Synergistic Effects of Factors Influencing Species and Habitats

It is difficult, and perhaps impossible, to separate individual causal factors that influence habitats or SGCN. Multiple factors are closely linked in cause and effect relationships across spatial and temporal scales. Adverse effects from multiple ecosystem stressors can have cumulative effects that are much more significant than the additive effects alone, with one or more stressors predisposing biotic organisms to additional stressors (Paine et al. 1998). For example, reduced fire frequency from a century of fire suppression is partly responsible for conditions that have allowed major outbreaks of several phytophagous insects (Peet 1988). Further, unusually dry periods and/or climate changes reduce available soil moisture causing water associated stress, reduced xylem pressure and pitch production in trees. These conditions allow insects to bore into and infect and kill trees. Affected stands with high tree mortality quickly accumulate dead standing and downed woody fuels. In turn, these conditions greatly increase the risk of catastrophic, stand-replacing wildfire and subsequent insect attack on trees injured or weakened by the fire (Gara et al. 1985).

To further illustrate the interactive and synergistic effects of these factors, consider historic grazing practices that reduced fine fuels and affected natural fire cycles. This condition, in combination with a century of fire suppression and multiple years of drought has created un-natural stand and fuel conditions, making forest and woodland habitat types increasingly susceptible to stand-replacing catastrophic wildfires. Add to this mix, insects and diseases linked with decreased forest health. The overall impact converts late-successional mixed conifer forests to early-successional grasslands, shrublands and recovering forests. Roads contribute to habitat fragmentation and are linked as well to other major habitat altering factors such as timber removal, fire ignition and suppression, fuel wood collection, and recreation.

The effects of climate change on ecosystems and species are likely to be exacerbated in areas that have already been substantially affected by human activities such as habitat loss and fragmentation, air and water pollution, and the establishment of invasive species. Habitat fragmentation decreases the ability of plant and animal species to migrate in response to changing conditions or species requirements. Invasive species are most successful in ecosystems already disturbed by anthropogenic activities (Elton 1958). Climate change may act as a form of disturbance creating opportunities for invasive species to colonize and displace native species (Malcolm and Pitelka 2000). When suitable habitat conditions disappear or shift faster than populations can adjust, the likelihood of species extirpation or extinction increases (Malcolm et al. 1998).

Many of the factors discussed above coincide in the same geographic area. Given the synergistic effects of multiple factors, it is difficult to understand the overall impact these factors will have on New Mexico landscapes, habitats, or Species of Greatest Conservation Need. In addition, it is difficult to understand which habitats may have higher risk of being altered by multiple factors. However, we conducted a simple analysis by summing magnitude scores of each of the 43 generic factors within each key habitat (See Approach chapter for details). This approach, while is not perfect, gives us a basis for understanding the possible synergistic effects, and where we might need further clarification on the outcomes of these factors.

The highest possible cumulative magnitude score for any habitat is 344 (see Approach Chapter). However, the top score of any key habitat was 165 (ephemeral natural catchments). Perennial marsh/cienega/spring/seeps and riparian habitats also yielded high cumulative magnitude scores (158 and 156, respectively) (Fig 4-6). Magnitude scores for each key habitat within category of factors that influence habitats are provided in Appendix L. Using cumulative magnitude scores as an indicator of the potential synergistic effects of all factors, these 3 key habitats may be at a higher risk of alteration by multiple factors than other habitat types in New Mexico. Likewise, aquatic habitats may be more likely to be altered than terrestrial habitats, with the exception of riparian habitats.

This information may be displayed spatially, allowing us to enhance our understanding of geographic areas where synergistic effects of potential factors may influence some habitats greater than other habitats (Fig. 4-7). Given this spatial representation, aquatic and riparian habitats statewide, areas in the shortgrass prairie in eastern New Mexico, and Madrean systems in the Gila National Forest may have several factors, that when placed together, influence the integrity of these habitats. These are key areas to investigate and enhance our understanding of factors that influence habitats.

Figure 4-6

Click figure for larger view.

Figure 4-6. Cumulative magnitude scores of 43 factors that influence key habitats in New Mexico. This analysis assists in the identification of key habitats which may have the highest risk of being altered by synergistic effects of factors that influence habitats. Methods of calculating magnitude scores are presented in the Approach chapter. Magnitude scores for each key habitat by category of factors that influence habitats are provided in Appendix L.

Figure 4-7

Click figure for larger view.

Figure 4-7. Magnitude scores of factors that influence habitats associated with terrestrial and aquatic land cover types. This spatial representation is designed to enhance our understanding of geographic areas where synergistic effects of potential factors may influence some habitats more than others. This analysis should not be used to locate small parcels of land.