Ecosystem Changes & Invasive Species: Fifth Grade Science
Ecosystems undergo continuous changes through gradual ecological succession or catastrophic disturbances, often severely disrupted by non-native invasive species that alter food webs.
How Ecosystems Change Over Time
Ecosystems are not static; they are dynamic, ever-changing ecological networks. Environmental changes occur on different timescales:
- Short-term changes occur rapidly: a seasonal freeze, a flash flood, a lightning-sparked forest fire, or a volcanic eruption. Organisms must flee, adapt, or perish.
- Long-term changes occur over decades, centuries, or millennia: shifting tectonic continents, advancing ice ages, or gradual ecological succession.
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| PRIMARY VERSUS SECONDARY SUCCESSION |
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| Succession Type | Starting Surface State | Pioneer Species Involved |
+---------------------+-------------------------+--------------------------+
| Primary Succession | BARE ROCK; zero soil | Lichens and mosses break |
| | exists (cooled lava, | rock into early soil; |
| | retreating glacier) | takes 1,000+ years |
| Secondary Succession| SOIL ALREADY EXISTS; | Fast-growing grasses and |
| | after fire, flood, or | weeds; reaches mature |
| | abandoned farmland | climax forest in 100 yrs |
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Ecological Succession: The Rebuilding Process
Ecological succession is the predictable, orderly process of gradual community replacement over time:
- Primary Succession starts on bare, sterile rock where no soil has ever existed (such as after a basalt lava flow or underneath a melted glacier). Pioneer species like hardy lichens arrive as airborne spores, clinging to rock and secreting mild acids that weather minerals into fine dust. As lichens die and decompose, their organic matter mixes with pulverized rock to form the very first thin layer of soil. Mosses follow, then grasses, shrubs, and eventually a towering climax forest over thousands of years.
- Secondary Succession begins in an area where an existing ecosystem was disturbed by a forest fire, tornado, or logging, but the soil remains intact. Because nutrient-rich soil is already present, seeds of grasses and annual weeds germinate immediately. Within decades, fast-growing pines yield to mature hardwood oaks and maples, reaching a stable climax community much faster.
Native versus Non-Native Species
- Native Species: Organisms that naturally occur, evolved, and adapted within a specific geographic ecosystem over thousands of years (e.g., American bison on North American prairies).
- Non-Native (Introduced) Species: Organisms that have been transported by human activity—accidentally or intentionally—outside of their historical geographic range into a new environment (e.g., honeybees brought to North America from Europe). Many introduced species are harmless or beneficial.
Invasive Species: Ecological Disruptors
An invasive species is a non-native organism that, when introduced into a novel ecosystem, multiplies aggressively, spreads out of control, and causes severe environmental, economic, or human health damage.
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| WHY DO INVASIVE SPECIES MULTIPLY SO FAST? |
| |
| 1. ABSENCE OF NATURAL PREDATORS |
| - In their native homeland, specific predators, parasites, and |
| diseases keep their populations in check. In the new environment, |
| native predators do not recognize or eat them! |
| |
| 2. HIGH REPRODUCTIVE CAPACITY (R-Selected) |
| - Invasive species produce thousands of offspring or seeds rapidly. |
| |
| 3. AGGRESSIVE COMPETITIVE ADVANTAGE |
| - They outcompete native species for food, sunlight, water, space. |
| |
| 4. ENVIRONMENTAL GENERALISTS |
| - They tolerate wide ranges of temperature, pollution, and diets. |
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Infamous Invasive Species Examples
- Kudzu Vine ("The Vine That Ate the South"): Introduced to the southeastern United States from Japan in 1876 as an ornamental shade vine and erosion-control plant. In warm southern climates, kudzu grows up to a foot a day, smothering entire native forests, telephone poles, and abandoned houses beneath a dense blanket of leaves that blocks sunlight and kills native trees.
- Zebra Mussels: Native to Eastern Europe, microscopic zebra mussel larvae were transported across the Atlantic Ocean inside the freshwater ballast water tanks of cargo ships and discharged into the North American Great Lakes in 1988. Zebra mussels filter plankton so aggressively that they starve native fish, cover native clams, and clog municipal water intake pipes, causing billions of dollars in damage.
- Burmese Pythons: Native to Southeast Asia, pet pythons released into the Florida Everglades have multiplied into tens of thousands of giant apex predators. Because Florida's native mammals have no evolutionary experience with giant constrictor snakes, pythons have wiped out over 90 percent of marsh rabbits, raccoons, opossums, and foxes across Everglades National Park.
- Cane Toads: Introduced to Australia in 1935 to eat sugar cane beetles. The toads failed to eat the beetles, but multiplied into hundreds of millions. Because cane toads secrete deadly bufotoxin poison from their shoulder glands, native Australian predators (crocodiles, dingoes, monitor lizards) that bite them die within minutes.
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| HOW INVASIVE SPECIES DISRUPT FOOD WEBS |
| |
| BEFORE INVASION (Balanced): |
| Native Phytoplankton ---> Native Zooplankton ---> Native Fish |
| |
| AFTER ZEBRA MUSSEL INVASION (Collapsing): |
| Millions of Zebra Mussels devour ALL Phytoplankton! |
| --> Native Zooplankton starve to death |
| --> Native Fish populations collapse due to lack of food |
| --> Water becomes abnormally clear, causing toxic weed blooms |
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Controlling Invasive Species
Managing invasive species requires coordinated scientific strategies:
- Physical / Mechanical Control: Hand-pulling invasive weeds, hunting pythons, erecting electric barrier fences in rivers to block Asian carp.
- Chemical Control: Applying targeted herbicides or pesticides, though care must be taken to prevent poisoning native wildlife.
- Biological Control: Introducing a natural predator or parasite from the invasive species' native homeland. Biological control must be rigorously tested in quarantined laboratories for years to ensure the introduced control organism does not attack native species!
- Prevention and Policy: Inspecting shipping containers, requiring international cargo ships to exchange ballast water in open ocean, and banning the commercial import of dangerous exotic pets.
Exercises and Solutions
Exercise 1: Comparing Primary and Secondary Succession After a Wildfire. A massive forest fire sweeps through a pine forest in California, burning all trees to black ash but leaving a thick layer of mineral-rich soil and unburned underground roots. Explain whether the subsequent ecological recovery will be primary or secondary succession, describe the first pioneer plants that will appear, and explain why this recovery occurs faster than on cooled volcanic lava.
Solution 1: The ecological recovery will be secondary succession. Secondary succession occurs because the existing soil was not destroyed by the fire; soil, organic nutrients, and underground seed banks remained completely intact. The first pioneer plants to appear will be fast-growing annual grasses, wild forbs, and fireweed whose dormant seeds germinate quickly when sunlight strikes the ash-fertilized soil. This recovery occurs vastly faster than on cooled volcanic lava because primary succession on lava requires centuries for lichens and physical weathering to break bare solid rock into the very first millimeters of soil. In secondary succession, rich soil is already present, allowing the ecosystem to rebuild in decades rather than millennia.
Exercise 2: Analyzing the Impact of Zebra Mussels on Great Lakes Fisheries. Explain why zebra mussels were able to multiply from a few stowaway individuals into billions of mussels covering the Great Lakes, identifying two distinct ecological advantages they had over native mussels, and describe how their feeding behavior harms native game fish.
Solution 2: Zebra mussels multiplied aggressively because of the absence of natural predators and their high reproductive capacity. In their native European habitat, specialized fish and waterfowl eat zebra mussels, but North American freshwater fish did not recognize them as prey. Furthermore, a single female zebra mussel can release up to one million eggs annually. They possessed two distinct competitive advantages over native mussels: they produce sticky thread-like anchors (byssal threads) that allow them to attach to any hard surface (including smothering the shells of native mussels), and their microscopic veliger larvae swim freely in currents, dispersing across hundreds of miles. Their voracious filter-feeding behavior strips the water column of virtually all microscopic phytoplankton, the foundational food source for native zooplankton; without zooplankton, the larvae of native game fish (such as walleye and yellow perch) starve, causing commercial and sport fishery populations to collapse.