Effects of Non-native,

Introduced, Exotic,

Nonindigenous, Alien,

and/or Invasive Species

on Biodiversity

“It is not just nuclear bombs and wars that threaten us, though these rank very high on the list: there are other sorts of explosions…ecological explosions”

-- Charles Elton (1958)

Biological Invasions are Natural …it’s just that the pace has changed

Costs of Exotic Species • Direct: $137 billion per year in U.S. (> $26 billion

for control) • Can dramatically alter species composition and

ecological processes (e.g., fire, nutrient cycling) • Can lead to declines and extinctions of native

species and local populations and reduced resilience of communities

• End result may be biotic homogenization – loss of floral and faunal distinctions among regions and reduced among-site variability at multiple levels of biological organization

9.1 Accumulation of established estuarine and marine invaders in San Francisco Bay

• Many native trout are threatened by genetic introgression from introduced trout – a form of biotic homogenization

Florida Has Significant Problems • Florida and Hawaii lead the nation in problems

caused by non-native species (with possible exception now of cheatgrass-infested states)

• Many taxonomic groups in Florida have non-natives composing 25% or more of their species

• Feral pigs have destroyed native vegetation over much of the state and reduced populations of native animals

• Melaleuca has altered fire regimes and probably lowered water tables

• Water hyacinth and hydrilla have reduced light penetration and phosphorous concentrations in rivers and lakes

Policy: Maintain a huntable population

Phases of Invasion

• Immigration • Survival of immigrants in new range • Transient (where many introduced

populations go extinct) • Persistence (naturalization) • Actual Invasion – but only a few

naturalized species become pests!

I = R x A x E where: I = impact of exotic species R = range size A = abundance E = per capita effect

What Traits Characterize Successful Invaders?

• “R-selected” – high reproductive rate and good dispersal capacity

• Good luck – few natural enemies in new area

Why do only a few introduced species become pests?

• Enemy release hypothesis – invaders have reduced natural enemies in new range

• Biotic resistance hypothesis – interactions with native species, including enemies, limit the impact of invaders

• Both hypotheses are supported in recent studies of invasive plants and their fungal and viral enemies (e.g., Mitchell and Power 2003) and in vertebrates and their parasites (number of parasite species and degree of infection are much lower in exotics than in natives; e.g., Torchin et al. 2003)

Case 9.4 (A) Invasive species are often those that escape their herbivores from their native habitat

Biological Control Poses Considerable Risks

• The species introduced to control exotic pests often become pests themselves and have significant impacts on native species and ecosystems, especially if they are not the specialized predators or parasites on the exotic host that they were assumed to be.

• Biocontrol agent may act as a resource subsidy, hosting large populations of predators or parasites that have negative effects on native species.

Genetic Paradox: How do introduced populations persist and adapt through

population bottlenecks?

• Loss of genetic variation associated with founder effects and genetic drift should compromise the ability of introduced populations to adapt – unless the bottleneck allows purging of lethal recessives or actually favors rapid adaptation

• Some species circumvent potential loss of genetic variation associated with bottlenecks through multiple introductions

• There may be no bottleneck – many introductions involve a large number of propagules (especially with aquatic species, e.g., those transported by ballast water)

• Some recent research show both bottlenecks (reduced variation within haplotypes) and admixture (mixing of multiple haplotypes in founded populations)

Source populations (native range)

From Asunce et al. 2011, Science.

US introduction

Worldwide introductions

Driver vs. Passenger Model for causes and consequences of invasions

• The driver model predicts that invaded communities are highly interactive, with native species being eliminated by competition with exotics. Communities are usually assumed to be saturated with species (i.e., no vacant niches).

• The passenger model predicts that invaded communities are structured mostly by non-interactive environmental factors. Communities are not saturated, either by nature or as a consequence of disturbance.

Does Competition from Exotics Threaten Biodiversity?

• Most regions absorb more species by invasion than they lose by extinction – hence, communities are not saturated, except at very small (e.g., 1 m2) scales, as in grasslands and the intertidal zone (Sax et al. 2007)

• In contrast to intertrophic interactions (e.g., predation, parasitism, disease) and habitat loss, competition from introduced species is not as likely to cause extinctions of native species (e.g., Davis 2003)

• Counter-examples? What about those exotic vines that overgrow native vegetation, robbing them of sunlight? This is interference competition, and may occur over many hectares (not so small)

Effects of Balsam Wooly Adelgid on Fraser Fir, Smoky Mountains

Port Orford Cedar (Chamaecyparis

lawsoniana) threatened by non-native Phytophthora lateralis root fungus, spread by forest roads

Native Hawaiian honeycreepers are going extinct, in large part due to introduction of mosquitoes and avian malaria

Elton’s (1958) Biotic Resistance Hypothesis

Species-rich communities are more stable and therefore less susceptible to species invasions.

Are More Diverse Systems More Resistant to Invasion?

• Theory: Species-rich communities less susceptible to invasion because resources are utilized more completely (fewer “empty niches”) – exotics as drivers

• In small-scale experiments (grasslands, sessile marine invertebrates, microbial communities, etc.) species-rich communities are more resistant to invasion

• At broader scales, species-rich communities (including hotspots) are usually more susceptible to invasion

• When immigration is enhanced by high propagule pressure or disturbance, diversity and invasibility may be positively correlated

• High environmental heterogeneity tends to reduce population expansion of invasive species

• The environmental conditions that contribute to high richness of native species (at broad scales) also favor high species richness of nonnatives.

• Two recent papers in Ecology (Fridley et al. 2004, Herben et al. 2004) show that the observed relationship between native and exotic species richness (negative at small scales, positive at large scales) is consistent with a null or neutral model of community assembly, where the number of species is constrained by the number of individuals an area can support.

Correlation of native to non-native plant species richness in counties for each state (AL shown as example)

From: Stohlgren et al. (2003)

Correlation of native and non-native species richness by state

From: Stohlgren et al. (2003)

9.10 The ratio of introduced to native species for 177 sites and regions around the world

9.13 (B) Number of introduced plant species vs. number of native plants

In South African reserves…

Exotics Can Drive Evolutionary and Ecological Change in Native Species

• In Australia, snakes have responded to invasion of cane toads (Bufo marinus) with a (probably) genetically based increase in body size and reduction in gape size (Phillips and Shine 2004).

• Meanwhile, cane toads have been evolving longer legs, which facilitate more rapid dispersal (Phillips et al. 2006).

• Introduced foxes have dramatically altered plant productivity and composition in the Aleutians. By reducing seabird populations, foxes have reduced delivery of nutrients (guano) from sea to land, thereby reducing nitrogen availability and plant productivity (Croll et al. 2005). Vegetation has changed from grassland to tundra.

What Determines Invasion Success? Von Holle and Simberloff (2005)

• Three hypothesized determinants of biological invasion outcome were manipulated in forest plant communities in Virginia: resident diversity, physical disturbance and abiotic conditions, and propagule pressure.

• The physical environment (flooding regime) and the number of established resident species had negligible effects on invasion success.

• Propagule pressure overwhelmed ecological resistance.

Recent Findings: • Suding and Gross (2006): Native and exotic

plant richness were positively correlated in a Michigan grassland-savanna, but the addition of native seed increased native species richness relative to exotics. Native enrichment was increased by burning combined with native seeding.

• Seabloom et al. (2006): In California, exotic and native richness are positively correlated and both are correlated with net primary productivity. However, native species richness peaks at high elevation, whereas exotic species richness peaks in low-lying coastal areas, where the highest richness of imperiled native species also occurs.

Recent Findings: • Davies et al. (2007): Native and exotic plant

richness were positively correlated at three spatial scales in California serpentine grasslands. However, at the smallest scale, the relationship was positive at low-productivity sites and negative at high-productivity sites (where competitive exclusion occurs).

• Fridley et al. (2007): Although ecosystems rich in native species are likely to be hotspots for exotic species, reduction in native species richness can exacerbate invasions…so (my interpretation), maintain ecological integrity by limiting disturbance by roads, livestock grazing, and other human activities.

Human-disturbed and road-accessible sites tend to have more exotics. Exotic

invasions often increase directly with road (and sometimes trail) density.

Amur Honeysuckle (Lonicera maackii): Forest edges in Ohio woodlots are local foci for invasion,

but landscape factors controlling propagule pressure, such as distance to nearest town, determine geographic spread

Livestock: A bigger threat than all other nonnatives to imperiled plant and animal species in the U.S.? (Gurevitch and Padilla 2004)

Parker et al. (2006): Native herbivores promote resistance to plant invasions, whereas exotic herbivores (to which native plants have not been selected to resist) facilitate invasions and trigger an “invasional meltdown”

9.13 (A) Number of introduced plant species vs. number of human visitors

In South African reserves…

Will impacts of Invasive Species Increase or Decline Over Time?

• Impacts of fire ants (Solenopsis invicta) in central Texas diminished over 12 years, with native ant and arthropod diversity returning to pre-invasion levels (Morrison 2002)

• Potential reasons for reduced impacts over time: 1)

depleted resource base, 2) acquisition of natural enemies, 3) other coevolutionary phenomena

• On the other hand, many introduced species show a lag

time between initial introduction and subsequent rapid expansion, caused by 1) lag phase in population growth curve, 2) time needed for adaptation to new environment, 3) accumulated genetic variation from multiple introductions (e.g., Anolis sagrei)

Will impacts of Invasive Species Increase or Decline Over Time?

• Seabloom et al. (2006) note that exotics spread ahead of the front of human development in California, and are still on the move. They predict that exotics will rapidly spread throughout California in coming years. Hence, California has a significant “invasion debt.”

• But effects of exotics are not all bad. In the broad sense, exotics promote evolutionary diversification among both exotic and native species and create new hybrid lineages (Vellend et al. 2007).

• Over a broad span of time, which will be more important – biotic homogenization or increased diversification?

• Is excessive worrying about exotic species – and a militaristic approach to their control – an indication of xenophobia and racism?

Exotics are Not Always “Bad” • Monarch butterflies in California winter

preferentially in exotic Eucalyptus groves • The Endangered Southwestern Willow

Flycatcher prefers to nest in tamarisk over native riparian vegetation (willows, etc.)

• The declining Tricolored Blackbird nests preferentially in Himalayan blackberry bushes

• Dominant exotic grasses apparently help Garry Oak savannas maintain their open structure by preventing succession in the absence of fire

• Several exotic annual grasses help stabilize roadside slopes and other erosion-prone sites better than native species

The dingo (Canis lupus dingo) presumably was brought to Australia by

humans from SE Asia ca. 4,000 BP. Though technically non-native, it has

developed distinctive characteristics and acts as an apex predator, exerting

top-down control of populations of opportunistic non-native species. Lethal

control fractures dingo social structure and leads to population increases of

invasive mesopredators and herbivores (Wallach et al. 2010, Ecology Letters)

Research Questions • How does the type (e.g., anthropogenic vs.

natural), intensity, spatial scale, and frequency of disturbance affect susceptibility to invasion?

• What are the pathways for the arrival and spread of non-native species, and how might these pathways be blocked or filtered to reduce invasions without engendering new problems?

• To what extent are intentional introductions of non-native species for commercial purposes leading to invasions of wild habitats and accompanying problems?

Research Questions (cont.) • To what extent do other environmental changes, such

as global warming, habitat fragmentation, air and water pollution, and increased nitrogen deposition affect the rate and extent of invasions?

• By what mechanisms do particular invasive species alter the structure, function, or composition of ecosystems?

• Under what conditions are changes in ecosystems caused by non-native species invasions reversible?

• What are the most effective measures for controlling or eradicating particular non-native species?

• Given the large number and advanced stage of invasion of non-native species in Florida, what are the taxonomic and geographic priorities for control?

Interesting Hypotheses (Callaway et al. 2002, Sax et al. 2007)

• Species invasions are facilitated where native predators and pathogens are mostly specialists (and thus unable to prey on introduced species) and native mutualists tend to be generalists (which may facilitate exotic species)

• Frequency of generalists vs. specialists among predators and mutualists varies among communities – e.g., tropical rainforests have more generalist predators but specialized mutualists, and are less easily invaded than temperate communities

9.17 The process faced by policymakers who must decide which nonindigenous species to eradicate or prevent