Fragmentation Begets Fragmentation
The latest argument in ecologists’ fragmentation debate
Not long ago, you could hear the melodious rustle of wind in the tall grasses across Iowa. Now, most of it is filled with the more discordant sounds of wind in corn fields. Less than 0.1% of Iowa’s tallgrass prairie remains. The rest has been replaced by industrial farms.
When habitat shrinks, it often also fragments into smaller and more isolated patches. Iowa is an extreme case of this and is where Rob Fletcher, now professor of landscape ecology and conservation at the University of Florida, first became interested in habitat fragmentation. But habitat loss and fragmentation are everywhere there is human development. Habitat loss is one of the main reasons biodiversity is declining. The question of whether habitat fragmentation is harmful even when the amount of habitat does not change is highly contested. To make matters more complicated, scientists disagree about whether habitat fragmentation should be studied at the scale of individual patches or collections of patches. A study Fletcher recently co-authored in Ecology is the latest argument in this fragmentation debate.
The fragmentation debate concerns whether it is more important to conserve small, isolated patches or the few large ones that remain. “That can have real consequences in terms of how people manage land,” says Fletcher.
Less than an hour from the University of Florida lies the Ordway-Swisher Biological Station. The smell of pine and songs of birds fill the air. Though in the summer, “You may not notice the sounds and the smells because you’re sweating so badly,” says Bob Holt, University of Florida biology professor who co-authored the study. The grasslands there are dotted with prickly pear cacti. For insects, individual prickly pears are habitat patches. Areas of the grassland with more, smaller prickly pears that are further apart are more fragmented and more difficult for the insects to move between.
The debate, however, did not begin with prickly pear, and it goes back decades. In the past few years, it has flared up again. In a 2017 article, Lenore Fahrig, professor of biology at Carleton University, found that 76% of responses to habitat fragmentation were positive out of 118 studies that analyzed habitat fragmentation independent of habitat loss. Then, Fletcher and some others published an article, “Is habitat fragmentation good for biodiversity?” In it, they say Fahrig’s conclusions were “drawn from a narrow and potentially biased subset of available evidence” and “should be interpreted cautiously.” Fahrig and some others rebutted with another article, “Is habitat fragmentation bad for biodiversity?” in which they argue their evidence was not biased but was focused on loss and fragmentation at the scale that it is relevant — collections of patches rather than individual patches. James Watling, a professor at John Carroll University who co-authored that paper, says they showed that habitat fragmentation is rarely bad for biodiversity, and it is more important to conserve small, isolated patches. “We can write them off as having very little conservation value and focus on preservation in these pristine landscapes or we can think about what we can do to actually invest in preservation and protection of these remaining patches,” he says.
In the article Fahrig published in 2017, she reviewed 5,000 studies of habitat fragmentation and found that only 11 analyzed habitat fragmentation independent of habitat loss, and none analyzed habitat fragmentation at multiple scales. It is difficult to isolate habitat fragmentation and loss in natural systems, so Fletcher’s research group decided to do some experiments. “This experiment cuts to the heart of the core issue in the debate,” says Nick Haddad, a biology professor at Michigan State University who was not involved in the study.
Fletcher chose prickly pear as the habitat for his experiments because they are easy to remove and plant. He chose to study the effects of manipulating this habitat on a few insects — the native cactus bug, the non-native leaf-footed bug, the cochineal scale insect, and the cactus weevil — because they all feed and breed only on prickly pear. (Plus, they are “incredibly cute,” says Fletcher.)
In the first experiment, Fletcher’s research group removed prickly pear either randomly or in clusters. In the second, they removed prickly pear to create collections of patches with the same total area but different numbers and sizes of patches. They then observed how many insects were in each patch.
Research technicians worked year-round, up to six people each day, planting prickly pear, removing it, and observing the insects. Their work was delayed by some deer who devoured the prickly pear. The research technicians planted new ones. They returned a week later, and the deer had done it again. So, they planted new ones once more and, this time, added a fence.
Fletcher’s research group found that habitat fragmentation, independent of habitat loss, was harmful to the bugs both at the scale of individual patches and at the scale of collections of patches. Smaller patches had fewer insects, as did areas where there were more patches that were further apart. “Our experiment was the first to show the relationship between those patterns,” says Fletcher. They suspect these effects may be because it is more difficult for the insects to move between patches that are more fragmented, and that movement is important for their reproduction.
“It’s this brilliant experiment that shows that we can’t consider one factor in isolation, we really have to be cognizant of the effects of habitat loss and of fragmentation in reducing biodiversity and harming ecological systems,” says Haddad. These findings support the argument that it is more important to conserve large patches of habitat.
But the debate will likely rage on (in the form of scientific papers whose authors occasionally grab drinks together). Fletcher himself says a big question is still “how much can we take from this study and apply it to other situations?” Watling, who reviewed the paper, has the same question. “This is a very highly specialized species-habitat relationship, which is not necessarily representative of plenty of other species of conservation concern,” he says. Unlike the insects Fletcher’s research group studies, most animals depend on more than just one plant. And it is important to not only know how fragmentation affects four insects but how it affects biodiversity as a whole. Even so, Fletcher thinks his research group’s findings are transferrable to other species and habitats. “A lot of the patterns that we see with an insect,” for example, “we’re seeing with a bird that flies across the state of Florida,” he says. It is even more important to research whether these findings hold true for biodiversity more broadly. “The next level for this study is to move beyond the few species that were studied to whole communities of species,” says Haddad.
Back in Iowa, few large patches of prairie remain. If habitat fragmentation is indeed harmful to most species in most habitats, Iowans should do everything they can to protect those surviving rolling hills of golden grasses. In an ideal world, the question of which patches are more important to conserve would not be that relevant. Iowans could protect the large patches as well as the small ones — the ones stowed between gravel roads and corn fields, railroads and soybeans. But, at least for now, that is not our world.
