Species Invasions II CH. 9

What factors determine whether an invasion succeeds or fails? ▪ Which species are the most successful invaders? Which communities are the most invasible? ▪ Invasion pathway with a sufficient propagule pressure (number, frequency and quality of organisms) must exist ▪ Invader’s physiological tolerances & life history must match the new environment well enough for survival and reproduction ▪ Interactions between invader and native species must be sufficient for survival and reproduction ▪ Caveat: the above can all interact

Propagule pressure ▪ By definition, all invasions require a pathway that can transport species beyond their native range ▪ The particular invasion pathway will determine the propagule pressure (quality, quantity and frequency of arriving organisms) ▪ Two commonly recurring predictors of a successful invader are: 1. an association with human activity 2. a previous history of successful invasion (due in part to a larger pool of source populations)

Propagule pressure ▪ Most invasions begin as small populations ▪ The must tolerate abiotic and biotic conditions of a new environment ▪ They must also face problems unique to small populations, including Allee effects, demographic stochasticity and environmental stochasticity ▪ Allee effect: positive correlation between population density & individual fitness. Mechanisms: mate finding, group defense, sex ratios, inbreeding depression ▪ Demographic stochasticity: variability in population growth rate with population density, independent of annual rates of survival & reproductive rates; due to random differences among individuals in survival and reproduction

Propagule pressure ▪ Environmental stochasticity: unpredictable spatiotemporal variation in environmental conditions ▪ The higher the propagule pressure – the larger & more frequent the initial populations – the higher the chances of a successful invasion ▪ Propagule pressure is not constant: invasion pathways grow & shift, particularly due to human trade & transport

Which species are good invaders? ▪ Characteristics that enhance invasibility: ▪ High fecundity, ability to spread vegetatively, parthenogenetic or hermaphroditic reproduction and broad physiological tolerances & diet ▪ Some successful invaders will not have these traits, and some organisms with these traits may not be successful invaders ▪ An invasion pathway may favor different characteristics than the establishment process

Success as a “quirk of fate” ▪ Argentine ants in their native range exhibit high inter-colonial aggression ▪ Invasive populations, which are a major threat to native species, are much more cooperative among colonies, but maintain their aggression towards other species ▪ This altered social behavior appears to have a genetic basis, and is found only in the introduced populations

Success as a “quirk of fate” ▪ Enemy release: the few introduced individuals that found a new population happen to be free of a parasite or disease ▪ Introduced populations are likely to acquire new parasites and diseases in the new environment, BUT a sufficient delay may enhance its initial prospect of success

Invaded community characteristics ▪ Every community is invadable given the right introduced species ▪ If some communities are more easily invaded than others, conservation efforts could be prioritized to protect vulnerable communities ▪ There are 2 primary features: ▪ First, the climate and habitat must be hospitable to potential invaders (e.g., the Sahara Desert may not be hospitable to most species) ▪ Second, the species richness, interaction strength, and trophic structure of the community, must be able to accommodate new species (see biotic resistance hypothesis)

Biotic resistance hypothesis (Elton, 1958) ▪ Biotic resistance hypothesis: species rich systems are more stable and therefore less susceptible to species outbreaks and invasions ▪ A proposed mechanism is that in species-rich communities the greater overall use of resources reduces the available niche space for prospective invaders ▪ Needs further study

Disturbance hypothesis for species invasions ▪ Disturbance – either natural or anthropogenic – makes a community more invadable ▪ This could be due to several reasons: ▪ Disturbance may make resources more available, allowing invaders to get a toehold ▪ New forms of disturbance may disfavor native species that are not adapted to it ▪ Invaders themselves can agents of disturbance, altering natural disturbance regimes (fire, hydrology) by causing physical disturbance themselves, and by simply consuming other species

Resource availability & susceptibility to invasion ▪ Increased resource availability increases a plant community’s susceptibility to invasion ▪ The resource availability at a given time (A) could increase due to an increase in resources (B), a decline in resource use by a resident species (B), or both (C) (Davis et al., 2000)

How are species introduced? ▪ Unintentional pathways ▪ Species can hitchhike on any type of human transport, from airplanes to cars and especially ships: ▪ Chinese trade to India & East Africa began in the 1400’s; Spanish, British & Dutch vessels to North America by 1500; Russian exploration of the north Pacific in the 1700’s. The wooden hulls of these sailing ships were encrusted with entire communities of marine organisms including crabs, barnacles, mussels, tunicates, worms and smaller organisms ▪ Ship ballasts were rock and soil that were used for stability during ocean travel, but deposited on arrival. These ballast materials contained seeds and soil invertebrates, including the European earthworm that now alters the soil structure and plant diversity in Australia, New Zealand & North America

How are species introduced? ▪ By the late 1800’s wooden ships with solid ballasts were replaced by metal ships with water ballasts. Water is taken in through a grate, allowing plankton and even larger organisms to enter. Any that survive the voyage are released alive upon arrival

How are species introduced? ▪ Intentional pathways ▪ Agriculture, aquaculture, recreation, erosion control, biological control & ornamental purposes ▪ As Europeans colonized the globe, they longer for familiar species from home, and back home a growing market demanded the import of exotic species from abroad ▪ E.g., Acclimatization Society of the United Kingdom ▪ Later governments introduced species, especially for fishing

How do we manage species invasions? ▪ Eradication may be impossible ▪ Control is next option 1. Physical control (trapping, hunting, digging, pulling, collecting) 2. Chemical control (pesticides, herbicides, medication) 3. Biological control (enhancement of native or introduction of nonnative predators, parasites or diseases) 4. Multiple aproaches

Pitfalls of biological control 1. We may introduce a new species of pest 2. The biocontrol agent may ‘jump’ into other species 3. The biocontrol agent may spread into other countries

Prevention - U.S. policy on invasive species

Invasive species control: conservation vs. ethics ▪ If we have a proven effective control method, conservation trumps animal ethics; that is, (ethical) killing individual animals is warranted for the sake of the invaded community ▪ If however, our control method is not effective in at least partially reversing the damage, then conservation does NOT trump animal ethics; that is, we can be guilty of animal cruelty