonlands Conservation Plan.
Invertebrates play several critical roles in the Preserve ecosystem. Many invertebrates, such as beetles and pill bugs, act as scavengers that break down dead animal and plant matter, recycling the nutrients back into the ecosystem. Invertebrates are also a food source for numerous species, including the golden-cheeked warbler. The warbler is an insectivore, meaning that it feeds exclusively on insects. Another crucial function performed by invertebrates is the pollination of flowering plants. Butterflies will visit flowers to collect nectar and inadvertently carry pollen from one flower to another. This is an example of mutualism, where both species benefit from interaction. Other major pollinators include bees, moths, wasps, flies, and beetles.
Although the mammals and birds of the Bull Creek Nature Preserve can often prove elusive, insects are the one animal guaranteed to be seen while hiking. The Preserve literally hums with the sounds of these small six-legged invertebrates. What may sound like a constant buzzing is actually one of the many forms of complex communication used by insects in locating mates, finding food sources, defending against predators, and caring for their young. For instance, mosquitoes are attracted to each other by the unique sound of their buzzing wings. Some insects such as beetles and lacewings will communicate potential dangers by sending vibrations through plants. In a particularly fascinating example, honey bees perform elaborate ‘waggle dances’ in the shape of a figure eight to tell other bees where to find a new field of flowers.
Insects develop and grow in distinct stages—a process known as metamorphosis that can happen in a variety of ways. For instance, in gradual metamorphosis, insects like grasshoppers hatch from eggs as small, wingless nymphs and grow larger through several molts, or shedding of their exoskeleton, before eventually developing wings and becoming adults. Young dragonflies called naiads develop underwater and emerge into the air as flying adults. This is called incomplete metamorphosis. In an example of complete metamorphosis, caterpillars hatch from eggs and grow as larvae before spinning a cocoon and entering into an inactive pupal stage. After undergoing extensive internal changes, a moth will break out from the cocoon as a fully-formed adult.
Spiders are another common invertebrate that can be found on the Bull Creek Nature Preserve—often by walking through the webs that stretch across the trail. Orbweaving spiders like the black-and-yellow argiope build these circular, spiraling webs between tree branches, using a sticky silk to trap flying insects. Look for the actual spider around the edge, waiting for prey by monitoring a signal or trip line attached to the web. Although orb webs may be the most recognized spider web, many species trap insects using different forms of webs or capture insects without using a web at all. For instance, grass spiders build flat sheet webs along the ground with a tube or funnel leading off from the center of one edge. The spider will hide in the bottom of the funnel and dart out to capture approaching prey. Jumping spiders and wolf spiders have excellent vision and will often actively hunt insects on the ground, while tarantulas will hide in burrows and feed on passing prey. Spiders greatly benefit the Preserve ecosystem by capturing and controlling the local insect population. In addition, webs can be used by birds such as the golden-cheeked warbler for nesting material.