Why Native Plants?

How Native Plants Restore Terrestrial and Aquatic Habitats

Native Plants and Terrestrial Habitats

Reedy Creek Environmental has been tremendously influenced by the work of Dr. Doug Tallamy, an entomologist at the University of Delaware.  He is also the author of the book: “Bringing Nature Home: How You Can Sustain Wildlife With Native Plants”.  I highly encourage you to read the book and visit his website devoted to promoting the use of native plants: http://www.bringingnaturehome.net/ .

The major argument for increasing the use of native plants hinges on the fact that most insect herbivores are “specialists”.  The vast majority of butterfly, moth, beetle, and other insect adults are very picky about where they lay eggs.  The classic example is the monarch butterfly which only lays eggs on members of the milkweed genus, Asclepias.  But there are thousands of other examples.  Some insect species such as the zebra swallowtail butterfly only lays eggs on a single species of host plant.  In the case of the zebra swallowtail, that plant is pawpaw, Asimina triloba.  If pawpaw was eliminated from our environment, we would also lose zebra swallowtails.

But the consequences of replacing native vegetation with non-native plants goes far beyond any direct effect on a relatively small number of insect species dependent on a given host plant.  The negative impact reverberates through the entire food web and ecosystem.  Consider that most birds feed their young nestlings exclusively on caterpillars.  As the landscape is covered with turf grass and non-native oriental species, there are far fewer caterpillars available and the parents have to work harder and harder to feed their young.  Dr. Tallamy and his students have conducted a variety of field experiments showing that non-native plants support much less insect diversity as well as far fewer caterpillars than native plants.

Can’t the insects just “adapt” to using non-native plants?  NO!!  The behavior of insect specialists has resulted from millions of years of evolution alongside their plant hosts.  The current state of affairs reflects eons of chemical warfare waged between plants and insect herbivores.  Most plants produce chemicals designed to discourage leaf damage and each insect specialist has evolved mechanisms to cope with the specific combination of defense chemicals produced by its host plant.  In addition, insect specialists have evolved to be very adept at “finding” the right host plant which can be quite a “needle in the haystack” problem.

In summary, native plants underpin the entire food web and the insects that feed on plants are essential components supporting the larger animals.  Any replacement of native plants tears at the fabric of the terrestrial ecosystem by reducing insect diversity and biomass.  Any time you replace lawn or non-native plants with native species, you are restoring the local ecosystem.


Native Plants and Watershed Restoration

The link between native plants and watershed restoration requires a little background information about water quality, especially in urban and suburban areas.  As a former biologist with the Department of Environmental Quality (DEQ), I helped document subpar water quality in Reedy Creek, Upham Brook, Proctors Creek, Powhite Creek, Jordans Branch, Swift Creek, North Run, and other streams in the City of Richmond, Henrico County, and Chesterfield County.  I never observed acceptable water quality (based on DEQ criteria) in any urban or suburban stream with more than minimal development activity.

To understand the negative impact of traditional development, it is useful to first consider how a good quality stream and watershed function.  In a watershed with minimal human impact that is dominated by forest, approximately 90% of the rainwater that hits the watershed infiltrates into the ground.  From there, some of the water migrates into deep groundwater, some of the water migrates into the shallow groundwater, and some of the water remains available in the soil for plants.  The key is that only 10% of the water ever reaches the local stream directly via overland flow.  And the water entering the stream gets there over a relatively long period of time because the tree canopy, permeable soils, and complexity of the forest floor all act to slow down the flow of water.

By contrast, developed watersheds are covered by large amounts of impervious surfaces such as roads, rooftops, sidewalks, and parking lots as well as semi-impervious surfaces such as lawns. When rain hits these surfaces, most of the water quickly runs off and is usually collected in storm drains and/or ditches that are designed to transport the water to the nearest stream as quickly as possible.  As a result, a very large volume of water reaches the stream and it gets there rapidly.  In other words, the stream becomes very “flashy”.  Compounding the problem, the runoff picks up pollutants along the way such as oil, fertilizers, pesticides, herbicides, and pet waste.  This combination of excessive stormwater volume and high levels of pollutants takes a devastating toll on the living resources of a stream.

The long-term solution to water quality issues is to mimic healthy, natural systems to the extent practical.  For new development, that means preventing polluted runoff through low-impact development and green infrastructure.  For existing development, that means implementing best-management practices (BMPs) such as rain water harvesting (rain barrels and cisterns), Bayscapes, rain gardens, and tree planting.  In particular, the vegetative solutions increase stormwater infiltration and filter out pollutants so they never reach the local stream.

The diagram below shows why native plants are critically important in restoring healthy watersheds.  On the far left is a representation of turf grass.  Note that the root system of turf grass is extremely shallow – only a couple of inches deep.  That explains why turf grass needs constant watering in the summer as well as applications of fertilizer to look “good”.  Grass is unable to reach the water and nutrients located deeper in the soil.  By contrast, native perennial grasses and flowers have very deep root systems.  These native plants are drought resistant and do not require application of fertilizers.  In addition, those long roots provide channels for water infiltration deep into the ground.  Landscapes with thriving native plants are far superior to lawns when it comes to infiltrating rain water.

Image result for diagram of native plant root systems