Brian Miller
GEOG 491-Lab #5 Report
Agricultural Zone
• What relationships exist between farm characteristics and the percent guava by farm?
Figure 1 (1_bwmiller.jpeg) shows the overall distribution of guava in the agricultural zone and nearby Park land. To gain an understanding of land use and guava cover I examined farm characteristics in relationship to percent guava by farm (Figure 2; 2_bwmiller.jpeg). Classes of guava cover shown in Figure 2 were determined using the natural breaks classification method. Charts 1 and 2 show that percent of land used for pasture and agriculture have a negative relationship with the percentage of guava on farms. In other words, as the amount of land used for pasture or agriculture increases, the amount of guava decreases. A similar trend is found for the number of cattle owned by farm proprietors (Chart 3). Each farm may have one or more fields, and field-level analyses are consistent with the farm-level findings. The date of last clearing of these fields also has an inverse relationship with percent guava cover (Chart 4). Chart 5 shows that fields used for pasture or crops have lower proportions of guava than unused fields.
• What, if any, proximity relationships exist between percent guava by field and (a) roads and (b) buildings?
Qualitatively it appears as though there is increased guava next to roads (Figure 1); however, the percent guava by field is similar for fields that intersect and do not intersect roads (Figure 2; 2_bwmiller.jpeg). Quantitative summary of these fields verifies this observation (Table 1). The presence of buildings appears to have a negative affect on guava cover on farms (Figure 2 and Table 2). This may be due to the fact that buildings indicate relatively permanent settlement and land use, as opposed to land abandonment.
Mean Percent GuavaDo not intersect rd / 35.46%
Intersect w/rd / 35.09%
Table 1. Mean percent guava for fields that intersect and do not intersect roads (Isabela Island, Galapagos, Ecuador).
Mean Percent GuavaField without a building / 37.71%
Field with a building / 27.91%
Table 2. Mean percent guava for fields with and without buildings (Isabela Island, Galapagos).
• Assign each farm a category of “abandoned,” “partially abandoned,” or “active” based on percent guava present and the farm characteristics in the attribute table. Are fields or farms next to abandoned fields/farms more likely to contain a higher percentage of guava?
The activity of farms is shown in Figure 3 (3_bwmiller.jpeg). Farm Activity was classified based on the total percent of the farm used for crops and pasture. The three classes (“abandoned”, “partially abandoned”, and “active”) were based on the natural breaks classification method where 0-20% total farm use is “abandoned”, 21-60% use is “partially abandoned”, and 60-100% use is “active”. The mean percent guava on these farms is shown in Table 3. These data show that as farm activity increases, mean guava cover decreases. It is also worth noting that the one farm with 0% use had 81.15% guava cover, reinforcing the relationship shown in Table 3. The guava cover of a field is also related to its intersection with abandoned or partially abandoned fields (Table 4).
Farm Status (n) / Mean Percent GuavaActive (9) / 30.78%
Partially Abandoned (3) / 51.26%
Abandoned (4) / 67.67%
Table 3. Farm activity and mean percent guava on Isabela Island, Galapagos.
Mean Percent GuavaIntersects (43) / 36.54%
No intersect (6) / 26.45%
Table 4. Guava cover for fields that intersect and do not intersect fields that are abandoned or partly abandoned, Isabela Island, Galapagos.
• Is there spatial clustering of guava in the agricultural zone or in individual fields, and does this spatial clustering provide clues to land use practices that could affect guava distribution?
It appears that there is clustering of guava in the agricultural zone and also within fields (Figure 1). For one, actively used farms have a lower percentage of guava cover, so land abandonment allows for the clustering of guava on these farms. There is also clustering within farms, probably because land is not used uniformly within each farm. These clusters of guava allow for colonization of newly disturbed and abandoned land.
For the Park zone
• Is there a relationship between guava in the Park and distance to the agricultural zone?
The agricultural zone discussed above lies within the Galapagos National Park. Although there is a specified boundary of the agricultural zone, there is a relationship between guava cover in the Park and distance to the farm boundary (Figure 4; 4_bwmiller.jpeg). The amount of pixels classified as guava decreases with increasing distance from the agricultural zone (Chart 6).
• Is there a relationship between guava distribution in the Park and percent guava in fields bordering the park?
Yes, there is a relationship between guava distribution in the Park and percent guava in fields bordering the park (Chart 7). This pattern is evident in Figure 5 (5_bwmiller.jpeg). Field guava cover is a stronger predictor of the number of pixels in the adjacent National Park land (see next question). This is probably due to finer scale of measurement of guava cover and more accurate allocation of Park land.
• How about between guava distribution in the park and characteristics of farms bordering the
Park?
There is a weak positive relationship between the percent of guava on farms and the amount of guava on adjacent National Park land (Chart 8). However, this is not an extremely accurate measure of the influence of farms on nearby land for two reasons. First, the amount of guava on national park land was calculated as the number of pixels, rather than a percent of land cover. Also, the study area extent does not include National Park land adjacent to the southernmost farms (Figure 6; 6_bwmiller.jpeg).
Summary of Preliminary Findings
Guava (Psidium guajava) is an introduced fruit tree on the Galapagos Islands of Ecuador. Dispersal of this non-native species is thought to be related to human land uses, particularly livestock and abandoned agricultural land. This study examines the distribution of guava in the highlands of Isabela Island as it relates to these human land uses. Figure 1 (1_bwmiller.jpeg) provides some indication of the prevalence of this non-native species within the study area. The aforementioned analyses provide empirical evidence of the relationships discussed below.
Guava prevalence in the agricultural zone appears to be positively related to land abandonment (i.e., land abandonment is associated with more guava). Measurements of guava cover as related to the date of last clearing as well as the amount of land used for pasture or agriculture provides evidence of this relationship. This notion is further supported by the relationship between guava cover in fields that intersect abandoned or partially abandoned fields. Therefore, if land has been used for agriculture or pasture, it is best if land use continues in order to discourage guava establishment and spread to other fields. It is likely that there are feedbacks between land use and guava invasion. Land use can permit the invasion of guava due to soil disturbance and the removal of natural vegetation; however, continued land use can prevent the establishment of guava over large areas. Guava can also impact land use. Due to the labor required to remove guava, its presence could discourage agriculture development, and thus encourage agriculture expansion to undisturbed sites. The relationship between roads and guava cover is not as clear, and the measurements used in this study may not be sufficient to detect a relationship. I assumed that the intersection of roads and fields is the important measurement for guava spread, but this measure may not be capturing the relationship between road development and guava cover. Roads may provide a disturbed area allowing for guava establishment only right next to the road.
The relationships between guava and land use in the agricultural zone are relevant to the persistence of ecosystem function in the National Park as well. Clearly, areas of the Park that are closer to the agricultural zone exhibit higher levels of guava cover. The above analyses assume that farms/fields are confined to the agricultural zone and that farmers do not use the Park for pasture or agriculture. It is worth studying Park land adjacent to the agricultural zone to determine if this is in fact true. Vegetation studies are needed to determine if Park land adjacent to the agricultural zone is being used for pasture or agriculture, or has been disturbed in some way.
Understanding the drivers of guava establishment and spread is relevant to local agricultural productivity as well as the conservation of biodiversity in the Galapagos Islands. Continued monitoring of guava cover and farm conditions is recommended. In order to address feedback mechanisms future studies should incorporate metrics of agriculture extensification and intensification. Natural disturbances (e.g., fire) and their relationship to guava cover and control should also be investigated. Also, household surveys should be used to understand reasons for land abandonment and livelihood strategies. Longitudinal farm data would be particularly helpful in addressing the dynamics of guava cover and land use over time. These analyses provide evidence that the agricultural zone activities are relevant to National Park management; therefore, integrated ecosystem management across developed and undeveloped land is essential.