One of the persistent questions in our interminable debate with native plant advocates is whether or not native vegetation provides superior habitat for wildlife compared to existing non-native vegetation. At the heart of that question is the closely related question of whether or not more insects are found in native vegetation than in non-native vegetation. That’s because insects (and other arthropods) are near the bottom of the food web. If there are fewer insects, there are probably fewer birds and other animals that eat insects. We have told our readers about many studies that find equal abundance and diversity of insects in native compared to non-native vegetation, so we won’t repeat them, but here’s a brief list of those studies and links to them for new readers:
- Dov Sax (Brown University) found equal diversity of plant and animal species in native oak woodland compared to eucalyptus forest in Berkeley, California.
- Arthur Shapiro (UC Davis) reported that many species of California native butterflies use both native and non-native plants and some no longer have native host plants.
- Robert Stebbins (UC Berkeley) reported finding significant numbers of insects and vertebrates in eucalyptus forest in the East Bay.
- Igor Lacan and Joe McBride (UC Berkeley) found equal abundance of benthic micro-organisms in riparian eucalyptus forest compared to native riparian woodland.
- The California Academy of Sciences in San Francisco found equal diversity of insect orders in all quadrants of their “living roof.” Two quadrants were dominated by non-native plants and two by native plants.
Does “restoration” of native vegetation increase insect populations?
In this post we will consider this issue from a slightly different angle: can insect population or diversity be increased by “restoration” of native vegetation? Even if we accept the premise of native plant advocates that native vegetation supports greater abundance and diversity of insects, can that population be “restored” by eradicating non-native vegetation and replacing it with native vegetation? That question is answered with a resounding “NO” by a study that compared arthropod abundance and diversity in undisturbed (predominantly native vegetation), disturbed (predominantly non-native vegetation), and disturbed sites 5 and 15 years after restoration. (1) Restoration methods described in the study are mowing followed by disking and seeding, disking and seeding, planting of container stock, and clearance by hand. All sites were irrigated initially. No mention is made of herbicide use or prescribed burns to eradicate non-native vegetation. The vegetation type in all 15 sites in Southern California was coastal sage scrub. This is the dominant vegetation type along the coast of California and is the goal of many restoration projects in the San Francisco Bay Area. Many species of both native and non-native vegetation in the study sites also exist in the Bay Area.
The study used pitfall traps to collect arthropods in these sites. Arthropods are invertebrates that include insects, arachnids (spiders), and crustaceans (aquatic species not relevant to this study). Arthropods are further divided into guilds such as herbivores, predators, scavengers, and parasites. Because of the method of collecting in pitfall traps, few herbivores were found. Here are some of the findings of this study:
- “Arthropod diversity at undisturbed and disturbed sites was greater than at sites that were 5 and 15 years following restoration.”
- “Number of arthropod species was not significantly different among undisturbed, disturbed, and restored sites.”
- “Vegetation at disturbed and undisturbed sites differed significantly; older restorations did not differ significantly from undisturbed in diversity, percent cover, or structural complexity.”
- “Vegetation characteristics did not differ significantly between the newly restored site and disturbed sites.”
- “…arthropod communities at all restored sites were, as a group, significantly different from both disturbed and undisturbed sites.”
- “As found in other studies of other restoration sites, arthropod communities are less diverse and have altered guild structure.”
Here is the concluding discussion of this study:
“Of the restoration sites sampled, none had developed an arthropod community that resembled undisturbed or disturbed native coastal sage scrub. Restoration sites in general exhibited lower arthropod diversity and a preponderance of exotic arthropod species. The time elapsed since revegetation effort had no discernible effect on arthropod community structure; there was no gradual return of the community to a more natural structure over time”.
“Restorations” do not improve arthropod abundance or diversity
This study found that arthropod population and diversity was the same in disturbed (non-native) and undisturbed (native) vegetation. When disturbed vegetation was “restored” arthropod population was maintained but the composition of the arthropod community was significantly changed even 15 years after the restoration was completed. There were more “exotic” species of arthropods in the restored sites even though the vegetation was similar to the undisturbed sites of native vegetation. The restored vegetation was native, but its arthropod occupants weren’t.
However, the birds and other animals that prey on those insects don’t care if the insects are native or non-native. Much like humans, animals are not concerned with the nativity of their food. The non-native apple you are eating is just as tasty whether you are eating it in its native range in Central Asia or where it has been introduced. If you have an apple tree, you know the birds and squirrels enjoy the apples too and the bees and other pollinators enjoy the apple blossoms. Most of what we eat is not native, yet many people are obsessed with the nativity of vegetation, claiming that animals require native vegetation even though humans don’t.
An important caveat
The predominant vegetation type in the San Francisco Bay Area is coastal scrub, which is also the vegetation type in the study of arthropod populations. This suggests that if a similar study were conducted here, the results might be similar. However, there is one very important difference between the restorations studied in Southern California and the restorations in the Bay Area. Land managers in the San Francisco Bay Area are using large amounts of herbicides to destroy non-native vegetation. The study in Southern California reports no herbicide use in restoration sites. It seems likely that herbicides sprayed in restoration projects in the Bay Area would decrease the population of arthropods. We would like to see a study that tests that hypothesis.
There is more to an ecosystem than plants
The veneration of native plants has become a national obsession. Demands for eradication of non-native plants are supported by many fictions to justify these destructive projects. One of those fictions is that wildlife requires native vegetation. We have found no empirical evidence to support that assumption. The study we are reporting today is yet more evidence that restoring native plants does not restore an ecosystem. In this case, after 15 years of effort, land managers were eventually successful in establishing a population of native plants. However, these “restored” native landscapes did not support a population of insects and spiders that were comparable to either the undisturbed native landscape or the unrestored non-native landscape. We have been looking for some legitimate reason to engage in these destructive projects for over 15 years. We have yet to find any justification for spraying our public lands with herbicides or destroying hundreds of thousands of healthy trees. We will keep looking.
(1) Travis Longcore, “Terrestrial Arthropods as Indicators of Ecological Restoration Success in Coastal Sage Scrub (California, USA),” Restoration Ecology, December 2003, Vol. 11 No 4, pp.397-409