The project "Affirmative Teaching and Innovative Learning and Teaching in Gymnasiums within the Framework of the Croatian Qualifications Framework" (2013–2015) was initiated by the Fifth Gymnasium (Zagreb) and implemented in cooperation with four additional high schools (from Vukovar, Pakrac, Knin, and Metković) and the Faculty of Science, University of Zagreb. The project involved biology teachers, faculty experts, and approximately 1,000 students.

The project had the following objectives:
1. Development and modernization of the education system and qualifications in gymnasiums,
2. Modernization of existing and development of advanced curricula for gymnasiums based on learning outcomes,
3. Development of teaching capacities for the implementation of innovative learning and teaching.

Three completely new interdisciplinary curricula were created: Research and recreation in nature, Project teaching - sustainable development and Urban environment design. As part of the Research and recreation in nature sub-project and curriculum, one of the botanical teaching topics (predicted for the duration of 20 hours) related to Methods of measuring the size of plant populations. The goal of this mini-project was for students to adopt and differentiate three basic methods of estimating population size: A) frequency measurement, B) density measurement and C) coverage estimation.

The outcomes were to enable students to:
1. explain the term "inventory",
2. analyse and calculate the size of plant species populations,
3. analyse the surface areas (coverage) of all plant species in the analysed area and
4. compare populations from different localities or habitats or studied in different periods (over time).

Namely, while the flora inventory answers the question "what we have", and flora mapping "where we have", monitoring or the process of repeated observation of "something we have somewhere" provides us with information about the movement (trend) of species populations in a certain area. Species populations can be stable in a certain area (their abundance is more or less constant and uniform), however, very often species populations show a negative trend (their abundance is decreasing) or a positive trend (their abundance is increasing), which is most often directly related to processes caused by human activity (use of habitat resources, implementation of protection measures, etc.). During the implementation of this subproject, students, together with their mentors, conducted research and analysis of plants in the area of Papuk Nature Park (eastern Croatia), and the most important results of this work will be presented and briefly discussed. The students adopted the fact that measuring the population sizes of selected plant species (e.g. endemic, endangered, protected, etc.) is crucial for assessing the status of a species, i.e. the trend to which a particular plant taxa is exposed in a certain geographical area or habitat type. We can conclude that the results of this part of the project fit into all three goals of the project and all three new interdisciplinary curricula. Additionally, tasks of this type can make students aware of changes in the environment and sensitize them to rare/threatened species, as well as to the dangers of the spread of invasive plant species. The added value of this sub-project is that it allows students to independently design their own project based on the adopted methods.

Keywords: Botany, education, fieldwork, Research and recreation in nature curriculum, secondary school students.