Community Ecology Bio 147/247 Human Impacts 1: The Ecology of Biological Invasions What is an exotic species? Human-mediated redistribution of species Introducing a species into a community in which it did not evolve Invaders can be plants, insects, reptiles, amphibians, mammals, microbes, pathogens, etc. Not all exotic species are problematic (although this may change through time or in different locations) What is the difference between a naturalized exotic species and an invasive exotic species? Ecological impacts: displaces native species either directly, e.g., through competition/ predation, or indirectly, e.g., by altering disturbance regimes, often leading to wholesale alteration of communities Why do we care? Invasive species are the 2 nd leading cause of extirpations/extinctions worldwide Disproportionately great toll on rare and endemic species (which contribute to biodiversity in a unique way and represent novel evolutionary trajectories) Homogenization of once-unique regional floras and faunas Costs the US government $4 billion in lost productivity, eradication/restoration each year NOTE: add 2 pgs reading in Morin see syllabus San Francisco Bay: Accelerating Invasion Rate in a Highly Invaded Estuary. Science 1998. 279: 555-558 San Francisco Bay and Delta may be the most invaded estuary and possibly the most invaded aquatic ecosystem in the world. Potential causes of increased exotics & loss of native species: shipping fish/oyster stocking introductions for biological control plant exotic vegetation for marsh "restoration" and erosion control floods after big snow melt hypersaline after a drought Man-made Natural Native S 1
Are some communities inherently more invasible than others? Using Community Ecology theory to understand the ecology of invasions: the multiple ways in which niche opportunities can arise. Diversity: theory and data give mixed results/predictions BUT there is a very reasonable/important resolution Ex 1a: Invasive plants growing in tussock vegetation at one place along a CA stream (Levine 2000): Grass Plantain Thistle Interpretation: 2
Q: So, why in nature do more diverse communities tend to be more invaded? Ex 1b: Same system as 1a, but at different places along the stream (Levine 2000). Ex 2: Different grassland sites (Stohlgren et al. 1999). Interpretation: Q: How to explain the opposing observations? What is the critical difference between Ex 1a and Ex 1b (or Ex 2)?? 3
Different ecological processes operate at different spatial scales (competition is a local process; community-level diversity is a larger scale process). (Shea and Chesson 2002) 4
Potential explanations: why are some communities are more invaded than others: (We ve already covered some of these.) 1. Empty niche or low functional group diversity 2. Disturbances 3. Enemy Release/Escape Hypothesis: escape their (specialist) enemies during colonization. Opuntia cactus in Australia and Cactoblastis? 4. Mutualisms: facilitation in harsh environments. 5. Competition: out-compete the native species for resources. (See also #1). 6. or have novel weapons (e.g., allelopathy) 7. Species may also evolve in their new landscapes: Evolution of increased competitive ability (EICA hypothesis). Summary: These seemingly disparate findings are all important concepts of community ecology, and summarized as niche opportunities. (Shea & Chesson 2002). The outcome of any introduction will be determined by niche opportunities that fluctuate in time and in space. Different species will be more/less able to exploit them. Which species become invasive in which communities, and when, may remain irritatingly idiosyncratic (Carla D Antonio). We can better understand retrospectively why they succeeded but prediction is still difficult and may not be possible in all cases. Niche opportunities: Organisms engage in multiple negative and positive interactions simultaneously and do so in an environment in which resources fluctuate. This can give rise to unexpected outcomes. 5