PATCH DYNAMICS AND THE TEMPORAL EFFECTS OF CLIMATE ON MONTANE
ISLAND PATCHES OF VARIED CONNECTIVITY
D OMENIC
D’A
MORE
Graduate Program of Ecology and Evolution, Cook College, Rutgers University
I
NTRODUCTION
A variety of model systems have been used to explain the diversity of species in patches such as marine islands and isolated forests. The purpose of this study is to compare and contrast two of these methods, island biogeography theory and patch mosaic, focusing on changes in size and variable connectivity in isolated montane forest patches.
MacArthur and Wilson (1963) first proposed the island biogeography model by analyzing oceanic islands off the coast of a mainland. The basis of the theory is that the number of species on an island is related to the balance between extinction rate and colonization rate. If these rates are the same then the number of species will remain at equilibrium; i.e. the number of species lost will be offset by colonizing species. An increase in extinction relative to the colonization rate will result in decreased diversity and an increase in colonization relative to extinction will lead to an increase in diversity. In theory, extinction rate should correlate with island area and immigration rate should correlate island isolation (or distance from mainland or colonization source) (Brown 1971; Lomolino 1996). The influence of two factors can result in the “rescue effect”, which is the tendency for species turnover to be lower on less isolated islands.
Therefore, increased immigration can prevent extinction (Brown and Kordric-Brown 1977). The abiotic and biotic characteristics present on an island are also determining factors and the “sea” between islands is a consistent “fatal” environment (Pickett and Rogers 1997). All of these
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factors will determine the diversity equilibrium that will eventually be reached (Simberloff and
Wilson 1969).
Nestedness is a component of island biogeography. Patterson (1987) defined the nested subset hypothesis whereby the species-poor island communities are actual subsets of the species rich ones. The implication is that the species on the most depauperate islands should also occur on islands with higher species richness. A gradient is formed relating decreasing diversity to increasing isolation (i.e. as one moves away from the mainland, islands become less diverse).
Since nestedness is dependent upon isolation values, relative nestedness values are highest in a system that is regulated by immigration and isolation (Lomolino 1996).
Island biogeography was one of the first clear methods of how to approach patchiness
(Pickett and Rogers 1997). The island biogeography model is similar to a patch-matrix model in that islands are patches with defined boundaries and the matrix is the surrounding sea. Patch dynamics in landscape ecology relates to how the patches in a heterogeneous environment undergo changes in structure and/or function either spatially and/or temporally (Pickett and
Rogers 1997; Turner et al. 2001). The creation, change and elimination of patches in the
“shifting mosaic” are essential parts of patch dynamics. This patch mosaic model deals with the relative affects of the matrix, or the area between patches. The matrix may facilitate or impede movement between patches, and may facilitate or impede movement more for certain species than for others. Connectivity is the ability of certain organisms to travel from one patch to another and can be related to island biogeography in that increased connectivity is correlated with an increase in relative immigration rates between patches/islands (Tischendorf and Fahrig
2000: Bowman et al. 2002). Connectivity can be species specific, with certain species having
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higher immigration rates than others. Nestedness values will therefore be increased for systems where there is increased connectivity (Lomolino 1996).
I review the biogeography of the Great Basin and the Cordillero-Madrean area of southwestern North America from the perspective of small mammals. This system has montane forest “islands” of pinon-juniper type vegetation at high altitudes on mountaintops located within a “sea” of low altitude woodland and scrub. The montane forest system has been studied from the perspective of island biogeography but less so from a patch mosaic perspective. I reviewed the formation of these patches and areas also looked at areas with either varying degrees of connectivity or no connectivity at all. The goal of this review is to evaluate how different levels of connectivity between patches can be based on 1) changes in patch area and isolation, 2) the degree of impediment imposed by compositional differences in the matrix 3) and species specific immigration rates. These causes are not mutually exclusive and are major components of the patch mosaic theory. With this information, I will then incorporate and compare the patch mosaic model to island biogeography theory in order to determine which is more appropriate for assessing this system.
L ITERATURE R EVIEW
Montane Patch Formation: the Influence of Climate
The entire area considered here has undergone a comparable history (Wells 1983: Rickart
2001). The existing warm deserts of the Southwest United States represent expansions that grew in the Holocene to replace the flora that was endemic in the late Pleistocene. During the last glacial maximum (approximately 18 thousand years ago) a cooler, wetter glacial climate prevailed. The vegetation of the pinon-juniper forest (Pinus longaeva, Picea engelmannii and
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Juniperus communis) , which is now located at high elevations as montane forest patches, was contiguous across the Great Basin at this time (Grayson 1993: Rickart 2001). The historic distributions of these species were lower than their present distributions by about 600 to 900 m.
These forests were connected throughout the landscape. As temperatures rose in relation to the retreat of glacial climate, the pinon-juniper forest retreated to higher altitudes up mountains and became isolated patches. The majority of the low altitude mammal species that thrive in the montane forest and could not adapt to warmer conditions, were forced to move up to high altitudes along with the vegetation. Fossil evidence of Neotoma (the packrat endemic to pinonjuniper forests) exhibited a range in the Pleistocene that was at much lower altitudes than presently seen. Low altitude environments became dry deserts (variable amounts of xeric desert scrub and scattered woodlands) that were not beneficial to these mammals. They are now isolated within montane patches located at higher elevations on mountaintops (Wells 1983).
Using a modeling approach, McDonald and Brown (1992) suggested that further increases in temperature would push the montane forests to even higher altitudes, which would reduce patch area. This model assumed that there was no immigration or connectivity between patches. Island biogeography theory states that area directly relates to extinction (MacArthur and Wilson 1963: Brown 1971). Thus, an increase in temperature in this instance would result in a decrease in area and consequently biodiversity. Model results show that boreal habitats would be reduced by 35% with a temperature increase of 3° C. This increase would be similar to the warming that occurred at the end Pleistocene which resulted in vegetation zones being displaced up by 500 m in elevation. Based on a tight coupling between vegetation and small mammals, the model predicted that 9-62% of all the mammal species in montane patches would go extinct
(McDonald and Brown 1992).
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Zero Connectivity: an Extinction Driven Model
To determine whether or not the montane forest communities were an example of the island biogeography model, mammals were surveyed on mountaintops in the Southern Great
Basin of Nevada (Brown, 1971), where mountaintops served as isolated patches of unique habitat surrounded by a matrix of xeric desert scrub. Consequently, Brown (1971) found that there was virtually no immigration between patches in the Southern Great Basin, making the rate of colonization (and therefore connectivity) effectively zero. In addition, the biodiversity also increased on montane forests with larger area, but had a negative relationship with isolation.
This agreed with island biogeography in that area related to extinctions, but deviated from the theory in that isolation appeared to have no bearing on diversity. Brown (1971) proposed that the “island” patches underwent colonization during a time when the landscape was transitioning from greater connectivity to greater fragmentation (c. late Pleistocene). Since then, connectivity has decreased to negligible values and colonization rates have essentially stopped due to the intermountain region (matrix surrounding the mountain tops), becoming an uninhabitable xeric habitat (Brown, 1971: 1995). Brown (1971) called extinction without immigration the
“relaxation model”. The model assumes that since there is no immigration, the isolated communities will decline because of extinctions (Lomolino and Davis, 1997). Brown’s relaxation model is further supported greater genetic isolation and hence greater genetic variability in several widespread species among different montane forest patches. This is due to reduced mating between populations because of the lack of connectivity (Rickart, 2001).
Nestedness does not play a major role in this system due to a lack of colonizations (Cutler,
1991). The combination of the formation and isolation of patches over time and the matrix
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functioning as a harsh filter, has resulted in no immigration between patches, and hence zero connectivity.
V ariable Connectivity: the Importance of Immigration
Further study of the southwestern area has shown that immigration may play a factor in the overall dispersal range of some small mammals further south of Brown’s (1971) study of the
Southern Great Basin, (from here on called the “Southwest”) (Lomolino et al., 1989). An assessment of the “Southwest”, in Arizona, New Mexico, southern Utah and Colorado, showed the matrix to have many woodland areas interdispersed with xeric scrub instead of a solely xeric vegetation matrix separating pinon-juniper forest patches. Mammals such as the packrat,
Neotoma, and many others were able to cross woodland and colonize the isolated montane forest patches. Consequently for this area, the increase in woodland and decrease in montane forest during the late Pleistocene may have decreased connectivity, but did not eliminate it. Even though area is still a large influence, immigration through woodland must play a factor in these specific patch dynamics. Therefore, diversity is significantly correlated with both area
(extinction probability) and current isolation (immigration potential). Species such as Neotoma have higher connectivity because they can migrate across woodlands easier, making the matrix of the landscape less of a barrier, and therefore colonize patches easier than other species.
Species such as the Least Chipmunk, Eutamias minimus, have high resource requirements and a larger size. These mammals typically immigrate little due to the matrix being more of a barrier for them. This results in low connectivity for that specific species and area having more of an influence (Lomolino et al., 1989).
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The yellow-nosed cotton rat, Sigmodon ochrognathus, portrays a similar scenario of varied connectivity around the southern Cordillero-Madrean region (which is further south than both the “Southwest” and the Southern Great Basin, ranging from New Mexico to northern
Mexico). The distribution of this rodent has been moving northward during the last fifty years
(Davis and Dunford, 1987) and has only recently been noticed in the Cordillero-Madrean. The species is endemic to montane forests in high elevations. Other species of the genus Sigmodon thrive in the woodland/scrub and outcompete the S.
ochrognathus (Hall, 1981). Since the landscape in and around the Cordillero-Madrean is similar to that of the Great Basin (montane forest islands surrounded by a “sea” of woodland and scrub) one would assume this movement would be impossible if there was no connectivity and the species did not successfully immigrate.
Research has suggested that this species is capable of a marginal existence in the matrix and it is using this for immigration; essentially “island hopping” northward. In this particular instance, the matrix serves as a filter and connectivity is reduced but not eliminated (Davis and Dunford,
1987).
Lomolino and Davis (1997) also studied the Cordillero-Madrean region. The results were similar to Lomolino et al. (1989) in that the Cordillero-Madrean region seems to be more strongly correlated with immigration because species richness significantly declined as isolation increased. They found as one moves further north, the intermountain matrix acts like more of a barrier because it starts become to more xeric desert scrub and less mammal species can cross it successfully. Immigration directly relates to connectivity, so the overall connectivity between patches decreases as one moves further north, until eventually one would reach the Southern
Great Basin, where there is no connectivity whatsoever (Brown, 1971: 1995: Lomolino and
Davis, 1997). There were also species specific levels of connectivity. Overall, species like
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Microttus mexicanus and Sciurus alberti appear to be better at immigrating. Nestedness values of each of these faunal groups seemed to be correlated highly with immigration and less with area. Conversely, species like Lepus americanus , Chlethrionomys gpperi and Phenacomys intermedius appear to be poorer at immigration because they have been observed on less isolated islands. This indicates and more area influenced dynamics.
The northern-most area of the Great Basin (north of Brown’s (1971) study; called the
Northern Great Basin from here on) showed a deviation from the “relaxation model” too.
Grayson and Livingston (1993) observed the species that were fully isolated to montane patches in Brown’s (1971) study, in the intermountain matrix. The matrix in the Northern Great Basin is also mostly xeric scrub, but some woodland. Sylviagus nuttalli (Nuttall’s Cottontail) and
Marmota flaviventris (Yellowbellied Marmot) were observed in both the montane patches as well as the matrix. This displays that immigration is not zero (at least with these species) and there is low yet existent connectivity in this region. The effectiveness of the Northern Great
Basin intermountain habitat as a filter is species dependent, allowing at least some immigration.
Nestedness in this data set also shows that there is immigration in these certain species. The nestedness value was higher in the Northern Great Basin than in the Southern Great Basin
(Cutler, 1991). Even though some immigration does occur, patch area still played a major role in influencing certain species. Many of these mammals, like Ochotona princeps (Pika) are still fully isolated; the matrix is impeding all immigration. They are influenced totally by extinction and not immigration (Lomolino and Davis, 1997). Grayson and Livingston (1993) also believe that many of the mammals will also show less immigration as one moves further south into the
Southern Great Basin due to an increase in the relative level of xeric scrub.
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Figure 1 illustrates relative immigration with isolation for all regions. The Cordillero-
Madrean and “Southwest” species are less isolated and have higher immigration. Movement north into the Southern Great Basin shows impeded immigration and increased isolation. The
Northern Great Basin has a small but noticeable amount of immigration. Isolation is directly related to the relative amount of xeric scrub. The Southern Great Basin is all xeric arid scrub, which has been shown to be a harsh filter, impeding immigration altogether for the mammals studied. Connectivity is zero as a result. The amount of woodland increases and xeric scrub decrease as one moves both south and north of the Southern Great Basin. Less xeric scrub and the more woodland in the matrix of these areas facilitates immigration and increased connectivity between patches. Therefore, the vegetation in the matrix directly affects the amount of immigration and connectivity between patches. Lastly, certain species exhibit more movement than others due to there specific ability to immigrate across the woodland in the matrix. It seems no species studied here can only cross xeric scrub successfully.
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F IGURE 1 : How immigration relates to isolation in southwestern North America. The Y-axis “probability of immigration” is synonymous with connectivity. The amount of isolation is directly correlated with the relative amount of xeric scrub in the matrix. Connectivity between patches is a function of the relative amount of xeric scrub. The Cordillero-Madrean region has very high immigration/connectivity due to a lower amount of xeric scrub and the Southern Great Basin has almost no connectivity due to the matrix being essentially all xeric scrub (adapted from Lomolino and Davis, 1997).
D
ISCUSSION
Although the theory of island biogeography may work well in explaining many habitats, the results of this study suggest it to not be a sufficient enough explanation for the montane forest patches of southwestern North America. Although island biogeography theory is based on isolation and area of patches, it assumes that these factors are constant. This is not the case because the montane patches formed due to a temperature increase, and have decreased in area and moved up in elevation (Wells, 1983: Rickart, 2001). McDonald and Brown’s (1992) model showed that this trend is expected to continue if temperature continues to increase. Areas with zero connectivity will suffer diversity losses due to reduced area. If connectivity is above zero
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diversity will be reduced, because the distance between patches will most likely expand as the area shrinks, resulting in increased isolation, decreased immigration, and decreased connectivity.
Since patch area and isolation can (and most likely will) change due to temperature and other abiotic factors, an approach that considers this, such as the patch mosaic model, should be used to understand this dynamic.
Brown (1971) displayed a system that is totally affected by the area of patches and extinctions within these patches. This system has zero connectivity, but all other areas (Northern
Great Basin, “Southwest”, and Cordillero-Madrean) have different levels of connectivity due to variable immigration. Island biogeography states that isolation, as in the distance from one patch to another, directly influences immigration. But isolation is directly correlated with the relative amount of xeric scrub in the matrix. Immigration and connectivity are functions of the components of the matrix. This displays that the landscape components in the matrix are major factors in impeding or facilitating immigration between patches and should be considered an influence on isolation. Island biogeography considers the role of the matrix to be constant throughout the landscape (Pickett and Rogers, 1997), but the inconsistency is what promotes most of the variable connectivity in this system. A patch mosaic approach considers the matrix as a fundamental factor in relative immigration rates and therefore is a more appropriate model for this system.
In these areas where there was connectivity between patches, the rates of immigration differed based on the species. When crossing the same matrix certain species like M. mexicanus and S. alberti were better at crossing the matrix, and therefore had higher connectivity than others like L. americanus (Lomolino and Davis, 1997). S. ochrognathus was able to “island – hop” due to a certain amount of connectivity, but was not as successful in the same matrix as
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others of its genus with higher connectivity. Since many species had different connectivity in parts of the matrix that was relatively consistent, connectivity must have been species specific.
The patch mosaic model encourages a species specific scale when studying movement through and between patches (Turner et al., 2001) and therefore is a more thorough approach.
In conclusion, the main factors affecting connectivity in Southwest North American montane mammal communities are the temporal changes of patch area and isolation, composition of the matrix, and the species specific immigration ability. To better understand these factors, it is essential to incorporate a patch mosaic model that takes these factors into consideration.
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