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EcologyJournal of Animal Ecology
Peer-reviewed journal article

Functional response predicts invasiveness but not trophic impact

Marine CourtoisiD, Chloé SouquesiD, Yann VoituroniD, Loïc TeulieriD, Vincent MédociD, François‐Xavier Dechaume‐MoncharmontiD

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Abstract

Biological invasions are a major driver of biodiversity erosion mainly because invasive species show greater trophic impact than their non-invasive counterparts. The experimental paradigm for assessing this trophic impact is the functional response (FR) test that describes the relationship between per capita consumption rate and resource density. Two key parameters are then assessable and comparable between populations and species: the space clearance rate (attack rate, a) measuring predatory efficiency at low prey densities, and handling time (h) representing the time required to capture, handle, and digest prey. This test is frequently conducted to compare non-invasive and invasive species and shows that invasive species have a higher FR than non-invasive species (characterized by higher space clearance rates and lower handling times) which would explain both their invasion success and their ecological impact. However, it appears that whether FR parameters differ between invasive species sampled in their native versus invasion range has never been tested, implicitly assuming that FR measures can be extrapolated to the entire range of distribution. Using a phylogenetically corrected comparative analysis of 269 FR observations from 45 freshwater fish species (23 non-invasive species and 22 invasive species), we confirm that invasive species exhibited higher FR than non-invasive species. However, this pattern holds true only when considering invasive species sampled in their native range. Invasive species studied in their invasion range displayed functional responses comparable to non-invasive species, with similar space clearance rates and handling times. Additionally, space clearance rates decreased with temperature in non-invasive species but tended to increase in invasive species from invasive introduction ranges, suggesting that climate warming may exacerbate competitive asymmetries. Together, these results indicate that high FR predispose species to invasiveness, but also challenge the assumption that FRs measured in the native range of a species can be directly extrapolated to predict its trophic impacts elsewhere. Our findings call for greater consideration of biogeographic context when using functional responses to assess invasion risk and ecological impact. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=109 SRC="FIGDIR/small/666957v2_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@c9f9aaorg.highwire.dtl.DTLVardef@262f48org.highwire.dtl.DTLVardef@ea5148org.highwire.dtl.DTLVardef@17e84ce_HPS_FORMAT_FIGEXP M_FIG C_FIG

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In-depth summary

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Claude Opus 5.5 · xhigh effort · 8 Nov 2026 · from the full text of version 2 (26 Jan 2026) · CC BY 4.0

Across 269 functional responses from 45 freshwater fish species, invasive species exceed non-invasive species only when sampled in their native range; introduced populations match non-invasive species.

Comparative functional response (FR) experiments are used to predict an invader's trophic impact and invasiveness. Yet most invasive-species FR come from native-range populations, and an earlier FoRAGE fish analysis neither separated sampling range nor modelled phylogeny; splitting donor from introduced populations shows that native-range FR may flag invasiveness but not impact in recipient communities.

Key results

  1. ShownWhen all invasive populations are pooled, invasive fish have higher space clearance rates, shorter handling times and higher FR ratios than non-invasive species. The effects are weaker when parameters are standardized by biomass.

    Invasive vs non-invasive, standard units: a posterior mean -1.07, 95%CrI [-1.63; -0.55], pMCMC = 6.7×10^−4; h 0.82 [0.18; 1.54], pMCMC = 0.022; FRR -1.8 [-2.83; -0.94], pMCMC = 3.0×10^−4. Biomass units: a pMCMC = 0.043, h pMCMC = 0.056, FRR pMCMC = 0.019. · Effect of the invasive status; DISCUSSION

  2. ShownSeparating invasive species by sampling range gives a much better fit. Native-range donor populations have clearly higher FR than introduced populations, and the pattern holds after removing Salmonidae and in a paired model of three species.

    Model 2 (sampling status) beat Model 1 by ΔDIC > 6 for all standard-unit parameters. IS-NR vs IS-IR posterior means: a 0.88 [0.49; 1.27], pMCMC < 3×10^−4; h -1.18 [-1.89; -0.44], pMCMC = 1.3×10^−3; a/h 2.11 [1.22; 2.97]. Paired three-species model: a 0.46 [0.30; 0.63], pMCMC < 3×10⁻⁴. · Effect of the sampling status; Tables

  3. SuggestedIntroduced (invasion-range) populations show no detectable difference from non-invasive species in a, h or a/h. The overall invasive advantage therefore comes from donor populations; it does not appear to arise after introduction.

    IS-IR vs NIS, standard units: a -0.56, 95%CrI [-1.15; -0.02], pMCMC = 0.062; h 0.038 [-0.83; 0.82], pMCMC = 0.92; a/h -0.51 [-1.55; 0.50], pMCMC = 0.33. The IS-IR group has 11 species and 61 estimates. · Effect of the sampling status; Invasive status and geographic range of sampling; DISCUSSION

  4. ShownThe effect of assay temperature on space clearance rate depends on sampling status: clearance rate falls at warmer temperatures in non-invasive species but not in introduced populations. Handling time does not depend on temperature.

    Status × temperature interaction for a: ΔDIC > 12. NIS slope -0.095, 95%CrI [-0.136; -0.0526], pMCMC < 3×10^−4; IS-IR shows a non-significant positive tendency (Table S3). Same pattern for a/h (ΔDIC > 2). The most parsimonious h model omits temperature. Biomass-unit results are consistent. · Effect of the temperature; DISCUSSION

  5. ShownAll FR parameters carry a strong phylogenetic signal, strongest for space clearance rate, so cross-species FR comparisons need to account for shared ancestry.

    Lynch's phylogenetic heritability > 0.50 for every parameter. a = 0.93, 95%CI [0.89; 0.96] in standard units, and 0.82 [0.72; 0.91] in biomass units. · Phylogenetic signals; Phylogenetic tree

Methods, data and reproducibility

Comparative analysis of standardized type II (Holling disk) FR parameters (a, h, a/h) from FoRAGE, with invasive status taken from GRIIS. Log-transformed parameters were fitted with Bayesian phylogenetic mixed models (MCMCglmm), and models with invasive status, sampling range and temperature were compared by DIC, in both per-capita and biomass units.

  • Standardized FoRAGE v5 type II FR estimates for freshwater fish predators in 3D arenas, with reported temperature and sampling range; compiled set deposited on Zenodo · freshwater fish (45 species) · n = 269 estimates, 45 species, 40 studies (353 identified) · 10.5281/zenodo.18377688
  • Biomass-standardized subset (a and h scaled by predator and prey mass) where prey body mass was reported · freshwater fish · n = 254 estimates for h and a/h
  • Non-invasive species (NIS), all sampled within their native range · freshwater fish · n = 23 species, 19 studies, 101 estimates
  • Invasive species sampled in their native range (IS-NR, donor populations) · freshwater fish · n = 13 species, 20 studies, 107 estimates
  • Invasive species sampled in their invasion range (IS-IR, introduced populations) · freshwater fish · n = 11 species, 13 studies, 61 estimates
  • Invasive species with FR measured in both native and invasion ranges, used for paired within-species comparison · fish (Gambusia affinis, Lepomis macrochirus, Poecilia reticulata) · n = three species
  • GRIIS country-level checklists used to assign invasive status and native range to each FR observation · freshwater fish · n = 22 invasive species classified
Methods & tools
FoRAGE functional response database (version 5), GRIIS (Global Register of Introduced and Invasive Species), Holling type II disk equation, MCMCglmm (Bayesian phylogenetic mixed models), FishPhyloMaker (R package), Deviance information criterion (DIC) model selection, Lynch's phylogenetic heritability, R (version 4.4.3), PRISMA screening flowchart
Data & accessions
https://doi.org/10.5281/zenodo.18377688
Reagents & environment
R 4.4.3; MCMCglmm assuming Brownian trait evolution, 3×10^5 iterations, 300 burn-in, thinning interval 100, inverse-Wishart prior; FishPhyloMaker for the phylogeny; FoRAGE version 5 (20 December 2024).
Reproducibility gaps
No analysis code repository is linked; only the compiled dataset is deposited on Zenodo. Package versions for MCMCglmm and FishPhyloMaker are not given.
Limitations (6)
  • The analysis relies on published FR compiled in FoRAGE, so publication bias toward positive invasive-vs-native findings cannot be ruled out. The authors suggest their estimates may be an upper bound on the true effect. · stated by the authors
  • Only species that went on to establish invasive populations are included. Because FR data on failed invaders are unavailable, the idea that high native-range FR helps a species establish abroad cannot be formally tested. · stated by the authors
  • A direct within-species comparison of native and invasion ranges was possible for only three species, and Gambusia affinis had a single introduced-range observation, so its pattern could not be confirmed. · stated by the authors
  • The study is limited to freshwater fish, whose FR literature is uneven and biased, and to public data in which some regions and taxa are under-represented. Whether the results hold for other taxa is untested. · stated by the authors
  • The finding of no difference between introduced and non-invasive populations is borderline. The standard-unit a contrast has 95%CrI [-1.15; -0.02] (pMCMC = 0.062), and the biomass-unit analysis without Salmonidae reports a significant difference (pMCMC = 0.0060). · noted in this summary
  • The warming inferences rest on differences in laboratory assay temperature across studies, not on within-population thermal experiments. Species identity, protocol and acclimation could therefore confound the temperature-by-status interaction. · noted in this summary

Who should read:Read this if you use comparative functional response experiments to screen fish invaders or predict their trophic impact, and want to see how sampling range, phylogeny and temperature change the conclusions.

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