So what impact do these plants have on native plants? One way to answer this question is to measure how much of the ground is covered by each species of plant so their percentages of ground cover can be compared.   

This is an aerial of Crawford Island. We’ve done research there to determine which species of plants dominate the dredge spoil, and what the diversity of species is. The task is simple; just imagine a line across the island. We call this line a transect. Then stretch a tape measure across it as in this slide, and simply record how many centimeters are intercepted above and below by each species at zero to 1, 1 to 2, 2 to 3, and greater than 3 meters. We took size data so we can measure how many new plants are growing, and to describe the understory vegetation beneath different types of trees. 

When the data are plotted on a graph it looks like this. On the horizontal axis we have the width of the island in meters. On the vertical axis we have the percentage of each meter that covered by native vegetation less than 1 meter in height, and by non-native vegetation greater than 3 meters in height. 

Native vegetation was depicted at this height because it represents replacement plants. Three important factors are apparent from this graph:
1. Native vegetation is more open, allowing access by ground-foraging animals like birds and reptiles 2. Non-native vegetation tends to dominate at 100% cover, choking out all other species. 
3. Where non-native plants comprise the canopy, there is almost no native vegetation beneath it. For approximately 100 meters, nearly half the width of the island, there is no native vegetation replacing itself. This area is utterly dominated by Australian pine, to the detriment of every other species found on the island. 

Next is a photograph of this destroyed habitat near meter number 165. In this picture, and the one that comes after it, we can see how Australian pine completely dominates the landscape. Notice how the fallen needle-like branches cover the ground like a carpet. These branches release toxic chemicals into the soil that prevent other plants from being able to grow beneath the Australian pines. This is call allelopathy, and in this way the Australian pines control or eliminate competition by other plants. The chemicals released by Australian pines are known to be carcinogenic. The effect of these chemicals leaching into the lagoon is unknown.   

The smaller dead stems that you see are the native Florida privet or wax myrtle, and have probably been killed by a combination of shading and allelopathy. Brazilian pepper also releases poisonous compounds into the soil. 

There are of course people who like these trees. And in this slide we see pelicans roosting in them. One of the arguments I’ve heard is that pelicans need these trees to rest in. But we have to remember that pelicans were here for many thousands of years before the Australian pines were and they’ll do just fine in native trees, as they have for millennia. 

This is a snowy egret walking across the top of that algae bloom we have in Jones Creek behind 4^th Street South in April of 2007. It’s actually using this algae bloom as habitat, finding fish and small snails to feed on. 

In the next slide we see an endangered manatee actually feeding on this algae. These are two protected species using this algae bloom, but I don’t think anyone would argue that this algae is worth preserving just because manatees are eating it. It’s much the same case with Australian pine. It’s there so pelican roost in it, but they don’t require it. Australian pine feeds nothing and really provides no habitat.

Continued Here