Species Invasions and Competition
For many years, competition was one of the suggested reasons for major faunal turnovers. It was hypothesized that wily little mammals outcompeted the dinosaurs (or drove them to extinction) by eating the eggs out of the dino nests. Dinosaurs were believed to have earlier outcompeted the therapsids (mammal-like reptiles) as part of their rise to world dominance. And, of course, following the rise of the Isthmus of Panama, invading placental mammals drove the native South American population of Marsupial mammals to extinction.
How much evidence was there to support the competition hypothesis? Not much. As with many hypotheses, it was simply an interpretation of an observed pattern (i.e., we have dinos and small mammals below the K/T boundary, but only small mammals above the boundary). We know that the dinos went extinct. We know that the mammals survived and prospered. We interpret that the survivor was somehow better than the group which went extinct. In all of these cases there was little actual evidence of direct competition, and proponents of competitive displacement were hard put to offer convincing arguments which clearly showed how the survivors were adaptively superior.
Does this mean that competition never leads to extinction? No, because we have numerous, well documented examples of competitive displacement in the world today, but no convincing evidence that competition can lead to the displacement of entire groups of animals.
So let us examine characteristics of species invasions to see what allows them to occur, how they affect communities, and some examples of recent invasions.
Whether we are talking about zebra mussels, kudzu, mosquito fish or fire ants; successful non-indigenous invading species tend to share certain characteristics:
- they are pioneer species with short generation times and high reproductive rate (r-species).
- they exhibit vegetative, clonal, or single parent (i.e., gravid or pregnant female can colonize) reproduction.
- they have high dispersal rates.
- they can be long-lived.
- they have high genetic variability.
- they are phenotypically plastic.
- they are abundant in their native range which is broad.
- they are often gregarious, and are habitat generalists.
- they are polyphagous (i.e., have broad diets).
- they are often human commensals.
All the above characteristics can really be summed up by two general statements:
1) these species are characterized by fast reproduction which is unaffected by an initial small population size.
2) these species exhibit broad environmental tolerance.
While these characteristics make a species a potent potential invader, they also require an community ripe for invasion. In effect, the community has to have certain characteristics which allow it to be invaded, and increase the likelihood of the invasion's success. These characteristics include:
- the community's being climatically matched with the invader's original habitat.
- the community being in an early successional stage (i.e., mainly indigenous "r"-species).
- a low diversity of native species in the community.
- an absence of predators on the invading species.
- an absence of native species morphologically or ecologically similar to the invader.
- an absence of predators or grazers in the evolutionary history of the indigenous species ("naive prey").
- a low connectance food web.
- an anthropgenically disturbed environment.
Note that a community with these characteristics will in effect offer the invader a free ride, because it will face no predation, competition or defense from the indigenous species. Any community with these characteristics will be likely to exhibit large invasion effects from non-indigenous species with the characteristics listed above.
So how do these invading species affect the community? We can look at some very simple models (below) to see how food chains can be disrupted.
As noted in (A) a food chain based on a single plant can be totally destroyed due to the loss of that plant. Numerous Hawaiian bird species have gone extinct which were highly adapted to a single food source plant. Loss of the plant due to invasion of exotic plants (introduced by humans), or due to overgrazing by feral pigs and goats (introduced by humans) lead to the extinction of the birds which were totally dependant on them. While (B) and (C) illustrates cases of predator loss (B) or plant loss (C) with little affect on the overall community, (D) illustrates a case where the loss of a predator results in an explosion of the herbivorous prey population, leading to the extinction of two plant species due to extreme overgrazing. In both (A) and (D), the loss of a keystone species results in a number of secondary extinctions and the loss of diversity in the community.
We can now look at some examples of exotic (invading) species and their affect on the indigeneous community.
BURROS in the American Southwest
This is an interesting example because the Burros (Equus asinus) have a clear adaptive advantage over the Bighorn Sheep with whom them compete for food. The burros are actually descendants of the African Wild Ass which is endemic to arid regions between Algeria and Somalia in Africa. They were introduced to North America by Spanish explorers in the 16th Century as beasts of burden who could tolerate the dry, hot conditions of the American West, and were later widely employed by prospectors (who isn't familiar with the image of the prospector with his heavily laden burro?). Unfortunately, when the prospectors gave up they generally abandoned their burrows, which quickly gave rise to a large population of feral burros in California, Arizona and Nevada.
So what's the great adaptive advantage? Simple, the burros have a cecal (hindgut) digestive system which allows them to use a high fiber diet. They intially chew their food to shatter the cell walls, but leave the breakdown of the high fiber parts of the plant to take place in the intestine. Ruminants (cud-chewers), on the other hand, must break all their food down to very small size before they can be fully digested. High fiber plant material requires a number of trips from the mouth (chewing) to the rumen (digesting) and back again before the particles become small enough to pass through the gut. When dealing with high fiber but low nutritional value plant material the ruminants derive very little value from their food.
Now consider the variable diet of the burro. Two areas studied yielded the following data for burro diet:
Area Grasses Forbs Browse
1 4% 30% 61%
2 61% 11% 28%
Obviously very different, but it did not matter to the burros. Unfortunately for the ruminants in the area, the burros go after the low fiber, high nutrient plants first (which are the main food resource for the ruminants), and when those are gone they simply shift to the high fiber, low nutrient plants. The shift doesn't bother the burros, but the ruminants are in trouble. As a result, the ruminant population declines, and eventually (without human intervention), the burros would have driven the Bighorn Sheep to extinction.
THE BROWN TREE SNAKE
Here we have a case of a predator with no natural controls driving prey populations to extinction.
The Brown Tree Snake (Boiga irregularis) will reach lengths of up to 2 meters. The snake is native to Australia, north through New Guinea, but was introduced to Guam, the Mariana Islands and other parts of the western Pacific following World War II probably as a stowaway in military shipping containers.
Bird species bagan to disappear from the middle of the island of Guam in the 1960's, at about the same time the snake started to appear in large numbers. After a number of initial incorrect hypotheses (pesticides, disease, introduced rats, feral cats, etc.) the cause of the bird decline was attributed to the snake. The snake is nocturnal, a good climber of trees and apparently finds birds' eggs and chicks in the nest quite tasty. The birds were totally unaware of the danger (naive prey) and never had the time to develope an avoidance strategy.
The extinction of native birds on Guam has been so extensive that the snakes are now switching to native lizards as their main prey.
Preventing the snake from invading Hawaii has now become a major goal of conservationists.
THE OPPOSSUM SHRIMP
Our last example deals with the introduction of the Oppossum Shrimp (Mysis relicta) into Flathead Lake and its tributaries in Montana. The shrimp was introduced by people who thought that it would be a good additional food source for the Kokanee salmon, a game fish. Unfortunately, the shrimp disrupted the food chain by eating most of the zooplankton in the lake (copepods and cladocerans) which actually caused a crash in the salmon population, which then caused a crash in the Bald Eagle population (which preyed on the salmon as their main food source - note graph in lower right corner of figure below).
Most modern exotic species problems have been caused inadvertently by humans. However, they do give us insight into the possibility of competitive interactions leading to the extinction of species. The burro example clearly illustrates an adaptive advantage of one species over others in a given environment, while the Brown Snake illustrates the introduction of a new predator among naive prey, and the Oppossum Shrimp illustrates the disruption of a food web with the introduction of a new species.