Background for Teachers

TPW Magazine, January 2009

Texas is home to an amazing array of plants and animals. But why are some species plentiful while others become rare, threatened or endangered? And what difference does it make if a species dies out?

Threats to Species Survival

Habitat Loss and Change

All species depend in some way on habitat, that is, food, water, shelter, space in a suitable arrangement. Some plants and animals were once more widespread, but people replaced natural areas to build towns, grow food, mine or otherwise use land and water resources. As we convert habitat for our needs, wildlife sometimes loses out. The Houston Toad is adapted to breed in shallow, ephemeral, natural ponds. The Houston Toad once was found in Houston, but people paved over its habitat, replaced native plants with exotic (non-native) vegetation and altered natural ponds and water drainage. The Houston Toad now lives in a small area just east of central Texas. The endangered White Bladderpod grows on a special type of rock that is now mined for road paving material.

Sometimes the habitat isn't totally destroyed, but is changed to the point that the plants and animals can no longer survive. Habitat fragmentation divides habitats into smaller segments or creates divisions between parts of habitat. Human-caused fragmentation such as fencing, plowing or roads, may create devastating barriers to food, water or shelter for some species. Diverting water also fragments habitat and affects plants and wildlife. Concho Water Snakes, especially juveniles, need flowing streams with rocky bottoms. Alterations of the river channel due to construction projects have caused changes in water depth that affect the snake's ability to find food and cover. The Interior Least Tern nests on inland river sandbars. Again, alteration of rivers by man has changed historical water flow, causing nesting problems for this bird. Pollution contaminates land and water resources. For some species that are sensitive to pollution, they must find new homes, or if that's not possible, they die. Humans have also prevented naturally-occurring disturbances such as fire or floods. Certain plant species depend on fire or flood as part of their life cycle. Red-Cockaded Woodpeckers once occurred throughout the Southeast U.S. in a long-leaf pine forest kept open by frequent fire. Preventing those fires allowed shrubs to grow up and choke out the woodpecker's habitat.

Invasive Species

Introduced or non-native species compete with native species for food, water and space. Dumping home aquarium water into streams introduced the Ramshorn Snail into the San Marcos spring ecosystem. This hungry snail has decreased plant cover for some native fish and salamanders. Similarly, the Channeled Applesnail is spreading in the Houston area. Many scientists theorize that the imported fire ant is out-competing Texas native red ants which are the Texas Horned Lizard's primary food. This might contribute to the decline in Texas Horned Lizards. Nuisance aquatic plants (PDF 929.8 KB) such as hydrilla, water hyacinth, and giant salvinia have invaded many Texas waterways. These introduced plants often grow rapidly, displacing more beneficial native species, and they can travel from one watershed to another by way of boat propellers, bilges, and livewells.

Climate Change

Climate change, whether warming or cooling, affects where plants and animals can survive, their growing seasons, migration, breeding and feeding patterns. Climate change can affect rainfall, water levels and distribution of vegetation. It may allow invasive species to expand their ranges and possibly out-compete or spread disease to rare species. Additionally, climate change may adversely affect hydrologic cycles and increase habitat fragmentation as the water and land respond to precipitation and temperature shifts. If climate change causes an increase in sea level, it would affect any organism endemic to the coast such as Tharp's rhododon or Green Island echeandia. Fluctuations in freshwater inflow and sea level rise would alter salinity in coastal bays and estuaries, affecting wintering birds and nursery areas for many saltwater fish.

Specialists Versus Generalists

Some species are specialists, meaning that they have very specific requirements for food, water sources or living spaces. These specialists are often adapted to eating one type of food, certain nesting or breeding conditions or living in only one type of habitat. They can be quite successful as long as specific habitat needs are available, but are unable to change food sources or move to other locations to improve survival. For example, the Golden-cheeked Warbler is dependent on mature juniper trees in Central Texas. The Attwater's Prairie Chicken requires coastal prairie with a mix of native grasses at different heights. Farming, over-grazing, conversion to coastal bermudagrass, urbanization and industrial expansion have caused the loss of 97% of the critically endangered Attwater's Prairie Chicken habitat.

Small Population Numbers

Most endangered plants and animals have few numbers and a limited distribution or range. In both cases, the plants and animals become more vulnerable to habitat destruction or other people-caused problems. In Texas, cave invertebrates are naturally rare and are extremely dependent on their dark cave environments. Spring ecosystems are naturally rare in Texas and are the home to the endangered blind salamander, San Marcos salamander, Fountain Darter, Comanche Springs Pupfish, and the Leon Springs pupfish. Many plants are tied to a particular geologic formation or soil type that is not widespread. Among these plants are Texas poppy-mallow, Texas prairie dawn, white bladderpod, Davis' green pitaya and the Nellie Cory cactus.

Small Number of Young and Long Gestation Periods

Some animals give birth to only one or two young every one or two years. While the populations of these animals drop, it takes much longer for them to recover because of a low birth rate. Some animals become extinct before they have time to make a comeback. Endangered and Threatened animals of Texas that suffer from this problem are the Whooping Crane (one or two chicks and the adults may not produce every year), the Bald Eagle (one or two chicks per year), and the Ocelot (one or two kittens per year).


Animals that migrate rely on specific habitat types and locations for their journey. Losing any one of those habitats along their migration route could be fatal. Also, migration itself can be dangerous for animals. For example, a hurricane may kill an entire flock of migrating birds. The endangered Whooping Crane migrates annually over a very long distance with many hazards along the way such as power lines, fences, lack of forage and resting areas, or hunters who mistake them for snow geese, a game bird.

Collected or Hunted in the Past

Before good hunting and conservation laws, some animals or plants were hunted or collected for human use without considering the impact to that species. For example, in the past Texas Horned Lizards were often collected for the pet trade. Many types of cacti have been collected to the point that they are now very rare in the wild. Years ago, Gray Wolves were killed almost to extinction as part of government extermination programs and have yet to recover their numbers in Texas.

Changes in the ecosystem

Some plants and animals are rare because an essential element for their life cycle is now missing. An ecosystem is a system of many parts, each playing a role in the success and sustainability of that ecosystem. Some species have lost other plants and/or animals that they depended on to complete part of their life cycle.


Why are we concerned about rare species?

All species contribute to biological diversity, often referred to as biodiversity. Good biodiversity contributes to environmental health and sustainability of ecosystems. Biodiversity is measured three ways: the variety of species in an area, the genetic variation within a species gene pool and the variety of habitat types in an area.

When a species is lost or becomes rare, its role or niche in the ecosystem is also lost. Imagine a structure losing one of its supports, or a strand of web breaks. Each species that is lost or reduced weakens the ecosystem. Other species and habitat types are affected. At what point will the entire system fail? And since ecosystems are linked, what effect will a damaged ecosystem have for another ecosystem?

When population numbers for a species decrease, the gene pool decreases as well. This makes the population more vulnerable.

In 1973, Congress passed the Endangered Species Act, recognizing that endangered wildlife, fish and plants have an "esthetic, ecological, educational, historical, recreational and scientific value to the Nation and its people." In addition to their role in an ecosystem, rare and endangered species are

  • a source of critical discoveries in medicine and science,
  • play key roles as biological controls for agriculture,
  • and serve as environmental barometers -- an early warning of dangerous pollution or other environmental changes. (For example, the absence of certain mussels, fish and aquatic invertebrates help identify problems with water quality.)

How You Can Help

  • Preserve water sources in their natural state.
  • Support controlled burns (fires) or naturally-started wildfires if safe.
  • Save water for wildlife.
  • Prevent water pollution, run off down storm drains and into rivers and streams.
  • Support large tracts of habitat and habitat corridors.
  • Control fire ants.
  • Stop aquatic hitchhikers.
  • Never release non-native species into the wild on purpose or accidentally.
  • Preserve native vegetation.
  • Do not buy or collect rare species from the wild.
  • Report violations to game wardens.

Additional Reading

Endangered and Threatened Animals of Texas

Attwater's Prairie Chicken Recovery Efforts - The Nature Conservancy