Death by Crab Spider, part two

Crab spiders that hunt in flowers prey on pollinating insects. Thus, pollinating insects tend to avoid flowers that harbor crab spiders. We established this in part one. Now we ask, what effect, if any, does this interaction have on a crab spider infested plant’s ability to reproduce? More importantly, what are the evolutionary implications of this relationship?

In a study published in Ecological Entomology earlier this year, Gavini, et al. found that pollinating insects avoided the flowers of Peruvian lily (Alstroemeria aurea) when artificial spiders of various colors and sizes were placed in them. Bumblebees and other bees were the most frequent visitors to the flowers and were also the group “most affected by the presence of artificial spiders, decreasing the number of flowers visited and time spent in the inflorescences.” This avoidance had a notable effect on plant reproduction, namely a 25% reduction in seed set and a 15% reduction in fruit weight. The most abundant and effective pollinator, the buff-tailed bumblebee, was deterred by the spiders, leading the researchers to conclude that, “changes in pollinator behavior may translate into changes in plant fitness when ambush predators alter the behavior of the most effective pollinators.”

Peruvian lily (Alstroemeria aurea) via wikimedia commons

But missing from this discussion is the fact that crab spiders don’t only eat pollinators. Any flower visiting insect may become a crab spider’s prey, and that includes florivores. In which case, crab spiders can benefit a plant, saving it from reproduction losses by eating insects that eat flowers.

In April of this year, Nature Communications published a study by Knauer, et al. that examined the trade-off that occurs when crab spiders are preying on both pollinators and florivores. Four populations of buckler-mustard (Biscutella laevigata ssp. laevigata) were selected for this study. Bees are buckler-mustard’s main pollinator, and in concurrence with other studies, they significantly avoided flowers when crab spiders were present.  Knauer, et al. also determined that bees and crab spiders are attracted to the same floral scent compound, β-ocimene. This compound not only attracts pollinators, but is also emitted when plants experience herbivory, possibly to attract predators to come and prey on whatever is eating them.

buckler-mustard (Biscutella laevigata) via wikimedia commons

In this study, the predators called upon were crab spiders. Florivores had a notable impact on plants in this study, and the researchers found that when crab spiders were present, florivores were significantly reduced, thereby reducing their negative impact. They also noted that “crab spiders showed a significant preference for [florivore-infested] plants over control plants.”

And so it is, a plant’s floral scent compound attracts pollinators while simultaneously attracting the pollinator’s enemy, who is also called in to protect the flower from being eaten. Luckily, in this case, buckler-mustard is easily pollinated, so the loss of a few pollinators isn’t likely to have a strong negative effect on reproduction. As the authors write, “pollinators are usually abundant and the low number of ovules per flower makes a few pollen grains sufficient for a full seed set.”

crab spider on zinnia

But none of these studies are one size fits all. Predator-pollinator-plant interactions are still not well understood, and there is much to learn through future research. A meta-analysis published in the Journal of Animal Ecology in 2011 looked at the research that had been done up to that point. Included were a range of studies involving sit-and-wait predators (like crab spiders and lizards) as well as active hunters (like birds and ants) and the effects of predation on both pollinators and plant-eating insects. They concluded that where carnivores “disrupted plant-pollinator interactions, plant fitness was reduced by 17%,” but thanks to predation of herbivores, carnivores helped increase plant fitness by 51%. This suggests that carnivores, overall, have a net positive effect on plant fitness.

Many pollinating insects have an advantage over plant-eating insects because they move quickly from flower to flower and plant to plant, unlike many herbivores which move more slowly. This protects pollinators from predation and helps explain why plant-pollinator interactions are not disrupted as easily by carnivores. Additionally, as the authors note, “plants may be buffered against loss of pollination by attracting different types of pollinators, some of which are inaccessible to carnivores.”

But again, there is still so much to discover about these complex interactions. One way to gain a better understanding is to investigate the effects of predators on both pollinators and herbivores in the same study, since many of the papers included in the meta-analysis focused on only one or the other. As far as crab spiders go, Knauer, et al. highlight their importance in such studies. There are so many different species of crab spiders, and they are commonly found on flowers around the globe, so “their impact on plant evolution may be widespread among angiosperms.”

In other words, while we still have a lot to learn, the impact these tiny but skillful hunters have should not be underestimated.

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Grasshoppers – More Friend Than Foe?

Major outbreaks of grasshoppers can be devastating. A plague of locusts of biblical proportions can decimate crop fields and rangelands in short order. Clouds of grasshoppers moving in and devouring every plant in sight makes it easy to see why these insects are often seen as pests. Even small groups of them can do significant damage to a garden or farm. Yet, grasshoppers and their relatives have great ecological value and are important parts of healthy ecosystems. Love them or hate them, they are an essential piece of a bigger picture.

Grasshoppers are in the order Orthoptera, an order that includes katydids, crickets, wetas, and a few other familiar and not so familiar insects. Worldwide, there are more than 27,000 species of orthopterans. These insects mostly feed on plants; many are omnivorous while others are exclusively herbivorous. They are most commonly found in open, sunny, dry habitats like pastures, meadows, disturbed sites, open woods, prairies, and crop fields. Most insects in this order are fairly large, making them easy to identify; yet they don’t seem to receive the same level of human attention that charismatic insects like bees and butterflies do. In Field Guide to Grasshoppers, Katydids, and Crickets of the United States, the authors defend this diverse group of arthropods: “Grasshoppers often are thought of as modest-looking brown or green insects, but many species in this family are brightly colored, and some of the most dull-colored species rival butterflies in beauty when they spread their wings in flight.”

photo credit: wikimedia commons

photo credit: wikimedia commons

The voracious appetite of grasshoppers and their preference for plants can influence ecosystems in many ways. Certain plants may be favored over others, which affects the diversity and distribution of plant communities. Grasses are a particular favorite, despite being high in hard to digest compounds like lignin, cellulose, and silica. As grasshoppers consume vegetation – up to their body weight per day – digested materials return to the soil where soil dwelling organisms continue to break them down. In this way, grasshoppers and their relatives are major contributors to nutrient cycling. Returning nutrients to the soil results in increased nutrient availability for future plant growth. In fact, one grassland study found that despite short-term losses via grasshopper herbivory, plant growth was enhanced in the long-term due in part to accelerated nutrient cycling.

Because grasshoppers are such prolific consumers, their robust bodies are loaded with nutritious proteins and fats, making them a preferred food source for higher animals. Reptiles, raccoons, skunks, foxes, mice, and numerous species of birds regularly consume grasshoppers and related species. While many adult birds feed mostly on seeds and fruits, they seek out insects and worms to feed their young. Nutrient-packed grasshoppers are an excellent food source for developing birds. Humans in many parts of the world also find grasshoppers and crickets to be a tasty part of their diet.

Grasshoppers provide food for other invertebrates as well. The aforementioned field guide refers to the fate of grasshoppers and certain species of blister beetles as being “intimately linked,” because the larvae of these blister beetles feed exclusively on grasshopper eggs. Several species of flies and other insects, as well as spiders, also feed on grasshoppers and other orthopterans.

grasshopper on blade of grass

In short, grasshoppers play prominent roles in plant community composition, soil nutrient cycling, and the food chain. When grasshopper populations reach plague proportions, their impact is felt in other ways. From a human perspective, the damage is largely economic. However, their ability to thoroughly remove vegetation across large areas can be environmentally devastating as well, particularly when it comes to soil erosion and storm water runoff. The USDA’s Agricultural Research Service considers grasshoppers “among the most economically important pests” and cites research estimating that they are responsible for destroying as much as 23% of available range forage in the western United States annually. A paper published in the journal, Psyche, references a period between 2003-2005 in Africa where locusts were responsible for farmers losing as much as 80 to 100% of their crops.

This level of devastation is relatively rare. In Garden Insects of North America, Whitney Cranshaw states that of the more than 550 species of grasshoppers that occur in North America, “only a small number regularly damage gardens…almost all of these are in the genus Melanoplus.” Like most large, diverse groups of organisms, many grasshopper species are abundant and thriving while others are rare and threatened. Human activity has benefited certain species of grasshoppers while jeopardizing others. In general, grasshopper populations vary wildly from year to year depending on a slew of environmental factors.

Differential grasshopper (Melanoplus differentialis) - one of the four grasshoppers that Whitney Cranshaw lists as "particularly injurious" in his book Garden Insects of North America. (photo credit: www.eol.org)

Differential grasshopper (Melanoplus differentialis) – one of the four grasshoppers that Whitney Cranshaw lists as “particularly injurious” in his book Garden Insects of North America. (photo credit: www.eol.org)

A plague or outbreak of grasshoppers is a poorly understood phenomenon. It seems there are too many factors at play to pin such an occasion on any one thing. Warm, sunny, dry weather seems to favor grasshopper growth and reproduction, so drought conditions over a period of years can result in a dramatic increase in grasshopper populations. But drought can also limit plant growth, reducing the grasshoppers’ food supply. Natural enemies – which grasshoppers have many – also come into play. It seems that just the right conditions have to be met for an outbreak to occur – a seemingly unlikely scenario, but one that occurs frequently enough to cause concern.

Grasshoppers and fellow orthopterans are fascinating insects, and their place in the world is worth further consideration. For an example of just how compelling such insects can be, here is a story about crickets from Doug Tallamy’s book, Bringing Nature Home:

“Male tree crickets in the genus Oecanthus attempt to lure females to them by making chirping songs with their wings. The loudest male attracts the most females, so males often cheat a bit by positioning themselves within a cup-shaped leaf that amplifies the song beyond what the male can make without acoustical help. Each male chews a hole in the center of his cupped leaf that is just large enough to accommodate his raised wings during chirping. This ensures that the sound projects directly from the center of the parabolic leaf for maximum amplification.

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