Using a considerably larger sample size and suite of predictor variables than our previous reviews, our results both support findings of prior studies and provide new insights into the role of project design, national context, local community characteristics, and outcome synergies. Although we do not find a systematic effect of national level variables on CBC outcomes, we do find some support for the importance of community characteristics, and strong support for success depending on several aspects of project design. Before discussing the wider implications of our results, we start by reviewing potential effect modifiers in this study, and how they may affect interpretation of our results.
Reasons for heterogeneity
While there is evidence that CBC can be an effective conservation tool, it is clearly not always successful. Here we discuss the ways in which particular variables might affect CBC success.
The lack of evidence for significant effects of national level indicators on projects success fails to support our hypotheses (NC1 & NC2), which represent the view that transparent and effective national governance influences project success through various mechanisms. These mechanisms might include enhanced trust as well as conditions that facilitate valuing future returns from resources more highly among its citizens. This result also runs counter to the finding from the only other quantitative study of national indicators, that HDI is important for successful fisheries co-management
Conversely, project design variables were important for successful CBC outcomes. In fact, three of the four project design hypotheses (H-PD1, -PD-3, and -PD4) were supported to some degree, consistent with previous reviews emphasizing the importance of (a) building local institutional capacity
 and training and skills development
[15, 22], (b) equitable distribution of benefits including avoidance of elite capture
[16, 104], (c) engagement with local institutions and cultural beliefs and traditions
[14, 105, 106], (d) the provision of social capital and other intangible social benefits
[15, 16, 23], and (e) participation in rule making and day-to-day decision-making associated with the project
[16, 19, 77]. In our study, capacity building was a significant predictor of all but attitudinal outcomes indicating that it may be a particularly important component of project design. In addition to the straight-forward skills that may be necessary to maintain the project, Baland and Abraham
 argue that capacity building may also help combat elite capture by ensuring that project finances are not simply funneled to a handful of local leaders.
Several examples from projects included in our review illustrate the importance of these project design variables. For instance, in a project in Yunnan, China, community members were trained to monitor resources
. Through this process the community gained an awareness of broader forest management needs beyond timber and drafted their own rules for sustainable harvesting of forest products. A project in Tanzania had similar results in that participatory monitoring efforts led to reductions in wildlife traps found in local forests and general improvements in forest quality
. Additionally, evidence from a project in Costa Rica suggests that providing opportunities for community learning was an important component of successful and sustainable sea turtle egg-harvesting
Finally, although not considered a feature of project design in our study, we found (unlike previous reviews
[20, 55]) that projects that have been established for a longer period of time are more likely to have economic success than more recently initiated projects. This result provides evidence that development opportunities and income generation may not emerge quickly and that CBC projects may require time before measureable economic success is achieved.
On the other hand, community characteristics were less important for CBC success than were project design variables. Only two hypotheses in this domain - H-CC2 and, to a lesser extent H-CC3 - were supported. Strong tenure rights were related to economic success and both population size and local culture and institutions affected behavioral success. This does not mean that community characteristics are unimportant. In fact, many reviews point to aspects of local context that are thought to be key to securing successful outcomes, such as a supportive local belief system
[15, 20] and well defined property rights and local tenure regimes
[15, 19, 22]. In addition, we want to reiterate that the local institutions variable could not be included in the multivariate analysis for ecological outcomes (see the paragraph on separation problems in Section 3.9.2 Multivariate analysis). This issue leaves open the question of whether supportive local cultures and effective local institutions are important for ecological success.
Among the cases in our sample, there are several examples of the importance of local context. Scanlon and Kull
 note that cultural pride and a shared sense of belonging (as well as the provision of economic benefits) were important for attitudinal and behavioral success in a community conservancy in Namibia. Further, Bajracharya et al.
 note that a history and culture of cooperation in addition to strong traditional management institutions (Ban Samiti) were crucial for the behavioral and ecological success of the Annapurna Conservation Area in Nepal. Finally, secure sea tenure provided an important foundation for the success of a marine protected area in the Solomon Islands
. These anecdotes provide examples of some of the ways in which local cultural contexts can affect project success. Local contexts might relate to lower rates of discounting the value of future resource availability, make communities responsible for broader environmental impacts (externalities), motivate sustainable resource use, increase accountability, and/or increase the salience of trust, reciprocity, and social norms
[48, 58]. However, our findings again suggest that well-designed projects can, in many instances, overcome unfavorable features of the local cultural and institutional context.
Several variables that we expected to be significantly associated with project success were not. In the realm of community characteristics, we found no evidence that market access was positively or negatively associated with any measure of success. This is important because some individual studies and reviews show positive associations between project outcomes and some measures of market integration
[13, 21, 23], while others find that market integration can lead to greater resource extraction and ecological degradation
[44, 77, 100, 113]. The lack of a significant association between market integration and any of the outcome variables may be attributable to two factors. First, we found coding for market integration to be problematic due to the limited information often presented in the project evaluations we coded. Authors of several previous reviews have also noted similar difficulties in adequately extracting measures of market integration
[19, 78]. Second, it is likely that market effects are contingent on any number of other variables including the nature of the resources in question, the size and make-up of the community, and the type of market. These aspects of market integration deserve closer quantitative examination.
Local ecological conditions are also mentioned frequently in the literature
[15, 16, 21, 77], but had no discernable impact in our study. Few articles in our sample provided detailed information about ecological characteristics so it is not entirely surprising that our coarse measure of ecoregion status was not associated with project outcomes. Future research could benefit from including measures of habitat types, elevation, rainfall and other ecological characteristics from outside sources.
In addition, there are several variables for which we attempted to collect information but that we omitted from the analysis because of insufficient information in the projects in our sample. These variables could have affected our results in a number of ways. For instance:
– Local ecological conditions like rainfall, elevation or predominant habitat type could affect the abundance and spatial heterogeneity of valuable resources in any given year or set of years, which may not be captured in short-term project evaluations. Such variation could influence the ecological outcomes that are reported at a given point in time.
– Historical inter-community and intra-community dynamics may affect levels of trust and cooperation as well as resource use patterns in ways that may not be visible to researchers.
– Projects related to resources that are fundamental for subsistence may resonate more strongly in communities and thus result in greater community participation in all stages of the project than those related to resources that create a fundamental hardship for local peoples (e.g. land for agriculture vs. crop damage from wildlife). That is, levels of local participation may be higher for projects targeting resources that have a particular level of importance to a community.
– The implementation of national policies related to CBC can differ greatly between nations
[5, 6] making it difficult to ascertain how important national policies actually are for CBC outcomes.
The degree and type of involvement of local or international NGOs and/or multilateral aid organizations could conceivably affect project outcomes. Such organizations may provide funding, consultation, or any number of other services and the nature of the organization (local, international, conservation-focused, development-focused, etc.) could influence the way a project is designed and implemented and how long funding is provided and for what activities. Similarly, one reviewer suggested that consideration be given to overlap in funders in the event that major funding organizations may influence the ways in which projects are designed and implemented. Unfortunately, many projects have multiple sources of funding making it difficult to discern how much funding came from a given source, not to mention how much influence such funding would have allowed. That said, future research could consider the potential that projects funded by the same agency or organization are designed and implemented in similar ways.
In short, there is the potential that additional factors could directly affect outcomes or mediate the effect of particular predictors on outcomes.
Review limitations and potential biases
The review was somewhat limited by the omitted variables noted above, although this was unavoidable since it depended on the information presented in the articles we reviewed. A larger set of search terms (e.g. community forest management, community forestry, community fisheries, community wildlife management, payment for ecosystem services, co-management) might have also increased the sample size. Given (a) the number of variables identified through theory, modeling, and empirical studies that potentially influence CBC outcomes, and (b) the multiple scales at which those variables operate (household, community, region, nation), a larger sample could help further tease out key relationships between predictors and outcomes and aid in examining the role of national socio-economic and political contexts.
This shortcoming is partially offset by fact that the current sample is nearly five times larger than that from Brooks et al.
, and over twice as a large as that use in the analysis by Waylen et al.
. Further, it is unlikely that there would be any systematic bias as a result of the terms that we did or did not use in the search.
The generally short time frame of many of the projects in our sample (an average of 7.7 years from the initiation of the project to the date research was conducted) may limit our ability to determine whether certain successful outcomes might presage other types of success in the future. For instance, while resource use may be considered to be economically beneficial at the time the data were collected, we have no evidence that the harvest of that resource is sustainable over the long term. Similarly, the relatively short time frame also precludes us from fully understanding the nature of synergies and tradeoffs.
As for any review there is the potential for publication bias, in which authors fail to publish or report on projects that have not produced successful outcomes. However, given (a) the controversy and debates that highlight multiple perspectives on CBC and suggest that there is as much interest in documenting failure as documenting success, (b) the fact that 26%
 of all outcomes were reported as failures, and (c) the absence of a relationship between author discipline and any of the four outcomes that would indicate a disciplinary bias in reporting results, we feel that publication bias presents a minimal problem for this analysis.
It is also important to note that the 136 CBC projects in our analysis may not represent a random sample of CBC projects because information on all existing CBC projects is not available. It would be impossible to obtain a true random sample of projects because of the absence of documentation and reporting on all CBC projects. However, because we selected cases without knowledge of the outcomes, we believe it is appropriate to generalize the findings beyond this sample.
Additional biases may include,
 bias arising due to poor study designs,
 author bias/conflict of interest, and
 bias in how interventions are described and which aspects are reported. It is impossible to eliminate or even identify all these sources of bias, though it is important to be cognizant of them.