Landscape Corridors and found that connecting otherwise isolated habitat patches could lead to faster rates of spread (Hess, 1994) By exploring these effects through a model, hypotheses could be initially tested and the study’s findings could be used to appropriately guide tests in real landscapes (e.g., Johnson and Haddad, 2011) In another example, Earn et al (2000) determined that landscape connectivity may lead to negative effects by promoting connectivity-mediated synchrony For evolutionary effects, Orrock (2005) explored how corridors affect rates of adaptive evolution and determined that the effects can be positive or negative, depending on the frequency of disturbance When disturbance is rare, corridors increase the fixation of beneficial alleles and the loss of harmful alleles (Orrock, 2005) However, when disturbance rates are high, the opposite occurs Empirical Evidence Observational Studies Observational field studies of corridor effects have provided insight into corridor function by incorporating realistic landscape features such as heterogeneous resources and large spatial scales Although observational studies provide a very important endpoint on the continuum of understanding corridor effects, they are also limited because they can be confounded by features that co-occur in natural landscapes (e.g., connectivity, area, and patch-shape effects; see Separating Connectivity Effects from Confounding Factors) and may lack true replication Here the author includes what others have referred to as ‘‘quasi-experiments’’ (Rosenberg et al., 1997) or ‘‘natural experiments’’ (Gilbert-Norton et al., 2010) as observational studies since they not include randomized manipulations or true controls One of the earliest studies of corridor effects in real landscapes was reported by MacClintock et al (1977), who demonstrated that bird species richness was higher in a single forest patch that was connected to other forested habitat by a corridor than in another unconnected forest patch Notably, this early study was conducted at the community level, which remains a challenge for ecologists to study at landscape scales However, this study contained a single replicate, so although the study’s findings were intriguing they were not especially robust Several observational studies have made use of existing shelterbelts (wooded habitats planted to provide windbreaks within agricultural landscapes) that connect otherwise isolated woodlots within agricultural settings Haas (1995) found that shelterbelts in south central North Dakota, USA, promoted the movement of Robins and Thrashers Harvey (2000) found that shelterbelts in Monteverde, Costa Rica that were connected to forest habitat had higher forest tree-seedling densities and greater numbers of forest tree species This was likely due to frugivorous birds moving more frequently through the connected shelterbelts because the density and diversity of bird-dispersed tree species were especially high These studies suggested that corridors (i.e., shelterbelts) promoted movement of birds and bird-dispersed seeds, but because the size and spacing of shelterbelts was not controlled, these studies were unable to determine how corridors functioned 469 A quasi-experimental study of corridor effects on diversity was conducted by Schmiegelow et al., (1997) using habitat patches of different sizes that were either connected or unconnected to adjacent existing habitat However, this study did not control for the area of the corridor Surprisingly, they found weak effects of corridors and the highest diversity was found in the smallest patch size, which contradicted theoretical predictions However, area and connectivity effects were confounded in this study because the corridors in their system were wider than the smallest habitat patches In addition, patches were nonrandomly located so connected patches were always adjacent to riparian areas, resulting in location and connectivity being confounded This study demonstrated the importance of controlling for the various mechanisms by which corridors can work and for the quality of the surrounding habitat matrix Machtans et al (1996) also conducted a quasi-experiment before and after the harvesting of forests that were adjacent to riparian buffer strips They measured the frequency of bird movements through these landscapes and found that buffer strips enhanced movement of juveniles by promoting connectivity and maintained the movement rates of adults They also found that corridors were used less frequently when interpatch distances were small, suggesting that corridors may be more important at larger spatial scales where movement is disproportionately enhanced by the presence of corridor One of the most robust approaches that can be used in observational studies is to infer landscape connectivity from genetic similarity among individuals because it is a measure that integrates past organism movement across landscapes For example, Hale et al (2001) measured gene flow in British red squirrels in the UK They found that recent plantings of conifer forests, which connected habitat fragments in the north of England with the south of Scotland, increased the amount of genetic mixing in squirrel populations In another example, McRae and Beier (2007) determined genetic similarity for several types of organisms and then correlated this with modeled measures of landscape connectivity They found an especially effective metric was the amount of landscape ‘‘resistance,’’ analogous to electrical resistance, which was strongly correlated with the genetic similarity of the organisms studied Such tight links between models and data hold promise for predicting the movement of organisms across large landscapes Experimental Studies Experimental studies can overcome some of the inherent challenges of observational systems in that they can control for various factors (e.g., area, connectivity, patch shape) that would otherwise be confounded They also can use higher levels of replication that can provide robust tests of corridor effects Controlled and replicated experiments, however, are logistically difficult to conduct at large scales, so most experiments have been conducted using microcosms, mesocosms, or small field plots Such studies lack the realism of actual landscapes where corridors are being implemented but allow for a more complete understanding of the mechanisms underlying corridor effects In the following sections,