Consequences of climate-induced range expansions on multiple ecosystem functions

The study addresses how climate-driven species range expansions alter ecosystem functioning in natural systems. Using a 30-year census of larval caddisfly assemblages in subalpine ponds, the authors combine long-term community composition with species functional traits (N and P excretion rates and detritus processing) to predict species’ contributions to key processes. They focus on the dominant resident Limnephilus externus, subdominant residents (Asynarchus nigriculus, Agrypnia deflata), and three range-expanding species (Limnephilus picturatus, Grammotaulius lorretae, Nemotaulius hostilis). Primary hypotheses: (1) Successive range expansions will reduce abundance of the dominant resident L. externus and its historically large relative contributions to nutrient supply and detritus processing, while range-expanding species will increase in abundance and contributions. Secondary objective: (2) Assemblage evenness will increase with new species and declines in dominance, increasing functional redundancy and decreasing aggregate variability in species’ contributions to ecosystem processes. The work leverages trait-based scaling to mechanistically predict species’ effects where direct in situ isolation of animal-driven N and P supply is difficult.

The paper situates range expansions as common under climate change and notes that community composition changes can alter ecosystem processes (citing global evidence of rapid range shifts). It highlights trait-based frameworks linking organismal physiology to ecosystem fluxes, while acknowledging that abiotic factors and biotic interactions also influence processes. Prior work in the study system shows caddisflies dominate detritivore biomass and coarse detritus breakdown; animal-driven nutrient supply can constitute large proportions of demand, with high interspecific variation in excretion and processing rates. Broader literature cited includes effects of biodiversity and evenness on ecosystem functioning and variability, the role of consumer stoichiometry, and invasion biology predictions based on traits and life histories. This foundation supports a trait-based approach to predict consequences of compositional change over long timescales.

Study system: Subalpine ponds in the Mexican Cut Nature Preserve (3560 m) in Colorado with >60 kettle-pond habitats. Annual caddisfly community censuses were conducted 25 of 30 years (1989–2019) using four 0.33 m² D-net littoral sweeps per pond between late June–mid-July across 3–7 permanent ponds (plus semi-permanent and temporary ponds). Focal taxa include resident L. externus (dominant), As. nigriculus, Ag. deflata, and range expanders L. picturatus (arrived 1998), G. lorretae (2006), N. hostilis (2016). Functional traits: species-specific nutrient (N, P) excretion rates and detritus processing rates from prior studies; excretion consistent within species and varies predictably with instar, while interspecific differences are large; lab- and in situ-derived processing rates align within ~20% and predict assemblage-level processing additively. Predictions of ecosystem process contributions: For each pond-year, species’ contributions were calculated as products of average density, final instar mass, larval development rate, and either excretion rate (for N, P supply) or detritus processing rate. A random sampling framework (1000 iterations per pond-year) incorporated variation in species-specific rates and corrected for sampling bias and missing pond morphometry: each iteration simulated an “average permanent pond” by sampling hydroperiod-specific normal distributions for pond area and proportion of habitable littoral area, rescaling densities to per m² of total pond area. Species-specific contributions were averaged across iterations to estimate pond-year contributions; species’ totals were summed to get assemblage totals, and species’ relative contributions (proportion of assemblage total) were computed. Species were grouped as dominant resident (L. externus), subdominant residents (Ag. deflata, As. nigriculus), and range-expanding species (L. picturatus, G. lorretae, N. hostilis) for comparisons. Aggregate variability: For each year, aggregate variability in species’ contributions (per process) was computed as the sum of among-pond variances of species’ contributions plus 2× the sum of covariances among all species pairs; values were square-root transformed to return to original process units and improve normality. Statistical analyses: A categorical factor “Range Expansion” defined four periods: pre-expansion (≤1997), 1st expansion (1998–2005), 2nd (2006–2015), 3rd (2016–2019). Mixed-effects models (R 4.0.2; lme4) assessed effects of Year, Range Expansion, Group/Species, and interactions on species’ relative abundances and relative contributions; Pond was a random effect with a third-order autocorrelation structure (ACF) for Year nested within Pond. For assemblage-total predicted processes, Year and Range Expansion (and interaction) were fixed effects. Post hoc slope (trend) comparisons within expansion periods used emmeans with linear contrasts vs slope=0. Evenness (Pielou’s J) was related to aggregate variability via linear models.

- Species composition and contributions: Species’ relative abundances changed over time (significant species × year × range expansion; Table S1), as did relative contributions to processes (Table S2). No directional trends in abundances or relative contributions during the 1st (1998–2005) or 2nd (2006–2015) expansions (linear contrasts p > 0.05). - Third expansion effects (2016–2019, N. hostilis): L. externus and As. nigriculus declined by ~1.9 ind/m²/year (p=0.012; p=0.054), while N. hostilis increased by ~1.4 ind/m²/year (p < 0.001) and surpassed L. externus in abundance by 2018. L. externus relative contributions declined annually by 11.2% for P supply (p < 0.001) and 13.9% for detritus processing (p < 0.001); range-expanding species’ contribution to P supply increased by 14.5% annually (p < 0.001). Subdominant residents showed no trends in contributions during any expansion and remained relatively low (<38% of P; <37% of detritus; p=0.917; p=0.464). - Nitrogen contributions: Subdominant resident Ag. deflata consistently supplied large proportions of N (~70.8%) across all expansions. It matched/exceeded L. externus N contributions in 75.6% of pond-year cases despite equal/greater density in only 22.8% of cases. - Total assemblage process rates: No detectable trends in total predicted caddisfly contributions to N supply, P supply, or detritus processing over 30 years (Figure S1A–C). - Pre-expansion declines (early 1990s): Prior to any range expansions, L. externus, Ag. deflata, and As. nigriculus declined by 1.7, 1.3, and 1.6 ind/m²/year (L. externus p < 0.001; Ag. deflata p=0.027; As. nigriculus p < 0.001). L. externus relative contributions to P and detritus declined by 9.0% annually (P: p=0.002; detritus: p < 0.001). - Range-expander performance: N. hostilis matched/exceeded L. externus relative abundance ~19.4× and ~24.2× more often than L. picturatus and G. lorretae, respectively (noting fewer surveyed ponds post-2016). It exceeded L. externus contributions more frequently than the prior two expanders combined by factors of 7.1× (P), 4.5× (N), and 6.9× (detritus). - Evenness and variability: Evenness (Pielou’s J) declined over time (F1,23=5.73, p=0.029, R²=0.20) and did not differ among or change within expansion periods. Aggregate variability in species’ contributions was negatively related to evenness for all processes: P (F1,23=17.61, p < 0.001, R²=0.42), N (F1,23=5.74, p=0.025, R²=0.17), detritus (F1,23=17.83, p < 0.001, R²=0.43). From least to most even assemblages, aggregate variability declined by 10.3× (P), 9.9× (N), and 3.7× (detritus).

Findings show that shifts in species’ relative functional roles can precede detectable changes in total ecosystem process rates. During the first two expansions, the dominant resident L. externus effectively regulated P supply and detritus processing; a subdominant resident, Ag. deflata, consistently regulated N supply due to its high per-capita excretion trait. The third expansion by N. hostilis altered this balance: L. externus lost dominance in P supply and detritus processing while the range-expanding group increased P contributions, indicating rising redundancy and potential functional displacement of the dominant resident. These results suggest that range expansions do not necessarily alter total ecosystem process rates, possibly due to energetic constraints or resource limitations, but can redistribute functional roles among species. Life history traits, particularly N. hostilis’s distinct developmental phenology (autumn hatching, early larval development when competition and predation are reduced, potentially extended by later winters), likely underpin its demographic success and enhanced contributions. Evenness declined over time, and lower evenness was associated with higher aggregate variability in process contributions, highlighting that dominance by a single taxon can amplify variability in ecosystem functioning. Cross-taxa context indicates that while other invertebrate groups (e.g., dipterans, zooplankton) may buffer total animal-driven nutrient supply, they are unlikely to substitute caddisflies for coarse detritus processing, implying process-specific sensitivity to compositional change.

Dominant and subdominant resident species can regulate ecosystem processes through sequential range expansions by taxa with similar life histories, but expansions by species with distinct life histories can shift relative functional roles and potentially displace dominant residents. Trait-based scaling combined with long-term community data provides mechanistic predictions of species’ contributions when direct measurements are challenging. If the N. hostilis expansion persists, it may functionally replace L. externus and could increase total process rates. Future research should test the generality of these patterns across systems, incorporate trait novelty relative to entire communities, assess buffering by non-focal taxa for different processes, and continue long-term observations to capture transient versus persistent dynamics.

- Scope limited to caddisflies: long-term range shift data and trait-based predictions were applied only to caddisflies; contributions are relative to the caddisfly assemblage rather than all fauna. - Estimation approach: ecosystem process rates are predicted from traits and densities rather than measured directly each year; although validated previously, predictions rely on assumptions (e.g., additive effects, consistent species-specific rates). - Sampling and scaling: densities were derived from littoral D-net sweeps; to correct spatial bias and missing morphometry, an average pond simulation was used, which introduces model uncertainty. - Incomplete pond metadata: lack of total area and habitable area for all ponds necessitated simulation-based rescaling. - Temporal and spatial coverage: not all ponds were sampled annually; some were surveyed opportunistically; the third expansion period is short (2016–2019) with fewer ponds, potentially limiting power to detect trends. - Generalizability: outcomes may be system- and process-specific; other taxa can buffer nutrient supply but not detritus processing, complicating extrapolation to whole-ecosystem functioning. - Potential constraints: total process rates may be constrained by energetic equivalence or detrital resource supply/quality, limiting detectable changes in totals despite shifts in roles.