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Niche Theory: Is there room for everyone?

September 3, 2013

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 invasive 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.

 

5 Comments leave one →
  1. Kenneth Gibson permalink
    September 11, 2013 9:14 pm

    When the eucalyptus have crowded all the humans off this continent, I’ll have the pleasure of having been right :-)

    Webmaster: Native plant advocates say many silly things. This is one of the silliest. Eucalypts are rarely invasive. Many native plants spread far more rapidly than eucalyptus or any other non-native. Coyote Brush is probably the best example of an invasive native plant. The word “invasive” is used by native plant advocates exclusively to describe non-native plants. It’s a rhetorical tool used to justify the eradication of non-native plants.

    Here are two Million Trees posts that provide scientific evidence that eucalypts are not invasive: http://milliontrees.me/2010/05/19/alien-invaders-another-scary-story/.

    http://milliontrees.me/2012/11/06/more-evidence-that-eucalypts-are-not-invasive/

    And here is a Million Trees post that provides a photographic example of a local eucalyptus forest that has not expanded over a period of 130 years: http://milliontrees.me/2011/03/26/photographic-evidence-that-eucalypts-are-not-invasive/

  2. November 3, 2013 6:42 am

    I guess the very nature of scientific study is to record results actually seen and verified, but I agree, many less reputable sources set out to prove what they expect rather than what is seen. I think also in nature, too much is variable and when looked at in controlled studies, findings are not always accurate if not under exact conditions of that which is in the natural settings. There are too many variables weighing in on a natural setting, demanding many disciplines to report findings. Weather is an example. It could affect breeding one year and not the next. It can affect a predators ratio compared to its prey. Stress factors, like maple trees producing excessive seed one year but not the next. A stressed tree is easily susceptible to disease and chewing insects from its roots to its leaves. I am honestly not sure how science can be definitive on findings and this leads to many of these contradictory and alarming reports. The bees are a great example. They have many external factors affecting them, not just competition between bee species. They happen to like clover too.

    • November 3, 2013 7:03 am

      There is no question in our minds about the truth of what you say about the variability of conditions in nature. And scientists who do not have preconceived notions are willing to say that. We are fortunate to have an advisor who is a distinguished professor of ecology with 40+ years of experience in the field. When we ask him questions, his most frequent answer is “We don’t know.”

      We wish that native plant advocates would be more humble about the assumptions they make about nature. The fact is, humans know very little about how ecosystems operate and that’s why there are often unintended consequences of “restoration” projects.

      • November 3, 2013 7:38 am

        Exactly. Science sometimes operates in a vacuum too protecting research funding. I was a biology major before architecture and was very involved in research. I changed my major when we were required to operate on live animals subjects with the inevitable outcome of death. I refused and was given a failing lab grade rather than making an alternative option of working on deceased subjects. I had a 4.0 GPA too so the lab grade was not much of an affect in my 4th year. I contested it an switched majors and retained the 4.0 until graduation. I saw too much hypocrisy in the science field and too much information not being shared. It is why I am a bit questioning on these many studies. I do put all my trust in science though. The people citing only certain relevant studies are doing a disservice. Other disciplines need to be involved when looking at things as complex as what we find in nature.

        • November 3, 2013 8:21 am

          We seem to have a great deal in common. It takes some independent thinking to question the conventional wisdom about the superiority of native species and the supporting assumption that all non-native species are harmful. One of my high school teachers said in her recommendation letter to a college that I was “a little argumentative.” I still take that as a compliment, though it may not have been intended as such.

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