A casual look at the world of plants and insects may suggest that nothing much is going on, but the constant struggle is anything but quiet. In this “eat, fight, or be eaten” world, scientists continue to learn how species’ various defense mechanisms, both direct and indirect, have emerged concurrently in life’s history to yield balanced, healthy ecosystems. These symbiotic relationships—sometimes involving multiple species—provide evidence for a Creator’s intelligent agency.
Symbiosis refers to a close association of two or more distinct species of life that mutually benefit one another at a cost to each of them that is less than the benefits. We’ve all seen the common example of bees and fruit trees. Bees grow a lot of long hairs that they do not directly need. These hairs are designed so that the pollen of fruit tree flowers sticks to them and rubs off when the bees encounter the tissues of flower pistils. Fruit tree flowers are designed with pistils that provide a nectar reward to visiting bees and an aroma that attracts them. The bees benefit from these nectar rewards and the fruit trees benefit from the bees pollinating their flowers. Each species pays a price for their mutualistic relationship but receives much more than what they pay for. In the case of the bees and fruit trees the symbiosis is obligate: neither species would survive without the other. Both must be created at the same time and place.
Symbiosis Studies Reveal Optimization
In a recent issue of Ecology Letters a team of ten ecologists reviewed years of scientific literature on plant-predator symbiosis.1 They pointed out that the complex and indirect nature of plant-predator symbiosis led their peers to conclude that it must be rarely manifested. However, their survey of the scientific literature led them to conclude that this indirect symbiosis was not rare but in fact, appears to be ubiquitous or nearly ubiquitous.
Plant species are constantly at risk of being wiped out by herbivores. This risk is not only a problem for plants but also for herbivores. If herbivores wipe out plant species by overconsumption, they lose their food sources and join the plant species in extinction.
Scientists have learned that ecological optimization pervades life history. The relationships among microbes, vegetation, herbivores, carnivores, parasites, and detritivores (animals that feed on decaying plant and animal tissues) appear to be consistently optimized for the benefit of all life. It takes a Mind to know what specific designs in one species would benefit that species and simultaneously maximally benefit all other species in the diverse categories of Earth’s life.
Defenses against Being Eaten
Ecologists have observed how plant species around the world possess direct defenses against herbivores. These defenses are optimally designed so that plants can survive herbivory while preventing the herbivores from overgrazing and suffering population collapses. Examples include the thorns and spikes on cacti, hair on thistles, bad-tasting chemicals in conifer needles, waxy coatings on the leaves of ice plants, and toxins in tobacco and tea leaves. Plant species that are the slowest growing and most exposed to environmental stresses and, hence, least tolerant to aggressive herbivory, possess these direct defenses to the greatest degree.
In a few plant species, ecologists observe far more complex indirect defenses. Here, a certain plant species will reward specific predators that feed on the herbivores that feed on the plants. These reward features are designed to ensure that enough predators remain in the vicinity of the plants to prevent herbivores from overgrazing.
A well-known example of a sophisticated indirect plant defense can be seen in flowering plants that grow extrafloral nectaries—sugar-producing glands outside of the flowers. These extrafloral nectaries serve no purpose in the plant reproduction. Rather, they are designed to attract and feed certain species of insects. These insects on the plants act as bodyguards, preventing other herbivores from feeding on other parts of the plants that would threaten the plants’ survival.2
Another well-known example involves plant species that provide homes for spiders that, in turn, feed on the insects that threaten to overgraze the plants. In a review written twenty-two years ago, a team of four ecologists wrote that such indirect plant defenses, rather than being a secondary line of defense against herbivores, rank as the most potent defense line.3
The team of ten ecologists discovered that examples of indirect plant defenses are far more varied and complex than previously thought. In their scientific literature review, they found diverse plant species that emit aromas to attract predator species that prey on the herbivores that feed on the plants. They also found equally diverse plant species that grow hair tufts that house predatory mites or insects. For some plants, the hairs are sticky and trap insects, which attracts predators that then also feed on other herbivores attempting to graze on the plants. They also noted that many tree and bush species are architecturally designed to make it easier for birds to attack the herbivores feeding on the trees and bushes.
Complex Indirect Symbiosis Appears to Be Designed
As a result of their study, the ten ecologists concluded that complex designs were far more numerous than anyone realized. Complex indirect plant defenses, rather than being rare and manifesting only one or two defense designs, were likely ubiquitous and manifesting multiple independent indirect defense designs. They noted that research on indirect plant defenses was nowhere near exhaustive. Thus, they anticipated that many more indirect plant defense designs remain to be discovered and that if ecologists are diligent they will find that indirect plant defenses are indeed ubiquitous.
Such continued research will yield practical results and food for thought. A greater understanding and application of indirect plant defenses could provide farmers with the means to protect their crops from herbivore pests without the use of pesticides. This new understanding of the diversity, complexity, and pervasiveness of indirect plant defenses provides yet more evidence that a Mind lies behind the ecological optimization observed throughout life’s history.
Featured image: Extrafloral Nectaries Attracting Predator Ants
Image credit: Schwitzke et al., Anthropod-Plant Interactions 9 (2015): 497–505
- Ian S. Pearse et al., “Generalising Indirect Defence and Resistance of Plants,” Ecology Letters 23, no. 7 (July 2020): 1137–52, doi:10.1111/ele.13512.
- Barbara L. Bentley, “Extrafloral Nectaries and Protection by Pugnacious Bodyguards,” Annual Review of Ecology and Systematics 8 (November 1977): 407–27, doi:10.1146/annurev.es.08.110177.002203.
- Arne Janssen et al., “Review Behaviour and Indirect Interactions in Food Webs of Plant-Inhabiting Arthropods,” Experimental & Applied Acarology 22 (September 1998): 497–521, doi:10.1023/A:1006089924336.