Agricultural areas are open and green, and it is often thought that they can provide suitable habitats for wildlife. Yet reality is not that simple, and some agricultural operations can have negative effects.
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At the end of the summer, Israel is dry and hot, but irrigated vineyards provide a green refuge and plenty of juicy, sugar-rich grapes. Many vineyards that grow grapes for the wine industry are mechanically harvested. A harvesting machine shakes the vines, dropping the grapes onto a conveyor belt that carries them on to the winery. Numerous animals live among the vines and may also fall and be collected during the harvest. After making several observations, we suspected that common chameleons (Chamaeleo chamaeleon) are attracted to vineyards and inhabit them. We wondered what happens to them when the vineyards are harvested at the end of the season. While we are not concerned about the premature death of a few chameleons, but rather, we are concerned about the creation of an ecological trap that could lead to the collapse of entire populations of chameleons and other wildlife.
Of the many decisions animals must make during their lives, choosing a habitat is one of the most crucial. The differences between high- and low-quality habitats are reflected in various factors including the amount of available food and shelter from predators, and the presence of potential mates. Since directly assessing habitat quality is complex, many species have evolved the ability to base their choices on cues that indicate habitat quality.
An “ecological trap” occurs when the environmental cues that animals depend on no longer accurately reflect the quality of the habitat. This typically happens as a result of human influence on the ecosystem. In such cases, animals fail to correctly assess habitat quality. In more severe scenarios, they are actually attracted to low-quality habitats. This is highly concerning because individuals from high-quality habitats may migrate to lower-quality ones. These processes may cause the collapse of otherwise sustainable populations [1].
Modern agriculture covers vast areas that expand every year, significantly impacting biodiversity. To understand some of these effects, surveys and monitoring are conducted to assess the presence of different species in agricultural fields. It is tempting to think that the presence of wildlife in agricultural areas means that these lands are “environmentally friendly,” and some even refer to them as “ecological corridors” connecting natural habitats. However, monitoring is often not carried out throughout the entire agricultural cycle, and it may not always account for the biology of specific species inhabiting the area. Since presence data is only collected at certain points in time, it is difficult to draw conclusions about the overall impact. Moreover, the attraction of wildlife to agricultural areas does not necessarily indicate their success there. As previously mentioned, such areas can even create an ecological trap.
Returning to the vineyards. A study from Dr. Kolodny’s lab at the Hebrew University examined whether vineyards might constitute an ecological trap for chameleons and other animals. We spent four nights in a waste container at an industrial winery in Israel, sorting through the refuse for every vertebrate we could find. Whenever possible, each carcass was assigned to a specific grape shipment. This allowed us to determine the origin of each carcass and the number of animals that were included in each shipment. Thus, we could calculate animal density based on the weight of the harvested grapes.
As we suspected, the most frequent species was the Mediterranean chameleon, accounting for 72 of the 105 reptiles that we collected. The Egyptian rock agamas came in second with 26 individuals collected. Additional species were collected in smaller numbers, including three frogs, two lizards, two geckos and one snake. Since chameleons were the most prevalent, we decided to focus on them. To determine whether an ecological trap was involved, we had to find out if the chameleons were attracted to the habitat that ultimately caused their deaths, or if they were in the vineyards for other reasons. This is a very difficult question to answer directly, so we relied on several types of indirect evidence [2].
First, we dissected all the collected chameleons and checked their digestive systems for grape remains. Second, we examined the reproductive status of all the females. In addition, we compared the body condition of the chameleons with data from the literature on other populations of the same species. Live chameleons were released back into the wild.
Of the 60 chameleons found dead, about half unequivocally contained grape remains in their digestive tracts. This is surprising, as chameleons are considered carnivores. Of the 24 females we collected, all but one contained maturing eggs. The body condition of the chameleons we collected was similar to that of chameleons from natural populations.
The body condition and reproductive status of the chameleons we collected suggest that they are healthy chameleons with high reproductive potential. They are not individuals in poor condition that were forced into the vineyards. The presence of grapes in their digestive systems indicates that they were drawn to the vineyards because of the availability of grapes.
The grape harvest in Israel takes place shortly before the chameleon egg-laying season. Because common chameleons have a short lifespan (some researchers believe they usually breed only once in their lifetime), it is likely that many of the harvested chameleons died before reproducing. This is not the case for starred agamas, whose egg-laying season occurs earlier.
In summary, it is difficult to unequivocally state that vineyards constitute an ecological trap for chameleons. However, in light of our findings, this seems like a plausible possibility. Similar phenomena may occur among many wildlife species inhabiting various agricultural crops worldwide. We recommend that future studies take this into account. Solutions must preserve agricultural yield while minimizing harm to biodiversity and preventing agricultural landscapes from “siphoning” individuals from natural populations, which could cause their collapse. All stages of the agricultural cycle must be considered, as well as how specific farming practices interact with the life cycles of different species.
The study described in this article [3] was conducted by Liran Sagi, Akiva Topper, Oded Keynan, Amos Bouskila and Oren Kolodny.
Hebrew editing: Smadar Raban
English editing: Gloria Volohonsky
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PhD student at the Department of Ecology, Evolution & Behavior at the Hebrew University of Jerusalem.
