DOMINO EFFECTS AND SECONDARY EXTINCTIONS
Lecture 8 NRE 220
Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 2: pages 35-53
How does the removal of one species (an extinction event) or the addition of one species (an invasion event) affect the entire biological system ? Are there "ripple" or domino effects ? This question opens up the entire issue of the "balance of nature" and the structure of communities.
Examples of domino extinctions:
seeds of the tree Calvaria major, now found exclusively on the island of Mauritius, must pass through the abrasive gut of a large animal in order to germinate. None of the animals currently on Maritius have that ability. The dodo (25 kg pigeon), hunted to extinction in the late 17th century, probably was the key to recruitment in this species. Only a few very old trees now survive.
blackfooted ferret was once very abundant in the western prairies. It preyed upon prairie dogs and used their burrows to nest in. Poisoning of prairie dogs has greatly reduced their abundance, and the blackfooted ferret is now the rarest mammal in North America.
Polynesians exterminated a number of large land birds (moas) in New Zealand. A large eagle went extinct also, probably due to the loss of their prey.
On the other hand, these aren't very subtle examples, and there aren't many examples of domino extinctions. In perhaps the majority of cases, no obvious consequences are known. The American chestnut is virtually extinct due to the blight disease, with apparently no secondary effects. One might expect the loss of associated insects. Ditto for passinger pigeon.
Perhaps domino extinctions aren't yet common. Perhaps the coming wave of tropical extinctions will have greater domino effects. Domino extinctions may be hard to document. We need to understand how secondary extinctions can occur. Their basis lies in ecological studies of species interactions.
Food webs and Species Interactions:
Food webs are the beginning point of the story. The connections of herbivore with plant, predator with prey, and competitors within a trophic level provide many pathways for one species to affect others directly and indirectly.
starfish in the rocky intertidal zone feed preferentially on mussels. In the absence of predators, mussels occupy all of the available space and a simpler community results. Predation enhances diversity in this system, and its removal has far-reaching effects. Origin of term "keystone" predator.
kangeroo rats in southwestern deserts: desert plants are strongly influenced by the consumers of their seeds -- several species of ants and several species of kangaroo rats. Experiments show that the largest rat influenced the numbers of all of the smaller species of rat, several of the ants, and significantly altered the vegetation. Because large rats selectively consume large seeds, which are from the largest plants, removal of the large kangaroo rat turns a grassland into a region of shrubs.
strong and weak interactions:
These examples show how species interactions can result in subtle and unpredictable influences on biological communities. When one species had profound effects on many others, disproportionate to its abundance, it is referred to as a keystone species. However, the majority of species are not keystone species and domino extinctions may not result from their disappearance.
1. a domino effect occurs when a significant change in the abundance (or presence) of one species results in further change to the species composition or relative abundance of other species in a community. aka "ripple effect" and "secondary extinction". From a conservation perspective, how does the removal of one species (an extinction event) or the addition of one species (an invasion) influence the entire biological system?
2. a keystone species is one whose presence and activities strongly affect other species in the community. Classic example is starfish study by Paine. Influence typically is via food web interconnections (e.g. starfish, kangaroo rat) but can be through habitat modification (elephant).
3. Evidence based on extinction events is surprisingly sparse (e.g., dodo/calvaria major, prairie dog/blackfooted ferret). But why not passenger pigeon and American chestnut?
4. Evidence based on invasion events is plentiful (e.g., mysid shrimp in Flathead Lake)
Does complexity promote stability?:
Once considered almost a theorem of ecology that "complexity promoted stability". A food web with many species seemingly provides more alternative pathways, so each predator has alternative prey species. Pest outbreaks seemed more frequent in agricultural systems, which are biological simpler and less diverse than natural meadows. The species-rich tropics seemed to have few pest outbreaks, whereas the species-poor polar regions were known for their oscillations in population abundance -- lemmings and arctic hare, for example.
Recent experimental evidence (coursepack, week 2) is supportive. When drought affected Minnesota meadows, productivity declined most in least diverse plots. Plant production was greater in most species-rich assemblages grown in UK greenhouses. More diverse systems may be more productive and more resilient. Hence loss of species may cause lowering of resilience and productivity.
Transparencies: 1. starfish and its community 2. Hypothetical food web, 3. Flathead Lake ecosystem. Slides: rocky intertidal, kangaroo rat.