Ants used to scapegoat our urban forest

Ants are important members of the ecosystem.  They improve the fertility and consistency of the soil.  They distribute plant seeds.  They are both predators of and food for other insects as well as birds and omnivorous mammals.  Therefore, their abundance in an ecosystem is often considered an indicator of its health.

Today we will report on a study of ant populations in San Francisco’s “natural areas,” parks that were designated over 15 years ago for restoration and preservation of native plants.  This study reaches this conclusion:

“The results of this study indicate that natural areas within urban parks play a critical role in supporting ant biodiversity. Many habitats in the natural areas of San Francisco’s parks support healthy, diverse ant communities. Areas of non-native forest, however, reduce this diversity. Maintaining open grasslands, reducing tracts of non-native forest, removing the invasive understory, and thinning forest canopy may all help support a healthier ant community and ecologically valuable parks.”  (emphasis added) (1)

Could this ant study be the first example we have found of evidence that native plants benefit wildlife and conversely that our non-native urban forest is less valuable for wildlife?  We have examined this study to determine how it reached this conclusion.  We have compared this study to similar studies that report different findings.  We reached the conclusion that this study does not support its conclusion that “reducing tracts of non-native forest…may…help support a healthier ant community and ecologically valuable parks.” 

The relationship between ant communities and soil moisture

The ant study used pitfall traps to survey the abundance and diversity of ant populations in 24 “natural areas.”  It also measured the moisture of the soil in proximity of the traps.  The ant study found that soil moisture and ant abundance and diversity were positively correlated at low levels of moisture, but that high levels of moisture found in eucalyptus forest were negatively correlated with abundance and diversity of ants: 

We will tell you how this ant study used this empirical observation of the relationship between soil moisture and ant populations to reach its conclusion that non-native forests must be “reduced” to achieve “ecologically valuable parks.” 

Generalizing about “urban forests”

This study of San Francisco’s ant population asks us to believe that its negative assessment of San Francisco’s urban forest applies to all urban forests:  “Urban forests are structurally different than natural forests.  Besides being smaller, fragmented, and more isolated than non-urban forests, urban forests also show increased canopy cover, greater disturbances due to human traffic and pollution, and differences in leaf litter accumulation.” (1)

We don’t think it is possible to generalize about all urban forests.  Here are two sources which suggest that the ant study has over-generalized about urban forests and ants found in them:

• According to the US Forest Service survey of urban forests, San Francisco has one of the smallest tree canopies in the country.  Only 11.9% of San Francisco is covered by the tree canopy, compared to 20.9% of New York City.  According to that survey, San Francisco’s urban forest removes 141 tons of pollutants per year compared to 1,677 tons of pollutants removed by New York City’s urban forest.
• Ants are found in some urban forests.  A study in Toledo, Ohio and Detroit, Michigan compared ant populations in urban habitats (forests in city parks, community gardens, and vacant lots).  (2) The study found greater diversity of ant species in forests than in other habitat types, but fewer ants.  They found 26 species of ants in the forest, 20 in vacant lots, and 14 in gardens.  They found no correlation between various characteristics of vegetation and ant diversity or abundance.  Soil moisture was not measured by this study.  Generalizations about urban forests derived from one study in San Francisco clearly do not apply to Toledo and Detroit.

More soil moisture in forest with a closed canopy

The ant study in San Francisco predicts greater soil moisture in a forest with a closed canopy and dense understory:

“A combination of high soil moisture, dense canopy cover, and dense understory (habitat complexity) may help explain the lack of ground-foraging ants in urban forests.” (1)  The study associates those characteristics specifically with the eucalyptus forest:  “Within forest types examined, eucalyptus forests contained significantly more soil moisture than other forest types and also had lower ant richness and abundance.”  (1)

We don’t think these generalizations can be applied neither to all eucalyptus forests nor solely to eucalyptus forests:

• The density of eucalyptus forest in the San Francisco Bay Area varies widely according to data presented recently by Professor Joe McBride to the Commonwealth Club:

Location	Average Number of Trees per Acre
Presidio, San Francisco	163
Land’s End, San Francisco	364
Tilden Park, Berkeley	540
East Ft Baker, Marin County	1795

In other words, not all eucalyptus forests have closed canopies.

• The density of understory in the eucalyptus forests of the Bay Area also varies widely.  One of the densest understories exists on Mount Sutro, which is the location of the Interior Greenbelt, where the ant study reports finding no ants.  In drier locations, such as Bayview Hill, there is little understory in the eucalyptus forest, which may be why the ant study reports finding ants there.  Bayview Hill is on the east side of San Francisco and therefore receives much less fog than Mount Sutro, which is closer to the ocean.
• Eucalyptus forest is not unique in often having a closed canopy.  Native redwood forest also has a closed canopy:  “Many meters above the ground, the branches of trees, especially those of redwood, merge to form a ceiling, or canopy.”  (3)

Fog and soil moisture

The ant study describes the relationship between fog and soil moisture in San Francisco:  “The increased moisture in eucalyptus is due to the fact that summer fog tends to condense on eucalyptus leaves and branches and drip down to the soil below.  Such fog drip can add as much as 42 cm of water to eucalyptus forest during a single summer.”  (1)

Fog in San Francisco is unrelated to the fact that its forest is predominantly eucalyptus:

• 

  Although redwoods did not live in San Francisco when Europeans arrived in 1769, they lived there in the distant past.  Native redwoods now exist only on the coastal fog belt of California.  Fog is essential to their survival:  “During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves.  When trees were absent, the average annual input from fog was only 17%, demonstrating that trees significantly influence the magnitude of fog water input to the ecosystem…The results presented suggest that fog, as a meteorological fact, plays an important role in the water relations of the plants and the hydrology of the forest.”  (4)

• Fog exists along the northern coast of California because the interior is hot and the ocean is cool.  When the cool ocean air meets the hot air from the interior, fog forms.  The existence of fog has nothing to do with the species composition of the forest.  Any tall tree is capable of condensing the fog, which then drips to the forest floor, providing water to both the trees and their understory.  The eucalyptus forest is not to blame for this sequence of events.

The nativity of the urban forest is irrelevant to the ants

The ant study implies that there are few ants in San Francisco’s urban forest because the forest is not native to San Francisco:  “…reducing tracts of non-native forest…may all help support a healthier ant community and ecologically valuable parks.”

Soil moisture is the operative variable in predicting abundance and diversity of ant populations.  The nativity of the vegetation is irrelevant to the ants:

• If the urban forest in San Francisco was native redwoods, it would precipitate equal amounts of fog, resulting in equal amounts soil moisture.  The ant population would probably be similar.
• A study of ant populations in the central Appalachian Mountains found the same relationship between soil moisture and ant populations in native forests:  “Fewer ants, lower number of species, and lower ant diversity were found at sites with higher elevation and soil moisture.”  (5)
• A study of ant populations in Northern California grasslands found that the characteristics of the soil were better predicators of ant populations than the types of vegetation:  “Plants were less important than soil attributes in explaining variation in overall ant species richness and abundance…”  (6) Chemical composition and consistency (sand vs. clay) of soils were evaluated by this study, but not soil moisture

“Science” in the service of nativism

We consider this ant study a classic demonstration of nativism.  In this case, soil moisture was confounded with the non-nativity of forest in San Francisco.  The nativity of San Francisco’s forest is irrelevant to the amount of soil moisture.  Any closed canopy forest of tall trees would precipitate equal amounts of fog and have a similar impact on ant populations. 

The study speculates that the allelopathic properties of eucalyptus may have a negative impact on the ants, but offers no evidence.  We have found no evidence of allelopathic properties of eucalyptus.  Nor do we think that the existence of ants should be the sole criterion for “ecological health.”  Would we demand the destruction of redwood forests so that we could have more ants?  We doubt it.

However, we must also give credit where credit is due.  The ant study reports that the existence of the non-native Argentine ant does not have a negative impact on the populations of native ants.  They report that the Argentine ants occupy the perimeter of the “natural areas” where native ants generally are not found.  This is a refreshing departure from the usual nativist claims that all non-native plant and animal species have negative impacts on native species.

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(1)    Kevin M. Clarke, et. al., “The influence of urban park characteristics on ant communities,” Urban Ecosyst, 11:317-334, 2008

(2)    Shinsuke Uno, et. al., ”Diversity, abundance, and species composition of ants in urban green spaces,” Urban Ecosyst, 13:425-441, 2010

(3)    UC Berkeley Botanical Garden, “Plants and Their Environments”

(4)    T.E. Dawson, “Fog in the California redwood forest:  ecosystem inputs and use by plants,” Oecologia, 117-4:476-485, December 1998

(5)    Changlu Wang, et. al., “Association Between Ants and Habitat Characteristics in Oak-Dominated Mixed Forests,” Environmental Entomology, October 2001

(6)    April Boulton, et. al., “Species Richness, Abundance, and Composition of Ground-Dwelling Ants in Northern California Grasslands:  Role of Plants, Soil, and Grazing,” Environmental Entomology, February 2005