How Spiders Increase Plant Diversity

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.

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.

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]

Further Reading: [1] [2]

 

The Accidental Grain - How Rye Evolved Its Way Into Our Diet

Humans have been altering the genomes of plants for a very long time. Nowhere is this more apparent than in the crops we grow. These botanical mutants are pampered beasts compared to their wild congeners. It is easy to see why some traits have been selected over others, whether it be larger leaves or fruit to munch on, smaller seeds to keep them out of our way, or tough rinds to make shipping easier. However, not all of our crops have been consciously bred for our consumption. Just as many weed species are adapting to herbicides today, some species of plant were able to adapt to the more archaic methods of early farming, which allowed them to avoid the ever watchful eye of the farmer.

This concept is known as Vavilovian mimicry (sometimes referred to as crop mimicry) and it is named after the Soviet botanist and geneticist Nikolai Vavilov (who was later imprisoned and starved to death by Stalin because of his firm stance on basic genetic principles). The idea is rather simple. At its core it involves artificial selection, albeit unintentional. A wild plant species finds certain forms of agriculture appealing. It becomes an apparent weed and the farmer begins to deal with it. Perhaps this plant is a close relative and thus looks quite similar to the crop in question. As the farmer weeds out plants that look different from the crop, they may be unintentionally selecting for individual weeds that more closely resemble the crop species. Over enough seasons, only those weeds that look enough like the crop survive and reproduce, sometimes to the point in which the two are almost indistinguishable.

Rye is an interesting example of this idea. Wild rye (Secale montanum) was not intentionally grown for food. It was a weed in the fields of other crops like wheat and barley. Both wheat and barley are annual plants, producing their edible seeds at the end of their first growing season. Wild rye, however, is a perennial and does not produce seed until at least its second season. Therefore, most wild rye plants growing in wheat or barely fields are killed at the end of the season when the field gets tilled. However, there are some mutant rye plants that occasionally pop up and produce seeds in their first year.

It is believed that these mutant annual rye were harvested unintentionally and reseeded season after season. Over time, other traits likely developed to help push rye into the spotlight for these early farmers. Like many wild grasses, wild rye has weak spindles (the part that holds the seed to the plant). In the wild, this allows for efficient seed dispersal. On the farm, this is not a desirable trait as you end up quickly losing the seeds you want to harvest. Again, by accidentally selecting for mutants that also had thicker spindles and thus held on to their seeds, farmers were unintentionally domesticating rye to parallel other cereal crops. It is believed that oats (Avena sterilis) also originated in this manner.

Photo Credit: Lotte Grønkjær (http://bit.ly/1xMEfVw)

Further Reading: [1] [2]