Utilizing Patch Dynamics to Characterize Biological Invasions

Kristen A. Ross

Landscape Ecology 2003

Problems Posed by Biological Invasions

Throughout the last several decades the literature on invasive species and their effects on natural systems has dramatically increased (Elton 1958, Drake et al. 1989, Lodge 1993, Mooney and Hobbs 2000). Consequently, the list of potential invaders continues to increase. As public awareness has expanded and educational forums have begun about the negative impacts invasive species can have on natural and agricultural systems, the question of prevention and predictability of invasion by exotic species dominates the science of invasion biology.

Invasive species have severe economic impacts on human dominated landscapes and perilous consequences for biodiversity. They have been shown to have effects at different spatial scales from the population level (Sakai et al. 2001) to the ecosystem level (Vitousek 1986). An increasing array of scientists from different disciplines (e.g. weed science, ecotoxicology, population biology, genetics, evolutionary biology, epidemiology) are becoming involved in some aspect of invasion biology (Sakai et al. 2001). The threat that invasive species pose to endangered populations, water systems, protected areas and landscapes, has prompted the scientific community to look beyond anecdotal evidence (e.g. Elton 1958) and focus on finding a way to predict when and how an area can become invaded by nonindigenous species. Assuming some predictability is possible, contributions from other disciplines such as landscape ecology can offer an alternative perspective for understanding the spread of invasive species.

Very few studies in the biological invasion literature examine invasions at a landscape level or utilize the concept of patch dynamics to characterize the movement of invasive species spread (but see Lathrop et al. 2003). An alternative method to describe and possibly predict invasion patterns is to utilize landscape metrics to examine invasion patterns. Patch dynamics, as well as biological invasions, occur on a wide variety of spatial and temporal scales. Levine and D’Antonio (1999) and Lonsdale (1999) suggested that determining the predictability of invasion lies in understanding the appropriate spatial and temporal scale at which the invasive species are affecting the landscape.

Patch Dynamics and the Invasion Process

Turner et al. (2001) define a patch as a component within a landscape that differs in appearance or structure from its surroundings. The matrix of a landscape, the general dominant cover type within which patches exist, is characterized by high connectivity (Turner et al. 2001). According to Forman (1995), the concept of patch dynamics considers the event or agent that creates patches and the species composition within those patches over time. Patch dynamics can occur across a variety of spatial and temporal scales (Pickett and White 1985). The nature of how patches are distributed across a landscape will affect resource availability within the system, the survival of the organisms living within those patches, and the rate at which new invasions (or patches) occur (Picket and White 1985). The dynamics of patch formation within a matrix are influenced by several factors including disturbance, environmental change, and nutrient or resource availability. How patches are formed and which factors drive their dynamics depends on scale, land use history, and human influences. Likewise, all of these factors similarly influence the success of invasive species. Consequently, I propose that biological invasions become patches in a landscape that contains a matrix dominated by native vegetation (see Figure 1 below). These invasion patches consist of single or multiple species whose dynamics are affected by abiotic, biotic and disturbance factors.

Traditionally, disturbance has been described as the instigator of invasion. Many authors have explored the influence disturbance has on the introduction of invasives into landscapes (Fox and Fox 1986, Drake et al.1989, Bergelson et al.1993, Cox 1999). Disturbance can facilitate invasion by creating patches within a landscape. Invasive, non-native, or ruderal species can take advantage of a change in resource availability from a gap, opening, or other opportunity after a disturbance. The establishment of these species can inhibit successional trajectories or other ecosystem processes within the gaps, and therefore alter ecosystem function.

Because invasions can modify the landscape structure leading to the creation of patchiness within the matrix, Crooks (2002) describes exotic species that directly modify ecosystems as “ecosystem engineers.” The invader species can affect and shape the habitat resulting in cascading effects on the native organisms as well as increasing or decreasing landscape complexity (Crooks 2002).

Biological Invasions Create Patches

The invasion itself (whether facilitated by disturbance or not) can create patchiness within a landscape such that patches of these invaders spread throughout the matrix dominated by a native cover type. As a result, the landscape (e.g. a forest) contains patches, gaps or other surface area changes that differ from the matrix. Lord and Norton (1990) describe the alien invasion of a native cover type at fine scales as structural fragmentation. In New Zealand, the native patchiness that characterized grassland communities was facilitated by fire disturbance prior to European settlement. Post-European settlement altered the fire regime, introduced grazing, and contributed to the creation of patches of non-native grasses across the matrix of native grassland species. The matrix, once dominated by native vegetation, became increasingly invaded such that it was eventually dominated by non-native species and the native grasses made up the patches within the grassland. This “fragmentation” or non-native patch formation within a matrix of native vegetation was driven by the type of human disturbance and specific site conditions (Lord and Norton 1990). As well, Royle and Lathrop (2002) used GIS to determine the changes that occurred temporally and spatially in hemlock dominated forests affected by the hemlock wooly adelgid (Adelges tsugaea), an invasive insect introduced from Asia. The effects of the adelgid infestations create patches of varying stages of decline within the native forested landscape. Monitoring the differential rates of decline leads to questions about the rate of the spread of the adelgid and its effects on the successional trajectory of the hemlock mixed-conifer forests (Rolye, pers. comm.).

Invasion Patterns

Many studies on biological invasions describe invasion events that have occurred across a variety of landscapes. These studies do not often include an explanation of the way that an invasion moves across a landscape matrix. Shigesada and Kawasaki (1997) summarize two types of invasion pattern. The first type is the traveling periodic wave. The invasion spread changes shape periodically while continuously expanding its range. The speed of the advancing wave depends on whether the front of the wave finds favorable or unfavorable habitat patches. An example is the expansion of Phragmites australis (common reed, hereafter Phragmites) that has spread across the Atlantic coastal marshes throughout the past 30 years (Rice et al. 2000). A Phragmites invasion front can periodically spread depending on whether the underground rhizomes encounter appropriate conditions for establishment (Lathrop et al. 2003).

The second type of pattern is the traveling irregular wave. The shape of the invasion front expands irregularly due to the irregular variation in the environment (Shigesada and Kawasaki 1997). The Argentine ant (Linepithema humile) invades coastal southern California scrub habitat in an irregular pattern depending on landscape fragmentation and the native arthropod community composition (Suarez et al. 1998). Due to the dynamic nature and behavioral ecology of native communities, the Argentine ant invasion front expands irregularly. In general, the biological traits of the organism play a role in patterns of invasion. Describing and understanding invasion patterns can lead to better predictability about further expansion.

Factors that Influence Invasion Pattern

The patterns the invasions create are influenced by the intrinsic characteristics of the invader, its method of dispersal, and the favorability of available habitat. The reproductive method, mobility, growth structure, and resource capture capability are examples of life history traits that will determine whether a species can establish, grow, and disperse in an alien environment. The dispersal patterns of invaders also dictate how an organism is spread throughout its environment (Shigesada and Kawasaki 1997). First, the scattered colony model of dispersal (see Figure 2) describes the way invaders extend their range into a surrounding matrix by random diffusion while at the same time dispersing individuals far from the front. Second, the coalescing colony model of dispersal (see Figure 3) describes the situation when individuals do not migrate very far from the core or parent population, but continue to expand resulting in a coalescing of the parent population and the satellite populations (Shigesada and Kawasaki 1997). Some invader species can follow both models depending on the environment. Finally, if the invader does not find suitable habitat it will not establish or it may require several introductions for establishment and spread to occur.

Importance of Patch Metrics to Invasion Biology

To understand the patterns created by invasions, the life history traits and the different spatial scales under which native and non-native species operate should be considered when studying the spread of invasives (Crooks 2002). Incorporating the concept of patch dynamics from the landscape ecology context is a useful tool that invasion biologists can utilize to characterize invasion patch composition and configuration and to determine effects of invaders on ecological processes. Patch dynamic analysis can also lead to management strategies designed to limit invasion spread.

Patch Composition

Exploring patch composition of an invaded landscape will provide insight into the species diversity and density within patches and the extent to which the landscape has become invaded. For example, Rice et al. (2000) used aerial photography to compare past and current patches of Phragmites australis to determine spread in the Chesapeake Bay area. Creation of patches of invasive species such as Lonicera maackii is somewhat dependent on whether frugivorous birds and other types of concurrently fruiting vegetation inhabit the Lonicera patches (Hutchinson and Vankat 1997). Studying patch composition may also indicate whether the invasion of the native matrix by other non-native species has been facilitated by the original invader. For example, Kourtev et al. (1999) suggest that large patches of Berberis thunbergii located in forests throughout the Northeast create a positive feedback situation for other invasive understory plants and exotic earthworms through efficient utilization of soil nitrate.

Patch Configuration

In addition, examining the configuration of these invasion patches within the landscape provides information about the proximity of different invasion patches, how the invader population may be shifting, and dispersal rates within the landscape. Patch edge and area metrics can provide clues to the increased vulnerability of the matrix to further invasion. Measuring size and density of an invasion patch will help to determine how the invader spreads throughout the matrix. Will it create few large patches or many smaller patches? Patch size will depend on the type of invader and the native matrix resistance to invasion. Mortensen et al. (1998) studied the spatial structure and patch behavior of weeds within crop fields. They found that patch density and edges are largely determined by site characteristics. The shape of the patch will be determined partly by the invasion resistance of the surrounding matrix and partly by the invasiveness and mobility of the non-native species. Lathrop et al. (2003) found that Phragmites has three common patterns of expansion across a marsh: circular (clonal) spread, directional (linear) spread, and advancing fronts away from linear features. They also found that during the early stages of invasion, patchiness increased. During later stages of invasion, patchiness decreased as the individual patches coalesced. Measuring patch shape will also help to predict the invasibility of matrices of similar quality and characteristics that have not yet been invaded. Studying the connectivity and adjacency of invader patches can provide insight into further spread and whether native matrix cover types will recover from invasion. Unfortunately, most invasion biology research has not incorporated these patch dynamics metrics into their experimental analysis.

Describing invasions in terms of patch dynamics will help scientists monitor changes in the landscape and understand alterations of ecosystem processes due to the occurrence of the invader. Several invasive species can alter their environment and affect habitat complexity (Crooks 2002). Traditionally, it is thought that Phragmites is an aggressive invader that is detrimental to salt marsh communities. When comparing multiple patches of Phragmites over time at different sites Rice et al. (2000) found that it is not expanding its range where it has become well established. Identifying these patches is useful to indicate potential areas where changes in the water table, salinity, disturbance regimes, erosion or deposition have occurred. Also, it may be more useful to target smaller Phragmites patches that are not well established for control purposes. Furthermore, Lathrop et al. (2003) studied patches of Phragmites that influence tidal exchange of energy, matter, and organisms. They found that Phragmites does alter the tidal drainage network. Dominant Phragmites patches decrease overall plant diversity ultimately affecting faunal diversity in wetlands as well. Royle and Lathrop (2002) documented defoliation patches of hemlock wooly adelgid. Tracking these patches of forest death is useful in predicting where forest successional patterns may be altered. Patches with high hemlock mortality may experience altered ecosystem function. Loss of foliage leads to an increase in light that affects soil moisture, soil pH, and reduces carbon inputs. Large quantities of standing dead biomass will increase fire potential. In another example, Posey (1988) studied the introduction of Zostera japonica, a coastal seagrass that created patches in mid-intertidal zones in Oregon. The introduced patches directly affected site characteristics. Mean sediment grain size decreased and sediment volatile organics increased. These direct changes created indirect effects on faunal species richness. The effects depended on the age of the patch of seagrass and its location in the intertidal zone. Suarez et al. (1998) found that Argentine ant invasion patches create more homogeneity within coastal scrub habitat because native ant diversity decreases. The elimination of native ants could have negative effects on soil ecosystem processes.

Finally, managers responsible for exotic species control will benefit from studies that incorporate patch dynamic analysis of invasions. For example, the use of landscape ecology metrics has implications for biological control implementation that targets invasive species. It may become easier to predict how the biological control species will inhabit the patches. In the case of the hemlock wooly adelgid, mapping patches of infestation can lead to the prediction of further infestation. Site-specific management strategies for the control of the adelgid can be implemented to reduce spread (Royle and Lathrop 2002). Posey (1988), Norton et al. (1995) and Suarez et al. (1998) suggest that managers should take into account the landscape level effects and scales of patchiness of invasive species when designing strategies for their control.

Conclusion

Invasions occur at differing scales across a variety of landscapes. Invasions and their measured pattern could be used to help predict future invasions. More studies in invasion biology should adopt a multi-faceted approach that incorporates examination of the biology of native species and exotic invaders, site characteristics that promote or resist invasibility, and landscape ecology metrics that help to describe current invasion patterns in a landscape. An interdisciplinary approach to invasion biology through the integration of a variety of ecological tools and analysis will possibly result in progress toward preventing the homogenization of the world’s biodiversity.

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