Monday, May 21, 2007

Ecological Traps

We often assume that animals always behave in their own best interests, but this is patently not true. Because their access to and ability to assess information is limited, animals make choices based on their evolutionary history. If, in the past, obeying a certain environmental cue lead to fitness-increasing benefits, then organisms that followed that cue would have more descendants than those that did not.

However, environmental cues are not always consistent or reliable. Animals may obey an environmental cue even if it presently has negative consequences because they are unable to tell the difference. Unfavorable habitats that present cues associated with favorable habitats are termed ecological traps. I suspect a good example of an ecological trap might be found in El Salvador. According to an article by Alberto Barrera, "An artificial lake in El Salvador brimming with sewage and industrial waste is mystifying scientists by attracting thousands of migratory and sea birds."

"Environment ministry ornithologist Ricardo Ibarra said birds may be attracted by the sandy beaches, crawling with insects, that appear around the edge of the lake in the dry season."

Presumably beaches crawling with insects might have cued good habitats in the past, but, in this case, may be associated with high levels of chromium and lead.

"The contamination is bound to be harming birds that feed and nest there, possibly making them too weak to be sure of making their migratory flights or affecting the strength of the shells of their eggs, he warned."

Another example of an ecological trap is the mayfly. Ordinarily mayflies lay their eggs on pond surfaces, but lately have been laying their eggs en masse on road surfaces. Scientists have shown that mayflies the polarized light reflected from roads closely mimics that from pond surfaces. Unable to tell the difference, mayflies have been laying their eggs on roads with obvious negative consequences.

Photo by Impactmedia.


  1. The classic example might be moth night attraction to light sources.

    Presumably moths find lunar navigation useful, but in the age of streetlights the dominant local night time light source might not be lunar.

    This results in concentrations of moths near the lamps, evidence of the carnage is on my porch floor, directly below the lights.

    There might be one positive outcome to this trap, namely I've seen bats feeding near some local lights, I assume attracted by the higher local concentrations of flying insects.

  2. Thank you for your article.

    I like the description " ....environmental cues are not always consistent or reliable..."

    However, the cues make the traps. It coule be in resources requirement for the animals. How can we avoid the situation happened when we manage a new habitat for animals?

    Could you explain and give me more examples? Your descriptions are so simple and clear to help me to solve some problems.

    Thanks a lot.

  3. There's some other examples, incl. baby sea turtles who use the rising sun to orientate to the ocean. Lights on land can confuse the turtles, causing them to migrate inland.

    Avoiding these problems requires better knowledge of animal behavior and natural history.

    Some good reviews available here:

  4. Thank you for your answer and informations. I will read those papers.

    I still have one question.

    I am a waders researcher, and a PHD student, too. I had thought about the problems about the human-made wetland, or called constructed wetland. Most of them are used for wasted water treatment, and we also could find many cases with good efficiency.

    But the benefit is only happenend in the water treatment. I did not find any articles discussed if the animals utilize those human-made wetland, then the animals will meet what kind problems.

    Is it a one kind of "ecological trap"?

    Thank you.

  5. I think I should read the two articles first, then asked my question. I find the answer.

    So, if we want to define it is the ecological traps or not, we need to assess and evaluate the habitat quality first. right ?

    The animals are trapped by wrong informations and cues. And if we want to avoid the accidents happened,we can use different and more habitats for their living and breeding. Like the source and sink in the landscaape ecology ?

    Do I touch the point ?

  6. Yes, I think that is exactly the point. Most development takes place in the absence of knowledge of ecological impact. In order to determine whether a habitat is a trap or not, assessing habitat quality is required. Most studies approach this by estimating growth rates in "natural" habitat vs. trap. With respect to source/sink theory in ecology, trap = sink i.e. population growth is negative.

  7. Thank you for your answer.

    So, if some birds choose a area for breeding, but the suddently precipation destory all nest, then the breeding success rate is very bad. At this case, can we said, the area is a trap?

    In fact, the birds are hard to find a good quality habiat for breeding. In the definition of traps, it would need two different quality habitat, one is better, another is not good. When the animals did not have any choices to get, can we said they are in the ecological traps?

    Or, it is another issues to discuss?

    About the population growth rate, I had a idea about the rare species, Black faced sponbill.

    In the world census this year, the total population is about 2000. In Taiwan, there are more than 1000 wintering individuals at a small protected area.

    I found the the population census in last five years, I found the population growth rate is so dramatical. The total population is double in the last five years.

    I can not sure the increasing population is result in the conservation work. But I can sure the population growth rate is too quickly. Maybe it would arrive the carrying capacity of the wintering site soon.

    The population from 1990-2009

    I am very glad to discuss those questions with you. Thank you for your help.

  8. I think a trap needs to be something more than a sudden environmental catastrophe e.g. flood. Instead, I define as a more systematic and pervasive decline in habitat quality that presents the same cues used by organisms to indicate high quality habitat.

    Having no habitat choice would preclude a trap. Recall the mayfly example. The polarized light cue still indicates high quality habitat e.g. pond, but can also indicate low quality habitat e.g. road. What is required is a new cue that allows mayflies to discriminate between roads and ponds, but they have not yet evolved the ability to make that discrimination.

    An interesting thing about growth rates is that the higher they are, the more chaotically total population size fluctuates. In other words, with greater population growth rate, population rises and falls are greater and could potentially lead to extinction (counterintuitively). I think this has been mathematically demonstrated before somewhere, but I can't remember where.

  9. The models shoule be in the ecology textbook. I think. I also forgot how to calculate. I need review my textbook.

    I like that you said, " but they have not yet evolved the ability to make that discrimination ". It let me think the road kills, and bird collisions by tower or building. Although it looks like accidents, but in other words in fact they didn't have enough time to "know" what they met.

    Thank you for your reply. It's very nice for me to discuss those with you.