User:Mskaer/Invasion Biology of California Grasslands

Invasion Biology of California Grasslands describes the study of exotic invasive species in grassland habitat of the state of California and focuses on plant species.

Overview
Exotic invasive species often dominate the vegetative community in California grasslands. Time of introduction for these species varies from 200 or more years, to within recent decades. Cause of introduction has been intentional (e.g. forage species) or unintentional (contamination of seed crops, livestock transport). Annual grasses of European origin are particularly dominant. These species have wide-ranging negative impacts on grassland ecosystem functioning and services and management attempts to date have had mixed results.

History
Heady and Burcham describe the recent history of grasslands as undergoing a series of temporally distinct waves of invasion: oats (Avena spp.), filarees (Erodium spp.), and black mustard (Brassica nigra) were among the earliest to arrive (before 1860); soft chess (Bromus hordeaceus), ripgut brome (B. diandrus), and foxtail barley (Hordeum murinum) spread in the 1860s-1870s; silver hairgrass (Aira caryophyllea), red brome (B. madritensis ssp. Rubens), and tocalote (Centaurea melitensis) spread in the late 1800s; barbed goatgrass (Aegilops triuncialis), medusahead (Taeniatherum caput-medusae), and yellow starthistle (C. solstitialis) are the most recent invaders and are actively spreading. The pre-invasion (prior to European settlement) composition of California grasslands is debated. Traditionally, it was thought that native perennial bunchgrasses, particularly purple needlegrass (Nassella pulchra), dominated. However, recent papers have suggested that perennial grasses may have only dominated in more wet, coastal grasslands, whereas annual forbs dominated drier valley grasslands. Some researchers even refer to these historical annual forb-dominated systems as ‘forblands’ rather than grasslands.

Causes
Intensive year-round livestock grazing and increased crop agriculture were major drivers of the dramatic decline in the native flora that occurred in the in the latter half of the 1800s. These factors, and others, were likely synergistic in their influence on the state-change of California grasslands from native to non-native annual-dominated flora.

Domestic Livestock
Livestock grazing is implicated in the conversion of California’s grasslands, but ironically the use of domestic grazers is now considered an effective management tool, although results have been mixed (see Management Strategies). Intense soil disturbance and compaction, nutrient addition, and herbivory associated with ill-managed livestock grazing would have had a dramatic impact on California’s historical grasslands that were not exposed to such extreme pressures during their recent evolutionary past. Introduced European species have evolved in association with heavy human and livestock disturbance, and may have thus been favored by the intense grazing that was concurrent with European settlement in the 1800s.

Crop Agriculture
Crop agriculture has been shown to be the best predictor for presence or absence of native perennial grasses. An analog to the modern-day practice of ‘flushing the seedbank’ and other destructive farming practices, such as non-rotational planting, used throughout the valley and coast grasslands would have had a substantial influence on the conversion of these systems from perennial-dominated to exotic annual-dominated flora.

Other Factors
Another factor that may have facilitated the invasion of California’s grasslands was the high abundance of burrowing animals. Schiffman argues that the soil disturbance associated with these animals selected for annual life history in plants prior to European settlement. When exotic species were introduced in the mid to late-1800s, the high frequency of disturbance would have favored those European species, which tend to be well-adapted to disturbance and have high fecundity. Other studies have argued that key traits such as early germination, fast growth, and tolerance of drought were the major contributors to the success of exotic species invasion. There is also evidence suggesting Native Californians regularly burned land for creation or maintenance of grasslands, and this type and frequency of disturbance would have created opportunities for exotic species to invade. Finally, recent research suggests exotic grasses, such as the oat species, may have been disease-facilitators for barley and cereal yellow dwarf virus, which affected the native grasses disproportionately.

Differential Invasion
Although many of the most common invasive species occur in all grassland types, exotic annuals are most dominant in valley grassland. Coastal grasslands have been more affected by perennial exotic grasses: velvetgrass (Holcus lanatus), tall fescue (Festuca arundinacea), harding grass (Phalaris aquatica), orchard grass (Dactylis glomeratus). It has been shown that species composition and diversity can influence susceptibility of grassland systems to invasion, particularly at the local level. Native perennial species have been shown to be strong competitors with exotic annual species in seed-addition experiments, which indicates that seed and seedling limitation of native species may be a major factor in susceptibility of California’s grasslands.

Some unique grasslands with specific associated soil types (e.g. serpentine grasslands) have been resistant to invasion, though soft chess is relatively dominant and barbed goatgrass has been increasing in these serpentine grasslands recently. It has been suggested that genetic factors may play an important role in these invasions (see Genetic Adaptation).

Genetic Adaptation
Many introduced invasive species have been shown to undergo adaptation in response to their new environment. For example, tall fescue, which is problematic in coastal grasslands, has been shown to shift genetically as a result of selection pressures from grazing. Rose clover (Trifolium hirtum) and slender wild oat have demonstrated ecotypic adaptation specific to different habitats in their new ranges. Red brome and cheatgrass (Bromus tectorum), both ranked as being of high concern by the California Invasive Plant Council (see Table 6.1), have shown genetically-based differential flowering phenology and seed mass in different habitats. Genetic factors have also been suggested to be important in the invasion success of barbed goatgrass. These examples demonstrate a potential for exotic invaders to become stronger competitors as they become better adapted to their new environments.

Another genetic trait found in introduced species is high levels of inbreeding resulting from a genetic bottleneck event at the time of introduction (a few pioneer seeds carry limited genetic diversity). For example, species such as slender wild oat (Avena barbata), soft chess, and some populations of yellow starthistle demonstrate lower rates of outcrossing and higher levels of inbreeding compared to populations in the species’ home ranges.

Current State of Invasion
With Florida, the state of California leads the US in total number of non-native species. Almost a third of the sustained exotic invasive species in the state occur in grassland systems, 66 of which are considered as being of moderate or high concern. These species are listed in Table 6.1. It is estimated that 9.2 million hectares of California grassland are dominated by non-native species, an area which represents almost a quarter of the total area in the state (42.4 million hectares).

Market vs. Non-market Costs
California’s grasslands host a large part of the state’s diversity and are increasingly losing productivity as a result of exotic species invasions38. Exotic species incur actual market costs to public and private parties; see Agriculture for an example of a market cost. Non-market, or ecological, costs cause losses in “ecosystem services” (see Ecosystem Services article; Ecosystem Effects below).

Agriculture
Although exotic species impact crop agriculture, their impacts are more dramatic in livestock agriculture. These lands support the majority of the California’s domestic livestock production, which represents over a third of the state’s agricultural revenue, second only to crops39. In the United States, invasions by weeds result in an estimated $2 billion revenue loss per year. A relatively small 5% increase in one of the problematic invasive species would constitute a loss of $65 million each year just in California.

Some invasive species cause reductions in forage productivity or quality, increased costs of managing and producing livestock, reduced animal weight gains, reduced quality of meat, milk, wool, and hides, and livestock poisoning40. For example, medusahead can reduce livestock carrying capacity up to 80%, primarily through reduced forage (unpalatability to animals) and possible health consequences for the animals41-43. Yellow starthistle has also been shown to have dramatic market cost impacts44,45.

Ecosystem Effects
The ecological, non-market, effects of exotic species invasion of California’s grasslands are far reaching, and include altered competitive dynamics3, damage to forage and habitat for native wildlife, depleted soil nutrients, degraded water resources38, and changes in other ecosystem processes4,5,46. Late-season annual species, such as yellow starthistle, deplete deep soil water reserves47, and alter system hydrology cycles by undergoing transpiration farther into the summer dry season than most native species3,40 and thus interfere with groundwater storage and recharge47. Exotic species can also effect soil nutrients, as in the case of nitrogen-fixing plants or shrubs, which are problematic in coastal grasslands and alter community composition mainly by facilitating increases in other invasive species, such as exotic annual grasses48,49. There may also be increased nitrogen flux in grasslands dominated by annual grasses due to high rates of self-thinning and turnover50,51.

Invasive species can have more persistent seed banks and alter the community response to disturbance (small mammals, livestock grazing, fire) or other alteration (nitrogen deposition or climate change)3. These plants can also alter soil microbe communities, though the impact of this change is still unclear50. Exotic species also interact with microorganisms such as viruses, rusts, smuts, and bacteria, including the barley and cereal yellow dwarf virus mentioned in Causes, and seed head fungi on barbed goatgrass, both of which may favor exotic invasive species50. Finally, tremendous amounts of aboveground plant biomass accumulate each year from exotic annual species5,52. This increased biomass can have a negative effect on plant germination, recruitment, species richness, and relative dominance53-56.

Interactions with Climate Change
The climate in California is expected to warm substantially (2-6 °C) by 2100 as a result of increased greenhouse gases57. Nitrogen deposition and increases in CO2 concentration will also occur during this period58. While the overall climate pattern of California will not change (wet, cool winters and hot, dry summers), the temporal and spatial pattern of precipitation, form of precipitation, frequency and severity of events, and length of growing season is likely to change57. Model predictions do not yet agree on the direction or amount of rainfall change in California, but any change will have effects on competitive dynamics in California grasslands. Early research on this subject has shown spring rainfall increases have the most dramatic effects on community composition59.

These changes will have impacts on species interactions, species ranges, community assemblages and ranges, and ecosystem processes60. Exotic species will interact with these impacts in varying ways that are difficult to predict. End results will depend on the extent to which non-native invasive species and co-occurring native species are adapted to the new climate regime61,62.

Fire
One major possible change in California grasslands that will likely interact with impacts of invasive species is the frequency and severity of fire60. Introduced species can alter fire regimes, and it is possible that there will be a synergistic increase in fire intensity and frequency due both to climate change and exotic species invasion.

Nitrogen
Nitrogen inputs from nitrogen-fixing invasive shrubs and plants are already impacting coastal grasslands (see Ecosystem Effects), and increasing inputs of atmospheric nitrogen will likely further alter community compositions in favor of exotic species. Experimental research has shown that nitrogen addition causes total biomass increases, driven mostly by exotic annual grasses63, and that these inputs decrease overall diversity53.

Jasper Ridge Global Change Experiment
It is challenging to predict or experimentally determine responses to multiple factors of global change60; however, the Jasper Ridge Global Change Experiment (JRGCE) is one multi-factor experiment for investigating multiple climate change factors in grasslands, established in 1998. Results from this experiment have been detailed and complex, but generally, the interacting climate change factors have had impacts on productivity, phenology, and relative abundance of all species, including natives and exotics60.

Management strategies
Land management strategies are diverse, and selection and efficacy of these tools varies depending on management goals and locale. It can be particularly challenging to control invasive species while simultaneously attempting to maintain or augment native species. Categories and terminology in this section follow DiTomaso et al. 2007.

Mechanical
Many mechanical methods are time and labor intensive, but require little training. Hand labor includes hand pulling, weed whips, sling blades, clippers, shovels, hoes, mattocks, and Weed Wrenches40. The efficacy of these strategies has not been scientifically evaluated, though anecdotally, they have been useful for small infestations or for volunteer labor. A benefit of hand labor is the possibility for selectivity (e.g. removal of only target species). Mowing is a common method, particularly along roads and other right-of-ways40. Its efficacy depends on seasonal timing, and is most useful just preceding seed production. Mowing has been shown to be beneficial in controlling the noxious yellow starthistle64, but mowing or clipping has also been shown to shift composition in coastal grasslands from exotic annual grasses to exotic forbs65. The use of tilling is common in agricultural systems for invasive control40. Various types of tillage can disrupt rooting systems or bury plant parts, including seeds, thus controlling regrowth. However, this technique can increase atmospheric dust and soil erosion, and may not be practical for all applications. Thatch (plant debris) removal is another mechanical method of reducing the impact and competitive ability of invasive species, particularly annual grasses.

Biological
Biological control (see Biological Pest Control article) incorporates the use of insects or pathogens as a broad-scale method for reduction of invasive species. Typically, these insects or pathogens are not entirely host-specific, but are expected to focus on the target species40. Stringent requirements for research and approval for introduction of biological control agents (organisms) are designed to prevent unintended consequences (e.g. negative impacts on non-target native species).

Chemical
Herbicides are considered to be the most economical option for invasive species control and management. Some herbicides are developed for specificity on certain plant families (e.g. Asteraceae, the sunflower family), and even certain species (e.g. yellow starthistle and other thistles)66. Like mechanical methods, timing of herbicide application impacts effectiveness of the treatment.

Cultural
Livestock grazing, prescribed burning, and revegetation are also methods of control and management of invasive species. The success of grazing as a control method depends on a number of factors, including timing, intensity, type of grazer, and type of grassland. For example, cattle and goat grazing can be effective for management of yellow starthistle when plants are younger, but only goats are effective once the plants become spiny in later growth stages67. Prescribed burning has also been shown to be useful in some cases, but fire-escape risk, air quality problems, and liability issues are issues near populated areas40. Finally, revegetation with competitive desirable native species (seeds or seedlings; see Restoration Ecology and Revegetation articles) can achieve long-term suppression of exotic invasive species. Some of the most successful species for this purpose have been perennial bunchgrasses because they reduce the growth and reproduction of target weeds68, though the most appropriate species will depend on the goals and specific location intended for revegetation40.