Niche Theory: Is there room for everyone?

One of the basic tenets of invasion biology is “ecological niche theory.”  According to that theory, every species occupies a specific niche to which it is adapted.  That niche provides all the biological resources the species needs for its survival and reproduction.

A corollary to that theory is that when non-native species are introduced to that “niche” the native occupant is the loser in the inevitable competition for available resources.  The intruder has the advantage in this competition because its predators have not usually been introduced at the same time. This is the “predator release” corollary.

Bmblebee on Cotoneaster, Albany Bulb
Bumblebee on Cotoneaster, Albany Bulb

Adherents to ecological niche theory therefore routinely predict the demise of native species whenever non-native species are introduced.  Often, their belief in this inevitable competition leads them to see what they expect to see.  The prediction that the introduction of the European honeybee to the New World would eventually decimate populations of native bees is an example of this mindset.  Since honeybees were introduced to the New World over 400 years ago, it seems reasonable to expect to see some evidence of this consequence by now.

Seeing what we expect to see

Forgotten Pollinators was published in 1996, at the height of popularity of invasion biology. (1)  We consider it a valuable, interesting book, but we were not impressed with the chapter devoted to the belief of the authors that the European honeybee is competing with native pollinators, to the detriment of native pollinators.  Although the authors interviewed several other scientists who shared that belief, they were unable to offer any empirical evidence that supported their belief.

One of the studies cited in Forgotten Pollinators quantified the amount of nectar and pollen consumed by honeybees and compared that to the quantity of nectar and pollen required by native bumblebees.  Based on those calculations, they predicted the demise of bumblebees based entirely on the amount of nectar and pollen consumed by honeybees.  The study arrived at the preposterous conclusion that a single honeybee hive could reduce the population of bumblebees by 38,400.

This dire prediction is based on the assumption that there is a finite amount of pollen and nectar available.  Therefore, every scrap of food collected by a honeybee is a scrap of food taken from a bumblebee.  It also assumes that the bumblebee loses the competition 100% of the time and neither insect is capable of expanding its range in the unlikely event that there is in fact a finite amount of food available.  This type of “zero-sum” thinking pervades the nativist ideology, e.g., a job taken by an immigrant is presumed to be taken from a resident.

Looking for bad news…finding good news

White Sweetclover
White Sweetclover

The federal government has invested in many careers and large sums of money to prove the assumptions of invasion biology as well as funding eradication projects based on those assumptions.  For example, the University of Alaska, Fairbanks was awarded a federal grant for $493,000 to test the hypothesis that the existence of non-native sweetclover is drawing pollinators away from wild native food sources such as cranberry and blueberry bushes.  After several years, researchers have concluded, “…there’s usually room for everybody.” (2)

The researchers monitored 20 sites for two years.  They observed that the sweetclover patches “were actually attracting three times as many pollinators to native plants as they would otherwise get.”  The sweetclover attracted many different types of pollinators, including moths, flies, and wasps.  Consequently there were many more cranberries in the “invaded” patches and equal quantities of blueberries.

Suspicious of those findings, the researchers moved their project to a controlled setting.  Then they got mixed results, which seemed to depend upon variations in the weather.

Researchers are still intent to find negative consequences of the existence of sweetclover.  Now they are hoping to prove that the sweetclover is changing the composition of the soil, which they predict will eventually “crowd out” the native plant species.  Will they keep looking until they can report bad news?

Empirical evidence is absent

After reading this good news about the exoneration of sweetclover in the nativist blame game, we decided to revisit the accusations made by Forgotten Pollinators about honeybees causing the decline of native bee populations.  Our search of scientific literature published after the publication of Forgotten Pollinators in 1996, was very revealing and is best represented by a review article published in 2004, “Impact of the introduced honeybee on native bees:  A review.”  (3)

The review analyzes 28 studies conducted all over the world about the impact of honeybees on populations of native bees.  This is a summary of the analysis:

“Although previous studies investigating indirect measurements have been cited as evidence of competition between honey bees and native bees, many of these studies were compromised by low replication, confounding factors or poor interpretation.  Studies that are well designed and implemented may find the potential to impact negatively on native bees but the use of indirect measurements does not reveal [their impact on] long-term survival of native bees. 

More direct studies of the impact of honey bees on native bee survival, fecundity or population density have shown little evidence that the presence of honey bees has any impact on native bees.” (3)

As we often do on Million Trees, we conclude with this rhetorical question:  How does invasion biology survive in the absence of empirical evidence that supports its hypothetical assumptions?


(1)    Stephen Buchmann & Gary Paul Nabhan, Forgotten Pollinators, Island Press, 1996

(2)    “Invasives pollination study shows mixed results for Alaska berries,” Fairbanks Daily News-Miner, August 23, 2013

(3)    D.R. Paini, “Impact of introduced honey bee (Apis mellifera) on native bees:  A review,” Austral Biology, (2004) 29, 399-407.


Exterminating Animals: Wasted lives and money

While we object to the needless destruction of non-native plants and trees, we are even more concerned about the destruction of non-native animals.  Using the same justification, namely that non-native animals out-compete native species of animals, native plant advocates and their allies are equally committed to the eradication of non-native animals.  The list of targeted animals is long: e.g., non-native frogs, turtles, fish, bees, foxes, opossums, squirrels, deer, pigs,etc.  When populations of native animals increase in urban areas, they are called “subsidized predators” and added to the death list: e.g., raccoons, skunks, etc.  A native animal can land on the death list if its range expands, such that it becomes a competitor for a preferred, rare native animal.

First we will indulge in a brief digression on behalf of the non-native European honeybee, one of the few species of bee in the United States that produces honey.  If that’s insufficient reason to defend its existence, let us consider that the European honeybee is responsible for pollinating about one-third of all agricultural crops and orchards in our country(1).  Even without eradication efforts, the honeybee is in trouble.  In the past several years, about one-third of all hives have failed each year from multiple factors summarized as “Colony Collapse Disorder.”  Despite the obvious value of this non-native creature, it is being eradicated by the Nature Conservancy on its “restorations” in the United States because it is non-native and it is considered a competitor to native bees, which are not capable of pollinating many agricultural crops or making honey(2).  This seems to us a classic case of nativism shooting us in the collective foot.

Now we will turn to two efforts to exterminate animals that were both appallingly destructive, but more importantly, ineffective and clearly a waste of both lives and taxpayers’ money.  The first example is historical, illustrating that man’s efforts to manipulate nature to serve his purposes are not new and undoubtedly can be traced as far back as the historical record can take us.

California Quail, Wikimedia Commons

In this case, we will look at the efforts of the California Division of Fish and Game to increase the population of quail(3).  Quail are native to California, but their population exploded with the arrival of Europeans whose agricultural and grazing practices increased the food supply of the quail.  The population of quail in California reached its peak during the period 1860 to 1895 and thereafter began to decline as non-native annual grasses began to dominate the non-native herbaceous and leguminous plants that preceded them.  Since man’s view of nature is rather narrow in time, limited by his brief lifetime compared to the more slowly moving forces of nature, the California Division of Fish and Game perceived the decline in the quail population as a problem requiring remediation.  One of their proactive efforts was to exterminate all animals believed to be predators of the quail.  This “predator control” effort was summarized for one six-month period as follows:

“…between January 1 and July 1, 1931, [deputies of the Division of Fish and Game] have destroyed…:  38 coyotes, 33 bobcats, 684 house cats, 35 foxes, 43 coons, 8 weasels, 2 opossums, 1 badger, 5 wild and unclaimed dogs…365 sharp-shinned and cooper hawks, 3972 blue jays, 293 magpies, 81 crows, 49 butcher birds, 2 great horned owls, and 47 snakes.”(4)

The Division of Fish and Game hired an army of 45 full-time men in 1948 to continue this war on the perceived enemies of quail.  This extermination effort was not abandoned until 1957, when the Division of Fish and Game concluded that the quail population was not benefitting from this animal holocaust and that reduced food sources and cover, resulting from changes in land uses and consequent vegetation types was the reason for the declining quail population.  In other words, hundreds of thousands of animals lost their lives over a period of over 25 years for no reason whatsoever.

So, did we learn anything from that experience?  Clearly not.  Today there are nearly as many programs to eradicate non-native animals as there are non-native species.  We choose the brown-headed cowbird as an example of modern eradication efforts because it is occurring in California (5).

Although the brown-headed cowbird is native, it is perhaps one of our most reviled birds because it is a nest parasite, which means that it lays its eggs in the nests of other birds.  Their egg is usually larger than the eggs of the “host” bird and it hatches earlier than its nest-mates.  The result of these advantages is that the off-spring of the nest owner usually does not survive, but the cowbird chick survives to repeat this trick.  Because the range of the cowbird has been expanding, it has been blamed for the declining population of songbirds.

But does the cowbird deserve to be blamed for the decline in the songbird population?  Professor Stephen Rothstein (Department of Ecology and Evolution, UC Santa Barbara) says, “NO.”  He tells us that the range of the cowbird is not larger than its historic range.  At the time of the megafauna (e.g., wooly mammoths), about 15,000 to 20,000 years ago, cowbirds probably occupied their current range.  When the megafauna disappeared, the range of the cowbird shrank.  As in the case of the quail, we look to man for the explanation for why its range has expanded again:  the introduction of large grazing animals by Europeans has provided the cowbird with a substitute for its prehistoric food source.

Professor Rothstein tells us of several efforts to exterminate cowbirds on behalf of declining populations of songbirds and concludes that although songbird populations may have recovered in some cases, the extermination of cowbirds is not the likely explanation for their recovery.

Kirtland’s Warbler, female, Wikimedia Commons

The recovery of the Kirtland’s Warbler in Michigan is a case in point.  The warbler’s nesting habitat was well known to require periodic fire.  Yet, the scientific managers of this recovery project preferred to kill cowbirds rather than to risk human life and property by not suppressing fire.  Nearly 125,000 cowbirds were destroyed in a portion of a small peninsula in Michigan during the period 1972 to 2002.  Although nest parasitism declined significantly, the population of Kirtland’s warbler did not increase until over 20 years later after a large accidental forest fire.  In other words, the cowbird is a scapegoat for the choices made by man, in this case the suppression of fire.

Like the predator control project on behalf of the quail, cowbird extermination projects create jobs.  The Kirtland’s Warbler project continues to cost about $100,000 per year, although there is no evidence that the warbler is benefiting from it.  Rothstein speculates:   ”The money spent on cowbird control every year may total more than one million dollars.”   This creates a profit motive which Rothstein says results in a “control lobby” that advocates for continuing the program whether or not it is effective.  He believes that this money would be put to better use by addressing the underlying problems, such as habitat loss to development or reduced water levels that change vegetation types, as in the case of the declining population of the Southwestern Willow Flycatcher.

These are familiar themes to the readers of Million Trees:

  • That native plant advocates and their allies frequently confuse cause and effect, e.g., cowbirds are not responsible for declining songbird populations.
  • That man finds it convenient to scapegoat plants and animals for changes in the environment that are caused by man.
  • That people who make their living in these misguided “restoration” efforts have a vested interest in their continuation whether or not they are effective
  • That the waste of money and scarce resources prevents us from addressing the real issues
  • That choosing to replicate nature at some specific point in historical time is illogical because nature is constantly changing, not always in response to the actions of man

Postscript:  Here is a link to a radio story about another episode in the attempt to save the Kirtland’s warbler.  After killing 125,000 cowbirds (according to the ABA article), US Forest Service changed its mind about why the population of Kirtland’s  warblers was dwindling.  They decided that the problem was that the warbler required young trees of a specific species, which is germinated by fire.  So, they set a prescribed burn that caused a wildfire on a windy day, burning over 20,000 acres, destroying 41 homes, and killing a young man who worked for the Forest Service.  The population of Kirtland’s warblers rebounded.  Now the Forest Service says they must continue to kill cowbirds and set prescribed burns every year forever if the Kirtland’s warbler is to survive.  This radio program poses the question:  does this make sense?    


(3) A. Starker Leopold, California Quail, University of California Press, 1977. (N.B. A. Starker Leopold is Aldo Leopold’s son.)

(4) Ibid.

(5) Rothstein, Stephen, “Brown-headed Cowbird, Villain or Scapegoat?,” Birding [journal of American Birding Association], August 2004, 374-384.