Photo by mbarisson licensed under CC BY-ND 2.0. — Photo by mbarisson **licensed under** **CC BY-ND 2.0**.

When you think of gardening, alligators don’t readily jump to mind. Hang out long enough in places like the Everglades and that might change. I was only recently introduced to the concept of a “gator hole” and I must say, I was surprised what a quick search of the literature revealed. It turns out that alligators are important ecosystem engineers and do a wonderful job at increasing plant diversity in the wetlands they inhabit.

Throughout southeastern North America, gators change their behaviors with the seasons. During the rainy season, alligators can be found floating in open water or sunning themselves on land. Except when hunting, they don’t seem to do anything with much urgency. Their activity level changes during the dry season when water is in short supply. Gators don’t sit back and let nature take its course. They spring into action and create their own aquatic refuges.

As the surrounding landscape begins to dry, gators will excavate holes or pits in the soggy ground called gator holes. These holes hold onto water when most of the surrounding landscape isn’t. The process of digging a gator hole may seem destructive but it all must be placed in the context of the surrounding environment. Most gator habitat exists in low lying areas. In places like the Everglades, there isn’t much topography to speak of. When a gator excavates a gator hole, it creates variation in both hydrology and soil conditions.

Photo by Anita Gould licensed under CC BY-ND 2.0. — Photo by Anita Gould **licensed under** **CC BY-ND 2.0**.

Soils that have built up over time are lifted out of the hole and piled into mounds. Mounded soils are not only rich in nutrients, they also dry at different rates, creating a gradient in water availability. Plants that normally can’t germinate and grow in saturated soils find suitable spots to live up on the soil mounds while emergent aquatic vegetation fills in along the parameter. Plants that normally prefer to grow in deeper water can also establish within the gator hole itself. In the midst of fairly uniform marsh vegetation, a gator hole quickly becomes a hotbed of plant diversity. The differences in vegetation can be so stark compared to the surrounding landscape that some scientists can actually map gator holes using aerial scans simply by measuring the differences in infrared radiation given off by the leaves of all the different plants that establish around them.

Of course, all of that plant diversity has a huge effect on other organisms as well. Gator holes become important areas for various reptiles, amphibians, birds, and so much more. The paths that alligators take to and from their holes even act like fire breaks, changing the way fire moves through the landscape, which only increases the heterogeneity of the immediate area. Fish, though occasionally eaten, greatly benefit from the stability of water levels within a gator hole. All in all, gator holes are extremely important habitats. Not only do they support a high diversity of plants and animals alike, they make places like the Everglades even more dynamic than they already are.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4]

If healthy ecosystems are what we desire, we must embrace predators. There is no way around it. Because of their meat-based diets, predators can have serious effects on plant diversity. Generally speaking, as plant diversity increases, so does the biodiversity of that region. It's not just large predators like wolves and bears either. Even predators as small as spiders can have considerable impacts on not only plant diversity, but ecosystem processes as well. Before we get to that, however, we should take a moment to review some of the background on this subject.

The way in which predators mediate plant diversity falls under a realm of an ecological science called top-down ecosystem controls. In a top-down system, predators mediate the populations of herbivores, which takes pressure off of the plant community. It makes a lot of sense as a numbers game. The fewer herbivores there are, the better the plants perform overall. However, ecology is never that simple. More and more we are realizing that top-down controls have less to do with fewer herbivores than they do with herbivore behavior.

Herbivores, like any organism on this planet, respond to changes in their environment. When predators are present, herbivores often become more cautious and change up their behavior as a result. Such is the case of grasshoppers living in fields. Grasshoppers are incredibly numerous and can do considerable amounts of damage to plant communities as they feed. Picture swarms of locusts and you kind of get the idea.

Photo by Andrew Cannizzaro licensed under CC BY 2.0

Given the choice, grasshoppers will preferentially feed on some plants more than others. Such was the case when researchers began observing grasshopper behavior in some old fields in Connecticut. The grasshoppers in this study really seemed to prefer grasses to all other plants. That is unless spiders were present. In this particular system lives a spider known as the nursery web spider (Pisaurina mira). The nursery web spider is an effective hunter and the fact does not seem to be lost on the grasshoppers.

In the presence of spiders, grasshoppers change up their feeding behavior quite a bit. Instead of feeding on grasses, they switch over to feeding on goldenrod (Solidago rugosa). Although the researchers are not entirely sure why they make this shift, they came up with three possible explanations. First is that the goldenrod is much more structurally complex than the grass and thus offers more places for the grasshopper to hide. Second is that goldenrod fills the grasshoppers stomach in less time thanks to the higher water content of the leaves. This would mean that grasshoppers had more time to watch for predators than they would if they were eating grass. Third is that the feeding behaviors of both arthropods allows the grasshopper to better keep track of where spiders might be lurking. It is very likely that all three hypotheses play a role in this shift.

Photo by Tibor Nagy licensed under CC BY-NC 2.0

It's the shift in diet itself that has ramifications throughout the entire ecosystem in question. Many goldenrod species are highly competitive when left to their own devices. If left untouched, abandoned fields can quickly become a monoculture of goldenrod. That is where the spiders come in. By causing a behavioral shift in their grasshopper prey, the spiders are having indirect effects on plant diversity in these habitats. Because grasshoppers spend more time feeding on goldenrods in the presence of spiders, they knock back some of the competitive advantages of these plants.

The researchers found that when spiders were present, overall plant diversity increased. This is not because the spiders ate more grasshoppers. Instead, it's because the grasshoppers shifted to a diet of goldenrod, which knocked the goldenrod back just enough to allow other plants to establish. It's not just plant diversity that changed either. Spiders also caused an increase in both solar radiation and nitrogen reaching the soils!

In knocking back the goldenrod, the habitat became slightly more open and patchy as various plant species of different shapes and sizes gradually established. This allowed more light to reach the soil, thus changing the environment for new seeds to germinate. Also, because goldenrod leaves tend to break down more slowly, they can have significant influences on nutrient cycles within the soil. As a more diverse set of plants establish in these field habitats, the type of leaf litter that falls to the ground changes as well. This resulted in an overall increase in the nitrogen supply to the soil, which also influences plant diversity.

In total, the mere presence of spiders was enough to set in motion these top-down ecosystem effects. It's not that spiders eat more grasshoppers, it's that they are changing the behavior of grasshoppers in a way that results in a more diverse plant community overall. This is a radically different narrative than what has been observed with examples such as the reintroduction of wolves to the greater Yellowstone ecosystem yet the conclusions are very much the same. Predators have innumerable ecosystem benefits that we simply can't afford to ignore.

Photo Credits: [1] [2] [3]