Across much of the world, land tenure markedly delineates and influences biodiversity conservation values, typically with residual intact biotic communities most persistent in conservation reserves, whereas lands of contrasting use and tenure have biotic communities of substantially modified composition. In much-transformed landscapes, biodiversity conservation may be largely dependent upon the network of conservation reserves [1]. In landscapes characterised by far less transformation, such as much of the world’s tropical savannahs, other land tenure types may make a contribution to biodiversity conservation that more nearly equals (and/or provides a necessary buffer and complement to) that provided by conservation reserves. Notwithstanding their recognised or assumed significance for biodiversity conservation and some obligation to demonstrate that reserve management investments produce conservation outcomes, there has been relatively little systematic consideration of the effectiveness of reserves compared with lands of other tenures [2,3,4,5,6].

Northern Australia provides an example of an extensive largely intact landscape (<5% cleared or significantly transformed) [7], managed under a range of land tenures, particularly low-intensity pastoralism (comprising 75% of the total land area), Indigenous lands (15%) and conservation reserves (6%), all characterised by general retention of native vegetation [8]. Unsurprisingly, given the relatively minor levels of modification of their lands, some landholder groups have asserted that their management provides good conservation outcomes and, hence, that there is little need for further reservation. Indeed, the maintenance of biodiversity is an explicit condition of the regulation of the pastoral industry in the Northern Territory, Australia. With respect to Indigenous lands, it has been asserted that traditional management practices are likely to be particularly effective for the maintenance of biodiversity [9]. This latter claim has fuelled substantial development of policies and practice that equate biodiversity conservation with Indigenous land management and its resourcing [10,11,12] and the recent development of Indigenous Protected Areas as significant components of Australia’s national reserve system [13,14]. However, the most substantial supportive study [9] may be an insecure basis for such a claim, given that it focused on one relatively small area and lacked controlled comparison with lands subjected to contrasting management.

Here, we undertake a systematic and very substantial assessment of vertebrate biodiversity in different land tenure types across an extensive area (ca. 300,000 km²) of the monsoonal tropics of the Northern Territory. To focus the assessment particularly on land tenure and its related management and reduce extraneous environmental variability, we restrict the survey and analysis to one vegetation type, forests dominated by Eucalyptus miniata and/or E. tetrodonta. This forest type is the characteristic vegetation across much of northern Australia, with a total area of about 450,000 km² [7]. Our objective is to compare, between different land tenure types, the biodiversity (and also the intensity of possible threatening factors) across lands supporting this vegetation type.

The land tenure types we consider are pastoral lands, Indigenous lands and conservation reserves. We recognise that in this region, as elsewhere, there may not be a close match between land tenure and the intensity and form of management. However, in this region, conservation reserves are typically managed to reduce the incidence of feral animals (principally pigs Sus scrofa, cattle Bos indicus and water buffalo Bubalus bubalis) and weeds and to manage fire regimes in a manner that aims to reduce the incidence of high intensity uncontrolled late dry season fire [15]. Pastoral lands are managed with extensive (typical property sizes 1,000 to 10,000 km²), but typically low-intensity (stocking rates typically <20 cattle/km²) ranching for beef cattle, with occasional areas of more intensified manipulation (some clearing and planting of exotic pasture grasses, but little or no additions of nutrients). The management of Indigenous lands varies along a gradient from traditional intensive manipulation of fire to absence of any management (largely due to depopulation or abandonment). Note that in this region, some conservation reserves are Aboriginal lands now jointly managed; these were treated here as conservation reserves.

A related broad-based assessment of land tenure impacts upon biodiversity and environmental management in this region was undertaken recently [16]. The current study differs from that work in that our study is (purposefully) restricted to one vegetation type, provides a direct survey-based assessment of faunal biodiversity and relates disturbance measures and biodiversity assessed together for a large set of individual quadrats.

We note that two other study designs may provide more incisive assessments of the efficacy of conservation reserves—matched cross-fence comparisons of contrasting land tenure (or management) types (e.g., [17]) and controlled time-series studies that seek to measure divergence in conservation values between a recently-established conservation reserve and a nearby unreserved area. Using the first approach, recent studies have demonstrated significant differences between pastoral lands and ungrazed lands in the richness and composition of both their vertebrate and invertebrate faunas in forests of tropical north-eastern Australia [18,19], greater abundance of native mammals in conservation reserves compared with paired sites on pastoral lands in tropical north-eastern Australia [20] and significant divergence in vegetation and vertebrate faunas across adjacent sites of markedly contrasting fire history [21]. In northern Australia, two recent studies have chronicled change over time in fauna assemblages following de-stocking (change from pastoral areas to conservation reserves), although the magnitude of the resulting increased abundance in native mammals varied appreciably between these two studies [22,23]. Furthermore, some monitoring studies have demonstrated responses over time of vegetation and vertebrates in relationship to management inputs on conservation lands [24,25,26,27].

In contextualising this study, we note that our focus is solely on vertebrate fauna and that patterns may differ for other components of biodiversity. For example, there are now well-established demonstrations that some fire-sensitive plant species may prosper on lands subjected to some forms of traditional Aboriginal fire management, but decline in lands subjected to less intensive fire management [28,29,30,31,32,33,34,35,36,37]. We recognise the limitation in our ambit, but consider it a necessary counterpoint to the bulk of those previous related studies that have asserted the benefits to biodiversity of Indigenous management mainly using evidence from a restricted (but nonetheless important) set of fire-sensitive plant species. Furthermore, we consider that the inclusion here of birds, mammals, reptiles and frogs (totalling >400 species) is likely to provide a relatively broad mix of ecologies and environmental responses to land tenure and management and, hence, a reasonably robust baseline for ecological generalizations.

Across a very large data set of quadrats sampled consistently for vertebrate fauna and a single (extensive) vegetation type, we attempted to distil the influence of land tenure (and its partly matched associate of land management) on biodiversity. Although the resulting models indicate that species richness and abundance is associated also with some other environmental factors, the results demonstrate that there is a significant influence of land tenure on vertebrate species richness and abundance. Of the three land tenures (pastoral, conservation reserves and Indigenous lands), conservation reserves supported highest species richness and total abundance for most fauna groups considered. There was some variation amongst vertebrate groups in the magnitude of this response to land tenure, with the most marked response being the markedly reduced abundance of threatened species on pastoral lands. Our study does not provide a definitive explanation of this particular response, but we speculate that it may be because many now threatened species have an association with relatively dense ground-layer cover, which would be most depleted (by grazing) on pastoral lands.

The trend suggests that management on conservation reserves may be having some beneficial impacts upon (at least vertebrate) biodiversity relative to other land types. We note that the findings of this study relate to vertebrate fauna only and may not apply comparably to plants and invertebrates; however, some previous studies in the region have indicated some broad commonality between vertebrate and invertebrate faunas in responses to land use [18,19], and we note also that sampling across birds, mammals, reptiles and frogs provides a reasonably broad scope for ecological response.

This “biodiversity dividend” associated with conservation reserves appears to be small, but significant. In part, the limited effect of conservation reserve tenure may be due to the relatively recent (mostly post-1960s) establishment of many reserves, typically from areas formerly exposed to a substantial history of pastoralism. In part, the limited effect of conservation reserve tenure may be due also to the limited resourcing for management that characterizes most conservation reserves in this region and, consequently, the limited extent to which detrimental disturbance factors are being controlled in conservation reserves [50]. In part, the limited dividend may also be because, in this region, the non-reserve land tenures are also relatively benign for biodiversity (although the results indicate that this assumption may be invalid for threatened species). Note that our use of the term “biodiversity dividend” should not imply that there has been a bonus above and beyond background rates of biodiversity. Indeed, given some broad-scale declines of biodiversity in this region across all land tenures [45], the biodiversity dividend for conservation reserves may be better interpreted as a biodiversity loss that is relatively less than the losses on other land tenures.

It may be argued that the higher values reported here for vertebrate biodiversity on conservation reserves than on other extensive land tenures may be due to deliberate establishment of those reserves on areas known to have supported high biodiversity value: that is, that the high biodiversity value preceded the conservation management. We suggest that this is not the case, because the location of conservation reserves in this region has been driven mostly by opportunity, by scenic value or by recreational potential [51].

The results also demonstrate a contrasting constellation of disturbance regimes operating across these different land tenures, with such differences in the incidence or impacts of threats presumably contributing to the observed differences in vertebrate species richness and abundance. The differences between land tenures in disturbance regimes observed in this study are consistent with some previous observations in this region, including a study restricted to the small rainforest patches embedded within the extensive savannah woodlands [52]. Both studies reported that weed impacts were highest on pastoral lands, then conservation reserves, then Indigenous lands; and both studies reported that pig impacts were greatest in conservation reserves, then pastoral lands, then Indigenous lands. However, the previous rainforest study reported that fire impacts were most severe for rainforests on pastoral lands, whereas no such trend was evident in our data [52]. Two other studies also reported an incidence of weed impacts that was higher on pastoral lands than in conservation reserves or Indigenous lands, with the relatively low incidence of weeds on Indigenous lands, most likely to be due to remoteness from major development rather than a consequence of intrinsic management [16,53].

This study provides no substantial support for previous assertions (notably [9]) of the superior value of Indigenous land management for fostering biodiversity. The broad regional analysis reported here indicates that, when corrected for variation in rainfall and rockiness, quadrats on Indigenous lands typically supported fewer frogs, reptiles, birds and mammals than comparably-sampled quadrats in the same environment on conservation reserves. However, we note that the management of Indigenous lands considered in this study varied substantially across a gradient from lands where traditional intensive management was being continued to lands that had been largely depopulated and neglected. Hence, our results here reflect land tenure per se rather than management.

Across northern Australia, there is now substantial investment by governments and a substantial policy framework for supporting environmental management actions by Indigenous people on Indigenous lands. This support is clearly providing important socio-economic contributions to disadvantaged communities, and there is evidence that involvement in management is producing health benefits [54,55]. However, while there may be some reasonable supposition that such management may produce some biodiversity benefit, particularly for some fire-sensitive plant species [9,13,14,56], there is as yet no compelling evidence to support such happy conflation more generally. The results reported here suggest that a far more rigorous evidence-based assessment of the environmental outcomes of this investment is required [57]. In considering such supporting evidence, we note the bluntness of the data we report here (one-off inventory, of species richness and total abundance) and recognise that a more sensitive and targeted measurement (and comparison) of management efficacy would be provided by monitoring of biodiversity trends comparably across lands of different tenure and exposed to contrasting management objectives, methods and resource investment.

We also recognise that there is increasing blending of Indigenous land management skills and conventional park management (the “two tool-box” approach [14,58]) on conservation reserves and Indigenous lands across much of northern Australia and increasing resources available for conservation management, particularly on Indigenous lands [14]. Such a recent development is likely to provide some increases in biodiversity outcomes, particularly for lands that have until recently been largely depopulated and unmanaged [30].

For pastoral lands, our results demonstrate that biodiversity values are being reasonably well maintained, albeit with values generally less than for conservation reserves. However, this is not the case for the set of threatened species, where the values for pastoral land were substantially (>50%) less than for conservation reserves. This contrast reinforces the point that conservation value may be poorly measured by simply the richness and abundance of broad taxonomic groups. The result here for threatened species suggests that pastoral lands may provide relatively little conservation benefit for these species and, indeed, that pastoralism across most of these north Australian landscapes may have contributed to the currently threatened status of some species. This conclusion, from broad-scale comparison, complements some findings of highly localised (typically cross-fence contrasts) that have demonstrated localised detrimental impacts of pastoralism on biodiversity [18,20,22].

This study provides correlative evidence of a response to land tenure and a foundation for subsequent research. Such research may provide stronger and more direct evidence if it involves ongoing monitoring of biodiversity at sites where land tenure (and/or management) has changed [22] and/or more intensive studies of habitat quality and ecology for species for which abundance varies between land tenures.

These results demonstrate a biodiversity dividend, albeit relatively small, that is attributable to the existence of conservation reserves, even within extensive largely intact landscapes. This conclusion, from our extensive regional-scale sampling, is broadly consistent with results from the few other studies (whose sampling has mostly involved other approaches (notably cross-fence comparisons) at smaller geographic scales) [18,19,20,21].

In this region, the continued existence of natural landscapes across the far more extensive matrix in which these reserves are embedded may be a major reason why these reserves maintain their biodiversity values [59]. Although less than for conservation reserves, Indigenous lands and pastoral lands are here shown to have substantial biodiversity value, notwithstanding their contrasting management regimes, intensity and objectives. Given the relatively small proportion of lands currently devoted to conservation reserves in this area, these unreserved lands make a very substantial contribution to regional biodiversity conservation. This conclusion suggests that there is substantial justification for investing in biodiversity management for lands of all tenures and managing this ecologically continuous land for biodiversity outcomes without excessive concern for cadastral boundaries. This is a markedly different situation to many other parts of the world, where broad-scale land-use impacts outside reserves result in a more marked contrast in biodiversity values between reserved and unreserved lands and probably ultimately lead to erosion of park values through fragmentation effects [60,61,62].

In this extensive natural landscape, lands of all tenures may contribute substantially to biodiversity conservation, but conservation reserves make the most substantial contribution. However, even in the largest and best-resourced conservation reserve in this region (Kakadu National Park), some components of biodiversity are currently undergoing considerable decline, suggesting that the management of threats even in conservation reserves is insufficient to meet the conservation need; that the biodiversity dividend reported here for conservation reserves may instead be simply a biodiversity detriment that happens to be less than on other land tenures [27,45].