When land managers unintentionally create ecological traps

The Be'er Sheva fringe-fingered lizard (Acanthodactylus beershebensis). Image credit, Arnon Tsairi.Sometimes the well-intentioned efforts of land managers can have unintended negative consequences for threatened species.

In a new case study in the journal Conservation Biology, researchers from Israel show how efforts by the national forestry agency to improve scrubland habitat in the Negev desert actually created an ecological trap for the highly endangered, endemic lizard, Acanthodactylus beershebensis.

The project involved retaining moisture and nutrients with pits and dykes to establish a mosaic of trees and natural areas.

Acanthodactylus beershebensis are impaled on branches by predatory birds. Image credit, Arnon Tsairi.When the lizards disappeared from the project area within 11 years, it appeared that the trees might have been creating perching opportunities for birds such as the southern gray shrike and common kestrel that prey on the lizard.

Agency staff countered that since the lizard had disappeared in both altered and unaltered areas the project was not responsible (and it was slated for expansion).

However, as Dror Hawlena and fellow researchers showed, the planted trees created an ecological trap that masked the negative impacts. An ecological trap occurs when a population prefers low-quality habitat areas.

This can create a population sink in which areas with rates of mortality greater than reproduction attract individuals from other locations thereby depleting the overall population numbers little by little.

The researchers hypothesized that an ecological trap had arisen because the lizard was "not adapted to identify possible perches as cues for increased predation" given the rarity of trees in their natural environment.

To test this hypothesis the researchers planted "artificial trees" which included poles rung by barbed wire - the barbed wire substituted for the thorny branches on which shrikes impale their prey. They then sampled for the lizard at the artificially planted area and a control site to see how the increase in birds of prey affected the population.

They found, as expected, that mortality of the lizard increased at the artificially planted site. Yet despite this decrease in survival, they found that the number of hatchlings and the population density increased. Meanwhile, at the control site, the number of hatchlings stayed constant and the population density declined.

After discounting alternative explanations, the researchers determined that "hatchlings dispersed into plots with artificial trees in a manner that indicated they perceived the quality of these plots as similar to the surrounding, unmanipulated landscape."

The study shows that seemingly minor human disturbances can create ecological traps even though conventional conservation metrics give little warning. The study also illustrates how applied research can achieve tangible, positive outcomes for conservation.

With their research findings in-hand, the study authors were able to convince the forestry agency to abandon afforestation efforts in the remaining lizard habitat and instead support establishment of a large sanctuary to protect the specialized species of the scrubland.

--Reviewed by Rob Goldstein

HAWLENA, D., SALTZ, D., ABRAMSKY, Z., & BOUSKILA, A. (2010). Ecological Trap for Desert Lizards Caused by Anthropogenic Changes in Habitat Structure that Favor Predator Activity Conservation Biology DOI: 10.1111/j.1523-1739.2010.01477.x