Month: August 2012

Darwin’s Finches: An opportunity to observe evolution in action

The finches on the Galápagos Islands are called Darwin’s finches because of the important role they played in the development of his theory of natural selection and evolution of species.

Galapagos Islands, satellite photo. Daphne Major is too small to be visible.

Charles Darwin spent five weeks on the Galápagos Islands in 1835, near the end of a five year expedition.  Although he noticed the similarity of the birds on the different islands, he didn’t realize they were all related to one common ancestor until he returned home.  Fortunately, he collected many specimens of the birds to bring home for study.  It wasn’t until those specimens were examined by an ornithologist that he learned they were 13 species of finches, distinguished primarily by variations in the size of the bird and its beak size and shape.

Unfortunately, he hadn’t recorded which islands the specimens were from, so the implications of their differences were somewhat of a mystery.  He lamented in Voyage of the Beagle, “It is the fate of every voyager, when he has just discovered what object in any place is most particularly worthy of his attention, to be hurried from it.”

But Darwin was no dummy, so despite lacking the data necessary to prove his point, he speculated in his memoir, “…in the thirteen species of ground-finches, a nearly perfect gradation may be traced from a beak extraordinarily thick, to one so fine, that it may be compared to that of a warbler.  I very much suspect that certain members of the series are confined to different islands…”

Such development of new species from a common ancestor in response to varying environmental conditions is called adaptive radiation.  Species also diverge from one another to reduce competition by specializing in a particular food forage type or technique.  Nearly 200 years later, science has proven Darwin’s hunch, but just as he had no way of knowing how long this process of speciation took, modern science still cannot answer that question.

Darwin’s finches continue to change in response to changing conditions

Large ground finch (Geospiza magnirostris). Linda Hall Library

Rosemary and Peter Grant have studied the finches on two Galápagos Islands (Daphne Major & Genovesa) for about thirty years.  Nearly every year they visited the finches, weighing and measuring every appendage of the birds, especially their beaks.  They banded the birds so they could follow their breeding success. They also measured their food:  how much food but more importantly how accessible the food is to the birds such as the difficulty of opening seeds.

The availability and type of food is what determines the shape and size of the birds’ beaks.  In a year in which there is plenty of rain, there is usually plenty of food which is relatively easy for the birds to eat.  When it doesn’t rain, the birds are reduced to the difficult task of trying to crack open a large, hard seed pod.  That’s when a big bird with a big beak has an advantage.   

Extreme weather is therefore a “selection event,” a time when not every bird is equipped to survive.  And the birds that survive are best equipped for those extreme conditions.  When the conditions improve, the bird that survived the hard time is not necessarily best equipped for the good times.

These are the principles of natural selection, but they were largely theoretical until the Grants spent many years watching the birds and how they survived such selection events.  They had the good fortune to witness two such events in the first twelve years of their study.

The drought

In the fifth year of the Grants’ study, 1977, there was a severe drought.  After one short storm in early January, there was no more rain for the remainder of the year.  In January, there were 1,300 finches on the island they studied that year.  At the end of the year, there were less than 300 finches left on the island.

The Grants measured and weighed the birds that survived the drought.  Then they returned to their lab at Princeton University to study their data:

  • Not a single finch was born and survived on the island in 1977
  • The surviving birds were 5-6% larger than the dead birds
  • The average beak size of the birds that survived was 11.07 mm long and 9.96 mm deep.  The average beak size of the birds that did not survive was 10.68 mm long and 9.42 mm deep.  These critical differences were too small to see with the naked eye, but became evident when the measurements were analyzed by computer.  This makes a strong case for scientific measurement verses anecdotal observation, which passes for “evidence” amongst native plant advocates.
  • Few female birds survived the drought, presumably because male birds are larger than females.

In the years following that drought, sexual selection played an important role in maintaining the population of larger birds with larger beaks.  Because the female birds were scarce, they could be very selective in their mates.  Who did they choose?  Of course, they chose the males with the traits that allowed the birds to survive the drought year.  When the ratio of males to females is more even, sexual selection plays a less important role in natural selection in monogamous species such as the finches.

The flood

Here on the West Coast, we are familiar with the weather phenomenon of El Niño, the nickname given to a heavy rain year resulting from an unusually warm ocean current.  In 1983, we experienced the strongest El Niño on record, as did the Galápagos Islands. 

In 1983, the Grants witnessed the reversal of the results of the 1977 drought:  “Natural selection had swung around against the birds from the other side.  Big birds with big beaks were dying.  Small birds with small beaks were flourishing.  Selection has flipped.” *

Lessons learned

Darwin’s finches give us reason for optimism about the future.  Nature can and will respond to changes in the environment.  Natural selection is not just an historical process that stopped when The Origin of Species was written nearly 200 years ago.  Natural selection is operating at all times, whether we notice it or not. 

However, the loss of nearly 80% of the birds on a Galápagos Island during a severe drought is not cause for celebration.  Although the species survived, hundreds of individual birds did not.  So, we are quick to add that our confidence in the adaptive abilities of nature is not an argument for abusing the environment.

Climate change has caused extreme weather events which are undoubtedly selection events for many species of plants and animals.  Unless we take action to reduce greenhouse gas emissions we can predict more of such events.  Destroying millions of trees solely because they are not native is irresponsible given the contribution their destruction makes to the greenhouse gases causing climate change.

*************************

*Jonathan Weiner, The Beak of the Finch, Vintage Books, 1994

Integrating new species into the food web

Zebra mussels, open underwater with siphons out. Creative Commons

We have been reading panic-stricken news reports about zebra mussels for over 10 years, but we weren’t paying much attention until a recent news report that they have arrived in California.  We decided it was time to educate ourselves about this “invasive species.”

Zebra mussels and their close relative, the quagga mussel, arrived in the Great Lakes Region of the United States in 1988, probably in the ballast water of big ships.  Although they are native to southern Russia and Ukraine, they are now found throughout Europe and England.

The negative side of the ledger

What the mussels lack in size, they make up for in numbers.  Though they are tiny—about the size of a dime–they are prolific breeders capable of creating big colonies rapidly.  They are a fresh-water mussel which means they exist where there are often water treatment facilities that supply our drinking water.  Their larvae are microscopic so they can enter water treatment facilities through the intake pipes and clog the system. 

They filter huge quantities of water, consuming plankton (microscopic plants and organisms) depriving other animals of nutrition.  This filtering of the water also increases water clarity and light penetration, changing the entire ecosystem in complex and unpredictable ways.

The positive side of the ledger

Where the mussels have gained a foothold, they have quickly entered the food web.  A monitoring program was started soon after mussels were found at Long Point Bay in Lake Erie.  The first sampling done in 1991 found mussels in 27% of the sampling stations, an estimated 1,189 tons of mussels.  By 1992, mussels were found in 80% of the sites, an estimated 4,536 tons of mussels.  (1)

In 1992, the monitoring program also started conducting stomach analysis of ducks killed at Long Point Bay.  Three species of duck (Greater and Lesser Scaup and Bufflehead) were found to be feeding heavily on the mussels.  Between 1993 and 1995 the population of mussels declined significantly from the highpoint of 4,536 tons to only 758 tons in 1995.  The population of the duck predators increased correspondingly during the same period of time. (1)

The authors of this study speculate that the mussels were also depleting their food source at the peak of their population and that they had exhausted available attachment sites, but the scientists believe duck predation was the primary reason for the declining population of mussels.  As always, there are many variables operating simultaneously in the ecosystem, and it isn’t possible to isolate one from the others.  (2)

Ducks aren’t the only predators of the mussels.  Crayfish are apparently capable of consuming large quantities of the mussels.  And some fish eat the mussels.  One study found that yellow perch didn’t eat the mussels in 1994, but a later study in 2004 reported that the perch were eating the mussels.  Plankton waste from the mussels settles on the lake bottom and the bottom feeders benefit from that fall out.

There is a downside to this story, however.  Remember that the mussels filter the water as they eat.  In addition to filtering plankton, they also filter pollutants and contaminants.  Researchers assume that the predators of the mussels are consuming those pollutants which then become a part of the food chain.  The mussel-consuming ducks at Long Point Bay apparently had elevated levels of contaminants in their tissue compared to ducks that consume only aquatic plants. (2)

What should we do?

According to the news story about the mussels in a local paper, the California legislature is considering increasing the registration fee for boats which would raise about $5 to $8 million dollars.  Although the news story isn’t clear about how this money would be used, let’s assume for the sake of argument that it would be used to prevent the spread of these mussels beyond the 25 lakes in California where they are now found.  That would apparently involve more inspection of boats being put into the water where the mussels don’t presently exist.  If that’s the plan, we enthusiastically endorse it.  Prevention is the best medicine, as they say.

But once the mussels have arrived, all scientists agree that eradicating them is not a realistic option.  Therefore, dousing them with chemicals—which is one of the recommended treatments—will undoubtedly do more harm than good. 

New species quickly become a part of the landscape.  Our initial reaction to them tends to be negative because we are suspicious of change.  In fact, there may be benefits that aren’t immediately evident and even if there isn’t an immediate benefit, they are often integrated into the environment over time.  Their populations often stabilize once they have exhausted available resources.  We should be patient because nature is resilient and our time frame is much shorter than nature’s time frame.    

Are we learning this lesson?

Broom, Redwood Park, Oakland, California

The California Invasive Plant Council (Cal-IPC) is dedicated to the eradication of non-native plants.  Scotch broom is one of their favorite targets for eradication.  Little progress has been made in this effort (see “Broom:  ‘I’ll be back’” and “Broom:  ‘I’m ba-ack’”) and recently Cal-IPC acknowledged this in their newsletter.  However, they urged their supporters not to lose heart because they reported that broom is now being browsed by herbivores.  So, what native plant advocates could not accomplish with manual labor and chemical warfare, the animals may accomplish by incorporating broom into their diets.  One hopes the animals aren’t eating broom doused with herbicides.

Cal-IPC also acknowledges in this article that broom does not grow in shade:  “Broom cannot tolerate heavy shade.  It usually established following logging or other activities that remove tree canopy.”  Could it be that they have finally noticed that the result of clear-cutting non-native trees in the East Bay hills is more broom, not more native plants?  We can only hope so.

There are pros and cons to every decision we make.  We don’t always know in advance what they are.  So, it pays to be cautious.  If we are patient, maybe nature will sort it out without our interference.  Particularly when our interference damages nature, we should exercise restraint.  We should give nature more credit for healing itself.  It has a much better track record than we do.

*******************************

(1)    Cox, George W., Alien Species and Evolution, Island Press, 2004

(2)    Petrie, Scott A., Knapton, Richard H., “Rapid Increase and Subsequent Decline of Zebra and Quagga Mussels in Long Point Bay, Lake Erie:  Possible Influence of Waterfowl Predation,” J. Great Lakes Research, 25(4) 772-782

Doug Tallamy refutes his own theory without changing his ideology

In our debates with native plant advocates, the scientist who is most often quoted to support their beliefs is Doug Tallamy who wrote an influential book, Bringing Nature Home:  How Native Plants Sustain Wildlife in our Gardens.    Professor Tallamy is an entomologist at the University of Delaware.

Professor Tallamy’s hypothesis is that native insects require native plants because they have evolved together “over thousands of generations.”  Because insects are an essential ingredient in the food web, he speculates that the absence of native plants would ultimately result in “ecological collapse” as other animals in the food web are starved by the loss of insects. (1)

Professor Tallamy freely admits that his theory is based on his anecdotal observations in his own garden, not on scientific evidence:  “How do we know the actual extent to which our native insect generalists are eating alien plants?  We don’t until we go into the field and see exactly what is eating what.  Unfortunately, this important but simple task has been all but ignored so far.”  (1)

This research has now been done to Professor Tallamy’s satisfaction by a Master’s Degree student under his direction.  The report of that study does not substantiate Professor Tallamy’s belief that insects eat only native plants.  In his own words, Professor Tallamy now tells us:

“Erin [Reed] compared the amount of damage sucking and chewing insects made on the ornamental plants at six suburban properties landscaped primarily with species native to the area and six properties landscaped traditionally.  After two years of measurements Erin found that only a tiny percentage of leaves were damaged on either set of properties at the end of the season….Erin’s most important result, however, was that there was no statistical difference in the amount of damage on either landscape type.” (2)

Corroborating Evidence

This finding that insects are equally likely to eat native and non-native plants may be new to Professor Tallamy, but it isn’t new to the readers of Million Trees.  We have reported many studies which are consistent with this finding.

Anise Swallowtail butterfly in non-native fennel
The English garden, where plants from all over the world are welcome

Specialists vs. Generalists

When debating with native plant advocates, one quickly learns that the debate isn’t ended by putting facts such as these on the table.  In this case, the comeback is, “The insects using non-native plants are generalists.  Insects that are specialists will not make that transition.”  Generalists are insects that eat a wide variety of plants, while specialists are limited to only one plant or plants in the same family which are chemically similar.

Professor Tallamy offers in support of this contention that only “…about 10 percent of the insect herbivores in a given ecosystem [are not specialists],” implying that few insects are capable of making a transition to another host plant.

However, categorizing insects as specialists or generalists is not a dichotomy.  At one extreme, there are some insects that choose a single species of plant as its host or its meal.  At the other extreme, there are insects that feed on more than three different plant families.  It is only that extreme category which has been estimated at only 10% of all phytophagous (plant-eating) insects.  The majority of insects are in the middle of the continuum.  They are generally confined to a single plant family in which the plants are chemically similar.

Putting that definition of “specialist” as confined to one plant family into perspective, let us consider the size of plant families.  For example, there are 20,000 plant members of the Asteraceae family, including the native sagebrush (Artemisia) and the non-native African daisy.  In other words, the insect that confines its diet to one family of plants is not very specialized. 

Soapberry bug on balloon vine. Scott Carroll. UC Davis

Professor Tallamy offers his readers an explanation for why specialist insects cannot make the transition from native to non-native plants.  He claims that many non-native plants are chemically unique and therefore insects are unable to adapt to them.  He offers examples of non-native plants and trees which “are not related to any lineage of plants in North America.”  One of his examples is the goldenrain tree (Koelreuteria paniculata).  This is the member of the soapberry (Sapindaceae) family to which the soapberry bug has made a transition from a native plant in the soapberry family in less than 100 generations over a period of 20 to 50 years.  Professor Tallamy’s other examples of unique non-native plant species are also members of large plant families which probably contain native members.  Professor Tallamy is apparently mistaken in his assumption that most or all non-native plants are unique, with no native relatives. 

The pace of evolution

Even if insects are “specialists” we should not assume that their dependence on a native plant is incapable of changing over time.  Professor Tallamy’s hypothesis about the mutually exclusive relationships between native animals and native plants is based on an outdated notion of the slow pace of evolution.  The assumption amongst native plant advocates is that these relationships are nearly immutable.

In fact, evolution continues today and is sometimes even visible within the lifetime of observers.  Professor Tallamy provides his readers with examples of non-native insects that made quick transitions to native plants:

  • The hemlock wooly adelgids from Asia have had a devastating effect on native hemlock forests in the eastern United States.
  • The Japanese beetle introduced to the United States is now eating the foliage of over 400 plants (according to Professor Tallamy), some of which are native (according to the USDA invasive species website).

These insects apparently made transitions to chemically similar native plants without evolutionary adaptation. If non-native insects quickly adapt to new hosts, doesn’t it seem likely that native insects are capable of doing the same?  That is both logical and consistent with our experience.    For example, the native soapberry bug mentioned above has undergone rapid evolution of its beak length to adapt to a new host.

Although Professor Tallamy tells us that the relationship between insects and plants evolved over “thousands of generations,” he acknowledges much faster changes in plants when he explains why non-native plants become invasive decades after their arrival:  “Japanese honeysuckle, for example, was planted as an ornamental for 80 years before it escaped cultivation.  No one is sure why this lag time occurs.  Perhaps during the lag period, the plant is changing genetically through natural selection to better fit its new environment.”  Does it make sense that the evolution of plants would be much more rapid than the evolution of insects?  Since the lifetime of most insects is not substantially longer than the lifetime of most plants, we don’t see the logic in this assumption.

Beliefs die hard

Although Professor Tallamy now concedes that there is no evidence that insects are dependent upon native plants, he continues to believe that the absence of native plants will cause “ecological collapse.”  In the same book in which he reports the study of his graduate student, Professor Tallamy repeats his mantra:  “…our wholesale replacement of native plant communities with disparate collections of plants from other parts of the world is pushing our local animals to the brink of extinction—and the ecosystems that sustain human societies to the edge of collapse.”

This alarmist conclusion is offered without providing examples of any animals being “pushed to the brink of extinction.”  In fact, available scientific evidence contradicts this alarmist conclusion. (3)

Here are more articles about the mistaken theories of Doug Tallamy:

  • Doug Tallamy claims that non-native plants are “ecological traps for birds.”  HERE is an article that disputes that theory.
  • Doug Tallamy claims that native and non-native plants in the same genus are not equally useful to wildlife, but he is wrong about that.  Story is HERE.
  • Doug Tallamy advocates for the eradication of butterfly bush (Buddleia) because it is not native.  He claims it is not useful to butterflies, but he is wrong about that.  Story is HERE.
  • Doug Tallamy publishes a laboratory study that he believes contradicts field studies, but he is wrong about that.  Story is HERE.
  • Doug Tallamy speaks to Smithsonian Magazine, Art Shapiro responds, Million Trees fills in the gaps:  HERE
  • Doug Tallamy’s Nature’s Best Hope perpetuates the myth that berry-producing non-native plants must be eradicated because they are less nutritious than the berries of native plants.  Available HERE. 
  • Doug Tallamy believes we must prevent hybridization.  Hybridization is a natural process that increases biodiversity and enables plants and animals to adapt to changes in the environment.  Available HERE.
  • There is NO evidence to support Doug Tallamy’s claim that insect populations are declining because of the existence of non-native plants.  Available HERE.

*********************

(1)    Tallamy, Doug, Bringing Nature Home, Timber Press, 2007

(2)    Tallamy, Doug, “Flipping the Paradigm:  Landscapes that Welcome Wildlife,” chapter in Christopher, Thomas, The New American Landscape, Timber Press, 2011

(3)    Erle C. Ellis, et. al., “All Is Not Loss:  Plant Biodiversity in the Anthropocene,” http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0030535

Biological Control: Another dangerous method of eradicating non-native species

We were recently reminded of the use of biological controls to eradicate non-native species when we learned that Australian insects may have been illegally imported to California to kill eucalyptus, which had been virtually pest free until 1983.  So, an article in the New York Times about the development of a fungus for the purpose of killing cheatgrass (Bromus tectorum) caught our attention.  The fungus has been given the ominous name, Black Fingers of Death, for the black stubs of cheatgrass infected with the fungus.

Cheatgrass, Bromus tectorum

Cheatgrass is one of the non-native grasses that have essentially replaced native grasses throughout the United States.  It was probably introduced with ship ballast and wheat seed stock in about 1850.  As we have reported, native grasses were quickly replaced by the non-native grasses which tolerate the heavy grazing of domesticated animals brought by settlers.    Native Americans had no domesticated animals.

Biological controls have frequently caused more serious damage than the problems they were intended to solve.  Therefore, we would hope that their intended target is doing more damage than the potential damage of its biological control.   We must ask if the cure is worse than the disease.  And in this case, we don’t think the damage done by cheatgrass justifies inflicting it with the Black Fingers of Death.

The track record of biological control

Biological control is the intentional introduction of animals, pests, microbes, fungi, pathogens, etc., for the purpose of killing a plant or animal which is perceived to be causing a problem.  The ways in which some of these biocontrols have gone badly wrong are as varied and as many as the methods used.

Introduced species of plants are said to have an initial advantage in their new home because their pests and competitors are not always introduced with them.  This is the “enemy release hypothesis” popular amongst native plant advocates to explain the tendency of non-native plants to be invasive.  However, this is usually a temporary advantage which is exaggerated by native plant advocates who do not seem to recognize the speed with which native species can adapt to new species, and vice versa.

Therefore, a popular method of biological control is to import the predator or competitor of the non-native species which is considered invasive.  This is only effective if the pest is selective in its host.  There are many examples of such introductions which did not prove to be selective:  “For the United States mainland, Hawaii, and the Caribbean region, Pemberton (2000) listed 15 species of herbivorous biocontrol insects that have extended their feeding habits to 41 species of native plants…” (1)  Although most of the unintended hosts were related to the intended hosts, some were not.

Similar shifts from target to nontarget species have occurred for biocontrol agents of animal pests:  “For parasitoids introduced to North America for control of insect pests Hawkins and Marino (1997) found that 51 (16.7%) of the 313 introduced species were recorded from nontarget hosts.  For Hawaii, 37 (32.3%) of 115 parasitoid species were noted to use nontarget hosts…biological control introductions are considered to be responsible for extinctions of at least 15 native moth species [in Hawaii].”  (1)

There are also several cases of biological controls escaping from the laboratory setting before they had been adequately tested and approved for release.   A virus escaped the laboratory in Australia and killed 90% of the rabbits in its initial spread through the wild population.  Very quickly, the virus evolved to a less fatal strain that killed less than 50% of the rabbits it infected.  A second virus was then tested and also escaped its laboratory trial and has spread through the rabbit population throughout Australia.

A fly being considered for introduction to control yellow starthistle apparently escaped and damaged a major cash crop of safflower in California according to a study published in 2001, illustrating the risks of biocontrols to agriculture.

This is but a brief description of the diverse ways in which nature has foiled the best efforts of the scientists designing biological controls for non-native species of plants and animals.  The source of this information (1) therefore concludes, “…many releases of species have inadequate justification…The first goal of research must be to show that the introduced biological control agent will not itself cause damage.”  Given this wise advice, we will return to the question, “What damage is being done by cheatgrass and does that damage justify the introduction of The Black Fingers of Death?”

Why is cheatgrass considered a problem?

Cheatgrass is one of the many non-native annual grasses which have replaced the native grasses which were not adapted to the grazing of domesticated animals.  Cheatgrass is a valuable nutritional source for grazing animals when it is green and loses much of its nutritional value when it dries.

Grazing is only one of the types of disturbance which create opportunities for non-native grasses to expand their range into unoccupied ground.  Fire is another disturbance which gives cheatgrass a competitive advantage over native grasses because it uses available moisture and germinates before native grasses can gain a foothold on the bare ground cleared by fire.

Cheatgrass is said to increase fire frequency by increasing fuel load and continuity.  Unfortunately, increasing levels of CO₂ (carbon dioxide) in the atmosphere is increasing the fuel load of cheatgrass:  “…the indigestible portion of aboveground plant material [of cheatgrass] …increased with increasing CO₂.” (2)

Carbon dioxide is the predominant greenhouse gas which is contributing to climate change.  And increasing frequency of wildfires is one of the consequences of the higher temperatures associated with climate change.  Therefore, one of the causes of the expanding range of cheatgrass is increasing levels of the greenhouse gases contributing to climate change.  Rather than address the underlying cause, we are apparently planning to poison the cheatgrass with a deadly fungus.

If we are successful in killing the cheatgrass, what will occupy the bare ground?  Will native grasses and shrubs return?  Will whatever occupies the bare ground be an improvement over the cheatgrass which has some nutritional value to grazing animals?  The US Forest Service plant database gives us this warning, “Care must be taken with methods employed to control cheatgrass so that any void left by cheatgrass removal is not filled with another nonnative invasive species that may be even less desirable.” 

Recapitulating familiar themes

The project to develop a deadly fungus to kill cheatgrass is another example of the issues that we often discuss on Million Trees:

  • Are the risks of the methods used to eradicate non-native species being adequately assessed and evaluated before projects are undertaken?
  • Are the underlying conditions—such as climate change–that have contributed to an “invasion” being addressed by the methods used to eradicate them?  If not, will the effort be successful?
  • Is the damage done by the “invasion” greater than the damage done by the methods used to eradicate the invader?  Is the cure worse than the disease?

We do not believe that these questions are being addressed by the many “restoration” projects we see in the San Francisco Bay Area.  Consequently, we believe that these projects often do more harm than good.

*************************

(1)    Cox, George W., Alien Species and Evolution, Island Press, 2004

(2)    Ziska, L.H.; Reeves III, J.B.; Blank, R.R. (2005), “The impact of recent increases in atmospheric CO2 on biomass production and vegetative retention of cheatgrass (B. tectorum): Implications for fire disturbance.”, Global Change Biology. 11 (8): 1325–1332,