Predator-induced collapse of niche structure and species coexistence | Robert M. Pringle et al. | ResearchPod

Transcript: Predator-induced collapse of niche structure and species coexistence

[[RP_SECTION:predator-impact-on-coexistence|Predator impact on coexistence]]

Alex: A six-year experiment on sixteen small Bahamian islands suggests that a top predator can undermine coexistence, apparently without eating much of its prey. The work is from Robert Pringle and colleagues, who manipulated lizard communities directly.

Sam: That cuts against the classic keystone model, where the predator suppresses the dominant competitor and makes room for everyone else. But if the predator isn't eating the prey, how do populations decline, and how do you get a mechanism out of an experiment like this? [[RP_SECTION:diet-and-spatial-mechanisms|Diet and spatial mechanisms]]

Alex: The diet data come first. They used DNA metabarcoding on fecal samples from the curly-tailed lizards, the predator in this system. The samples were mostly arthropods, and anoles turned up only rarely. So it looks like a generalist, and lethal predation doesn't account for the declines.

Sam: That only tells you what the predator isn't doing.

Alex: Right, so the mechanism is spatial, and the design is what lets you see it. Brown anoles, green anoles and curly-tailed lizards were added to the islands in different combinations. You can compare each species alone against the pairings and the full assemblage. Without the predator, brown anoles stay near the ground and green anoles occupy the arboreal space, so the two partition the habitat. The curly-tailed lizard patrols the ground, and perch-height observations show the brown anoles shifting significantly higher.

Sam: So the fear response pushes them into the trees, where the green anoles already are. [[RP_SECTION:niche-compression-and-competition|Niche compression and competition]]

Alex: Yes. That non-consumptive behavioral shift is the load-bearing piece. It collapses the niche separation, and interspecific competition intensifies in a smaller shared space. The paper reports population declines and, on some islands, extinction of the native anoles.

Sam: One thing doesn't sit right. The green anoles are the arboreal specialists. Shouldn't they win the competition in the trees?

Alex: The explanation offered is a priority effect. The brown anoles were established first and at higher abundance, so even a species better suited to the trees can be outnumbered. On top of that, the predator-driven compression made conditions unstable. I'd treat this part as the authors' interpretation rather than something the design isolates cleanly.

Sam: Fair. So what carries the main claim? [[RP_SECTION:experimental-design-and-analysis|Experimental design and analysis]]

Alex: Three things: the perch-height shift, the diet data showing infrequent predation, and the community-level outcome in the factorial design. The food-chain-length analysis is the clearest example of the last one. Adding either the predator or the second anole species increased food-chain length, but adding both together reduced it.

Predator-induced collapse of niche structure and species coexistence

Robert M. Pringle, Tyler R. Kartzinel, Todd M. Palmer, Timothy J. Thurman, Kena Fox-Dobbs, Charles C. Y. Xu, Matthew C. Hutchinson, Tyler C. Coverdale, Joshua H. Daskin, Dominic A. Evangelista, Kiyoko M. Gotanda, Naomi A. Man in ’t Veld, Johanna E. Wegener, Jason J. Kolbe, Thomas W. Schoener, David A. Spiller, Jonathan B. Losos, Rowan D. H. Barrett

Jun 5, 2019·14:52·user_upload·Original Paper

Keystone predationNiche partitioningNon-consumptive effectsTrophic omnivoryInterspecific competitionBrown anole (Anolis sagrei)

Listen

15 min

Briefing

Research Question

This study investigated how the introduction of top predators influences the coexistence of competing prey species. While classic ecological theory suggests that predators often promote biodiversity by preventing competitive exclusion (the keystone predation model), the authors tested whether predator-induced behavioral changes—specifically the fear-driven avoidance of predators—might instead collapse niche structure and destabilize coexistence.

Approach

Researchers conducted a six-year, whole-ecosystem manipulation on 16 small islands in the Bahamas. They used a 2x2 factorial design, introducing curly-tailed lizards (the top predator) and/or green anoles (a competitor) to islands already inhabited by brown anoles. The team tracked population trajectories, habitat use, and diet composition using DNA metabarcoding and stable-isotope analysis to determine how these species interacted and partitioned resources under different predation regimes.

Findings

Contrary to the keystone predation model, the presence of curly-tailed lizards did not facilitate the coexistence of the two anole species. Instead, the predator induced a behavioral shift: brown anoles, which typically occupy a more terrestrial niche, were forced into the arboreal habitat used by green anoles to avoid predation. This collapse of spatial niche partitioning forced the two species into direct competition for the same limited resources. Consequently, green anole populations were heavily suppressed and often went extinct on islands where the top predator was present. Furthermore, the study found that the top predator acted as a trophic omnivore, which shortened food chains by competing with the prey for arthropod resources.

Why It Matters

This research challenges the long-standing assumption that top predators generally enhance biodiversity. It demonstrates that the non-consumptive effects of predation—such as the fear-driven restriction of prey to specific microhabitats—can be a powerful force that limits species coexistence. These findings highlight the importance of considering spatial structure and prey behavior when predicting the outcomes of biological invasions.

Key Terms and Definitions

Intraguild Predation — A complex interaction where a predator species both competes with and consumes the same prey species.
Niche Partitioning — The process by which competing species use the environment differently in a way that helps them coexist.
Trophic Omnivory — The behavior of a consumer that feeds on multiple trophic levels, which can significantly alter food-web structure and food-chain length.
Keystone Predation — A model where a predator promotes prey diversity by disproportionately consuming a dominant competitor, thereby preventing it from excluding other species.
Refuge-Competition Model — A framework where predator-induced avoidance behavior forces prey into limited, shared refuges, thereby intensifying interspecific competition and destabilizing coexistence.

Predator-induced collapse of niche structure and species coexistence: Research Paper Podcast