Introduced species that overcome life history tradeoffs can cause native extinctions

Abstract

Introduced species threaten native biodiversity, but whether exotic species can competitively displace native species remains contested. Building on theory that predicts multi-species coexistence based on a competition-colonisation tradeoff, we derive a mechanistic basis by which human-mediated species invasions could cause extinctions through competitive displacement. In contrast to past invasions, humans principally introduce modern invaders, repeatedly and in large quantities, and in ways that can facilitate release from enemies and competitors. Associated increases in exotic species' propagule rain, survival and competitive ability could enable some introduced species to overcome the tradeoffs that constrain all other species. Using evidence from metacommunity models, we show how species introductions could disrupt species coexistence, generating extinction debts, especially when combined with other forms of anthropogenic environmental change. Even though competing species have typically coexisted following past biogeographic migrations, the multiplicity and interactive impacts of today's threats could change some exotic species into agents of extinction.

Fig. 1

Simulated species extinctions resulting from human-mediated species invasions. a Competition-colonisation tradeoff surface (grey line) of 20 coexisting species (blue circles) in a metacommunity, going from the best competitor (s₁) to the worst competitor (s₂₀). Species’ competitive ranks are inverse to the rank order of species’ colonisation abilities (this is the tradeoff). Note that, regardless of positions on the X-axis, differences in species’ competitive abilities are determined by rank only such that the competitive difference between s₁ and s₂ is the same as the competitive difference between s₁₈ and s₁₉. b Schematic drawing of disrupted tradeoff showing shifted position of designated “invader” (s₁₀, the intermediate competitor in the metacommunity, white circle indicates h₁₀ = 0) reflecting input of external colonists following Eq. 1 (i.e. increasing h₁₀ from 0 to 0.025 (yellow), 0.05 (red) and 0.1 (violet); shifted positions of s₁₀ on the figure were chosen to reflect the number of observed extinctions: 3 for h₁₀ = 0.025, 5 for h₁₀ = 0.05 and 7 for h₁₀ = 0.1); blue circles indicate “native” species that are superior competitors and thus unaffected by the invasion; grey circles indicate the ten natives that could be displaced by s₁₀. c Relative abundance (log scale) of each of the 20 species over time. Each line represents a species: the red line indicates the invader after elevating h₁₀ to 0.05; blue lines indicate persistent native species; grey lines indicate native species driven to extinction (i.e. relative abundance drops below 0.0001 of species’ original equilibrium abundance) as a consequence of increasing h₁₀. d A timeline of percentage of the 19 native species driven to extinction as a consequence of elevating h₁₀ to 0.025, 0.05 (same scenario shown in c) and 0.1

Fig. 2

Native species extinctions resulting from species invasion in 20-species metacommunities where the invader experiences an increase in external colonists (h_i), and a reduction in mortality (m_i). For each line in each panel, a new species has been introduced that would normally coexist with the 19 other species in the metacommunity given their natural colonisation and competitive abilities (i.e. the invader naturally falls upon the universal tradeoff surface). However, input of external colonists (h_i, shown in panel a) and reduced mortality (m_i, shown in panel b) (changes in h_i and m_i are both measured on the X-axis) of the invader allows it to overcome this tradeoff and cause extinctions. Where lines plateau, all inferior competitors have been driven to extinction. The competitive rank of the invaders is indicated by different coloured lines: s₁ = pink (superior competitor), s₇ = orange, s₁₃ = light blue, s₁₉ = dark blue. Lines indicate average effects across an ensemble of 250 metacommunities; shaded areas enclose 95% of the metacommunity responses

Fig. 3

Native species extinctions resulting from elevated disturbance and invasion of a species with a supply of external colonists (h_i). Disturbance increases the mortality rate of all 20 species in the metacommunity from 0.05 to a 0.06, and b 0.35. Other details as in Fig. 2. In all panels, species are lost from the metacommunity due to increased disturbance alone (i.e. when h_i = 0). Invasion of s₇ causes more extinctions than s₁ under high disturbance (shown in b) because the natural colonisation rate of s₁ (c₁) is too low to cope with elevated mortality, unless increases in h₁ compensate for it