Biotic Resistance Hypothesis

Biotic resistance hypothesis (also known as the diversity-invasibility hypothesis or Elton's hypothesis) is a foundational ecological principle proposing that resident biological communities with high native species richness and functional diversity are more resistant to the establishment, proliferation, and ecological impact of non-native invasive species than species-poor or disturbed communities. Formulated systematically by the British ecologist Charles S. Elton in his 1958 monograph The Ecology of Invasions by Animals and Plants, the hypothesis links biodiversity conservation directly to ecosystem stability across terrestrial, freshwater, and marine habitats of the biosphere.

While Elton drew his initial inference from broad natural history comparisons—contrasting the vulnerability of remote oceanic islands and simplified agricultural monocultures with the apparent stability of undisturbed tropical forests—subsequent experimental and theoretical research has revealed that biotic resistance operates through specific mechanistic pathways whose net relationship with native diversity flips across spatial scales, a phenomenon termed the invasion paradox.

Biotic resistance does not arise from species counting alone; rather, it emerges from trophic and competitive interactions among resident organisms that limit the demographic growth rate (r) of arriving propagules:

Resource Preemption and Niche Complementarity

According to resource-ratio theory (Tilman's R* rule), a newly arriving propagule can establish in a community only if unconsumed limiting resources—such as soil nitrate (NO₃⁻), orthophosphate, light, rooting space, or water—exceed the invader's minimum zero-net-growth requirement. In a diverse native assemblage exhibiting niche complementarity, species differ in phenology (e.g., cool-season C₃ vs. warm-season C₄ grasses), rooting architecture (combining shallow fibrous roots with deep taproots capable of hydraulic redistribution), and light interception profiles. Together, complementary native guilds draw available soil nitrogen, moisture, and understory irradiance down to low steady-state concentrations, leaving little open niche space for exotic colonists.

The Sampling Effect and Dominant Competitors

Even when niche complementarity is modest, high-diversity plots have a higher statistical probability of containing at least one competitive native species whose functional traits closely match—and outcompete—the arriving exotic species. This sampling (or selection) effect often pairs with biochemical interference mechanisms, such as rhizosphere juglone allelopathy or antimicrobial secondary metabolites produced by native mycorrhiza associations, which suppress exotic seed germination and seedling survival.

Resident Enemies: Predators, Herbivores, and Pathogens

While the enemy release hypothesis notes that invaders leave behind coevolved specialist enemies from their native range, resident generalist herbivores, granivores, omnivorous crustaceans, and soil oomycete pathogens still impose substantial mortality on newly introduced propagules. In intact marine benthic communities and freshwater microcosms such as phytotelmata, predation by native top consumers frequently prevents exotic larvae from reaching reproductive maturity even when abiotic conditions are favorable.

For decades after Elton's 1958 work, field ecologists puzzled over contradictory empirical findings: small-plot grassland experiments consistently showed that higher native species richness reduced exotic invasion, whereas regional vegetation surveys across landscapes, states, and river basins showed that hotspots of native biodiversity also harbored the highest numbers of naturalized non-native species.

Resolved by Jonathan Levine, Carla D'Antonio, and Tom Stohlgren in the late 1990s and early 2000s, this invasion paradox arises from scale-dependent covariance between intrinsic biotic resistance and extrinsic environmental heterogeneity:

At the neighborhood scale (1 m²), climate, soil fertility, and disturbance history are uniform across experimental replicates; thus, adding native plant guilds directly reduces open light and soil nitrate, revealing the negative causal effect of biotic resistance. Across broad landscapes (100 km²), however, benign climates, fertile alluvial floodplains, and heterogeneous topography simultaneously promote high native species richness and high exotic colonization ("the rich get richer"), masking local competitive resistance unless environmental covariates and propagule pressure are statistically controlled.

Even in diverse native communities, biotic resistance can be temporarily or permanently overridden by two factors: fluctuating resource availability and abiotic edaphic stress filters. As formulated in Davis, Grime, and Thompson's (2000) fluctuating resource theory of invasibility, any pulse of unused resources—whether triggered by wildfire canopy removal, nitrogen eutrophication, overgrazing, or allochthonous nutrient inputs such as seabird guano subsidies—briefly relaxes competitive resource preemption and opens an invasion window.

Conversely, in abiotically harsh environments such as ultramafic serpentine soils characterized by low calcium-to-magnesium ratios and elevated nickel concentrations, native plant cover and biomass are naturally sparse. In these edaphic islands, abiotic physiological stress—rather than biotic competition—serves as the primary barrier excluding non-adapted exotic grasses, until atmospheric nitrogen deposition fertilizes the soil and enables fast-growing exotic annuals to overcome the edaphic filter.

What is the invasion paradox in community ecology?

The invasion paradox is the observation that native species richness and exotic species richness are negatively correlated at small neighborhood scales (1–4 m² plots) due to direct competitive biotic resistance, but positively correlated at large landscape and regional scales because favorable climate, high soil fertility, and habitat heterogeneity support high numbers of both native and non-native species.

Does high native biodiversity completely prevent biological invasions?

No. High native diversity reduces the establishment rate and lowers the biomass and ecological dominance of arriving exotic species, but it rarely reduces invasion probability to zero—especially when high propagule pressure or sudden resource pulses (such as fire, soil disturbance, or nutrient fertilization) create temporary windows of unconsumed resources.

How do niche complementarity and the sampling effect differ in biotic resistance?

Niche complementarity occurs when multiple native species with different root depths, phenologies, and nutrient requirements partition resources so thoroughly that almost none remain for an invader. The sampling effect occurs when a diverse community is statistically more likely to include one highly dominant native competitor that single-handedly suppresses the invader.

1. Elton, C. S. (1958). The Ecology of Invasions by Animals and Plants. Methuen & Co. https://doi.org/10.1007/978-1-4899-7214-9
2. Levine, J. M., & D'Antonio, C. M. (1999). Elton revisited: A review of evidence linking diversity and invasibility. Oikos, 87(1), 15–26. https://doi.org/10.2307/3546992
3. Fridley, J. D., Stachowicz, J. J., Naeem, S., Sax, D. F., Seabloom, E. W., Smith, M. D., Stohlgren, T. J., Tilman, D., & Von Holle, B. (2007). The invasion paradox: Reconciling pattern and process in species invasions. Ecology, 88(1), 3–17. [https://doi.org/10.1890/0012-9658(2007)88[3:TIPRPA]2.0.CO;2](https://doi.org/10.1890/0012-9658(2007)88[3:TIPRPA]2.0.CO;2)
4. Kennedy, T. A., Naeem, S., Howe, K. M., Knops, J. M. H., Tilman, D., & Reich, P. (2002). Biodiversity as a barrier to ecological invasion. Nature, 417(6889), 636–638. https://doi.org/10.1038/nature00776
5. Levine, J. M., Adler, P. B., & Yelenik, S. G. (2004). A meta-analysis of biotic resistance to exotic plant invasions. Ecology Letters, 7(10), 975–989. https://doi.org/10.1111/j.1461-0248.2004.00657.x