Factors Influencing Ecosystem Integrity

Chapter 3 Chapter 3 – Factors Influencing Ecosystem Integrity Ecosystems are not defined so much by the objects they contain as by the processes that regulate them. — Christensen et al. 1989 Key Questions to change is not limitless and some organisms • What are the primary natural processes and human activities that are clearly more adaptable than others. In gen- drive change in the composition, eral, the complete assemblage of native plants structure, and extent of southern and animals is most likely to be maintained California mountain and foothill when environmental conditions remain within ecosystems? their natural historic range. Thus we need to develop an understanding of what the historic • How are those processes and or natural ranges of variability are for various activities currently affecting the ecological processes. These serve as the refer- landscape and what can be ence conditions to which present day determined regarding the natural range of variability? conditions can be compared. The closer current conditions are to reference conditions, the • What current trends are apparent and greater our confidence that all the ecological what threats or opportunities are components will continue to persist (for de- presented by them? tailed description of the “reference variability” This chapter considers the primary pro- concept see Manley et al. 1995). This is the cesses and activities that modify ecological basis for the “coarse-filter” approach to con- communities in the mountains and foothills. servation described in chapter 2. These include (1) natural disturbance pro- In our analysis, we first asked the assess- cesses such as fire, flood, drought, and ment task group to identify and describe the outbreaks of insects or disease; (2) human uses key interactions between landscape elements of the land for development, resources, and and the change agents (e.g., ecological pro- recreation; and (3) the spread (often uninten- cesses or human activities) currently shaping tionally) of new elements such as air pollution them. Then we examined those interactions, and non-native plants and animals. using the best available information on refer- Each of these “change agents” acts upon ence conditions to assess how today’s dynamics the landscape and modifies its character. To compare with the historic range of variability. persist over time, plant and animal species Of particular interest is how the current man- must continually be able to survive, reproduce, agement of natural processes (e.g., fire or recolonize as the landscape changes. Thus suppression or streamflow regulation) has af- the disturbance processes that shape local eco- fected the species and habitats that evolved systems are as important to understand as the under the influence of those processes. physical components of those ecosystems. The element/process interactions identi- Since disturbance and climatic variability fied by the assessment task group are are inherent components of natural systems, summarized in the following sections. A sepa- all species have adaptations to survive in a rate matrix was developed for each of the six changing world. Yet, their ability to respond large-scale vegetation mosaics described in 61 chapter 2. The matrices were used to clarify anecdotal (e.g., accounts of a few early inhab- the effects of different activities and prioritize itants); (2) incomplete and less reliable the those having the biggest impact on species further back in time you go (e.g., official conservation and ecosystem integrity. We be- records of past fires); or (3) based on indirect lieve this information will be useful when the information that is subject to varying inter- time comes to formally describe desired con- pretations (e.g., fire scar analyses, vegetation ditions and management priorities. What patterns observed in old aerial photographs, follows is a description of the primary change and patterns observed in nearby areas where agents and how they influence ecosystems. fire suppression is less of a factor). Unfortu- nately, this body of evidence does not provide The Influence of Fire clear, unequivocal answers; thus, there will always be different interpretations of how things Experience has taught us that we cannot used to be. prevent fire. In my opinion, it is better to In this assessment we examined various have a fire every year, which burns off a ... theories and supporting evidence on how small area, than to go several years ... and today’s fire regime may differ from historic have a big one denuding the whole watershed conditions and what is relevant about those at once. changes. We focused primarily on factors in- — William Mulholland, 1908 fluencing the persistence of native ecological Fire is a primary agent of change in veg- communities and how today’s conditions com- etation patterns across the southern California pare with what is known about the natural landscape. The distribution, composition, and range of variability. In this section we present structure of almost all plant communities in some of the local fire history data, describe this region are influenced by fire, and as the key concerns, and try to identify where there quote above illustrates, prominent southern is strong scientific consensus and where there Californians have long recognized its impor- is not. tance and offered opinions on how it should be managed. Has the Fire Regime Changed? The significance of fire in shaping south- There is little argument that the fire re- ern California wildlands is reflected in a large gime in southern California has changed as body of scientific research on the subject. It is human populations have grown and fire sup- not by coincidence that the Forest Service re- pression practices have become increasingly search station in southern California is the effective. Even assuming that Native Ameri- Forest Fire Laboratory and, as the name im- cans actively burned, there are many more plies, is largely devoted to fire research. Yet, ignitions today with the combination of hu- even with the extensive scientific attention man- and lightning-caused starts, and those given this subject there are widely divergent ignitions are more concentrated along the in- opinions among researchers regarding historic terface between urban development and patterns of fire frequency, size, and intensity wildlands. However, most of those ignitions (Zedler 1995). This uncertainty can largely be are quickly suppressed unless conditions are attributed to the inherent difficulties associ- conducive for rapid fire spread. Most acres now ated with studying a phenomenon that affects burn in human-caused fires (Davis and large areas, recurs sporadically over time in- Michaelsen 1995; Conard and Weise 1998), tervals that range from a few years to several presumably because these fires can arise at any hundred, and whose individual events vary time and thus have the greatest chance of ig- widely in extent and severity. niting vegetation during key periods when Fire history information provides valuable conditions are prime for fire spread (e.g., dur- insights, but the sources suffer from being (1) ing extended heat waves or when “Santa Ana” 62 Chapter 3 or “sundowner” conditions bring high tem- Studies looking solely at southern Califor- peratures, ultra-low humidity, and high winds). nia fire history records do not support the It is reported that 10 percent of the fires hypothesis that fires have become fewer and in southern California wildlands account for larger. Using fire perimeter maps that extend over 75 percent of the area burned (Strauss et back roughly ninety years, Conard and Weise al. 1989; Keeley et al. 1999). Thus, most ar- (1998), Weise et al. (in press), and Keeley et eas currently burn in large fires. It is unclear al. (1999) did not find statistically significant whether this is a natural, historic pattern or increases in average fire size over the recorded an artifact of fire suppression. Keeley et al. time period. Keeley et al. (1999) also found (1999) analyzed fire history records and found that most southern California counties have that this pattern dates back to at least 1910. had a statistically significant increase in the However, we have no way of knowing if it was number of reported fires per decade. These the pattern prior to the arrival of Europeans. authors conclude that large fires, usually Most of the vegetation that burns in south- fanned by fall Santa Ana weather conditions, ern California is chaparral, which is well have always been a dominant component of known for its tendency to go from being dif- southern California’s fire regime and this has ficult to burn under moderate conditions to not been changed by fire suppression activi- exploding into a fire storm under extreme con- ties. Moritz (1997) came to a similar ditions (P.H. Zedler, Univ. of Wisconsin, in conclusion in a statistical analysis of Los Pa- litt. 1998). Thus, chaparral has characteristics dres National Forest fire history data, but he that make it naturally susceptible to large fires. did detect a significant decline in the frequency However, it also seems evident that fires in of small fires. It is suggested that suppresson today’s environment are able to grow signifi- may actually be helping maintain something cantly in size only when weather and fuel approximating the historic fire regime by neu- moisture conditions are severe enough to limit tralizing the large increase in human-caused the effectiveness of suppression actions. A re- ignitions (Conard and Weise 1998; Keeley et sulting hypothesis is that active suppression al. 1999). has changed the fire regime such that fires are It is likely that these differing findings and now less frequent but likely to be larger in size interpretations each have some merit. It is un- and more severe in intensity. doubtedly true that large fires occurred Support for this hypothesis is found in historically; there is evidence of such fires in comparisons of fire patterns in southern Cali- old charcoal deposits (Byrne et al.

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